A novel surrounding refractive index sensor based on the polarization characteristics of thin-cladding long-period fiber grating

•This paper proposes a method based on the photoinduced birefringence of thin-cladding long-period fiber grating for surrounding refractive index measurement.•The core mode and cladding modes for optical fibers of various cladding radii are analyzed with the three-layer fiber model.•The sensing principle is analyzed using the full-vector coupled mode equations and the Stokes parameters, and the effects of LPFG and incident light parameters on the sensing properties are investigated.•The method needs a tunable laser source for the measurement of output light polarization states.

In this paper, a new method to measure the surrounding refractive index (SRI) is proposed. It is based on the core-only photoinduced birefringence of long-period fiber grating (LPFG). The relationship between Stokes parameters of output light polarization states and SRI is analyzed using full-vector coupled mode equations. Compared to the conventional LPFG SRI sensors, this sensing method features a fairly good linearity performance and reasonably high resolution over a much broader SRI measurement range from 1 to 1.46 which may be applied to the humidity and vapor sensing.