Coherent Ph oton Sources ÂNonlinear Photonic ... - Photonics Journal

Coherent Photon SourcesNonlinear Photonic Effects 

Nano-PhotonicsBio-Photonics - Silicon Photonics 

Photonics Materials and Engineered Nanostructures 

Plasma Photonics, Terahertz and Microwave Photonics 

Integrated Photonic Systems

IEEE Photonics Journal Breakthroughs in Photonics 2010 

Editorial 

Breakthroughs in Photonics 2010 

Table of Contents 

Breakthroughs in Photonics 2010 . . . . . . . . . . . . . . . . . . . . . . . . C. S. Menoni 244 

Coherent Photon Sources from Far Infrared to X-Rays 

Current Status on High Average Power and Energy Diode Pumped Solid State 

Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J.-C. Chanteloup and D. Albach 245 

Next-Generation Light Sources in 2010 . . . . . . . S. G. Biedron and S. V. Milton 249 

Fundamentals of Light Propagation and Interaction; Nonlinear Effects 

Nonlinear-Optical Probe for Ultrafast Electron Dynamics: From Quantum Physics 

to Biosciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Zheltikov 255 

Photonics Materials and Engineered Photonic Structures 

Semiconductor Core Optical Fibers . . . . . . . . . . . . . . . . . . . . . . . . . S. Morris, 

J. Ballato, T. Hawkins, P. Foy, B. Yazgan-Kokuoz, C. McMillen, R. Stolen, and R. Rice 259 

III-Nitride Optoelectronic Devices: From Ultraviolet Toward Terahertz . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Razeghi 263 

New Materials for Short-Pulse Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Druon, F. Balembois, and P. Georges 268 

Single-Photon Counting Detectors . . . . . . . . . . . . . . S. D. Cova and M. Ghioni 274 

Biophotonics 

Optical Bioimaging 2010: Seeing More, Deeper, Faster . . . . . . D. D. Sampson 278 

Nano-photonics 

New Design Principles for Nanoplasmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . A. I. Fernández-Domínguez and S. A. Maier 284

IEEE Photonics Journal Breakthroughs in Photonics 2010 

Metal-Cavity Nanolasers . . . . . . . . . . . . . . . . . . S. L. Chuang and D. Bimberg 288 

Graphene Nanophotonics . . . . . . . . . . . . . . . . . . . . . . . . F. Xia and P. Avouris 293 

Silicon Photonics 

Optical Interconnects Using Plasmonics and Si-photonics . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . N. Pleros, E. E. Kriezis, and K. Vyrsokinos 296 

Plasma Photonics 

Emergence of Plasma Photonics . . . . . . . . . . . . . . . . . . . . . . . . . . J. G. Eden 302 

Terahertz Photonics 

Advances in Terahertz Waveguides and Sources . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . H. Pahlevaninezhad, B. Heshmat, and T. E. Darcie 307 

Microwave Photonics 

Recent Breakthroughs in Microwave Photonics . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . I. Gasulla, J. Lloret, J. Sancho, S. Sales, and J. Capmany 311 

Integrated Photonic Systems 

Future of Transmission Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Hirano 316 

Major Accomplishments in 2010 on Optical Fiber Communications . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. E. Willner, Z. Pan, and M. I. Hayee 320 

Toward the Shannon Limit of Spectral Efficiency . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L.-S. Yan, X. Liu, and W. Shieh 325 

Breakthroughs in Optical Wireless Broadband Access Networks . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X. Zhang, B. Hraimel, and K. Wu 331

IEEE Photonics Journal 

Editorial 

Breakthroughs in Photonics 2010 

Breakthroughs in Photonics is an annual Special Issue of the IEEE PHOTONICS JOURNAL that 

showcases major accomplishments across the broad spectrum of Photonics Science and 

Technology within the last year. 

Breakthroughs in Photonics 2010 contains 19 invited comprehensive refereed reviews written by 

experts that cover progress in the areas of Coherent Photon Sources from Far Infrared to 

X-Rays, Fundamentals of Light Propagation and Interaction; Nonlinear Effects, Photonics 

Materials and Engineered Photonic Structures, Biophotonics, Nano-photonics, Silicon 

Photonics, Plasma Photonics, Terahertz Photonics, Microwave Photonics, and Integrated 

Photonic Systems. 

The topics of the invited reviews are a subset of a broader and intense activity in Photonics 

worldwide and reflect on the scope of the journal. The selection of the topics and invited 

contributions is carried out by the IEEE PHOTONICS JOURNAL Editorial Board. 

I am indebted to the authors, the IEEE PHOTONICS JOURNAL Editorial Board, editorial staff, and the 

IEEE Publications Department for their invaluable contributions to this Special Issue of the IEEE 

PHOTONICS JOURNAL. 

Carmen S. Menoni, Editor-In-Chief 

Vol. 3, No. 2, April 2011 Page 244

IEEE Photonics Journal High Average Power and Energy DPSSLs 

Current Status on High Average Power and 

Energy Diode Pumped Solid State Lasers 

Jean-Christophe Chanteloup and Daniel Albach 

(Invited Paper) 

Laboratoire Utilisation des Lasers Intenses (LULI), Ecole Polytechnique, 

CNRS, CEA, UPMC, 91128 Palaiseau, France 

DOI: 10.1109/JPHOT.2011.2140097 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received March 9, 2011; revised March 16, 2011; accepted March 22, 2011. Date of current 

version April 26, 2011. Corresponding author: J.-C. Chanteloup (e-mail: jean-christophe.chanteloup@ 

polytechnique.fr). 

Abstract: With ongoing efforts in Europe, the United States, and Asia on power production 

through inertial fusion, intense work has been focused on proposing and studying Diode 

Pumped Solid State Lasers (DPSSLs). Such drivers should be able to deliver 1 to 10 kJ at a 

repetition rate in the 10-Hz range and a wall-plug efficient nearing 10%. Recent achievements 

will be presented with emphasis on 100 J-class prototypes, which are currently being built. 

Index Terms: Solid lasers, power lasers. 

High-Power laser Energy Research (HiPER) [1]–[5] in Europe, Laser Inertial Fusion Engine 

(LIFE) [6], [7] in the United States, and Generation of Energetic Beam Ultimate (GENBU) [8] in 

Japan are scientific programs that are dedicated to demonstrating the feasibility of laser driven 

fusion [1] as a future energy source. Fusion energy is an attractive, environmentally clean power 

source using sea water as its principal source of fuel. Demonstration of the scientific proof of 

principle is expected between 2011 and 2012 as part of the National Ignition Fusion (NIF) and Laser 

Méga Joule (LMJ). These programs rely on two main pillars, respectively, associated with fusion 

physics and laser engineering (through Diode Pumped Solid State Laser (DPSSL) studies). 

LIFE laser system [9]–[11] is designed considering the 15 years of experience the Lawrence 

Livermore National Laboratory (LLNL), acquired on the Mercury laser Diode Pumped Solid State 

Laser (DPSSL), which demonstrated 61 J at 10 Hz in 2008 [12]. Aiming at delivering several kJ at a 

10 to 20 Hz repetition rate with a 9 10% efficiency, the foreseen architecture would rely on a dual 

amplifier in cavity (NIF-like scheme). It relies on gas cooled Nd 3þ doped phosphate glass, while 

Yb 3þ or Tm 3þ are still considered as alternate doping ions for ceramics or crystal host matrices like 

YAG, Y2O3, S-FAP, CaF2 or SrF2, whereas in 2010, assessment of basic properties of several new 

laser glasses was performed, and several subscale prototype experiments are foreseen for 2011, 

like new Pockels cell technology, near field spatial filtering, or advance laser diode pulser. 

GENBU is a milestone in Institute for Laser Engineering (ILE, Osaka, Japan) laser development 

for fusion reactor drivers [8]. The main laser will deliver 1 kJ at a repetition rate ranging from 50 to 

100 Hz in the picosecond regime. The two-stage amplifier relies on cryogenically cooled Yb 3þ :YAG 

with the original Total Reflection Active Mirror (TRAM) architecture [13], [14]. ILE work relies on a 

decade long experience on DPSSL with the High Average-power Laser for Nuclear Fusion 

Application (HALNA) program, which demonstrated 21 J at 10 Hz in 2008 [15]. 



Fig. 1. (Right) Existing and foreseen European high energy DPSSL facilities distribution over a log–log 

energy versus repetition rate map. (Left) Summary of ongoing and foreseen high energy DPSSL 

programs in Europe in 2010. 

Considering recent shut down (Mercury, USA) and reconversion (HALNA, Japan) of experimental 

high energy DPSSL programs for IFE, the momentum is now pointing toward Europe in the 

beginning of this new decade. Let us therefore concentrate now on European efforts largely linked 

to the HiPER program (ns and ps regime) and to Extreme Light Infrastructure (ELI; http://www. 

extreme-light-infrastructure.eu/), which is a consortium of European laboratories where, although at 

a lower energy level than HiPER, DPSSL activities are going on as well. Fig. 1 gives an overview of 

European landscape on high-energy DPSSL physics. 

The HiPER program, which is part of European Authorities roadmap since 2006, is currently in its 

preparatory phase (2008–2011). Twenty six European partners share expertise to study ignition 

scheme and laser driver design. The power-to-grid demonstration is expected at the horizon of 

2035–2040 after testing the key reactor components. Three DPSSL schemes are explored in 

conjunction with 100 J test bed prototypes [2], [3], [16]: 

• The Science and Technology Facilities Council Rutherford Appleton Laboratory (STFC-RAL), 

United Kingdom, proposes a kJ scheme based on high pressure Helium cooled slab amplifiers 

at cryogenic temperature [17]. The proposed architecture is similar to the LLNL Mercury 

program with a noticeable difference in terms of gain medium: ceramic Yb 3þ :YAG in place of a 

S-FAP crystal. In order to experimentally explore that option, STFC-RAL started, in 2009, the 

DIPOLE program. A 10-J prototype was commissioned in 2010 with a He cryo-cooled gas 

amplifier hosting four 55-mm diameter co-sintered ceramics YAG slabs (Yb 3þ as lasing ion and 

Cr 4þ in periphery for cladding). 

• The Institut für Optik und Quantenelektronik at the Friedrich Schiller Universität (IOQ-FSU) 

Jena, Germany, proposes a HiPER scheme based on angular and spectral multiplexing 

extraction through Yb 3þ :CaF2 slabs. The gain medium is cryo-cooled using a Helium gas high 

pressure flow in a quite similar way to the STFC-RAL proposed HiPER amplifier scheme. IOQ- 

FSU operates the DPSSL Petawatt Optical Laser Amplifier for Radiation Intensive experimentS 

(POLARIS) laser system for several years and is deeply involved into Yb:CaF2 growth. 

POLARIS current energy achievement is 8 J on a daily basis. 

• The Centre National de la Recherche Scientifique (CNRS) Laboratoire pour l’Utilisation des 

Lasers Intenses at the Ecole Polytechnique, Palaiseau, France (LULI-CNRS) proposes a 

HiPER scheme based on cryo-cooled active mirror amplifiers with a static Helium cell at low (10 to 

100 mbar) pressure. Six amplifiers in a double pass configuration would be required to reach the 

10 kJ unit beam requirement for HiPER. This innovative cooling scheme will be explored in the 

LULI DPSSL program Lucia. In 2010, after activation of its first water-cooled active mirror amplifier 

head, this laser was able to deliver around 7 J at 2 Hz [18]. 



To complete the overview of European efforts toward high energy DPSSLs, let us mention other 

important projects currently under development: 

• The Institute of Physics of the Academy of Science (Prague, Czech Republic) launched the 

High-average power pulsed lasers (HiLASE) project in January 2011, which aims to deliver 100 J 

at 10-Hz laser pulse trains by 2015. Two amplifier architectures will be simultaneously explored in 

the first phase: thin disk with back conductive cooling and cryo-cooled slab disks with Helium at 

high pressure. 

• The Helmholtz-Zentrum Dresden-Rossendorf (HZDR, Dresden, Germany) is also exploring 

DPSSL systems, although at a smaller scale. Recent developments have shown a 1.5-J diodepumped 

laser using an Yb 3þ :YAG slab at room temperature [19], with the option for Yb 3þ doped 

CaF2. 

Lucia, which is the LULI DPSSL program, relies on active mirrors cooled from the HR coated 

back surface of Yb 3þ :YAG crystals (see Fig. 2). All crystals are large enough (60 mm diameter for 

the main amplifier for a 24 mm extraction beam) to help circumvent transverse oscillations due to 

ASE. Over the past two years, most of the efforts were dedicated to ASE [20] and thermal [21] 

management. 300- J 7 ns pulses produced by the oscillator are amplified in two active mirror 

multipass preamplifers to reach around 100 mJ before entering the main amplifying stage for a fourpass 

extraction layout, leading to a 7-J pulse train [18]. Recent improvements in injected beam 

quality allowed us to overcome the 10-J threshold level in 2011. 

Lucia is used as a test bed for further development related to the HiPER or ELI programs. Gain 

medium engineering is among the key aspects currently explored by the Lucia team [22]; gradient 

doped (several at% per cm) and large (10-cm-diameter range) Yb 3þ :YAG crystals have been 

successfully grown in collaboration with Laserayin Tekhnika csc (www.laser.am) [23]. 

Another promising axis of research is dedicated to the design of an efficient, tunable, and longterm 

reliable cooling architecture for a large-disk laser amplifier at cryogenic temperature. The Lucia 

second amplifier head will indeed rely on cryogenic cooling with a thin, low-pressure gas cell 

located at the HR side of the active mirror gain medium. This innovative cooling concept is relevant 

for HiPER/ELI as well. 

Finally, let us mention the very active field of DPSSLs relying on chirp pulse amplification. Recent 

energetic achievement [24]–[26] has indeed demonstrated that 100 mJ to J level short pulses can 

be produced in the 10–100-Hz repetition rate range, opening the way to 1-to-10-W average power 

applications requiring ps and fs pulse durations. 

References 

Fig. 2. (Right) Lucia laser layout pictured with oscillator and amplifying stages. (Left) Four pass 

extraction scheme illustration at the amplifier level. 

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[8] H. Furuse, Y. Takeuchi, T. Nakanishi, A. Yoshida, R. Yasuhara, T. Kawashima, H. Kan, N. Miyanaga, and J. Kawanaka, 

BRecent progress in GENBU laser,[ in Proc. 6th Int. Workshop HEC-DPSSL, Versailles, France, Sep. 8–10, 2010. 

[9] A. Bayramian, BLIFE laser system update,[ in Proc. 6th Int. Workshop HEC-DPSSL, Versailles, France, Sep. 8–10, 

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[10] J. A. Caird, V. Agrawal, A. Bayramian, R. Beach, J. Britten, D. Chen, R. Cross, C. Ebbers, A. Erlandson, M. Feit, B. Freitas, 

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C. W. Siders, E. Stappaerts, S. Sutton, S. Telford, J. Trenholme, and C. P. J. Barty, BNd:Glass laser design for laser 

ICF fission energy (LIFE),[ in Proc. 18th Top. Meeting TOFE, San Francisco, CA, Sep. 28–Oct. 2, 2008. 

[11] J. A. Caird, V. Agrawal, A. Bayramian, R. Beach, J. Britten, D. Chen, R. Cross, C. Ebbers, A. Erlandson, M. Feit, 

B. Freitas, C. Ghosh, C. Haefner, D. Homoelle, T. Ladran, J. Latkowski, W. Molander, J. Murray, S. Rubenchik, 

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[12] A. Bayramian, J. Armstrong, J. G. Beer, R. Campbell, B. Chai, R. Cross, A. Erlandson, Y. Fei, B. Freitas, R. Kent, 

J. Menapace, W. Molander, K. Schaffers, C. Siders, S. Sutton, J. Tassano, S. Telford, C. Ebbers, J. Caird, and C. Barty, 

BHigh-average-power femto-petawatt laser pumped by the Mercury laser facility,[ J. Opt. Soc. Amer. B, Opt. Phys., 

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[13] J. Kawanaka, Y. Takeuchi, A. Yoshida, S. J. Pearce, R. Yasuhara, T. Kawashima, and H. Kan, BHighly efficient 

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[14] H. Furuse, J. Kawanaka, K. Takeshita, N. Miyanaga, T. Saiki, K. Imasaki, M. Fujita, and S. Ishii, BTotal-reflection activemirror 

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[15] R. Yasuhara, T. Kawashima, T. Sekine, T. Kurita, T. Ikegawa, O. Matsumoto, M. Miyamoto, H. Kan, H. Yoshida, 

J. Kawanaka, M. Nakatsuka, N. Miyanaga, Y. Izawa, and T. Kanabe, B213 W average power of 2.4 GW pulsed 

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J. L. Collier, and B. J. Le Garrec, BLaser concepts for a rep-rated multi-kJ ICF-driver of the HiPER facility,[ in Proc. 

ICUIL Conf., Watkins Glen, NY, Sep. 26–Oct. 1, 2010. 

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IEEE Photonics Journal Next-Generation Light Sources in 2010 

Next-Generation Light Sources in 2010 

Sandra G. Biedron and Stephen V. Milton 


Colorado State University, Department of Electrical and Computer Engineering, 

Fort Collins, CO 80523 USA 

DOI: 10.1109/JPHOT.2011.2135846 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received February 28, 2011; revised March 18, 2011; accepted March 22, 2011. Date of 

current version April 26, 2011. Corresponding author: S. G. Biedron (e-mail: biedron@engr.colosate. 

edu). 

Abstract: As electron beam quality and control continue to improve, so do the capabilities of 

free-electron lasers (FELs). Significant advances have occurred in the field over the last few 

years and have greatly enabled the end users of the FELs, i.e., those doing cutting-edge 

research via use of the extremely high-brightness FEL light pulses. After a brief review of the 

basics of FELs, we describe the present frontiers of FEL research and development (R&D), 

along with several of the most recent demonstrated advances. 

Index Terms: Photon sources, coherent sources, free-electron lasers, tunable lasers, 

particle accelerators, synchrotron radiation, linear accelerators. 

1. FEL Basics 

A high-energy electron traveling through a sinusoidally varying transverse magnetic field will emit 

light with a characteristic wavelength 

¼ u 

2 2 ð1 þ K 2 =2Þ 

where u is the period of the magnetic field, is the normalized electron beam energy, and K is the 

normalized peak magnetic field strength. 

If a bunch of N electrons are randomly distributed in phase space (with a bunch length longer 

than the wavelength given above), and if there is no significant interaction between the electrons 

(each electron radiates independently), the resultant electromagnetic (EM) wave from each electron 

has a random phase and, therefore, adds incoherently (the field grows like the square root of N and 

the power is proportional to N). 

In a free-electron laser (FEL), the electrons are forced to radiate in phase, i.e., coherently [1]. This 

is done by allowing the ensemble of electrons to both move in the sinusoidally varying magnetic 

field of an undulator or wiggler magnet and interact with a strong EM field of wavelength equal to the 

resonant wavelength above. This interaction forces the electrons into Bmicrobunches[ spaced by 

the resonant wavelength. Once this begins, the electrons in the microbunches radiate coherently 

and enhance the present EM field. Exponential growth is inevitable until saturation is reached. The 

resultant coherent signal emitted by the electron bunch is then many orders of magnitude higher in 

power than the incoherent signal from an unmicrobunched beam. 

An interesting aspect of FELs is that they are continuously tunable in wavelength via either 

changing the electron beam energy or strength of the magnetic field K . There is also no classical 



limit on how short a wavelength is attainable by an FEL; however, quantum effects ultimately create 

a limit in the very hard X-ray regime. 

2. Key Ongoing FEL Research and Development Areas 

Technology enhancements are driving the capabilities of FELs into new realms. Improvements in 

the electron beam brightness (high charge in a small 6-D phase space) have lead to major 

advances in the wavelengths that are obtainable [2]. 

Timing and synchronization control is also critical to FEL performance, particularly if used in 

seeded configurations or in user applications requiring pump-probe configurations. Enhancements 

in this area have been able to show timing and synchronization stabilities over hours that are sub 

10s of femtoseconds (fs) over kilometers [3]. 

The use of a seed source to both improve the temporal coherence and intensity stability of the 

FEL is also of major interest and is being pursued by just about all new amplifier proposals or 

projects that are either operational or under construction [4]. 

In other areas of the FEL landscape, energy-recovery linacs continue to improve in performance 

and are enabling high-average powers required by the user community [5], the desire for 

compactness is driving novel subsystems such as laser/plasma wakefield accelerators and mini 

undulators [6], and there are concepts to extend FEL oscillator performance into the X-ray 

wavelength regime [7]. 

3. Some Highlights of 2010 

There have been a number of advances over the past couple of years in FELs, and we will highlight 

some, but certainly not all, of them. Those interested in the current status of FELs should refer to the 

large volume of work presented at the annual International Free-Electron Laser Conferences [8]. 

Below are a few of the more recent accomplishments around the world. 

After a near flawless commissioning 2 years ago, the Linac Coherent Light Source (LCLS) at the 

SLAC National Accelerator Laboratory has delivered high-quality, very-high-peak power X-ray 

pulses to a wide range of experiments [9]. With its up-to-14.35-GeV electron beam, X-ray output 

pulses at photon energies of 8 keV, peak powers upward of 9 GW, and pulse lengths below 100 fs 

make it ideal for the exploration of materials of subnanometer size and now, most recently, of a 

noncrystalline biological sample with a single optical pulse [10]. In 2010, they tested a second 

harmonic afterburner scheme [11] to achieve higher powers on the desired harmonic. This is one of 

a few schemes proposed by the FEL community to increase the harmonic powers in FELs [12]. 

Also, the LCLS has measured the harmonic content of the FEL [13]. The harmonic content agrees 

with the code predictions and with the harmonic to fundamental ratios seen in previous, longer 

wavelength comparisons [14]. 

In a separate, but related, experiment at SLAC, the concept of echo-enabled harmonic 

generation (EEHG) was successfully tested [15]. Here, a seed laser is used, together with energy 

dispersion, in a manner that imprints on the electron beam microbunching structure at very high 

harmonics of the seed laser wavelength. This prebunched beam can then be made to radiate at this 

new wavelength. EEHG represent a very real possible means of achieving near-direct seeding into 

the X-ray wavelengths. 

The FEL Division at Thomas Jefferson National Accelerator Laboratory (JLAB), who are most 

noted for their work involving the use of an energy-recovery linac in achieving a world record in FEL 

average power (14 kW in the infrared), have now, with an addition to their system, extended their 

capabilities into the vacuum ultraviolet (VUV) range and have delivered 10-eV (124 nm) FEL 

pulses. A hole out-coupling mirror on their UV line operating at 370 nm in the fundamental delivered 

this VUV harmonic light to a calibrated VUV photodiode. They measured 5 nJ of fully coherent light 

in each 10-eV micropulse, which represents approximately 0.1% of the energy in the fundamental. 

An example of their early success of operating at 400 nm is shown in Fig. 1, where the average 

output power indicated is 100 W [16]. 



Fig. 1. Jefferson Lab high average power UV demo at 400 nm fundamental. (Upper left) Scatter from 

high reflectivity mirror. (Upper right) Average power. (Lower left) Light scatter from power probe. (Lower 

right) Time-dependent diagnostic. 

Fig. 2. First measurements of seeded coherent emission from FERMI@Elettra. Green shows the time 

profile of a single pulse (the photodiode is in saturation due to the intensity of the FEL pulse). Yellow 

shows a series of (left) seeded FEL pulses and (center-right) no seed. 

Another recent addition to this new breed of FEL user facilities is the FERMI@Elettra FEL User 

Facility (FERMI) at the Sincrotrone Trieste in Basovizza, Italy. At FERMI, the first of two FEL lines 

was recently completed and commissioning started. FERMI utilizes a unique seeding scheme 

called High-Gain Harmonic Generation (HGHG) [17] in order to reach shorter wavelengths, stabilize 

the FEL power, and enhance the longitudinal coherence. Also, it is unique that FERMI uses what 

are called APPLE II-type undulator magnets that allow the facility to provide the user community 

variable polarization from linear to both helicities of circular polarization, permitting an additional 

means of probing the samples under study. Once fully functional, FERMI will provide users with 

photons in the wavelength range of 100 nm to 4 nm. Notably, FERMI achieved first coherent 

emission in December 2010 [18]. The first signals are shown in Fig. 2. 

Another facility in Italy, i.e., Sorgenta Pulsata e Amplificata di Radiazione Coerente (SPARC), in 

Frascati, Italy, is making exciting progress [19]. Although not designed as an end-user facility, 

SPARC was constructed with a full user facility, i.e., Sorgenta Pulsata e Amplificata di Radiazione 

X-ray (SPARX), in mind. Its goal is as an R&D test bed to further the understanding of beam 

generation for FELs, the seeded FEL process in various configurations, and benchmarking FEL 

codes. 2010 was a particularly successful year for them as they were able to successfully lase in a 



Fig. 3. Recent aerial picture of the SPring-8 research grounds, including the soon-to-be completed 

XFEL. 

variety of seeding configurations, thus paving the way for other user facilities using seeding as a 

primary means of improving and controlling the quality of the FEL pulses. 

Meanwhile another method of seeding the FEL was pursued at the SPring-8 Compact Self- 

Amplified Spontaneous Emission (SASE) Source (SCSS) Test Accelerator in Harima Science Park 

City, Japan. Following their success of operating the SASE-based FEL at VUV wavelengths, and in 

preparation for their current project the X-ray FEL, they have added a seed source based on the 

output of a high-harmonic generation (HHG) table-top laser system. With this, they have been able 

to achieve direct seeding of the FEL at 61 nm and have amplified the signal by many orders of 

magnitude [20]. 

4. Coming Soon 

This new breed of light source, i.e., the modern FEL user facility, has been dubbed the BNext- 

Generation[ synchrotron light source or, in other circles, 4th-generation light sources. They are 

distinguished by extremely high peak brightness (photons/sec/mm 2 /mrad 2 /ð0:1% BWÞ compared 

with the current 3rd-generation light source and are very complimentary in nature to these 

3rd-generation sources. (For a more complete list of light sources and a further description, see 

the website http://www.lightsources.org.) Although there are now a few 4th-generation light source 

user facilities constructed and operating, there are a number on the way. Below is a list of some 

of these. 

1) The European X-ray Laser LaboratoryVXFEL [21] 

• This 3.4-km-long X-ray laser facility is Europe’s answer to the LCLS. It starts in Hamburg, 

Germany, and runs underground to the outskirts of the city, where the experimental hall will be. 

2) LCLS II [22] 

• This is an upgrade to the LCLS I that includes an additional FEL line optimized for soft X-rays 

along with additional experimental stations. 

3) SwissFEL [23] 

• To be located at the Paul Scherrer Institute in Switzerland, this FEL will use a combination of 

either SASE operation, seeded operation with an HHG source, or possibly even the more 



exotic but perhaps even more capable concept EEHG to provide a broad user community a 

selection of output beam properties, spanning the EUV region to hard X-rays. 

4) SPring-8 Joint project for FEL [24] (Fig. 3) 

• This is the follow-on project to the SCSS mentioned above. It is scheduled for first beam 

tests in 2011 and will be a SASE FEL utilizing some very interesting concepts such as a 

thermionic electron source and in-vacuum undulators. It is Japan’s entry into the field of 

upcoming 4th-generation X-ray light source user facilities. The goal will be to achieve hard 

X-rays for a broad user community. 

5. Summary 

2010 proved to be exciting for FEL experiments conducted by many enthusiastic teams. Concepts 

for improving FEL performance continue to be developed, and several FEL-based next-generation 

light sources are being delivered for use by users of synchrotron light source laboratories 

worldwide. 

Acknowledgment 

The authors wish to thank our fellow FEL colleagues who have contributed significantly to the 

field. Special thanks go to G. Neil and S. Benson (JLAB), P. Emma (SLAC), our many colleagues at 

Sincrotrone Trieste, L. Giannessi (ENEA, Team SPARC), and to T. Shintake (SPring-8) for their 

contributions to this article. 

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IEEE Photonics Journal Probe for Ultrafast Electron Dynamics 

Nonlinear-Optical Probe for Ultrafast 

Electron Dynamics: From Quantum 

Physics to Biosciences 

Aleksei Zheltikov 


Department of Physics and Astronomy, Texas A&M University, College Station, 

TX 77843-4242 USA 

Physics Department, International Laser Center, M. V. Lomonosov Moscow State University, 

Moscow 119992, Russia 

DOI: 10.1109/JPHOT.2011.2142180 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: A. Zheltikov (e-mail: zheltikov@physics.tamu. 

edu). 

Abstract: Recent discoveries in ultrafast optical science and advances in laser technologies 

offer unique tools for probing the extremely fast dynamics of bound and free electrons, 

giving the key to identifying the key scenarios and understanding the decisive early 

episodes of light-induced processes in physics, chemistry, and biology. 

