Laser Feedback and New Principle of Heterodyne Interferometry

Among the obstacles of a stable interference pattern in precise long-path laser interferometers the optical feedback through scattering in lasers is insidious. If even the perfect stabilized lasers are used, the accuracy of interference measurements can be aggravated by few orders because of a rather weak optical feedback. The new principle of extra-resonator heterodyne interferometry based on a laser frequency modulation (FM) of light that overcomes this difficulty is described. The multi-mirror cavity model of precise laser device is used to estimate the effect of the performance degradation both in the long-path geophysical laser strainmeters and in the equal-arm gravitational wave interferometers. The developed FM-interferometry technique should advance the precision of laser instruments and extend the area of their application