Studies On The Key Technologies In The Mode-division-multiplexing Based Optical Fiber Communication System

To satisfy the increasing demand of global broadband services,researches on Tb/s or beyond optical fiber communication systems,which is an urgent task of the next generation optical communication networks,have been implemented widely in recent years.Coherent optical orthogonal frequency division multiplexing(CO-OFDM)technology is one of the effective ways to realize the transmission systems of such scale so far.Multicarrier optical source is one of the key components of CO-OFDM based optical transmission systems.For single mode fiber(SMF),the capacity is approaching the Shannon limit due to Kerr non-linearity.To further increase the capacity of the optical fiber communication systems,we need to exploit the novel space degree with few mode(FMF)or multimode fiber(MMF),i.e.multiple fiber modes supported.The study of this paper is involved in the field of mode division multiplexing(MDM)based fiber communication,including the multicarrier optical source and the mode converter used in mode division multiplexing.In optical communications,optical frequency comb can be used as multicarrier optical source simplifying the system architecture of the conventional wavelength division multiplexing(WDM)systems,which consist of hundreds of individual lasers.So high-quality optical frequency comb is one of the key components to realization of transmission capacity of Tb/s or beyond in CO-OFDM based optical transmission systems.Optical frequency comb generated by recirculating frequency shifter(RFS)based on in-phase quadrature(IQ)modulator is suitable for the optical communication systems of such scale.To realize more elastic optical communications,we have proposed a novel optical frequency comb generation scheme based on a dual-polarization IQ modulator.Meanwhile,mode division multiplexing is one of the main ways to further increase the capacity of optical transmission system.Fundamental mode(FM)is needed to be converted to higher order mode(HOM)before being multiplexed in the MDM systems.There are many ways for realization of mode conversion,such as spatial phase plate,spatial light modulator or mode coupling,etc.Due to the advantages of high conversion efficiency,easily-made and convenience,the mode converter based on all-fiber fused-type mode selective coupler(MSC)has been studied widely.Thus,we have also done some studies on this kind of mode converter.The main contents of this paper are as follows:In the first part,we mainly present the background and the significance of our studies.We first briefly review the development of optical fiber communications,and then we introduce our research topic of mode division multiplexing from the point of increasing the transmission capacity of optical fiber communications.At last,we specifically describe the two key technologies in our studies,including the generation of optical frequency comb and the conversion of optical fiber modes.In the second part,we present the relevant theories of optical frequency comb,and propose a novel optical frequency comb generation scheme and analyze its performances of theoretically and experimentally.The performances of the optical comb generation scheme based on recirculating frequency shifter(RFS),including steady state and signal-noise ratio(SNR),are first reviewed in detail.Then we propose a novel optical frequency comb generation scheme based on the dual-polarization IQ modulator,and derive the transfer function and output of the ideal and practical models of the scheme.At last,we build up an experiment to study the performances of the generated optical combs.In the third part,we show our relevant research on the all-fiber fused-type mode selective coupler(MSC).We first introduce the mode selective coupler serving as mode converter.Then we detail the principle of the mode conversion of the MSC,i.e.the mode coupling theory,and theoretically describe the key factors of mode conversion.At last,we show the experimental results,then discuss the impact factors of fabricating the mode selective coupler and mode conversion,and come up with improvement ways correspondingly.In the last part,the summary and expectation of our work are described.