Generation of coherent far-infrared radiation using lasers

A review is given of methods used for generating coherent far-infrared radiation. They include the isolation of the frequency difference between two lasers, SRS by polaritons, optical pumping of gases, and the development of the electrical-discharge laser. Basic principles underlying the realization of the phenomena on which the above methods of generating far-infrared radiation are based are discussed. The properties of the working media, i.e., nonlinear properties of crystal converters (exploited in isolating the difference frequency and in SRS by polaritons) are described. Methods of producing inversion of level populations in gases used in lasers (pumped optically or by an electrical discharge) are analyzed. Specific schemes and device designs for the far infrared are described, and the possibilities and likely performance of these schemes are discussed. The review does not cover electron-beam generators (backward-wave tube, gyrotrons, etc.).