Page 213 - High Power Laser Handbook

182 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 183

SBS cell
MO Amplifiers
Polarizer λ/4 plate

Clean Aberrated Hypersonic wave
input input
caused by
electrostriction
Aberrations

Focal region
Clean Invetred in SBS liquid
output output

Figure 7.10 SBS-based phase conjugate mirror configured with a MOPA.

that upon the return transmission through the aberrated PA, the orig-
inal planar wavefront is recovered at the MOPA output port.
PCMs can be formed using a variety of linear and nonlinear phys-
ical mechanisms and materials. Although SBS works particularly
well with pulsed, high-peak-intensity SSLs, lower-threshold PCMs
have been demonstrated using SBS in multimode fibers and in free
30
space using photorefractive or thermal gratings. 31
Despite the attractive simplicity of a PCM, they are not always
feasible to implement on an HP SSL. Each PCM mechanism, whether
SBS, thermal, or photorefractive, constrains the incident laser’s oper-
ating regime. For example, SBS in Freon has a high threshold and
requires a long interaction length to build up sufficient reflectivity
from the acoustic grating; thus, it does not work well with anything
other than pulsed, single-frequency lasers with long coherence
lengths. The dynamic range in power of most thermal PCM configu-
rations is also limited. Typically the reflection from a PCM is signifi-
cantly less than unity, requiring a high-gain geometry to avoid
substantial loss of efficiency. Finally, the conjugation range of any
PCM will be limited—essentially, the input wavefront aberrations
must be of sufficiently low amplitude and spatial frequency such that
the beam does not break into separate spots near the focus. This yields
a set of conjugate returns from each spot with uncorrelated phases
that will not yield planar output after the second PA pass.
7.5.3 Adaptive Optics
Adaptive optics (AO) provides a more flexible and engineerable
means of wavefront control than phase conjugation. This capability
32
comes at the cost of added complexity in the form of active control