Core–cladding mode recoupling based fiber optic refractive index sensor

We present a compact and robust fiber optic refractive index sensor based on a tilted fiber Bragg grating (TFBG) inscribed in a thin-core fiber that is spliced to a standard single-mode fiber (SSMF). Due to the core mismatch between the SSMF and the thin-core fiber, cladding modes generated from the TFBG can be back into SSMF for detection. Since the effective refractive index of a cladding mode is dependent on the surrounding refractive index (SRI), the cladding-mode resonances can be used for measurement. We fabricate the sensor probe and test it in a series solution with different SRIs, and the experimental results agree with the theoretical analysis. The repeatability of the sensor is also tested in the same SRI range. We demonstrate its capability and simplicity to use this sensor for a wide range SRI measurement.