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Journal of the Optical Society of America B

Self-bending of light beams in photorefractive phase conjugators

V. V. Eliseev, A. A. Zozulya, G. D. Bacher, Jack Feinberg

Published Mar 1, 1992• 19 citations
Publisher

Abstract

Stimulated photorefractive phase conjugators often exhibit well-defined curved beam paths that cannot be explained by simple beam fanning. We propose a model of these apparently curved paths as a series of straight-line segments, with beams propagating in both directions along these segments. These line segments spring from the amplification of scattered light between regions of the crystal already containing counterpropagating pump beams. As these line segments form, they create new interaction regions that generate new segments, thereby making the final beam path appear to be curved. Application of our model to a single-interaction-region mutually pumped phase conjugator shows that the threshold coupling strength required for the appearance of these new segments is only slightly higher than the threshold for the phase conjugator itself.

DOI
10.1364/josab.9.000398

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Stimulated photorefractive phase conjugators often exhibit well-defined curved beam paths that cannot be explained by simple beam fanning. We propose a model of these apparently curved paths as a series of straight-line segments, with beams propagating in both directions along these segments. These line segments spring from the amplification of scattered light between regions of the crystal already containing counterpropagating pump beams. As these line segments form, they create new interaction regions that generate new segments, thereby making the final beam path appear to be curved. Application of our model to a single-interaction-region mutually pumped phase conjugator shows that the threshold coupling strength required for the appearance of these new segments is only slightly higher than the threshold for the phase conjugator itself.
In recent years a variety of photorefractive phase conjugators have been demonstrated that rely on stimulated light beams.1-6 These self-pumped and mutually-pumped phase conjugators are all closely related7 and differ only in the number and angle of their input beams. Here we propose a detailed though necessarily simplistic description of how light beams spring up inside a photorefractive crystal, and why under some conditions these stimulated light beams appear to follow curved paths inside the crystal. We show that these curves are composed of a number of straight line segments which spring up by stimulated scattering in the photorefractive crystal.
The threshold for self-pumped optical phase conjugation by means of backward stimulated photorefractive scattering has been dramatically lowered by inserting a retroreflecting screen into the path of the beam after the beam leaves a BaTiO3 crystal. Stable reflectivities of as much as −60% and good phase-conjugate fidelity have been demonstrated in a crystal that has too low a gain–length product to exhibit unseeded backward stimulated scattering. Precise control of the threshold for stimulated scattering is demonstrated by adjusting the seed intensity. Reflectivity and fidelity are experimentally characterized at 515 nm as a function of the following parameters: pump and seed intensities, crystal angle with respect to the incident beam direction, crystal interaction length, crystal–lens separation, and aberration strength. Operation at 633, 730–800, and 839 nm is demonstrated also.
We identify three separate regimes in the evolution of self-pumped phase conjugation in photorefractive media. These are evident from the time evolution of transmission, omnidirectional scatter, and phase-conjugate reflection intensities measured experimentally. Initially, beam fanning occurs, manifested by depletion of the transmitted light to one-third of its initial value. This is followed by a gradual increase in the omnidirectional scatter intensity caused by spreading of the light paths in the crystal, until almost all the crystal is flooded. Only much later does the phase-conjugate beam appear and grow to half of the input intensity at the expense of both the transmitted and scattered light. The three processes persist for different durations: fanning is appreciably faster than the other two processes. Each regime is characterized by different beam spread within the crystal, which is observed in top-view images. We postulate physical mechanisms to describe the observed phenomena.
We present a plane-wave analysis of a recently demonstrated self-pumped phase conjugator. This device uses four-wave mixing to produce the phase-conjugate replica of an incident optical wave. All the waves are derived from the single incident wave: there are no externally supplied pumping beams. We consider the case of four-wave mixing in two interaction regions coupled by simple reflection. We calculate the phase-conjugate reflectivity as a function of coupling strength, taking into account imperfect coupling between the two interaction regions, and show that there is a threshold coupling strength below which the reflectivity is zero and above which the reflectivity is multiple valued. We also compute the coupling strength per unit length for a photorefractive crystal of barium titanate.