Research And Application Of Key Technique For Multiplexing Of The Fiber Bragg Grating Sensor

In recent years, fiber Bragg grating (FBG) sensing technology has become an important topic of the sensoring field. It has been widely investigated and applied due to its advantages. The demodulation cost of a single FBG sensing system is not competitive because a complex and expensive equipment is required. Only throughing multiplexing technique to set up a sensing network, in order to highlight the advantages of FBG. With the subjects supported by the National Nature Science Foundation of China (Grant No.60877043), the Provincial Nature Science Foundation of Heilongjiang Province (Grant No.F201435), the Provincical Science and technology Foundation of Heilongjiang Province (Grant No.GC08A514), the Science and technology research institute of Shenyang railway bureau "The evaluation of the railway bridge strengthening effect" and the High Level Innovation Teams of Heilongjiang University (Grant No.HD-028). The FBG vibration sensor, the multi-longitudinal mode fiber laser sensor and its multiplexing are studied and applied, and obtained some innovatice research results. The main works are as follows:First, the development history and classification of the FBG are reviewed. The passive and active FBG sensing technology and the multiplexing technology of the FBG sensor are introduced, systematically. The research target of this paper is provided.Second, the coupled mode theory of the FBG is provided. Base on the theory, a passive FBG vibration sensor was developed, and its sensing properties have been studied experimentally. The structure parameters of a railway bridge has been monitored using our sensor based passive wavelength and space division multiplexing system, the number of multiplexed FBG sensor is 37.Third, based on the energy-level transition theory, the principle of the fiber laser as well as the threshold and phase of laser oscillation have been analized in theory. Then the sensing theory of the multi-longitudinal mode fiber laser sensor was introduced, in the multi-longitudinal mode fiber laser sensor, the beat frequency signal generated by the interference between multi-longitudinal modes acts as a sensing signal, and shifting of the beat frequency is used to reflect the physical paremeter change. Three multi-longitudinal mode fiber lasers with different laser cavity structure have been designed, and a stress test platform is built to study their stress-frequency responses.In order to improve the sensitivity and the SNR, a high precision accelerometer based on π phase-shift DFB fiber laser on a triangular cantilever beam structure was developed. The acceleration and frequency response has also been studied through the contrast test.Fourth, by applying the principle that the multi-longitudinal mode fiber laser sensor on different laser cavity length will have different beat frequency spacing, while combining with the frequency (wavelength) division multiplexing (FDM) technology, three active FBG sensing system multiplexed different laser cavity structure have been exploited. These active sensing systems realize the function of multi-point sensor addressing, and the problem of the crosstalk between multiple sensors and the pump light power distribution in these active FBG-FDM sensing system have also been resolved, successfully. Finally, shift responses of the strss-beat frequency of these active sensing systems are studied experimentally by using a micro-displacement platform to stretch the resonant cavity length changing the beat frequency shift. The beat frequency signal noise sources of these systems are also analyzed.Fifth, the wavelength division multiplexing fiber loop ring-down spectrum (WDM-FLRDS) sensing system based on the wavelength selection of FBG has been proposed. The sensing principle of FLRDS, the multiplexing principle of the WDM-FLRDS sensing system, the maximum multiplexing number of sensors and the crosstalk between the ring-down pulse signals have been analized in theory. At last, the pressure-ring-down time responses of this WDM-FLRDS sensing system are studied by using microbend pressure device. The experiment results are agreed with the theoretical analysis.