Abstract

Photorefractive two-wave mixing can be regarded as a real-time holographic interferometer. By using this technique we can observe a slowly varying phase change or vibration mode. If the phase of signal beam is changed, we observe an interference pattern between the pump beam diffracted by phase grating and that of transmitted-signal beam. The pump beam and the new signal beam generate a new phase modulation in the crystal and, at the same time, erase the old phase modulation. The visibility of the interference fringes decreases with time; the time constant of the photorefractive index change in BaTiO₃ is about 10 s under a 1 W/cm² irradiation at 488 nm. The time necessary for phase modulation depends not only on the total intensity in crystal but also on the ratio of the signal beam intensity to the pump beam intensity. We experimentally investigate the temporal behavior of phase modulation in a crystal in which the phase of signal beam shifts suddenly. We analyze the response of making and erasing phase modulation in the crystal by using coupled wave equations and a simplified photorefractive material equation. We find that the large coupling coefficient of materials such as BaTiO₃ brings about rapid fast generation of phase modulation.

© 1990 Optical Society of America