A Stable Heterodyne Interferometric Transducer

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Abstract
With conventional interferometric transducers, the light from a gas laser is reflected from a test piece and mixed with reference light to produce interference fringes as the test piece moves. A detector converts the light variations into electrical signals, which can then be analyzed to determine surface displacement. To determine target direction and to improve signal-to-noise characteristics, optical heterodyning is employed. This technique mixes light from the target with reference light that has been offset in frequency by, for example, an acousto-optic modulator. Thus, a stationary target gives rise to a signal at the offset frequency and a non-stationary target causes the received signal to deviate up or down from the offset frequency depending on target direction. Significant improvements in the transducer described above have been realized with the use of a semiconductor laser as the light source and by enclosing the beam paths in optical fibers. These two advances make the transducer more flexible, and also improve reliability and performance. However, the design of a solid-state, fiber-optic transducer presents a unique set of problems which must be solved before the technique can be used in an industrial environment. These problems include: 1) noise pickup in the fiber leading to the test piece; 2) feedback (or back reflections) of light into the laser diode; 3) modal noise introduced when the fibers are flexed; and 4) speckle effects which can cause signal dropout. Solutions to the above problems have been devised and implemented in a prototype interferometric transducer developed at United Technologies Research Center.

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