| Since the emergence of the optical fiber communication system,its communication capacity has maintained exponential growth for decades.Recently,the capacity growth of the optical fiber communication system is gradually approaching the bottleneck,and one of the critical restraining factors is the nonlinear interference(NLI)introduced by the nonlinear effect in fibers.Additionally,it is necessary to find novel capacity expansion methods that are compatible with current systems.Focusing on the critical issues of increasing communication capacity in optical fiber communication systems,two aspects of researches have been carried out in this thesis.On one side,it has been investigated including the elimination and compensation methods for the NLI in coherent fiber communication systems;On the other side,the strictly symmetrical same-wavelength bidirectional(SSSWB)technique has been researched which is highly compatible with current systems.Especially,those critical issues restricting the application of the SSSWB technique in coherent fiber communication systems have been handled competently.In this thesis,the main contents and achievements related with the NLI in coherent fiber communication systems are clarified as follows:(1)In the fiber communication system with distributed Raman amplifiers,a quasilossless fiber link has been realized with insertion of Bragg fiber gratings.Simulation results exhibited that a quasi-lossless fiber link with length of 61.5 km could be obtained,and the optical power variation of channels in C-band was constricted below 1.2 dB.An approximate practical realization for the quasi-lossless fiber link is proposed,which provides the physical support for obtaining the analytical solution of the nonlinear Schr(?)dinger equation through the nonlinear Fourier transform technique.(2)On compensating for the NLI,contrastive research has been carried out between the two standard digital back propagation algorithm realization schemes based on timedomain perturbation theory,including the schemes where the digital back propagation procedures are implemented in the digital signal processing of the transmitter and the receiver respectively.Based on theoretical analyses and simulations,a conclusion has been obtained: As for the digital backward propagation technology based on the time-domain perturbation theory,the truncation error in the pre-compensation structure at the transmitter will accumulate in the practical fiber link,thus deteriorating the performance of NLI compensation.In contrast,the post-compensation scheme possesses higher perturbation approximation accuracy and better NLI compensation performance due to the additional consideration of the practical nonlinear phase shift.This conclusion provides guidance for low-complexity NLI compensation algorithms.In terms of novel technologies for boosting the communication capacity,three critical problems encountered by the SSSWB coherent fiber communication system have been thoroughly investigated including the nonlieanr inteference,Rayleigh scattering noise and reflection interference.All these three problems have been solved competently.Moreover,pioneering demonstrations of the SSSWB coherent fiber communication systems with ultrahigh capacity have been achieved in this thesis.The details are described as follows:(3)As for the NLI problem,the gaussian noise model of SSSWB coherent fiber communication systems has been theoretically deduced,and a simplification of the model has been proposed.Based on the analyses of the integration expression related with the power spectral density of the NLI,an important conclusion has been obtained: Since the four-wave mixing efficiency between the bidirectional signals is quite low,thus,there is merely weak nonlinear interaction between the bidirectional signals.Therefore,the limitation of the NLI on the SSSWB technique can be removed rationally,which lays the foundation for the SSSWB technique in coherent fiber communication systems.(4)One of the main restricting factors of the SSSWB coherent fiber communication system,Rayleigh scattering noise,has been investigated,which includes the origin of the Rayleigh scattering noise,its influence on the system and the corresponding suppression method.Based on the analyses of the Rayleigh scattering light and signal light,a suppression solution of the Rayleigh scattering noise has been found,and the functional analysis is utilized to deduce the condition where the scattering noise can be optimally suppressed,which is: the optical fiber link should satisfy the lossless condition.Further,considering the advantages of distributed Raman amplifiers in achieving quasi-lossless fiber links,the pump power and the total input signal power have been optimized through the gradient descent method,in order to ensure that the performance of the practical SSSWB coherent fiber communication system can approach the theoretical limit,which has verified the theory and reveled the capacity limit of the SSSWB system.(5)In aspect of the other critical limitation factor of the SSSWB system,which is the reflection interference,it has been mainly researched including the formation mechanism of the reflection interference,its influence on the system and the corresponding elimination method.Taking the advantage that the reflection interference is predictable,and considering the importance of carrier phase information in coherent fiber communication systems,a reflection interference elimination method has been proposed which is assisted by the linear frequency modulated training sequence.Through the proposed method,the reflection interference can be nearly eliminated,and experiments have been carried out to verify the effectiveness of our proposed method.Finally,based on the competent solutions of these three critical problems,the thesis has demonstrated the pioneering completely strictly symmetrical same-wavelength bidirectional fiber communication systems in the fiber with length of 90 km and 130 km where the modulation formats are DP-16 QAM and DP-QPSK respectively,and the baudrate of the 160 channels in C-band is set to 28 GBaud evenly. |
