All-Fiber Michelson Interferometer Proposed as Unique Sensor

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.

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.

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.