Investigation On The Novel Fiber Bragg Grating Sensing Technology

Thanks to its advantages of immunity to strong electromagnetic interference, corrosion resistance, low welding dissipation, high sensitivity, small volume and multiplexed sensing capability, fiber Bragg grating (FBG) as one of the most promising optical passive components has been widely applied in various sensing fields in recent years. Using FBG for high temperature safety monitoring has a broad application prospect, therefore, the study of the heat resistance FBG sensing system has always been a hot research topic. FBG acoustic emission (AE) technology is a novel detection technology which combines the advantages of FBG sensing technology and AE detection technology and has shown a good development potential in the field of structural health monitoring. Some foreign research institutes have studied this technology; however, relatively few studies have been reported in china.This dissertation aims at addressing some hot theory and application issues in the field of high temperature measurement and AE sensing based on FBG. A new heat resistant FBG sensing system has been designed and implemented based on researches of high temperature FBG sensing properties and multi-FBG demodulation technique; by using the established improved matrix data fusion model to fuse the raw temperature data of the new system, a high reliability can be achieved; theoretically and experimentally studied the key factors affecting the FBG AE sensing system sensitivity under low amplitude AE waves; in addition, a novel strain-insensitive and relocatable resonant FBG AE sensor with enhanced sensitivity is demonstrated. The main research contents are as follows:(1) Detailed the development and research status of the application of FBG; summarized and analyzed the research progress and level of FBG temperature sensing; summarized the current status and development trends of FBG AE sensing.(2) To overcome the shortcomings suffered by the existing heat resistant FBG sensing system, such as limited accuracy, slow response and poor anti-interference capability, a FBG temperature sensing model is established after an in-depth analysis of high temperature FBG properties. The temperature sensing system constructed with a novel tubular heat-resistance FBG sensor and a wavelength demodulating system based on InGaAs detector and diffraction technology is also designed. The novel tubular FBG sensor has advantages of small volume, high sensitivity and linearity, the optimized FBG sensor array can improve the reliability of the system and the novel wavelength demodulating system can eliminate the noises disturbances from the optical path. Experimental analysis has verified the sensing characteristics of the novel FBG sensor. According to the20℃～290℃temperature measurement results, the average goodness-of-fit of sensing system’s temperature curve is0.9991, the system has a sensitivity of0.0258nm/℃, accuracy of±0.6℃, response time of less than16s and the maximum temperature fluctuations±0.7℃. Moreover, the sensing network is steady enough for high temperature measurement.(3) In view of the problem of the traditional support degree matrix method can’t show the temperature data reliability in the whole measurement range, an improved support degree matrix model is established on the basis of the in-depth study of the principles and implementations of multi-sensor and data fusion algorithms. The improved support degree matrix model is used to fuse the raw temperature data in the heat-resistant FBG sensing system. Temperature measurement experiments show that the total absolute temperature estimated error of the system based on the improved support degree matrix model is significantly smaller than the error values of the other three fusion methods (averaging method, batch estimation method and the traditional support degree matrix method). According to the experimental results, the system based on the data fusion model has high accuracy and good reliability when a sensor in the sensor array is failed. Therefore, the system can be used in the practical projects for temperature measurement.(4) Considering the practical needs of AE wave detection, the refractive index distribution of an uniform FBG under AE wave is revealed after analysis of the FBG transfer matrix theory and research of the interaction principle between AE wave and FBG. On the basis of this study, the FBG AE sensing model is established. Combining with the actual measured main parameters of FBG and the simulated parameters of AE wave, the simulated reflection spectrum of FBG under AE wave can be achieved. Through the simulation the impact of the amplitude and wavelength of AE wave on the characteristics of FBG reflection spectrum is mainly analyzed. The FBG reflection spectra at the initial time and the spectra at the different times in one AE wave period are simulated under AE waves with different wavelengths and amplitudes. The simulation results indicate the qualitative or quantitative impact of the amplitude of AE wave and the ratio between the wavelength of AE wave and the grating length of FBG on the characteristics of the FBG reflection spectra. The aforementioned research on the numerical simulation of the FBG reflection spectra under AE waves provides an effective theoretical basis for FBG AE wave detection.(5) Through detailed analysis of the principle, advantages and disadvantages of the high frequency FBG demodulation method, a FBG AE sensing system based on tunable narrow band laser demodulation technology is built, which exhibits the advantages of simple construction, high wavelength resolution, high demodulation speed and perfect sensitivity. The sensitivity model of FBG AE sensing system under a low-amplitude AE wave is established on the basis of a in-depth study of the impact of the grating length of FBG on the sensitivity of the AE sensing system. The sensitivity of the FBG AE sensing system based on FBGs of different lengths is theoretically simulated. Additionally, using the simulation model, the main influencing factor on the sensitivity of the ultrasonic sensing system with different lengths gratings is first investigated. In the following AE detection experiment, the AE wave responses of the sensing system with six FBGs of different lengths are obtained, respectively. The theoretical analysis and the experimental results both indicate that the sensitivity of the system is primarily determined by the slope of the FBG spectral linear range. The conclusion will be useful for sensitivity improvement of the FBG-based ultrasonic and acoustic emission sensing system.(6) A novel resonant FBG AE sensor is demonstrated for AE wave detecting. Taking advantage of the one end free configuration and the relocatable acoustic coupling method, the sensor is insensitive to mechanical strain applied to the monitored structure and can be easily redeployed. Response experiments of the novel sensors with different sensing lengths show that the first-order resonance frequency decreases as the sensing length increases. The theoretical and experimental values of the first-order frequency match well, the error is less than±2kHz; as the sensing length decreases, the6dB bandwidth of the first-order resonant peak increases, and the detected resonant peak bandwidth is less than7.5kHz. Tensile test has verified the strain-insensitive characteristic of this FBG-AE sensor. Comparison tests between the novel FBG-AE sensor and the traditional fully bonded FBG-AE sensor demonstrate that the sensitivity is enhanced by about1.2times. The new designed sensor possesses good resonant frequency response to the standard AE signals as well.