Amplification of light by a drift-type photorefractive crystal

Abstract

The photorefractive effect is a bulk phenomenon in which the local index of refraction is changed by the spatial variation light intensity. Such a condition has been observed in crystals like bismuth silicon oxide (BSO) and barium titanate (BaTiO3), and these photorefractive crystals (PRCs) have been extensively studied, particularly for their application in optical storage, optical computing, and image processing.
For many applications, the significant optical process is the formation of a grating by two coherent light beams. This process, referred to as two-wave mixing, can result in the amplification of a weak optical signal at the expense of a strong reference beam. The gain coefficient is determined by the amplitude of the index grating, as well as the phase shift between the index grating and the interference pattern. Diffusion-type crysals (such as BaTiO3) generally have large gain coefficients due to their correspondingly large electro-optic coefficients. Drift-type crystals (i.e. BSO) have smaller electro-optic coefficients and can attain high gain coefficients only in the presence of an external electric field. Consequently, the gain mechanism of drift-type crystals is more complicated. Drift-type PRCs have optimum gain with moving gratings (as opposed to the stationary gratings of diffusion-type crystals), with velocity dependence on factors such as external field, grating wavelength and the intensity of the pump beam.
Optimum gain properties of drift-type crystals have been previously reported [Opt. Commun. 38, 249 (1981), J. Appl. Phys. 58, 45 (1985)]. Certain assumptions, however, which may not be applicable under the usual experimental conditions were made in the derivation of the analytical expressions for the optimum gain and the corresponding grating velocity. These are manifested in discrepancies between theoretical results and some experimental observations. In this paper, a general equation for the index of refraction, which takes into account the effect of controllable parameters such as the external field, is derived from Kukhtarev's material equations. Theoretical curves are fitted to experimental data, allowing the determination of ratios of parameters specific to the crystal used.