Page 212 - High Power Laser Handbook

180 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 181

a lens to correct for thermal focusing. Computer-controlled fabrica-
tion methods, such as magnetorheological finishing (MRF), provide
the capability to manufacture custom surface-relief profiles in silica
and other substrates, with spatial frequencies ~1 per mm and strokes
(wavefront amplitude) of multiple waves. 26,27 SPPs have been dem-
onstrated to increase brightness from both stable and unstable
resonators. 23,28
Although SPPs do provide simple methods of correction, they
can be cumbersome to implement in a high-fidelity system. Gain
module OPD can be rigorously calculated using numeric models, but
in an HP SSL, residual OPD is often driven by uncontrolled compo-
nent variations rather than by deterministic design; therefore, an SPP
must be custom fabricated for each laser. This requires that the laser
first be built and its wavefront measured at full power before the SPP
can be made. Moreover, any change in the laser’s thermal profile due
to changes in operating power, component degradation, or the influ-
ence of the SPP itself on the extracting beam can invalidate the old
23
wavefront map and require installation of a new SPP. Finally, it is
difficult to achieve ~λ/10 fidelity given the accumulated tolerances
in wavefront measurement, SPP manufacturing, and final installation
and alignment; thus, even with an SPP, it is difficult to directly obtain
near-diffraction-limited beams from large apertures.
To further correct laser wavefronts, dynamic methods are often
employed that can respond in real time to changes in the laser’s
aberrations.

7.5.2 Phase Conjugation
Phase conjugate mirrors (PCMs) represent attractive dynamic meth-
ods for wavefront correction of high-power lasers. A PCM differs
from a regular mirror in that it reflects the conjugate of an incident
wavefront. For example, whereas an incident diverging beam would
still be diverging after reflection from a regular mirror, it would be
converging after reflection from a PCM. This phase conjugation pro-
vides automatic correction of laser and optic wavefront aberrations
and beam jitters without active electronic controls.
One particularly successful implementation of PCMs in HP SSLs
has used stimulated Brillouin scattering (SBS) in liquid Freon. The
29
basic concept, implemented in a MOPA configuration, is shown in
Fig. 7.10. The low-power beam with a planar wavefront is incident on
the PA from the left. Upon the first pass through the PA, the beam is
amplified and aberrated. The aberrated beam is then focused into the
cell containing a Brillouin-active material. Electrostriction of the
material near the beam focus creates a longitudinal acoustic grating
whose transverse phase profile is identical to the optical wavefront of
the focused beam. After Bragg reflection from this moving grating,
the return beam has the conjugate wavefront of the forward beam, so