Page 365 - High Power Laser Handbook

334 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 335

reveal that the main thermal impedance is determined by the heat sink
for pump spots larger than ~450 mm in diameter and pump power den-
2 7,21
sities around 10 kW/cm . Although this does not make it impossible
to further scale the pumped area, and thus the output power, the per-
formance will be affected by the temperature increase, and the loss in
efficiency will ultimately cancel the benefits of a larger size. However,
further scaling of the pump spot size is feasible using heat sink materi-
als with a better thermal conductivity, such as diamond.
As an example, we discuss the thermal management of the VEC-
SEL currently generating the highest CW power in the fundamental
10
transverse mode (Fig. 13.3). The structure’s GaAs wafer has been
removed, and mounted on a diamond heat sink. In Fig. 13.3a, the
output power is shown as a function of the pump power. Already at
a pump power of 30 W, an output power of 12.6 W is generated at
42 percent total optical-to-optical efficiency. The maximum output
power of 20 W is achieved for 50 W of pump radiation at 40 percent
efficiency. At 30 W pump power, the incident pump power density is
16.6 kW/cm²; at 50 W, it is 27.6 kW/cm². Figure 13.3b shows the
calculated temperature difference between the maximum tempera-
ture in the gain region and that of the heat sink using a standard
finite-element simulation. The temperature increase as a function of
the pump mode radius is given for the two discussed pump intensi-
ties (27.6 kW/cm²: solid gray line; 16.6 kW/cm²: dashed gray line).
The vertical line indicates the 240-mm pump radius used in the
experiment. At the highest pump intensity, we obtain a temperature
increase of 40 K for the 240-mm pump radius. A comparison with an
unprocessed gain structure on a 600-mm thick GaAs wafer shows the
importance of thermal management for power scaling (black curve).

100
20 50 80 Gain structure
on GaAs
Output power (W) 15 30 Opt.-to-opt. efficiency (%) ∆T (K) 60 Gain structure ~4×
40
on diamond
10
40
20
5
10 20
0 0 0
0 10 20 30 40 50 10 10 10 10
Incident pump power (W) Mode radius on gain (µm)
(a) (b)
Figure 13.3 (a) Output power of the currently highest fundamental mode continuous-
wave VECSEL versus the incident pump power. The mode radius is 240 µm, and the
gain structure is mounted on a diamond head spreader. (b) Finite-element simulation
of the heating of the gain structure versus the mode radius on the gain at a fixed
pump and heating intensity. The dashed lines correspond to an incident pump power
density of 16.6 kW/cm , or 30 W of incident pump power in (a); the solid lines
2
correspond to 27.6 kW/cm , or 50 W of incident pump power. 10,24
2