Optically nonlinear solid crystal media, with spatially periodic electric fields applied thereto, have been described in the prior art for the purpose of enhancing desired optical parametric interactions. See: S. E. Miller, Bell System Technical Journal, Vol. 48, pp. 2189-2219 (1969): "Some Theory and Applications of Periodically Coupled Waves"; and U.S. Pat. No. 3,617,912 issued to S. E. Miller on Nov. 2, 1971. These parametric interactions enable the conversion in a solid crystal medium of an optical input wave beam of given optical frequency (and wavelength) to an optical output wave beam of a different frequency (and wavelength). However, the solid crystal media required for these parametric devices are rather expensive to fabricate in order to yield sufficiently high quality, impurity-free optical nonlinear materials with tolerable optical loss. In addition, a solid medium in an optical parametric device has a tendency to crack or break up under the strains produced during operation by the relatively high-intensity optical waves required for reasonable efficiency of optical frequency conversion. In a second harmonic generator, for example, the second harmonic output intensity increases as the square of the input intensity, thereby necessitating the relatively high optical input intensity. Therefore, it would be desirable to have an optical second harmonic generator in which the optical nonlinear medium is not subject to the problems of high fabrication costs and of breakage during operation.