A sensing structure consisting of an abrupt taper spliced uniformly in to a fiber Bragg grating （FBG） is proposed and experimentally demonstrates refractive index （RI） and temperature measurements. Cladding modes are generated in the fiber through the abrupt taper containing the FBG. Most modes are reflected by the FBG at shorter wavelengths and reenter the launch fiber after passing through the abrupt taper. Spectral integrals are used to measure the power generated by the cladding and core modes. A sensitivity of-83.97 nW/RIU for ambient RI and a temperature sensitivity of 10 pm/℃ are obtained. No cross- sensitivity problems exist between ambient RI and temperature measurement.
The reflective spectrum power and the bandwidth of the fiber Bragg grating （FBG） under gradient strain are researched and experimentally demonstrated. The gradient strain is applied on the FBG, which can induce FBG bandwidth broadening, resulting in the variation of reflective power. Based on the coupled-mode theory and transfer matrix method, the segmental linear relationship between the gradient strain, the reflective power, and the bandwidth is simulated and analyzed, and the influence of the FBG length on the reflective spectrum is analyzed. In the experiment, the strict gradient stain device is designed; the experimental results indicate that the reflective optic power and the bandwidth of the FBG under gradient stain are concerned with the length of the FBG. Experimental results are well consistent with the theoretical analysis, which have important guiding significance in the FBG dynamic sensing.
