Nonlinear optics methods in conversion of spatially modulated signals

The interaction of light waves in crystals with a quadratic nonlinearity is considered. It is shown that when one of the interacting waves carries information on the image of an object, such information can be transferred to a different wavelength, providing the basis for a parametric image converter. The results are given of experimental measurements of the noise and sensitivity of parametric-conversion detectors. It is shown that when both interacting waves carry information on an object, the interaction in a nonlinear crystal allows us to obtain correlation functions of the initial images of the objects. Suitable systems make it possible to determine experimentally the correlation function of the images of simple objects. By way of example, a description is given of a system which makes it possible to remove phase distortions introduced by an optical system in the far-field zone.