AbstractsComputer Science

Fiber nonlinearity mitigation using mid-span spectral inversion in long-haul coherent optical OFDM systems

by Mohammad Monir Morshed

Institution: Monash University
Department: Department of Electrical and Computer Systems Engineering
Year: 2015
Keywords: Optical communications; Coherent optical communications; Cross-phase modulation; Four wave mixing; Optical phase conjugation; Fiber nonlinearity; Kerr effect; Mid-span spectral inversion (MSSI); XPM; SPM; FWM; Self-phase modulation
Record ID: 1032464
Full text PDF: http://arrow.monash.edu.au/hdl/1959.1/1145346


Optical fiber communication systems have become the backbone of today’s communication networks due to their enormous bandwidth, over several terahertz (THz), enabling capacities of 100 Tb/s and beyond. Almost all of world’s long-haul internet traffic is carried by these optical backbone networks. Despite the fact that the internet bubble ended in the early 2000s, its traffic has been constantly increasing at an astounding rate of 75% per year. In addition to that, new emerging video-centric applications such as IPTV will continue to increase the demand on the underlying optical backbone networks. As a result, ten to twenty years from now, optical networks will have to carry vastly increased amounts of data. However, recent research shows that fundamental limits in optical backbone networks are being approached. These limits are imposed by noise generated from inline amplifiers used to boost up the signal and the intrinsic nonlinearity of conventional standard single mode fiber (S-SMF). In order to meet long-term needs and challenges, therefore, research in wideband optical subsystems enabling high capacity long-haul transmission must be urgently pursued. One approach to break through the current capacity limit is a combination of using advanced modulation formats like coherent optical Orthogonal Frequency Division Multiplexing (CO-OFDM) and fiber nonlinearity mitigation techniques. In OFDM, the orthogonal property of the sub-carriers allows formation of an almost rectangular spectrum, which increases spectral efficiency. However, at the high powers required for higher order modulation formats, the nonlinearity in the fiber causes nonlinear mixing between the subcarriers, restricting the maximum allowable power below nonlinear threshold, and hence constraining the total capacity and distance. The PhD thesis proposed using mid-span spectral inversion (MSSI) that uses optical phase conjugation (OPC) module to mitigate the fiber nonlinearity in CO-OFDM systems. Using MSSI, the spectrum of the first-half of the link (from the Tx to the OPC module) will be inverted by Four-Wave Mixing (FWM) of the OFDM signal with a pump wave. The spectrally inverted signal is then selected to pass through the second half of the link. Because the signal is spectrally inverted, the second half of the link should undo the dispersion and nonlinearity of the first half of the link. During the project, a detailed analytical formalism to describe the performance of the OPC module has been developed. This aids the design and improvement of fundamental performance of OPC module. The first experimental demonstration of using MSSI in a coherent system has been made using dual polarization CO-OFDM systems carrying 1.21-Tb/s over 800 km. A design outline for optimum performance using MSSI has been developed. Two novel methods for improving the fundamental performance of MSSI have been proposed. The first method splits the nonlinear element into two parts, inserting a notch filter to remove the pump and then reinserting the pump into the second part of…