Distributed Fiber Sensing Of Multi-dimensional Strain And Shape Sensing Based On OFDR

Optical Frequency Domain Reflectometry(OFDR)is an emerging distributed optical fiber measurement and sensing technology.The OFDR measurement fiber uses Rayleigh backscatter shift to sense temperature,vibration and strain.Because OFDR distributed sensing has high sensing sensitivity and spatial resolution,it is especially suitable for two-dimensional strain sensing and three-dimensional shape sensing,which is difficult to achieve by traditional distributed optical fiber sensing.Two-dimensional strain sensing is realized by densely arranging optical fibers on the test piece,which has important application potential in high-intensity monitoring occasions such as aircraft skin and industrial equipment.The three-dimensional shape of multi-core fiber can be reconstructed by measuring the strain distribution of three cores in multi-core fiber by OFDR.It also has great application potential in the fields of robot control and medical endoscopy.This paper intends to carry out two-dimensional strain sensing and three-dimensional shape sensing research based on OFDR.The main work is as follows:(1)A two-dimensional strain sensing method using a fiber arrangement of Archimedes spiral is proposed.Because the Archimedes spiral has a simple relationship between the radial radius and the polar angle,the strain change in the one-dimensional information is corresponding to the two-dimensional coordinates,that is,the twodimensional strain information is formed.Two-dimensional strain sensing with different strain load angles can be realized by OFDR demodulation by using the fiber arrangement of the Archimedes spiral.(2)Focusing on the shape sensing principle based on optical frequency domain reflection technology,and deducing the Frenet-Serret equation describing the characteristics of space curve,the frequency drift of back-reverse Rayleigh scattering spectrum is related to the bending radius.The algorithm provides a theoretical basis.A shape reconstruction algorithm based on Frenet frame is proposed.The apparent curvature of the curve can be obtained under the strain condition of each core of the multi-core fiber.The curve deflection can be obtained by deriving the fiber bending direction.The Frenet frame,curvature and torsion combined with the Frenet-Serret equation can obtain the tangent vector at each position of the curve.The integral of the tangent vector at each position of the curve can be used to reconstruct the spatial curve.(3)Based on the OFDR distributed shape sensing experimental system,combined with the proposed shape sensing algorithm,the reconstruction experiments of twodimensional and three-dimensional shapes are carried out,and the reconstruction error analysis is carried out.A shape reconstruction using a known radius circle is proposed to obtain the strain calibration coefficients required for the multi-core fiber Rayleigh scattering spectral frequency shift and shape reconstruction,which avoids the calibration coefficient deviation caused by the additional calibration process.Accurate reconstruction of the fiber shape with a radius of curvature of 1 m is achieved by optimizing the spatial resolution and spectral refinement factor,and the reconstruction of many complex three-dimensional shapes is realized by the optimized spatial resolution and spectral refinement factor.