Abstract

Bismuth silicon oxide (Bi₁₂SiO₂₀, BSO) is widely used as a photorefractive material for two-beam coupling (TBC) gain and holographic grating recording applications.¹ As is the case for most photorefractive materials, BSO suffers from several inherent reflectivity induced problems due to its high index of refraction (n = 2.61 at 514 nm). Multiple internal reflections of the writing beams interact among themselves to write transmission gratings with periods at or near the primary grating period. Furthermore, interference of the incident beams with the internally reflected beams gives rise to additional reflection gratings. Both types of interaction combine to reduce the overall grating modulation strength and to increase the potential for crosstalk and cross coupling, particularly for the case of multiplexed holograms. Reducing these problematic multiple reflections by means of a simple double layer bandpass antireflection coating² increases the TBC gain coefficient and the diffraction efficiency by increasing the effective grating modulation depth. The writing sensitivity of the crystal (as expressed by the grating rise time) and the dependence of the diffraction efficiency on the incident beam polarization were also investigated for both uncoated and AR coated crystals.

© 1989 Optical Society of America