Fiber Bragg Grating Sensing Key Technologies

In this thesis, we have systematically investigated the key technologies for fiber Bragg grating (FBG) sensor to be applied into temperature and pressure measurement inside long-distance oil pipelines. The main body is composed of following parts: the sensitivity enhancement, the encapsulation modification, and the wavelength division multiplex of FBG.Lower response sensibility to environment temperature and pressure is a fatal problem of bare FBG sensor. Based on the knowledge of existing technologies for sensibility enhancement, a method is proposed to improve the temperature sensitivity, in which a FBG is fixed on a plate stuff or a tube with a kind of high temperature-resistance pastern. The experiments show that the temperature sensitivity coefficient is about 3 times that of bare FBG in the temperature range from 0 to 90℃. According to the requirement of temperature and pressure measurement in oil pipelines, the combination of cantilever and Bourdon tube is put forward to realize the tunable sensibility and measuring range of fiber grating pressure sensor. The maximum pressure response sensibility reaches 0.188nm/MPa in the pressure range of 0-10MPa, about 63 times that of bare fiber grating sensor. Moreover, the encapsulated sensors have excellent linearity and repeatability.Chirp appears usually in FBG encapsulated by conventional process, together with fluctuating linearity and repeatability. This restricts the application of FBG to a great degree. To overcome this drawback, we designed a tunable pre-strain frame. The temperature sensing test on 44 FBG, which are encapsulated in this way, demonstrated that, their response sensibility is enhanced greatly no matter on a plat or in a tube. Furthermore, they have excellent linearity, even with the value of above 0.99 for most of them. The coefficient of temperature response sensibility varies from 1.94×10^-5 to 2.21×10^-5/℃, which is in good agreement with theoretical prediction (2.08×10^-5 /℃). Therefore, it can be stated assuredly that our new encapsulation approach can guarantee absolutely the relevant parameters of FBG, such as response sensibility, repeatability and linearity.We further studied the performance requirements for wavelength division multiplex and the number limitation due to signal-to-noise ratio and wavelength band. The stability of band width and reflectivity is improved tremendously through optimization of response sensibility and modification of encapsulation technology. And finally the quasi-distributed network is established with a bandwidth of 45nm for temperature, pressure and strain sensing by wavelength division multiplex, in which 20 FBG can be used simultaneously.