Page 369 - High Power Laser Handbook

338 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 339

–18
–21
2
order of 10 to 10 cm . Semiconductor lasers, on the other hand,
rely on allowed transitions with upper-state lifetimes of a few nano-
seconds or less and thus have cross sections higher by several orders
of magnitude (see Table 13.1). Semiconductor lasers exhibit signifi-
cantly lower saturation energy E , according to
sat
hν
⋅
E = A F = A⋅ (13.1)
sat sat σ +σ
em abs
with mode area A and saturation fluence F , as well as photon energy
sat
hν and absorption and emission cross sections σ and σ , respec-
abs
em
tively, at the signal wavelength. This lower saturation energy gives
rise to dynamic gain saturation during propagation of the pulse
through the active region (Fig. 13.5c). This dynamic gain saturation is
then partially or fully recovered between two consecutive pulses.
As shown in Fig. 13.5b, the SESAM’s slow recovery time, in com-
bination with a constant gain saturation, leads to a long net gain win-
dow after the pulse. Intuitively, one would expect background noise
behind the pulse to be amplified as well. However, because the pulse’s
leading edge is absorbed each time the pulse hits the absorber, the
center of the pulse is shifted backward each roundtrip, and amplified
noise is swallowed by the pulse after some roundtrips. Numerical
simulations reveal that the pulse duration can be even more than one
order of magnitude shorter than the SESAM recovery time. To
52
achieve shorter pulse durations in the femtosecond regime, further
pulse stabilization mechanisms are necessary. This can be achieved by
introducing a well-balanced amount of self-phase modulation (SPM)
and group delay dispersion (GDD) into the cavity. The resulting pulses
can be considered as solitons; therefore, the typical mode-locking pro-
cess in solid-state TDLs is known as soliton mode locking. 53–55
In most VECSELs, the following dynamic gain saturation process,
depicted in Fig. 13.5c, takes place: A net gain window is opened by
saturating the SESAM. This temporal window is closed by the
dynamic saturation of the gain induced by the pulse in the active
region. This mechanism is typical for lasers with larger gain cross sec-
tions, such as semiconductor and dye lasers. To enable stable mode
56
locking with an open net gain window, it is obvious that the SESAM’s
saturation energy must be smaller than that of the gain, so that the
absorber saturates first. In addition, a shorter net gain window can be
obtained when the absorber recovers faster than the gain.

13.4.2 Different Operation Regimes
Continuous wave (CW) TDLs deliver several kilowatts of output
9]
57
power and several 100 W in diffraction-limited beam quality. Such
performance enables mode-locked operation with average output pow-
ers and pulse energy levels that cannot be obtained directly from any