| Fiber communication network is the basis of information society and the belt of national industrialization and informatization. Rcent years, as the fast development of the HDTV, video on demand and mobile broadband service, it puts forward an urgent demand on the requirement of bandwidth and capacity in optical communication network. As the deployment of40Gb/s WDM system and the emergence of100Gb/s optical communication system, the future trend of both transmission and access network is prone to400Gb/s～1Tb/s or even higher speed. Seeking a new theory of optical signal transmission and signal processing mode has become one of major challenges for the future optical communication technologies.There are two difficult and urgent problems must to be solved in today’s fiber communications system:the first one is the approaching capacity limit of fiber communication system. According to the Cisco statistics, the capacity of single fiber will reach200Tb/s by2020. Traditionally, the spectral resource is thought to be unlimited. However, the available bandwidth in fiber communication system is about100nm, in which the mature used part is less than60nm. As the increasing demand on system capacity, it requires the system to further improve the spectral efficiency (SE), which needs to explore the new dimensional and high-order modulation formats. The second one is caused by the emerging new services such as cloud computing. The traditional DWDM system is not suitable for the development of network, and the flexible resource allocation and variable speed are the trends of future network.In view of this, this paper focuses on the two difficult problems existing in today’s fiber communication system:the first one is the increase of system spectral efficiency and capacity limit, and the second one is the improvement of signal performance and flexibility. The first term is realized by the transmission theory of multi-dimensional multi-level optical signal and ultra-dense optical orthogonal wavelength multiplexing. The second term is achieved by the DSP compensation of optical signal as well as identification and scrambling theory of variable rate optical signal.The main innovative research efforts of this paper are summarized as follows.1. Transmission theory of multi-dimensional multi-level orthogonal optical signal1) The multiplexing/de-multiplexing of multi-dimensional signal and the interference suppressionThe traditional fiber communication system has utilized single dimension of optical light, such as amplitude or phase. However, the light is a kind of vector signal, which has multiple dimensions of phase, amplitude, polarization and angular. In order to increase the spectral efficiency, this paper proposes a novel multiplexing and control theory of vector information in optical frequency division multiplexing (OFDM) system, including transmission scheme based on polarization shift keying (PolSK), phase shift keying (PSK) and OFDM modulation. The simulation and experiment are executed to demonstrate the feasibility of the proposed scheme. The corresponding results were published as series papers on the top journal of Optics Express, and one of these papers is cited by21times.2) High-order optical signal based on multi-differential amplitude and phase modulationFor high-order optical signal, complicated channel estimation must be performed to ensure the signal performance, which is realized through training sequence or pilots. In order to improve the system performance, people often increases the pilots or estimation tap for the digital compensate algorithm, which ignores the realization of the algorithm, including the complexity and the information redundancy. This paper proposes and experimentally demonstrates a novel multi-ifferential amplitude and phase modulation format for high-order optical signal for the first time. It doesn’t require any channel estimation at the receiver due to the differential detection during demodulation. The proposed scheme can reduce the redundancy and complexity while maintaining a high SE for optical system. The experimental results show that the multi-differential amplitude and phase modulated optical signal has equivalent BER performance of mQAM modulated optical while keeping a better resistance to fiber nonlinearity. The efficiency of information is also improved by11%. The corresponding results were published on Optics Express, and the research group of Irish Cork University thought the proposed algorithm is creative in optical transmission system (seen in OE, vol.21, no.2,2013).2. Research on performance of ultra-dense WDM optical signal1) Optical signal design based on side-lobe suppressionDue to the Shannon limit, the signal speed of single carrier cannot increase infinitely. The ultra-dense multi-subcarriers parallel transmission has been a preferred scheme for future single channel Tbit transmission, which has also been widely approved in the industry area. However, there is one key problem awaiting to be solved—the interference among subcarriers. If the interference can be reduced, the SE or transmission performance of ultra-dense optical signal can be further improved. This paper proposes a novel ultra-dense optical signal based on side-lobe suppression, studies the generation principle of the signal and analyzes the improvement of interference by both theory analysis and experiment. The results show that the method based on side-lobe suppression can reduce the interference among optical carriers and improve the transmission performance. The interference among subcarriers is firstly reduced by11.5dB. The corresponding results were published on the top journal of Photonics Technology Letters (PTL) in2011and2012, and also presented in OFC’2011. Meanwhile, the author won the first prize of SARFT High-tech Research