Effect of gain saturation on the oscillating modes of optical masers

Because an active maser medium exhibits nonlinear gain saturation, the oscillating modes of an optical maser are expected to be somewhat different from those of the passive resonator. Statz and Tang [5] have obtained some numerical results for an active resonator with a pair of parallel-plane, infinite-strip mirrors. We have reformulated the problem for active resonators with circular mirrors of both parallel-plane and confocal geometries and have obtained numerical results using an iterative method of solution. We find the cardinal features of the active modes, such as mode patterns, diffraction losses, and resonant frequencies, to be essentially the same as those of the passive modes, even for unsaturated gains as high as three and a half dB per pass. The mode that predominates in an active Fabry-Perot resonator is found to be the lowest-order (TEM00) mode. However, the predominating modes in an active confocal resonator are found to depend on the Fresnel number; the larger the Fresnel number, the higher is the mode order. The study includes computations of field distributions, diffraction losses, and phase shifts of the steady-state predominating modes and of their output intensities as functions of unsaturated gain, saturation parameter, mirror transmissivity, scattering loss, and resonator geometry.