Speckle-free image amplification by two-wave coupling in a photorefractive crystal

A technique of speckle-noise reduction for an image amplified by two-wave coupling with a photorefractive crystal is proposed. The spatial coherence for the axis along the fringe formed in the photorefractive crystal is reduced to remove the speckle, but the spatial coherence for the axis vertical to the fringe is unaltered. This is obtained by rotating a mirror to deflect the illumination beams one dimensionally for both the signal and reference patterns during the exposure of the hologram. Because of the imageholography configuration, the images of the signal and the reference do not move on the photorefractive crystal as the two beams deflect. The beam deflection can be faster than the response of the photorefractive crystal. Some experimental results for the image amplification with the technique developed are presented, and they are compared with results obtained without speckle-noise reduction. Fringe deformation caused by deflection of the signal and reference beams is analyzed.