High-sensitivity and high extinction ratio fiber strain sensor with temperature insensitivity by cascaded MZI and FPI

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In this study, a hybrid cascade strain sensor of a Mach-Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI) with high sensitivity and high ER based on Vernier effect is proposed. The two interferometers are separated by a long single-mode fiber (SMF). The MZI is used as the reference arm, which can be flexibly embedded in the SMF. The FPI is used as the sensing arm and the hollow-core fiber (HCF) as the FP cavity to reduce optical loss. Simulation and experiments have proven that this method can significantly increase ER. At the same time, the second reflective face of the FP cavity is indirectly spliced to increase the active length to improve the strain sensitivity. Through the amplification of Vernier effect, the maximum strain sensitivity is -649.18pm/με, and the temperature sensitivity is only 5.76pm/∘C. The magnetic field was measured by combining the sensor with a Terfenol-D (magneto-strictive material) slab to verify the strain performance, and the magnetic field sensitivity is -7.53nm/mT. The sensor has many advantages and has potential applications in the field of strain sensing.