A Fiber-Optic Displacement Sensor Based on High-Precision Differential Phase Measurement

A fiber-optic displacement sensing scheme based on high-precision detection of differential phase is proposed, with advantages of simple structure, low cost, high precision, large dynamic range, polarization independent, and remote sensing ability. The designed displacement sensor is composed of a sensing fiber, a reference fiber, a Fabry-Perot laser diode, five couplers, two photodiodes and a signal processing circuit. The reference fiber is used for temperature compensation. The optical path difference (OPD) between the two fibers varies with the stretch of the sensing fiber. By high-frequency sinusoidal modulation of the laser source, the OPD can be converted to the differential phase between the reflected lights from the endfaces of the two fibers. The differential phase is detected by cross-correlation method. Experimental results show that, the resolution of the static displacement measurement has been achieved to be 1.4 μm. In addition, the dynamic range is estimated to be 2.66 m. Furthermore, the designed sensor shows the capability of dynamic displacement measurement.