Direct measurement of the amplitude and the phase of photorefractive fields in KNbO_3:Ta and BaTiO_3

Abstract

Real-time direct measurement of the amplitude and the phase of photorefractive fields is performed for all stages of photorefractive grating formation including saturation. The photorefractive processes in tantalum-doped potassium niobate and nominally pure barium titanate crystals are investigated as a function of the grating spacing. The diffusion and photovoltaic transport lengths, the Debye screening length, and the net trap concentration are determined. Independent direct measurement of the photorefractive phase permits a more accurate and reliable comparison between experimental results and existing models. The measurement technique involves the diffraction of an amplitude-modulated laser beam by an acousto-optic Bragg cell operating with the same laser modulation frequency. The photorefractive gratings are formed by the interference of the transmitted and diffracted components of the beam. This nondestructive measurement technique produces excellent sensitivity and signal-to-noise ratio. The heterodyne detection scheme results in a dynamic range of 50-dB and a phase measurement accuracy of 2°.