2.1 Ultrafast solid-state lasers - ETH - the Keller Group

72 2.1.3 Overview of ultrafast solid-state lasers [Ref. p. 134
has been demonstrated [04Lor]. With the QD-SESAM we can resolve the saturation issue for
higher pulse repetition rates (i.e. shorter cavities) with nearly equal laser beam sizes in the gain
and the absorber to obtain a stable cavity design. This requires a lower saturation fluence. With
QD-SESAMs we can obtain both a low saturation fluence and a sufficiently low modulation depth
with the optimization of the dot density and the design of the structure (i.e. moving from a antiresonant
to a resonant design) [07Maa]. This is not possible with quantum-well absorbers. With
an optimized QD-SESAM consisting of only one single self-assembled InAs quantum-dot layer at
low growth temperatures we succeeded to push the repetition rate of passively modelocked VEC-
SELs up to 50 GHz [06Lor]. In addition, these QD-SESAMs allowed for the first demonstration of
passively modelocked VECSEL with an integrated saturable absorber layer in the gain structure
[07Bel, 07Maa]. This will ultimately offer the potential for wafer-scale fabrication and operation at
even higher repetition rates. We refer to this device as the Modelocked Integrated eXternal-cavity
Surface Emitting Laser (MIXSEL). Such lasers could become an enabling technology basis for
ultra-compact high-repetition-rate laser devices suitable for cost-efficient high-volume fabrication.
The ultimate goal is to extend the excellent results with optically pumped VECSELs to electrical
pumping. However, this is not just a simple extension even though very promising results have
been achieved in the cw regime with 500 mW average output power [03McI]. Initial mode-locking
results reported however only very moderate average output power well below 100 mW [04Zha2].
For comparison, it is also instructive to consider briefly the performance of pulsed edge-emitting
semiconductor diode lasers, which can exhibit the highest pulse repetition rates of any optical
source. The obvious advantages of compactness, efficiency of pumping, and ease of manufacture
and integration make these sources primary candidates for applications such as optical time-domain
multiplexing, microwave carrier generation and optical clock recovery. The efficiency of direct modulation
of the diode current falls off exponentially with increasing frequency above the diode relaxation
resonance, which lies typically in the range 1–10 GHz: Thus the highest repetition frequencies
are achieved using mode-locking of monolithic diode lasers, with gain, saturable absorption and/or
external modulation all built into a single chip. Mode-locked edge-emitting diodes are immensely
versatile in repetition frequency, from individual gain-switched pulses, through the microwave region
of the spectrum and up to THz. The various schemes developed to realize lasers of this type
have been reviewed by Avrutin et al. [00Avr]. Passive mode-locking, with a reverse-biased saturable
absorber section included in the monolithic cavity, is particularly well-adapted to the generation
of ultrashort pulses at high repetition rate because it does not require electrical modulation, which
imposes a bandwidth limitation on repetition rate, and also impresses phase structure on the pulses.
The first demonstration of such a monolithic device was reported by Vasil’ev et al. [89Vas], who
reported a 100-GHz train of 2.5-ps pulses from an AlGaAs/GaAs injection laser, corresponding to
fundamental mode-locking of the 380-μm long cavity. The highest output power to date is 250 mW
at 4.3 GHz [06Pla]. Unfortunately, such high average power cannot be extended to pulse repetition
rates well above 10 GHz because gain guiding at higher current densities gives rise to higher-order
transverse modes. In addition, edge-emitting semiconductor lasers have strongly asymmetric beam
profiles, which often need to be corrected with precisely mounted lenses. Typically, the same epitaxial
layer forms both the gain (with a forward-biased section) and the saturable absorber (with
a reverse-biased section) – and can therefore not be optimized independently. The long interaction
length in the device introduces significant dispersion and nonlinearities. It is also challenging to
fabricate an edge-emitting laser cavity length with a very precise pulse repetition rate and have
this laser synchronized to an external reference clock. A more extensive recent review of ultrashort
pulse generation with edge-emitting semiconductor lasers is given in [95Jia, 95Vai, 03Del, 00Avr].
Higher output power can be achieved from Semiconductor Optical Amplifiers (SOAs) [92Del,
95Del]; it is outside the scope of this review. However, mode-locking of SOAs in external cavities
currently attracts considerable interest. It involves extreme pulse chirping, so that the amplifier is
re-pumped during the passage of the pulse. Stretching and external recompression of these pulses
is accomplished using chirped fiber Bragg gratings, with dispersion > 1600 ps nm −1 . A system of
Landolt-Börnstein
New Series VIII/1B1