Research On Key Technologies Of Optical Fiber Strain Measurement Based On Broad Rayleigh Spectrum Detection

Strain measurement is one of the important research contents in the fields of structural health monitoring,experimental mechanics,and precision measurement.The difference in the mechanical properties of the structure itself and the complicated load distribution make the strain field of the structure have the characteristics of large strain range and uneven spatial distribution.These characteristics put forward higher requirements for the measurement range,spatial resolution and measurement accuracy of the strain measurement method.The optical fiber strain sensor can measure the strain field of the structure by sticking the optical fiber to the surface of the measured structure.Among them,the distributed optical fiber strain measurement method based on the principle of optical frequency domain reflectometer has the advantage of much higher spatial resolution than other optical fiber sensors,and has attracted the attention of scholars in recent years.However,this method still has problems such as imperfect measurement models,limitations of detection method,and inconsistent spatial measurement characteristics,which severely restrict the improvement of the measurement range and other performance in the measurement system.With special consideration for the problems and shortcomings of the existing distributed strain measurement method based on the principle of optical frequency domain reflectometer,this thesis carried out the theoretical research and experimental verification of the distributed optical fiber strain measurement method based on broadband Rayleigh scattering spectrum detection.High-precision distributed optical fiber strain measurement with large strain measurement range and high spatial resolution is realized.The main contents of this thesis are as follows:With regards to the lack of a complete measurement model in the existing measurement,which leads to the lack of theoretical guidance for the improvement of the strain measurement range,a distributed optical fiber strain measurement theoretical model based on Rayleigh scattering spectrum phase analysis is established.This model quantitatively described the mathematical relationship between the various parameters in the optical frequency domain reflection system and the different phase components of the Rayleigh scattering spectrum,established the mapping model between the incident signal light band,the Rayleigh detection spectrum and the strain,and determined the two types of strain detection methods in the optical frequency domain reflectometer system.By imposing constraints on the phase difference between adjacent sampling points in space,the linear correspondence between the strain measurement range and the spectral detection bandwidth is determined.This model completes the existing distributed measurement theory based on the principle of optical frequency domain reflectometer,and provides a theoretical basis for realizing distributed optical fiber strain measurement with a strain measurement range of 0.01 ?.With regards to problem that the detection bandwidth of optical fiber Rayleigh scattering spectrum is limited due to the low tuning range of the existing signal light modulation technology,a method for expanding the detection bandwidth of optical fiber Rayleigh scattering spectrum based on precise splicing of multi-band spectra is proposed.The method mathematically proves the uniqueness of the local Rayleigh scattering spectra of the characteristic fiber section on the optical frequency determination.Then by selecting the characteristic fiber and extracting its local Rayleigh spectrum,and determining the splicing position of the Rayleigh spectrum of the adjacent band of the measuring fiber according to the optical frequency determination,the high-precision splicing of the fiber Rayleigh scattering spectrum is realized.This method can complete the high-precision acquisition of the splicing positions of adjacent bands without the help of any external wavelength calibration equipment,and effectively expand the detection bandwidth of the optical fiber Rayleigh scattering spectrum in the measurement system.In the experiment,the distributed feedback laser array was used to construct a multi-band sweep frequency interferometry system.The results show that the method can achieve 35.013 nm broadband Rayleigh scattering spectrum detection,and the splicing error between bands is less than 2 pm.With regards to the problem that the spatial consistency of the measurement characteristics in the existing strain solution method is difficult to guarantee,the limitations of the traditional strain solution method are analyzed,and a distributed optical fiber strain calculation method based on the optimal solution calculation of the Rayleigh scattering spectrum correlation evaluation function is proposed.This method constructs a single-peak Rayleigh scattering spectrum correlation evaluation function,transforms the strain solution problem into calculating the optimal solution corresponding to the maximum value of the Rayleigh spectrum correlation evaluation function within the detection bandwidth constraints.The optical fiber spatial dislocation that dominates the difference in spatial measurement characteristics is eliminated at the maximum value,ensuring the consistency of the spatial measurement characteristics.Based on the above research,a distributed optical fiber strain measurement system based on multi-band sweep frequency interference was built.The measurement performance of the measurement system is experimentally verified by the standard optical fiber stretching device.The experimental results show that the spatial resolution of strain is 8 mm,the measurement range is 0.01 ?,and the expanded uncertainty is better than 15 ?? within a measuring length of 7 m.Distributed strain measurement was carried out on the composite material plate under concentrated force and the optical fiber laid on the flexible plate under deformation.The results proved the feasibility of the proposed distributed optical fiber strain measurement method in non-uniform strain field measurement.