Chapter 12 - Interferometric Sensors

Abstract

Advancements in the field of lasers and optical fibers have opened up the use of interferometers as sensors for a number of physical quantities. The chapter describes the use of interferometry as sensors—namely, rotation lasers, fiber interoferometers, laser feedback inferometers, Doppler interoferometers, vibration interoferometers, interoferometric magnetometers, and adaptive optics. Ring-laser rotation sensors offer several advantages over mechanical gyroscopes including fast warm-up, rapid response, large dynamic range, insensitivity to linear motion, and freedom from cross-coupling errors in multiaxis sensing. For these reasons, they have been used widely in missile guidance and inertial navigation systems. The passive ring interferometer eliminates the problem of mode locking by using an external laser and measuring the differential shift in the resonant frequency for the two directions of propagation. Fiber interferometers can also give very high sensitivity for some types of measurements, because they make possible very long paths with very low noise, in a small space, permitting the use of sophisticated detection techniques. The first application of fiber interferometers was in rotation sensing, where a closed multi-turn fiber-optic loop was used, instead of a conventional cavity with mirrors, to increase the effective area of the ring. Other applications of fiber interferometers take into account the fact that the optical path length changes when a fiber is stretched and is also affected by the ambient pressure and temperature, so that an optical fiber can be used as a sensor for a number of physical quantities.