We consider in detail the piezoelectric properties and the optical activity of (110)-cut cubic photorefractive crystals with respect to the optimization of two-beam coupling. An expression is obtained for the azimuth of two interacting linearly polarized waves at which the maximum energy exchange occurs. The expression for the maximum intensity of one of the waves is also obtained. It is shown that for each light-wave azimuth there are three local energy-exchange maxima, corresponding to three different orientations of the grating vector in the cut plane. The optimum conditions for energy exchange in a BSO crystal with thickness d = 2.19 mm are realized at azimuths ψ ≅ 23° and ψ ≅ 13° for orientation angles θ ≅ 55° and θ ≅ 305°, respectively. The results of the experimental study of two-beam coupling in BSO are in good agreement with theory.

Orientation and polarization effects of two-beam coupling in a cubic optically active photorefractive piezoelectric BSO crystal

A holographic interferometer that uses two-wave mixing in a photorefractive (Bi12SiO20) crystal under an applied ac field is described. The interferometer uses a repetitive sequence of separate record and readout times to obtain quasi real-time holographic interferograms of vibrating objects. It is shown that a good signal-to-noise ratio of the interferometer is obtained by turning off the object illumination and the applied ac field during readout of the hologram. The good signal-to-noise ratio of the resulting holographic interferograms enables phase measurement, which allows for quantitative deformation analysis.

Dynamic holographic interferometry by photorefractive crystals for quantitative deformation measurements.

We study two-beam interaction in photorefractive sillenite crystals in reflection geometry and derive analytic expressions for the signal gain and its polarization direction in the presence of a strong pump beam. The crystal is cut normal to one of the crystallographic axes, and no external electric field is applied. Crystal absorption and its natural optical activity are taken into account. The coupling effects are strongly dependent on the polarization directions of the interacting beams. We determine the optimal polarization directions and optimal crystal thickness that give maximum signal gain. Experimental results are in excellent agreement with theoretical calculations.

https://hal-iogs.archives-ouvertes.fr/hal-00877996/document

Polarization properties of self-diffraction in sillenite crystals: reflection volume gratings

The coupled-wave theory is extended to analyze the two-wave mixing interaction under an alternating electric field in a sillenite crystal. Within the undepleted pump beam approximation, an analytical expression which allows prediction of the phase, the polarization state, and the amplitude of the amplified beam is derived. It is shown that all these features usually oscillate. These predictions are confirmed by experiments. >

Polarization properties of two-wave mixing under an alternating electric field in BSO crystals

Photorefractive materials have been extensively used in
recent years as real-time recording media for optical holography.(^1,2) One prospective application of real-time holography is in the area of optical information processing; for example, the correlation between two mutually incoherent images has recently been demonstrated in real time in a four-wave mixing
geometry. (^3) Often, however, the information to be processed exists only in incoherent form. High performance spatial light modulators(^4) are thus necessary in many optical information processing systems to convert incoherent images to coherent replicas for subsequent processing. We report in this Communication the successful demonstration of real-time incoherent-to-coherent images transduction through the use of holographic recording in photorefractive crystals. Several possible configurations and experimental results are presented.

Photorefractive Incoherent-to-Coherent Optical Converter,

Doppler-enhanced self-diffraction in two-beam coupling experiments with sillenite crystals exhibits pronounced optical polarization effects due to the concomitant presence of natural optical activity and electric-field-induced linear birefringence. Coupled wave equations that describe the polarization properties, exclusive of self-diffraction effects, have previously been derived for the two principal crystal orientations most commonly used in photorefractive recording. In this paper, the coupled wave equations are combined with a linearized model of the photorefractive recording process (single trap level, single mobile charge species) to analyze the impact of self-diffraction effects on the polarization state evolution. Numerical solutions of these equations yield optimum configurations for enhanced gain and improved image contrast in two-wave mixing with such materials. In addition, inclusion of optical activity in the model emphasizes the contribution of this effect to the apparent reduction of the effective electrooptic coefficient of bismuth silicon oxide crystals.

R. Johnson

Papers

Polarization properties of enhanced self-diffraction in sillenite crystals