Enabled Key Technologies For Mode Division Multiplexing Optical Transmission

With the development and popularization of Internet,the services including stream media,tele-working,online game,and social network become part of our daily lives.The demand of Internet based services,such as high-definition television,video on demand,and mobile communication,stimulate the urgent increase of network bandwidth.Due to the limitation of fiber nonlinearity,the available capacity of stand single mode fiber(SSMF)cannot satisfy the need of future network bandwidth requirement.Therefore,it is of great significance to explore the spatial dimention of optical fiber and utilize the orthogonal spatial modes as independent channels for information transmission,in order to further improve the capacity of single optical fiber.Consequently,mode division multiplexing(MDM)using few-mode fiber(FMF)has attracted worldwide research interests.In this dissertation,the enabled techniques for the MDM transmission are investigated,including the implementation and characterization of mode-division multiplexers,the investigation of mode dependent characteristics arising in the FMF,and MDM transmission experiments.The main conclusion of this thesis are summarized as follows:(1)The mode coupling of the FMF is investigated under condition of fiber bending and twisting.When the FMF is bent or twisted,the strength of mode coupling among the vector modes are numerically studied.The results show that the mode coupling within the same mode group is easy to occur,while the mode coupling between mode groups is so weak as to be reasonably neglected.(2)Based on the spatial phase modulation of liquid crystal on silicon(LCOS),a mode division multiplexer with off-focus operation is designed and realized with advantage of polarization division multiplexing(PDM)compatibility.Arbitrary linearly polarized(LP)mode arising in the FMF can be excitated with high mode selectivity and can be multiplexed with the fundamental mode.The positon tolerance of free-space optical devices is improved due to the off-focus operation,leading to easy implemention and mantainance.The influence of phase modulation dependent loss(PMDL)on the mode selective conversion is studied and the corresponding mitigation techniques are proposed.(3)In order to characterize the mode dependent properties of photonic lantern based mode division multiplexer,a simple power transfer matrix measurement scheme is proposed.Based on backward optical power detection and numerical calculations,the power transfer matrix can be obtained.According to the power transfer matrix,the mode dependent properties can be derived,including insertion loss and mode extinction ratio.Photonic lanterns with various both operation modes and mode selectivity are experimentally characterized,and the results verify the easy operation and correct function of the proposed scheme.(4)According to the dipole radiation and surface wave excitation theory,the theoretical model of Rayleigh backacattering in the FMF is put forward and verified through experimental measurement.The relationship of Rayleigh backscattering strength and mode distribution with respect to the forward propagation mode is comprehensively investigated.Through numerical calculations,the mode dependent characteristics are obtained,including the influence of fiber structure parameters.The Rayleigh backscattering is found to exist in every guided mode with individual ratio.In particular,the mode having the same modal distribution as the forward mode dominates the Rayleigh backscattering.Finally,the Rayleigh backscattering in various two-mode fibers are experimentally measured,and the results are in well accordance with the theoretical results.(5)The multiplexing transmission of 12-spatial and polarizational modes over 40 km FMF is experimentally demonstrated.Especially,the trellis coded modulation(TCM)over 8-PSK modulation format in the MDM system shows favorable coding gain.The results of the MDM transmission over 6-LP mode fiber show that the advanced coding scheme,TCM-8PSK,can get better performance in comparision with traditional QPSK modulation format,while maintaining the same effective spectral efficiency.After 40 km transmission,the TCM-8PSK modulation can get 1.93 dB OSNR gain.Moreover,the 3-mode fiber loop transmission setup is developed.Both the mode dependent loss of the FMF link and gain of the few-mode Erbium doped fiber amplifier need to be well managed in order to achieve stable and reliable transmission.