Mode pulling in ring lasers

Expressions are derived for the dispersion of a laser gain medium at arbitrary values of the power density within the laser. These expressions apply to multimode operation of a laser where the mode spacing is less than the pressure-broadened atomic linewidth. The dispersion calculations are used to predict the spectra of beats between adjacent oscillating modes as a function of parameters characterizing the laser cavity and the gain medium. Experiments aimed at testing the accuracy of the threoretical predictions are described. It is shown that the theory gives a reasonably good description of the variation of the center frequency of the beat spectrum with internal power density. However, for reasonably high power densities, the observed width of the beat spectrum considerably exceeds that predicted. This discrepancy is related to an effective width of each oscillating mode.