2.1 Ultrafast solid-state lasers - ETH - the Keller Group
2.1 Ultrafast solid-state lasers - ETH - the Keller Group
2.1 Ultrafast solid-state lasers - ETH - the Keller Group
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72 <strong>2.1</strong>.3 Overview of ultrafast <strong>solid</strong>-<strong>state</strong> <strong>lasers</strong> [Ref. p. 134<br />
has been demonstrated [04Lor]. With <strong>the</strong> QD-SESAM we can resolve <strong>the</strong> saturation issue for<br />
higher pulse repetition rates (i.e. shorter cavities) with nearly equal laser beam sizes in <strong>the</strong> gain<br />
and <strong>the</strong> absorber to obtain a stable cavity design. This requires a lower saturation fluence. With<br />
QD-SESAMs we can obtain both a low saturation fluence and a sufficiently low modulation depth<br />
with <strong>the</strong> optimization of <strong>the</strong> dot density and <strong>the</strong> design of <strong>the</strong> structure (i.e. moving from a antiresonant<br />
to a resonant design) [07Maa]. This is not possible with quantum-well absorbers. With<br />
an optimized QD-SESAM consisting of only one single self-assembled InAs quantum-dot layer at<br />
low growth temperatures we succeeded to push <strong>the</strong> repetition rate of passively modelocked VEC-<br />
SELs up to 50 GHz [06Lor]. In addition, <strong>the</strong>se QD-SESAMs allowed for <strong>the</strong> first demonstration of<br />
passively modelocked VECSEL with an integrated saturable absorber layer in <strong>the</strong> gain structure<br />
[07Bel, 07Maa]. This will ultimately offer <strong>the</strong> potential for wafer-scale fabrication and operation at<br />
even higher repetition rates. We refer to this device as <strong>the</strong> Modelocked Integrated eXternal-cavity<br />
Surface Emitting Laser (MIXSEL). Such <strong>lasers</strong> could become an enabling technology basis for<br />
ultra-compact high-repetition-rate laser devices suitable for cost-efficient high-volume fabrication.<br />
The ultimate goal is to extend <strong>the</strong> excellent results with optically pumped VECSELs to electrical<br />
pumping. However, this is not just a simple extension even though very promising results have<br />
been achieved in <strong>the</strong> cw regime with 500 mW average output power [03McI]. Initial mode-locking<br />
results reported however only very moderate average output power well below 100 mW [04Zha2].<br />
For comparison, it is also instructive to consider briefly <strong>the</strong> performance of pulsed edge-emitting<br />
semiconductor diode <strong>lasers</strong>, which can exhibit <strong>the</strong> highest pulse repetition rates of any optical<br />
source. The obvious advantages of compactness, efficiency of pumping, and ease of manufacture<br />
and integration make <strong>the</strong>se sources primary candidates for applications such as optical time-domain<br />
multiplexing, microwave carrier generation and optical clock recovery. The efficiency of direct modulation<br />
of <strong>the</strong> diode current falls off exponentially with increasing frequency above <strong>the</strong> diode relaxation<br />
resonance, which lies typically in <strong>the</strong> range 1–10 GHz: Thus <strong>the</strong> highest repetition frequencies<br />
are achieved using mode-locking of monolithic diode <strong>lasers</strong>, with gain, saturable absorption and/or<br />
external modulation all built into a single chip. Mode-locked edge-emitting diodes are immensely<br />
versatile in repetition frequency, from individual gain-switched pulses, through <strong>the</strong> microwave region<br />
of <strong>the</strong> spectrum and up to THz. The various schemes developed to realize <strong>lasers</strong> of this type<br />
have been reviewed by Avrutin et al. [00Avr]. Passive mode-locking, with a reverse-biased saturable<br />
absorber section included in <strong>the</strong> monolithic cavity, is particularly well-adapted to <strong>the</strong> generation<br />
of ultrashort pulses at high repetition rate because it does not require electrical modulation, which<br />
imposes a bandwidth limitation on repetition rate, and also impresses phase structure on <strong>the</strong> pulses.<br />
The first demonstration of such a monolithic device was reported by Vasil’ev et al. [89Vas], who<br />
reported a 100-GHz train of 2.5-ps pulses from an AlGaAs/GaAs injection laser, corresponding to<br />
fundamental mode-locking of <strong>the</strong> 380-μm long cavity. The highest output power to date is 250 mW<br />
at 4.3 GHz [06Pla]. Unfortunately, such high average power cannot be extended to pulse repetition<br />
rates well above 10 GHz because gain guiding at higher current densities gives rise to higher-order<br />
transverse modes. In addition, edge-emitting semiconductor <strong>lasers</strong> have strongly asymmetric beam<br />
profiles, which often need to be corrected with precisely mounted lenses. Typically, <strong>the</strong> same epitaxial<br />
layer forms both <strong>the</strong> gain (with a forward-biased section) and <strong>the</strong> saturable absorber (with<br />
a reverse-biased section) – and can <strong>the</strong>refore not be optimized independently. The long interaction<br />
length in <strong>the</strong> device introduces significant dispersion and nonlinearities. It is also challenging to<br />
fabricate an edge-emitting laser cavity length with a very precise pulse repetition rate and have<br />
this laser synchronized to an external reference clock. A more extensive recent review of ultrashort<br />
pulse generation with edge-emitting semiconductor <strong>lasers</strong> is given in [95Jia, 95Vai, 03Del, 00Avr].<br />
Higher output power can be achieved from Semiconductor Optical Amplifiers (SOAs) [92Del,<br />
95Del]; it is outside <strong>the</strong> scope of this review. However, mode-locking of SOAs in external cavities<br />
currently attracts considerable interest. It involves extreme pulse chirping, so that <strong>the</strong> amplifier is<br />
re-pumped during <strong>the</strong> passage of <strong>the</strong> pulse. Stretching and external recompression of <strong>the</strong>se pulses<br />
is accomplished using chirped fiber Bragg gratings, with dispersion > 1600 ps nm −1 . A system of<br />
Landolt-Börnstein<br />
New Series VIII/1B1