NONLINEAR PROPAGATION EFFECTS IN FIR OPTICALLY PUMPED AMPLIFIERS

The nonuniform pump intensity distribution perpendicular to the propag ation direction causes an index of refraction and gain inhomogeneity o f the medium with signal frequency dispersion, for instance, in the am plifier type optically pumped lasers. The resulting waveguiding and ga in focusing caused gain line distortion, which is investigated experim entally and theoretically in a methanol model material with a comparat ively low pumping power level. The narrowing of the spectral region of amplification is observed with a decreasing pump beam size. This obse rvation is interpreted as the result of propagation of the signal FIR beam in a waveguide induced by the pumping beam. This waveguiding is p redicted to be a dominant mechanism in the amplifier type FIR lasers p umped by focused, high intensity short pulses.