Index Terms: Ultrafast photonics, nonlinear optics, light–matter interactions. 

The interaction of electromagnetic radiation with electrons is the key mechanism whereby the 

laser field acts upon matter. Laser fields with a sufficiently high intensity, which is typical of modern 

short-pulse laser sources, give rise to field-induced ionization of atoms and molecules, generating 

free electrons. Tunneling of an electron through a potential barrier distorted by a high-intensity light 

field is a key elementary event that launches a broad diversity of light-induced effects in physics, 

chemistry, and biology, turning on the clock in an ultrafast light–matter interaction. Since such an 

ionization occurs on an extremely short time scale, i.e., often faster than 1 fs, long before any other 

laser–matter interaction processes become effective, it defines time zero for cascades of 

complicated physical, chemical, and biological transformations in matter [see Fig. 1(a)]. While 

these later phases of light–matter interactions can be accessed with a number of advanced laser 

techniques using ultrashort light pulses [1], methods enabling a clear identification of the decisive 

early episodes in light–matter interactions related to extremely fast electron dynamics are still under 

development. 

The cutting-edge technologies of attosecond spectroscopy developed in recent years are 

primarily based on the detection of the yield of charged particles [2]–[4]. These methods have 

resulted in revolutionary breakthroughs in our understanding of fundamental electronic dynamics in 

the gas-phase media and on solid surfaces. However, the detection of ultrafast electronic dynamics 

in the bulk of condensed-phase systems, including real-life chemical and biological systems, calls 

for further developments [5]–[7]. Recent discoveries in ultrafast nonlinear optics suggest new ways 

of confronting these challenges. Experiments performed at the Vienna University of Technology 

show [7] that, upon an accurate discrimination against the signal related to atomic nonlinear-optical 

susceptibilities and harmonics related to electron rescattering on parent ions, the spectra of optical 



Fig. 1. (a) Timeline of ultrafast electron dynamics and electron-initiated processes in the field of ultrashort 

laser pulses, including electron photoionization through tunneling or a multiphoton process, electron 

salvation, capture of electrons into antibonding molecular orbitals, electron autodetachment, dissociation 

of molecules, recombination of free electrons, and dynamics of vibrational wave packets. (b) Spatiotemporal 

map of a few-cycle laser pulse propagating through an ionizing medium. (c) Schematic of nonlinear-optical 

bioimaging using coherent Raman scattering. MPA: multipass amplifier; OPA: optical parametric amplifier; 

SHG: second-harmonic generation. 

harmonics and wave mixing can provide a wealth of information on electron tunneling dynamics. 

This approach allows subfemtosecond electron ionization dynamics to be detected without 

attosecond pulses but using a fraction of the field cycle of an ultrashort laser pulse as a 

subfemtosecond probe. More generally, oscillations of electromagnetic field in a few-cycle pulse are 

ideally suited to probe ultrafast dynamics of ionization [8], [9]. This argument is illustrated by the 

spatiotemporal map of a few-cycle pulse in Fig. 1(b). Compression of field half-cycles, which is 

clearly visible in this map, visualizes ionization-induced blue shift, while the defocusing dynamics, 

which are seen on the trailing edge of the pulse, maps both the spatial and temporal profiles of the 

electron-density buildup within the laser pulse. 

In a standard beam-focusing geometry, laser-induced ionization is accompanied by the 

absorption of laser radiation along the entire beam-propagation path. A transverse profile of free 

electrons generated under these conditions displays a maximum on the beam axis, giving rise to a 



negative lens, which defocuses the laser beam. These phenomena limit the efficiency of the 

interaction of laser radiation; maximum field intensity and the maximum electron density in the focus 

of the laser beam; prevent laser radiation energy from being efficiently deposited in a gas, liquid, or 

solid target; and impose serious restrictions on material micromachining, as well as on spectral and 

temporal transformation of high-power ultrashort laser pulses. In particular, in the filamentation 

regime, beam defocusing induced by free electrons limits the field intensity and the density of free 

electrons at levels dictated by the balance of beam focusing due to the positive lens related to the 

third-order nonlinear susceptibility of the medium (Kerr effect) and defocusing caused by the negative 

lens induced by the transverse profile of free-electron density [10]. Recent studies have shown [8] 

that a multibeam ionization of gas and condensed-phase media with interfering ultrashort laser 

pulses can help to substantially increase the maximum field intensity and the density of free 

electrons attainable in the focus of the laser field relative to the regime of single-beam ionization. 

Multibeam ionization schemes have been shown to offer new solutions for laser micro- and 

nanomachining, micro- and nanosurgery, spectral and temporal transformation of ultrashort light 

pulses, as well as remote sensing of the atmosphere. Subfemtosecond changes in the local 

refractive index induced in the regime of multibeam tunneling ionization enable the high-speed 

switching of optical signals. 

In silicon photonics, which offers an advanced platform for the creation of broadband on-chip 

components and integrated networks for emerging optical information technologies [11], ultrafast 

electron dynamics can help to implement ultrafast switches and finely tunable frequency comb 

generators [12]. In recent experiments [13], a photonic platform integrating a silicon nanowaveguide 

ring resonator and a photonic-crystal fiber (PCF) frequency shifter has been developed. These 

studies demonstrate that the ringdown response of a silicon nanowaveguide ring resonator can be 

efficiently controlled through ultrafast light-induced free-carrier generation by a femtosecond laser 

pulse. Recent theoretical studies, on the other hand, show [14] that the enhancement of 

multiphonon tunneling recombination of free carriers in strong laser fields offers a channel whereby 

ultrafast carrier-density dynamics in a semiconductor can be controlled by properly shaped laser 

pulses. This regime of laser–solid interaction enables an ultrafast switching of optical and electric 

properties of semiconductor materials, suggesting new strategies for laser micro- and nanomachining, 

optical data processing, and ultrafast plasmonics. 

Modern optical technologies offer a broad variety of powerful methods and tools for chemically 

selective high-resolution microspectroscopy and imaging [see Fig. 1(c)] of biochemical processes 

and biological objects [15]–[18]. Because of the generic I N scaling of an N-photon response of a 

material to a laser field with intensity I, high laser intensities are needed to provide a high sensitivity, 

a high signal-to-noise ratio, and a high image-acquisition speed in nonlinear-optical neuroimaging. 

The flip side of bioimaging with high laser intensities is an increased risk of irreversible light-induced 

modifications and damage of biotissues [19]. Accumulation of free electrons generated by ultrashort 

laser pulses with intensities below the single-pulse laser damage threshold tends to initiate 

cascades of unwanted processes in biotissues, including the formation of reactive oxygen species, 

causing the death of cells, as well as DNA-strand breaking by low-energy electrons due to the rapid 

decay of transient molecular resonances localized on DNA constituents [20]. These issues raise 

concerns regarding the noninvasiveness of nonlinear-optical imaging techniques, calling for indepth 

quantitative studies of ultrafast ionization phenomena accompanying nonlinear-optical 

interactions of laser pulses with brain tissue. Recent experiments at M. V. Lomonosov Moscow 

State University demonstrate [21] that free-electron generation that accompanies coherent anti- 

Stokes Raman scattering (CARS) of ultrashort laser pulses in brain tissue [see Fig. 1(c)] manifests 

itself in a detectable blue shift of the anti-Stokes signal. Experimental studies suggest that this blue 

shift can be used to quantify the ionization penalty of CARS-based neuroimaging. 

The advent of laser systems capable of routinely generating fully controlled few-cycle light pulses 

leads us to rethink and redefine the concepts of fast and slow in natural sciences. In this new era of 

ultrafast science, methods for detecting and understanding the early phases of light–matter 

interactions are in great demand. Nonlinear optics is among the most promising approaches, as it 

opens routes to unexplored territories and otherwise inaccessible fundamental events and 



processes, giving the key to identifying and understanding the decisive early episodes of lightinduced 

processes in physics, chemistry, and biology. 

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[5] A. M. Zheltikov, A. A. Voronin, M. Kitzler, A. Baltusˇka, and M. Ivanov, BOptical detection of interfering pathways in 

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[6] A. M. Zheltikov, A. A. Voronin, R. Kienberger, F. Krausz, and G. Korn, BFrequency-tunable multigigawatt sub-half-cycle 

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IEEE Photonics Journal Semiconductor Core Optical Fibers 

Semiconductor Core Optical Fibers 

S. Morris, 1 J. Ballato, 1;2 T. Hawkins, 1 P. Foy, 1 B. Yazgan-Kokuoz, 1 

C. McMillen, 3 R. Stolen, 1 and R. Rice 4 


1 Center for Optical Materials Science and Engineering Technologies (COMSET), 

School of Materials Science and Engineering, Clemson University, Clemson, SC 29634 USA 

2 Holcomb Department of Electrical and Computer Engineering, Clemson University, 

Clemson, SC 29634 USA 

3 Department of Chemistry, Clemson University, Clemson, SC 29634 USA 

4 Northrop Grumman Space Technology, Redondo Beach, CA 90278 USA 

DOI: 10.1109/JPHOT.2011.2135847 

1943-0655/$26.00 Ó2011 IEEE 


version April 26, 2011. Corresponding author: J. Ballato (e-mail: jballat@clemson.edu). 

Abstract: Presented here is a review of recent efforts to date in the field of semiconductor 

core optical fibers. Various processing techniques have been employed to fabricate such 

fibers and control the degree of crystallinity achieved within the fiber cores. Following the 

brief review of recent progress is a more in-depth look at the molten core approach, which 

allows for long lengths of highly crystalline semiconductor core fibers to be achieved. 

Index Terms: Optical fiber, semiconductor, silicon, germanium. 

Recently, much progress has been made in the field of semiconductor core optical fibers, 

extending the field of silicon photonics from a planar waveguide form to an optical fiber-based 

technology. When combined with an appropriate cladding glass, such highly crystalline semiconductor 

core optical fibers have significant potential for Raman fiber devices, mid- and long-wave 

infrared sensing and power delivery, and terahertz guided wave structures. Their mid-infrared 

transmission capabilities have generated much interest in their use in the biomedical industry, 

where there is an unmet need for robust infrared waveguides in a variety of dental and medical 

procedures. Several techniques have been employed in the fabrication of silicon optical fiber. In 

addition to the work conducted at Clemson University (USA), researchers at Virginia Tech (USA), 

the Massachusetts Institute of Technology (USA), the University of Erlangen-Nuremberg 

(Germany), and collaborations between Pennsylvania State University (USA) and Southampton 

University (U.K.) have made significant contributions to this growing field. 

The current state of the art includes several different techniques which have been employed in the 

fabrication of silicon optical fiber. Sazio et al. [1]–[6] have explored high-pressure microfluidic chemical 

deposition to deposit silicon inside pure silica microstructured optical fiber (MOF) templates for 

the fabrication of silicon fibers. MOFs are created by stacking and fusing glass capillary and rod 

arrays into performs which are subsequently drawn. The semiconductor deposition was performed 

by flowing a silane/helium mixture through the capillary hole. Since this is done at a temperature 

range where the material would remain amorphous (400 C–500 C), subsequent annealing is used 

to control the silicon polycrystallinity. Crystal grain sizes were reported to be around 0.5–1 m. 

Initially, high optical transmission losses of around 50 dB/cm at 1550 nm have been reported for 

amorphous samples with losses decreasing with both increased annealing temperature and 

increasing wavelength. Transmission losses were also determined for polycrystalline samples. The 



Fig. 1. (Left) Elemental analysis of the glass-clad germanium core optical fiber. (Right) Scanning 

electron micrograph showing the central silicon core and glass cladding. 

lowest reported loss was approximately 5 dB/cm at 1550 nm [7]. Optical characterization revealed 

effective mode areas comparable with the effective area of a single mode fiber’s fundamental mode. 

Effects such as tensile strain induced at the silicon core–silica cladding interface during cooling 

result in a red-shift of the Raman spectra of the silicon tubes with respect to that of single crystalline 

silicon [8]. 

Scott et al. [9], [10] have used powder-in-tube fabrication methods in which powder silicon is 

packed into a silica tube, pulling a vacuum and evacuating the preform, in order to minimize silicon 

oxidation. This powder-in-tube technique produced fibers, drawn at around 1600 C, which averaged 

an overall length of approximately 7 cm. Thermal expansion induced microcracks, and other 

such irregularities, at the boundary between the core and the cladding materials resulted in high 

optical losses. Based on results of electron dispersive spectroscopy (EDS), silicon and oxygen 

were both present in the cladding glass. However, silicon was the sole element confirmed throughout 

the core. Electron backscatter diffraction (EBSD) confirmed large grained polycrystallinity 

aligned with the fiber axis, along with several crystalline orientations throughout the fiber. 

Our group at Clemson University employs a more conventional draw tower fabrication process 

which has shown success in fabrication of long lengths of optical fibers [11], [12]. The Bmolten core[ 

technique employed requires that the core semiconductor material melt at a temperature where the 

cladding glass softens and draws into the optical fiber. In order to achieve this, a semiconductor rod 

is sleeved inside a tube of cladding glass, the draw temperature of which is greater than the melting 

temperature of the core. The molten semiconductor core is therefore contained within the constraints 

of the inner walls of the cladding glass tube. Efforts have primarily been focused on silicon 

[11], germanium [12], [13], and indium antimonide [14] cores and their respective cladding glass 

selections. To date, more than 250 m of crystalline germanium core fiber have been fabricated. 

Fig. 1 provides the compositional profile along a line crossing the full diameter of the core, as well as 

the interface into the cladding. Also shown is a cross-sectional view of a silicon core fiber. 

As crystallographic strain can be induced by thermal expansion mismatch between the core and 

the cladding materials, crystallographic orientation data have been obtained on glass clad germanium 

optical fibers in order to determine the nature of the crystallinity of the material. These data 

were obtained by single crystal X-ray diffraction (XRD) and EBSD. Reflection profiles of initially 

obtained axial photographs revealed a local single crystalline character in the fibers, as seen in 

Fig. 2, with sufficiently large grain sizes to reliably determine orientation. A preference for alignment 

in the h100i and h110i directions was exhibited close to the longitudinal axis of the fiber, while h100i 

orientations were likely to align with the fiber axis. This preference toward h100i and h110i orientations 

has often been observed in dendritic crystals of cubic symmetry. 

The ability to achieve high crystallinity over fairly long lengths of fiber begs the question as to the 

rate at which these crystals can be grown. The interplay between kinetics and thermodynamics also 

plays a strong role here, as crystallization of the core occurs without a seed crystal from which 



nucleation may occur. A spontaneous nucleation event must then occur during the draw process as 

the fiber cools, which gives an indication that there must exist a range of draw speeds and temperatures 

over which crystallization can occur over long distances. While crystalline core fibers 

have been achieved at draw rates of about 1 m/s, it is very likely that the draw speeds can be 

considerably higher [15]. 

Much progress has been made to this emerging field in a fairly short period of time. While several 

different methods, which have been reviewed here, have been employed to create these 

semiconductor core fibers, such fibers have proven to be of some difficulty to create. Future work 

aims to lower the attenuation of the fibers to below 1 dB/m, which will require reducing the amount 

of both scattering and absorption. Cladding glasses are being developed to better control the 

dissolution of oxide impurities and improve the thermal expansion mismatches, which should further 

reduce the losses. A significant amount of work remains in order to develop a more complete 

understanding of the role of diffusion, kinetics, thermodynamics, and thermomechanics in the 

optimization of these novel optical fiber material systems. 

References 

Fig. 2. Histogram of germanium crystallographic orientations nearest to the fiber longitudinal axis 

(from [13]). 

[1] N. Healy, J. R. Sparks, M. N. Petrovich, P. J. A. Sazio, J. V. Badding, and A. C. Peacock, BLarge mode area silicon 

microstructured fiber with robust dual mode guidance,[ Opt. Exp., vol. 17, no. 20, pp. 18 076–18 082, Sep. 2009. 

[2] B. R. Jackson, P. J. A. Sazio, and J. V. Badding, BSingle-crystal semiconductor wires integrated into microstructured 

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[3] D. J. Won, M. O. Ramirez, H. Kang, V. Gopalan, N. F. Baril, J. Calkins, J. V. Badding, and P. J. A. Sazio, BAll-optical 

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pp. 161112-1–161112-3, Oct. 2007. 

[4] P. J. A. Sazio, A. Amezcua-Correa, C. E. Finlayson, J. R. Hayes, T. J. Scheidemantel, N. F. Baril, B. R. Jackson, 

D. J. Won, F. Zhang, E. R. Margine, V. Gopalan, V. H. Crespi, and J. V. Badding, BMicrostructured optical fibers as 

high-pressure microfluidic reactors,[ Science, vol. 311, no. 5767, pp. 1583–1586, Mar. 2006. 

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[6] I. A. Temnykh, N. F. Baril, Z. Liu, J. V. Badding, and V. Gopalan, BOptical multistability in a silicon-core silica-cladding 

fiber,[ Opt. Exp., vol. 18, no. 5, pp. 5305–5313, Mar. 2010. 

[7] L. Lagonigro, N. Healy, J. R. Sparks, N. F. Baril, P. J. A. Sazio, J. V. Badding, and A. C. Peacock, BLow loss silicon 

fibers for photonics applications,[ Appl. Phys. Lett., vol. 96, no. 4, pp. 041105-1–041105-3, Jan. 2010. 

[8] C. E. Finlayson, A. Amezcua-Correa, P. J. A. Sazio, N. F. Baril, and J. V. Badding, BElectrical and Raman 

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pp. 100501-1–100501-3, Oct. 2009. 



[10] B. L. Scott, K. Wang, and G. Pickrell, BFabrication of n-type silicon optical fibers,[ IEEE Photon. Technol. Lett., vol. 21, 

no. 24, pp. 1798–1800, Dec. 2009. 

[11] J. Ballato, T. Hawkins, P. Foy, R. Stolen, B. Kokuoz, M. Ellison, C. McMillen, J. Reppert, A. M. Rao, M. Daw, S. Sharma, 

R. Shori, O. Stafsudd, R. R. Rice, and D. R. Powers, BSilicon optical fiber,[ Opt. Exp., vol. 16, no. 23, pp. 18 675–18 683, 

Nov. 2008. 

[12] J. Ballato, T. Hawkins, P. Foy, B. Yazgan-Kokuoz, R. Stolen, C. McMillen, N. K. Hon, B. Jalali, and R. Rice, BGlass-clad 

single-crystal germanium optical fiber,[ Opt. Exp., vol. 17, no. 10, pp. 8029–8035, May 2009. 

[13] C. McMillen, T. Hawkins, P. Foy, D. Mulwee, J. Kolis, R. Stolen, R. Rice, and J. Ballato, BOn crystallographic orientation 

in crystal core optical fibers,[ Opt. Mater., vol. 32, no. 9, pp. 862–867, Jul. 2010. 

[14] J. Ballato, T. Hawkins, P. Foy, C. McMillen, L. Burka, J. Reppert, R. Podila, A. M. Rao, and R. R. Rice, BBinary III-V 

semiconductor core optical fiber,[ Opt. Exp., vol. 18, no. 5, pp. 4972–4979, Mar. 2010. 

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IEEE Photonics Journal III-Nitride Optoelectronic Devices 

III-Nitride Optoelectronic Devices: From 

Ultraviolet Toward Terahertz 

M. Razeghi, Fellow, IEEE 


Center for Quantum Devices, Department of Electrical Engineering and Computer Science, 

Northwestern University, Evanston, IL 60208 USA 

DOI: 10.1109/JPHOT.2011.2135340 

1943-0655/$26.00 Ó2011 IEEE 


version April 26, 2011. Corresponding author: M. Razeghi (e-mail: razeghi@eecs.northwestern.edu). 

Abstract: We review III-Nitride optoelectronic device technologies with an emphasis on 

recent breakthroughs. We start with a brief summary of historical accomplishments and then 

report the state of the art in three key spectral regimes as follows: 1) Ultraviolet (AlGaNbased 

avalanche photodiodes, single photon detectors, focal plane arrays, and lightemitting 

diodes); 2) Visible (InGaN-based solid state lighting, lasers, and solar cells); and 

3) Near-, mid-infrared, and terahertz (AlGaN/GaN-based gap-engineered intersubband 

devices). We also describe future trends in III-Nitride optoelectronic devices. 

Index Terms: III-Nitrides, AlGaInN, AlGaN, InGaN, AlGaN/GaN, ultraviolet, avalanche 

photodiodes, single photon detector, focal plane array, light-emitting diode (LED), solid state 

lighting, solar cell, intersubband devices, terahertz (THz). 

III-Nitrides (AlGaInN) are a unique semiconductor material system offering a wide direct bandgap 

that can be flexibly tuned over the complete spectral range from deep ultraviolet ( 6.2 eV) to nearinfrared 

( 0.7 eV). Two key spectral regimes (Ultraviolet (UV) and Visible) have drawn most of the 

attention historically. However, recently, higher control over material and interface quality and growth 

engineering has enabled the use of intersubband (ISB) transitions to access a new spectral regime: 

Terahertz (THz). In all three of these key regimes, III-Nitride optoelectronic devices promise more 

reliability and higher efficiency. In addition, III-Nitrides being environmentally friendly, robust, and 

compact diversify their application span from everyday life to the military and even into outer space. 

In 1992, Prof. M. Razeghi joined Northwestern University and founded the Center for Quantum 

Devices (CQD). In January of 1994, through a collaboration between Prof. Razeghi and Aixtron, 

the World’s first commercial reactor designed for the growth of GaN, i.e., the Aixtron 200-4/HT 

metal-organic chemical vapor deposition (MOCVD), was designed and installed at Northwestern 

University. The CQD immediately entered the growing III-Nitride arena and began a journey that 

would leave behind a legacy of pioneering research and numerous World’s first discoveries, 

covering everything from material development, to light-emitting diodes (LEDs) and laser diodes 

(LDs), to UV photodetectors, focal plane arrays (FPAs), and avalanche photodiodes (APDs) [1], [2]. In 

this paper, we review world’s first demonstrations and present current state-of-the-art III-Nitrides 

research from UV toward THz. 

The UV region is very important as many biological agents (such as anthrax and the plague) are 

luminescent in the UV. Scattering of short-wavelengths in the atmosphere also enables non-line-ofsight 

secure ground-based communications. Similarly, strong absorption of UV in the ozone layer 



Fig. 1. (a) (Lower left) Schematic of a back-illuminated single photon detector. (Background) Scanning 

electron micrograph of a processed array of single photon detectors. (b) (Top right) RT electroluminescence 

of the fabricated hybrid green LED; inset shows scanning electron micrograph. (c) (Top 

left) Negative differential resistance phenomena observed in resonant tunneling diodes (RTDs) at RT 

and 77 K; inset shown optical micrograph of a RTD. 

below 280 nm creates a so-called Bsolar-blind[ regionVfree of background noiseVfor terrestrial 

applications, as well as promises secure space-to-space communications. UV emitters and 

detectors also find applications in astronomy for cosmic events analysis and in space exploration. 

These unique applications are only possible by compact and high performance UV emitters and 

detectors. The AlxGa1 xN material system covering the 200–365 nm regime is an excellent material 

choice for these devices. 

By the mid-1990s, despite large lattice mismatch to sapphire substrate ( 16%), high-quality AlN 

and AlGaN regrowth was established, and UV GaN photovoltaic diodes and AlxGa1 xN 

photoconductors ð0 X 1Þ were demonstrated. Shortly thereafter, improved p-doping in AlGaN 

enabled shortest wavelength AlGaN photodiodes and improved quantum efficiency ( 68%) of 

solar-blind ð 280 nmÞ p-i-n photodiodesVleading to integrated III-Nitride optoelectronic devices 

such as 320 256 FPAs [1], [2]. 

Currently, with the maturing growth and processing technology, more advanced detector 

structures benefiting from avalanche gain mechanisms are being realized in order to outperform 

bulky UV photomultipliers. These state-of-the-art UV APDs possess gains of 51 000 [see Fig. 1(a)], 

and (via Geiger-mode operation) enables UV single photon detection with efficiencies as high as 

33% [3]. The use of III-Nitride APDs presents key advantages such as lower operation voltages, 

much reduced sizes, and no need for cooling, which enable the fabrication of more compact, lower 

power, and all-solid-state APD/complementary metal–oxide semiconductor integrated arrays, which 

are suitable for integration into satellites, airplanes, and military vehicles for secure communication 

and aerial countermeasures. Although these results motivate single photon UV FPAs, further 

material and growth improvements are essential for these applications to replace existing systems. 

In the UV emitter side, since the first demonstrations of 280-nm and 265-nm solar-blind UV LEDs 

in the early 2000s, recent material improvements have led to deep UV LEDs with 210-nm emission. 

However, the performance of solar-blind ð 280 nmÞ UV LEDs still suffers from material quality, 

and their wall-plug-efficiency (WPE) is limited to 3% [4]–[6]. For the UV LDs, the situation is much 

worse. Due to a lack of cleavage planes, mirror formation is quite difficult, and optical confinement 

layers with higher aluminum content than the active layer degrade the device quality and, hence, 

the performance. Today, UV LDs cannot be realized for G 335 nm [4]–[6]. Despite these 

problems, recent electron beam excitation of AlGaN quantum wells demonstrated 40% efficiency at 

240-nm emission [4]–[6]. This motivates further work on high-power efficient AlGaN solar-blind 

LEDs as well as solar-blind LDs. A high-quality freestanding (FS) AlN substrate with controlled 

crystal directions (polar versus nonpolar) could enable such breakthroughs in this wavelength 

regime. 

Visible LEDs are important for solid-state lighting (SSL), which holds promise for more energyefficient, 

longer lasting, more compact, and lower maintenance substitutes for today’s incandescent 

and fluorescent light sources. The total annual energy consumption in the U.S. for lighting is 

approximately 800 TW-h at a cost of $80 billion to the public. This corresponds to greenhouse gas 

emissions equivalent to more than 70% of the emissions from all the cars in the world. Thus, novel 



solutions to lighting with higher efficiency are critical to reducing global energy consumption and 

helping to lower greenhouse gas emissions. 

The human eye is sensitive only to light in the visible spectrum, ranging from violet ð 400 nmÞ 

through red ð 700 nmÞ. However, the human eye is most sensitive to green ð 555 nmÞ, and 

green light strongly affects the human perception to the quality of white light. Although ultrabright 

and efficient blue InGaN-based LEDs (WPE 50.8%)/LDs (WPE 24.3%) are readily available 

(that enabled blue-ray technology), the performance of green LEDs (WPE 5%) is still far from 

adequate for lighting use. This Bgreen gap[ prevents the generation of high performance white 

LEDs based on color mixing. Instead, state-of-the-art white LEDs employ blue LEDs with phosphor 

down conversion, lowering the WPE ( 39.5%) [7], [8]. 

LEDs based on InxGa1 xN alloy are currently the most promising candidates for fulfilling the green 

gap. However, the high indium content ( 30%) required in the active layers for green emission causes 

indium leakage problems and quantum confined stark effect (QCSE). In particular, the high indium 

content of the InxGa1 xN enabling green light emission becomes unstable and diffuses at the elevated 

substrate temperatures that are necessary for p-GaN capping, and the resulting carrier overflow due 

to QCSE degrades the spectral quality and performance. This is conventionally prevented by thinner 

InGaN quantum well and thicker GaN quantum barrier designsVthat is not ideal for emitter 

standpoint. These tradeoffs between theoretical and experimental optimal designs lead to efficiency 

droopVespecially for higher indium content green emittersVlowering the LED performance from blue 

toward green. However, the emerging FS GaN substrates show promise, whereas demonstration of 

high power green LDs (9 50 mW) motivates new applications such as hand-held projectors [7], [8]. 

Recently, ZnO/InGaN-based novel green LEDs [see Fig. 1(b)] have been introduced [9], [10]. 

Using a similar approach in the blue counterpart, these hybrid LEDs achieved as high as 35% WPE 

and have been shown to perform as well as conventional ones but at less cost. As indium and 

gallium are rare materials and ZnO has a small lattice mismatch to GaN ( 1.9%), more of these 

ZnOV(In)GaN hybridizations are expected as higher quality ZnO materials and emerging 

affordable ZnO substrates come into play [9], [10]. All these worldwide efforts will soon enable 

SSL in our daily lives. 

Today’s world uses energy at a yearly rate of 13 TW. The reserves of fossil fuels that currently 

power society will fall short of this demand over the long term, and their continued use produces 

harmful side effects such as pollution that threatens human health and greenhouse gases 

associated with climate change. Our primary source of clean, abundant energy is the sun. The sun 

deposits 120 000 TW of radiation on the surface of the earthVfar exceeding human needs, even in 

the most aggressive energy demand scenarios. 

