Abstracts Category : Other

Limitations to the quality of optical phase conjugationby stimulated Brillouin scattering

by Thomas Richard Moore

The process of optical phase conjugation bystimulated Brillouin scattering is studied both theoretically andexperimentally, with emphasis on aspects that apply tophase-conjugate lasers. A theoretical formalism based ondecomposing the optical field into a set of orthogonal modes ispresented; this formalism is then used as the basis for a numericalmodeling effort. </br>Many of the steady-stateaspects of stimulated Brillouin scattering are investigatedincluding: the distribution of energy in the interaction region,the spatial effects of distributed noise, phase-pulling of theStokes wave and the effects of adding smooth aberrations to thepump beam. Among other things, it is shown that for focusedGaussian beams the distance from the focal point of the lens to thefront of the Brillouin medium defines the pump depletion region. </br>The study of SBS with pump beams that includesmooth aberrations is used to investigate the limitations ofoptical phase conjugation by stimulated Brillouin scattering. It isshown that the presence of intensity maxima that do not coincidewith a region in which the phase is varying rapidly in space canlead to poor phase conjugation; this situation can occur when thepump beam contains a significant amount of a smooth aberration suchas astigmatism. It is determined that the limitations imposed bythe presence of smooth aberrations on the pump beam may be avoidedby ensuring that the SBS process occurs in the regime where thepump beam is virtually undepleted. </br>Dynamiceffects caused by the stochastic initiation of the stimulatedBrillouin scattering process are also investigated. It is shownthat both the amplitude and phase fluctuations, which are thesignature of the initiating noise, may be suppressed by a judiciouschoice of pump geometry. In particular, by insisting that theproduct of the Brillouin linewidth and the transit time of lightthrough the medium be less than unity, almost all phase andamplitude noise on the Stokes beam can be suppressed. </br>In concluding the work, the design andconstruction of an efficient, inexpensive and robust Q-switchedphase-conjugate laser is presented. The effects of feedback intothe laser are eliminated by utilizing an entirely solid-state ringoscillator with a color-center Q-switch. The laser has been shownto produce transform-limited pulses from 40 to 400 ns long withenergy up to one Joule per pulse and near diffraction-limited beamquality