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336 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 337

Loss Loss Loss
Gain Gain Gain

Pulse Pulse Pulse
Time Time Time
(a) (b) (c)
Figure 13.5 Mechanisms of pulse shaping and stabilization using saturable
absorbers. (a) Fast saturable absorber: Net gain window is opened and
closed by the saturable absorber. (b) Slow saturable absorber and no
dynamic gain saturation: Long net gain window is opened by the saturable
absorber, and amplified background noise behind the pulse is swallowed with
a timing delay of the pulse caused by an attenuation of the leading edge by
the absorber in each roundtrip. (c) Slow saturable absorber with dynamic
gain saturation: Net gain window is opened by the saturable absorber and
closed by gain saturation.

a pulse. Depending on the recovery time of the SESAM and the satu-
ration dynamics of the gain, we can distinguish among three distinct
mechanisms of stable pulse formation.
The first mechanism, shown in Fig. 13.5a, relies on a fast, satura-
ble absorber with a recovery time shorter than the pulse duration.
Standard SESAMs, which are based on absorption bleaching in the
valence band to conduction band transition using quantum wells or
bulk materials, usually do not have such a dominant short recovery
time in the femtosecond-pulse-width regime. Thus, this mechanism
is particularly important for Kerr lens mode locking, 48–50 a technique
that has not yet been applied to disk lasers.
In the case of a slow saturable absorber, the recovery time is lon-
ger than the pulse duration, leading to two further mechanisms of
pulse formation. For gain materials with small gain cross sections, we
do not observe any significant change of the saturation of the gain for
each pulse—the so-called dynamic gain saturation. In this case, the
gain remains constant and is only saturated with the laser’s CW
power (Fig. 13.5b). This case is of particular importance for rare earth–
doped solid-state gain materials, because the 4f transitions, which are
relevant for most laser processes in this material group, are only
weakly allowed. They exhibit long lifetimes in the order of hundreds
of microseconds to several milliseconds. According to basic laser
equations, they consequently have low-gain cross sections in the
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