Page 281 - High Power Laser Handbook

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

Besides the results on maximum power or efficiencies which are
35
presented in the mentioned paper , there is especially one important
feature in Eq. (10.32): the strong influence of the thermal load param-
eter C (cf. Sec. 10.5.1) and of the internal loss b inside the resonator.
This relation also holds for efficiency calculations.
Based on considerations similar to the ideas presented in Sec.
10.5.10 and Fig. 10.13, a slightly different expression for the reduced
lifetime was found: 36
τ
τ ASE ≈ exp(2 g + gEi(2 gr h + ) 2 p / ) 2 gEi(2 g) (10.33)

This can be approximated by

r  r 2 
τ ~ τ p exp −  p g (10.34)
ASE h   h  
With the lifetime reduction from Eq. (10.33), a similar dependence of
maximum output power from the internal loss and the thermal load
parameter as in Eq. (10.32) can be derived. The results are clearly
beyond actually possible or planned thin-disc designs, but even more
than 20 MW from one disc seem feasible if the internal loss b inside
the resonator would be reduced to 0.25 percent—but requiring a
pump spot diameter of ~5.5 m.
The achievable efficiencies are small (less than 10%), but follow-
ing both papers, 35,36 higher efficiencies are possible with a slightly dif-
ferent optimization. With an internal loss of 0.25 percent, 1 MW laser
power will be possible with nearly 50 percent optical-optical effi-
ciency. Only a pump spot diameter of 20 cm will be required for this
laser power.

10.6 Thin-Disc Laser in Continuous-Wave Operation

10.6.1 High Average Power
Very high laser output power can be achieved from one single disc by
increasing the pump spot diameter while keeping the pump power
density constant. 37, 38 The highest output power reported for a single
disc is 6.5 kW. 39
Figure 10.18 shows one example for high output power with high
efficiency from a single disc (Trumpf Laser). More than 5.3 kW of
power has been achieved with a maximum optical efficiency of more
than 65 percent. This high efficiency of the thin-disc laser results in a
very high electrical efficiency for the total laser system—greater than
25 percent for industrial lasers with 8-kW output power and a beam
propagation factor M² of less than 24.