Distributed refractive index sensing based on U-bent fiber through spectral detection of Rayleigh scattering

Abstract

We developed a distributed refractive index (RI) sensor based on high performance optical frequency-domain reflectometry (OFDR) by simply bending a piece of standard single mode fiber (SMF) in a U shape. In the U-bent region, cladding modes are excited, which can reach to the boundary of the SMF to sense external RI variation. The cladding modes are then coupled back to the core mode and interfere with the fundamental mode. Thus, the fundamental mode can carry the varied RI information, and distributed index sensing is achieved by measuring the wavelength shifts of the local Rayleigh backscattered spectra. Thanks to the high signal SNR of OFDR, that compensating the bending induced loss, the proposed sensor can be bent in a small bending radius so that a high sensitivity of RI could be achieved. In the experiment, index sensitivity of 39.08 nm/RIU is achieved by imposing a bending radius of 4 mm, when the RI ranges from 1.3330 to 1.3773. Additionally, the proposed sensor maintains buffer coating intact, which boosts its practicability and application flexibility.