The Nonlinearity Of Few-mode Fiber Transmission And Suppression

To meet the traffic progression demands, space has been introduced as a new dimension for space-division multiplexing(SDM) to sustain the capacity growth in optical transmission systems. One way to exploit SDM is to utilize few-mode fibers(FMF) with each independent channel on an orthogonal spatial mode to increase the overall number of parallel channels, thus enabling higher capacity for fiber transmission. Analogous to single-mode fiber, nonlinear effects may eventually limit the information capacity in long-haul FMF transmission systems, as fiber nonlinearities generate distortions that rise faster than the signal power.In this paper, the nonlinear phenomena in FMFs have been systematically discussed, such as mode self-phase modulation, modal cross-phase modulation and so on. Above all the nonlinear phenomena, our research focuses on the hotspot of the nonlinearity in FMFs at home and abroad-- "Inter-mode four-wave mixing(IM-FWM)".The IM-FWM effect to the advanced modulation optical signals in FMFs has been theoretically and numerically studied in order to propose a method to reduce it. Firstly, starting from fiber nonlinear principle, the mechanism of various types of IM-FWM with two, three or four spatial modes in FMFs has been discussed, which makes a summary of extended nonlinear modeling for IM-FWM in FMFs supporting more modes. Secondly, different forms of multi-mode non-linear Schr?dinger equation(MM-NLSE) for two, three or four spatial modes are derived, along with up to twelve types of wave/mode distributions and their corresponding phase matching conditions. Thirdly, by simulation, an analysis of the effects of the wavelength, mode configuration, DMGD, CD and RMC upon IM-FWM efficiency in FMF has been made. Finally, according to the theoretical analysis and simulation result, a suppression method which balances different factors has been presented and simulated to reduce IM-FWM efficiency. The result shows that the IM-FWM efficiency has been reduced 20 dB for DPSK and 4QAM, 6dB for 16 QAM. The wavelength configuration in the method is set according to ITU suggestion, which will provide a good reference for the future optical network construction.