Fiber nonlinearity mitigation of WDM-PDM QPSK/16-QAM signals using fiber-optic parametric amplifiers based multiple optical phase conjugations

We demonstrate fiber nonlinearity mitigation by using multiple optical phase conjugations (OPCs) in the WDM transmission systems of both 8 × 32-Gbaud PDM QPSK channels and 8 × 32-Gbaud PDM 16-QAM channels, showing improved performance over a single mid-span OPC and no OPC in terms of nonlinear threshold and a best achievable Q2 factor after transmission. In addition, after an even number of OPCs, the signal wavelength can be preserved after transmission. The performance of multiple OPCs for fiber nonlinearity mitigation was evaluated independently for WDM PDM QPSK signals and WDM PDM 16-QAM signals. The technique of multiple OPCs is proved to be transparent to modulation formats and effective for different transmission links. In the WDM PDM QPSK transmission system over 3600 km, by using multiple OPCs the nonlinear threshold (i.e. optimal signal launched power) was increased by ~5 dB compared to the case of no OPC and increased by ~2 dB compared to the case of mid-span OPC. In the WDM PDM 16-QAM transmission system over 912 km, by using the multiple OPCs the nonlinear threshold was increased by ~7 dB compared to the case of no OPC and increased by ~1 dB compared to the case of midspan OPC. The improvements in the best achievable Q2 factors were more modest, ranging from 0.2 dB to 1.1 dB for the results presented

A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems

Optical communication systems represent the backbone of modern communication networks. Since their deployment, different fiber technologies have been used to deal with optical fiber impairments such as dispersion-shifted fibers and dispersion-compensation fibers. In recent years, thanks to the introduction of coherent detection based systems, fiber impairments can be mitigated using digital signal processing (DSP) algorithms. Coherent systems are used in the current 100 Gbps wavelength-division multiplexing (WDM) standard technology. They allow the increase of spectral efficiency by using multi-level modulation formats, and are combined with DSP techniques to combat the linear fiber distortions. In addition to linear impairments, the next generation 400 Gbps/1 Tbps WDM systems are also more affected by the fiber nonlinearity due to the Kerr effect. At high input power, the fiber nonlinear effects become more important and their compensation is required to improve the transmission performance. Several approaches have been proposed to deal with the fiber nonlinearity. In this paper, after a brief description of the Kerr-induced nonlinear effects, a survey on the fiber nonlinearity compensation (NLC) techniques is provided. We focus on the well-known NLC techniques and discuss their performance, as well as their implementation and complexity. An extension of the inter-subcarrier nonlinear interference canceler approach is also proposed. A performance evaluation of the well-known NLC techniques and the proposed approach is provided in the context of Nyquist and super-Nyquist superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial

Experimental demonstration of performance enhancement in non-linearity limited optical fibre systems

This thesis presents a study of the nonlinear limits of coherent, long-haul, optical fibre transmission systems and studies the capabilities of digital and all-optical nonlinearity compensation techniques to enhance their performance. By deriving the theoretical description of optical fibre nonlinear Kerr effects, this thesis presents theoretical, numerical, and experimental evidence showing that the compensation efficiency of deterministic nonlinear impairments in OPC assisted transmission system is highly dependent on the span length. This document shows that the deployment of multiple OPCs, in a system limited by deterministic signal-signal nonlinear interactions, can negate the performance enhancement achieved by a single OPC. I have derived, and verified by simulations, closed form equations that accurately represent the ultimate nonlinear threshold of the nondeterministic nonlinear signal-noise interaction limit in discretely amplified and quasi-lossless Raman optical fibre transmission systems. This nondeterministic nonlinear threshold can be unveiled when deploying ideal nonlinearity compensation techniques and can be minimised by deploying multiple OPCs.In this thesis, I have experimentally shown that the performance enhancement achieved bymid-link OPC when deployed in discretely amplified transmission system is highly dependent on the bandwidth of the signals propagating along the system. The experimental results have shown that the OPC enhances the reach of discretely amplified transmission system by 43%,32%, and 24% for 2x28Gbaud, 4x28Gbaud, and 8x28Gbaud of PM-QPSK signals,respectively. Also, I have experimentally demonstrated the highest reported reach enhancement of 72% (compared to EDC system) for 3.6Tbps (30x30Gbaud PM-QPSK, spectral efficiency of 3.6bps/Hz); when deploying a mid-link OPC in distributed Raman system

Multi-channel all-optical signal processing based on parametric effects

Two different experiments that use parametric effects for the processing of multiple signals in a single fiber are reviewed. The first experiment uses optical phase conjugation to mitigate the effects of nonlinearity in transmission, whereas the second uses multiple phase-sensitive amplifiers to regenerate six different channels

Impact of Frequency Shift on Nonlinear Compensation Using Optical Phase Conjugation for M-QAM Signals

Nonlinear compensation using optical phase conjugation (OPC) have been considered a promising technique to increase the reach of high-speed fiber-optic transmission systems. OPC-based nonlinear compensation employs an optical phase conjugation located at a middle of the fiber link to generate a complexed conjugated signal with respect the signal in the first half of the link for propagation in the second half. OPC technique assumes a symmetry for signal propagating in the first and second half to obtain a perfect nonlinear and chromatic dispersion. However, as most of practical OPC schemes are realized by nonlinear effects such as four-wave mixing or a combination of second-harmonic generation and difference frequency generation, the frequency shift induced by OPC affects the signal symmetrical requirement for nonlinear compensation because the chromatic dispersion is different for the first and second half transmissions. In this paper, we investigate the impact of frequency shift on the nonlinear compensation using OPC for high symbol rate, high level modulation format signals. This will be important to understand the tolerance of the OPC techniques against such a practical condition for actual system implementations

Performance limits in optical communications due to fiber nonlinearity

In this paper, we review the historical evolution of predictions of the performance of optical communication systems. We will describe how such predictions were made from the outset of research in laser based optical communications and how they have evolved to their present form, accurately predicting the performance of coherently detected communication systems

Kerr nonlinearity in optical communication systems

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

Optical fibre technologies for future communication networks

New transmission technologies need to be developed to satisfy the ever increasing demand for communication traffic. This paper reviews some recent research on optical fibre technology that aims at addressing this challenge

Nonlinearity compensation in multi-rate 28 Gbaud WDM systems employing optical and digital techniques under diverse link configurations

Digital back-propagation (DBP) has recently been proposed for the comprehensive compensation of channel nonlinearities in optical communication systems. While DBP is attractive for its flexibility and performance, it poses significant challenges in terms of computational complexity. Alternatively, phase conjugation or spectral inversion has previously been employed to mitigate nonlinear fibre impairments. Though spectral inversion is relatively straightforward to implement in optical or electrical domain, it requires precise positioning and symmetrised link power profile in order to avail the full benefit. In this paper, we directly compare ideal and low-precision single-channel DBP with single-channel spectral-inversion both with and without symmetry correction via dispersive chirping. We demonstrate that for all the dispersion maps studied, spectral inversion approaches the performance of ideal DBP with 40 steps per span and exceeds the performance of electronic dispersion compensation by ~3.5 dB in Q-factor, enabling up to 96% reduction in complexity in terms of required DBP stages, relative to low precision one step per span based DBP. For maps where quasi-phase matching is a significant issue, spectral inversion significantly outperforms ideal DBP by ~3 dB