In this paper we investigate the mode structure in an unstable optically pumped FIR oscillator as a function of the pump beam uniformity. It is shown that the effective output coupling is sensitively dependent on the pump beam intensity profile and the degree of saturation. As an example a telescopic D2O resonator of nominally 75% output coupling is discussed. The mode structure varies from a stable configuration with zero loss in the small signal range to saturated intensity distributions with output coupling factors exceeding the geometrical value.
M. R. Siegrist, M. R. Green, P. D. Morgan, and R. L. Watterson, "Mode structure in the unstable resonator of an optically pumped FIR laser: an investigation," Appl. Opt. 19, 3824-3829 (1980)