Research On Equalization Technology Of Mode Division Multiplexing System Based On Few Mode Fiber

Mode division multiplexing is a technology that utilizes mode orthogonality in a few mode fiber to transmit multiple signals simultaneously between different modes.At present,due to the nonlinear effect of single mode fiber,its capacity is also approaching the "Shannon limit".Therefore,new multiplexing technology need to be considered to break the current technical bottleneck to meet the higher demand for network bandwidth.Few mode fiber is a new transmission medium,the number of supported modes between the multimode fiber and the single mode fiber.Compared with multimode fiber,the nonlinear damage inside the few mode fiber is less,which is more conducive to long distance transmission.Few mode fiber having orthogonality,it is possible to realize the signal transmission between different modes each mode.The mode division multiplexing technology based on the few mode fiber relies on several modes of the few mode fiber to transmit signals,and realizes the doubling of the fiber transmission capacity,which has become the key technology for large-capacity fiber transmission.In practical applications of few mode fiber,factors such as uneven refractive index distribution,eccentricity of the core,and slight bending caused by external pressure will cause coupling between different modes in the fiber,that is,mode coupling.In addition,the differential mode group delay caused by the difference in mode group speed between modes in a few mode fiber will cause pulses at different times to overlap and form inter-symbol interference.Therefore,to recover the source signal at the receiving end of the mode division multiplexing system,the MIMO equalization of the digital signal processing part at the receiving end is essential.The main content of this paper is the research on the equalization technology of the few mode fiber mode division multiplexing system.Firstly,the mode orthogonality in a few mode fiber is proved by the Maxwell equations.Then analyze the link damage of the few mode fiber transmission in the mode division multiplexing system.In order to describe the damage and interaction between modes,a matrix transmission model is used to mathematically model the transmission link of the mode division multiplexing system.In order to explore the transmission characteristics of the MDM system,MATLAB is embedded in VPI Transmission Maker 9.1 to build a 2 × 2 MDM simulation system.For different transmission distances,this paper involves two types of reception schemes,namely coherent detection and direct detection,at the receiving end of the mode division multiplex transmission system.When the receiver uses coherent detection,it introduces the time domain equalization and frequency domain equalization in detail,and makes a detailed derivation of the LMS algorithm and the RLS algorithm,and then the DSP processing part uses the time domain and frequency domain LMS and RLS algorithms to equalize in the simulation system,respectively.Based on the traditional time domain LMS algorithm,a VLMS algorithm is proposed,which has a faster convergence speed than the traditional LMS algorithm.In the direct detection mode division multiplexing system proposed in this paper,KK joint MIMO equalization is used to recover the source signal.The paper studies two types of optical single-sideband modulation systems,DDMZM and virtual carrier.