Page 198 - High Power Laser Handbook

166 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 167

Figure 7.2 SSL slabs cut from Nd:YAG boules.

(Fig. 7.2). Boule length is also limited by doping gradients that arise
from increasing concentrations of the dopant in the melt as growth
6
proceeds. The maximum clear aperture that can be cut from such a
boule is typically about one-third its diameter due to the need to avoid
7
low-optical quality areas in the boule that exhibit growth striations.
A significant development in the past decade has been the emergence
of high-optical-quality microcrystalline ceramics, which have largely dis-
placed bulk crystalline hosts in high-power SSLs. These ceramic materi-
8
als are fabricated from high-purity crystalline nanopowders that are
pressed and sintered into the desired final shape. Because interstitial
regions between the individual microcrystal domains are much smaller
than an optical wavelength, the sintered material can exhibit excellent
transparency and homogeneity. The sintering fabrication process elimi-
nates the size constraints imposed by crystal growth and has enabled the
production of finished YAG pieces with greater than 10 x 10 cm clear
2
apertures, including co-sintered structures comprised of different doping
concentrations or entirely different materials (see Chap. 11). The spectro-
scopic, thermal, and mechanical properties of finished ceramics tend to
be nearly identical or superior to those of crystalline YAG. However,
ceramic YAG has been shown to be somewhat more resistant to thermal
stress fracture than crystalline YAG, because there are no contiguous
9
cleave boundaries and more energy is required to propagate a crack
between crystal domains than in a single-crystal lattice.

7.2.3 High-Average-Power SSL Materials
Virtually all HAP SSLs are based around YAG that is doped either with
Nd or Yb and that emits near 1064 nm or 1030 nm, respectively.
3+
3+
Several factors are responsible for these two materials’ dominance of