Noise reduction in photorefractive image amplifiers and applications

(Received 20 February 1993, accepted 2 April 1993)

Abstract
We propose and experimentally demonstrate two original methods for suppressing the beam fanning noise of photorefractive amplifiers. The first one involves two wave mixing in a slowly rotating crystal, while the second requires to tilt the crystal with respect to the recording beams. This method applies to materials which exhibit a sharp resonance of the gain versus the pump-probe beam angle. With both methods a large improvement of the S/N ratio is achieved and virtually noise free image amplifiers are demonstrated. In Bi ₁₂SiO ₂₀, subpicowatt optical signals are detected and amplified. By using combinations of LiNbO ₃/Bi ₁₂SiO ₂₀, as storage and amplification crystals respectively a long term readout (more than 20 hours) of a photorefractive memory is achieved.