The Research On Distributed Stress Sensing Based On Optical Frequency Domain Reflectometry

Optical frequency domain reflectometry(OFDR) is an advanced distributed optical fiber sensing technology. Compared with optical time domain reflectometry(OTDR),OFDR has several significant advantages of high spatial resolution, high sensing accuracy and high sensitivity. So it has been widely used in military and civilian application. Firstly, the components and principle of OFDR are introduced and its main parameters are analyzed. Secondly, the sensing mechanism using fiber Rayleigh scattering is discussed. Thirdly, the nonlinearity caused by tunable laser and its compensation methods are studied. Finally, the experiment system based on OFDR for distributed stress sensing is established.The main works of the thesis are listed as follows:1. The distributed stress sensing theory of OFDR is elaborated. The OFDR system based on Mach-Zehnder interferometer is theoretical investigated. Besides, the principle of distributed stress sensing based on local Rayleigh backscattering profile is discussed.The theories show that the position of the measured point is determined by the beat frequency of OFDR and the wavelength shift of local Rayleigh scattering is proportional to the change of stress or temperature.2. Three nonlinearity compensation methods are proposed and demonstrated. The phase noise model is established to analyze the nonlinearity caused by tunable laser.Deskew filter, non-uniform fast Fourier transform and one-dimensional interpolation are employed to compensate the nonlinearity. The results demonstrate that deskew filter is only capable for a weak nonlinearity, and non-uniform fast Fourier transform is found to be only working at short distance and one-dimensional interpolation can compensate best at long distance with a strong nonlinearity. Compared with the former two methods,one-dimensional interpolation is proved to be more effective to improve the spatial resolution and more suitable for distributed stress sensing based on OFDR.3. The distributed stress sensing experiments based on OFDR are carried out. The results demonstrate that a spatial resolution of 5 mm is achieved at a fiber sensing length of 31 m with a wavelength scanning range of 80 nm while the spatial resolution is deteriorated to 13.4 cm at a fiber sensing length of 180 m due to the stronger nonlinearity with the wavelength scanning range of 6.144 nm. We also demonstrate thata stress difference in a range of from-400 g to 400 g can be measured with a stress resolution of 0.25 g when the wavelength scanning range is 6.144 nm. The scale factor of the wavelength shift and stress value is 13.135 pm/g under the goodness of fit R2 of0.99999 with a stress accuracy of 8.48%.