Page 372 - High Power Laser Handbook

340 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 341

Although today’s ultrafast solid-state TDLs typically operate in
the average output power regime of several tens of watts, the output
power of mode-locked VECSELs is often in the order of 100 mW. The
first passively mode-locked VECSEL was also realized in 2000 and
had an average output power of 21.6 mW in 22-ps pulses at a repeti-
76
tion rate of 4 GHz. Since then, the average power has been scaled to
2.1 W with a pulse duration of 4.7 ps and a similar repetition rate.
38
Further scaling above the 10-W regime seems feasible, as nearly an
order of magnitude higher output power in fundamental mode oper-
ation has recently been demonstrated. 10
Table 13.1 shows a comparison of the output data and other
parameters of the VECSEL and the Yb:YAG-based TDL with the high-
est average output powers in mode-locked operation.

Repetition Rate
Section 13.4.1 explained that VECSELs and solid-state TDLs rely on
different pulse formation mechanisms: Whereas mode-locked VECSELs
rely on dynamic gain saturation, the gain in TDLs is saturated to a
constant level according to the laser’s average power. In addition,
TDLs, with their low-gain cross sections and long upper-state lifetimes,
carry the risk of unwanted Q-switching instabilities. The reason for
these instabilities is that pulses with higher energies within typical
noise fluctuations can saturate the SESAM more strongly and there-
fore reduce their losses. The gain then must respond fast enough with
additional saturation; otherwise, there will be positive feedback, and
the noise pulse will be able to increase its energy further. This would
destabilize the damping of relaxation oscillations and could lead to
stable Q-switched mode locking (QML) or only Q-switching insta-
bilities. In a stable QML regime of operation, the laser output consists
of mode-locked pulses underneath a Q-switched envelope. (Typi-
cally, the mode-locked pulse repetition rate is in the order of 100 MHz,
determined by the laser cavity length, whereas Q-switching modula-
tions usually have frequencies in the kilohertz region, similar to the
typical relaxation oscillations of TDLs.) One way to protect a laser from
QML is to decrease the repetition rate, thus increasing the pulse energy
87
in such a way that the gain saturation sets in faster. Hönninger et al.
developed a stability criterion for the minimum required pulse energy
E in comparison to the saturation energies of the gain material E sat,gain
p
and the saturable absorber E sat,abs :
2
E
E > E sat,gain sat,abs D R (13.2)
p
This equation shows that low saturation energies and a low modula-
tion depth DR are also beneficial to prevent the laser from Q-switching
instabilities. For the femtosecond laser, the QML threshold is typi-
cally five times lower, because the soliton effect, together with the