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Annual Meeting Optical Society of America

Incoherent phase-conjugate backscattering in photorefractive media

Mordechai Segev, Amnon Yariv

Published Jan 1, 1991• 0 citations
Publisher

Abstract

Phase-conjugate backscattering in photorefractive (PR) materials, which can be considered a special case of two-beam coupling, involves interaction between two counterpropagating beams via light-induced reflection gratings. Since this backscattering results from interference between the beams, it requires mutual temporal coherence. Moreover, when one beam is self-generated, the input beam is required to have a long coherence length.

DOI
10.1364/oam.1991.mj2

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Phase-conjugate backscattering in photorefractive (PR) materials, which can be considered a special case of two-beam coupling, involves interaction between two counterpropagating beams via light-induced reflection gratings. Since this backscattering results from interference between the beams, it requires mutual temporal coherence. Moreover, when one beam is self-generated, the input beam is required to have a long coherence length.
The demonstration of optical phase conjugation in photorefractive materials has been accomplished by numerous methods, including i) externally-pumped four-wave mixing,1ii) self-pumped (passive) phase conjugation,24iii) stimulated backscattering,5,6and iv) mutually-pumped phase conjugation.7-10Stimulated backscattering and self-pumped phase conjugators have only a single incident beam. On the other hand, externally-pumped four-wave mixing has two auxiliary pumping beams in addition to the incident probe beam. Different still is the cross-conjugation of the two incident beams in mutually pumped phase conjugators. To use these conjugators in a given application, the compatibility between the processes generating the phase-conjugate wave(s) and the application must be considered. The distinguishing features of the different conjugators translate to advantages or disadvantages for certain applications.
Several mutually pumped phase conjugation configurations have been experimentally demonstrated and theoretically studied in photorefractive media.1 Although these materials are very efficient at low operating power, the process is inherently slow due to the need to generate a large number of photo-induced charge carriers.2 This paper examines the possibility of the same process occurring in a transparent Kerr medium which offers the advantage of faster response times. Due to an electrostrictive nonlinearity in the medium, a photon from either of the two input beams can be spontaneously converted into a frequency downshifted Stokes photon and an acoustic phonon, a process better known as Brillouin scattering.3 The presence of phonons due to noise will also help in Stokes scattering the incident photons. Each pair of incident beam and its coherently generated scattered beam will then write a grating that is moving at the phonon velocity. Reading of this moving hologram by the other incoherent input beam may generate the cross-readout phase conjugated beam that is frequency shifted from the read beam by the frequency of the acoustic field. The nonlinear medium thus acts like a phase conjugate mirror to both of the incident beams. Since the read-write processes occur simultaneously, the dynamics of the beam coupling are described by NFWM equations in the nonlinear medium with appropriate boundary conditions. Our calculations show that it is indeed possible to phase conjugate the two mutually incoherent input beams simultaneously when the coupling strength is above a threshold value. If one of the input beams is very weak compared to the other input beam, it is phase conjugated with a very high reflection coefficient. We have also examined the effects of seeding the phase conjugated beams.
The effect of biasing illumination on the degenerate four-wave mixing (DFWM) in absorbing photorefractive (PR) media has been investigated. The input beams are considered to be spatially uniform. The analysis has been made both for transmission and reflection geometry. Considering pump beam depletion and the slowly varying amplitude approximation, nonlinear coupled wave equations are obtained and have been solved numerically by a shooting method. The effect of different parameters such as input beam intensities ratio, thickness of the interacting medium on the phase conjugate reflectively (PCR) has been investigated. It has been observed that PCR can be controlled to a large extent by controlling the biasing intensity.