High-sensitivity optic fiber strain sensor with low crosstalk in temperature via cascading a tunable Fabry-Perot cavity and a Sagnac interferometer

The optical Vernier effect (OVE) has received much attention from researchers in the fiber sensing field due to its remarkable capability to enhance measurement sensitivity. To the best our knowledge, we first propose and demonstrate a highly sensitive optic fiber strain sensor based on OVE, utilizing a tunable Fabry-Perot interferometer (FPI) and a Sagnac interferometer (SI). The reference element FPI consists of an open cavity (i.e., air cavity) formed by two end faces of single-mode fibers positioned on high-precision electric motors. Consequently, the free spectral range (FSR) of the FPI can be precisely controlled by adjusting its cavity length with assistance from these high-precision electric motors. The sensing element SI is constructed using a section of polyimide-coated Panda-type polarization-maintaining fiber (PMF) through a 3 dB coupler. When the FSRs of the FPI and SI are similar, they are cascaded to generate OVE, which can amplify the sensitivity of the fiber sensor. The experimental results show that the sensitivity of the single SI structure for measuring the strain is only 10.2 pm/με. Meanwhile the strain sensitivity of the cascaded open-cavity FPI and SI structure is significantly increased to approximately 199.98 pm/με. It is evident that the strain sensitivity of the cascaded FPI and SI structure is more than 19.6 times higher than that of the single SI structure. The designed optic fiber sensor exhibits a low temperature cross-sensitivity of 0.341 με/°C and good stability. Hence, it can be considerable potential for applications in strain monitoring environments.