Principles of Fiber-Optic Interferometry

Abstract

Optical interferometers are well-known for their ability to make high-precision measurements of optical path difference (OPD) or changes that may be induced by a physical displacement or a refractive index change in the interferometer. Various configurations of interferometers had been demonstrated using traditional light sources with relatively short coherence lengths, long before the invention of the laser in 1960; these conventional interferometers are well identified with the founders of modern optics, such as Newton, Young and Michelson. However, the applications of optical interferometers were limited greatly by the poor spatial and temporal coherence and directionality of the available optical sources. The invention of the laser in the early 1960s had a dramatic impact on optical interferometry as much larger OPDs could be measured due to the greater temporal and spatial coherence and the much better directionality of the laser, for example the He-Ne laser. Also, the measurement precision could be improved considerably due to the greater brightness of the laser. Vibration measurement with sub-Angstrom resolution had been demonstrated [1]. Laser-based interferometry has become a standard technique for distance and vibration measurement. However, such measurement systems are generally restricted to laboratory environments due to the fundamental requirement to maintain the relative alignment of the internal optical beams constant, as this critical alignment can be easily perturbed by random noise unless the instrument is properly engineered. With the development of low-loss optical fibers and their associated fiber-optic components, all-fiber-optic versions of many of the classical interferometers have been introduced. The incorporation of fiber optic components into the interferometer allows the construction of robust and versatile systems capable of remote operation, lending to the generation of general purpose sensors for any physical parameter that affects, directly or indirectly, the OPD within one of the fiber arms. This has made it possible to use optical interferometers outside the laboratory for the measurement of many physical variables. Although fabricating an optical interferometer from optical fibers solves many of the alignment problems discussed above, the nature of the mode of operation of the optical fiber, correctly termed an optical waveguide, can give rise to significant problems due to its modal characteristics (birefringence).