Page 363 - High Power Laser Handbook

332 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 333

AR coating (~1 µm)

Gain material
(100–300 µm)
AR coating (~1 µm)
Active region (~1 µm)

HR coating HR coating
(~4–5 µm) (~4–5 µm)

Metallic layer Metallic layer
VECSEL Solid-state TDL
Figure 13.2 Comparison of the composition of a VECSEL and a TDL disk. Although
the coatings are of comparable thickness, the active region in the VECSEL is roughly
two orders of magnitude thinner than the TDL crystal.

Despite the similarities outlined above, there are basic differences
in terms of thermal management between solid-state TDLs and
VECSELs. The most obvious difference is the thickness of the active
region (see Fig. 13.2), which in both cases is sandwiched between a
roughly 1-mm thick AR coating and a 4- to 5-mm thick HR distributed
Bragg reflector (DBR) coating. Although typical active regions of
VECSELs exhibit a thickness of around 1 mm, the significantly lower
3+
absorption efficiency of Yb -doped materials requires a thickness on
the order of 100 mm to achieve a good absorption efficiency, even for
the multipass pump concept described previously. Furthermore, the
thermal conductivity for semiconductors is much higher than for
suitable crystalline insulator host materials of Yb ions (e.g., YAG).
3+
Consequently, the normalized thermal resistance of a semiconductor
disk is much lower than that of a YAG disk (see Table 13.1), which
allows for significantly higher pump power densities of more than
2
30 kW/cm in VECSELs, even in single-mode operation. In contrast,
10
2
the pump intensity in a solid-state TDL is typically below 15 kW/cm and
is even lower for fundamental mode operation (see Table 13.1).
23
However, the typical pump beam diameters in VECSELs are smaller,
leading to a higher absolute thermal resistance—that is, to a larger
temperature increase for the same heating power. The high thermal
conductivity of the semiconductors requires a heat sink material with
an even higher thermal conductivity. According to the scaling law for
lasers in the thin-disk geometry, the output power increases linearly
with the pump and laser mode area if the pump density is kept con-
stant and the heat flow is dominated by a one-dimensional propaga-
tion into the heat sink.
As an example, we consider a 5-mm thick AlGaAs VECSEL struc-
ture directly mounted on a copper heat sink. Numerical calculations