Abstract

When an optical amplifier is injected with a very weak optical wavefront, its output is generally corrupted by a strong background noise that limits the signal-to-noise ratio in the detection plane. In photorefractive amplifiers, beam fanning arises from the amplification of the scattered light caused by crystal imperfections and local index perturbations. It is responsible for the optical noise observed in the detection plane. In this paper, we propose and experimentally demonstrate the detection of weak coherent wavefronts carrying the spatial information of an image. The technique relies on photorefractive two-wave mixing amplification with a phase modulated signal beam.^1-4 Figure 1 shows the operating principle. A fraction of a laser beam, prior to illuminating the object to be detected (O), is phase modulated via a reflection on a piezomirror. When the piezomirror is excited with a sinusoidal voltage at frequency f, the object is illuminated with a multifrequency beam that include the f₀ and the f₀ f components, where f₀ is the frequency of the laser.

© 1993 Optical Society of America