Research On Theoretical Designs And Applications Of Mode-Division Multiplexing Fibers

The transmission capacity of single-mode fiber(SMF)based communication systems is limited by the nonlinear Shannon limit,thus unable to meet the demand of ever-increasing data traffic.Fortunately,the spatial position multiplexing and mode-division multiplexing(MDM)techniques based on the transverse spatial dimension still exist massive capacity promotion space.Therefore,the design and fabrication of MDM specialty fibers and related MDM fiber-optic communications have become one of the global research hotspots over the last decade.This dissertation carries out theoretical and experimental studies on MDM fibers,including conventional fiber,ring-core fiber(RCF),degeneracy-lifted multi-mode fiber(MMF),erbium-doped fiber(EDF),etc.The major contents are summarized as follows.(1)Researches on basic theories of different kinds of mode bases in fiber.The spatial distributions including intensity,phase and polarization of four kinds of commonly used mode bases,i.e.eigenmodes,linearly polarized(LP)modes,circularly and linearly polarized orbital angular momentum(OAM)modes,are characterized in detail.Meanwhile,the transformation relationship and performance for fiber MDM transmission of these four mode bases are discussed.(2)Theoretical researches on the OAM mode properties in conventional fibers.1)The division of OAM mode group,broadband characteristics,and impacts of external perturbations and intrinsic defects in conventional OM3 MMF are investigated.2)The OAM mode properties including tolerance to ellipticity/bending,fiber attenuation and bandwidth in standard G.652 SMF at visible wavelengths are investigated.The simulation results indicate that the two widely deployed conventional fibers may support low-crosstalk multi-channel OAM mode group multiplexing communications.(3)Design and fabrication of RCFs.1)An air-core strongly guiding RCF enabling above 10^-4 effective index separation between adjacent modes(except degenerate odd and even modes)is designed.2)A weakly guiding RCF with effective index separation above10^-4 among the 1^st to the 12^th order OAM mode groups,which also possesses low loss,small dispersion and ultra-low nonlinearity is designed.3)A kind of MDM scheme with simplified multiple-input multiple-output(MIMO)equalization based on the above weakly guiding RCF is proposed.The algorithmic complexities of simplified MIMO and traditional global MIMO in both time and frequency domains are compared to each other.4)Three kinds of low-loss(?0.26 d B/km)weakly guiding RCFs with F-doped cladding which possess below-20 d B/km crosstalks for higher-order mode groups are designed,fabricated and experimentally characterized.(4)Design of degeneracy-lifted MMFs supporting fully separated eigenmodes.1)By combining geometric structure birefringence and stress birefringence,a PANDA-type elliptical-core MMF which is able to support 24 eigenmodes with effective index separation above 1.35×10^-4 over the whole C band is designed.2)By further employing the elliptical ring-core structure,a bow-tie elliptical ring-core MMF supporting 52 eigenmodes with effective index separation above 1.09×10^-4 over the whole C band is designed.3)A solid-core photonic crystal MMF featuring asymmetrical core is designed,which is able to support 51 eigenmodes with effective index separation above 1.37×10^-4 or 24 eigenmodes with effective index separation above 1.10×10^-3 over the whole C+L band and has ultra-low confinement loss.(5)Design and characterization of OAM MDM EDFs.1)A double-cladding and weakly-guiding RCF based cladding-pumped and gain-equalized OAM MDM EDF is designed,which can provide a gain above 20 d B for all 14 OAM modes with gain difference below 0.63 d B over the whole C band.2)The amplification performance of the 1^st order OAM modes is experimentally characterized using a practically fabricated erbium-doped RCF.The measured small signal gain is above 14 d B over the whole C band.(6)Experimental studies on OAM MDM fiber-optic communication systems.1)A 1.1-km full-duplex bidirectional OAM mode multiplexing communication system is experimentally demonstrated.The uplink and downlink each transmit x-polarized and y-polarized OAM_±1 modes.2)By employing the low-loss RCF,ultra-low-loss SMF,and homemade all-fiber OAM mode(de)multiplexer,100-km heterogeneous all-fiber communication system,seamlessly connecting the RCF-based high-speed sub-network with two OAM modes(l=1,2)multiplexing and the SMF-based low-speed sub-networks,is demonstrated in the experiment.