Abstract

Experimental results are presented for an optical phase rectifier using nonlinear optical processing. We show that this device can be used to convert complex amplitude multiplicative optical noise on a positive scalar image to additive optical noise. Our implementation of phase rectification uses diffusive photorefractive two-beam coupling in a thin crystal of barium titanate. A weak plane wave reference beam is amplified point by point to saturation using the noisy image as a strong pump. At saturation all the power at a given point in the noisy pump is transferred to the corresponding overlap point in the weak plane wave. The amplitude of the reference becomes that of the pump but, because the beam coupling occurs in the diffusion regime, the phase of the reference remains uniform after amplification. The converted image can be subjected to additional noise reduction using methods for dealing with additive (signal dependent) noise,¹ or multiplicative positive scalar noise.² Such treatment is possible because the Fourier transform of the converted image shows the diffraction pattern of the signal embedded in a broad and weak speckle pattern. Alternative implementations of real-time optical rectification using optically addressed spatial light modulators require that the output amplitude of the spatial light modulator be specifically adjusted to be the square root of the input intensity. Cascading an initial optical phase rectifier with a Fourier-plane phase-preserving thresholding device¹ dramatically reduces multiplicative noise with good optical efficiency and provides a coherent signal output.

© 1991 Optical Society of America