Page 308 - High Power Laser Handbook

276 So l i d - S t at e La s e r s Heat-Capacity Lasers 277

Nd: YAG 4-slab system
12 1
Output reflectivity = 85%

10
0.8
Intensity
Output intensity (kW/cm 2 ) Gain coefficient (%/cm) 6 Fluence 0.6 Output fluence (J/cm 2 )
8

0.4
4
Gain coefficient
0.2
2

0 0
0 0.1 0.2 0.3 0.4 0.5
Time (ms)
Figure 11.10 Calculated output variables for a four-slab Nd:YAG oscillator.

As an example of the type of parameter studies that may be
done, in Fig. 11.11 we show the output power as a function of slab
count and output-coupler reflectivity. The equivalent unstable reso-
nator magnification (M = 1/ R ), as well as the measured output
oc
power for the M = 1.5 case, is shown. As expected, systems with
higher slab count tend to optimize at higher values of magnification
due to the increased amount of gain in the propagation direction.
Finally, we show in Fig. 11.12 the dependence of output power on
the temperature of the slab. As mentioned earlier, the output power is
reduced as the temperature is increased, due both to thermal popula-
tion of the lower laser level and to thermal depopulation of the upper
laser level. The calculation shown is for a seven-slab system, produc-
ing roughly 75-kW output power at the initial temperature of 300°K.
For relatively limited temperature increases of 100°K, the output
power at the end of the burst is about 80 percent of the initial power.
We have found that a typical temperature rise/pulse is approximately
0.05°K. Thus, a 10-s burst at 200 Hz raises the temperature on the
order of 100°K.