Most present production of solar power is based on crystalline silicon cells: the first-generation 

technology. The second generation, which is now starting to be commercialized, is based on thinfilm 

cells and cells made from inexpensive oxide semiconductor materials coated with lightsensitive 

dyes and from photoactive organic polymeric materials. These approaches may yield 

much lower costs but, at present, have significantly lower conversion efficiencies. The gamechanging 

breakthrough needed from third-generation cells is both lower cost and very high 

conversion efficiency, which will require entirely new paradigms for photon capture and conversion. 

High-efficiency solar cells (SCs) can be produced currently by combining semiconductor materials 

in a tandem cell structure to capture far more of the energy in sunlight. The trouble is that the cost 

per unit area of these cells is 200 times more expensive than first-generation cells. 

With the recent revision of the InN bandgap as 0.7 eV, the InGaN material system is useful for 

photovoltaic applications due to the possibility of fabricating not only high-efficiency multijunction 

(MJ) SCs but third-generation devices such as intermediate-band SCs based solely on the nitride 

material system as well. While the maximum reported efficiency for a SC is 40% at 364 suns, 

which is achieved by a triple-junction GaInP-GaInAs-Ge tandem, such devices are approaching 

maturity in terms of efficiency limits. For tandem SCs, it is more convenient to grow MJ InGaN SCs 

in a MOCVD equipment than to grow the present InGaP-based MJ SCs using different kinds of 

semiconductors. More importantly, the lattice-match and the thermal expansion match between the 

layers in InGaN tandem SCs could be much better within the same alloy system. Detailed balance 



modeling indicates that in order to achieve practical terrestrial photovoltaic efficiencies of greater 

than 50%, materials with bandgaps greater than 2.4 eV are required. In addition to the wide 

bandgap range, the nitrides also demonstrate favorable photovoltaic properties such as low effective 

mass of carriers, high mobilities, high peak and saturation velocities, high absorption coefficients, 

and radiation tolerance. Recent works on InGaN SCs demonstrated conversion efficiencies of 

2%Vrequiring for further material and design improvements [11], [12]. 

Terahertz wavelengths penetrate through nonconductors (fabrics, wood, and plastic), enabling a 

more efficient way of performing security checks (e.g., at airports). Being a nonionizing radiation, THz 

radiation is environmentally friendly, enabling a safer analysis environment than conventional X-ray 

based techniques. Due to deep penetration depth through body and tissue selectivity, THz waves are 

employed in medicine for cancer cell detection, as well as for bone analysis. THz spectroscopy can 

also be used to enable identification of pharmaceutical ingredients (for example, in drugs) and 

explosives. Thanks to the large longitudinal optical phonon energy (90 meV), III-Nitrides is a 

promising candidate for room temperature (RT) operation of THz quantum cascade lasers (QCLs). 

Realization of a THz QCL requires precise control over material and interfaces necessary to form the 

intersubband levels and allow injection via tunneling between levels. 

Recently, many groups have worked on III-Nitride ISB devices and demonstrated ISB transitions 

from near- to mid-infrared wavelengths (1–5 m) at RTVlimited by sapphire absorption. By 

switching to silicon substrate, ISB transitions were demonstrated up-to THz wavelengths at 4 K [13], 

[14]. Another significant demonstration has been regarding the injector part of the QCLs–quantum 

tunneling. With the recent commercial availability of lattice-matched FS GaN substrates and 

polarization-free-engineered AlGaN/GaN active layers on nonpolar substrates, reliable and 

reproducible RT negative differential resistance in resonant tunneling diodes was demonstrated, 

proving for the first time (irrespective of growth technique) that reliable and reproducible quantum 

tunneling is possible in III-Nitrides [see Fig. 1(c)] [15]. These recent experimental demonstrations 

provide motivation toward the eventual realization of RT THz QCLs based on III-Nitrides. 

In conclusion, since early 1990s, interest in III-Nitride optoelectronics has continued to increase: 

first, thanks to UV and visible regimes and, more recently, to ISB devices from the infrared toward 

THz. With continuing developments in material growth and design and emerging FS substrates, this 

miracle material system is bound to penetrate increasingly more into research areas and our lives. 

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IEEE Photonics Journal New Materials for Short-Pulse Amplifiers 

New Materials for Short-Pulse Amplifiers 

Frédéric Druon, François Balembois, and Patrick Georges 


Laboratoire Charles Fabry de l’Institut d’Optique, 91127 Palaiseau Cedex, France 

DOI: 10.1109/JPHOT.2011.2135845 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received February 27, 2011; revised March 16, 2011; accepted March 23, 2011. 

Date of current version April 26, 2011. Corresponding author: F. Druon (e-mail: frederic.druon@ 

institutoptique.fr). 

Abstract: Laser amplifiers seek high power, efficiency, and short pulse durations. Research 

laboratories in this field have focused their investigations toward new laser materials that can 

be efficiently diode pumped, sustain high-power pumping, and have broad emission 

bandwidth to achieve ultrashort pulse amplification. This is why, for more than ten years now, 

new Yb-doped materials have been intensively investigated. In the actual state of the art, 

they represent the more promising and successful materials for these kinds of applications. 

In this paper, we will do a short review of the last and more impacting discoveries and 

demonstrations in this field over the last few years. 

Index Terms: Laser, solid laser, optical amplifier, ultrafast laser, diode-pumped laser. 

Directly diode-pumped femtosecond lasers delivering high power is one of the hottest and most 

challenging current topics. From this point of view, many international researchers are investigating 

the use of Yb-doped materials. In fact, it is admitted now that only ytterbium-doped crystals can 

provide efficient femtosecond amplifiers at high repetition rate and high energy. Over the past 

decade, laser development using Yb-doped materials, especially crystals, has become one of the 

most active fields in laser research. It is now widely recognized that Yb-doped crystals have a 

significant potential in the development of directly diode-pumped high power and ultrashort lasers 

[1], [2]. This is possible thanks to the simple electronic-level structure based on only two manifolds 

of the Yb 3þ laser active ions and the reduced Bquantum defect[ between pump and laser photons. 

This leads to high pumping efficiencies, very favorable thermal properties, and their broad emission 

bands, due to a strong electron–phonon coupling, thus allowing ultrashort-pulse generation. Since 

the laser performance of Yb-doped crystals is strongly correlated to the crystal-host properties, an 

important international research activity has been focused on the search for innovative Yb-doped 

crystals to improve efficiency, high average power, and pulse duration. 

For amplifiers, two main approaches are described in the literature. The first one considers 

classical and well-known Yb-doped materials such as glasses or YAG (Y3Al5O12). In these works, 

the main development novelty is concentrated on the amplifier architectures involving new crystal 

geometries with YAG and new large-mode-area fiber geometries with glass. The second approach 

concerns the materials themselves in order to find the perfect crystal gathering high thermal 

conductivity, high gain, and broad emission bandwidth. In fact, since pulse duration is directly 

correlated to the spectroscopy of the Yb ions imbedded in their crystal, the crystal host choice is 

crucial to having good spectroscopic properties. The influence of the crystalVincluding the Stark 

effect of the electric field, electrophonon, and vibronic interaction of the latticeVis very important on 

the spectra of this rare-earth dopant. It directly impacts the intensity and the broadness of the 



Fig. 1. Thermal properties versus spectroscopic properties for different Yb-doped crystals. Yb:CALGO 

and Yb:CaF2 clearly exhibit atypical properties. 

emission spectrum lines. On the other hand, the potential to sustain high power is directly correlated 

to the capacity of the host matrix to well evacuate the heat (thermal conductivity, typically) brought by 

high-power pumping, which is still an issueVeven if the thermal loads with Yb 3þ remains exceptionally 

low compared with other dopants. The high thermal conductivity is then a very important 

parameter to consider for high-power amplifiers. 

Over the past decade, many novel crystals have been proposed for a new generation of 

femtosecond diode-pumped solid-state lasers. On the one hand, high average powers and very 

efficient lasers crystal such as tungstates (Yb:KYW [3]–[5], [38] and Yb:KGW) or orthosilicates 

(Yb:YSO [6] and Yb:LuSO) have been investigated. For amplifier systems (especially in the 

industry [5]), tungstates are exclusively used since they gather (which is atypical) reasonable 

emission spectral bandwidth and high cross sections. On the other hand, ultrashort-pulse-generation 

crystals such as borates (Yb:GdCOB [12] or Yb:YCOB [13], [14] Yb:BOYS [7]) or silicate (Yb:SYS 

[8], [9]), vanadates (Yb:YVO4 [16], Yb:GdVO4, and Yb:LuVO4 [17]) have been investigated. However, 

in amplifiers, the gain of these crystals that is relatively low, which imposes a high number of 

passes with a strong impact of the gain narrowing, which was demonstrated at low (100 J) [10], [11] 

and high (12 mJ) energy [15]. Moreover, the thermal conductivity is also an issue for these crystals 

that cannot deliver high output average powers in standard amplifier configurations. An alternative is 

to investigate crystals with high thermal conductivities (around 10 W/m/K for undoped crystals) with 

narrower spectral bandwidths, such as Yb-doped sesquioxide [18] or YAG. The narrow bandwidths 

can be overcome in oscillators with a strong amount of Kerr nonlinearity to broaden the spectrum 

beyond the natural emission [19]–[21], but this cannot be extended to ultrashort amplification 

because of the gain narrowing effect. 

Another alternative way is to use exceptions to the basic rule, which accordingly in simple-matrix 

crystal phonons, propagate well (thus with a high associated thermal conductivity) but do not have 

enough disorder to permit broad bandwidths, while on the other hand, disordered materials allow 

sufficiently different environments for Yb 3þ to generate broad bandwidths but have a low thermal 

conductivity caused by their disorder. The strategy is to find highly structured crystal to allow high 

thermal conductivity but with atypical spectral properties (see Fig. 1). 

Both CaGdAlO4 (CALGO) and CaF2 have relatively good thermal properties with a thermal 

conductivity (for undoped crystals) of around 10 W/m/K. Measurements of the 2-at% Yb:CALGO 

thermal conductivity yielded 6.9 W/m/K and 6.3 W/m/K along the a and c axis, respectively. This is 

similar to values obtained in Yb:YAG [23]. In the case of the CaF2, the thermal conductivity values 

of undoped fluoride crystals are equal to 9.7 W/m/K [24] and decreases down to 6 W/m/K when 

doped at 2.6% in Yb 3þ . With these kinds of values, one can expect narrow bandwidth for the 

spectra. Nevertheless, this is not the case, while the reason for each crystal it is not the same. 

In the CALGO crystal structure, Ca 2þ and Gd 3þ equally share the same crystallographic site and 

can both be substituted by Yb 3þ ions. This leads to the large inhomogeneous broadening in the 

emission spectrum. The presence of a plateau in the gain cross section between 1000 nm and 



1050 nm can be explained by the exact complementarity of two different sites in the host. This 

peculiarity explains the unusually flat emission spectrum of Yb-doped CALGO crystal [25]–[27]. 

In the Yb:CaF2 and its isotypes such as Yb:SrF2, the broad bandwidth is due to substitution of 

Ca 2þ or Sr 2þ ions by Yb 3þ ions that create hexameric clusters inside the matrix due to the unbalance 

valence of the substituted ions with Yb 3þ . These clusters appear for Bheavily[ doped fluorites 

id est above 0.5% at doping. The Yb 3þ ion cannot then be considered as a single isolated ion. This 

leads to a broad emission spectrum. Thanks to the cluster arrangement of the Yb 3þ , the absorption 

and the emission spectra of the fluorites are relatively broad and represent an exception in the 

realm of Yb-doped crystals with good thermal properties. Moreover, the Yb:CaF2 and its isotypes 

such as YbSrF2 have a very long fluorescence lifetime above 2 ms (among the longest, with Yb: 

LnCOB, for Yb-doped laser materials). This makes them even more attractive for amplifiers since 

this allows a better storage of the energy [28]–[35]. 

This is why we have intensively investigated and characterized these promising novel crystals 

within the strong collaboration between the laboratories expert in material science such as CIMAP 

Caen, LCMCP Paris, and LCFIO Palaiseau, and why, additionally, a large number of ambitious 

scientific projects aim at using partly or entirely this technology to explore new fields of physics, 

such as attosecond physics at MPQ München, X-ray lasers MBI Berlin, proton generation for 

cancer treatment at FZD Dresden, and high field physics (electron and proton acceleration, nuclear 

physics) through the French Institut de la Lumière Extrême (ILE) and the corresponding European 

ELI project. We can also mention fusion projects (Genbu in Japan, HiPER in Europe), where 

efficiency at kilowatt average power level is a key parameter to achieving positive overall gain in 

next-generation fusion power plants. 

In the actual state of the art, the results for amplification chains using these new materials have 

demonstrated the shortest pulses ever produced with high-power, high-energy, and efficient amplifiers. 

Siebold et al. have successfully developed a terawatt system within the Polaris project [34], 

[35]. The production of 197-mJ, 192-fs pulses has been demonstrated at 1 Hz with the potential for 

higher extractable energy with improvements on the crystal and coating quality. 

On the other hand, at high repetition rate, seeded with pulses from an Yb:CALGO oscillator, a 

Yb:CaF2 amplifier delivering short pulses ( 180 fs) [36], [37] at up to 1 kHz repetition rate has been 

demonstrated. The shortest pulse duration generated is 178 fs, which corresponds, to our best 

knowledge, to the shortest pulses for a room-temperature Yb-doped-crystal amplifier. The corresponding 

energy is 1.4 mJ before compression (620 J after), at a repetition rate of 500 Hz for 16 W of 

pump power. The bandwidth is 10 nm centered at 1040 nm. At 10-kHz repetition rate, 1.4 W of 

average power before compression is obtained, corresponding to an optical–optical efficiency 

of 10%. 

The future prospects for these new materials for amplifiers will mainly consist in adapting to these 

crystals the novel amplifier architectures and crystal geometries already developed with Yb:YAG. 

In fact, another interesting point to note concerns the parallel progress done on geometries 

specifically developed for high-power lasers based on crystals. These architectures involving 

crystals with a high surface/volume ration such as thin-disk [38]–[40], slab [41], and crystalline-fiber 

[42] have allowed strong improvements in the Yb:YAG amplifier performances, especially in terms 

of power. For example, the INOSLAB experiment has allowed a record of average power for 

femtosecond pulses with an average output power of 1.1 kW, a peak power of 80 MW, and a 615-fs 

[41] pulse width overwhelming then the fiber based amplifiers previous record [43]. On the other 

hand, very promising architectures closed to fibers but involving crystalline matrices (so-called 

single crystal fibers) are also very promising in terms of high power and pulse duration preservation. 

In fact, very recently, a fiber-crystal amplifier has demonstrated [42] 330-fs pulses with an 

average power of 12 W. This is the shortest pulse duration ever produced by an Yb:YAG amplifier. 

Moreover, the gain in the single crystal fiber can reach a value as high as 30 in a simple double 

pass configuration. 

The other interesting method under exploration for Yb:CaF2 concerns cryogenic cooling. In fact, 

such as in Yb:YAG [44] and YLF [45], the laser amplification at low temperature will allow, with 

Yb:CaF2, better performance in terms of average power and efficiency [46]. First, the cryogenic 



cooling improved the thermal conductivity with a factor 7 from room temperature to LN2 temperature, 

for example. Second, the cross sections are also enhanced by a factor of 3. On the other 

hand, the emission peaks tend to narrow, which unfavored ultrashort pulse amplification. Nevertheless, 

preliminary experiments [45] with Yb:CaF2 have been realized, and amplification of short 

pulses is possible. 

In conclusion, the main advances in the field of new materials for amplifiers really focus on the 

purpose of obtaining more average power in the ultrashort-pulsed regime and have mainly involved 

research on new Yb-doped materials. Among them, the actual breakthrough seems to come from 

exceptionally broad materials with high thermal conductivities such as Yb:CaF2 or Yb:CALGO, 

which has been confirmed by excellent experimental performances: amplification up to the TW and 

record in pulse duration. In the same time, architectures optimal for very high power such as crystalfibers, 

thin-disks, and slab configuration have been developed for more standard crystal like Yb:YAG 

and have really overcome the actual limitations in terms of high-power amplification. The next 

generation of amplifiers for ultrashort pulses would certainly combine these technological advances 

to produce ultrahigh-power, ultrashort-pulse amplifiers with applications for precise ablation of a 

broad range of materials, from dielectrics to metals, to be used in industrial applications, such as 

metal drilling or texturing in the automotive field, or medical applications, such as eye surgery, and is 

expected to find mass-production applications in the semiconductor industry for its unique ability to 

achieve selective ablation, which is particularly required in the photovoltaic industry. For academic 

applications, the high repetition rate (high-power) short-pulse amplifiers are also an important issue 

such as, for example, in high harmonic generation. 

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[31] A. Lucca, G. Debourg, M. Jacquemet, F. Druon, F. Balembois, P. Georges, P. Camy, J. L. Doualan, and R. Moncorgé, 

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[33] F. Druon, D. N. Papadopoulos, J. Boudeile, M. Hanna, P. Georges, A. Benayad, P. Camy, J. L. Doualan, V. Ménard, and 

R. Moncorgé, BMode-locked operation of a diode-pumped femtosecond Yb:SrF2 laser,[ Opt. Lett., vol. 34, no. 15, 

pp. 2354–2356, Aug. 2009. 

[34] M. Siebold, S. Bock, U. Schramm, B. Xu, J. L. Doualan, P. Camy, and R. Moncorgé, BYb:CaF2VA new old laser 

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[35] M. Siebold, M. Hornung, R. Boedefeld, S. Podleska, S. Klingebiel, C. Wandt, F. Krausz, S. Karsch, R. Uecker, 

A. Jochmann, J. Hein, and M. C. Kaluza, BTerawatt diode-pumped Yb:CaF2 laser,[ Opt. Lett., vol. 33, no. 23, 

pp. 2770–2772, Dec. 2008. 

[36] A. Puglys, G. Andriukaitis, A. Baltusˇka, L. Su, J. Xu, H. Li, R. Li, W. J. Lai, P. B. Phua, A. Marcinkevicius, M. E. Fermann, 

L. Giniunas, R. Danielius, and S. Alisˇauskas, BMulti-mJ, 200-fs, cw-pumped, cryogenically cooled, Yb,Na:CaF2 

amplifier,[ Opt. Lett., vol. 34, no. 13, pp. 2075–2077, Jul. 2009. 

[37] S. Ricaud, F. Druon, D. N. Papadopoulos, P. Camy, J.-L. Doualan, R. Moncorgé, M. Delaigue, Y. Zaouter, A. Courjaud, 

P. Georges, and E. Mottay, BShort-pulse and high-repetition-rate diode-pumped Yb:CaF2 regenerative amplifier,[ Opt. 

Lett., vol. 35, no. 14, pp. 2415–2417, Jul. 2010. 

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IEEE Photonics Journal Single-Photon Counting Detectors 

Single-Photon Counting Detectors 

Sergio D. Cova, Fellow, IEEE, and Massimo Ghioni, Senior Member, IEEE 


Dipartimento di Elettronica e Informazione, Politecnico di Milano, 20133 Milano, Italy 

DOI: 10.1109/JPHOT.2011.2130518 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: S. D. Cova (e-mail: sergio.cova@polimi.it). 

Abstract: Various new developments for array detectors based on Silicon Single-Photon 

Avalanche Diodes (SPADs) were reported. Improved Si-SPAD technologies brought higher 

detection efficiency in the red wavelength range. Higher performance was attained with 

InGaAs/InP SPADs by employing fast circuit techniques and by monolithic resistor-detector 

integration. New InGaAs(P)/InP SPAD array detectors provide remarkable performance in 

the near-infrared range (NIR). Photon detection at longer wavelengths (up to 3.5 m) was 

pursued with antimonide SPADs and Superconducting Single-Photon Detectors (SSPD). 

Index Terms: Photon counting, photon timing, array detector, avalanche diode. 

Photon counting applications require high detector performance (high photon detection efficiency 

(PDE), low dark count rate (DCR), and small photon timing jitter), but they also require reliability, as 

well as ease of implementation, miniaturization, and integration in systems. This review will deal 

with detectors that satisfy such requirements and not with other remarkable cases that represent 

demonstrations of detection principles based on appealing physical effects. Many applications (e.g., 

analytical techniques in life sciences, 3-D imaging, or laser detection and ranging (LADAR), etc.) 

concern the visible spectral range (VIS); others of high interest (e.g., quantum key distribution 

(QKD) and eye-safe LADAR) concern the near-infrared range (NIR). The two ranges will be separately 

considered. A basic issue must be well focused: advanced analog detectors (backilluminated 

charge-coupled devices (CCDs), etc.) have ultraweak dark current and measure very 

weak photon fluxes; therefore, when and why are photon-counting detectors advantageous? 

Essentially, it is when the measurement time is very short, e.g., with high frame-rate imaging, 

fluorescence correlation spectroscopy (FCS), fast optical pulses, etc. The reason is the electronic 

readout noise of analog detectors. At short measurement time, the readout noise is dominant over 

the dark-current noise and sets the sensitivity limit to analog detectors, whereas it simply does not 

exist in photon-counting detectors. 

In the VIS Photomultiplier Tubes (PMTs), the classic photon counting detectors are progressively 

replaced by Silicon microelectronic detectors. A new micro-PMT technology has also been announced, 

with miniaturized multiplier structure built with microelectromechanical systems technology 

[1]. Microsystem integration prospects are open by new developments in Single-Photon 

Avalanche Diodes (SPADs), which are the digital detectors that work in Geiger avalanche mode 

above the breakdown level. Incidentally, SPAD devices are also the basic element of Silicon 

PhotoMultipliers (SiPMs), which are intended to replace PMTs as multiphoton pulse detectors. 

Silicon technology with submicron resolution makes possible SPAD array detectors that are suitable 

for 3-D imaging, i.e., with high number of pixels, adequate filling factor, and smart pixels with 

integrated electronics. Remarkable results have been obtained in 130-nm technology in 



Fig. 1. The 32 32 imaging array detector developed at SPADlab [8] in 350-nm HV-CMOS technology. 

(a) Smart pixel structure and (b) pictorial view of the 2-D array architecture. 

collaboration by various groups [2]. A 32 32 pixel SPAD array with an in-pixel Time-to-Digital 

Converter (TDC) [3] was developed and experimented in 3-D imaging and Fluorescence Lifetime 

Imaging (FLIM) [4]. Extension to 160 128 pixels has been announced [5]. A tough basic issue 

must be faced: Inherent features of superscaled technologies conflict with detector performance. 

The thin junction depth limits the PDE, the high electric fields enhance the DCR, and afterpulsing 

effects are strong. Peak PDE 25% was attained at 500 nm wavelength, falling below 5% at 800 nm, 

with a DCR of hundreds of counts/s for a detector diameter of less than 10 m. Specific modifications 

have also been devised within the standard 130-nm technology and experimented with 

significant results [6]. The trend to superscaled technologies to attain higher system integration is 

anyway hindered. For instance, working in 90-nm scaled technology, SPAD devices have been 

demonstrated but with remarkably lower performance [7]. 

An attractive alternative is given by high-voltage complementary metal–oxide semiconductor 

(CMOS) technologies (HV-CMOS) for automotive and industrial control electronics. Their scaling is 

less marked, but technological features for accommodating high-voltage devices are also favorable 

to SPAD junctions. In 350-nm HV-CMOS technology, SPADs with noteworthy performance have 

been obtained, and a 32 32 pixel SPAD array with smart pixels (see Fig. 1) has been demonstrated 

at our laboratory (SPADlab) [8]. Better than 35% peak PDE is attained at 450 nm wavelength, 

decreasing to 8% at 800 nm, with DCR in the range of kcounts/s for 20- m diameter SPADs. 

In-pixel information storage capability (similar to a CCD) is given by an 8-bit counter. Noteworthy 

results have been obtained in challenging experiments [9]. Work is in progress on a 32 32 SPAD 

array for 3-D imaging with an in-pixel TDC [10]. 

Technologies with dedicated features can better exploit the SPAD detector performance, and 

arrays of SPADs can also be implemented with moderate scaling, although with significant limitations 

to the system integration. Linear 1 8 array and 2-D 6 8 arrays were developed at SPADlab 

in a custom 1- m technology, with pixels of 50 m diameter, 9 50% peak PDE at 550 nm, reduced 

to 15% at 800 nm, and a DCR of few hundreds of counts/s [11]. Significant results were obtained in 

demanding applications, such as multispot FCS with single-molecule detection [12]. In-pixel 

circuitry was added with modifications to the custom technology [13], and further evolution is 

under way. 

New developments in single detectors are mainly concerned with wide sensitive area and enhanced 

PDE in the red wavelength range (600 nm to 900 nm) of high interest for fluorescencebased 

techniques in biomedical applications. PDE 9 70% at 670 nm with diameter 9 100 m was 

announced for a new commercial detector [14] but with technology unsuitable to integration and 

producing high-voltage devices with high power dissipation. To obtain red-enhanced PDE, lowvoltage 

operation, and suitability to monolithic integration, a new SPADlab custom technology was 

developed [15]. A deeper depletion layer with a new profile of the electric field was devised and 



designed for optimal tradeoff between operating voltage, avalanche triggering probability, DCR, and 

timing jitter. 

The PDE in the NIR is extremely low for classic PMTs and 1% for PMTs with advanced 

photocathodes, which have high photon timing jitter and DCR. Photon counting was first extended 

to the NIR with Ge SPAD and consolidated with InGaAs/InP SPADs, which are now the workhorse 

for experiments. A thorough review of their state of the art is available [16]. Compound semiconductor 

devices have physics and technology that is inherently more complex than Silicon and do 

not benefit of such a huge research effort. They are strongly plagued by carrier generation centers 

and other point defects that affect the SPAD performance. Carrier trapping and delayed release in 

deep levels produce strong afterpulsing. In recent years, significant progress has been achieved in 

fabrication technology and device design, but important limitations remain, particularly concerning 

the capability of working in free-running mode or with very high counting rate in gated mode. 

To circumvent these limitations at least in part, fast electronic circuit techniques have been 

exploited, and intriguing results have been reported with ultrafast detector gating in a selfdifferencing 

arrangement [17]. New devices, called Negative-Feedback Avalanche Diodes 

(NFADs), integrate a high-value thin film resistor in the chip to obtain passive quenching with 

very low parasitic capacitance and, thereby, fast operation with reduced avalanche charge and 

afterpulsing effect [18]. 

Very remarkable new developments of 2-D arrays of NIR-sensitive SPADs have been reported. 

With improved planar technology (contaminant reduction, uniform processing, etc.), monolithic focal 

plane arrays (FPA) with 32 32 SPADs of 34 m diameter were developed. InGaAsP/InP structure 

was employed for operation at 1064 nm and InGaAs/InP for 1550 nm [19]. Results are particularly 

striking for the InGaAsP/InP array, with high uniformity, yield and reliability, and high performance 

level: PDE better than 40% and DCR G 20 kcounts/s in operation at 250 K. The array detector 

chip was hybridized to CMOS integrated circuitry that enables independent time-of-flight measurements 

for each pixel with subnanosecond jitter for LADAR applications with frame rates up to 

200 kHz. 

Pioneering work was carried out at the Lincoln Laboratory at the Massachusetts Institute of 

Technology for extending photon counting deeper into the NIR [20]. For 2- m wavelength photons, 

devices of 30 m diameter with InGaAsSb absorber layer were developed for operation at 77K. 

Arrays of 1000 pixels interfaced with a CMOS readout circuit were demonstrated in a 3-D imaging 

system with acceptable performance for applications. Further extension to 3.4- m wavelength was 

pursued by developing devices with an InAsSb absorber. Correct SPAD operation of these devices 

was demonstrated, and detection of 3.4- m photons was verified, but with very low PDE and very 

high DCR, which is not suitable for applications. 

The extension of photon counting to longer wavelengths was also pursued with Superconducting 

Single-Photon Detectors (SSPDs) based on NbN nanowires. Novel device structures with strip 

width reduced to 55 nm and multiple nanowires in parallel were developed and experimented up to 

3.5 m wavelength with appreciable PDE and low DCR [21]. For operation at telecom wavelengths 

(1.3–1.55 m), the SSPD technology is well established and provides single-photon detectors 

capable of free running operation with good PDE, low DCR, and timing jitter better than 100 ps, 

although there is a fairly high cost and moderate miniaturization and system integration. A new 

achievement aiming to improve the system integration has been reported, namely, a chip with fourchannel 

SSPD integrated into an optical cavity structure [22]. 

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IEEE Photonics Journal Optical Bioimaging 2010 

Optical Bioimaging 2010: 

Seeing More, Deeper, Faster 

David D. Sampson 


Optical+Biomedical Engineering Laboratory, School of Electrical, Electronic and Computer Engineering 

and Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, 

Crawley, WA 6009, Australia 

DOI: 10.1109/JPHOT.2011.2128304 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received February 27, 2011; accepted March 7, 2011. Date of current version April 26, 2011. 

