Two flat parallel end faces of a nonlinear crystal (such as lithium niobate) are composed of a Purot Cavity. The signal frequency and the invalid frequency correspond to the two resonance peaks of the resonant cavity. When the flux density of the exhaust light is large enough, the energy is transferred from the pumping light to the signal oscillation mode and the invalid oscillation mode, and the followed is these The emission of coherent radiant energy on the frequency. The oscillator can be tuned by changing the temperature, electric field or the like.
