Fiber Refractometer Based on a Fiber Bragg Grating and Single-Mode–Multimode–Single-Mode Fiber Structure

A refractive index (RI) sensor based on a novel fiber structure that consists of a single-mode–multimode–singlemode (SMS) fiber structure followed by a fiber Bragg grating was demonstrated. The multimode fiber in the SMS structure excites cladding modes within output single-mode fiber (SMF) and recouple the reflected cladding Bragg wavelength to the input SMF core. By measuring the relative Bragg wavelength shift between core and cladding Bragg wavelengths, the RI can be determined. Experimentally we have achieved a maximum sensitivity of 7:33nm=RIU (RI unit) at RI range from 1.324 to 1.439