A bismuth silicon oxide crystal is used in the diffusion regime as a dynamic recording medium in a real-time holographic interferometer based on anisotropic self-diffraction. This device is connected with an interferogram-analysis method that uses the phase-shifting technique for quantitative measurement of diffusive-reflecting object deformations. In addition to the usual error sources in phase shifting, the temporal interferogram erasure is studied and is found weakly perturbative for the measured phase. It is shown that quantitative measurements are possible for low-intensity object beams (8 μW/cm2) and a large observed area. A practical situation of defect monitoring in a composite structure is presented.

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Phase-shifting real-time holographic interferometry that uses bismuth silicon oxide crystals

Beam coupling effects in an optically active photorefractive crystal of Bi12SiO20 are very much influenced by the polarization direction of the interacting beams. Analytic expressions for the signal amplification in the diffusion regime have been derived for two commonly used crystallographic configurations,G∥ andG∥ ,G being the grating vector. It is shown that only the orientation is suitable for signal amplification. The results obtained for the diffusion regime can be applied, as a first approximation, to the more common and more intricate drift regime in which a bias electric field is applied to the crystal to enhance the coupling effect.

Influence of the polarization direction of two-beam coupling in photorefractive Bi12SiO20: Diffusion regime

The Lattice Compatibility Theory (LCT): An attempt to explain Urbach tailing patterns in copper-doped bismuth sillenites (BSO) and germanates (BGO)

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

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.

Polarization properties of enhanced self-diffraction in sillenite crystals

We study the polarization properties of volume phase gratings induced in photorefractive materials that are both optically active and linearly birefringent, such as Bi12SiO20, Bi12GeO20, and Bi12TiO20. Analytic expressions for the diffraction efficiency and for the orientation and the ellipticity of the diffracted light have been derived for the 〈110〉 and 〈001〉 configurations under the assumption of a nondepleted readout beam. The physical parameters that determine the polarization characteristics of the diffracted light are the orientation of the readout vector, the crystal thickness, the optical activity, and the electric-field-induced linear birefringence. Our results are valid for arbitrary values of these parameters. The particular cases of zero optical activity, zero linear birefringence, a thin crystal, or a specific readout-vector orientation can be deduced from the general formulas. The analytic expressions that we have derived permit an optimal choice of different parameters for the image-processing and beam-coupling applications of these materials.

D. Rouède

Papers

Efficiency and polarization characteristics of photorefractive diffraction in a Bi 12 SiO 20 crystal