Spatial coherence of optical harmonics and propagation of randomly nonisotropic light beams

The results are given of an investigation of the spatial coherence of a second harmonic excited in a nonlinear crystal by multimode laser radiation. It is shown experimentally and theoretically that, in general, the harmomic field is randomly nonisotropic. The correlation radius of this field is highest along the direction of sideways drift of energy in the nonlinear crystal. The characteristics of the propagation of such harmonic beams hi linear media are investigated. It is found that, during propagation, the initially circular cross section of the harmonic beam becomes strongly elongated and its field becomes randomly isotropic. These properties of harmonics should be allowed for in the design of multipliers of frequencies of high-power multimode beams and in their use in holography, heterodyne detection, etc.