Filmed extrinsic Fabry–Perot interferometric sensors for the measurement of arbitrary refractive index of liquid

A filmed extrinsic Fabry–Pérot interferometric (FEFPI) sensor for the measurement of arbitrary refractive index (RI) is experimentally demonstrated. The FEFPI sensor was constructed by aligning two fiber endfaces, on which the multilayer dielectric mirrors are deposited. There is a fixed reflection at the filmed fiber endface no matter how much the RI is, and we can obtain a clear interferogram with a high visibility. A resolution of up to 1.64 × 10−5 refractive index unit is achieved in our experiment.