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All-Fiber Michelson Interferometer Proposed as Unique Sensor

Photonics Spectra
Nov 2006
Measurement of mechanical parameters could be temperature-independent.

Breck Hitz

Researchers at Harbin Engineering University in China have demonstrated an all-fiber Michelson interferometer that they suggest could be used as a displacement sensor, an accelerometer or a flow-velocity sensor. Unlike many other fiber optic sensors, theirs would be independent of environmental temperature and pressure changes because both arms of the Michelson would be affected equally by such changes.

The researchers constructed the interferometer by splicing a dual-core fiber onto the end of a single-mode fiber and coating a 40-percent-reflecting silver surface onto the end of the dual-core fiber (Figure 1). Bending the latter induced a phase difference between the backreflected light in the two cores. The researchers calculated the magnitude of the phase difference as a function of bending, taking into account both the change in refractive index and the change in physical length of the cores.

PRMichelson_FIg1.jpg

Figure 1. The all-fiber Michelson interferometer comprises a dual-core fiber spliced to the end of a single-mode fiber. The two cores of the dual-core fiber are the arms of the interferometer, and the tapered zone between the fibers is analogous to the beamsplitter in a conventional Michelson. LD = laser diode; PD = photodiode.


After a bit of arithmetic, they concluded that the phase difference was linearly proportional to the vertical deflection of the end of the dual-core fiber; i.e., δφ = k(y/l), where y is the vertical displacement of the end of the dual-core fiber (“vertical” as indicated in Figure 1), l is the length, and k is a constant taking into account the optical and mechanical parameters of the fiber.

PRMichelson_FIg2.jpg
Figure 2. The researchers measured micron-scale deflections of the end of a 40-mm-long dual-core fiber. The individual points represent experimental data, which fit well to the solid straight line.


As with any Michelson, the amplitude of the light returned from the device is a measurement of the phase difference between the arms. Thus, when the scientists illuminated the interferometer with 670-nm light from a diode laser, they could infer the vertical displacement of the end of the dual-core fiber from the intensity of the returning light (Figure 2). They envision their interferometer serving as an accelerometer, for example, by orienting the fiber orthogonal to the acceleration, so that even a very small acceleration would produce a measurable displacement of the fiber end.

Optics Letters, Sept.15, 2006, pp. 2692-2694.


GLOSSARY
interferometer
An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces; it can measure a length in terms of the length of a wave of light by using interference phenomena based on the wave characteristics of light. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers. The basic interferometer components are a light source, a beamsplitter, a reference...
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