Research On Digital Equalization Techniques Of Polarization Multiplexed Coherent Optical Communication Systems

Dispersion equalization is one of the key technologies for modern fiber opticcommunication systems. Compared with the optical dispersion compensation, theelectronic digital equalization has smaller size and lower cost, and it can achieveexcellent performance with the combination of digital signal processing. Digitalequalization techniques become one of the hot research fields of optical fibercommunication. Benefit from the development of ultra-large-scale digital integratedcircuit technology, as well as the M-ary modulation, coherent detection andpolarization division multiplexing (PDM) technology in the field of opticalcommunications, digital equalization has become an essential technology of the nextgeneration100Gbit/s optical networks.Coherent detection can not only improve the receiver sensitivity, but alsomaintain the linearity of chromatic dispersion and polarization mode dispersioneffects, and is suitable for the detection scheme of long-haul fiber opticcommunication systems. Polarization division multiplexing does not change the fiberlink, making the system transmission capacity doubled via increasing the complexityof the transmitter and receiver. The combination of polarization division multiplexing,coherent detection and M-ary modulation often referred to as single-carrier systems.Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulationscheme. It increases the flexibility of the system, and provides the ability of a100Gbit/s and above system transmission capacity. This research mainly investigates thedigital dispersion equalization technologies for polarization multiplexed coherentsingle-carrier system and optical orthogonal frequency division multiplexing system.The work in this thesis is supported by the Hebei Province Natural ScienceFoundation (No. F2008000116) and the National Natural Science Foundation (No.60572018). The main contents are as follows:1. After analyzing the nonlinear Schrodinger equation of optical pulses above 30ps in the long-haul optical fiber, the effect of group velocity dispersion(GVD) and polarization mode dispersion (PMD) on optical signaltransmission is studied, and the compensation principle of polarization modedispersion is analyzed. The linear channel model of the polarizationmultiplexed coherent single-carrier system and optical orthogonal frequencydivision multiplexing system is studied, and a matrix channel model of fiberchromatic dispersion (CD) and polarization mode dispersion is analyzed.Simulation model for112Gbit/s polarization multiplexed coherentsingle-carrier system and optical orthogonal frequency division multiplexingsystem are created.2. A butterfly finite impulse response (FIR) filter based structure is proposedfor the digital dispersion equalizer of100Gbit/s polarization multiplexedcoherent single-carrier system, which adopt least mean square (LMS)algorithm and recursive least squares (RLS) algorithm as the adaptivealgorithm. Simulation analyses of the equalization performance of twoalgorithms updated equalizer under different channel impairments arecarried out. The simulation results show that: the equalization performanceof the recursive least squares algorithm better than the least mean squarealgorithm, with the configuration of the25-tap equalizer, recursive leastsquares algorithm can compensate1760ps/nm residual chromatic dispersionand104.9ps mean differential group delay (DGD) caused by polarizationmode dispersion simultaneously with2.74dB optical signal to noise ratio(OSNR) penalty, and has2.23dB performance improvement compared withthe least mean square algorithm under the same conditions.3. A butterfly infinite impulse response (IIR) filter based structure is proposedfor the digital dispersion equalizer of100Gbit/s polarization multiplexedcoherent single-carrier system, which adopts least mean square algorithm asthe adaptive algorithm. The equalization performances of the designedequalizer are analyzed by simulation under different channel linearimpairments. The simulation results show that: with the configuration of21feed forward taps and2feed back taps, the equalizer can compensate 1440ps/nm residual chromatic dispersion and95ps mean differential groupdelay simultaneously with2.95dB power penalty.4. An improved intra-symbol frequency domain average algorithm using theKaiser window function is proposed to improve the channel estimation andequalization performance of100Gbit/s polarization multiplexed coherentoptical orthogonal frequency division multiplexing system in noisyenvironments. The channel estimation and equalization performances of theimproved algorithm and original algorithm are analyzed through simulationunder different system conditions. The simulation results show that: theKaiser-window-based improved algorithm has improved performance thanthe original algorithm. To compensate the group velocity dispersion andpolarization mode dispersion of1000km transmission fiber and achieve thebit error rate upper limit required by forward error correction (FEC) code,the improved algorithm need an OSNR of15.73dB,0.24dB lower than theoriginal algorithm. To compensate the group velocity dispersion andpolarization mode dispersion of900km transmission fiber, the optical signalto noise ratio penalty of the improved algorithm is3.01dB,0.35dB lowerthan the original algorithm.