| While the demand of informational society for Internet services is dramatically increasing, access network has become a bottleneck between high-capacity local area networks and backbone networks. Passive Optical Network (PON), which is now a hot research topic in the network research field, is believed to be the best candidate for the next-generation access network. Now ATM-based APON technology, Ethernet-based EPON technology, and based on the general frame agreement GPON technology used TDM / TDMA (TDM) access technology, it may be referred to TDM-PON. However, as the number of user increases and new businesses continue to emerge, the demand for bandwidth will be higher. The MAC protocol based on TDM is very complex and difficult to upgrade to greater bandwidth. Thus, the WDM technology will be applied to the optical access network(WDM-PON).WDM-PON allows use independent wavelength communications between each users and OLT. This provides a fixed point-to-point virtual connection for each ONU and OLT. Now WDM-PON usually use spectral slicing light source and AWG as multiplexer/demultiplexer. Spectral slicing will lead to great insert loss and linear crosstalk. Because of the large temperature coefficient of AWG, it is necessary to adopt channel wavelength drift and monitoring technology. This will lead to increased initial investment, and access network is very sensitive to the cost, So WDM-PON is difficult to be put into practice.As the volume of data downlink access network,a good transition method is upgrade PON using TDM and WDM PON technology composite (CPON). Because DWDM wavelength spacing is narrow, the power stability multi-wavelength light source with narrow linewidth and multiplexer/demultiplexer with high isolation and small loss become the key to the transmission system. Usually laser's linewidth is narrow. There will be no spectral width outside the designated channel as long as wavelength spacing is large enough. In the receiver, optical detector sensitivity in a very wide spectral range is high. When channel isolation is not good enough, less signal leakage will affect the receiver sensitivity. Therefore, the isolation of the demultiplexer must be higher.Under this background, the author inquired a large number of literatures, analyzed a wide range of architectures of WDM-PON, and develops a new passive optical network upgrade scheme instructed by processorbased on FBG demultiplexer. Consequently, this thesis will unfold the study focus on this point, conducted a simulation and experimental tests.Firstly, this paper introduces the principles and standards of DWDM technology. The performance and advantages of various demultiplexers also are introduced and compared. According to the features of access network, the scheme using fiber grating as demultiplexer is proposed. Compared with other demultiplexers, FBG is simple in structure, high coupling efficiency, and low cost and suit to access network. The narrowband filter which combine Low-cost coupler and fiber grating is placed in the user terminals. The demultiplexer composed by passive optical splitter and FBG filters .Thus we completed the upgrade of the system but have no changes in structure of network. It will not only increase the system's capacity, but also maintain the flexibility of network.Secondly, we also design a model on OptiSystem according to the basic CPON transmission experiments system. To analyze the performance of the model, we simulate and testify the effect of WDM on the traditional system. Then, we modify the model with different DWDM components and analyze some system index such as BER,Q factor,Crosstalk,Eye-diagram, optimizing the system. The results show that the introduction of WDM device caused a certain amount of power loss,but greatly increasing the number of users and access speed. Crosstalk will be in the range of restrictions as long as reasonable restrictions of light emission wavelength and demultiplexer bandwidth.Finally, the passive optical network downlink direction transmission experiments system based on WDM was established. The system has two channel .The wavelength spacing was 0.8nm. One channel was constant rate of NRZ coding, another was multi-rate modulation。The largest insert loss of channel is 12.5dB, crosstalk less than -36dB.The eye-diagram is ideal and the eyelid is thin. The sensitivity of multi-speed receiving is -32.28 dBm and one of constant rate receiving is -27.09dBm.The power penalty is about 0.52dB.Through studies relatively error rate, Q factor, eye diagram and other indexes, prove the feasibility and effectiveness of the system.In summary, the passive optical access system with the introduction of WDM technology is the main task of this paper. WDM technology application in the Passive Optical Access System downlink direction can greatly improve theutilization of optical bandwidth. Power loss, crosstalk, bit error and other aspects of the transmission characteristics of the system are tested. The factors that various demultiplexers affect the transmission quality are also analyzed. I hope that these researches could provide the worthy theories and the technique support for the high-speed, low-cost passive optical access systems and bring great economic benefits. |
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