thông qua 22). ZnGeP, có thể điều chỉnh trên phạm vi này với một biến thể của về 4 ", phạm vi góc cạnh nhỏ nhất, CdSe sẽ cần khoảng 14", phạm vi góc lớn nhất. AgGaS, không hiển thị một loạt các điều chỉnh bất thường phẳng khoảng 4,2 ym. | 334 Norman p. Barnes through 22 . ZnGeP could tune over this range with a variation of about 4 the smallest angular range CdSe would require about 14 the largest angular range. AgGaS does display an unusually flat tuning range about pm. Besides this the tuning curves are in general similar except for the direction of the curvature. As such selection of the best nonlinear crystal would probably be based on considerations other than the phase matching curves. Angle degrees FIGURE 15 Phase-matching curve for LiNbO. for a pump. 9. PERFORMANCE Optical parametric oscillators have developed from their initial stage where they were little more than a curiosity. Initial performance was limited by lack of high optical quality nonlinear crystals nonlinear crystals with relatively small nonlinear coefficients and limited pump laser performance. In addition optical parametric oscillators were in competition with dye lasers in the visible and near infrared. Pulsed dye lasers have an advantage because laser-pumped dye lasers do not necessarily require high beam quality from the pump laser. In essence dye lasers can serve as an optical integrator convening a fixed-wavelength pump laser with relatively poor beam quality into a tunable laser with a better beam quality. In the face of these difficulties optical parametric oscillators enjoyed limited commercial applications for a considerable time. However several increases in optical parametric oscillator technology have improved the viability of these devices. Pump 7 Optical Parametric Oscillators 335 Pump Optical quality of the nonlinear crystals has improved. Optical quality improvements have occurred both in the form of decreased absorption and decreased distortion. For example LiNbO3 crystals were found to suffer from optically induced refractive index inhomogeneities. It was found that in part these problems could be traced to Fe impurities. By decreasing the Fe impurities the susceptibility of
