Page 290 - High Power Laser Handbook

258 So l i d - S t at e La s e r s Thin-Disc Lasers 259

in the amplifier to achieve the desired pulse energy. Therefore, it is
very important to design the resonator in a way that the resona-
tor’s internal losses are as small as possible. Otherwise the effi-
ciency is reduced dramatically.
Using a picosecond oscillator (pulse duration 1.8 ps) as a seed
laser allows for the amplification of picosecond pulses up to nearly
5 mJ energy at 1 kHz repetition rate and up to 1 mJ at 20 kHz. 62–64 Due
to gain narrowing in the amplifier, the pulse duration of the ampli-
fied pulses is extended to 4 ps. The beam is also for these pulses
nearly diffraction limited. Using higher repetition rates, about 0.3 mJ
65
at a repetition rate of 200 kHz were reached. The average laser
power is approximately independent of the repetition rate at such
high repetition rates.
Also Yb:K(WO ) ) is used as the laser active medium in thin-disc
4 2
pulse amplifiers. One advantage of this material is its much broader
gain spectrum compared to Yb:YAG. Therefore even shorter pulses
66
can be generated and amplified. In one experiment GTI mirrors
were used for keeping the pulses short during the amplification. As a
seed laser, a Yb:Glass oscillator that delivered pulses with a pulse
length of about 500 fs and an energy of about 1 nJ was used. These
pulses were amplified to more than 100 mJ with a pulse duration of
less than 900 fs.
It is also possible to omit the GTI mirrors. Using seed pulses with
a duration of 270 fs and no compensation of the increasing pulse
duration in the amplifier, but compressing the pulses after amplifi-
cation with a grating compressor, a pulse length of 250 fs was demon-
strated at an output energy of 116 mJ and at a repetition rate of 40 kHz,
67
using Yb:KY(WO ) ). It is remarkable that this result was achieved
4 2
without chirped pulse amplification (CPA). Due to the large beam
diameter inside the amplifier (2 mm in the Pockels cell) the pulse
lengthening by non-linear effects could be limited to pulse duration
values below 1 ps.
This result was further scaled with an Yb:KLu(WO ) ) thin-disc.
4 2
Figure 10.29 shows the pulse energies for different pump powers at
50 kHz repetition rate. Up to 395 mJ could be reached with this setup.
Figure 10.30 shows the autocorrelation traces for different pulse ener-
gies. With a pulse energy of 315 mJ, the pulse duration after amplifica-
tion and compression is significantly shorter than the seed pulse
duration. This is due to the spectral broadening of the pulse in the
BBO crystal used for the Pockels cell.

10.7.4 High Pulse Energy Thin-Disc Lasers
It is also possible to generate higher pulse energies with thin-disc
amplifiers. As already mentioned, 280 mJ at 100 Hz and 25 ns were
reached with cavity dumping. With a regenerative amplifier, also
240 mJ at 100 Hz and 8 ns were realized. 58