Kerr nonlinearity in optical communication systems

  • Abdallah Ali

Student thesis: Doctoral ThesisDoctor of Philosophy


Single-mode fibers (SMFs) are approaching their nonlinear capacity limit soon, andthe new technology for increasing the capacity per fiber like space-division multiplexing(SDM) is not ready yet. Therefore, mitigating the nonlinearity in SMFs becomesan important aspect of the current research. Optical phase conjugation (OPC) comesas a promising method for simultaneous compensation of dispersion and nonlinearityin optical fiber link for a broadband signal and in real-time. However, it is limitedby the power and dispersion symmetry requirements around the mid-link OPC device.The power symmetry almost has been achieved by using Raman amplification.Another scheme for achieving the power symmetry was by achieving that for thenonlinear effective region instead of the total link through shifting those regions byadding a dispersive element collocated with the OPC. In this thesis, the two methodswill be investigated in improving the symmetry around the mid-link OPC eitherthrough simulation or experimentally. A mathematical analysis for the latter methodwill be performed by estimating the four-wave mixing (FWM) power from the interactionsof three tones propagating through the optical fiber. Then a closed-formformula for the nonlinear noise power from the transmission of Nyquist-shaped wavedivision multiplexed (WDM) signal will be driven. The closed-form will be used inpredicting the performance of a system employing mid-link OPC with lumped amplification.In order to verify the mathematical results, simulations were run and give a good agreement with the theory. The closed-form formula is verified experimentally through the transmission of 4.08Tb/s WDM signal over 600km with mid-link OPC and 75km span is added after the OPC to improve the symmetry. The Raman amplification scheme in improving the power symmetry around the mid-link OPC has been tested with a real-time transceiver which proves the potential application of mid-link OPC in a real commercial system. The dispersion slope effect on the nonlinearity modeling is studied and a figure of merit is developed to predict when the dispersion slope needs to be considered in the models to give accurate results.The different OPC designs based on optical fiber are discussed and a wavelength shift-free OPC design is presented.
Date of Award2021
Original languageEnglish
SupervisorAndrew Ellis (Supervisor)


  • optical fiber
  • coherent optical communications system
  • Kerr nonlinearity
  • nonlinearity mitigation
  • optical phase conjugation

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