ABSTRACT
Lasers are basically discrete-wavelength sources of coherent radiation involving stimulated emission between quantized energy levels in an amplifying medium. Optical parametric devices (OPD) are powerful solid state sources of broadly tunable coherent radiation capable of covering the entire spectral range from the near UV to the mid IR and can operate from continuous-wave down to the femtosecond time domain. For a variety of reasons, however, the OPD technology never quite developed to the same level of maturity as the laser technology. One obvious reason for the slow development of OPD technology is that the devices require laser pump sources. The parametric process is one of the most elementary nonlinear optical processes. It is obvious that the basic three-photon parametric process can be generalized to those involving four or more photons corresponding to the higher order terms in the induced polarization in the nonlinear medium.
