Photorefractive crystals

As described in section 7.7, photorefractive crystals can be used as recording materials for holography. In such crystals, exposure to light frees trapped electrons and induces a redistribution of charge within the crystal. The spatially varying electric field produced by this charge pattern modulates the refractive index of the crystal through the electro-optic effect, resulting in the formation of a volume phase hologram. If the incident light distribution changes, a new charge pattern and a new hologram are formed in a characteristic time τH. The dynamic and adaptive properties of holograms recorded in photorefractive crystals have opened up new and interesting possibilities in holographic interferometry.

Time-average interferometry of vibrating surfaces

Photorefractive crystals of the sillenite family (BGO, BSO, and BTO) have been used most commonly for holographic interferometry because of their relatively high sensitivity to light. One of the earliest applications of such photorefractive crystals to holographic interferometry made use of the ability of a BSO crystal to generate a phase-conjugate image in real time [Huignard, Herriau & Valentin, 1977]. A typical optical system used to study the vibration modes of a loudspeaker is shown in fig. 17.1.

In this case, time-average fringes are formed because the crystal integrates over the characteristic time τH required to build up the hologram, which is usually much longer than the period of the vibration. With such an arrangement, it is possible to observe the changes in real time, as the excitation frequency is changed.