Page 277 - High Power Laser Handbook

246 So l i d - S t at e La s e r s Thin-Disc Lasers 247

An instructive question is the extractable energy of a quasi-CW
pumped thin-disc. The reduced duty cycle reduces the need of thick-
ness optimization and the non-lasing condition facilitates the numer-
ical handling of the differential equation.
The “model system” is a Yb:YAG disc with a thickness of 600 mm,
Yb concentration 4.5 percent and a pump power of 16 kW (pump power
density 5 to 6 kW/cm²), but with a duty cycle of only 10 percent (for the
calculation of the average temperature in the active area). As a first
result, Fig. 10.14 shows that in this case, the ASE will strongly reduce
the achievable gain, and the gain will be saturated after less then 1 ms.
As the system is intended for energy extraction, it is also useful to
look at the extractable energy. In Fig. 10.15, this is done for discs with
different thicknesses. The product of doping concentration and thick-
ness was kept constant to facilitate the comparability of the results.
The influence of the thickness on the temperature is small due to the
low duty cycle. Obviously, the classical strategy of making the disc
very thin is no longer suitable at this energy level; the thickest disc
reaches the highest gain and also the highest extractable energy.
Up to now, all calculations were done with a HR coating which is
totally reflecting at all angles and wavelengths. Besides the technical
difficulties to realize such a coating, it is also beneficial to use a coating
with some loss for the ASE. Figure 10.16 shows results obtained with a
so-called “ideal” coating with a reflectivity of only 75 percent for angles

60
Double pass gain (%) 40

20
Without ASE
With ASE

0
0 500 1000 1500 2000 2500 3000
Time (µs)
Figure 10.14 Temporal development of the gain in a thin-disc at quasi-cw
pumping (10 percent duty cycle), with and without ASE. Doping concentration
4.5 percent, thickness 600 µm, pump power 16 kW, pump spot radius
9.8 mm.