Page 287 - High Power Laser Handbook

256 So l i d - S t at e La s e r s Thin-Disc Lasers 257

600

550

500
Pulse duration (ns) 450 190 W pump power

400

350
160 W pump power
140 W pump power
300
110 W pump power
80 W pump power
250
60 W pump power
200
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Repetition rate (kHz)
Figure 10.25 Pulse duration of the q-switched thin-disc laser as a function
of the repetition rate for different pump power levels.

Figure 10.25 shows the pulse length of the pulses as a function of
the pulse repetition rate for different pump power levels. At low rep-
etition rates, the pulse duration is about 250 ns, whereas for higher
repetition rates, the pulses become longer—up to 570 ns at a 13-kHz
repetition rate. The reasons for these long pulses are the length of the
resonator (840 mm for fundamental mode operation) and the rela-
tively low gain per roundtrip of the disc—and, hence, the relatively
high reflectance of the outcoupling mirror. These restrictions in repeti-
tion rate and pulse duration (limited to pulse durations longer than
200 ns for the setup used) could be overcome by using thin-disc ampli-
fiers, which are described in Sec. 10.7.3. Alternatively, also the cavity
dumped operation described in Sec. 10.7.2 is a very flexible scheme
concerning pulse durations and repetition rates.

10.7.2 Cavity-Dumped Operation of the Thin-Disc Laser
Several possibilities exist for extracting the energy that is stored inside
a cavity. In the setup shown in Fig. 10.26 either the thin film polarizer
can be used as outcoupling mirror or the second harmonic generation
(SHG) in the SHG crystal can be used to extract the energy from the
cavity. 57,58
Applying the full quarter-wave voltage to the Pockels cell, the
outcoupling can be switched to 100 percent, creating pulses of some
tens of nanoseconds. By applying only a small voltage, one can reach a
kind of “cavity leaking” instead of cavity dumping with longer pulses.
In this case, the pulse duration and pulse energy can be controlled very