Optical Time-domain Reflectometry For Few-mode Optical Fibers

The extremely low loss of silica fibers has enabled the telecommunication revolution,but single-mode fiber-optic communication systems have been driven to their capacity limits.As a means to overcome this capacity crunch,space-division multiplexing(SDM)using few-mode fibers(FMF)has been proposed and demonstrated.For the fundamental physical process of OTDR,Rayleigh scattering in single-mode fibers has been studied long time ago and the associated theories were well established.However,the Rayleigh scattering in few-mode fibers has not been systematically studied.In this thesis,we study the principle and applications of optical time-domain reflectometry in few-mode fibers.The content contains five parts as listed below:(1)A general expression was derived for calculating the back-scattered power of optical time-domain reflectometry in the few-mode fiber.A scalar theory for Rayleigh scattering,including intra-and inter-modal Rayleigh scattering,was established.(2)It was discovered theoretically that forward inter-modal Rayleigh scattering is a mechanism of intrinsic crosstalk in few-mode optical fibers.Although the power of Rayleigh scattering is significantly smaller that the power of excitation light,Rayleigh scattering can keep increasing while the excitation light is propagating along the fiber.(3)Experimentally,intra-and inter-modal Rayleigh backscattering was measured in step-index and graded-index few-mode optical fibers using a special modemultiplexer/de-multiplexer.The measured results matched the theoretical results with a maximum difference of 0.28 dB,proving the validity and the effectiveness of the theory established.(4)A vector theory of Rayleigh scattering in polarization-maintaining few-mode optical fibers was established.Analysis showed that the vector theory becomes identical when it deals with the Rayleigh scattering in the same polarization as the excitation light and Rayleigh scattering is zero in the orthogonal polarization of the excitation light.(5)Inter-modal crosstalk in the same and orthogonal polarizations as the excitation light was measured in a polarization-maintaining few-mode optical fibers.The thesis established the theoretical foundation for optical time-domain reflectometry in few-mode optical fibers and provided technical methods for characterizing few-mode optical fibers(including polarization-maintaining few-mode optical fibers).In particular,the measurement of the inter-modal crosstalk in few-mode optical fibers will impact the research in space-division multiplexing and few-mode lasers and laser amplifiers.