Characterization of thin real-time holographic media by dithered four-wave mixing

Abstract

Photosensitive optical media of the types that are used in real-time holography generally exhibit both photorefractive and photochromic response. The parameters that characterize each of these types of response can be extracted by detailed quantitative analysis of four-wave mixing experiments in the Raman–Nath regime. This principle has been tested using reactive eyeglass lenses. The techniques of Fourier optics have been applied to the analysis of diffraction from amplitude/phase gratings produced in the photochromic glass by laser illumination. Dithering of a laser-induced fringe pattern produces a combination of two-wave and four-wave mixing that can be analysed in terms of amplitudes proportional to products of Bessel functions of different orders. The incident and diffracted beams are predicted, and observed, to be modulated at the dither frequency and its harmonics. The small dither amplitudes that were employed, made possible by use of a phase-sensitive detection technique, allowed Bessel function expansions to be truncated after only a few terms for the purpose of parameter estimation. The technique has allowed the very small refractive index change associated with dispersion of the photo-induced absorption in silver halide doped photochromic glass to be measured for the first time.