Research On High-speed And High-precision Distributed Strain And Birefringence Sensing Based On OFDR

Optical Frequency Domain Reflectometry(OFDR)technology is an emerging fiber-optic distributed sensing technology.Its high sensitivity,high spatial resolution and high signal-to-noise ratio have attracted much attention.It has an irreplaceable role in the fields of device inspection,structural health monitoring,and 3D shape sensing.However,the current OFDR technology still has some bottlenecks:lack of theoretical sensitivity derivation for Rayleigh scattering sensing,lack of the method to accurately obtain the birefringence of the fiber with fewer polarization states,urgent need for demodulation methods with high stability and high sensitivity,urgent need for higher test rates at longer test distances.In view of the above problems,this paper proposes a Rayleigh scattering sensing theory based on perturbation method,a local birefringence measurement method based on a similar Mueller matrix,a distributed high-sensitivity static strain sensing method based on differential relative phase,and a real-time dynamic strain sensing method based on GPU and completes the relevant experimental verification.The main innovations of this paper are as follows:(1)The Rayleigh scattering sensing theory based on perturbation method is proposed.The perturbation method is used to theoretically analyze the selectivity of Rayleigh backscattering,and the theoretical temperature strain sensing sensitivity of Rayleigh scattering sensing based on Bragg condition is obtained.The theoretical sensing sensitivity is consistent with the test values of Rayleigh backscattering sensitivity.Compared with the traditional frequency domain Rayleigh scattering sensing theory,this theory is also applicable to distributed sensing based on OFDR continuous weak grating fiber and Rayleigh scattering enhanced fiber.(2)A similar Mueller matrix distributed birefringence solution method and a twostate polarization OFDR system are proposed.The similarity relationship between the measurement matrix and matrix of the fiber under test is constructed by using two input polarization states,and the local birefringence measurement of the fiber-to-bemeasured is realized by the trace of the measurement matrix.Bending,twisting,and transverse pressure-induced birefringence were measured in single mode fiber and two kinds of spun HiBi fiber using this method,where the system can measure a minimum birefringence phase delay of 0.0004 rad and a maximum fiber under test length of 100 m.(3)A distributed high-sensitivity static strain sensing method based on OFDR differential relative phase is proposed.Strain measurement is achieved by measuring the differential relative phase of the two measurements of the reference state and the test state,which eliminates the OFDR common-mode residual nonlinear phase while helping to eliminate rapidly changing phase warpped rates along the fiber.Using all grating fiber as the fiber to be tested can obtain stronger backscatter than single mode fiber.In the experiment,the measurement range of the static strain change is 0 to 25 μεthe sensing spatial resolution is 4.35 cm,the minimum measurable strain change is 0.25με,and the total length of the fiber under test is 36 m.Compared to the crosscorrelation method using the same data,the differential relative phase method can provide better sensing performance for measuring small strain changes at the same spatial resolution.(4)A GPU-based OFDR real-time distributed dynamic strain and shape sensing method and system is proposed.Due to the large number of parallel processing steps involved in OFDR distributed sensing,the use of GPU can significantly speed up data processing.By analyzing the parameters and performance of tunable light source,capture card,and GPU,a method for optimizing the number of threads and blocks in GPU stream processors is proposed.Based on OFDR strain demodulation steps,various kernel functions are constructed and the selection of the number of each kernel function block is discussed.In the experiment,the total processing time of GPU parallel computing is improved by about 81 times compared with the total processing time of serial computing in CPU.The measurement rate of this system is up to 60 Hz.The system realizes dynamic strain sensing,the sensing spatial resolution is 20 cm,the sensing range is 200 m,the measurable vibration frequency is 20 Hz,and the minimum peak change is 2.5 με.At the position of 200 m,two strained regions with a separation of 20 cm can be clearly distinguished.In addition,some complex 3D shapes were reconstructed using a GPU-based OFDR strain measurement system,multi-core fibers and the Frenet-Serret framework.