Characterization Of Distributed Fiber Optic Birefringence And Application Of The Composites Detection Based On PA-OFDR

This paper describes a polarization analyzing optical frequency domain reflectormeter?PA-OFDR?which is the first distributed polarization analysis system using binary magneto-optical rotators in an optical frequency domain reflectormeter?OFDR?.The paper mainly focuses on distributed birefringence characteristics of single-mode fiber?SMF?,which includes the bending-induced birefringence coefficient?BBC?measurement,the residual birefringencemeasurement,thepressure-inducedbirefringencecoefficient?PBC?measurement,etc.,and the application of the system in the detection of composite bending deformation based on PA-OFDR.The main contents and innovations are as follows:1.We describe a novel distributed polarization analysis system which analyzes the variations of polarization state of the backscattered light along the SMF and then the birefringence information can be derived.Here PA-OFDR has a birefringence measurement resolution of<2×10^-7,a birefringence spatial resolution of 0.25mm,a reflection spatial resolution of 10?m,a measurement range of 100 m,and a backscattering sensitivity of-130dB.We first make 12 fiber loops with different radii in a length of SMF under test?SMF-UT?to induce different well defined local birefringence values and then use the PA-OFDR to measure the birefringence distribution along the fiber.The theory of bending-induced birefringence was verified and the BBC of the SMF-UT was accurately measured for the first time to be 6.601×10^-10m².2.we obtain the birefringence values of the 12 fiber loops of different radii one at a time using a non-distributed polarization analysis?NPA?system and obtain a BBC value of6.490×10^-10m²,agreeing well with our distributed measurement with a relative error of only1.68%.As an added confidence,the accuracies of the experimental data are cross-checked with both distributed and non-distributed methods.In addition,we measure the residual birefringence of the SMF with both distributed and NPA systems and obtain consistent results with a relative error of only 0.59%.3.We performed a distributed measurement of the pressure-induced birefringence of SMF-UT using the PA-OFDR system and determined the PBCs of different fibers,which the PBC of SMF without coat is 9.012×10^-8m/N,the PBC of SMF with ordinary coat is5.839×10^-88 m/N,and the PBC of SMF with golden coat is 8.707×10^-88 m/N,and verified the theoretical formula of pressure-induced birefringence.At the same time,The accuracy of the PBC value of uncoated SMF is was verified using a NPA system and obtain a PBC value of8.977×10^-88 m/N,agreeing well with our distributed measurement with a relative error of only0.39%,which further proves the feasibility of distributed polarization analysis systems in distributed fiber optic pressure sensing with high spatial resolution and accuracy.4.We measured the birefringence of the SMF which embedded into the carbon fiber reinforced plastics with varying degrees of bending deformation to reflecte the variations of internal pressure of CFRP.This work further proves that such a distributed polarization analysis system has a valuable application in material detection.