Reflective Refractometer Based on a Thin-Core Fiber Tailored Multimode Fiber Bragg Grating

A temperature-independent reflective refractometer configuration based on a core-diameter-mismatch is proposed and experimentally demonstrated. The configuration consists of a short section of thin-core fiber (TCF) with a multimode fiber (MMF) tip inscribed with a fiber Bragg grating (FBG). The TCF excites cladding modes into the downstream MMF via the mismatch-core splicing interface, and the parts of the core and cladding modes are reflected back to the lead-in fiber core by the MFBG. The recoupling efficiency of the core and cladding modes is highly dependent on the surrounding refractive index (SRI) of the TCF and MMF. Experimental results show that some recoupled modes show a high sensitivity to SRI. Power-referenced and temperature-independent SRI measurement with improved sensitivity for resolutions with SRI values near 1.33 is achieved through the wavelengths of selective mode monitoring.