Decoupling Measurements Of Temperature And Strain Using A Distributed Optical Fiber In Low Temperature Environment

With the continuous development of modern science and technology and the expansion of engineering applications,more and more engineering structures(such as in-orbit spacecraft,cryogenic storage tanks,superconducting structures,etc.)operate in extreme multi-field environments such as extremely low temperatures and strong magnetic fields.The accurate and effective characterization of the basic mechanical and multi-field properties and behaviors associated with these structures is not only the basis for their design and operation,but also an important issue related to their safe operation,performance and service life evaluation.In extreme multi-field environments such as extremely low temperatures and strong magnetic fields,conventional mechanical and temperature-based multi-field testing methods based on electromagnetism principles usually suffer from reduced performance or even failure,as well as vulnerability to interference,compensation difficulties,single-point measurements,and other disadvantages.It is of great scientific significance and engineering application to develop fiber optic sensor technology that is resistant to electromagnetic interference,light,thin,and can be measured in a distributed manner,and to explore new methods for experimental measurements in low-temperature multi-field environments.Fiber-optic sensors usually have dual sensitivity to temperature and strain,and the cross-response information of both is reflected in the wavelength change or frequency change of fiber-optic measurements simultaneously.Based on Rayleigh scattering optical frequency domain reflection(OFDR)distributed fiber optic sensing technology,this paper carries out the research of a new method to decouple the mechanical deformation and temperature field measurement of structures in low temperature environment.First,we carried out an experimental study on the compensation test of strain and temperature in distributed optical fiber at low temperature,which is the basis of fiber sensing and testing.Based on the parameters related to the thermal-optical coefficient of the fiber and the thermal expansion coefficient of the structure,we adopted two compensation means to measure temperature and mechanical strain at low temperatures,and determined the applicability of the distributed fiber temperature/strain sensing principle at low temperatures and the effectiveness of temperature/strain measurements using the compensation method.Secondly,unlike the existing widely used conventional techniques for compensation and decoupling measurement of two fibers of different types,we develop a decoupling model for temperature and strain field measurement of distributed fibers in low-temperature environments based on a single fiber circular/arc geometric configuration wiring method.Unlike the ambient temperature case,the fiber temperature-sensitive parameters are significantly dependent on temperature in the low-temperature use environment,and the temperature nonlinear dependence of the fiber temperature-sensitive parameters needs to be considered.On this basis,the relationship between the harmonic coefficients of the Rayleigh backscattered frequency drift signal of the fiber and the strain/temperature field parameters is obtained,and the decoupling of the measured temperature and strain signals is achieved.Based on the established single fiber temperature and strain decoupling theory,we built an experimental platform for low-temperature and mechanical loading measurements,and carried out decoupling measurements and verification experiments under single and combined temperature and strain loading for the fiber circular and circular arc wiring methods,respectively.The experiments include single uniform field loading,single gradient field loading,two uniform combined loading and two non-uniform combined loading experiments.The results show that the distributed fiber can achieve decoupled temperature and strain measurements,and the experimental results are in good agreement with the theoretical results;compared with the fiber circular wiring mode,the segmented arc wiring mode can also achieve good decoupled measurements,and its wiring mode is flexible and can be suitable for small test space,which expands the application range of fiber decoupled measurements.This study provides a new idea and method for the measurement of mechanical and physical quantities in low-temperature multi-field environments,which is simple and easy to implement for the decoupled measurement of temperature and strain in a single fiber.