Abstract

When two coherent plane waves interfere within a photorefractive crystal, the resulting periodic intensity pattern causes a periodic modulation of the refractive index of the crystal. For the case of purely diffusive transport of the photorefractive charges, there is a Δϕ = 90° phase shift between the intensity pattern and the refractive-index pattern. However, if there is any externally applied or internally generated (by the photovoltaic effect, for example) dc electric field in the crystal, the phase shift Δϕ will differ from 90°. We present a method for accurately determining the phase shift Δϕ and thereby the magnitude of any dc electric fields inside the photorefractive crystal. We use two- beam coupling in the crystal to deplete an already weak probe beam by an intense pump beam. The phase of the probe beam is weakly modulated sinusoidally at a frequency ω, causing a variation in the transmitted intensity of the probe beam. By measuring the ratio of the ω and 2ω components of the transmitted beam, the phase shift Δϕ may be obtained.¹ We use this technique to measure the steady state and time-evolving phase shift Δϕ in barium titanate and lithium niobate crystals.

© 1988 Optical Society of America