Page 282 - High Power Laser Handbook

250 So l i d - S t at e La s e r s Thin-Disc Lasers 251

6 70

60
5
50
Laser power (kW) 3 40 Optical efficiency (%)
4

30
2
20
1
10

0 0
0 2 4 6 8 10
Pump power (kW)
Figure 10.18 Output power and optical efficiency from a single disc.
(Courtesy of Trumpf Lase)

An alternative way to scale the output power is to use several
discs in one resonator. Figure 10.19 shows the design of a laboratory
setup for high beam quality in which four discs are coupled together
in one resonator. Figure 10.20 shows the output power and the optical
efficiency of such a laser as function of the pump power. The high
beam quality is made possible by the concept of neutral gain modules.
For this concept, the discs are optically combined to modules which
40
have a minimum effective optical length and refractive power.
Figure 10.21 shows a further example of power scaling by combina-
tion of several disks in one resonator, delivering more than 20 kW of
output power, but with reduced beam quality.

10.6.2 Fundamental Mode, Single Frequency and
Second Harmonic Generation (SHG)
High-power thin-disc lasers in the kilowatt-power range are typically
operated with a beam propagation factor (beam quality) M² of about
20 (i.e., the laser beam’s focusability is 20 times worse compared the
theoretical limit M² = 1). This is sufficient for the typical demands of
welding or cutting applications. Beyond this beam quality, the thin-
disc laser design also offers the possibility to operate high-power
lasers in the fundamental mode (M² = 1) 31,41–43 due to the disc’s small
thermal effects and small optical distortions.
Using a resonator design which has a stable fundamental mode
diameter of 70 to 80 percent of the pump spot diameter, it is possible
to achieve high laser output power with high optical efficiency. This