Novel circulatory and regenerative multiple-beam interferometry methods making use of optical maser oscillations of extreme monochromaticity based on stimulated emission present interesting possibilities for investigating with high-precision various effects of motion and of fields on the propagation of light. These methods inherently permit increasing considerably the accuracy of the historical relativistic experiments of Michelson, Sagnac, and others, and have also potential applications to studies of other radiation propagation effects in magnetic, electric, and gravitational fields, and in moving refractive media, and to the modulation of coherent radiation. These propagation effects can produce frequency splits of the optical-maser oscillations, and the frequencies of the resulting optical beats are a measure for those effects; frequency measurements can be accomplished with high accuracy by using primary frequency standards, e.g., atomic clocks. Contrary to the many proposed applications of optical masers which are directed towards utilizing the high-spatial coherence of the wave fields, the present subject is thus primarily concerned with the narrow-frequency bandwidths possible with these novel radiation sources. The high accuracies possible with the new interferometric methods described should lead to quantitative results which up to now were either impossible to achieve, or only by experiments on very large scales.
ADOLPH H. ROSENTHAL, "Regenerative Circulatory Multiple-Beam Interferometry for the Study of Light-Propagation Effects," J. Opt. Soc. Am. 52, 1143-1147 (1962)