Low cross-sensitivity and sensitivity-enhanced FBG sensor interrogated by an OCMI-based three-arm interferometer.

An FBG sensor interrogated by an optical carrier microwave interferometry (OCMI)-based three-arm Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. In our sensing scheme, the interferogram generated by interfering the three-arm-MZI middle arm with the sensing arm and the reference arm respectively is superimposed to produce a Vernier effect to increase the sensitivity of the system. The simultaneous interrogation of the sensing fiber Bragg grating (FBG) and the reference FBG by the OCMI-based three-arm-MZI provides an ideal solution to the cross-sensitivity problems (e.g. temperature vs. strain) associated with conventional sensors that produce the Vernier effect by cascading optical elements. Experimental results show that when applied to strain sensing, the OCMI-three-arm-MZI based FBG sensor is 17.5 times more sensitive compared to the two-arm interferometer based FBG sensor. And the temperature sensitivity is reduced from 371.858 KHz/°C to 1.455 KHz/°C. The prominent advantages of the sensor, including high resolution, high sensitivity, and low cross-sensitivity, make it a great potential for high-precision health monitoring in extreme environments.