Corresponding author: D. D. Sampson (e-mail: David.Sampson@uwa.edu.au). 

Abstract: The year 2010 has seen many major developments in optical bioimagingVtoo 

many to fully survey here. We concentrate on breakthroughs that impact on future bioimaging 

of live animals and humans, including advances in resolution, imaging depth, 

speed, and function. 

Index Terms: Biomedical optical imaging, high-resolution imaging, optical microscopy. 

Optics and photonics-based bioimaging begins with microscopy of cells on glass slides and ends 

with medical imaging of tissues and organs. 2010 saw the continued expansion and increase in 

importance of bioimaging. In this review of selected advances in 2010, we restrict ourselves to 

optical imaging and omit near-field and probe-based techniques. 

A major thrust in microscopy has been the push for improved resolutionVparticularly beyond the 

diffraction limit, so-called super-resolution microscopy, and 2010 has seen several breakthroughs 

that set the scene for future advances. Researchers are demanding more than just higher resolution, 

however. To better understand basic biology and disease processes, biologists are increasingly 

looking beyond cells to live animal models, and bioimaging tools have been driven to keep pace. 

In vivo imaging brings with it many challenges, including overcoming the effects of overlying tissue, 

access to organs deep in tissue, the provision of functional capability, and adequate speed to 

observe important processes and overcome the effects of motion. These issues are critical in small 

animal imaging, which is a major driver in biomedical science. In medical imaging of humans, 

additional issues arise. The problems of accessing the imaging site and observing the desired target 

are compounded by the more stringent requirements on safety and levels of invasion demanded for 

imaging in humans. Many of these issues have been significantly advanced in 2010. With this focus 

in mind, we survey bioimaging and examine selected developments. 

1. Resolution Beyond the Diffraction Limit 

The ever-growing demand in biology for improved resolution has stimulated the development of 

super-resolution microscopy techniques [1], which are so called because they surpass the 

fundamental diffraction limit first described by Abbe in 1873. Enormous strides have been taken in 

recent years [2], [3], and the first super-resolution microscopes have become commercially available. 

To date, three approaches have emerged, based, in broad terms, on nonlinear fluorophore 

responses to reduce the emission spot size [4], multiple stochastic localizations of single molecules 



to build up an image from a sequence of events [2], [3], or spatial-frequency engineering by 

structured illumination [5]. Despite the advances represented by these approaches, the monitoring 

of dynamic processes at super-resolution without affecting a cell’s physiology or viability remains an 

elusive goal. 

In 2010, activity in super-resolution techniques remained strongVboth their use in biological 

imaging and their technical development [6]–[8]. An issue often overshadowed by the spectacular 

images produced is the computational overhead. Algorithms to speed up stochastic super-resolution 

methods have been the focus of several investigators in the last year [7], [8]. Another intriguing 

computational line of research [9], [10] is the application to optical super-resolution of compressed 

sensing signal processing techniques [11]. Compressed sensingVa field that has exploded in recent 

yearsVis generally directed toward reducing sampling requirements without loss of information. It 

incorporates the prior knowledge that a sample is sparse in a given basis set used to describe it. In 

optics, this can be real space and, thus, may apply to sparse objects such as cells, filaments, or 

vesicles. Compressed sensing is able to use this knowledge to extrapolate beyond the spatial 

frequency cutoff of an (low-pass) imaging system. Early results are promising, although in common 

with other methods [12], [13], resolution improvement has only been shown in a low-numerical 

aperture system. Super-resolution of better than half the wavelength of light is expected soon, 

however. This computational technique could be applied to many bioimaging approaches and, 

therefore, has a potentially very wide impact. A major test will be how well it reconstructs images of 

interesting biological objects and with what resolution. How it compares with conventional wide-field 

deconvolution microscopy [14] is another open question. 

2. Wavefront EngineeringVDeeper Imaging 

In optical microscopy, scattering of the light incident on, or generated by, the sample is always a 

source of problems. This holds true however it is generated, whether by fluorescence, harmonic 

generation, other nonlinear processes, or just plain elastic scattering. Even for the thinnest biological 

samples (cells and tissue sections), confocal fluorescence microscopy can experience 

significant resolution degradation in specimens only tens of micrometers thick [15]. In essence, 

wavefronts are corrupted by refraction and diffraction in the overlying medium, in much the same 

way as stars are blurred and twinkle when viewed through the intrinsic turbulence of the atmosphere. 

Adaptive optics was first developed to address this issue in astronomy. The detected 

wavefront is directly measured and the results used to remove the aberrations by deforming a 

telescope’s mirror in a feedback loop. Applying adaptive optics to microscopy is challenging. Point 

sources are not generally available, wavefronts cannot readily be measured within a sample, 

aberrations in entering and exiting beams can be different, and multiple scattering further complicates 

matters. 

The earliest work applying adaptive optics to microscopy was performed on the retina. By removing 

the aberrations caused by the eye, spectacular images of rods and cones were obtained 

[16]. Early work in biological confocal microscopy [17] produced more modest improvements, but 

the recent demonstration of correction in two-photon images of mouse embryos, based on iterative 

image processing, shows greater improvement [18]. In 2010, two groups advanced wavefront 

correction, with impressive new results. A group led by Dholakia employed a modal method [19] in 

which the sample is sequentially illuminated by a set of orthogonal modes. These were not the 

previously employed Zernike modes [shown on the left side of Fig. 1(a)] [17] but are instead squareshaped 

modes tiling the Fourier plane in which the wavefront altering device, i.e., the spatial light 

modulator, is located. Measuring each mode’s interference with a single reference mode sequentially 

optimizes the modes. Conveniently, the sample is always a Bguide star[Va microsphere point 

object in situ. The results are striking; optical trapping of beads is demonstrated through a 40- mthick 

turbid overlayer. A group led by Betzig [20] has put forward an alternative image-based zonal 

adaptive optics method [shown on the right side of Fig. 1(a)]. It is based on indirectly measuring the 

wavefront and correcting it in discrete zones that tile the entrance pupil of the imaging. Both methods 

demonstrated high-fidelity reconstruction of a point object through a turbid overlayer although, in 



Fig. 1. Imaging deeper in turbid media. (a) Adaptive optics, showing modal and zonal approaches to 

sequential wavefront correction. (b) Use of a diffuse scatterer as a focusing lens. (c) Modified Bessel 

beam with Bself-healing[ properties. 

using up to 900 modes [19], it is a laborious process. The zonal method [20] was used to correct 

cellular images in the basal layer of a 300- m-thick fixed brain tissue slice, with notable but more 

modest improvement. 

So how far can we penetrate into tissues today without resorting to adaptive optics? With confocal 

microscopy, images can be produced from a few hundred micrometers into highly scattering 

samples. The utility of such images is evidenced by current clinical use in dermatology [21]. 

Multiphoton and nonlinear microscopy extend imaging depths toward 1 mm, and optical coherence 

tomography extends imaging depths still further into the 2–3 mm range [22], sacrificing resolution to 

capture gross morphology. Eventually, multiple scattering causes the loss of image information and 

light more resembles a diffusion process. Until recently, this diffusion regime was thought to require 

approaches in which a property of the diffusion process, such as transit time, was the only rich 

source of information. In principle, if we neglect absorption, such multiply scattered light retains its 

image information if only we knew how to reverse the process. In recent years, several approaches 

to doing this have been demonstrated [23]–[25]. In 2010, Vellekoop et al. [25] have shown subdiffraction 

spot-size focusing using a thin highly scattering layer as the focusing element! A spatial 

light modulator controlled by a learning algorithm optimizes the incident phase front focused by the 

highly scattering layer in a lengthy sequential process broadly similar to those used in [19] and [20]. 

The experiment demonstrates the surprising result that scattering in a random medium placed 

behind a lens can be used to improve the focusing resolution to beyond the Abbe limit of the lens 

[see Fig. 1(b)]. Such an approach could be used to focus to a spot much smaller than the wavelength 

of light used, which is a feat already achieved with microwaves [26]. 

The focus on manipulation of wavefronts to achieve higher quality images and smaller focused 

spots raises the general question of what types of beams would be optimum for imaging through 

scattering heterogeneous media. In 2010, Bessel beams have received renewed attention in this 

regard [27]. These beams, which are typically generated by an annular amplitude mask or by its 

phase-mask near equivalent, i.e., an axicon, contain a narrow central lobe and many sidelobes and 

retain their intensity profile over an extended axial range [see Fig. 1(c)]. They have been investigated 

in optical coherence tomography [28] to overcome the attenuation away from focus caused 

by the beam spread in an axial (depth) scan. In 2006, Leitgeb et al. [29] showed excellent images 

using Bessel beam illumination and Gaussian beam detection. In 2010, while developing narrow 



extended beams for selective plane illumination microscopy, the Bself-healing[ property of Bessel 

beams has been highlighted. Such beams are surprisingly highly effective in avoiding the shadowing 

effects of large overlying structures, which is a potential explanation of the high image quality in 

[29]. New general methods for generating related complex beam shapes have also been put forward 

in 2010 [30]. It is interesting to consider whether the Bessel beam is the optimum for imaging in turbid 

media. 

3. Speed 

Higher speeds in bioimaging serve a variety of purposesVcapturing dynamic events, avoiding 

motion artifacts, or making acquisition times, especially for 3-D imaging, practical. Optical coherence 

tomography leads the way in high-speed acquisition, breaking the Megahertz barrier in 2010 

[31]. Huber et al. beat the previous record by a factor of 50 demonstrating a sustained rate of up to 

20.8 million depth scans per second, 14 600 frames/s, and one full volume in 25 msVremarkably 

without sacrificing image quality. 

Increased speed is required to make in vivo imaging feasible in various applications of nonlinear 

microscopy. It is a key issue in performing in vivo calcium imaging of neuronal network activity in the 

mouse brain. The strategy to increase the speed of two-photon microscopy, in this case, was to use 

acousto-optic scanners and a random access scanning protocol [32]. Dramatic technological 

improvements have recently led to the first in vivo mouse and human skin images using stimulated 

Raman scattering microscopyVan exceptional technical achievement [33]. 

A powerful new development in drug discovery is the ability to screen chemical libraries for 

efficacy on living organisms, such as the roundworm C. elegans and the zebrafish. Such capabilities 

represent impressive feats of engineering, in synthesizing microfluidics, manipulation, and 

high-speed microscopy and microsurgery. In recent work [34], high-throughput screening of zebrafish 

larvae, including laser microsurgery, was achieved at the rate of one every 20 s, executing a 

procedure that would take more than 10 min to perform manually. 

4. Imaging Live Animals and Humans 

2010 has seen the increasing sophistication of label-free optical microscopy methods. Olivier et al. 

[35] have developed a label-free protocol using second- and third-harmonic generation microscopy 

for imaging live cells in whole embryos without damage. By imaging unstained whole zebrafish 

embryos over their first ten cell-division cycles, they made the important observation that celldivision 

dynamics contrast markedly with conventional descriptions of this process. 

Two-photon microscopy continued to enable advances in neuroscience in 2010. It has been used 

to monitor tumor metastasis in the mouse brain, with sensitivity to single cells, and even their stages; 

differentiating between dormant, regressing, and proliferating cells over many weeks [36]! Using a 

two-photon enhanced phosphorescent nanoprobe, partial oxygen pressure in mouse cortical 

microvasculature has been measured [37], and high-speed calcium imaging with two-photon microscopy 

has enabled improved time-resolved measurement of brain neuronal activity [32]. 

To access internal organs, the challenges of propagation through overlying tissue can be circumvented 

by delivery of optical beams direct to the site via physical meansVand endoscopic 

confocal microscopy in mouse models has shown increases in sophistication and performance in 

2010, including demonstrations of tracking cells [38] and monitoring tissue structures over months 

[39]. In solid tissues, the need for miniaturization to minimize trauma to the tissue makes imaging 

more difficult, but needle-based microscopes are also steadily advancing [40]. 

In humans, confocal endomicroscopy is being explored for a wide range of clinical applications 

[41]. Many of these are directed toward cancer detection: generally in situ diagnosis for biopsy or 

surgical guidance. An important target is the esophagusVif precancerous changes are detected 

then most cancers can be prevented. Current biopsy methods often fail to detect precancer 

(dysplasia) through failing to hit the precancerous tissue in biopsy sampling. A 2010 study of 68 

patients comparing confocal fluorescence endomicroscopy with standard four-quadrant biopsy [42] 

shows that challenges remain. Another appealing approach to cancer detection in situ was first 



demonstrated more than a decade ago. Light scattering spectroscopy [43] is used to detect the 

average increase in cell nuclear size and its spread caused by cancer. New results in 2010 [44] 

show promise for prescreening the esophagus for biopsy site selection. The method uses a side 

viewing, rotating translatable probe to record white light reflectance spectra in orthogonal polarizations 

over large areas of the esophagus. Maps produced by the technique revealed a high 

sensitivity of 92% and specificity of 96% in a double-blind comparison with biopsy reports, albeit on 

a small eight-patient cohort. 

In 2010, several groups have reported important baseline studies on the capacity of optical 

coherence tomography to delineate tumor morphology in solid human tissues [45], [46]. Surprisingly, 

given the length of time it has been in existence, such studies are in their infancy. Progress 

has been impeded by the slow development of the capability to make accurate comparisons with 

histology. This, in turn, has necessitated 3-D imaging, software tools for comparing images, and 

strong engagement with adventurous pathologists. Early results are encouraging, but much more 

work is required to establish clear benchmarks. 

This incomplete survey of bioimaging in 2010 highlights the diversity and sophistication of this 

surging and dynamic field. Fundamental developments in image formation and the rapid 

development of adaptive optics and wavefront engineering set the scene for major advances in 

future years, but as can be seen, pushing existing tools to their limits is enabling sophisticated 

functional imaging in situ in animal models, and inroads are being made in human imaging as well. 

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IEEE Photonics Journal New Design Principles for Nanoplasmonics 

New Design Principles for Nanoplasmonics 

A. I. Fernández-Domínguez and S. A. Maier 


Department of Physics, Imperial College London, London SW7 2AZ, U.K. 

DOI: 10.1109/JPHOT.2011.2127469 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received February 18, 2011; accepted March 7, 2011. Date of current version April 26, 

2011. Corresponding author: S. A. Maier (e-mail: s.maier@imperial.ac.uk). 

Abstract: During the last year, innovative designs that exploit the strong dependence of 

plasmonic modes on geometry have been proposed, offering new prospects on the tailoring 

of the optical properties of metal nanoparticles. On one hand, Fano resonances enable the 

opening of a narrow spectral window where light scattering is strongly inhibited. On the other 

hand, transformation optics makes possible the transfer of capabilities from infinite to finite 

systems, which leads to an efficient nanoconcentration of broadband radiation in 

nanostructures. 

Index Terms: Plasmonic Nanostructures, Fano Resonances, Transformation Optics. 

When light interacts with a metallic nanoparticle (NP), its conduction electrons are driven by the 

incident electric field. The coupling of the external radiation and the collective electronic oscillations 

within the metal gives rise to the so-called localized plasmon resonances (LPRs). The mixed 

electromagnetic-wave and surface-charge nature of LPRs has grabbed much attention during the 

last decade, as it allows one of the main constraints of classical optics, the diffraction limit, to be 

overcome [1]. This fact makes possible the control of the flow of light and its interaction with matter 

at the nanoscale. Thus, the excitation of LPRs in metal NPs leads to striking effects such as the 

drastic increase of the nanostructure effective cross section, the subwavelength localization of 

electromagnetic energy in its vicinity, or the highly directional scattering of radiation out of the 

system [2]. 

The characteristics of LPRs depend strongly on the shape and size of the particle sustaining 

them. This fact enables the spectral tuning of the aforementioned effects within the visible range, 

which has found applications in technological areas such as sensing, biomedicine, or photovoltaics 

[3]–[5]. Until recently, the manipulation of the electromagnetic behavior of metal NPs was limited to 

the modification of the resonant frequency of dipolar LPRs, which, due to their strong radiative 

character, govern the interaction of the structure with free space radiation. However, metal NPs do 

not only support bright dipole-like resonances but higher order multipolar modes associated with 

charge oscillations with more complex symmetry features as well. These are usually termed dark 

modes, since they couple only weakly to light. Therefore, the damping of these dark resonances is 

mainly caused by metal absorption, rather than radiative losses, which makes them spectrally 

narrower than dipolar LPRs. 

The intrinsic dark nature of multipolar LPRs seemed to prevent their exploitation in nanooptics. 

However, during the past few years, various NP configurations have been proposed where bright 

and dark modes occur in the same frequency window. This provides the latter with a key role in the 

interaction of these structures with light. In these designed systems, dark LPRs are not excited by 

the incoming radiation but by near-field coupling with the bright dipolar mode that resonates at the 



Fig. 1. Plasmonic design principles enable the accurate control of the spectral response of metallic 

nanoparticles. In the left panel, the exploitation of Fano resonances in a dolmen-shaped nanobar 

configuration (scale bar: 100 nm) leads to the polarization-dependent suppression of its scattering cross 

section within a narrow frequency window [6]. In the right panel, transformation optics enables the 

transfer of the broadband behavior of an infinite plasmonic system (a metal/dielectric/metal sandwich) to 

a finite one (two touching nanospheres), which also gives rise to a strong concentration of light at the 

contact point of the NPs [17]. The calculations correspond to Ag spheres of 25 nm radius, and the 

longitudinal electric field in the insets is evaluated at 600 nm. 

same frequency. In analogy to atomic physics, the destructive interference between the bright 

(broad) and dark (narrow) excitation channels available for the incident radiation leads to the 

formation of Fano resonances. These give rise to sharp and narrow dips in the scattering spectra of 

the supporting nanostructure, whose origin is the effective inhibition of radiative losses linked to the 

excitation of dark resonances in the system. 

The left panel of Fig. 1 renders two experimental extinction spectra for a dolmen-shaped gold 

nanostructure supporting a Fano resonance at 750 nm [6]. The geometry is conceived so that its 

constituents, i.e., monomer and dimer, sustain a bright (dipolar) and a dark (quadrupolar) mode 

within the same frequency window. The hybridization of these plasmonic modes yields, for the 

appropriate incoming polarization (blue), the appearance of a Fano-like minimum in the cross 

section for the composite structure. More recently, Fano effects have been reported in a wide range 

of plasmonic systems [7] and metamaterials [8], and their sharp spectral profile is being exploited in 

the design of devices such as biological sensors or active photonic waveguides [9]. 

During the last decade, transformation optics [10] has become the theoretical framework driving 

the development of metamaterials science. This elegant tool, which exploits the invariance of 

Maxwell’s equations under coordinate transformations, provides the link between a desired 

electromagnetic effect and the material properties required for its occurrence [11]. In this context, it 

establishes how the electromagnetic constitutive relations must be modified within a metamaterial 

structure in order to achieve a given optical response. Similar ideas have been also transferred to 

plasmonics, and the routing of surface plasmon polaritons through transformation optics has been 

reported lately [12], [13]. 

In the last year, several theoretical works have recovered the original purpose of transformation 

optics, which was first thought as a strategy to ease the solution of Maxwell’s equations [10], by 

applying it to the analytical treatment of the interaction of light with metal NPs. The approach is as 

follows: Using a spatial transformation, singular NP configurations can be mapped into more 



manageable plasmonic systems, where electromagnetic fields can be calculated analytically. By 

transforming back to the original frame, the optical properties of the initial nanostructure are known, 

and simple expressions for magnitudes such as the cross section or the near-field enhancement 

can be obtained. Apart from the deep physical insight that this method offers, the analytical 

treatment of singular plasmonic geometries, such as crescents [14], wedges [15], or touching 

nanowires [16] and spheres [17], has also opened the way to the solution of one of the paradigms of 

modern photonics: the design of nanometric devices able to collect and concentrate light efficiently 

within a wide spectral range. 

The bottom right panel of Fig. 1 sketches the transformation optics procedure for the case of a 

dimer of touching nanospheres, which are mapped into a metal/dielectric/metal geometry under a 

coordinate inversion [17]. The upper panel plots the comparison between the absorption spectra for 

a single Ag sphere (black) and a dimer of touching spheres (red) of 25 nm radii. Note that, whereas 

the cross section for the single sphere is negligible far from its dipole LPRs, the dimer collects light 

efficiently within the whole visible range (shaded). Remarkably, this effect is accompanied by the 

focusing of the broadband incident radiation at the contact point of the NPs, which yields extremely 

high field enhancement factors in its immediate vicinity (see insets). This effect has potential 

applications in technological areas such as solar photovoltaics or Raman spectroscopy [14]. 

The physical origin of the efficient light harvesting capabilities of touching NPs is also revealed by 

transformation optics. The equivalence between the original and the transformed systems enables 

the interpretation of the physical mechanisms behind this phenomenon as resulting from the 

capability of the surface plasmon modes in the planar geometry to transport energy along its flat 

interfaces. It is important to note that the inversion illustrated in Fig. 1 is only one of the whole set of 

possible mathematical transformations, which makes this approach an extremely versatile tool for 

the design of novel functional plasmonic structures. 

In summary, Fano resonances and transformation optics are elegant concepts that make 

possible the design of plasmonic NPs showing unexpected, and technologically promising, optical 

properties. We have shown that the exploitation of Fano resonances allows the molding of the 

spectral response of composite metal nanostructures within narrow frequency windows. In turn, 

transformation optics ideas convey the broadband behavior of propagating surface plasmons to 

subwavelength NPs, also providing a highly efficient strategy to achieve nanoconcentration of light 

in the contact point of touching geometries. Although the nanofabrication of structures where these 

effects are optimized remains challenging, experimental realizations indicating the validity of these 

ideas in different systems have already been reported. 

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IEEE Photonics Journal Metal-Cavity Nanolasers 

Metal-Cavity Nanolasers 

Shun Lien Chuang 1 and Dieter Bimberg 2 


1 Department of Electrical and Computer Engineering, University of Illinois 

at Urbana-Champaign, Urbana, IL 61801 USA 

2 Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany 

DOI: 10.1109/JPHOT.2011.2138690 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received February 19, 2011; revised March 24, 2011; accepted March 26, 2011. Date 

of current version April 26, 2011. The work at the University of Illinois at Urbana-Champaign was 

supported by the Defense Advanced Research Projects Agency’s NACHOS Program under Grant 

W911NF-07-1-0314. S. L.C. also thanks the support of the Humboldt Research Award from the 

A. von Humboldt Foundation. The work at Technische Universität Berlin was supported by 

Deutsche Forschungsgemeinschaft in the frame of SFB787. Corresponding author: S. L. Chuang 

(e-mail: s-chuang@illinois.edu). 

Abstract: Recent progress on nanoscale lasers, especially metal-cavity nanolasers, is 

highlighted. Inspite ofthe metal loss, metal cavities of subwavelength scales have been 

used successfully for semiconductor lasers byoptical or electrical pumping from low to room 

temperature. We focus on the demonstration of a substrate-free metal-cavity surfaceemitting 

microlaser operating continuous wave at room temperature with electrical injection. 

Index Terms: Semiconductor lasers, nanolasers, nanophotonics. 

Nanoscale lasers possess advantages such as low power consumption, an ultrasmall footprint, 

and ultrafast switching [1]–[3]. Potential applications include biochemical sensing [4], imaging [5], 

and intrachip and interchip short-distance optical interconnects [1], [2]. Practical nanolasers require 

electrical injection operation at room temperature in continuous-wave mode. Independent nanolasers 

can form dense arrays of subwavelength pitch for possible near-field scanning and optical 

atom traps. The smallest laser based on dielectric cavities requires an optical cavity with a dimension 

of half a wavelength in all three directions, which is often called the diffraction limit. During the 

last decade, photonic crystal lasers have been extensively studied as candidates for small lasers. 

However, to have a large quality factor for laser action, many periods of photonic crystal are 

required, making the size onthe order of several wavelengths. Toproduce alaser breaking the 

diffraction limit, one approach is to use the plasmonic effect [6]–[15] formed at the interfaces 

between the metal and semiconductor. In this case, both the physical and effective volume of the 

optical cavity can bereduced, although it would beat the expense of modal absorption due to the 

metal loss. By positioning the active materials such as quantum dots or quantum wells (QWs) at 

the peak of optical fields with an emission wavelength near the cavity resonance, itis possible to 

enhance the spontaneous and stimulated emission [12] and reduce the lasing threshold. There 

has been excellent progress inmicro-and nanolasers, especiallymetallic and plasmonicnanolasers 

[3]. Plasmonic nanolasers via optical pumping have been reported by [7] using a CdS nanowire as 

the gain medium ontop of asilver surface with a5-nm insulator gap. Nanoparticles with a gold core 

and dye-doped silica shell have been used to realize spaser-based nanolasers via optical pumping 

[8]. Electrical injection of metal-cavity lasers with a dimension that is less than a wavelength in one or 

two dimensions has been demonstrated in [9]–[11]. 



Fig. 1.(a) Schematics of our metal-cavity microlaser. The fabricated device has an active region of 

14 GaAs/Al0:2Ga0:8As quantum wells. Optical feedback is from the bottom silver and top hybrid/DBR 

mirrors with the surrounding metal sidewall. Silver encapsulation helps mode confinement and 

scattering reduction. (b) Current dependent spectra of the device lasing under CW current injection at 

300 K. (Inset: scanning electron micrograph of aSiNx-passivated cavity before metallization). (c) Light 

output power as afunction of injection current (L–I) curves at various temperatures (10 C–27 C) and 

the I–V curve at room temperature (27 C) [22]. 

In 2010, significant progress on micro- and nanolasers has been made, i.e., subwavelength 

nanolasers via optical pumping [16]–[19], nanopillar lasers on silicon substrate [20], electrical 

injection Fabry–Perot metal-cavity lasers at 240 K[21], and substrate-free metal-cavity surfaceemitting 

microlasers at room temperature [22]–[26]. At the University ofCalifornia at San Diego, 

metallo-dielectric subwavelength lasers [16] using an InGaAsP multiple quantum well (QW) active 

layer disk surrounded by an aluminum/silica bilayer shield as the cavity were made by optical 

pumping at room temperature. The importance of the optimized thickness of the insulating silica is 

emphasized to reduce the threshold gain for optical pumping at room temperature. The feedback is 

provided by a mode cutoff plug-instructure which forbids the propagating mode inside,thus achieving 

a high reflectivity mirror. At University ofCalifornia at Berkeley, subwavelength nanopatch lasers 

using top and bottom metals (gold) to form the nanocavity with InP/InGaAsP/InP materials with a 

physical volume ð0:019 3 0Þ were demonstrated at 78 K[17], [18] by optical pumping. Due to their 

resemblance topatch antennas inmicrowave technology,the structures emit lightfrom the sidewalls 

with constructive/destructive interferences in the surface normal direction and are suitable for beam 

divergence control. Polarization controllability has been demonstrated by tuning the geometryofthe 

nanopatches. Silver nanopan plasmonic lasers [19] with avolume of 0:56ð 0=2nÞ 3 have also been 

demonstrated at 8 K with a subnanometerlinewidthbyoptical pumping.Whispering gallery modes in 

silver defined cavity were identified in nanopan plasmonic lasers. Nanolasers using InGaAs 

nanopillars grown onsilicon substrate byoptical pumping at room temperature have also been 

reported by UC Berkeley [20]. Until recently, the electrical injection of metal-cavity semiconductor 

lasers has demonstrated significant progress, such as high-temperature (240 K) continuous-wave 

(CW) operation using aFabry–Perot type with emission from the bottom aperture byArizona State 

University and Technical University ofEindhoven [21], as well as a CW room temperature surfaceemitting 

microlaser bonded on silicon with a physical volume of12 3 0 by the University ofIllinois and 

the Technical University ofBerlin [22]–[26]. 

