Page 259 - High Power Laser Handbook

228 So l i d - S t at e La s e r s Thin-Disc Lasers 229

large surface-to-volume ratio, heat dissipation from the disc into the
heat sink is very efficient, thus allowing for operation at extremely
high volume power densities in the disc (up to 1 MW/cm³ absorbed
pump power density).
The crystal can be pumped in a quasi-end-pumped scheme. In
this case, the pump beam hits the crystal at an oblique angle. Depend-
ing on the thickness and doping level of the crystal, only a fraction of
the pump radiation is absorbed in the laser disc. Most of the incident
pump power leaves the crystal after being reflected at the back side of
the disc. By successive redirecting and reimaging of this part of the
pump power onto the laser disc, the absorption can be increased.
A very elegant way to increase the number of pump beam passes
through the disc is shown in Fig. 10.2. The radiation of the laser diodes
for pumping the disc is first homogenized either by fiber coupling of
the pump radiation or by focusing the pump radiation into a quartz-
rod. The end of either the fiber or the quartz rod is the source of the
pump radiation, which is imaged onto the disc using a collimation
optic and the parabolic mirror. In this way a very homogeneous pump
profile with the appropriate power density in the disc can be achieved,
which is necessary for good beam quality. The unabsorbed part of the
pump radiation is collimated again at the opposite side of the para-
bolic mirror. This beam is redirected via two mirrors to another part
of the parabolic mirror, where the pump beam is focused again onto
the disc, but this time from a different direction. This reimaging can
be repeated until all the (virtual) positions of the parabolic mirror
have been used. At the end, the pump beam is redirected back to the
source, thereby doubling the number of pump beam passes through
the disc. In this way, up to 32 passes of the pump radiation through
the disc have been realized and more than 90 percent of the pump
power will be absorbed in the disc.

Pump beam Pump beam
Parabolic
mirror

Outcoupling
mirror

Deflection Thin disc
prisms crystal on
heat sink

Figure 10.2 Pump design of the thin-disc laser with 24 pump beam passes.
(Courtesy of Institute of Laser Physics, University of Hamburg)