Distributed refractive index sensing based on tapered fibers in optical frequency domain reflectometry.

We present a distributed refractive index (RI) sensor using tapered optical fibers in optical frequency domain reflectometry (OFDR). RI of the external medium surrounding the tapered optical fibers is measured by the optical frequency shifts of the local back-reflection spectra in OFDR. By a spectrum interpolation, we can increase the resolution of RI measurements without decreasing the sensing spatial resolution. In our experiments, we realize a truly distributed RI sensing with a 4.25 mm spatial resolution and 2.1 cm measurement distance. We calibrate the relationship between the optical frequency shifts of the local back-reflection spectra and RI variation. RI ranges from 1.3574 to 1.3686 and the sensitivity is about 8565 GHz/RIU (68.52 nm/RIU) in the presented sensor. We also measure RI variation in a glycerol solution diffusion to verify the capability of distributed RI sensing by the presented sensor.