The size ofthe laser isnownot limited by the diffraction limit. Nanolasers will have a large impact 

on our technology if they are integrable to current electronic architecture. From an application point 

of view, nanolasers with integrability to current electronic platforms (i.e., silicon) will lead to 

advanced photonic integrated circuits. Several nanolasers have shown a promising future for 

integration either by direct growthof nanopillars(without metal coating) on a silicon substrate [20] or 

by stacking the devices onto the electronic platform. Wedemonstrated experimentally a metalcavity 

surface-emitting microlaser with metal on the top and surrounding sidewall and a bottomdistributed 

Bragg reflector (DBR), which lases at room temperature under CW operation (see Fig.1) 

[22]–[26]. The active region consists of14pairs of GaAs/Al0:2Ga0:8As QWs. Multiple QWs uniformly 

distributed in the active region are used to provide enough optical gain without worrying about the 

longitudinal standing wave (node/peak) effects. A17.5-pair n-doped quarter-wavelength DBR acts 

as both the feedback and the electron injector. The integration to silicon was demonstrated by flipchip 

bonding to a gold coated silicon substrate with the complete removal of the GaAs substrate to 



allow itssurface emission. The physical size after substrate removal is only 2.0 m in diameter and 

2.5 m in total thickness (avolume of 12 3 0 ),including the overall p i(QWs)-n(DBR) regions. Flipchip 

bonding with metal allows the integration of our metal-cavity lasers to various substrates, 

including silicon in our devices. Metal serves as a multifunction medium for reflector, contact, and 

heatsink. The round-trip resonance phase condition is satisfied by choosing the active layer 

thickness to match the boundary conditions at both top metal and bottom DBR for the metalconfined 

fundamental optical mode. Also, with a broadband reflector using metal, the detuning of 

the cavity mode with the gain peak can thus bereduced, compared with standard vertical-cavity 

surface emitting lasers [27]. Our devices were mounted on athermoelectrically cooled copper heat 

sink for measurements at 300 Kunder CW operation. Thermal management has been largely 

improved as aresult of efficient heat removal from the surrounding metal and the substrate-free 

configuration with bonding. Wehave measured the light output power asafunction of the injection 

current at temperatures from 10 to 27 C, showing temperature-stable operation with a 

characteristic temperature of425 K[22], [26]. The light output power is up to 7.5 W at 4.5 mA. 

We have also measured the laser linewidth and obtained a value of 0.67 A˚ (full-width at halfmaximum) 

at a bias of 2.8 mA. This is probably the narrowest measured laser linewidth among 

metal-cavity lasers with electrical injection, which are typically hard to measure due to their low 

power. A kink at3.2 mAbias current shows polarization switching behavior, which is confirmed by 

measuring the polarization resolved L–I curves and emission spectra at various bias currents [22]. 

We have also developed arigorous theoretical model, which takes into account the plasmonic 

dispersion in a nanocavity [12]–[14] and pointed out the importance of using the energy (instead of 

power) confinement factor [13]. Our theoretical formulation and the resultant rate equations have 

been applied to study nanolasers such as a nanobowtie laser [12] and a metal-cavity edge-emitting 

laser [13], [14] for the prediction of lasing threshold and light output power versus injection current 

(L–I curve). Tocompare our theory with experimental data [22]–[26], we first calculate the band 

structure ofthe GaAs/AlGaAs QWlasers and the optical gain spectrum as afunction of increasing 

carrier density. Wealso compared the amplified spontaneous emission spectra in the metal cavity 

with the measured asymmetrical electroluminescence spectra [see Fig. 1(b)] at various injection 

currents below threshold and obtained good agreement. The band edge of the QW spontaneous 

emission spectrum and the cavity resonance spectrum creates an asymmetrical lineshape. Our 

model result of the quality factor Qof556ofthe cold cavity is close to the measured value of580 at 

low injection current. We then model the measured light output power asafunction of the injection 

current based on our rate equations and show our theory agrees well with the experimental data 

shown in Fig. 1(c). Wefound that at a very small bias current below 0.5 mA, there is nolight 

emission until the spontaneous emission peak wavelength merges with the cavity resonance 

wavelength. Above 0.5 mA, the spontaneous emission starts to amplify significantly with increasing 

gain as the current increases. When the optical gain reaches threshold at 1.75 mA, the laser action 

starts to occur. We have further reduced the size ofour metal-cavity surface-emitting lasers by 

either shrinking the diameter or reducing the number of DBR pairs to only five or even zero, while 

maintaining areasonable quality factor for laser action. The results will be reported in the near 

future [28]. 

To recap, we have demonstrated experimentally a room-temperature metal-cavity surfaceemitting 

microlaser [22]–[26] and developed arigorous model for nanolasers with further reduction 

in size [13], [14]. Our theory explains the observed asymmetrical optical emission spectrum below 

threshold and the light output versus injection current (L–I curve). Nanolasers pose intriguing 

challenges for researchers in photonics, both intellectually and technologically. Due to their compactness 

in size and substrate-free and/or silicon compatibility, they are promising elements to 

bridge the gap between nanophotonics and silicon electronics. They have potential applications for 

ultrahigh density photonic integrated circuits with ultralow power consumption and footprint and 

ultrafast switching speed. The ultrahigh modulation bandwidth ofnanolasers has yettobe demonstrated 

experimentally [29]–[31]. Further research is necessary to reduce the metal losses in the 

cavity and to overcome the technological challenges of nanofabrication of nanoscale semiconductor 

lasers with electrical injection. 



In summary, wehave reviewed progress in nanolasers in 2010 with a more extensive discussion 

of our own contributions to metal-cavity nanolasers. We note that our brief review may be 

incomplete; nevertheless, we hope it will provide a stimulus for further research on nanolasers. 


The authors would like to thank C. Y. Lu, S. W.Chang, and A. Matsudaira at the University of 

Illinois at Urbana-Champaign and T. D. Germann and U. W.Pohl, at the Technical University of 

Berlin for their contributions. 

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[5] Y. Nakayama, P. J. Pauzauskie, A.Radenovic, R. M.Onorato, R. J. Saykally, J. Liphardt, and P. D. Yang, BTunable 

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[7] R.F.Oulton, V.J. Sorger, D.A.Genov, D.F.P. Pile, and X. Zhang, BA hybrid plasmonic waveguide for sub-wavelength 

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and U. Wiesner, BDemonstration of a spaser-based nanolaser,[ Nature, vol. 460, no. 7259, pp. 1110–1112, 

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[9] M. T. Hill, Y. S. Oei, B. Smalbrugge, Y.Zhu, T. de Vries, P. J. van Veldhoven, F.W.M.vanOtten, T.J. Eijkemans, 

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[10] M. T. Hill, M. Marell, E. S. P.Leong, B. Smalbrugge,Y.Zhu, M. Sun, P. J.van Veldhoven,E.J. Geluk, F. Karouta, Y. Oei, 

R. Nötzel, C. Z.Ning, and M. K. Smit, BLasing in metal-insulator-metal sub-wavelength plasmonic waveguides,[ Opt. 

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[11] M. T. Hill, BNanolaser beat the diffraction limit,[ IEEE Photon. J., vol. 2, no. 2, pp. 235–237, Apr. 2010. 

[12] S.W.Chang, C. Y. Ni, and S.L.Chuang, BTheory for bowtieplasmonicnanolasers,[ Opt. Exp.,vol. 16, no. 14, pp.10 580– 

10 595, Jul. 2008. 

[13] S.W.Chang and S.L.Chuang, BFundamental formulation for plasmonicnanolasers,[ IEEE J. QuantumElectron.,vol. 45, 

no. 8, pp. 1014–1023, Aug. 2009. 

[14] S. W.Chang, T.R. Lin, and S. L.Chuang, BTheory of plasmonic Fabry–Perot nanolasers,[ Opt. Exp., vol. 18, no. 14, 

pp. 15039–15 053, Jul. 2010. 

[15] C. Z.Ning, BSemiconductor nanolasers,[ Phys. Stat. Sol. B, vol. 247, pp. 774–788, 2010. 

[16] M. P. Nezhad, A.Simic, O. Bondarenko, B. Slutsky, A. Mizrahi, L. Feng, V.Lomakin, and Y. Fainman, BRoomtemperature 

subwavelength metallo-dielectric lasers,[ Nat. Photon., vol. 4, pp. 395–399, 2010. 

[17] K. Yu, A. M. Lakhani, and M. C. Wu, BSubwavelength metal-optic semiconductor nanopatch lasers,[ Opt. Exp., vol. 18, 

no. 9, pp. 8790–8799, Apr. 2010. 

[18] A. M. Lakhani, K. Yu, and M. C. Wu, BLasing in subwavelength semiconductor nanopatches,[ Semicond. Sci. Technol., 

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[19] S. H.Kwon, J. H.Kang, C. Seassal, S. K.Kim, P. Regreny, Y.H.Lee, C. M.Lieber, and H. G. Park, BSubwavelength 

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Sep. 2010. 

[20] R. Chen,T.T.D. Tran,K.W.Ng,W.S.Ko,L.C. Chuang,F.G. Sedgwick, and C. J. Chang-Hasnain, BAll-semiconductor 

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pp. 15–16. 



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IEEE Photonics Journal Graphene Nanophotonics 

Graphene Nanophotonics 

Fengnian Xiaand Phaedon Avouris 


IBM Thomas J. Watson Research Center, Yorktown Heights, NY10598 USA 

DOI: 10.1109/JPHOT.2011.2129591 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received March 8, 2011; accepted March 11, 2011. Date of current version April 26, 2011. 

Corresponding author: F. Xia (e-mail: fxia@us.ibm.com or fxia@alumni.princeton.edu). 

Abstract: Graphene, which is a single layer of carbon atoms assembled in a honeycomb 

lattice, has recently attracted significant attention, primarily due to its extraordinary electronic 

properties. In fact, its photonic properties are notless exciting. Graphene interacts with 

light strongly from ultraviolet to far infrared, and such interaction is tunable byelectric field. 

Moreover, although graphene itself is gapless, adirect, tunable bandgap can becreated by 

breaking its intrinsic crystallographic symmetry. These unique properties make graphene a 

promising candidate for various light detection, manipulation, and generation applications in 

an ultra-wide operational wavelength range. In this paper, we first discuss afew possible 

photonic applications based on the exceptional photonicproperties of graphene,followed by 

detailed presentation on graphene photodetectors. Finally, twomajor future directions on 

graphene nanophotonic research will be covered. 

Index Terms: Graphene, photodetectors, optical modulators, nanophotonics. 

In 2004, single-layer graphene was isolated from graphite using mechanical exfoliation, and its 

carrier transport properties were reported by Geim’s research group [1]. Since then, itquickly 

became one of the hottest research topics in condensed matter physics and semiconductor electronics. 

Intense interests in graphene mainly arise from the unique band structure of graphene, as 

shown in Fig. 1(a): Graphene is a semimetal with zero band gap and linear energy dispersion 

around the K ðK 0 Þ point, i.e., the so-called Dirac point. The behaviorsofthe carriers ingraphene can 

be described by the relativistic Dirac equation with zero effective mass. On the contrary, carriers in 

conventional semiconductors are governed by the nonrelativistic Schrödinger’s equation with finite 

effective masses. 

The unique band structure of graphene also leads to its striking photonic properties. Asingle 

graphene layer absorbs about 2.3% ( ,where is the fine structure constant) ofthe vertical 

incidence light in awide wavelength range due to interband transitions [2], when the Fermi-level in 

graphene is aligned with Dirac point energy, as shown in Fig. 1(b). This makes graphene a 

promising candidate for photodetectors at least from near-infrared to visible wavelength range. 

When the Fermi-level is tuned away from Dirac point energy by EF, the graphene is expected to 

become close to transparent to photons with energy below 2 EF, due to Pauli’s exclusive principle, 

as shown in Fig. 1(b). This tunable absorption property may beutilized to construct light modulators 

or switches.Finally,there are afewapproaches tocreate a directand tunablebandgap inotherwise 

zero gap graphene [3]–[6], which may also lead to useful applications in infrared nanophotonics. 

We first performed photocurrent imaging experiment ingraphene field-effect transistors (FETs). 

In this experiment, light from a helium-neon laser was focused on a graphene FET and a scanning 

mirror was used to scan the light spot across the device. The photoinduced current inexternal 



Fig. 1.(a) Band structure ofthe single layer graphene. (b) The alignment of the Fermi-level and Dirac 

point energies in intrinsic, p-doped, and n-doped graphenes (from top to bottom). 

Fig. 2. (a) A scanning electron micrograph ofagraphene photodetector with interdigitated metallic finger 

(in false color). Titanium (Ti) and Palladium (Pd) are used as source and drain electrodes, respectively. 

(b) The schematicband profileof such the graphene FET with Ti and Pd assource and drainelectrodes. 

Pd introduces heavy p-doping, while the doping introduced by Ti is much lighter. The red dashed line 

represents the Fermi-level, and the thick solid black line denotes potential profile. (c) The 10-Gbits/s 

eye-diagram obtained using the graphene photodetector shown in (a) in an optical communication link 

(adopted from [10]). 

circuit was then recorded as afunction of the light illumination position. Strong photocurrents are 

usually observed when the light is focused at the metal–graphene interface, due to the strong builtin 

electric field at the interface caused by the charge transfer between the metal and graphene. 

Such a built-in field separates the electron–hole pairs generated by photons and leads to 

photocurrent [7], [8]. We also measured the high-frequency photoresponse of such graphene FETbased 

photodetectors and found no significant photoresponse degradation, even at a light intensity 

modulation frequency of 40GHz [9]. This isdue to the high carrier mobility and large carrier velocity 

ingraphene,leading to avery large operational bandwidth.However, the external photoresponsivity 

in such photodetectors is much smaller than that of aconventional III–V based high bandwidth 

photodetectorforthe following two reasons. First, absorption ofvertical incidence lightinsingle-and 



few-layer graphene is rather limited. Second, the effective light detection area ismuch smaller than 

the light spot size, because the high built-in electric field only exists at the metal–graphene 

interface. 

In 2010, we showed that the external photoresponsivity of such graphene photodetectors can be 

improved greatly via a structure consisting of multiple interdigitated metallic fingers, as shown in 

Fig. 2(a) [10]. In this device, astrong internal field exists onall the metal–graphene interfaces, and 

the total effective light detection area is hence significantly enhanced. However, due to the symmetric 

nature ofthe internal field, photocurrents generated at the source and drain electrodes 

cancel each other out, and the net photocurrent inexternal circuit will be zero regardless of the gate 

bias, ifsource and drain electrodes are made from the same metal [10]. Therefore, we utilized 

different source (Titanium) and drain (Palladium) metals to resolve this problem. Different doping 

levels are introduced in graphene under source and drain electrodes, as shown in Fig. 2(b) [10]. By 

adjusting the back gate bias, the internal E-field at both source and drain electrodes can bealigned 

to the same direction, leading to greatly enhanced light-detection efficiency. Moreover, by applying 

a small bias between the source and drain, the detection efficiency can befurther improved. A 

device shown in Fig. 2(a) exhibits a maximum external photoresponsivity ofaround 6.1 mA/W at 

1.55 m light excitation, which is 15 times more efficient than the graphene photodetector 

reported in [9]. Such a photodetector was deployed in a10-Gbits/s optical communication link, 

and error-free recovery of optical PRBS was realized,as shown bythe complete open eye-diagram 

in Fig. 2(c). 

Future research on graphene nanophotonics will most likely focuses on the following two major 

directions. The first isdevelopment of integrated graphene optoelectronic devices for near-infrared 

optical communication and interconnects applications. High-performance, high-bandwidth photodetectors 

and optical modulators are within reach through the monolithic integration of large-scale 

singleorfew-layer graphene withsubmicron silicon photonic waveguides [11]. The second direction 

is the application of graphene in infrared and terahertz regimes [12]. For example, creation of a 

moderate direct bandgap in biased bilayer graphene may allow for widely tunable, mid-infrared light 

emission [3]–[5]. Terahertz imaging can also be another promising direction due to the strong 

absorption of far-infrared and terahertz light ingraphene resulting from intraband transitions [13]. 

References 

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IEEE Photonics Journal Optical Interconnects 

Optical Interconnects Using Plasmonics 

and Si-Photonics 

Nikos Pleros, 1 Emmanouil E. Kriezis, 2 and Konstantinos Vyrsokinos 3 


1 Department of Informatics, Aristotle University ofThessaloniki, 54006 Thessaloniki, Greece 

2 Department of Electrical and Computer Engineering, Aristotle University ofThessaloniki, 

54124 Thessaloniki, Greece 

3 Informatics and Telematics Institute, Center for Research and Technology Hellas, 

57001 Thessaloniki, Greece 

DOI: 10.1109/JPHOT.2011.2127470 

1943-0655/$26.00 Ó2011 IEEE 

Manuscript received February 18, 2011; accepted March 7, 2011. Date of current version April 26, 

2011. This work was supported by the European FP7 research program PLATON under Contract 

249135. Corresponding author: N. Pleros (e-mail: npleros@csd.auth.gr). 

Abstract: Optical interconnects have continued to experience significant advances during 

the last year, exploiting mainly relevant progress in silicon photonics technologies. This year 

has also marked the first attempts toward integrating silicon photonics with plasmonics and 

enabling the introduction of the small-size power-effective plasmonic structures in true 

interconnect applications. The first successful silicon–plasmonic coupling configurations 

have been reported, and switching by miniaturized thermo-optic plasmonic modules has 

been realized, resulting in the first efforts for high-throughput on-chip silicon–plasmonic 

router architectures. 

Index Terms: Optical interconnects, optical routing, plasmonics, silicon photonics. 

During the first decade of the 2000s, the field of optical communications has witnessed some of 

the most dramatic changes in both its application areas and its enabling technology quiver. Optical 

technology has gradually penetrated into shorter distance transmission links well below the 10-m 

range and is currently rapidly replacing copper cables in High-Performance Computing (HPC) and 

Data Center interconnects [1]. As parallel processing has turned to the accepted methodology for 

boosting HPC performance improvements, multiple processing cores are required to exchange a 

vast amount of information that can simply not be sustained by bandwidth-limited electrical interconnects 

[1]. This reality extends along a clearly shaped roadmap to finally bring optics Binto-thebox[ 

in order to address the steadily growing bandwidth need in the field of computing without 

leading, however, to new size and power consumption explosions. Today’s world-leading Supercomputers 

already employ thousands of km’s of optical fiber for their inter-rack connections in order 

to cope with aggregate data rates of several hundreds of gigabits per second and deliver Peta-flops 

computational powers [2], [3], requiring, however, a total area of afew hundreds of meters squared 

and several megawatts of power. 

Facing the era of Exascale processing powers [2], [4], HPC size and power consumption emerge 

as the main set of barriers in trying to accommodate increased aggregate traffic rates. This has 

spurred intense research over the last five years toward completing the turn to optically interconnected 

processing machines and deploying photonic chips for the entire range of hierarchical 

system-levels: inter-rack, backplane, on-card, and even on-chip [1], [6]. The potential to drive down 



energy and footprint requirements when increasing the utilization degree of optics has been 

recently highlighted by the demonstration of an8 8 hybrid optoelectronic router for 10-Gb/s 

optical packets [7], which has led to a10-fold reduction in energy consumption compared with the 

respective purely electronic routing solutions. Similar benefits were also demonstrated by the 

OSMOSIS optical packet switch prototype that offers a throughput of 2.5 Tb/s and extends 

complexity and line-rate capabilities to 64input/output ports and 40 Gb/s, respectively, bringing 

great promise for utilization in supercomputer infrastructures [8]. To this end, major microelectronic 

vendors have started to heavily invest in research on novel next-generation computing 

architectures relying on the employment of nanophotonics and photonic Network-on-Chip (NoC) 

configurations [9]. The BCorona[ [10] and BPROPEL[ [11] research projects are recent examples 

that exploit hybrid optoelectronic routing with nanophotonic switching matrices for routing data in 

Chip Multi- and Many Processor (CMP) architectures with hundreds of interconnected cores. 

Respective efforts pursued at Columbia University have been reported during the last year to 

evaluate the scalability, performance, and realizability of photonic NoC-supported CMP designs [9], 

also taking into account physical-layer performance analysis [12]. 

The technology of choice for next-generation chip-level optical interconnection seems to come 

again from the silicon industry [5], [13]. Silicon photonics is considered as the mainstream integration 

platform for optical circuitry, offering attractive characteristics like low-loss optical signal 

propagation, high integration densities, and CMOS-compatible fabrication processes [14]–[16]. 

Silicon waveguides with sub-micron dimensions can serve as broadband optical gateways with 

more than 200 nm bandwidth [17] and typical optical loss coefficients lower than 2dB/cm [18], [19]. 

Moreover, they have been demonstrated to successfully host most of the critical functions required 

for optical interconnect applications, including wavelength selective filtering elements [20], optical 

modulators [21], high-speed Ge-on-Si photodiodes [22], optical switching modules [23], and even 

optical lasing sources [24]. These rapid advances are gradually bringing silicon photonics into 

maturity sothat this is now considered as the ideal low-loss interconnection platform. 

The research outcomes of 2010 have additionally strengthened this belief, yielding some major 

technology breakthroughs: IBM’s Terabus project demonstrated a 24-channel transceiver module 

that is capable of providing Tbps-class bidirectional aggregate data rates for board-level interconnects 

[25]. Intel has succeeded in demonstrating Wavelength Division Multiplexed (WDM) Photonic 

Integrated Circuit (PIC) devices with up to 200-Gb/s aggregate traffic ratecapabilities [26]and active 

optical cable configurations with 4-wavelength 50-Gb/s transceiver hardware [27]. Fabrication 

processes have also made significant strides reporting sub-nanometer linewidth uniformities using 

CMOS technologies [28]. An important step has also been made toward highly functional modules 

through the integration of silicon with different material structures: Nonlinear organic materials combined 

with silicon waveguide technology have led to the successful demonstration of electrooptic 

modulation devices for higher modulation formats up to 100-Gb/s rates [29]. Similar remarkable 

achievements have also been made toward on-chip optical logic and memory circuits, originating 

through heterogeneous integration of III–V microdisklasersonsilicon-on-insulator (SOI) and leading 

to a novel optical flip-flop configuration with record low power consumption and footprint values [30]. 

However, with footprint and power consumption being the driving forces in next-generation chipscale 

broadband interconnects, the search for disruptive enabling technologies capable of driving 

performance even beyond the limits of silicon photonics has already been initiated. Plasmonics 

have recently emerged as avery promising technology platform for driving down optical circuitry 

size [31], accompanied by expectations to address on-chip territories that lie beyond the reach of 

photonics and electronics [32], [33]. Thisnew discipline relies on the propagation of electromagnetic 

waves known as Surface Plasmon Polaritons (SPPs) along a metal-dielectric interface. This leads 

to strong optical mode confinements, allowing for the deployment of optical structures with subwavelength 

dimensions, breaking, in this way, the size barriers of traditional diffraction-limited 

optics. On the same line, they allowfor seamless interfacing of optical beams with electronic control 

signals through the underlying metallic film, providing,in this way,anatural energy efficient platform 

for merging broadband optical links with intelligent electronic processing, ascurrently considered to 

be the mainstream approach in CMP chip-scale architectures. 



Fig. 1.(a) The layout of adual-ring 2 2DLSPP switch serving as the switching matrix on an SOI 

motherboard, shown in (b) in its design toward terabit-per-second routing interconnect applications. 

(c) 8 40 Gb/s-channel spectrum for both heated and unheated states of the switch at one output of 

the 2 2silicon-plasmonic router, revealing extinction ratio values higher than 5 dB for all eight 

channels. 

The interest inplasmonics has rapidly taken up during the last decade as awhole new class of 

plasmonic devices found its way to experimental demonstrations, mostly extending along different 

waveguiding [34]–[40] and passive filtering circuitry [41], [42]. Moreover, the area of Bactive plasmonics[ 

has also made its first important steps toward allowing the control of plasmon propagation 

[43], also resulting in the demonstration of the first plasmonic nanolaser cavities [44]. However, the 

aim for bringing plasmonics intro truepractical system-levelapplications at chip-scaleenvironments 

is certainly still in its infant years, with the main achievement reported so far coming from aKorean 

consortium that demonstrated four-channel 2.5 Gb/s board interconnects with Long-Range SPP 

waveguides serving as the transmission lines [45]. The main limiting factorinpushing plasmonics to 

additional system-level advances stems certainly from their high propagation losses, as most 

plasmonic waveguide structures restrict signal propagation over afew tens of micrometers due to 

internal damping of radiation in metal. 

In this perspective, 2010 can be designated as a milestone toward turning plasmonics into 

practical circuitry for interconnect applications. The interfacing of plasmonic structures with the 

outer worldhas been realized for the first time, demonstrating effective optical fiber input and output 

coupling [46]. At the same time, efforts for coping with the high plasmonic propagation losses have 

concentrated on interfacing them with low-loss photonic waveguides, allowing for purpose-driven 

switching between photonic and plasmonic modes. Within this frame, aconductor-gap-Silicon SPP 

waveguide structure [47] and a polymer-on-gold DLSPP ring resonator end-coupled to SOI waveguides 

have been presented [48], both reporting on low coupling losses of 1dB. 

These achievements have raised expectations for the combined exploitation of silicon photonics 

and plasmonics for interconnection purposes, with plasmonics being used only where active 

functionality is required, while turning to low-loss silicon photonic structures when passive circuitry 

is needed. This concept iscurrently pursued by the European research project PLATON that has 

been initiated by the authors and was launched at the beginning of 2010. PLATON envisions the 

deployment ofterabit-per-second routerfabrics relying on the deposition of DLSPP-based switching 

matrices on a SOI motherboard that will host all necessary waveguiding, multiplexing, filtering, and 

signal detection elements, as well as the electronic control driving circuit [49] (see Fig. 1). This 

consortium has already achieved remarkable advances in DLSPP switching configurations both at 

the experimental [50], [51] and at the theoretical level [52]–[54], exploiting thermo-optics to alter the 

stateof DLSPP waveguide ring resonator orMach–Zehnderinterferometric switching elements.We 

have also presented the first silicon–plasmonic 2 2 router architecture that is capable of providing 

up to 320 Gb/s throughput performance [49], relying on the employment of anovel dual-ring 



DLSPP switching module and indicating the potential for driving down size and power consumption 

metrics. On the same line,the efforts to advance and benefit from the functional portfolioof DLSPPs 

have led to the demonstration of power monitoring [55], as well as to a new type of Long-Range 

DLSPP waveguides that can enhance the propagation length of plasmons, allowing for increased 

interaction lengths between optical and electrical signals [56]. These advances canonlysuggestthat 

the successful merger of plasmonics with silicon photonics toward true interconnect routing 

applications is indeed headed in the right direction,rendering the next few years as averypromising 

period for on-chip silicon–plasmonic router implementations. 

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[41] E. Devaux, T. W. Ebbesen, J. C. Weeber, and A. Dereux, BLaunching and decoupling surface plasmons via 

microgratings,[ Appl. Phys. Lett., vol. 83, no. 24, pp. 4936–4938, Dec. 2003. 

[42] A. Boltasseva and S. I.Bozhevolnyi, BDirectional couplers using long-range surface plasmon polariton waveguides,[ 

J. Sel. Topics Quantum Electron., vol. 12, no. 6, pp. 1233–1241, Nov./Dec. 2006. 

[43] K. F. MacDonald, Z.L.Samson, M.I.Stockman, and N. I. Zheludev, BUltrafast active plasmonics,[ Nat. Photon., vol. 3, 

no. 1,pp. 55–58, Jan. 2009. 

[44] M. T. Hill, Y.-S. Oei, B. Smalbrugge, Y.Zhu, T. de Vries, P. J. van Veldhoven, F.W.M.vanOtten, T.J. Eijkemans, 

J. P. Turkiewicz, H. de Waardt, E. J. Geluk, S.-H. Kwon, Y.-H. Lee, R. Nötzel, and M. K. Smit, BLasing in metalliccoated 

nanocavities,[ Nat. Photon., vol. 1, no. 10, pp. 589–594, Oct. 2007. 

[45] J.T.Kim, J. J. Ju, S. Park, M. S.Kim, S.K.Park, and M. H. Lee, BChip tochipoptical interconnectusing gold long-range 

surface plasmon polariton waveguides,[ Opt. Express, vol. 16, no. 17, pp. 13 133–13 138, Aug. 2008. 

[46] J. Gosciniak, V. S. Volkov, S. I.Bozhevolnyi, L. Markey, S. Massenot, and A. Dereux, BFiber-coupled dielectric loaded 

plasmonic waveguides,[ Opt. Express, vol. 18, no. 5,pp. 5314–5319, Mar. 2010. 

[47] M. Wu, Z. Han, and V. Van, BConductor-gap-silicon plasmonic waveguides and passive components at subwavelength 

scale,[ Opt. Express, vol. 18, no. 11, pp. 11728–11 736, May2010. 

[48] R. M.Briggs, J. Grandidier, S. P. Burgos, E.Feigenbaum, and H. A. Atwater, BEfficient coupling between dielectric 

loaded plasmonic and silicon photonic waveguides,[ Nano Lett., vol. 10, no. 12, pp. 4851–4857, Oct. 2010. 

[49] N. Pleros, K.Vyrsokinos, S. Papaioannou, D. Fitsios, O. Tsilipakos, A.Pitilakis, E. Kriezis, A. Miliou, T. Tekin, M.Baus, 

M. Karl, D. Kalavrouziotis, G. Giannoulis, H. Avramopoulos,N.Djellali, J.-C.Weeber, L. Markey,A.Dereux, J. Gosciniac, 

and S. Bozhevolnyi, BTb/s switching fabrics for optical interconnects using heterointegration of plasmonics and silicon 

photonics: The FP7 PLATON approach,[ presented at the 23rd Annu. Meeting IEEE Photonics Soc., Denver, CO, 

Nov. 2010, Paper TuH2. 

