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

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 the nonlinear element. The second method uses a phase shift filter between the two nonlinear elements, to improve robustness in practical implementation. Both methods offer 1 dB of maximum signal quality improvement in a 10 × 80-km 4-QAM 224-Gb/s CO-OFDM system. In summary, this work has demonstrated by simulation and experiments that MSSI offers benefits to coherent optical systems, including OFDM systems. It has developed analytical formalisms that identify the performance-limiting mechanisms in optical phase conjugators based on third order nonlinearity, and has introduced two methods of mitigating these mechanisms. MSSI fell from favour in the mid-2000s due to its complexity. However, after this successful demonstration of fiber nonlinearity compensation using MSSI in a coherent system, MSSI has now drawn considerable attention recently. Subsequently, there have been demonstrations of Raman-enhanced MSSI and multiple phase-conjugations based coherent systems at the 2014 conference on Optical Fiber Communications (OFC)