Number 152 (Story #3), November 19, 1993 by Phillip F. Schewe and Ben Stein OPTICAL PARAMETRIC OSCILLATION (OPO) is a nonlinear optical phenomenon (the light output is not proportional to the input light) in which a light beam at one frequency can, by passing through a special crystal, be split into two beams at lower frequencies. The whole process can be tuned---creating in effect a multicolor laser system---by changing the refractive index of the nonlinear crystal, which in turn can be accomplished by rotating the crystal relative to the incident light beam. The OPO technique may prove to have applications in the study of quantum optics, in the activation (at specific wavelengths) of photosensitive drugs, in the monitoring of tiny amounts of environmental pollutants, in full-color compact television, and in sampling a system's spectrum over very short time intervals (time-resolved spectroscopy), an application in which the high peak power of pulsed lasers can only make the OPO process more efficient. (Physics World, Oct. 1993.)