The Research On Link Analyses And Signal Processing For Mode Division Multiplexing System In Few-Mode Fiber

Our current society is going through a time with the information technology developing rapidly. The Internet, big data, cloud service, IP broadcast and the next generation mobile communication technology make an increasing demand on broadband data transmission capacity, there is a urgent need to promote the transmission rate of optical fiber backbone network. Wavelength Division Multiplexing (WDM) and DWDM, EDFA, high performance fiber and coherent optical communication technique have been introduced to commercial fiber link, and greatly enlarge the transmission capacity and distance. Optical OFDM, higher order modulation formats, and polarization multiplexing have come into practice. However, the promoting of fiber capacity is limited by the natural nonlinearly of single mode fiber, and we are approaching the Shannon limit in several years. In order to break through the limit of single-mode optical fiber transmission, optical transmission technology based on spatial dimensions multiplexing, including mode multiplexing and multi-core fiber technology, is becoming a hot spot for researchers.The mode multiplexing technology based on few-mode fiber, transmitting several space-orthogonal modes, can exponentially improve the spectral efficiency of a single optical fiber. However, due to impairments in the fiber parameters and circuit bending, torsion and so on, there will exists coupling between different modes. At the same time, the different transmission speeds of modes result in dispersion. These two factors will lead to serious damage to the mode multiplexed signal, and will limit the system capacity. Thus, the mode multiplexing system based on few-mode fiber requires MIMO signal processing techniques to recover the distorted signals, and to improve system performance. This article will analyze the few-mode fiber link of the mode multiplexing system and study the signal processing technique based on theoretical research and simulation.The main contents and innovations of this paper are as follows:(1) This paper has studied the implementation of the key components in few-mode fiber MDM system, including the key methods and components of the transmitter and the receiver, such as mode converters, mode multiplexer and de-multiplexers. The spatial modes distribution and linear polarization modes in few mode fiber are studied. This paper also have an investigation on the design of few mode fiber for mode division multiplexing.(2) This paper has studied the linear effects of few-mode fiber transmission link, including the basic characteristics of few-mode fiber transmission, the mode coupling in transmission link, and modal dispersion. A complete transmission system model that contains intra-modal chromatic dispersion and inter-modal crosstalk effect in few-mode fiber is established, and the influence on modal signal transmission is simulated. According to the channel characteristics of the link, this paper analyzes link capacity and presents an optimization mechanism.(3) Based on the impact of mode signal transmission, this paper has studied the processing algorithms at de-multiplexing end for mode division multiplexing system, including the compensation for chromatic dispersion, and the use of CMA algorithm in MIMO blind equalization. The expanded MIMO RLSCMA algorithm has been applied into the system, and achieved a faster convergence speed, suggesting a stronger ability to track the changing complex channel.