Nonlinear propagation effects in FIR optically pumped amplifiers

Abstract The nonuniform pump intensity distribution perpendicular to the propagation direction causes an index of refraction and gain inhomogeneity of the medium with signal frequency dispersion, for instance, in the amplifier type optically pumped lasers. The resulting waveguiding and gain focusing caused gain line distortion, which is investigated experimentally and theoretically in a methanol model material with a comparatively low pumping power level. The narrowing of the spectral region of amplification is observed with a decreasing pump beam size. This observation is interpreted as the result of propagation of the signal FIR beam in a waveguide induced by the pumping beam. This waveguiding is predicted to be a dominant mechanism in the amplifier type FIR lasers pumped by focused, high intensity short pulses.