Low-noise amplification of ultraweak optical wave fronts in photorefractive Bi_12SiO_20

We propose and demonstrate a technique for suppressing the optical noise of the photorefractive amplifiers that exhibit a sharp gain resonance around an optimum pump-to-signal angle. The amplification of extremely weak images is achieved when the entrance face of the crystal is tilted with respect to the pump beam direction. In Bi(12)SiO(20) subpicowatt optical signals are detected with this technique.