[50] J. Gosciniak, S. I.Bozhevolnyi, T. B. Andersen, V.S. Volkov, J. Kjelstrup-Hansen, L. Markey, and A. Dereux, BThermooptic 

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Jan. 2010. 

[51] A. Dereux, K. Hassan, J.-C.Weeber, N. Djellali, S.I.Bozhevolnyi, O.Tsilipakos,A.Pitilakis, E. Kriezis, S. Papaioannou, 

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IEEE Photonics Journal Emergence of Plasma Photonics 

Emergence of Plasma Photonics 

J. G. Eden 


Laboratory for Optical Physics and Engineering, Department of Electrical 

and Computer Engineering, University of Illinois, Urbana, IL 61801 USA 

DOI: 10.1109/JPHOT.2011.2138691 

1943-0655/$26.00 Ó2011 IEEE 


version April 26, 2011. Corresponding author: J. G. Eden (e-mail: jgeden@illinois.edu). 

Abstract: The past several years have witnessed the emergence of asubfield of photonics 

in which the unique optical and electronic properties of microplasmas and gases are 

coupled with solid phase materials and optical components to realize photodetectors, 

transistors, photonic crystals, laser sources, and lighting. Highlights of novel plasma or gasbased 

photonic devices reported in 2010 are described. 

Index Terms: Photonics, plasma devices, bipolar transistors, microwave filters, glow 

discharge devices, displays, lighting, lasers, optical pumping, ultraviolet sources. 

In the last century, gasphase plasmasgavebirth to an arrayof photonic and electronicdevices.A 

generation of electron tubes incorporating a raregasor mercuryglow discharge, such as the OA-OE 

series of switches, modulators, and rectifiers, were instrumental in the operation of communications 

and audio systems. For more than two decades, Nixie tubes based on low-pressure neon plasmas 

were the alphanumericdisplayof choice.Aresurgence inplasma photonics is underway as aresult of 

the recent development of microplasma devices [1] in which low-temperature plasmas are confined 

within a cavity. Plasmas have long played a central role in laser physics and technology and the 

development of plasma-based lasers in the XUVand X-ray regions,inparticular, continues unabated 

[2]. However, the successful leveraging of micro and nanofabrication processes to miniaturize 

nonequilibrium(electron temperature neutral gas temperature) plasmas, combined with the benign 

behavior of microplasmas at pressures up to(and beyond) 1atm, has opened the doorto a series of 

novel photonic devices. Several highlights from 2010 illustrate the potential of this technology. 

Wagner et al. [3] announced an npn phototransistor in which the collector of anSi bipolar 

transistor was replaced by alow-temperature plasma. By coupling electron-hole ðe h þ Þ and 

electron-ion plasmas across a potential barrier with a strong electric field, a hybrid plasma/ 

semiconductor transistor having alight-emitting collector was realized. Ithas long been known that 

both plasmas are described by virtually identical relationships for diffusion, drift, and recombination 

(for example), but this linkage has not previously been exploited in optoelectronic devices. Fig. 1(a) 

is a simplified diagram of the plasma bipolar junction transistor (PBJT) reported in [3]. One 

beneficial aspect of incorporating e -ion plasma into an electronic device is that, because the 

background number density in the collector plasma is three to five orders of magnitude lower than 

those of most semiconductor crystals (such as 10 22 cm 3 for Si), the electron drift velocities in the 

gas phase are typically afactor of three to an order of magnitude or greater than those for itssolidstate 

counterpart. For aplasma in0.1 atm of neon ( 100 Torr), for example, the electron drift 

velocity is 6 10 6 cm/s for anelectric field strength of only 500 V/cm [4]. The collector current 

ðicÞVbase current ðiBÞ characteristics ofthe PBJT[see Fig.1(b)] showthat thisdevice behaves as a 



Fig. 1.(a) Generalized diagram (not toscale) of the cross section of the plasma bipolar junction 

transistor (PBJT). (b) ic iB characteristics for the phototransistor with 25Torr Ne in the collector. Both 

figures are reprinted from [3] by permission of the American Institute of Physics. 

phototransistor. Most significantly, [3] demonstrated that the collector plasma (and its visible/ 

ultraviolet radiative output) can be modulated and extinguished with G 1Vapplied to the emitterbase 

junction. Thus, twodrawbacks of plasma devices of the pastVphysical size and the requirement 

for hundreds of volts to modulate the plasmaVhave been overcome. The PBJT, as well 

as its microplasma-based photodetector [5], [6] and electron-injected transistor [7] predecessors, 

appear to have considerable promise for displays and optoelectronic applications demanding visible 

or ultraviolet emission capability or exceptional power loading and thermal dissipation properties. 

Examples of the latter include the power amplifiers in cell phone base stations. 

Tachibana, Sakai et al. have adapted microplasma technology from a different perspective to 

realize plasma photonic crystal (PPC) filters for the 10–100 GHz microwave region [8]–[10]. With a 

17 30 array of microplasmas having a lattice constant of 2.5 mm, for example, the Kyoto 

University group demonstrated a drop region near 62 GHz for both the transverse electric and 

transverse magnetic (TE and TM) modes. Owing to a bandgap in the -X direction, areduction of 

more than 80%in the transmission through the array was observed at 61.7 GHz. Inspired by the 

work ofTachibana et al., several groups reported [11]–[13]advances in PPCs in2010,including the 

design of tunable filters. Fan et al. [13] exploited the formation of square, superlattice, and 

hexagonal PPCs by self-organized filaments in dielectric barrier discharges to design filters whose 

spectral characteristics can be modulated both spatially and temporally. For the hexagonal crystal 



Fig. 2. Photograph of a6 6 in 2 ð 225 cm 2 Þ flat lamp, based on microplasma technology developed 

for general illumination (photo courtesy of Eden Park Illumination). 

structure, for example, bandgaps can be engineered for the TE mode at frequencies beyond 

100 GHz for lattice constants as small as 1.7 mm. 

A similar story is unfolding in applications of microplasma technology to lighting [14]. Fig. 2 is a 

photograph of alightweight lamp being developed for general illumination, which comprises an 

array of microplasmas in tandem with athin dielectric barrier plasma operating at a pressure near 

one atmosphere. Having an overall thickness G 4 mm and an active area of 6 6 in 2 ð 225 cm 2 Þ, 

these Blight tiles[ offer luminance values of 12 000 cd/m 2 , a luminous efficacy approaching 

30 lumens/W, and a color rendering index (CRI) above 80[15]. In contrast with other emerging 

lighting technologies, microplasma lamps exhibit virtually noBdroop[ in output with increasing 

electrical drive, even for luminance levels above 10 000 cd/m 2 .Astandard 36in 2 light Bengine[ 

weighs G 200 g and produces 300 lumens with an intensityoverthe entire surface of the lamp that 

is uniform to within 5%. 

Similar technology has resulted in the demonstration of small displays that are both flexible and 

transparent. Fig. 3 is a photograph of alow-resolution prototype display that was fabricated at the 

University of Illinois and incorporates an array of microcavities produced in a siloxane polymer by 

replica molding [14]. Each pixel comprises a microplasma emitting deep-ultraviolet radiation which, 

in turn, photoexcites a phosphor. Because thin polymer sheets are now available that are 

transmissive in the UV B and C regions, arrays such as that shown in Fig. 4are also of interest for 

biomedical applications. 

Closely related to photonic devices incorporating a plasma are those integrating a gas with an 

optical material or structure to yield a hybrid system offering new photonic functionality. In this vein, 

Benabid et al. of the University of Bath (U.K.) announced in 2010 the realization of compact 

multiwavelength lasers based on stimulated Raman scattering in hydrogen-filled photonic crystal 

fibers [16]. In these experiments, 2mofKagome photonic crystal fiber having an outer diameter of 

130 m was driven by a532 nm Nd:YAG laser providing 550-ps pulses and an average power of 

25 mW. With 20bar of hydrogen gas in the fiber, the Raman laser generated the 23 lines extending 

in wavelength from 353 nm(ultraviolet) to 712 nm (deep red) that are illustrated in Fig. 4.The 

efficiency for conversion of pump power into the stimulated Raman lines was 60% overall, and the 



Fig. 3. Photograph of a transparent, flexible display based on the photoexcitation of phosphors bythe 

deep-UV radiation generated by an array of microcavity plasmas (photo courtesy of the University of 

Illinois). 

Fig. 4. Stimulated Raman emission line spectrum in the visible and near-infrared (400–850 nm) 

produced by 550 psNd:YAG laser pulses in aKagome photonic crystal fiber filled with 20bar of H2. 

(c) and (d) Near-field (NF) and far-field (FF) intensity spatial profiles for the 567 nm line. (e) Comparison 

ofaportion of the laser spectrum in the visible with the absorption spectra for several biological markers 

(reprinted from [16] by permission of the Optical Society ofAmerica). 

average spectral power density was measured to range from 130 mW/nm in the visible to 7mW/nm 

in the ultraviolet. Both values are orders of magnitude larger than those available with conventional 

supercontinuum sources. One advantage of this single pass laser, relative to other Raman systems 

(such as the Si ring laser [18]), is the overall gain that is available byextending the length ofthe 

photonic crystal fiber. As emphasized in [16], applications of these multiline sources in biomedical 

diagnostics such as photoexciting chromophores for DNA sequencing or flow cytometry are 

particularly appealing. To that end, Fig. 4(e) (reprinted from [16]) shows the overlap of several lines 

of the H2 Raman laser in the visible with the absorption spectra of specific biological marker dyes. It 

should be mentioned that the Bath group also successfully demonstrated the generation of slow 

and superluminal light in an all-fiber optical system by integrating a 20-m section of hollow fiber 

filled with acetylene. Pulse delays as large as 7 ns per meter of hollow fiber are anticipated, and the 



incorporation of this technology into fiber optical gyroscopes is expected to increase the single-tonoise 

ratio significantly. 

Plasma-based optoelectronic devices have along and distinguished history and the achievements 

briefly reviewed here are only afew of the recent milestones. All attest to arenaissance of 

photonics devices that combine the best features of microplasmas and nonlinear optics in the gas 

phase with optical materials to gain access to a new generation of systems. 

References 

[1] K. H. Becker, K. H. Schoenbach, and J. G. Eden, BMicroplasmas and applications,[ J. Phys. D:Appl. Phys., vol. 39, 

no. 3, pp. R55–R70, Feb. 7, 2006. 

[2] J. J. Rocca, BCompact plasma-based soft X-ray lasers,[ IEEE Photon. J., vol. 2, pp. 217–220, Apr. 2010. 

[3] C. J. Wagner, P. A.Tchertchian, and J. G. Eden, BCoupling electron-hole and electron-ion plasmas: Realization of an 

npn plasma bipolar phototransistor,[ Appl. Phys. Lett., vol. 97, no. 13, pp. 134102-1–134102-3, Sep. 2010. 

[4] S. C. Brown, Basic Data of Plasma Physics, 1966. Cambridge, MA: MIT Press, 1967. 

[5] S.-J. Park, J. G. Eden, and J. J. Ewing, BPhotodetection in the visible, ultraviolet, and near-infrared with silicon 

microdischarge devices,[ Appl. Phys. Lett., vol. 81, no. 24, pp. 4529–4531, Dec. 2002. 

[6] N. P. Ostrom and J. G. Eden, BMicrocavity plasma photodetectors: Photosensitivity, dynamic range, and the plasmasemiconductor 

interface,[ Appl. Phys. Lett., vol. 87, no. 14, pp. 141101-1–141101-3, Oct. 2005. 

[7] K.-F. Chen and J. G. Eden, BThe plasma transistor: A microcavity plasma device coupled with a low voltage, 

controllable electron emitter,[ Appl. Phys. Lett., vol. 93, no. 16, pp. 161501-1–161501-3, Oct. 2008. 

[8] O. Sakai, T. Sakaguchi, T. Naito, D.-S. Lee, and K. Tachibana, BCharacteristics of metamaterials composed of 

microplasma arrays,[ Plasma Phys. Control. Fusion, vol. 49, no. 12B, pp. B453–B463, Dec. 2007. 

[9] O. Sakai, T. Sakaguchi, and K. Tachibana, BPhotonic bands in two-dimensional microplasma arrays. I.Theoretical 

derivation of band structures of electromagnetic waves,[ J. Appl. Phys., vol. 101, no. 7, pp. 073304-1–073304-9, 

Apr. 2007. 

[10] T. Sakaguchi, O. Sakai, and K. Tachibana, BPhotonic bands in two-dimensional microplasma arrays: II. Band gaps 

observed in millimeter and subterahertz ranges,[ J. Appl. Phys., vol. 101, no. 7, pp. 073305-1–073305-7, Apr. 2007. 

[11] L. Qi, Z. Yang, F.Lan, X.Gao, and Z. Shi, BProperties of obliquely incident electromagnetic wave in one dimensional 

magnetized plasma photonic crystals,[ Phys. Plasmas, vol. 17, no. 4,pp. 042501-1–042501-8, Apr. 2010. 

[12] X.-K. Kong, S.-B. Liu, H.-F. Zhang, and C.-Z. Li, BA novel tunable filter featuring defect mode of the TE wave from onedimensional 

photonic crystals doped by magnetized plasma,[ Phys. Plasmas, vol. 17, no. 10, pp. 103506-1–103506-5, 

Oct. 2010. 

[13] W. Fan, X.Zhang, and L. Dong, BTwo dimensional plasma photonic crystals in dielectric barrier discharge,[ Phys. 

Plasmas, vol. 17, no. 11, pp. 113501-1–113501-7, Nov. 2010. 

[14] J. G. Eden and S.-J. Park, BSheetlike microplasma arrays have many applications,[ in Laser Focus World, pp. 33–37, 

Jul. 2010. 

[15] J. G. Eden, S.-J. Park, C. M.Herring, and J. M.Bulson, BMicroplasma light tiles: Thin sheet lamps for general 

illumination,[ J. Phys. D:Appl. Phys., 2011, to bepublished. 

[16] Y.Y.Wang, F.Couny, P. S. Light, B. J. Mangan, and F. Benabid, BCompact and portable multiline UV and visible 

Raman lasers in hydrogen-filled HC-PCF,[ Opt. Lett., vol. 35, no. 8, pp. 1127–1129, Apr. 2010. 

[17] N. V. Wheeler, P. S. Light, F. Couny, and F. Benabid, BSlow and superluminal light pulses via EIT in a 20 m acetylenefilled 

photonic microcell,[ J. Lightwave Technol., vol. 28, no. 6, pp. 870–875, Mar. 2010. 

[18] O. Boyraz and B. Jalali, BDemonstration ofasilicon Raman laser,[ Opt. Exp.,vol. 12, no. 21, pp.5269–5273, Oct. 2004. 


IEEE Photonics Journal Advances in Terahertz Waveguides and Sources 

Advances in Terahertz 

Waveguides and Sources 

H. Pahlevaninezhad, B. Heshmat, and T. E. Darcie, Fellow, IEEE 


Department of Electrical and Computer Engineering, University ofVictoria, 

Victoria, BC V8P 5C2, Canada 

DOI: 10.1109/JPHOT.2011.2128303 

1943-0655/$26.00 Ó2011 IEEE 

Manuscriptreceived February 18, 2011; accepted March 8, 2011. Dateof currentversion April 26, 2011. 

Corresponding author: H. Pahlevaninezhad (e-mail: hpahleva@uvic.ca). 

Abstract: In this paper, we review recent progress toward efficient and versatile devices for 

terahertz applications. Low-loss transmission lines and waveguides are discussed, as well 

as potentially high-power photomixing devices based on carbon nanotubes. 

Index Terms: Terahertz, transmission lines, waveguides, photomixers, carbon nanotubes, 

terahertz spectroscopy. 

1. Introduction 

Recent technical innovation has made the terahertz (THz) frequency range more accessible for 

scientific and industrial applications. Numerous applications have been explored in areas such as 

security, drug analysis,inspection,and spectroscopy [1]–[3]. These applications benefit from the short 

wavelength,intermediatephoton energy,and uniquepenetration orabsorption properties of photons at 

THz frequencies. However, difficulties in generating, manipulating, and detecting THz radiation 

continue to plague mostapplications. Source powers tend to below. THz optics is costly, difficult to 

align, and has high loss. Detectors are expensive, inefficient, and usually require alignment with an 

optical pump source.These factors restrict THz applications toexploratory and scientific investigation. 

In terahertz time-domain spectroscopy (THz TDS), afemtosecond optical pulse is divided into 

two beams and focused on the emitter and detector. The generated THz wave from the emitter is 

then focused on the detector. Generally, this would require precise alignment of THz lenses, with 

associated loss. Confining THz radiation within waveguide structures offers tremendous potential 

advantages in size, performance, and versatility, driving research on many types of THz waveguides. 

Using transmission lines and waveguides canhelp the integration of THz systems, avoiding 

the difficulties associated with THz beam-shaping and beam-steering optics. In addition, output 

power must be increased. The most commonly used source of THz radiation for frequencies less 

than 3THz is a photoconductor-driven antenna fabricated on low-temperature Gallium Arsenide 

(LT-GaAs). Optical power limitations (largely due to thermal conductivity) and carrier dynamics limit 

output power from these devices to small fractions of amilliwatt. 

This review focuses on recent activity in the development of THz waveguide technology and new 

devices for the generation of THz signals with potentially higher power using carbon nanotubes 

(CNTs). We review recent progress with slot-line and two-wire waveguides operating at THz frequencies 

and propose a photoconductive device based on CNTs that has the potential to dramatically 

(200 ) increase available output power. 



Fig.1.(a)Slot-line in a homogeneous medium and (b) the electric field amplitude square foraslot-line in 

a homogenous medium. 

2. THz Transmission Lines and Waveguides 

Transmission lines, including microstrips, striplines, slot-lines, and coplanar striplines, arecritical 

components for microwave systems. However, loss and dispersion limit the applicability of conventional 

transmission lines at frequencies beyond 100 GHz. There has been intense interest in 

using metallic waveguides for THz applications [4]–[6]. One application for waveguide structures is 

spectroscopy. This is driven in part by the complementary set of spectral features provided relative 

to Raman spectroscopy. Rotational transitions of light polar molecules and low-frequency vibrational 

modes of large molecular systems can beprobed by THz spectroscopy. Other interesting 

directions for waveguides include a highly sensitive microfluidic sensor using the TE1 mode of a 

parallel-plate waveguide [7] and superfocusing of THz radiation to below =50 using plasmonic 

waveguides [8]. 

It has been shown that microstrips can beused as THz lines [9]. Of the many types of planar 

lines, however, the slot-line structure is more compatible with THz photoconductive switches, 

allowing high coupling efficiency of THz waves into the transmission line. Conventional slot-lines, 

which are formed by athin slot in a conductive coating on one side a dielectric substrate, have 

proven tohave high loss at THz frequencies due toelectromagnetic shock wave radiation. The slotline 

surrounded by a homogeneous medium [see Fig. 1(a)], on the other hand, can bealow-loss 

transmission line for THz waves as it avoids radiation loss into the substrate.Italso supports a TEM 

mode that is free from cutoff frequency and group velocity dispersion, making it suitable for 

broadband applications. Fig. 1(b) shows the electric field distribution of the TEM supported by a 

slot-line in a homogeneous medium. The field is mostly concentrated at the edges of the conductors. 

Knowing the field distribution, we recently estimated the absorption of the slot-line in a 

homogenous medium tobeabout 2cm 1 at 1 THz [10], which is anorder oflowerthan conventional 

lines. 

Pushing for lower loss, single [12] and two-wire waveguides [13] have been explored. The twowire 

waveguide offersboth lowloss and good coupling to most photomixers.Weevaluated two-wire 

waveguides for THz frequencies and developed an analytical expression for the TEM mode. 

Knowing the field distribution, weestimated less than 0.01 cm 1 loss [13] and good coupling 

efficiency [14] from typical THz sources. We fabricated a simple two-wire THz waveguide using a 

pair of 300 mdiameter gold wires separated by roughly 1 mm. The 10-cm-long wires are supported 

within a glass tube and stretched with modest tension between two plastic end caps. We 

measured experimentally picosecond pulses [see Fig. 2(b)] with significant amplitude after 10cm 

transmission using the setup shown in Fig. 2(a). By comparison, rotation of the waveguide by 

90 , such that the dipole field is orthogonal to that of the TEM mode, results in the anticipated 

extinction. 

3. Implication of New Materials 

Aside from guiding and managing emitted THz power, the power emitted and the receiver sensitivity 

are offundamental importance. There are many different approaches for generating THz radiation 

[14]. Photoconductive switches (PC switches) in pulsed mode and photomixers in continuous wave 

(CW) mode are most commonly used mainly because they are tunable and low cost. The structure 

consists ofafast photo-absorbing semiconductor layer on a THz-transparent substrate with an 



Fig. 2. (a) Experiment setup for two-wire waveguide and (b) signal measured after the waveguide when 

the dipole source polarization and the TEM mode polarization are parallel (upper waveform) and when 

they are perpendicular (lower waveform). 

Fig. 3. (a) Schematic view of a THz photomixer. Two lasers are focused on the gap to generate THz 

radiation. (b) Fabrication challenges for deposition of SWNTs in the gap. 

antenna structure to shape the radiation pattern generated by absorption of the optical pump 

(Fig. 3). New nanomaterials have been suggested to boost the efficiency of the conversion from the 

incident optical power to the output THz power [15]–[17]. Higher mobility nanomaterials, such as 

CNT and graphene, are the top candidates for this efficiency improvement [16], [17]. 

Semiconducting single-wall carbon nanotubes (S-SWNT) are of particular interest for THz 

photomixers and PC switches because they offer tunable optical absorption [18], [19]. Consideration 

of the basic theoretical dependencies of devices based on LT-GaAs, but with substitution of 

CNT parameters, suggests atwo-order-of-magnitude improvement inoutput THz power compared 

with LT-GaAs [18]. Further studies are necessary to address the dynamics of each parameter and 

consider the 1-D nature of CNTs in the PC gap [19]. The number of free photocarriers is the key 

parameter for engineering the SWNT film, and this number is affected by many parameters such as 

incident power, carrier life time, film optical density, filling fraction, etc., which are dependent on 

SWNTs chirality, purity, and fabrication method [20]. Estimation of number of free photocarriers in 

the SWNT film can then be followed by using the proper model toderive the conductivity ofthe film, 

which is atask that has been addressed in other applications of this material [20]. Estimating the 

dependence of the conductivity ofthe film (CNTs spanning the PC gap) on the optical pump, and 

using a circuit model for the photomixers, enables prediction of the THz output power. 

Fabrication of THz photomixers with a SWNT film as the PC material in the gap is challenging. 

CNTs ofthe appropriate chirality must be aligned spanning the gap with sufficient density to absorb 

the pump and with good electrical contactto the electrode (gold) structure. Since these are common 

challenges with CNTs, many methods that have been developed can beapplied to this challenge 

[21], [22]. An example of CNTs deposited across a5- mgap at the feed point of a20- mdipole 

antenna on high-resistivity Silicon is shown in Fig. 3(b). We continue working to improve deposition 

uniformity, alignment, and thickness. 



4. Summary 

We have reviewed recent work in waveguides and photoconductors for THz applications. Low-loss 

waveguides may simplify alignment and increase interaction length and versatility. CNT-based 

photoconductorsoffer possibilities for higher signal powerand more sensitive receivers. Challenges 

remain significant, but progress continues along the path to a next generation of improved THz 

systems. 

References 

[1] H. H. Mantsch and D. Naumann, BTerahertz spectroscopy: The renaissance of far infrared spectroscopy,[ J. Mol. 

Struct., vol. 964, no. 1–3, pp. 1–4, Feb. 2010. 

[2] M. R. Leahy-Hoppa, M. J. Fitch, and R. Osiander, BTerahertz spectroscopy techniques for explosive detection,[ Anal. 

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[3] Y. Watanabe, K. Kawase, T. Ikari, H. Ito, Y.Ishikawa, and H. Minamide, BComponent analysis of chemical mixtures 

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[6] M. Theuer, R. Beigang, and D. R. Grischkowsky, BHighly sensitive terahertz measurement of layer thickness using a 

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[9] M. Martl, J. Darmo, D. Dietze, K.Unterrainer, and E. Gornik, BTerahertz waveguide emitter with subwavelength 

confinement,[ J. Appl. Phys., vol. 107, no. 1,pp. 013110-1–013110-5, Jan. 2010. 

[10] H. Pahlevaninezhad, B. Heshmat, and T. E. Darcie, Slot-line for terahertz waves, to bepublished. 

[11] K. Wang and D. Mittleman, BMetal wires forterahertz waveguiding,[ Nature,vol. 432, no. 7015, pp. 376–379,Nov. 2004. 

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IEEE Photonics Journal Recent Breakthroughs in Microwave Photonics 

Recent Breakthroughs in 

Microwave Photonics 

I. Gasulla, J.Lloret, J.Sancho, S. Sales, Senior Member, IEEE, and 

J. Capmany, Fellow, IEEE 


ITEAM Research Institute, Universidad Politécnica de Valencia, 46022 Valencia, Spain 

DOI: 10.1109/JPHOT.2011.2130517 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: J. Capmany (e-mail: jcapmany@dcom.upv.es). 

Abstract: We present abrief review of recent accomplishments in the field ofMicrowave 

Photonics (MWP). Recent research across a broad range of MWP applications is summarized, 

including photonic generation of microwave, millimeter, and Terahertz waves; 

broadband optical beamforming for phased array antennas; tunable, reconfigurable, and 

adaptive microwave photonic filtering, as well as the application of slow and fast light effects 

to the implementation of tunable microwave phase shifting and true time delay operations. 

Index Terms: Microwave photonics, photonics generation, microwave photonic filtering, 

optical beamforming, slow and fast light. 

Significant worldwide advances have been achieved in 2010 in awide range of Microwave 

Photonics (MWP) applications, spanning different technology platforms, including integrated photonics 

based on III–V technologies and silicon-on-insulator (SOI),as well as photoniccrystals (PC). In 

this sense, integrated MWP is of strategic importance as it opens the door for MWP to benefit from 

potential low-costapproaches,reliability,and economies of scale. 

The first main area in which important progress has been reported is photonic generation of 

microwave, millimeter, and terahertz waves. An impressive advance regarding integrated MWP is 

reported in [1], where an ultrabroad-bandwidth arbitrary radio frequency generator based on a silicon 

photonic spectral shaper has been presented, that is capable of synthesizing burst radio-frequency 

waveforms with programmable time-dependent amplitude, phase, and frequency tunability up to 

60 GHz. The generator isbased on the wavelength-to-time conversion of the broad spectrum ofa 

mode-locked pulsed laser which has previously been shaped by a photonic integrated circuit (PIC) 

consisting ineightindependentring resonators (RRs),as shown in Fig.1.By independently thermooptical 

tuning the spectral characteristic of every RR,the broadband spectrum ofthe mode-locked 

laser output is carved, and this characteristic is translated to the time domain using a dispersive 

element. By replacing the output couplers ofthe RRs with tunable Mach–Zehnders (MZM), the 

amplitude ofthe bandpass canbeprogrammed, providing full amplitude and wavelength tunabilityof 

the resonances comprising the shaped spectrum and,therefore,the time characteristics ofthe burst 

signal. Two interesting figures of the described chip are worth mentioning: footprint of 0.1 mm 

1.2 mmonan SOI platform and 25 dB of total optical fiber to fiber loss. 

Still within the field of millimeter-wave generation, three high-data-rate demonstrations employing 

bulk optical components have presented in [2]. The first option is based on two cascaded MZMs for 

optical double-sideband suppressed carrier 60-GHz generation and subsequent broadband on–off 



Fig. 1. Attenuation control in an ultrabroad-bandwidth arbitrary radio-frequency generator based on a 

Silicon Photonic spectral shaper. (a) Mach–Zehnder arms incorporated in the through port near the 

rings. (b) Optical image showing two sets offinished RRs with microheaters and contact pads. Scale 

bar is 20 m. 

keying (OOK) modulation up to 12.5 Gb/s with a spectral efficiency of 0.5 bit/s/Hz. The sensitivity 

of the constructed coherent wireless receiver with 23-dBi horn antennas is above 46 dBm for 

error-free transmission of10.3125 Gb/s in anoutdoor environment over a wireless distance of50m. 

To improve the spectral efficiency, asecond option based on orthogonal frequency-division multiplexing 

(OFDM) using 16quadrature amplitude modulation (16QAM) has been proposed,achieving 

a wireless transmission of 27.04 Gb/s overan air link of 2.5 m. Athirdoption,which ismade upbya 

compact and cost-effective 60-GHz wireless transmitter comprising a mode-locked laser diode and 

an electroabsorption modulator, accomplishes error-free indoor and outdoor 5-Gb/s wireless 

transmission over distances up to 40 m. 

