Thinned Multicore Fiber Supermode Bragg Grating and Interferometer for Temperature-Compensated Refractive Index Sensing

A temperature-compensated refractive index (RI) sensor based on thinned strongly-coupled seven-core fiber (SC-SCF) supermode Bragg grating and interferometer is demonstrated. The SC-SCF spliced between two standard single mode fibers was first tapered to 55.0 μm and then chemically etched to a waist diameter of 15.6 μm, forming a supermode interferometer. Bragg grating was successfully written into the thinned SC-SCF through the phase mask side-illumination method and two resonant dips were observed in the transmission spectrum. The transmission spectra and RI response characteristics of the device were theoretically analyzed and simulated based on the modal interference theory through finite element method. The RI and temperature responses of the device were experimentally investigated, respectively. The measured RI sensitivity and temperature sensitivity of one of the interference fringe dips were 504.1 nm/RIU for RI in range of 1.335∼1.345 and −10.1 pm/°C, respectively. The main Bragg grating dip was found to be insensitive to RI and have a temperature sensitivity of 12.3 pm/°C. This device is attractive for temperature-compensated RI sensing applications.