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Conference on Lasers and Electro-Optics

Vector phase conjugation in photorefractive materials

Juan Francisco Lam

Published Jan 1, 1985• 0 citations
Publisher

Abstract

Current interest in the nonlinear optical properties of photorefractive materials stems from the fact that they are potential components in the design of parallel optical logic architecture. Barium titanate and bismuth silicon oxide are the two candidates being studied, and they show promise provided the polarization states of the input radiation fields are identical. This restriction can be removed if the medium can respond to the interference of two orthogonally polarized light fields.

DOI
10.1364/cleo.1985.thn2

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Journal of the Optical Society of America B•Oct 1, 1997•3 citations
Current interest in the nonlinear optical properties of photorefractive materials stems from the fact that they are potential components in the design of parallel optical logic architecture. Barium titanate and bismuth silicon oxide are the two candidates being studied, and they show promise provided the polarization states of the input radiation fields are identical. This restriction can be removed if the medium can respond to the interference of two orthogonally polarized light fields.
We demonstrate for the first time, to our knowledge, second harmonic optical phase conjugation, using the second spatial harmonics of a nonsinusoidal grating formed in a photorefractive material. The possibility of harmonic phase conjugation has been considered previously for artificial Kerr media.1 We have applied the same idea to barium titanate, since the existence of the higher-order gratings in this material has been known to us for some time. In our experiment, two 1.06-µm cw Nd:YAG laser beams wrote a grating in a poled photorefractive barium titanate crystal. Then a second harmonic of the YAG at 0.532 µm read out the grating in a counterpropagating direction to one of the writing beams. In this geometry, the usual Bragg condition is not satisfied. The reflectivity of the 0.532-µm signal was rather weak, ~0.01%, with ~0.1-mW reading beam power. The buildup time was around 3 s, with loosely focused writing beams with 50-mW power in each beam. The same experiment with a SBN crystal showed no significant harmonic phase conjugation. Finally, we stress that this effect is different from the previously published result.2
We have demonstrated experimentally that optical Not-OR (NOR) and Not-AND (NAND) operations can be done with a single setup. The setup is capable of processing parallel inputs. Two barium titanate (BaTiO3) crystals are used in the setup and photorefractive effects are employed. Instead of using the interference between the two input signals to be processed or the so called local incoherent erasing, the setup works on the phenomenon of self-pumped phase conjugation and the suppression of it by the input signals in ordinarily polarized state. In this way, the two input channels can be mutually incoherent.
We present an eight-dimensional matrix formulation of vector four-wave mixing. We study the case of a nonoptically active photorefractive for the transmission grating. The coupled equations are represented in a group formalism and irreducible forms are found. We show that when cross-polarization coupling is neglected, the system of equations is governed by two coupled SU(2) groups. We develop a theory that allows for pump depletion and electric-field-induced bulk birefringence. Analytical solutions are found when cubic 4¯3m materials are considered in two standard experimental geometries, i.e., electric fields perpendicular to the (110) crystal faces and electric fields perpendicular to the (001) crystal faces. Results are presented that show the effects of linear and circular polarized beams, and the use of these materials for vector phase conjugation is discussed. Solution routes to other geometries and noncubic photorefractive materials are also presented.