The fanning of beams resulting from amplified scattering poses a significant limit on the signal-to-noise ratio achievable in high gain photorefractive amplifiers.
The buildup and the decay of photorefractive two-beam coupling are experimentally demonstrated to be strongly dependent on the intensity ratio of the two beams (signal-to-pump ratio). The experiment, done with a BaTiO3 crystal, shows good agreement with the theory of two-beam coupling in the depleted-pump regime. The deviation of the response behavior from the small-signal regime is seen for ratios as small as ∼10−5. These results are significant for applications of two-beam coupling as gain media, especially for the elimination of amplified noise (fanning).
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 Bi12SiO20, subpicowatt optical signals are detected and amplified. By using combinations of LiNbO3/Bi12SiO20, as storage and amplification crystals respectively a long term readout (more than 20 hours) of a photorefractive memory is achieved.
The storage application of the light-climbing effect in a LiNbO(3):Fe crystal has been studied experimentally. A new type of photorefractive storage in LiNbO(3):Fe crystals, in which one could write the multipage simultaneously and at a later time read out the multistored pages, using only one readout beam simultaneously, has been demonstrated successfully. Furthermore the signal-to-noise ratio of the output images was high because the climbing lights suppressed the fanning noise scattering lights.