Gain Saturation and Output Power of Optical Masers

The nonlinear gain characteristics of optical maser amplifiers at high beam intensities, and the optimum cavity coupling of maser oscillators for maximum output power, are computed for maser media with homogeneous and inhomogeneous line broadening. An approximate expression is derived for the power output of a gas maser oscillating simultaneously at many longitudinal cavity resonances, based on the assumption that the gain saturates independently at each frequency. In each case, the decrease of maser gain with radiation intensity involves an empiric constant, or saturation parameter, which is characteristic of the active medium. Power and gain measurements at 1.15 μ on three He–Ne maser tubes of different diameter, in a cavity 1.75 m long, are found to satisfy the derived multifrequency power expression, and permit evaluation of the gain—saturation parameter for this gas mixture. The power expression, derived for a single transverse mode, is unexpectedly found to hold for multimode oscillations as well, within the range of measurements. From the measured saturation parameter and the derived expressions, the performance of amplifiers and other oscillators with the same active medium can be predicted.