Ultrahigh-resolution spectroscopy based on competition between ring-laser waves

A technique of nonlinear laser spectroscopy is described which employs power resonances in emission from a gas laser with a ring cavity. This technique makes it possible to detect the hyperfine structure of a Doppler-broadened absorption line even if the investigated gas has a small absorption coefficient; it is based physically on competition among the effects of spectral saturation of the gas line, spatial modulation of the laser medium, and frequency synchronization of opposed waves from the ring laser. The technique is used to study the hyperfine structure of excited-neon absorption lines which is due to the presence of neon isotopes in the absorbing medium as well as to measure precisely the Zeeman splitting of methane IR absorption lines. The resolution achieved in these investigations is approximately two millionths per cm. It is shown that the response and accuracy of a nonlinear ring-laser spectroscope for studying the hyperfine structure of weakly absorbing gases can be increased by using self-oscillation lasing modes.