22.1 INTRODUCTION

The potential of nonlinear optical phenomena as a means of providing coherent radiation was realized nearly four decades ago, soon after the invention of the laser. Nonlinear effects in electromagnetism had been observed as early as the late nineteenth century by Kerr, Roentgen, Kundt, and Pockels, and later by Raman in 1927 when he discovered spontaneous scattering of light into new wavelengths in passing through a transparent medium. However, it was not until 1961 that the first observation of coherent nonlinear optical effects was made by Franken et al.,1 who demonstrated second-harmonic generation of light in the crystal of quartz. This demonstration, which was made possible by the unprecedented optical intensities available from the laser, propelled the field of nonlinear optics and led the way for intensive theoretical and experimental efforts in this field. Soon after, the importance of optical parametric generation and amplification as powerful techniques for the generation of tunable coherent light in new spectral regions was recognized2 4 and the first experimental device based on this concept was reported in 1965.5 This demonstration initiated an intense surge of research interest in parametric devices and prompted an extensive search for nonlinear materials and laser pump sources. During this period, rapid progress was made in the development of parametric devices and intensive efforts were directed at the fundamental level toward the studies of crystal optics and the understanding...