Study Of Nonlinear Impairments And Their Compensation Methods In Coherent Optical Transmission System

With the explosive growth of the demand for Internet-related services,ultra-high speed,large capacity and long distance are now becoming the direction that optical fiber communication develops towards.The arise of coherent optical transmission enhances the bandwidth efficiency and the receiver sensitivity of system,and that making fiber nonlinearities the main limit in long-haul transmission systems.Thus this thesis mainly focuses on the nonlinearities compensation methods in coherent optical systems.The main contents are summarized as follow.Nonlinear Schrodinger Equation(NLSE)which describes the evolution of optical field in a fiber,and how the fiber nonlinearities impact the optical signal are introduced first.These give the basic concept for the design of compensation methods.Then Split-step Fourier Method and Finite-difference Method for solving the NLSE numerically are presented,which are the foundation of the following simulations.The theory of Digital Back-propagation(DBP)is introduced next.And the effect of DBP in fighting against the nonlinear impairments is demonstrated through numerical simulation using Matlab.The performance of DBP is strongly dependent on the step length,but small step leads to unacceptable computational burden due to increasing number of Fast Fourier Transform(FFT)operation.So the DBP with span-step length is a compromising choice which gives a relatively good improvement.Then,performance comparison between DBP in QPSK and 16QAM systems shows that the higher-order modulation formats degrade the algorithm due to their worse tolerance for nonlinear noise,while the higher residual dispersion in dispersion-managed link significantly improves the standard deviation of phase noise at the end of receiver.It is noteworthy that a compensation coefficient C for nonlinear phase shift is added in each simulation mentioned before,and the variation tendency of C in different systems is discussed and analyzed.The Volterra Series Nonlinear Equalizer(VSNE)operated in frequency domain is presented,starting from introducing the theory of VS which is a powerful tool in analyzing nonlinear systems.This equalizer compensates the linear and nonlinear impairments parallel,so it is still efficient even though the simulation found that it is inferior to DBP at high launch power.A time domain equalizer based on VS is presented,too.It compensates the nonlinear impairments adaptively using the training sequence sent along with the information symbols,while keeps transparent to system parameters.Simulations show that the equalizer improved the optimal Q values of 0.8dB,0.7dB,0.6dB after transmissions of 1600km,1760km and 1920km,respectively.Besides,the influence brought by different delay tap number is discussed,and found that there is a tradeoff between ASE noise and fiber nonlinearities in choosing tap number.At last,nonlinear perturbation pre-distortion(PPD)method and phase-conjugated twin waves transmission are briefly introduced.Both these methods are based on the perturbation analysis in optical fiber,and attracting intensive attention because of their possibility in reducing the computational complexity.