Low cross-sensitivity and sensitivity enhanced FBG sensor based on OCMI with three cascaded FBGs

This paper presents a highly sensitive, temperature-insensitive optical carrier microwave interferometry (OCMI) system using a cascaded three fiber Bragg grating (FBG) structure to generate an enhanced Vernier effect for sensing applications. The enhanced Vernier effect is created by superimposing the interferograms of two separate interferometers formed by pairing the sensing FBG with each reference FBG. Experimental and theoretical results show that in strain sensing, the sensitivity based on enhanced Vernier effect is -4.642 MHz/µε, which is 66.3 and 61.4 times higher than that of strain sensor based on two single interferometers used to generate the enhanced Vernier effect in this system respectively. Compared with the strain sensor based on traditional Vernier effect, this sensor has higher sensitivity, and in addition the amplification factor for measuring sensitivity can be easily adjusted by changing the spatial distance between the three cascade FBGs. Moreover, the temperature sensitivity is decreased from -38.318 MHz/°C to -71.384 kHz/°C with temperature compensation. The sensor exhibits high resolution, high sensitivity, and low cross-sensitivity, making it great potential for measuring small physical changes in complex environments.