Research On The Distributed Raman Amplification For Coherent Optical Fiber Communication Systems

With the rapid development of the Internet,the Internet of Things(IoT)and other technologies,there exists an ever-increasing demand for network bandwidth,which indicates a higher requirement of the transmission capacity of fiber-optic communication systems.Coherent optical communication technology,using advanced modulation formats and digital signal processing,has become an inevitable choice for the next generation high-capacity long-haul fiber-optic communication systems.Within such systems,optical amplifiers are one of the key components that are indispensable.Among all kinds of optical amplifiers,distributed Raman amplifiers(DRA)are extremely valuable in terms of both theretical research and system applications,because they have low-noise feature and can provide high and flat optical gain within a large and adjustable bandwidth.DRAs have been widely deployed in traditional intensity modulation/direct detection(IM/DD)systems.Theoretical modeling of Raman amplification is also well developed.However,we believe that in the context of coherent systems,the effect of Raman amplification on the polarization-multiplexed advanced modulation formatted signals in not properly addressed.In this thesis,we conduct a comprehensive theretical and experimental investigation on this scientific problem.The main contributions are as follows:(1)Using Jones vector based coupled nonlinear Schrodinger equations,we proposed a general signal transmission model,which can describe the evolution of the amplitude,the phase and the polarization states of the electromagnetic fields under Raman amplification.Both linear effects and nonlinear ones of optical fibers have been considered in the proposed model.We can utilize the model to characterize,design and optimize the coherent optical communication systems based on Raman amplification with arbitrary number of pumps,arbitrary direction pumping configurations and arbitrary amplification order.(2)We discovered and investigated the relative phase noise(RPN)caused by the interaction between Raman pump source and cross-phase modulation effects.Under certain reasonable assumptions,we provided analytical expressions of the noise and studied its statistical property.We experimentally measured its transfer function and verified that the RPN is low-pass in nature.Using Monte-Carlo simulations,we investigated the RPN-induced tramsision penalty under different pumping configurations and parameters for both single-carrier and OFDM systems.We discussed the ways to mitigate the RPN using optimized carrier phase estimation algorithms and tested their performances.(3)We discovered and investigated the relative polarization noise(RPolN)caused by the interaction between Raman pump source and cross-polarization modulation effects.By numerically solving the coupled nonlinear Schroding equations,we studied the statistical property of the noise and compared it to relative intensity noise(RIN)and RPN.It was found that the cut-off frequency of the RPolN transfer function is mucher higher than that of RIN and RPN.We analyzed the Raman pump RIN-induced impairments,including RIN transfer,RPN and RPolN,in both multi-span long-haul transmission systems and unrepeatered transimission systems.We also briefly introduced the algorithm that mitigates RPolN and presented its performance.(4)Using the proposed signal propagation model,we investigated the unrepeatered transmission systems based on distributed Raman amplifiers.The powers of signal and Raman pumps were optimized.We compared the tranmission performance under different amplification order and quantified the penalties caused by various noise souces.In assistance of Giles model,we provided an optimization method of remote optically pumped amplifiers that are commonly used in unrepeatered transmission systems.The length of the Erbium-doped fiber and the position of ROPA were jointly optimized.In summary,we proposed a universal signal propagation model that characterizes Raman amplification.We discovered and investigated,for the first time to the best of our knowledge,the phase and polarization noise induced by pump-signal nonlinearity.The results can be served as theoretical foundation for the characterization and optimization of distributed Raman amplifiers and signal transmission performance in coherent optical communication systems.The findings can provide a useful guideline to design the next-generation high-speed long-haul fiber-optic communication systems.