Theoretical Study Of Mode-selective Couplers Based On Optical Fiber Gratings

With the sharp rise of information exchanging around the global,it is urgent to improve the transmission capacity in communications to satisfy the increasing bandwidth requirements in recent years.As we all know,the technology of single-mode fiber(SMF)has been developed for more than 30 years and has been approaching the bandwidth limit,therefore it hardly break through the technological difficulties of improving the bandwidth capacity.In order to solve this problem,a new multiplexing technology based on a new degree of freedom for optical communications called mode division multiplexing(MDM)has been proposed and applied in recent three years.The DMD technology including orbit angular momentum(OAM)multiplexing has been regarded one of the most promising way to increase the data capacity in optical communications.The SMF just support one core fundamental mode,however,the few-mode fiber(FMF)can support several eigenmodes in fiber core.The DMD technology with the FMF as transmission media utilizes these modes as independent data channels,thereby increasing the transmission capacity by two or three times.In the development of MDM technology,the mode-selective couplers(MSCs)and mode add/drop multiplexers(MADMs)in transmitting and receiving ends become the key devices used to selectively add and drop modes to be multiplexed in the MDM transmission system.In this graduation thesis,several novel devices and constructions of MSCs and MADMs are proposed and then analyzed about their rationality and feasibility in detail.Based on mode coupling of optical fiber gratings,two design projects are for MDM technology,and one project is for OAM multiplexing technology,as follows:In chapter one,the introduction of optical communications and its history of technology development is carried out,related to the space-division multiplexing(SDM)including the multicore fiber(MCF)multiplexing and MDM technology,as well as OAM multiplexing.Among the key techniques in MDM transmission system,we introduce the techniques of MDM and MADM at home and abroad in detail.In chapter two,as to the coupled-mode equations of mode coupling in fiber gratings and between two parallel fibers,we give the detailed solution procedure,and make compensation for the deficiencies about the procedure presented in most textbooks and journals.In chapter three,two projects have been proposed for MSCs and MADMs in DMD transmission system.One is based on the design structures connected fiber Bragg gratings(FBGs),LPFGs with optical circulator.Another is based on two parallel LPFGs.Through theoretical simulation and analysis,compared with current proposed design structures,some advantages of the latter one have been revealed,for instance,the lower mode coupling interference when add and drop multiplexed modes.In addition,its coupling efficiency can reach 98%,and the 3dB bandwidth can approach 10 nm.In chapter four,firstly,we deduce the null-null bandwidth formula of reflective spectra for core fundamental mode through FBG,and simulate the spectra of tilted fiber Bragg gratings(TFBGs)under different tilt angles and modulation strength.We find the effect of fringe tilt of TFBGs on the bandwidth of their reflective spectra,that is,in some special angles such as 8?,14?and so on,the degree of compressing bandwidth is very remarkable.In addition,we proposed a novel generation method of fiber OAM modes based on tilted LPFGs inscribed in FMF that has a large numerical aperture,which of method is flexible and expedient,and has high coupling efficiency.It can provide a new sight to design the all-fiber generator of OAM modes.