Page 366 - 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

A 40-K temperature increase is already reached at a pump radius
below 30 mm at the higher pump intensity (or 60 mm at the lower
pump intensity). On the other hand, the diamond-mounted structure
is suitable for further power increase by enlarging the pump
diameter: Operating the laser at the 40 percent lower pump intensity
of 16.6 kW/cm² should allow for an increase of the pump spot radius
by roughly a factor of four, while maintaining the same 40-K temperature
increase. Considering this 16-fold increase of the pump area and the
slightly higher efficiency at the lower pump intensity, it should be
possible to increase the output power by nearly an order of magni-
tude to well above 100 W.
It is currently not clear which effects will finally limit the power
scaling in VECSELs. Additional challenges will arise at very large
pump radii, such as inversion depletion due to amplified spontane-
ous emission (ASE) inside the disk, which can strongly affect the
laser’s efficiency. 25
In solid-state TDL materials, the pump and laser mode diameters
can be scaled to several millimeters and even more than 1 cm, due to
the lower amount of generated heat per volume thanks to the lower
3+
quantum defect of Yb lasers and the lower pump power density.
Furthermore, the ratio of the total thermal impedance of the disk and
the heat sink, which is usually made from copper or copper tungsten,
is larger. Therefore, the heat will not accumulate in the heat sink. An
approach for overcoming the thermal limitations is to reduce the
quantity of generated heat. The main contribution results from the
quantum defect, which is the energetic difference between the pump
and laser photons. If the quantum defect is reduced, higher pump
3+
powers can be applied. For Yb -based solid-state TDLs, the quantum
defect is already very low. Yb:YAG is typically pumped at 941 nm,
and the laser wavelength is 1030 nm, resulting in a quantum defect of
less than 9 percent. However, rapid progress has been made in recent
years in developing new laser materials that are pumped directly into
3+
the zero-phonon line of the Yb ion. 26–34 Pump wavelengths around
975 nm reduce the quantum defect and thus the total generated heat
by nearly a factor of two.
In VECSELs, the quantum defect and the thermal load can be
reduced via in-well pumping. In this case, the pump wavelength is
chosen such that the incoming photons are only absorbed in the
35
quantum wells. The interaction of the pump light with the quantum
wells takes place in a small region a few nanometers in length,
which is much shorter than in barrier pumping, where the typical
interaction length is ~1 mm. Therefore, the fraction of absorbed pump
light in a single pass is significantly lower. The absorption efficiency
can thus be increased with the established multipass pump scheme
used for solid-state TDLs (see Fig. 13.1b). Another approach for
improving the absorption efficiency is based on resonant VECSEL