Method and apparatus for photorefractive pirouette display and

Abstract

A photorefractive pirouette display device can produce the sonogram of speech sounds by incorporating the short-time Fourier spectrum of speech waves as the input. The device requires a coherent source of optical radiation which can be obtained for a variety of different lasers. A photorefractive circuit is used to display a time-dependent signal, beam. An analog signal is imposed on the beam as temporal modulations on its intensity and/or phase, the pump beam is a plane wave constant amplitude. The signal beam interferes with the pump beam in the photorefractive crystal, the resultant intensity modulation pattern nonuniformly excites the charge carriers in the crystal. The excited charge carrier migrate in the crystal to form a microscope electric field, which produces an index grating in the crystal due to the Pockels effect. The crystal is rotating continually with angular velocity. The pump beam diffracts off of the holographic grating and reconstructs the signal beam. As the crystal rotates, the reconstructed signal beam rotates along a circular ring. In polar coordinates the angle .THETA. specifies a position on the ring. The diffracted light at angle .THETA. at a given time is denoted by A(t,1,.THETA.) where 1 is the crystal thickness. A(t,1,.THETA.) is proportional to the signal amplitude at an earlier time. The key to achieving the pirouette display is to ensure that any grating written at any time is always Bragg matched to the pump beam, even if the crystal is in continuous rotation. This requires that the pump beam be parallel (or antiparallel) to the rotational axis of the crystal.