Page 225 - High Power Laser Handbook

194 So l i d - S t at e La s e r s Zigzag Slab Lasers 195

overlap the seals near the entrance and exit areas of the slab. This can
easily lead to damage, because the evanescent waves penetrate into
the medium outside the slab a distance comparable to the wavelength
of light. Thus, if the O-ring material is even slightly absorbing, it can
char and damage the slab. This problem can be mitigated by slab and
beam injection geometry, which creates small dead zones along the
TIR face where seals may be placed without risk of damage, as shown
in Fig. 8.6b. The dead zones are areas along the TIR face where the
beam does not touch the slab faces. This, in turn, leads to small, unex-
tracted volumes in the slab. If we define the fill factor F as the ratio of
the beam footprint on the TIR surface to the total zigzag footprint, the
2
fraction of volume extracted η is given as
η = F(2 - F ) (8.7)
This equation indicates that even if the dead zone is as large as
20 percent of the beam footprint, the reduction in extraction efficiency
is proportional to the fraction of unextracted volume, which, in this
example, is only 4 percent.
An alternative to creating dead zones is the use of an evanescent
wave coating. Recent advances in coating technology have enabled
the deposition of thick coatings (2 to 3 µm) of low-index material,
such as fused silica or MgF . If the slab is designed such that the
2
total internal reflection is at the slab-coating interface, this type of
coating can isolate the beam from anything that may be on the out-
side of the TIR face.

Scaling Limitations
As mentioned earlier, traditional side-pumped slabs rely on the slab
thickness for absorption of 808-nm diode light. For efficient absorption
(> 70 percent) in Nd:YAG at a typical 1.1 percent doping level, the slab
thickness must be ~4 mm or thicker. For a slab height of around 3 cm
(i.e., the maximum available for monolithic crystalline slabs), the 4-mm
thickness limits the slab to an extracted power of about 1 kW before the
stress level reaches a significant fraction of the fracture stress. Thus, for
further scaling using the side-pumped geometry, the user must either
accept lower efficiency or devise a method for using the unabsorbed
diode light. The latter can be achieved by stacking two or more thinner
slabs side by side. This, however, creates nonuniform pumping in the
thin dimension, which can cause the slab to bow. Recent advances in
ceramic materials have eliminated the ~3-cm height limitation and can
provide further scaling through taller slabs.
For Yb:YAG, the doping level can be higher—up to 100 percent
3
(stoichiometric). In principle, this would enable thinner slabs. How-
ever, recent work on thin-disk lasers and slabs has shown anomalous
loss mechanisms for highly pumped slabs at doping levels beyond
4
7 to 8 percent. This forces the user to either compromise efficiency or
use multiple absorption passes with low-doped material as described
in Chap. 10.