The coupled-cavity laser interferometer with rationally related cavity lengths

Abstract

Single-ended commercial gas lasers are used to measure small changes in the optical length of an external cavity. The ratio of the length of the laser cavity to the length of the external cavity is chosen to be a rational fraction and this permits a wide range of external cavity lengths to be used in practice. The denominator of this ratio is set equal to the number of longitudinal lasing modes s, and this enables each mode to contribute sequentially to the measurement, producing one cycle of modulation in lasing intensity for each lambda /2s change in length. A short 7.5 mW helium-neon laser was used to record successive lambda /8 changes in length of external cavities that had a gross length ranging from one quarter to nineteen quarters of the laser cavity length. In principle greater sensitivities can be obtained by using longer gas lasers.