Real-Time Holographic Interferometry in Photorefractive BTO Crystals Using Optical Phase Conjugation with a Low Power Laser Diode

Abstract

We demonstrate holographic interferometry using photorefractive BTO crystals as reusable recording materials, optical phase conjugation with low power laser diodes and computer image processing for rapid and precise interpretation of the fringe patterns. The BTO crystals provide: good sensitivity and diffraction efficiency at the laser diode wavelength, without external electric field on the crystal, high resolution, unlimited reusability, control of writing and storage times. Two-wave mixing and four wave mixing configurations are built for evaluation of recording, storage and erasure times, as well as of diffraction efficiency and image quality. The second configuration proved some advantages: the reconstructed image and the direct image (seen by the TV camera) are not superimposed and the phase distortions introduced by some optical components and even by the BTO crystal can be corrected (by phase conjugation), which lead to better quality, low-noise double-exposure and time average interferograms. Comparison of the results obtained with He-Ne and diode lasers shows the possibility of using low power laser diodes in real-time, compact, low-cost holographic interferometers. Direct spatial phase reconstruction and efficient algorithms were used for fast computer image processing, compatible with the hologram recording time. The spatial information about some complex phenomena, which evolve at this time scale, can be obtained by these methods.