Long Distance Fiber Bragg Grating Sensing System

With the rapid development of large composite and concrete infrastructures in China, more advanced sensing systems are needed in the construction and monitoring of important projects. Fiber Bragg Grating (FBG) is ideally suited for real-time structural health monitoring of oil pipeline and dam etc because of its unique characteristic. FBG can be easily embedded or attached to the structures, and FBG sensor arrays are easily made for temperature and strain monitoring by connecting several Bragg gratings written at different wavelength serially in a line along a single fiber. The sensing transmission distance with a broadband light source is limited up to 25km mainly due to transmission loss of optic fiber, Rayleigh scattering and intrinsic loss of the fiber link. The purpose of this thesis is to study novel methods to realize long-distance and remote sensing with low cost and good practicability. The main points of the thesis are as follows:Firstly, the principles and application of FBG sensor are analyzed, and the existing methods of long distance FBG sensing system are investigated and compared. Optic amplification technology is analyzed and compared, and then EDFA and FRA are chosen to apply to the long distance sensing system.Secondly, a driving circuit of pump laser is designed and manufactured, which is based on Atmel 89C52 MCU, and peripheral circuits of the system are accomplished including A/D, D/A, EEPROM, LCD, pump protection, temperature controlling, signal conditioning, etc. The circuit is applied to EDFA and FRA in the sensing system.Thirdly, a novel method of long distance sensing system based on EDFA / FRA hybrid amplification is proposed. A broadband source is used to provide reflective light for FBG. The sensing distance is up to 50km and the SNR of sensing signal is 14dB. Then, a fiber ring laser source is used for replacing the broadband source, a tunable filter is used to differentiate sensing signals, the excellent SNR of 58dB is achieved along 50km optic fiber.Finally, the stability, multiplexing, temperature sensing, static and dynamic strain sensing experiments are carried out with sensing system based on a fiber ring laser. The results show that the system works well. There is good linearity between the wavelength and temperature or static strain, and the dynamic strain with frequency of 800Hz can be detected in the system. All results demonstrate that the feasibility of the sensing system,...