Study of physical layer impairments in high speed optical networks

The work done in this thesis focuses on the impact of transmission impairments in high speed optical networks. Specifically it focuses on the impact of nonlinear impairments in long haul fiber optic data transmission. Currently deployed fiber optic transmission networks are running on NRZ OOK modulation formats with spectral efficiency of only 1 bit/symbol. To achieve spectral efficiency beyond 1 bit/symbol, fiber optic communication systems running on advanced modulation formats such as QPSK are becoming important candidates. The practical deployment of QPSK based fiber optic communication system is severely limited by Kerr-induced nonlinear distortions such as XPM and XPolM, from the neighboring NRZ OOK channels. In this thesis we focus on the impact of nonlinear impairments (XPM and XPolM) in fiber optical transmission systems running on QPSK modulation with both differential and coherent detection. The dependence of impact of nonlinear impairments on SOP, baud rate of the neighboring NRZ OOK channels and PMD in the fiber, is analyzed in detail through numerical simulations in VPItransmission Maker®. In this thesis we also analyze digital signal processing algorithms to compensate linear and nonlinear impairments in coherent fiber optic communication systems. We propose a simplification of the existing method for joint compensation of linear and nonlinear impairments called ―digital back propagation‖. Our method is called ―weighted digital back propagation‖. It achieves the same performance of conventional digital back propagation with up to 80% reduction in computational complexity. In the last part of the thesis we analyze the transmission performance of a newly proposed hybrid WDM/TDM protection scheme through numerical simulation in VPItransmission Maker®. The transmission performance of the hybrid WDM/TDM PON is limited by impairments from passive optical devices and fiber optical channel. Acknowledgments I would like to thank all the people who helped me throughout the research period till now. I would specially like to thank my academic supervisor Prof. Lena Wosinska for accepting me as PhD candidate in school of information and communication technology, KTH, Sweden. Her guidance and open heartedness has helped me to a great extent to reach this stage of my research. I would then like to thank my co-supervisor Dr. Marco Forzati in Acreo AB whose guidance at every small step of my research lead to the publications that are discussed in this thesis. His support both moral and technical, motivated me to achieve better and higher targets for my research goals. Under his guidance I did …