Photorefractive Noise Suppression using Achromatic Gratings

Abstract : Photorefractive beam fanning is a well known effect in which a collimated beam of light is scattered into a broad fan as it propagates through a crystal which exhibits high two wave mixing gain. Beam fanning can be a very strong effect, removing almost all the light from the initial beam. It has been proposed that the fan originates from scattering off defects in the crystal, which then become amplified by the two beam coupling process. Beam fanning can be beneficial, for instance, in allowing self-pumped phase conjugation. But it can also be an undesirable competing nonlinear effect. For example, it may prevent high gain image amplification by two beam coupling, by robbing the pump and signal beams of light. In this letter, we demonstrate that two beam coupling, using achromatic gratings, can suppress beam fanning that would otherwise occur.