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

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... ications 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 ( [...]