Temperature-Compensated Refractive-Index Sensing Using a Single Bragg Grating in an Abrupt Fiber Taper

In this paper, temperature-compensated refractive-index (RI) sensing is realized by use of a single Bragg grating inscribed in a silica microfiber. The microfiber is tapered from a single-mode fiber with two short transition regions. The mode evolution and intermodal couplings are analyzed based on the coupled local-mode theory. Due to the sharp variation of transverse geometry along the fiber length, couplings between copropagating local modes are excited, and the energy of light can be distributed into different modes by the transition regions. The grating couples the incident light to phase-matched backward fundamental and higher-order modes at the individual wavelengths. The coupled higher-order modes can be partially retrieved as a result of the abrupt taper. As a result, several reflection peaks can be observed in the reflection spectrum. The peaks present different responses to surrounding RI and identical temperature sensitivity. Therefore, the temperature cross-sensitivity can be removed by measuring the wavelength separations. Compared with previous reports, the proposed method is simpler and more practical.