Limitations to the quality of optical phase conjugation by stimulated Brillouin scattering

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 1993.The process of optical phase conjugation by stimulated Brillouin scattering is studied both theoretically and experimentally, with emphasis on aspects that apply to phase-conjugate lasers. A theoretical formalism based on decomposing the optical field into a set of orthogonal modes is presented; this formalism is then used as the basis for a numerical modeling effort. Many of the steady-state aspects of stimulated Brillouin scattering are investigated including: the distribution of energy in the interaction region, the spatial effects of distributed noise, phase-pulling of the Stokes wave and the effects of adding smooth aberrations to the pump beam. Among other things, it is shown that for focused Gaussian beams the distance from the focal point of the lens to the front of the Brillouin medium defines the pump depletion region. The study of SBS with pump beams that include smooth aberrations is used to investigate the limitations of optical phase conjugation by stimulated Brillouin scattering. It is shown that the presence of intensity maxima that do not coincide with a region in which the phase is varying rapidly in space can lead to poor phase conjugation; this situation can occur when the pump beam contains a significant amount of a smooth aberration such as astigmatism. It is determined that the limitations imposed by the presence of smooth aberrations on the pump beam may be avoided by ensuring that the SBS process occurs in the regime where the pump beam is virtually undepleted. Dynamic effects caused by the stochastic initiation of the stimulated Brillouin scattering process are also investigated. It is shown that both the amplitude and phase fluctuations, which are the signature of the initiating noise, may be suppressed by a judicious choice of pump geometry. In particular, by insisting that the product of the Brillouin linewidth and the transit time of light through the medium be less than unity, almost all phase and amplitude noise on the Stokes beam can be suppressed. In concluding the work, the design and construction of an efficient, inexpensive and robust Q-switched phase-conjugate laser is presented. The effects of feedback into the laser are eliminated by utilizing an entirely solid-state ring oscillator with a color-center Q-switch. The laser has been shown to produce transform-limited pulses from 40 to 400 ns long with energy up to one Joule per pulse and near diffraction-limited beam qualit