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

88 2.1.4 Loss modulation [Ref. p. 134
the saturation fluence of ≈ 10 mJ/cm 2 is still rather high for stable solid-state laser mode-locking.
In comparison, epitaxially grown SESAMs typically have a saturation fluence in the range of
10 μJ/cm 2 depending on the specific device structure.
2.1.4.3.4 Historical perspective and SESAM structure
Semiconductor saturable absorbers have been used as early as 1974 in CO 2 lasers [74Gib] and 1980
for semiconductor diode lasers [80Ipp]. A color-center laser was the first solid-state laser that was
cw mode-locked with an intracavity semiconductor saturable absorber [89Isl]. However, for both the
diode and color-center laser, dynamic gain saturation supported pulse formation and the recovery
time of the slow saturable absorber was not relevant for pulse generation (Fig. 2.1.5a). In addition,
because of the large gain cross section (i.e. approximately 10 −14 cm 2 for diode lasers and 10 −16 cm 2
for color-center lasers) Q-switching instabilities were not a problem. This is not the case for most
other solid-state lasers, such as ion-doped solid-state lasers, which have typically 1000 or even
more times smaller gain cross sections. Thus, the semiconductor saturable absorber parameters
(Fig. 2.1.8 and Fig. 2.1.9) have to be chosen much more carefully for stable cw mode-locking.
We typically integrate the semiconductor saturable absorber into a mirror structure, which
results in a device whose reflectivity increases as the incident optical intensity increases. This
general class of devices is called SEmiconductor Saturable Absorber Mirrors (SESAMs) [92Kel2,
96Kel, 03Kel]. A detailed description and guideline how to design a SESAM for either passive modelocking
or Q-switching for different laser parameters is given in recent book chapters [99Kel, 03Pas].
SESAMs are well-established as a useful device for passive mode-locking and Q-switching of many
kinds of solid-state lasers. The main reason for this device’s utility is that both the linear and
nonlinear optical properties can be engineered over a wide range, allowing for more freedom in
the specific laser cavity design. In addition, semiconductor saturable absorbers are ideally suited
for passive mode-locking solid-state lasers because the large absorber cross section (in the range
of 10 −14 cm 2 ) and therefore small saturation fluence is ideally suited for suppressing Q-switching
instabilities [99Hoe1].
Initially, SESAMs for solid-state lasers were used in coupled cavities [90Kel2, 92Kel1], because
these early SESAM designs introduced too much loss for the laser cavity (Fig. 2.1.12b). In
1992, this work resulted in a new type of intracavity saturable absorber mirror, the Antiresonant
Fabry–Perot Saturable Absorber (A-FPSA) [92Kel2, 94Kel] where the absorber was integrated
inside a Fabry–Perot structure of which the bottom reflector was a high reflector (i.e. approximately
100 %) (Fig. 2.1.12c). This was the first intracavity saturable absorber design that allowed
for passive mode-locking of diode-pumped solid-state lasers without Q-switching instabilities. The
Fabry–Perot was operated at antiresonance to obtain broad bandwidth and low loss. The A-FPSA
mirror was mainly based on absorber layers sandwiched between the lower semiconductor and the
higher SiO 2 /TiO 2 dielectric Bragg mirrors. The top reflector of the A-FPSA provides an adjustable
parameter that determines the intensity entering the semiconductor saturable absorber and therefore
the saturation fluence of the saturable absorber device. Therefore, this design allowed for a
large variation of absorber parameters by simply changing absorber thickness and top reflectors
[95Bro1, 95Jun1]. This resulted in an even simpler SESAM design with a single quantum well
absorber layer integrated into a Bragg mirror [95Bro2] (Fig. 2.1.12d) – this was later referred to
as Saturable Bragg Reflectors (SBRs) [95Tsu].
In the 10-fs regime with Ti:sapphire lasers we have typically replaced the lower semiconductor
Bragg mirror with a metal mirror to support the required large reflection bandwidth
[96Flu1, 97Jun1]. No post-growth processing is required with an ultrabroadband monolithically
grown fluoride semiconductor saturable absorber mirror that covers nearly the entire gain spectrum
of the Ti:sapphire laser. Using this SESAM inside a Ti:sapphire laser resulted in 9.5-fs pulses
[02Sch2]. The reflection bandwidth was achieved with a AlGaAs/CaF 2 semiconductor Bragg mirror
[00Sch]. More recently a broadband SESAM was fabricated by increasing the reflection bandwidth
Landolt-Börnstein
New Series VIII/1B1