Simultaneous Temperature And Strain Sensing Utilizing Interaction Between Acoustic And Optical Modes In Fibers

The distributed temperature and strain sensing technology has broad application prospect in many fields such as oil and gas pipelines,power lines,bridges and buildings security monitoring.As one of the typical methods,the simultaneous temperature and strain sensing based on Brillouin scattering has the advantages of high accuracy,long measurement distance and high spatial resolution.A method for simultaneous temperature and strain sensing based on interaction between acoustic and optical modes in stimulated Brillouin scattering(SBS)process is proposed and studied by theoretical analysis,simulation and experimental measurements.(1)Following the physical process of SBS,the effects of the fiber material and structure on the excitation of the acoustic and optical modes are analyzed.Then,the tailoring effects of acoustic and optical modes field distributions on the shape of the Brillouin gain spectrum(BGS)are discussed.Finally,the relationships between the BGS with the temperature and strain variations are given.On this basis,the theoretical model is proposed to achieve the simultaneous temperature and strain sensing based on the interaction between acoustic and optical modes in fibers.And the calculation methods are also given respectively.(2)A multiple acoustic modes/single optical mode fiber is proposed.The multi-peak BGS which is contributed by the interaction between multiple acoustic modes and single optical mode is obtained.Therefore,the simultaneous temperature and strain sensing is expected to be achieved by utilizing the linear relationship between the multiple Brillouin frequency shifts(BFSs)of BGS with the temperature and strain variations.The effects of the fiber material and structure on the excitation of the acoustic and optical modes,as well as the tailoring of the BGS shape are analyzed by simulation.In addition,the effects of the fiber material and structure on the temperature and strain measurement ranges,sensing sensitivities and maximum errors are analyzed by simulation.The simulation results show that,by utilizing the multiple peaks of BGS in the proposed fiber,the maximum errors of temperature and strain may be 0.5~oC and 10.3??respectively.(3)An experimental device is built for measuring the frequency response characteristic of SBS process based on the multiple acoustic modes/single optical mode fiber.The experimental results show that the BGS do have two peaks,which makes it suitable for simultaneous temperature and strain measurements.On this basis,a measurement system is built for distributed temperature and strain sensing based on the multiple acoustic modes/single optical mode fiber.The principle of the system and the design method of each component module of the system are discussed in detail.The experimental results show that the multiple BFSs of the BGS do have a linear relationship with the temperature,and the distributed temperature measurement can be achieved over a 20km range.