FBG Based Compound Temperature And Strain Sensing Technology

Supported by the special fund project of Jiangsu Province for transforming scientific and technological achievements which is charged with Jiangsu Trigiant Photoelectric Communication Co Ltd and Southeast University, the fiber Bragg grating (FBG) temperature/strain sensor technology and its applications is investigated in this thesis. The works focus on to achieve simultaneous detection of temperature/strain and to monitor photoelectric composite cable in the base station and antenna system so that can set up the quasi distributed FBG array sensing system and ensure the stable operation for the mobile base station and the antenna system.Based on the coupled mode theory, firstly, the mathematical models for the amplitude and power coefficients of reflection/transmission waves in single mode FBG are derived. The transmission phase and delay characteristics are analysed and their mathematical models devised for the lightwave transming through the FBG, which can be defined as the time delay and group index, induced by the sidelobe in the transmission spectrum of FBG. Analytic results show the group index in the sidelobe regine of transmitted lightwave is proportional to the first order derivative of the lightwave phase to wavelength in a FBG length L so that induces the slow light. Then the compound sensing models based on the center wavelength shift for reflect lightwave and the group index for slow lightwave in the transmission sidelobe for same FBG are investigated and established. Based on above, the compound sensing scheme is proposed by simultaneously detecting the centre wavelength shift for reflected lightwave and the time delay for slow lightwave, which can sensor simultaneously the change of temperature and strain imposed on the FBG. Afterward, we simulate and analyze the relationships between the reflection/transmission spectra and the sensor parameters of FBG, as well as the slow wave group index characteristics near the sidelobe for different types of FBGs. The results show that slow light group index changes nonlinearly with the fringe visibility and gating length for the FBG. With the increase of the FBG length and the fringe visibility, the group index of FBG near the sidelobe of transmission spectrum can be significantly increased, and also the group index produced by uniform FBG is larger than those produced by chirp and apodization FBGs. Finally, we propose new scheme of compound FBG temperature/pressure sensor array system, discuss the timing sequence of single pulse and adjacent pulses in the sensor network, and determine the minimum distance and the smallest time interval in adjacent pulse, the arrangement of FBG array and the scanning period of optical source.