and Development Award in2012(ranked seventh). Shanxj, radio and television network group agrees to add this technical solution into the renovation project of "Triple Play".2) Tbit optical signal based on supercontinuumThe low cost dense paralleled subcarriers light source is the key point to realize Tbit optical transmission system. This paper studies the supercontinuum ultra-dense light source based on mode-lock laser and high-nonlinear fiber and proposes new schemes of Tbit radio over fiber access (ROF) and optical orthogonal frequency multiplexing access system. A2.56Tb/s ultra-dense optical access system is realized in the experiment, and the results show a good performance of the signal. The corresponding results were published on Optics Express in2011and2012, and achieved the largest optical access capacity in2012. The results were also reviewed to "reach the international advanced level" in the863Project Completion of "Key technologies in100Gb/s optical transmission system". This technology was later supported by the863Major Project "Key technologies in Tbit optical transmission system" to realize a Tbit demonstration project between WuHan and ShangHai, which would connect multiple10Gigabit Ethernet.3. Digital compensation of optical signal based on code modulation1) Pre-compensation based on LDPC and frequency domain pre-filteringA new pre-compensation scheme based on LDPC and frequency domain pre-filtering is proposed and demonstrated in PDM-16QAM orthogornal frequency division multiplexing optical signal. It can increase the tolenrance of fiber nonlinearity and improve the BER performance. In the experiment, a277.6Gb/s PDM-16QAM orthogornal frequency division multiplexing optical signal is transmitted over200km fiber successfully. An error-free performance is achieved at the optcal signa-to-noise ratio of14.7dB.2) OFDM signal compensation based on code division multiplexingA novel OFDM signal compensation based on code division multiplexing is proposed to increase the power budget of the fiber link and improve the receive sensitivity of signal. In the experiment, a4.68Gb/s code division multiplexed OFDM ROF signal is transmitted over25km fiber and8m air link successfully. Compared with the normal OFDM signal, the result breaks the transmission limit of air link under same receive sensitivity (extended by3m) and increases the system power budget. Besides, the optical beating noise in upstream link is an international problem in optical OFDM access network. The diversity gain of code division multiplexing has well solved this problem. The corresponding results are published on Optics Express twice. The proposed technology is cited by N. Cvijetic from NEC lab and the initiator of optical OFDM system, and they have both thought it as one of the most characteristic technologies applied in recent optical OFDM signal (seen in JTL, vol.30, no.4,2012; OFT, vol.17, no.5,2011).4. Variable rate transmission, adaptive identification and optical signal scrambling1) New scheme of variable rate optical transmissionAs the wide usage of DSP technology in optical communication system, the flat network by perception at physical layer has been a new research hotpot. This paper proposes a novel variable rate transmission scheme for optical orthogonal frequency division multiplexing signal. It can not only improve the capacity and flexibility of the network, but also be compatible with signals under different speeds. Based on this scheme, a109.92Gb/s transmission system is realized in the experiment, where the performance and interference of these variable rate signals are deeply analyzed. The power penalties are all less than0.5dB. The proposed system can accommodate different optical signals varying from lOGb/s to100Gb/s. The corresponding results are published on Optics Express. The IEEE/OSA fellow of A.M.J.Koonen from Eindhoven University of Technology has cited this method and thought the author study the dynamic bandwidth allocation on optical physical layer for the first time. The author also won the second prize of Outstanding Achievement Award of Ministry of Education (Natural Sciences) in2012(ranked fourth).2) Nonlinear dynamic scrambling technologyIn order to increase the physical security of the variable rate optical signal, this paper proposes novel scrambling/descrambling schemes based on nonlinear chaos and constellation rotation for the first time. A scrambling model with multi-level secure key and huge key space is built in this paper. The performance on security is deeply analyzed with theoretical and experimental method, and it gets a good experiment results. The corresponding results were continuously published as series papers on the journals of PTL, JLT and Optics Express (total6journal papers).In conclusion, in order to solve the problems about spectral efficiency and system flexibility in fiber communication system, this paper systematically and deeply investigates the multi-dimensional multi-level orthogonal optical signal and its compensation methods, and studies the technologies of variable rate transmission, adaptive identification and optical signal scrambling/descrambling. All the research work is elaborated from theoretical analysis, simulation and experimental study. The author has published more than30papers in top journals of IEEE Journal of Lightwave and Technology, Photonics Technology Letter, OSA Optics Express as well as international conference of OFC and ECOC. As first or corresponding author, he has published16papers indexed by SCI Second Region. These papers were cited more than100, one of which was cited by21times. The research work of this paper will provide references for the practical application of future flexible, large capacity optical transmission system. |
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