A new technique for generating narrow-linewidth microwave signals has been recently presented 

in [3]. It is based on beating multiple signals byusing coherence control of atime-delayed and 

frequency-shifted optical signals comb. Microwave signal generation at 11.25 and 30 GHz with a 

linewidth less than 100 Hz and sideband suppression 9 30 dB has been demonstrated. 

Within the area of optoelectronic oscillators, asimple method to extend the frequency tunability 

has been recently reported in [4]. The system core is aFabry–Perot laser diode which functions 

through external injection as a high-Q photonic microwave filter. The generated frequency is tuned 

over the operational bandwidth ofanelectrical amplifier (from 6.41 to 10.85 GHz) bychanging the 

optical wavelength and tuning the electrical phase shifter. More than 20dBof second harmonic 

suppression and a phase noise of 92.8 dBc/Hz at a 10-kHz offset frequency have been 

demonstrated. 

Optical phased-locked loops (OPLLs) constitute another interesting approach where some 

analog MWP functions can begreatly improved. The main advantage stems from the fact that, by 

reducing the path lengths, higher loop bandwidths can beachieved, while at the same time, very 

stable optical paths that allow low-noise coherent summing of optical signals are feasible. Some 

interesting preliminary results are reported in [5] with special emphasis onPIC-based OPLLs for 

coherent receivers for phase-modulated signals and phased-locked tunable lasers. Afundamental 

20-GHz offset locking and 300-MHz bandwidth have been demonstrated, and a phase-error 

variance 0:03 rad 2 was measured over a2-GHz measurement window. 

Terahertz wave generation is an emergent field ofinterest within MPW due to its potential 

advantages in material characterization, nondestructive testing, tomography imaging, and chemical 

or biological sensing. In this context, the European project Integrated photonic millimeter-wave 

functions for broadband connectivity (IPHOBAC) in relation to the interesting results achieved by 

consortium members in 2010, such as the development of a waveguide-fed, traveling-wave unitraveling 

carrier photodiode (TW-UTC-PD) [6], is worthy of mention. This device, integrated with 

different types of antennas, has been demonstrated as a tunable terahertz source covering 

frequencies up to 1 THz. Some promising results are narrow-band output powers of148 Wat 

457 GHz and 24 Wat914 GHz for devices integrated with resonantantennas,while there is also 

105 Wat255 GHz down to 10 Wat612 GHz for devices integrated with broadband antennas. 



Furthermore, the application of the frequency modulation (FM) technique is one of the most 

promising solutions for measuring distances in noninvasive imaging systems,as it has been reported 

in [7], where the FM of a350-GHz signal with a 6.7-GHz frequency deviation was experimentally 

demonstrated. In this sense, the work initiated in 2009 about serial time-encoded amplified microscopy 

(STEAM) technology [8] should also be mentioned. By means of optical image amplification, 

STEAM overcomes the fundamental tradeoff between sensitivity and speed that affects virtually all 

optical imaging systems. 

The second main MWP area where significant advances have been recently reported is 

photonic signal processing. This area embraces solutions for optical beamforming, photonic 

filtering, microwave phase shifting, and discriminators for phase-modulated links. 

A novel integrated optics approach for optical beamforming has been reported within the framework 

of the Smart antenna systems for radio transceivers (SMART) Dutch project [9], [10]. A 

nonplanar phased array antenna has been developed which consists of broadband Ku-band 

stacked patch antenna elements and a broadband optical beamformer network (OBFN) that 

employs optical RRs, integrated in low-loss complementary metal–oxide semiconductor compatible 

waveguide technology. The proposed OB is tunable and can operate in squint-free mode (that is, 

based on true time delays and notin tunable phase shifters). The broadband true time delay feature 

is achieved by a subsystem based on coherent optical combining using cascades of optical RRs as 

tunable elements. Aprototype based on eight elements was implemented featuring an optical 

sideband suppression of 25 dB, RF-to-RF delay up to 0.63 ns, atuning speed of 1ms, and a phase 

accuracy better than pi/10 rad in arange of 1–2GHz. 

As far as microwave photonic filtering is concerned, several groups have reported interesting 

resultssupported by different opticplatforms.Arecentreconfigurable approach [11] has implemented 

a filterthat isswitchablebetween highQbandpass and notch responses by tuning two tunableoptical 

bandpass filters (TOBFs). Positive and negative coefficients are obtained easily bydetuning the 

TOBF toget phase-intensity signal conversions in phase or out of phase. Afree spectral range 

(FSR) of 4.9 MHz and Q factor of 327 has been accomplished, while arejection ratio of42 and 

34 dB was achieved for the bandpass and notch filters, respectively. The operation frequency, 

which is limited by the shape and bandwidth ofthe TOBFs, was located around 20 GHz. 

Integrated-optics filtering led to various preliminary results based mainly onsingle-cavity RRs. 

For instance, [12] reports the results for aunit cell that could bean element of more complex lattice 

filters. This unit cell, which is integrated in InP-InGaAsP, is composed of two forward paths and 

contains one RR. By selectively biasing one semiconductor optical amplifier (SOA) and phase 

modulators placed in the arms of the unit cell, filters with a single pole, asingle zero, or acombination 

of bothcanbeprogrammed.Inparticular, forthe design reported in [12], the frequency tuning 

range spans around 100 GHz. The same group is nowworking on more complex designs and 

different unit cell configurations [13]. A hybrid version incorporating silicon photonic waveguides 

was recently reported [14]. Another more complex scheme has also been recently presented [15], 

where 1–2GHz-bandwidth filters with veryhigh extinction ratios ( 50 dB) have been demonstrated. 

With a power dissipation of 72 mW, the ring resonance can betuned (thermal control) by one free 

spectral range,resulting in wavelength-tunableoptical filters. Both second-and fifth-order RRs have 

been demonstrated. 

The impressive potential of photonic crystals is stimulating, in the context of MWP as well, the 

interest of researchers. The feasibility of exploiting PCs with the aim ofimplementing MWP filtering 

is demonstrated in [16]. Specifically, adesign of a2-D PCasymmetric Mach–Zehnder filter based 

on self-collimation effect is reported. Self-collimated beams, that is, diffraction-free beams, are 

effectively steered by employing line-defect beam splitters and mirrors in order tocreate the interferometric 

structure. Reflection-related problems at the input and output ports, which causes crucial 

performance limitations when using self-collimation,areminimized by adding antireflection layers.A 

full tunability over the FSR bandwidth of the notch response is controlled by acting on the 

geometrical properties of the defects periodical structure. 

The implementation of tunable phase shifting functionalities is also of great importance in many 

MWP applications. Several approaches using different technologies have already been presented 



in the literature. Within the integrated optics approach, atunable microwave phase shifter based on 

SOI dual-microring resonator has been recently demonstrated [17]. A quasi-linear phase shift of 

360 withRF-power variation lowerthan2dBat a frequency of40GHz is achieved.The phase shift 

is electrically controlled by means of two independent microheaters, which are used to alter the 

silicon effective index for FSR adjustment purposes, giving, as aresult, phase-shifting tunability. 

A novel method enabling high-accuracy full characterization of subpicosecond optical pulses 

based on temporal interferometry bymeans of an unbalanced temporal pulse shaping has been 

demonstrated in [18]. Since no fiber interferometer is used, the system stability is greatly improved. 

In addition, the temporal interference pattern can betuned by acting on the microwave modulation 

signal, which allows testing awide variety ofinput pulses. The feasibility of characterizing 550-fs 

width optical pulse train with arepetition rate of48.6 MHz is demonstrated using this approach. 

In addition, the recent demonstration of the first integrated discriminator for phase-modulated 

MWP links [19] is worth mentioning. The photonic chip consists of five optical RRs, where a dropport 

response of anoptical ring resonator (ORR) is cascaded with athrough response of another 

ORR to yield alinear-phase-to-intensity-modulation conversion. The balanced link exhibits high 

second- and third-order intercept points of46 and 36 dBm, respectively, which are simultaneously 

achieved at one bias point. 

Finally, webriefly state some of the most remarkable advances recently accomplished within our 

research group at the ITEAM Research Institute ofthe Universidad Politecnica de Valencia. The 

subarea of arbitrary waveform generation led to the proposal ofanovel photonic structure based on 

a microwave photonic filter fed by an optical source with areconfigurable power spectral distribution, 

adispersive element, and the combination of an interferometric structure with balanced photodetection 

[20]. This configuration provides a large degree offlexibility,in contrast withother optical 

techniques, since the generated waveform, which reaches operation frequencies up to 15 GHz for 

bandwidths from 1to8.75 GHz, can befully reconfigured by controlling the optical source power 

spectrum and the interferometric structure. Inorder to show the system potentialities, the waveform 

generator was adapted to multiband ultra-wideband (UWB) signaling format. 

The application of slow and fast light (SFL) effects to the implementation of both microwave 

tunablephase shifting and true time delay functionalities has been anotherfieldofintense research. 

Importantresultshave been obtained in the framework ofthe Europeanproject Governing the speed 

of light (GOSPEL) which are applicable to phase array antenna systems and dynamically reconfigurable 

filters. For the purpose of achieving true time delay, anovel scheme based on the combination 

of phase shifting and stimulated Brillouin scattering (SBS) in fibers, which exploits the 

separate carrier tuning technique onnarrowband resonances, was reported in [21]. The spectral 

shape of acomplex-valued two-tap filter (with a bandwidth of120 MHz at a central frequency of 

6GHz) wasshifted by making use ofthe SBS frequency tuning, allowing an FSR change regarding 

the fractional bandwidth onthe order of 20%. A tunable 360 MWP phase shifter, based on the 

exploitation of coherent population oscillations in active semiconductor waveguides, was demonstrated 

in [22]forabroad frequency range up to 40GHz. The proposed phase shifterisbased on the 

cascading of several SOAs, with in-between stages of optical filtering and regeneration, and can be 

reconfigured on a sub-nanosecond time-scale. 

MWP remains as one of the most active and multidisciplinary research fields within the photonics 

community. Significant activity and progress have recently been made in the areas of photonic 

generation and processing of microwave, millimeter, and terahertz waves, providing many advantageous 

features over their electronic counterparts. Among the different technology platforms 

reported last year, integrated photonics has gained special prominence, as compared with discrete 

components, and will be expected to experience an increasing impact in future years. 

References 

[1] M. Kahn, H.Shen, Y. Xuan, L.Zhao, S. Xiao, D. Leaird, A.Weiner, and M. Qi, BUltrabroad-bandwidth arbitrary 

radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,[ Nat. Photon., vol. 4, no. 2, 

pp. 117–122, Feb. 2010. 



[2] A. Stöhr, BPhotonic millimeter-wave generation and its applications in high data rate wireless access,[ in Proc. IEEE 

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[3] C. Pulikkaseril, S. M.Hanham, R. Shaw, R. A.Minasian, and T. S. Bird, BCoherence-controlled mm-wave generation 

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[5] L. A. Coldren, BPhotonic integrated circuits for microwave photonics,[ in Proc. IEEE Top. Meeting MWP, Montreal, QC, 

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[6] E. Rouvalis, C. C. Renaud, D. G. Moodie, M.J. Robertson, and A. J. Seeds, BTraveling-wave uni-traveling carrier 

photodiodes for continuous wave THz generation,[ Opt. Express, vol. 18, no. 11, pp. 11105–11 110, May2010. 

[7] H.-J. Song, K.-H. Oh, N.Shimizu, N. Kukutsu, and Y. Kado, BGeneration of frequency-modulated sub-terahertz signal 

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[8] K. Goda, K. K. Tsia, and B. Jalali, BSerial time-encoded amplified imaging for real-time observation of fast dynamic 

phenomena,[ Nature, vol. 458, no. 7242, pp. 1145–1149, Apr. 2009. 

[9] A. Meijerink, C. G. H.Roeloffzen, R. Meijerink, L. Zhuang, D. A. I. Marpaung, M.J. Bentum, M.Burla, J. Verpoorte, 

P. Jorna, A. Hulzinga, and W. van Etten, BNovel ring resonator-based integrated photonic beamformer for broadband 

phase array receive antennasVPart I: Design and performance analysis,[ J. Lightw. Technol., vol. 28, no. 1,pp. 3–18, 

Jan. 2010. 

[10] L. Zhuang, C. G. H.Roeloffzen, A.Meijerink, M. Burla, D. Marpaung, A.Leinse, M.Hoekman, R. G. Heideman, and 

W. van Etten, BNovel ring resonator-based integrated photonic beamformer for broadband phase array receive 

antennasVPart II: Experimental prototype,[ J. Lightw. Technol., vol. 28, no. 1,pp. 19–31, Jan. 2010. 

[11] Y. Yu, E. Xu, J. Dong, L.Zhou, X. Li, and X. Zhang, BSwitchable microwave photonic filter between high Q bandpass 

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[12] E.J.Norberg, R. S. Guzzon, S.Nicholes, J. S. Parker, and L. A. Coldren, BProgrammable photonic filters fabricated with 

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[14] H. W. Chen, A.W.Fang, J. D. Peters, Z.Wang, J. Bovington, D. Liang, and J. E.Bowers, BIntegrated microwave 

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[16] T.-T. Kim, S.-G. Lee, H.Y.Park, J.-E. Kim, and C.-S. Kee, BAsymmetric Mach–Zehnder filter based on self-collimation 

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[17] M. Pu, L. Liu, W. Xue, Y.Ding, H.Ou, K. Yvind, and J. M. Hvam, BWidely tunable microwave phase shifter based on 

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[18] C.Wang and J.Yao, BCompletepulse characterization based on temporal interferometry using an unbalanced temporal 

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IEEE Photonics Journal Future of Transmission Fiber 

Future of Transmission Fiber 

Masaaki Hirano 


Optical Communications R&D Laboratories, Sumitomo Electric Industries, Ltd., 

Yokohama 244-8588, Japan 

DOI: 10.1109/JPHOT.2011.2130519 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: M. Hirano (e-mail: masahirano@sei.co.jp). 

Abstract: In 2010, most high-capacity transmission experiments have been demonstrated 

over low-loss and/or low-nonlinearity dispersion unshifted fibers. Not only the research 

interest but the discussions of actual deployment of such linearity-enhanced fibers as well 

were starting to increase network capacity with systems based on greater than 100-Gb/s 

symbol rate. In addition, evolutional optical fibers that dramatically increase capacity in a 

single fiber, including multicore fibers and photonic crystal fibers, have been proposed. 

Index Terms: Optical Transmission Fibers, Fiber Nonlinear Optical Effects. 

Along with the rapid spread of bandwidth-hungry services, large volumes of data are required to 

be transmitted over along reach. As a corollary, the demand for broadband Internet-traffic continues 

to increase at about 2dBper year [1], and it is said that Bcapacity crunch[ in the very near 

future will become a possible reality [2]. A straightforward way to keep up with the explosive traffic 

growth is to increase the transmission capacity perasingle fiber, and therefore, there have been 

continuous and strong demands for advanced transmission fibers. The type of advanced fibers has 

been historically changing everyseveral years along with the development of transmission systems 

and signal processing [3]. 

In2010,the mostadvanced fiberfor high-capacity long-haul transmission experiments completely 

changed to linearity-enhanced fibers, that is, low-loss and/or low-nonlinearity dispersion-unshifted 

fiber. Actually, this class of fibers has been utilized as the transmission line in 10 out of 11highcapacity 

transmission experiments, which was presented as postdeadline paper in [4] and [5]. In 

addition,to further expand capacityoverthe coming decade, many differentkinds of evolutional fibers 

have been proposed and fabricated. 

Here, state-of-the-art linearity-enhanced fibers for long-haul systems will be described. Then, the 

required performance of transmission fibers for tomorrow’s high-capacity long-haul systems will be 

discussed. In addition, some prospects of evolutional fibers, including photonic crystal fiber (PCF), 

multicore fiber (MCF), and multimode fiber (MMF), will be also described. 

Recent capacity progress depends on the spectral efficiency increase using higher order signal 

formats with a coherent detection. Actually, 10-Tb/s transmission systems based on a100-Gb/s 

symbol rate with quadrature phase shift keying (QPSK) are launching the service in 2010. In the 

digital coherent transmission systems, adigital signal processor (DSP) that can equalize for practically 

any amount of linear transmission impairments has been utilized [6]. In this system, accumulated 

chromatic and polarization-mode dispersions are no longer obstacles, and therefore, 

dispersion compensation in the transmission line has become unnecessary. In fact, the larger 

chromatic dispersion improves transmission capacity and distance [7]. With such electronic 



Fig. 1.(a) Attenuation spectra of pure-silica core fibers with Aeff of 134 m 2 and the characteristics at 

1550 nmand (b) contour map ofrelative FOM normalized to SSMF. 

processing, required performances for optical system came to besimple, increasing the optical 

signal-to-noise ratio (OSNR) [8]. As for fiber properties, reduction of the attenuation and nonlinear 

coefficient ð Þ are essential, because the lower fiber attenuation can directly increase the output 

power for acertain input power, and the lower can allow the higher input power managing the 

transmission impairment due to Kerr nonlinear effects accumulated in afiber [9]. 

Pure-silica core fiber (PSCF) has an improved attenuation of 0.15 to 0.17 dB/km at 1550 nm[10], 

which is significantly lower than the attenuation around 0.19 dB/km ofastandard single-mode fiber 

(SSMF) doped with GeO2 in the core. Inorder to decrease the ,reduction of the nonlinear 

refractive index of n2 and enlargement of the effective area of Aeff are key issues, because the is 

defined as ¼ n2=Aeff 2 = ,where is the lightwave wavelength. The n2 is determined with the 

composing material, and a PSCF is about 10% lower ( 0.3 dB) than that of aGeO2-doped SSMF 

[11]. To decrease the ,itismore important toenlarge the Aeff. The challenge is to cope with poor 

macro- and microbending loss performance by employing an appropriate refractive index profile 

and alow Young’s modulus primary coating. Applying atrench-assisted profile, afiber with the Aeff 

of 120 m 2 , which is 1.5 times larger than that of SSMF, and the better microbending loss 

performance than SSMF’s, was demonstrated [12]. Another issue of large-Aeff fiber is the huge 

splicing loss to an existing SSMF because of a large amount of mismatching in mode-field diameters 

between fibers. Considering the splicing loss, we found that that the Aeff around 135 m 2 would 

be the most suitable as along-haul transmission fiber [13] and actually demonstrated PSCF with 

the Aeff of 134 m 2 and low attenuation of 0.161 dB/km having the equivalent bending sensitivities 

to that of actually deployed fibers, as shown in Fig. 1(a) [3], [13]. 

A lot of high-capacity transmission experiments through linearity-enhanced fibers were 

presented in 2010 [14]–[17]. For example, the record-high total capacity of 69-Tb/s transmission 

over 240-km-long PSCF with the Aeff of 110 m 2 and attenuation of 0.160 dB/km [14], and total 

capacityof12.5 Tb/s transmission over 9360 km-long large Aeff fiber of150 m 2 with the attenuation 

of 0.183 dB/km [15], was demonstrated, respectively. Here, the question is, Bwhich is the best fiber 

for high-capacity and long-haul transmission?[ In a system with the span length of L [km], the figure 

of merit (FOM) for afiber having the Aeff ½ m 2 Š and an attenuation of [dB/km] will be described as 

[3], [18] 

FOM ½dBŠ ¼10logðAeff Þ L: (1) 

The first term means that the allowable signal launched power limited by Kerr nonlinearities is 

determined with a product of Aeff ,and the second term represents the output power after 

L km-long fiber propagation. Therefore, the better ONSR a system has over afiber with the higher 

FOM[9]. Providing anSSMF with the of 0.19dB/km and Aeff of 80 m2 as areference,the relative 



FOM canbecalculated.Fig.1(b) shows the contour mapofthe relative FOMas afunction of Aeff and 

attenuation for span length L of 60 km and 120 km. Ascan be seen in Fig. 1(b), reported linearityenhanced 

fibers have improved FOMs by3to 5 dB as aresult of improvements to both Aeff and 

attenuation. Itshould benoted that the lower attenuation has the more advantageous impact on the 

FOM rather than the larger Aeff for the longer span length. Inorder to apply this class of linearityenhanced 

fibers to aterrestrial transmission system, there are still things to besolved, including 

establishment ofaunified standard and evaluation of mixability with various types offibers that have 

actually been deployed [19]. 

To further decrease the nonlinearity,aPCFwithdramaticallyenlarged Aeff of 220 m 2 withpractically 

low bend-induced loss was demonstrated [20]. However, its attenuation is ashigh as 1.2dB/km,and it is 

expected that the attenuation oflarge-Aeff PCF will be able tobereduced to a comparable value with the 

lowestattenuation of aPCF ever reported (0.18 dB/km) [21]. 

In orderto avoid the capacity crunch, the advent of some evolutional fibers is anticipated over the 

next decade, and new multiplexing schemes other than time and wavelength have been seriously 

considered [20]. MCF has several cores in a single fiber, and space-division multiplexing through 

the MCF isexpected to dramatically increase the transmission capacity [21]. In order to apply a 

MCF to along-haul transmission, the challenge is to reduce intercore crosstalk, and the efforts to 

manage the crosstalk were actively reported in 2010 [22]–[25]. We and Fini et al. independently 

revealed that the crosstalk is significantly affected by a bend in MCF and is a stochastic value, both 

with theoretical [22], [23] and experimental [22] evaluation. In2011, it is promising that an MCF 

having negligible crosstalk will be demonstrated. 

Propagation mode division multiplexing (MDM) using a multiple-input and multiple-output (MIMO) 

algorithm over MMF is also a hot topic, and transmission of two modes 4 Gb/s over a5km-long 

MMF was demonstrated [26]. The transmission capacity and reach are still not very impressive 

compared with that in today’s WDM systems, and the development of fiber structure suitable for the 

MDM and improvement of the MIMO algorithm are strongly expected. 

In summary, advanced transmission fibers are strongly expected in order to keep up with 

explosive increase of traffic growth. Linearity-enhanced PSCFs would bepromising because of the 

potential attenuation as lowas0.15 dB/km and Aeff as large as 135 m 2 .Forthe next decade, some 

Bevolutional[ fibers will be expected to prevent transmission capacity from crunch. 

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[11] T. Kato, Y.Suetsugu, and M. Nishimura, BEstimation of nonlinear refractive index in various silica-based glasses for 

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[13] Y.Yamamoto, M. Hirano, K. Kuwahara, and T. Sasaki, BOSNR-enhancing pure-silica-core fiber with large effective area 

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OTuI2. 

[14] A. Sano, H.Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, 

H. Yamazaki, Y. Sakamaki, and H. Ishii, B69.1-Tb/s(432 171-Gb/s) C-and extended L-band transmission over 240 km 

using PDM-16-QAM modulation and digital coherent detection,[ presented at the Opt. Fiber Commun./Nat. Fiber Optic 

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Turin, Italy, 2010, Paper Tu.3.C.4. 


IEEE Photonics Journal Optical Fiber Communications 

Major Accomplishments in 2010 on Optical 

Fiber Communications 

A. E. Willner, 1 Fellow, IEEE, Z.Pan, 2 Senior Member, IEEE, and 

M. I. Hayee, 3 Senior Member, IEEE 


1 Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90007 USA 

2 Department of Electrical and Computer Engineering, University ofLouisiana at Lafayette, 

Lafayette, LA70504 USA 

3 Department of Electrical and Computer Engineering, University ofMinnesota Duluth, 

Duluth, MN55812 USA 

DOI: 10.1109/JPHOT.2011.2131123 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: A. E. Willner (e-mail: willner@usc.edu). 

Abstract: Optical fiber communications continued to advance at a rapid pace in 2010. This 

paper presents a brief overview of the most exciting technological advances of 2010, eking 

out record capacity out of optical fiber through enhancing spectral efficiency and shrinking 

power consumption using modern optical fiber communication system designs. 

Index Terms: Optical fiber communication, coherent communications, optical modulation, 

optical signal processing. 

The rapidglobal proliferation ofthe Internetisdriving communication networks increasinglycloser 

to their limits while available bandwidth is disappearing due to ever-increasing network load. The 

fiber optic backbone has been, and will remain, the only viable solution for the next generation of 

ultrahigh-speed networks in the foreseeable future. Over the past decade, wavelength division 

multiplexing (WDM) and erbium-doped fiber amplifier technologies have been deployed to obtain 

systems with an aggregatecapacityexceeding 1Tb/s [1], [2]. However, the individual data rates per 

wavelength (channel) in these systems have been G 100 Gb/s, mainly limited by transceiver 

technologies using direct detection receiver with intensity and/or phase modulated transmitter. 

These transceivertechnologies define upperlimit on spectral efficiency and lowerlimit on power per 

channel tobeinjected in the fiber, which in turn sets limit on fiber nonlinearity. Consequently, the 

combined effectimposes a ceiling on maximumcapacity tobeobtained through a singleoptical fiber. 

To fulfill the future needs of bandwidth requirement, not only spectral efficiency of optical fiber 

communication system needs to beimproved, but power per wavelength needs to bereduced as 

well so that higher individual data rates per wavelength ofup to 1Tb/s can beachieved with total 

aggregate capacities exceeding 1 Pb/s. The work in this direction has already begun, and some 

great progress was seen in2010 which isexpected to further growin futureyears as shown in Fig.1 

where record capacity out of single fiber is shown versus the year in which that capacity was 

experimentally demonstrated. This is a modified figure of[3]. As can be seen from the figure, the 

1980s research trend was time division multiplexing (TDM) using single-channel optical fiber 

communication. In the 1990s, the trend evolved to WDM, and by the mid 2000s, coherent 

communication became the most dominant research trend to enhance capacity through a single 

optical fiber. 



Fig. 1.Capacity through a single fiber versus year, which was achieved through an experimental 

demonstration using TDM, WDM, and coherent communication technologies. The data have been 

extracted from published experimental demonstrations in the respective years. 

The most prominent research trend over the past few years in improving spectral efficiency and 

obtaining lower channel power has beenthe coherent transceiver technology, as opposed to direct 

detection technology. In addition, higher level modulation formats, and optical/electronic signal 

processing in conjunction with coherent communication technology help further increase spectral 

efficiency. Similarly, miniaturization of optical components through photonics integrated circuits(PICs) 

and advanced electronicsignal processing techniques keep the operating channel power even lower. 

Consequently,ahighertotal capacity through a singleoptical fiber canbeobtained.In the following,we 

will reviewthe majoraccomplishments in2010onachieving high spectral efficiency and least operating 

channel power for obtaining record overall capacities in optical fiber communication systems. 

Spectral Efficiency: Coherent optical communications in combination with multilevel modulations 

is considered as the next revolution of optical communication due to the potential of high 

spectral efficiency and high receiver sensitivity [4], [5]. In coherent optical communication, 

information is encoded onto amplitude, phase, and polarization of the electrical field ofthe light 

wave. Since information symbols can be encoded in all the degrees of freedom available in afiber, 

the coherent system permits use of spectrally efficient multilevel modulations. Some great progress 

was made in 2010 on coherent communication using multilevel modulations leading toward record 

capacity out of optical fiber systems [6]–[10]. Among short distance demonstrations, NTT’s 

postdeadline publications of OFC 2010 and ECOC 2010 are prominent, in which they reported 

spectral efficiencies of 6.4 and 9.0 b/s/Hz, respectively [6], [7]. These demonstrations were carried 

out with atotal of 432 171 Gb/s and 100 120 Gb/s channels using 16- and 64-quadrature 

amplitude multiplexing (QAM) formats with polarization division multiplexing (PDM) on240 km and 

160 km offiber, respectively. Similarly, AT&T Research Lab’s demonstration of 640 107 Gb/s 

over 320 km of transmission resulted in spectral efficiency of 8 bps/Hz [8]. Among long-haul 

distance demonstrations, Alcatel–Lucent’s experimental demonstration of 4-bps/Hz using ten 

224-Gb/s channels over 1200-km fiber isquite impressive [9]. For ultralong-haul distances, Tyco 

Telecommunications demonstrated 112 112 Gb/s transmission using quadrature phase shift 

keying (QPSK) with PDM over 9360 km, setting arecord spectral efficiency of 3.6 bps/Hz for 

transpacific distances [10]. 

