Reflective fiber refractometer based on fiber Bragg grating inscribed in thin-core-fiber

A novel reflective refractometer based on a fiber Bragg grating (FBG) inscribed in a thin-core-fiber (TCF) stub was demonstrated. The cladding modes are excited by the core diameter mismatch between the single-mode-fiber (SMF) and TCF. The core mode and specific cladding modes in the TCF are reflected by the FBG and partly recoupled into the back-propagating core mode in the SMF. Experimental results demonstrate that the power of the cladding mode reflection changes with the surrounding refractive index (SRI). The temperature-immune SRI measurement with linear sensitivity of 54.55 dB per refractive index unit has been achieved by the cost-effective power detection within the refractive index range from 1.33 to 1.42. To eliminate the measurement deviation caused by the fluctuation of light source, the simple self-referencing detecting has been carried out by measuring the ratio of the cladding mode reflection and the core mode reflection.