Hole Burning Effects in a He-Ne Optical Maser

A study has been made of pulling effects by the amplifying media on the TEM₀₀ modes of a helium-neon maser using a circular plane mirror Fabry-Perot cavity in which the mirror separation was known with precision. Approximate expressions are derived for mode pulling in homogeneously and inhomogeneously broadened optical masers. The experimental results suggest that the losses in the maser are not determined entirely by the mirror reflectance coefficient. A power-dependent splitting of the beat frequencies between simultaneously oscillating modes is explained in terms of a nonlinear frequency-dependent pulling effect arising from inhomogeneous broadening. The case of Lorentzian holes burned in a Gaussian line is treated specifically. It is suggested that the anomalous variation of beat frequencies with pumping rate results from hole repulsion effects and that the hole widths required arise from the combined effects of small-angle elastic scattering and stimulated emission. It is a consequence of the interpretation that the number of simultaneously oscillating even-symmetric modes in the helium-neon maser may easily be determined from the Fourier spectrum of the beat frequencies. A method is described by which the central cavity resonance may be stabilized near the center of the Doppler line in the case of three oscillating symmetric modes.