Over the past few years, orthogonal frequency-division multiplexing (OFDM) scheme has come 

out as an efficient coherent communication scheme in an attempt to enhance spectral efficiency. In 

2010, this trend continued, and many record-breaking experimental demonstrations were carried 

out using OFDM [11]–[14]. Among specificexamples of published research include anexperimental 

demonstration of a 400-Gb/s coherent PDM–OFDM transmission over 80 km in a 50-GHz 

frequency grid, resulting in 8 bps/Hz of spectral efficiency [15]. OFDM has also led the way to 

utilizing optical signal processing techniques that are inherently associated with low complexity and 

ultrahigh speed [16], [17]. Optical signal processing can provide some specialized functions that 

may not be matched by electronic counterparts, such as phase-sensitive amplification, transparent 

optical grooming/aggregation, advanced modulation format conversion techniques, and seamless 



Fig. 2. Maximum spectral efficiency demonstration versus transmission distance using coherent 

communication technologies in 2010. 

bandwidth scaling, all resulting in enhanced spectral efficiency and higher total capacity out of fiber. 

An example of such work is experimental demonstration of anoptical fast Fourier transform (FFT) 

scheme at a data rate of 392 Gb/s using passive optical components with OFDM [16]. It enables 

OFDM signal processing way beyond the bandwidth limitsof electronics,thereby possessing a huge 

potential of much higher spectral efficiencies. Another example is the demonstration of asingle 

source optical OFDM transmitter atthe data rates of 5.4 and 10.8 Tb/s and single receiver based 

upon optical FFT [17]. The transmitter atthe data rates of 5.4 and 10.8 Tb/s used 75 spectrally 

overlapped QPSK and 16-QAM signals, respectively, producing spectral efficiencies of 2.88 and 

5.76 bps/Hz. Fig. 2summarizes the maximum spectral efficiency demonstrated in 2010, using 

various modulation formats with coherent communication. 

Optical signal processing cannot only help OFDM systems achieve higher data rate and 

spectral efficiency but can help other coherent optical systems avoid complex implementation of 

electronic signal processing to further enhance the spectral efficiency as well [18], [19]. A notable 

experiment involving optical signal processing has shown 512-QAM coherent optical transmission 

by using an optical phase-lock loop. Apolarization-multiplexed 54-Gb/s data signal was transmitted 

at 3 Gsymbol/s with an optical bandwidth of 4.1 GHz [18]. Although, this experiment was 

carried out with single channel but achieving a data rate of54Gb/s possessing a bandwidth of only 

4.1 GHz suggests that spectral efficiency of more than 10 bps/Hz is onthe horizon. Another 

example of optical signal processing using multilevel coherent communication, is the demonstration 

of 128-Gb/s transmission on 610 km of standard single mode fiber using parallel optical signal 

processing at the receiver [19]. Although the achieved spectral efficiency was only 1.8 bps/Hz, this 

experiment demonstrates that employing optical signal processing relieves electronic signal 

processing, thereby enabling electronic dispersion compensation of 610 km of standard single 

mode fiber, which would otherwise have not been possible. 

The trend of using optical signal processing in combination with coherent signal processing and 

multilevel modulation formats is akey to increasing spectral efficiency in optical communication 

systems, as mentioned in the references above. However, at the same time, optical signal processing 

helps mitigate fiber degradations [19], as well as achieve important networking functions in 

all optical networks. Anexample ofthis kind of work is a demonstration of atunable and reconfigurable 

tapped delay line using conversion dispersion-based delays [20]. At this point, it is important 

to mention that many novel designs of multimode and multicore fibers emerged in 2010 to 

optically process the WDM signal by adding another dimension to either mitigate fiber degradations 

e.g., fiber dispersion, nonlinearity, and crosstalk [21]–[23], and/or help design access networks 

[23]–[26]. 

Power efficiency: Many examples of multilevel modulation formats using coherent communication 

leading toward record spectral efficiency and fiber capacity were summarized in the above 

section. Higher level modulation formats using coherent communication require complex highspeed 

electronic signal processing at the transmitter, as well as at the receiver end. Without 

miniaturizing the size of electronics both at the receiver and transmitter end, itwould not have been 



possible to implement such higher level modulation schemes using coherent communication. 

Therefore, alot of work on PICs producing miniaturized transmitter sources and high electronic 

signal processing at the receiver has been carried out over the past few years, and the trend 

continued in2010 as well [27]–[31]. Photonic integration enables cost-effective,reliable,and powerefficient 

scaling of optical networks and helps to realize a higher level of performance and 

functionality. PICs play an increasingly critical role in the realization of the functionalities that are 

required to advance optical communication systems. 

One more requirement for higher level modulation formats is to have higher operating channel 

power, which can furtherincrease fiber nonlinearities, thereby limiting the fiber capacity. Using PICs 

and high-speed electronic/optical signal processing can relieve the operating channel power, 

consequently enabling highly spectral efficiency, producing high-capacity optical fiber communication 

systems. There werequite afewresearch experiments using this line ofresearch in2010 [8], 

[32], [33]. All these experiments used electronic signal processing at the transmitter and/or receiver 

to enable higher level modulation formats, obtaining record capacity through the optical fiber. For 

example, in an experimental demonstration, with 16 PDM-QAM at 112-Gb/s transmission over a 

distance of 1440 to 2400 km of standard single mode fiber wassuccessfully carried out using 

nonlinear digital back propagation and novel electronic signal processing techniques [33]. 

It is also important to mention that minimizing power consumption may not only be necessary to 

achieve higher capacity byreducing fiber degradations and increasing spectral efficiency with 

effective signal processing, power, and energy-efficient networks may also be needed for 

environmental reasons. Some importantfoundational work was accomplished in this line ofresearch 

in 2010 as well [34]–[36]. 

Conclusion: In conclusion, toobtain record capacity needs of future generation optical fiber 

networks, the research trend is to use coherent communication with multilevel modulation formats. 

To enable multilevel modulation formats, optical and/or electronic signal processing at the 

transmitterand receiver ends is essential. Similarly, PICs are the key to implementing such complex 

optical and electronic signal processing schemes required for multilevel modulation formats using 

coherent communication. This trend continues to grow, and we will soon see more research and 

experimental demonstrations paving the way for spectral efficiencies of up to afew tens of bits per 

second per Hertz in the near future. 

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IEEE Photonics Journal Toward the Shannon Limit of SE 

Toward the Shannon Limit of 

Spectral Efficiency 

L.-S. Yan, 1 X. Liu, 2 and W. Shieh 3 


1 Center for Information Photonics and Communications, School of Information Science and 

Technology, Southwest Jiaotong University, Chengdu 610031, China 

2 Bell Labs, Alcatel-Lucent, Holmdel, NJ 07733 USA 

3 Department of Electrical and Electronic Engineering, University of Melbourne, 

Parkville, Vic. 3010, Australia 

DOI: 10.1109/JPHOT.2011.2127468 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: L. S. Yan (e-mail: lsyan@home.swjtu.edu.cn). 

Abstract: Progress in high-capacity optical communication systems in 2010 is reviewed, 

with spectral efficiency (SE) as the main figure of merit. Advanced modulation formats, such 

as quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing 

(OFDM), together with polarization-division multiplexing (PDM) and digital coherent 

detection, are playing key roles in approaching the Shannon limit of SE for optical fiber 

communication. Photonics is now entering into a new era of high SE on par with electronics 

(wireless), yet with orders of magnitude higher overall system capacity. 

Index Terms: Fiber optics, optical fiber communication, Shannon limit, spectral efficiency. 

Ever since Shannon disclosed the information capacity limit in a noisy communication link [1], 

researchers have put tremendous effort into approaching it in both wireless and optical links over the 

past few decades. Optical fiber communication systems and networks have been playing important 

roles in supporting the exponentially increasing information traffic around the globe [2], [3]. To grasp the 

sustainability of the capacity growth, the Shannon limit in optical fiber networks has been theoretically 

studied [4]–[8] and experimentally explored. There have been multiple Bhero[ experiments setting new 

records in high-capacity optical communication every year over the past three decades, and the year of 

2010 was no exception. These recent advances are underpinned by various key technologies, 

including advanced modulation, polarization-division multiplexing (PDM), digital coherent detection, 

digital signal processing, advanced coding, advanced fiber technologies, and photonic integration. 

Among these technologies, advanced modulation combined with digital coherent detection is the key 

enabler. Quadrature amplitude modulation (QAM) and coherent optical orthogonal frequency-division 

multiplexing (CO-OFDM) are the two popular modulation formats explored in lab demonstrations, with 

high potential to be commercially deployed in the near future. 

In this paper, we give a brief review on the major breakthroughs in the field of optical fiber 

communications in 2010, highlighting the enabling techniques and demonstrations with a focus on 

high spectral efficiency (SE). The intent of this review is not to present the pros and cons of various 

technologies, but rather to provide a literature survey for the photonics community. 



Fig. 1. (a) Historical evolution of spectral efficiency (updated figure from [8]). (b) Major high spectralefficiency 

demonstrations in 2010 showing the trend toward the Shannon limit. (c) Signal constellations 

of 16-QAM [33], 36-QAM [28], 64-QAM [29], and 512-QAM [30] demonstrated in 2010. 

2. Device and Subsystem Improvements 

We begin with the enabling techniques at the device and sub-system levels that are fundamental to 

enhance system performance. In 2010, various noteworthy implementations were accomplished. 

For signal generation, i) Yamazaki et al. demonstrated a 64-QAM modulator based on silica photonic 

integrated circuits (PLCs) and LiNbO3 phase modulators [9]; ii) Winzer et al. generated a 56-Gbaud 

PDM 16-QAM data stream using a single I/Q modulator [10]; and iii) Yi et al. [11] and Hillerkuss et al. [12] 

showed Tb/s OFDM signal generation from frequency-locked optical combs. For signal reception, 

i) Nagarajian et al. reported a 10-channel (45.6-Gb/s/channel) PDM differential quadrature phaseshift 

keying (DQPSK) InP receiver based on PLCs [13]; ii) Fischer et al. demonstrated a 128-Gb/s 

QPSK digital coherent receiver with parallel optical sampling [14]; iii) Shen et al. designed a 

polarization demultiplexer and polarization-mode-dispersion (PMD) compensator for a 112-Gb/s 

direct-detected PDM return-to-zero (RZ) DQPSK system [15]; and iv) Kaneda et al. showed a realtime 

2.5-GS/s coherent receiver for sub-band detection of a 53.3-Gb/s OFDM signal [16]. As a 

complementary approach to the high-speed electronic DSP-based PMD compensation and 

polarization demultiplexing, optical polarization tracking at a speed up to 59 krad/s was demonstrated 

in a 112-Gb/s PDM-RZ-DQPSK transmission experiment [17]. Another key breakthrough was the 

development of high-speed analog-to-digital converters (ADCs) at 56 Gb/s [18], which enabled the 

implementation of digital coherent receivers at 100-Gb/s and beyond [7]. 

3. Channel Data Rate Increase 

It is desirable to increase the data rate per wavelength channel beyond the currently commercialized 

100 Gb/s [8]. Using spectrally efficient reduced-guard-interval (RGI) CO-OFDM and high-speed 

ADCs, Liu et al. [19]–[21] demonstrated the detection of 448-Gb/s, 606-Gb/s, and 728-Gb/s signals 

with a single coherent detection front-end, which allows for a simple transceiver architecture for costeffective 

system upgrade. As the data rate of a channel continues to increase, the sampling speed of 

ADC will eventually limit the data rate that can be received in a single-detection step, and this calls for 

banded detection or optical time-division multiplexing (OTDM). Several groups have achieved Tb/s 

transmission using CO-OFDM with banded detection [22], [23] and OTDM with multiple parallel 

receivers [24]. A comprehensive study on the use of OFDM to form high-SE superchannel was 

reported by Chandrasekhar et al. [25]. An interesting scheme to approach the SE of OFDM using 

Nyquist WDM was recently proposed and studied [26]. 

4. SE Increase 

To increase the total capacity of optical fiber communication system within a limited bandwidth 

(generally determined by the bandwidth of optical amplifiers), one has to increase the channel SE. 

PDM is an effective technique to double the SE by carrying two independent data streams on two 

orthogonal polarization states at the same wavelength. Fig. 1(a) illustrates the SE increase enabled 



Fig. 2. (a) Historical evolution of the distance-SE product (in km-b/s/Hz) over the last six years. (b) An 

exemplary experimental setup utilizing enabling techniques such as 64-QAM, RGI-CO-OFDM, PDM, 

digital coherent detection, and low-loss low-nonlinearity fiber (after [21]). 

by polarization interleaving and PDM since 1990. Fig. 1(b) summarizes the major results on high- 

SE transmission obtained in 2010 [19]–[21], [27]–[30]. Typical recovered signal constellations of 

PDM-16QAM [27], OFDM/32QAM [20], PDM-64QAM [29], and PDM-512-QAM [30] are shown in 

Fig. 1(c). Note that the Shannon limit curve plotted in Fig. 1(b) is for the case with PDM but without 

the consideration of fiber nonlinearity. The nonlinear Shannon limit in the PDM case is still under 

active study [6]. Note also that the gap between the obtained SE and the Shannon SE limit 

becomes larger at higher SE, indicating larger fiber nonlinearity penalties and implementation 

penalties for signals with larger constellation sizes. 

5. Overall Performance Increase 

The results in Fig. 1 are plotted based on SE. Apparently the SE alone is not sufficient to evaluate 

the overall system performance. Here, we use two different figures of merit (FOMs): the product of 

transmission distance and capacity (Distance Capacity) and the product of transmission distance 

and SE (Distance SE). In addition to those already highlighted, there are also several Bhero[ 

experiments during 2010 achieving impressive overall performance, e.g., long transmission 

distance of a few thousands of kilometers and high capacity of tens of Tb/s with SE higher than 

3 bit/s/Hz) [31]–[34]. Various enabling techniques are employed in these systems to achieve these 

levels of performance. We further draw the FOM in term of Distance SE within the period of last five 

years (2005–2010) in Fig. 2(a). Notably, this FOM was essentially flat from 2005 to 2007 but 

dramatically increased after the emergence of the aforementioned enabling technologies such as 

QAM, OFDM, PDM, and digital coherent detection. An exemplary experimental setup [21] that 

utilized most of these technologies is shown in Fig. 2(b). In addition, ultra-large-area fiber (ULAF) 

was used to reduce both fiber loss and nonlinear coefficient in some of these hero experiments. 



6. Other Highlights 

As impressive as the above breakthrough lab demonstrations are field trials of high-speed (i.e., 

9 100-Gb/s/channel) data transmission carried out over existing commercial links reported in 

March 2010 [35], [36], especially Verizon’s test based on native internet protocol traffic at 100-Gb/s 

[35]. Such trials provide valuable information for the actual deployment of 100G Ethernet. More 

recently, a single-wavelength 112-Gb/s transceiver based on PDM-QPSK and digital coherent 

detection has become a commercial reality [37]. 

As more techniques are employed in optical communication systems to enhance the overall 

performance, some new challenging issues are appearing and have to be addressed. These 

include the well-known but complicated fiber nonlinearity issue. There are sustained efforts to 

mitigate or compensate certain types of fiber nonlinear effects [38], [39]. 

Another enabling technique that is worth mentioning is the optical signal processing approach in 

high-speed systems. In 2010, there were several papers covering different signal processing 

functionalities, such as phase-sensitive regeneration of DPSK signals [40], optical regeneration of 

PDM signals [41], and orthogonal tributary channel exchange for PDM signals [42]. Although these 

all-optical approaches are not yet sufficiently mature for practical implementations, they open new 

possibilities to further enhance the optical communication performance. 

7. Concluding Remarks 

In summary, numerous exciting advances took place in 2010 to push the limit of optical 

communication systems in terms of channel data rate, SE, overall capacity, and transmission 

distance. We highlighted these breakthroughs through several updated figures. Remarkably, recent 

optical fiber transmission demonstrations are already not too far away from the Shannon limit of 

single-mode fiber-optic transmission any more [6]. It is expected that riding on Moore’s law, future 

advances in electronics [43] will continue to both demand and enable further capacity growth in 

optical communications. It may be desirable to relax the nonlinear Shannon limit by using new fibers 

with lower loss and/or lower nonlinear coefficients and developing nonlinearity mitigation strategies. 

Furthermore, potential utilization of the spatial degree of freedom through novel fibers, possibly by 

means of MIMO techniques, is a new direction worth exploring [2], [44]. With the increase in 

capacity, the cost per bit needs to be reduced as well in order to allow for sustainable capacity 

growth. Therefore, advances in the area of photonic integrated circuits are deemed to be essential. 

Facing Bthe coming capacity crunch[ [2], one can be assured that continued Bresearch in this area 

is essential, challenging, and likely to be interesting[ [3], as well as rewarding, both intellectually and 

economically. 


The authors would like to thank Dr. P. Winzer from Bell Labs (Alcatel-Lucent) for valuable comments 

during the review preparation. 

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IEEE Photonics Journal Optical Wireless Broadband Access Networks 

Breakthroughs in Optical Wireless 

Broadband Access Networks 

Xiupu Zhang, 1 Bouchaib Hraimel, 1 and Ke Wu 2 


1 Advanced Photonic Systems Lab., Department of Electrical and Computer Engineering, 

Concordia University, Montréal, QC H3G 1M8, Canada 

2 Poly-Grames Research Centre, Department of Electrical Engineering, Center for Radiofrequency 

Electronics Research of Quebec, École Polytechnique de Montréal, 

Montréal, QC H3T 1J4, Canada 

DOI: 10.1109/JPHOT.2011.2129504 

1943-0655/$26.00 Ó2011 IEEE 


current version April 26, 2011. Corresponding author: X. Zhang (e-mail: xzhang@ece.concordia.ca). 

Abstract: Enabling technologies for optical wireless broadband access networks have been 

explored and developed with respect to optical components, modules, transmission 

systems, and networks. In this paper, we review the most significant accomplishments 

reported during 2010 with emphasis on radio-over-fiber (RoF) technology that is critical for 

the deployment of optical wireless broadband access networks in the near future. 

Index Terms: Microwave photonics, fiber optics systems, optical communications, 

integrated photonic systems. 

Optical wireless broadband access networks have attracted much attention from both the 

academic and industrial communities, resulting in much peer-reviewed literature published during 

2010, which reported innovations and accomplishments in the areas of components, modules, 

transmission systems, and networks. It has been known that high capacity wired access networks 

based on fiber-to-the-home (FTTH) technology have been successfully deployed in many countries. 

In the near future, optical wireless broadband access networks will be integrated into the FTTH 

infrastructure to cover a dedicated space or a whole metropolitan city. 

Optical wireless broadband access networks can be divided into two categories in terms of radio 

frequency (RF) carriers: 10 GHz and less (i.e., microwaves) and millimeter-waves. For the RF 

carriers of 10 GHz and less, optical wireless broadband access networks are mainly used to 

support current wireless signal distribution, such as for second-, third, and fourth-generation, and 

Wi-Fi, as well as future ultra wideband (UWB) and cognitive radio systems. Optical wireless 

broadband access networks with millimeter-wave carriers are mainly used for the realization of 

high-capacity wireless signal coverage for some dedicated places and environments, such as 

hospitals, office buildings, and airports. Such millimeter-wave optical wireless technologies are also 

being considered as one of the back-haul base-station connection technologies of emerging 

broadband wireless services. Although millimeter-wave optical wireless platforms have been 

studied and developed for many years, some dedicated component, system, and networking 

technologies have yet to be considered mature and should be further developed so that the distribution 

of millimeter-wave wireless signals can be realized in a cost-effective manner. The optical 

wireless broadband access networks may also be used to support wireless sensor networks, 

including radar and imaging applications. The distribution of wireless signals through optical 



Fig. 1. (a) Predistortion circuit layout and (b) measured RF power of RF carrier and third-order 

intermodulation distortion (top two lines: RF carrier power; bottom two lines: third-order intermodulation 

nonlinear distortion power). 

wireless broadband access networks can be classified into radio over fiber (RoF), intermediate 

frequency (IF) over fiber, and baseband over fiber or digitized RF over fiber, and among them, the 

RoF technology has been considered to be the most promising compared with the others. Therefore, 

we only highlight the accomplishments of enabling technologies related to the developments 

and innovations of the RoF technology during 2010. 

For the breakthroughs and innovations in lasers and photonic sources that are used for the RoF 

systems, a feedforward linearized laser was proposed to improve modulation linearity [1]. Optically 

injection-locked vertical-cavity surface-emitting lasers (VCSELs) or distributed feedback (DFB) 

lasers were also investigated with regard to having a broad modulation bandwidth that can be used 

for 60-GHz millimeter-wave over fiber systems [2], [3]. Additionally, a reflective semiconductor 

optical amplifier (SOA) located at a base station was proposed to replace a laser for an uplink [4]. Due 

to limited modulation bandwidth of the reflective SOAs, this technique can be used only for uplinks 

with the RF carrier frequencies of less than 3 GHz. Alternatively, a saturated SOA can be used at a 

base station to obtain a quasi-continuous-wave light for an uplink [5]. Two lasers with orthogonally 

polarized lights, i.e., one for downlink and the other for uplink, are located at central stations, and 

thus, no lasers are required at base stations or remote antenna sites [6]. Moreover, it was found that a 

broadband optical source combined with a Mach–Zehnder interferometer could be used to obtain 

multichannel light sources for wavelength division multiplexing (WDM) RoF systems [7]. 

Significant accomplishments in optical modulators for optical wireless broadband access networks 

have been made. We have proposed a mixed-polarization linearized Mach–Zehnder modulator 

(MZM) that leads to a more than 10-dB improvement of spur-free dynamic range (SFDR) [8]. 

Later, this technique was extended to linearize a polarization-dependent electro-absorption modulator 

(EAM), and a 10-dB SFDR improvement was achieved [9]. Instead of using a single MZM, a 

dual-parallel MZM can be used to improve modulation linearity [10]. We have also proposed and 

investigated a low-cost predistortion circuit to linearize optical modulators [11]. Fig. 1 shows the 

circuit layout and measured photodetected RF power of the RF carrier and third-order intermodulation 

distortion after 20 km of fiber transmission with and without using such a predistortion 

circuit before an EAM. It was shown that an 11-dB improvement of SFDR was achieved. This circuit 



can also be used for the linearization of a direct-modulation laser diode, an MZM, or a phase 

modulator, which will fully be reported later. More importantly, such a predistortion circuit can easily 

be designed for the broadband operation and, thus, can be used to linearize millimeter-wave over 

fiber systems. An alternative technique for linearizing optical modulators and thus RoF systems was 

also studied, which is based on optical phase modulation in the transmitter and remodulation and 

optical filtering in the receiver with coherent detection [12]. 

For microwave photonics including RoF technology, broadband and high-power photodiodes are 

required. Use of two cascaded photodiodes leads to 91-GHz bandwidth and 18-dBm RF power [13], 

and a single photodiode was reported to generate an RF power of 29 dBm at 5 GHz [14]. A highlinearity 

four-photodiodes array integrated with a power combiner was demonstrated to have an RF 

power of 8 dBm at 20 GHz [15]. An RF power of 0.7 dBm has been achieved at 60 GHz using a 

single InP photodiode [16]. A broadband InP photomixer integrated with millimeter-wave antenna 

was fabricated and generates an RF power of 4.5 dBm at 110 GHz [17]. Instead of using a 

broadband photodiode for millimeter-wave over fiber systems, a omplementary metal–oxide– 

semiconductor (CMOS) avalanche photodiode was demonstrated to directly up-convert an IF 

wireless signal to 60-GHz millimeter-wave signal at base stations, where the CMOS avalanche 

photodiode was used for both photodetection and harmonic mixing [18]. 

To develop low-cost millimeter-wave broadband RoF systems, efficient and innovative interfaces 

between wireless and photonic signals such as optical modulators and photodetectors should be 

made. Novel optical modulators based on Calcium Barium Niobate materials that provide electrooptical 

coefficients that are roughly three times higher than LiNBO3 were studied for very low-driving 

voltage applications [19]. In addition, a class of emerging optical modulators and photodetectors 

that make use of a substrate integrated waveguide (SIW) was proposed and demonstrated rather 

than conventional microstrip and coplanar waveguide (CPW) transverse eletromagnetic mode 

(TEM-mode) traveling-wave electrodes. In this way, millimeter-wave signals can be transmitted and 

processed with low-loss and self-packaging features with non-TEM mode propagation [20], [21]. 

For millimeter-wave based optical wireless broadband access networks, photonic frequency upand 

down-conversion have been considered as a new low-cost technique. It was found that an 

EAM can be used for simultaneous frequency up- or down-conversion and optical subcarrier 

modulation [22], [23], resulting in a low-cost base station. The frequency down-conversion from 

30 GHz to 4 GHz and optical subcarrier modulation using an EAM were validated using 

multiband orthogonal frequency division multiplexing (OFDM) UWB over 20 km of single-mode 

fiber, and an error vector magnitude (EVM) of less than 21 dB was obtained. An optical phase 

modulator or an MZM can also be used for the photonic generation of millimeter waves [24]–[28]. 

Using frequency quadrupling with an MZM, millimeter waves were optically generated with 

frequency hopping multiband OFDM UWB signals as optical subcarriers, and an EVM of less than 

21 dB after 20 km of fiber was achieved [25]. Using frequency doubling in an optical phase 

modulator, a 60-GHz millimeter-wave signal was successfully distributed over 10 km of fiber, 

together with microwave signal at 15 GHz and baseband signal, all at 2.5 Gb/s [27]. In addition, the 

use of two cascaded EAMs, an SOA, or highly nonlinear fiber was investigated for the generation of 

millimeter-waves [29]–[31]. It was shown that two cascaded EAMs may be efficient for frequency 

up-conversion if the two EAMs are carefully designed, which is considered a promising technique 

since the two EAMs can be integrated in one chip [29]. A full-duplex 62-GHz millimeter-wave over 

25-km fiber transmission system was verified, where an arrayed waveguide grating and an SOA 

were used for frequency up-conversion and wavelength reuse [30]. Frequency up-conversion was 

also obtained using four wave mixing in highly nonlinear fiber, and validated for 30 GHz 

millimeter-wave transmission over 25-km fiber at 2.5 Gb/s [31]. 

During 2010, a series of novel enabling techniques for microwave or/and millimeter-wave over 

fiber transmission systems have been investigated and demonstrated. We investigated optical 

subcarrier modulation using a low-cost EAM integrated laser for frequency hopping multiband 

OFDM UWB over fiber systems, showing that the system performance is limited by the EAM 

modulation nonlinearity induced nonlinear distortion [32], and is worse than that by the use of an 

MZM [33]. Therefore, maximum RF modulation power to drive an EAM is limited, resulting in low 



dynamic range of RoF systems. However, using the predistortion, mixed-polarization linearization, 

or other linearization techniques, the impact of EAM modulation nonlinearities was significantly 

reduced [9], [11], [12]. Transmission of millimeter-wave multiband OFDM UWB signals over fiber 

was also investigated using photonic frequency up- and down-conversion [22], [24], [25], as 

mentioned above. Transmission of both millimeter-wave wireless and baseband wired signals was 

validated with colorless WDM [27], [34], [35], showing that the future broadband wireless access 

can be incorporated into the FTTH infrastructure. A new WDM-RoF access network architecture 

supporting simultaneous transmission of 1.25 Gb/s wired and 63-GHz wireless signals was 

demonstrated using a reflective SOA [34]. Similarly, a 40-GHz millimeter-wave over 125-km fiber 

using four-wave mixing in an SOA for frequency up-conversion and a wired transmission, both at 

2.5 Gb/s in WDM-passive optical network (PON), was demonstrated [35]. An exciting demonstration 

was 300 GHz wireless transmission at 12.5 Gb/s based on RoF technology [36]. For very short-reach 

access, wireless over multimode or plastic fiber has also been found very promising to leverage the 

future broadband access networks [37]–[39]. Furthermore, It was demonstrated that RoF systems 

support future wireless multiple-input–multiple-output (MIMO) signals [40], [41]. Simultaneous 

transmission of multiservice MIMO wireless signals over in-building fiber to antennas for current 

wireless signals was evaluated [41]. 

Dynamic capacity allocation algorithms must be solved for RoF systems before their commercialization. 

A medium-transparent medium access control (MAC) was proposed and demonstrated 

in 60-GHz millimeter-wave over fiber networks and can be used for RoF over bus and RoF 

over PON with Poisson and burst-mode traffics [42]. Moreover, integrated Ethernet PON and WiMAX 

over fiber was investigated [43], showing that centralized scheduling is preferred for both Ethernet 

PON and WiMAX. Otherwise, fiber transmission of WiMAX is limited in fiber length. WiMAX over fiber 

distribution was validated in a field trial for 300-km/h high-speed train systems [44], indicating that 

fiber length is limited by time division duplex protocol of the current WiMAX standard. Dynamics 

using both 1-D (optical routing) and 2-D (optical routing and electrical subcarrier multiplexing) were 

demonstrated for wireless over multimode fiber systems [39]. Instead of using Internet protocol (IP) 

layer solution for multicasting services, a dynamic wavelength router was proposed and 

demonstrated to support multicasting, peer-to-peer, and dynamic capacity allocation [45], [46]. 

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