Nonlinear crystal, performance limit

As a final example to generate continuously tunable IR-laser radiation, I want to mention mixing the outputs of a fixed frequency laser and a tunable dye laser in a nonlinear process. Since different wavelengths normally have different refractive indices in transparent matter, an anisotropic medium is needed to phasematch the conversion of two propagating waves of different wavelengths. Another limitation of this technique is internal absorption of the IR-radiation by the nonlinear crystal. Nevertheless excellent performance of the difference laser system is possible, as has been verified by Pine [1,16/1,17]. For further details we refer to the article by T. Oka. 

Difference-frequency laser (cw) (see Section 20.4.2.1 for the generation of difference frequency) By using various nonlinear crystals, this laser can cover almost the entire mid-infrared region (the lower limit is about 550 cm ). Since the width of this laser line is very narrow, this laser is important for performing high-resolution time-resolved measurements in the gaseous state. 

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