Adaptive photorefractive interferometers for ultrasound detection

Abstract

The principle of optical detection of ultrasound consists in measuring the phase modulation induced by the small ultrasonic surface displacement upon a probe beam impinging on the surface. An interferometer is used to transform phase-modulated light scattered from the surface into a modulated electrical signal. Since usual material surfaces are rough, such an interferometer should operate with speckled beams. Two-wave coupling via dynamic hologram recorded in a photorefractive (PR) crystal is the simplest and efficient technique for demodulation of a transient phase shift buried in a speckled wave. PR crystal acts as a self-adjusted beam combiner providing holographic adaptation of an arbitrary object-beam wave front with that of the reference beam. Besides the wave front adaptation, the crystal also automatically stabilizes an average path difference between the interfering beams thus diminishing noise caused by random optical path difference fluctuations. Here we present a novel modification of the two-wave mixing technique based on the vectorial wave coupling in PR crystals of cubic symmetry such as semiconductors and sillenites. The linear regime of phase demodulation is achieved applying an alternating external field to the crystal. It allows us to design an adaptive interferometer with sensitivity approaching to the classical homodyne detection limit.