Mode spectra in ring and normal lasers

Observations have been made of the mode power spectra for both ring and normal He-Ne lasers operating at 6328 Å. The interest in such observations is twofold. First a proposed frequency-dependent loss mechanism, which arises through saturation of the medium dispersion, and the use of an aperture to control the losses in the laser cavity are evaluated. Secondly, this loss mechanism, in conjunction with the importance of mirror-image mode competition, is related to the observed asymmetries of the mode spectra from normal lasers. The mode spectra in a normal laser are typically asymmetric, with the bulk of the oscillation energy occurring to the low-frequency side of the gain curve. It is shown that for very low values of the aperture loss, the normally observed spectrum becomes degenerate with a spectrum that is its mirror image about the center of the gain curve. This observation is discussed in terms of mirror-image mode competition and the frequency-dependent loss. Calculations of the frequency-dependent loss permit the prediction of the directionally-dependent loss required in a ring laser to suppress one of the directions of oscillation. The values of this loss are typically a small fraction of the excess gain in either direction. It is shown that these predictions, together with some qualitative implications, are well borne out by experiment.