Research On Sensitivity Enhancement And Range Extension Of Optical Fiber Distributed Acoustic Sensing

Distributed acoustic sensing(DAS)based on phase-sensitive optical time domain reflectometer(?-OTDR)has gradually become a new generation of acoustic sensing technology due to its advantage of dense sensing points,high sensitivity and long sensing distance.DAS technology uses the phase information of backward Rayleigh scattering light of optical fibers and restores the small signals along the fiber to realize real-time monitoring of the change of external environment.At present,it has been widely used in perimeter security,pipeline safety,oil exploration,earthquake early warning,traffic monitoring,etc.The performance boost of DAS system has become a research hotpot topic in the field of optical fiber sensing all over the world.However,most of existing researches focus on improving the performance of light source,optical modulation,optical amplification and other optical devices as well as signal demodulation methods,while the research on the sensing fiber itself is almost blank.Although the standard single-mode fiber has been widely used for DAS technology,it is worth exploring how to further improve the strain sensitivity(sensitization)of sensing fiber.In addition,for long-distance sensing(extending range),most of the research focus on the amplification technology itself,the advantages of sensing fiber itself have not been fully played.Therefore,the breakthrough in the combination of special sensing fiber and amplification technology may become one of the trends of future research.Aiming at these two key scientific problems,thesis carried out theoretical analysis,simulation exploration and experimental verification,and the main innovative achievements are summarized as follows:(1)A new concept of cladding softened fiber is proposed and realized for the first time.The effect of acoustic pressure on fiber refractive index and length is explained at length from the elastic mechanics and elasto-optical effect,and the analysis model and calculation equation of strain sensitivity are obtained.The effects of Young's modulus,thickness and Poisson's ratio of each fiber layer on the strain sensitivity are analyzed by simulation,and the conclusion is drawn that Young's modulus and thickness of the cladding layer have the greatest effect on the strain sensitivity.And five kinds of cladding softened small-diameter fibers were fabricated by adding phosphorus at the cladding to reduce Young's modulus.It was found that there is a linear correlation between the concentration of phosphorus and the strain sensitivity,and the higher the concentration of phosphorus,the higher the strain sensitivity.The new cladding softened fiber proposed by the author has the characteristics of simple fabrication and high strain sensitivity,which paves a new way for further research on acoustic sensing fiber with higher sensitivity.The research achievements were published in the international journal Optics Express.(2)A new method combination of ultra-low loss fiber and erbium-doped fiber to achieve long-distance DAS is proposed.Combining first-order forward Raman amplification and erbium-doped fiber remote optically pump amplifier,the 120 km long-distance DAS based on the combination of ultra-low loss fiber and erbium-doped fiber was achieved experimentally.Through the combination of Raman amplifier power distribution model and erbium-doped fiber Giles gain model,the optimal combination mode of ultra-low loss fiber and erbium-doped fiber was explored by simulation.The optimal insertion position of erbium-doped fiber was determined by experiments.The results show that the single to noise ratio of the sensing fiber terminal is effectively improved to 31 d B,and the strain resolution reached 320.8 p?/?Hz.This new type of special fiber combination has opened up a new door for the subsequent realization of longer distance DAS.