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Soviet Journal of Quantum Electronics

Self-conjugation of a speckle pump beam in a loop containing a photorefractive crystal

A. V. Mamaev, Vladimir V. Shkunov

Published Nov 30, 1992• 3 citations
Publisher

Abstract

We show theoretically and experimentally that the optimum geometry for phase conjugation of speckle beams in a loop containing a photorefractive crystal is the "biting-its-own-tail" geometry with beam diameter preserved. We predict an increase in the threshold for the process by a factor of 1.5 due to the introduction of speckle in the beam to be conjugated.

DOI
10.1070/qe1992v022n11abeh003663

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Photorefractive Effects, Materials, and Devices•Jan 1, 1991•0 citations
We show theoretically and experimentally that the optimum geometry for phase conjugation of speckle beams in a loop containing a photorefractive crystal is the "biting-its-own-tail" geometry with beam diameter preserved. We predict an increase in the threshold for the process by a factor of 1.5 due to the introduction of speckle in the beam to be conjugated.
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.
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.
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.