Page 260 - High Power Laser Handbook

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

Using multiple pump beam passes through the disc results in a
thinner disc or a lower doping concentration, thus reducing such ther-
mal effects like thermal lensing and stress in the disc. Another advan-
tage of this system is that it increases the effective pump power density
(nearly 4 times for 16 pump beam passes); thus, on the one hand, the
demands on the pump diode’s power density (beam quality) are
reduced, while on the other hand, quasi-three-level laser materials
(e.g., ytterbium-doped materials) can be used with this design.
Quasi-three-level materials offer the ability to build lasers with
the highest efficiency. However, they are hard to operate, because the
energy difference between the lower laser level and the ground level
is small, leading to a significant thermal population of the lower laser
level. Some amount of pump power density is necessary simply to
reach transparency at the laser wavelength, making it necessary to
pump the material with high pump power density in order to reach
threshold without increasing the crystal’s temperature too much.
Using multiple pump beam passes through the crystal is thus the key
to achieving low threshold and high efficiency, because it simultane-
ously helps reduce the thickness of the crystal and the doping con-
centration. This decoupling of laser and pump beam absorption is
essential for operating quasi-three-level systems. The limit for the
possible number of pump beam passes through the disc is given by
the beam quality of the laser diodes, which determines the beam
diameter on the parabolic mirror and, hence, the number of positions
on the mirror that can be used. The better the beam quality of the
pump laser diodes, the higher the number of pump beam passes and
the higher the total efficiency of the thin-disc laser.
When operating the disc in this setup, it is easy to scale the output
power simply by increasing the pump spot diameter, keeping the
pump power density constant. In addition, there is no need to increase
the brightness of the pump laser diodes.

10.4 Possible Laser Materials
Nearly all classical laser materials can be operated in the thin-disc
design, especially if the absorption of the pump radiation is rather
high and the lifetime of the excited state is not too short. The first
material used with the thin-disc laser was Yb:YAG; with this material
3+
most of the high power or high energy results were reached. Yb has
two important benefits: a small quantum defect and no parasitic
effects such as upconversion, cross relaxation, excited-state absorp-
3+
tion and so on. Laser operation of Yb with the thin-disc laser has
been demonstrated in a large variety of host materials and also other
active ions were successfully operated in the thin-disc laser setup.
Table 10.1 gives an overview of successful combinations without
intending to be exhaustive.