Temperature Compensated Refractometer Based on Parallel Fiber Fabry–Pérot Interferometers

We experimentally demonstrate a temperature compensated highly sensitive refractometer based on integrated parallel Fabry-Pérot interferometers (FPIs). The sensor is constructed by splicing a fraction of hollow-core fiber between a seven-core fiber and a single-mode fiber, which simultaneously forms an open cavity and a sealed cavity. The two FPIs are mutually independent. The opening cavity provides entire interaction between filled liquid/gas and guided light with a high sensitivity of 1096.9 nm/RIU, but that of the closed one is on the contrary, i.e., insensitive to the external refractive index (RI) because of the sealed resonant cavity. The temperature sensitivities of the parallel FPIs in deionized water are -137.6 and 2.25 pm/°C, respectively. Therefore, such a refractometer could eliminate the temperature cross sensitivity, with ultra-compact size, low cost, and prompt access to liquid samples for direct RI measurement.