Research On Cross-Sensitivity Of Fiber Bragg Grating Sensors

As a booming sensor technology, Fiber Bragg Grating has many unique advantages such as strong anti-interference of electromagnetism capability, corrosion resistance, small size, light weight and high sensitivity, so it can be used widely in civil engineering, petrochemical industry, aviation and space flight. Therefore, finding solutions of pivotal problems about FBG in theory and practical application can bring important scientific significance and economic value. In order to make fiber Bragg grating sensor be practical and produced on large scale, it needs to take effective method to solve the question of cross sensitivity about temperature and strain. So the paper is mainly focused on this problem and detailed studies on some areas such as fiber grating theory and solutions are carried out.Firstly, this paper introduces basic theory of fiber grating and sensing mechanism. Embark on coupled mode theory, reflection spectrum characteristic of uniform fiber Bragg grating is studied. And further analysis about the change of reflection spectrum when different parameters is carried out using simulation. Then the table of bandwidth and the maximum reflectivity under different parameters is given. According to transport theory, strain and temperature characteristics of fiber grating are analyzed and the corresponding sensor model is verified at the same time. And then the change of FBG reflection spectrum under non-uniform strain is obtained using transfer matrix method and simulation. Two-dimensional and three-dimensional comparison diagram are also given.Secondly, based on detailed theoretical study on cross-sensitivity of fiber Bragg grating generation mechanism, this paper analyzes the solutions of cross sensitivity about strain and temperature at home and abroad. And then an effective and low complexity measurement method is introduced, which is based on Equi-intensity cantilever with pre-strain fiber Bragg grating as the sensing element. According to this method, a measurement system is established. Strain and temperature sensing properties of fiber Bragg grating are studied using this system. Then experimental results are analyzed, and relationship curved surface about Bragg resonance, wavelength temperature and strain is plotted. In addition, the measurement result impact of the non-linear relationship between temperature and wavelength is analyzed. Then the linear and nonlinear application scope of reflection wavelength and temperature is given. Linear responds to strain and temperature of bare grating part and pre-strain part are achieved in the linear measurable ranges of temperature. The measured strain sensitivity coefficient and temperature sensitivity coefficient is 0 nm/N,0.4082nm/N, 0.0128nm/℃and 0.1346nm/℃. The results show that the method needs only a light source, a demodulator and optical architecture is simple, so this system makes the actual measurement of temperature and strain facilitated.Finally, in order to determine the wavelength of the output signal when the peak moment occurs accurately in the process of solving cross-sensitive, a spectrum processing method is proposed which is used to filter noise and get precise peak at the request of low cost and high precision. This method can solve cross-sensitivity problem with high precision. On this basis, the paper analyzes the algorithm using experiment and simulation. According to this analysis, adaptive threshold wavelet can effectively improve SNR of the system and reduce reading errors caused by the noise. In addition, the error of the wavelength testing is 0.01nm before B-spline fitting and 0.0017nm after fitting under the same sampling precision and error is lowed about an order of magnitude. So B-spline fitting can improve the resolution for measurement of fiber Bragg grating wavelength shift and reduce detection errors of wavelength under the same sampling precision.