Novel fiber-optic distributed refractive index sensing method by optical frequency domain reflectometry

We proposed a novel fiber-optic distributed refractive index (RI) sensing method by optical frequency domain reflectometry (OFDR), which is qualified for most scenes such as solution diffusion process and multi-analytes detection that require distributed RI sensing ability. We use a section of multi-core fiber (MCF) as the sensing fiber, which consists of one central core and six outer surrounding cores. The MCF is etched by hydrofluoric acid until the outer cores are exposed to the air, thus the effective index of outer cores will be affected by the RI of the surrounding samples, which will lead the wavelength shift of the Rayleigh backscattering (RBS) spectrum. In order to realize distributed RI sensing, we introduce the OFDR technique, which has excellent ability of spatial localization. Then, by demodulating the wavelength shift of the local RBS spectrum, the distributed RI sensing is realized. The sensing length of 19 cm and the spatial resolution of 5.3 mm are achieved in experiment. The RI sensing range is as wide as 1.33~1.44 RIU, and the maximum sensitivity of 47 nm/RIU is obtained around 1.44 RIU. In addition, the central core can be used for temperature compensation because it keeps isolated to the surrounding sample. The experimental results indicate that the temperature sensitivity reaches 9.8 pm/°C during 25~80 °C. By temperature compensation, the RI measurement error induced by temperature disturbance can be eliminated. Finally, we successfully detect the diffusion process of glycerol in water using the proposed fiber-optic distributed RI sensor.