Research On The Distributed Torsion Sensing Based On Optical Frequency Domain Reflectometry And Helical Fiber

Distributed torsion sensing has a wide range of potential applications in the field of health monitoring of the torsional deformation of the bridge,real-time feedback of the motion parameters of the drill bit and the drill pipe during automated drilling operations,3D shape reconstruction of the probe and catheter in medical intervention and the multidegree-of-freedom torsional motion measurement of the bionic mechanical structure.Optical fiber torsion sensor has been widely studied due to its unique advantages of small size,lightweight,anti-electromagnetic interference and high sensitivity.However,most of the reported optical fiber torsion sensors are designed to realize single-point measurement,and there is a problem that distributed torsion measurement cannot be achieved.In response to the above problems,this paper first established the sensing theory of the optical frequency domain reflectometry(OFDR),then designed and built an OFDR distributed sensing system.By using the helical multi-core fiber as the sensing fiber,the research on distributed torsion sensing with direction recognition based on OFDR and helical fiber was carried out.The main contents are as follows:1)Theoretical model of OFDR was first deduced to analyze the important parameters such as spatial resolution and measured distance in the system.The sensing mechanism based on the measurement of the Rayleigh backscattering spectrum was expounded,and the key issues such as polarization fading effect and nonlinear tuning effect of light source in OFDR system were also analyzed.2)The OFDR distributed sensing system was established and optimized,focusing on the use of the polarization diversity receivers to eliminate polarization fading effects in the system,and the use of balanced receivers to suppress relative intensity noise in the system,and the use of one-dimensional interpolation algorithms to compensate for nonlinear tuning effect of the light sources.Distributed strain sensing in single-mode optical fiber was experimentally implemented to verify the functions of both the hardware and software systems.3)A distributed torsion sensing method and system based on OFDR and helical fiber were proposed.On the basis of establishing the theoretical model of torsion sensing in the helical fiber,it is clarified that the eccentric helical structure is the key to realize the torsion direction recognition.Both the theoretical model and numerical simulations showed that the torsion sensitivity is inversely proportional to the helical pitch.When the pitch is large,there is a nonlinear relationship between the shift of the Rayleigh backscattering spectrum and the applied torsion.When the pitch is small,there is an approximately linear relationship between the shift of the Rayleigh backscattering spectrum and the applied torsion.In the experiment,distributed torsion sensing was demonstrated by using the helical multi-core fibers with pitches of 15.4 mm and 6 mm,and the corresponding torsion sensitivities are 0.6 pm/(rad/m)and 1.9 pm/(rad/m),respectively.The adjacent distance between the sensing points reaches 9.4 mm.Finally,the comparison experiment of distributed torsion sensing based on non-helical structure fiber further confirms that the eccentric helical structure is the key to realize the direction recognition of distributed torsion sensing.