Intensity detecting refractive index sensors with long-period fiber gratings written in STS structure

A refractive index intensity detecting sensor with long-period grating written in the single-mode–thin-core–single-mode fiber(STS) structure is proposed and optimized theoretically. The sensor is composed of two single-mode fibers connected by a section of long-period fiber grating fabricated on thin-core fiber. After optimization and benefitting from the phase matching point, the loss peak of the structure can reach 62.8 d B theoretically. The wavelength of the characteristic peak is fixed at the phase matching point, so intensity detection can be achieved. The sensitivity can reach 272.5 d B/RIU. The structural optimization in this Letter provides a reference for the fabrication of an easy-made all-fiber sensor without extra cladding.