Frequency characteristics of a two-mode ring gas laser with a two-isotope active medium

An analysis is made of a mode-locked four-wave (two-mode) stimulated emission regime in a ring laser with a two-isotope active medium. The ranges of existence and stability of this regime are found as a function of such laser parameters as the pressure in the gaseous active medium, resonator perimeter, losses suffered by opposite waves, excess of the rate of pumping above the threshold, detuning, etc. A detailed study is made of the dependence of the difference between the frequencies of the opposite waves on the detuning in the mode-locked regime and this is done for arbitrary values of the laser parameters. The frequency characteristic is reduced to a form which is suitable for comparison with experimental results.