Page 269 - High Power Laser Handbook

238 So l i d - S t at e La s e r s Thin-Disc Lasers 239

10.5.5 Design Study for High-Power Thin-Disc Lasers
For the design of high power thin-disc lasers, two mounting designs
seem to be promising. The “classical” design is to solder the disc onto
a heat sink, the alternative is to use a transparent (e.g., undoped YAG)
supporting structure on top of the disc and apply the cooling directly
to the disc. Both concepts are sketched in Fig. 10.9. In this section,
they will be compared based on their mechanical behavior and their
thermal lens. In both cases, the Yb:YAG has a thickness of 180 mm and
a diameter of 60 mm. It is either soldered on CuW (thickness 1.5 mm)
or bonded to undoped YAG for direct cooling. The pump spot radius
is 11 mm, and the pump power is varied between 6.4 kW and 25.6 kW.
Based on the quasi-static model (c.f. Sec. 10.5.8, Fig. 10.12) this would
be sufficient for 14 kW of laser power with one disc.
In Fig. 10.10, the results of FEA calculations are given. The mechan-
ical behavior of both designs is quite different. Because the support
from the heat sink is missing, the directly cooled design is less stiff and
it shows tensile stress in radial direction. The classical design provides
better compensation of the azimuthal stress. Nevertheless, the total
temperature rise in the directly cooled design is smaller as there is no
additional thermal resistance of the heat sink and the solder layer. Due
to these lower temperatures, also the thermally induced stress inside
the disc is below the critical value of 130 MPa.
The heat sink does not only provide stiffness to the system, it does
also contribute to the deformation due to the temperature gradient.

AR coating
Yb:YAG
HR coating
Solder
CuW
Cooling fluid

AR coating
YAG
Yb:YAG
HR coating
Glue
CuW
Cooling fluid

Figure 10.9 Different mounting designs, top: Yb:YAG soldered on CuW (“classical”
thin-disc design); bottom: composite disc, directly cooled.