Unrepeatered 64QAM over SMF-28 using Raman amplification and digital backpropagation

Unrepeatered transmission over SMF-28 fibre is investigated using Raman based amplification. Experiments and simulations demonstrate a transmission up to 200 km (41 dB) span length using 28Gbaud 64 QAM modulation employing digital back propagation in DSP

2048-QAM transmission at 15 GBd over 100 km using geometric constellation shaping

We experimentally investigated a pilot-aided digital signal processing (DSP) chain in combination with high-order geometric constellation shaping to increase the achievable information rates (AIRs) in standard intradyne coherent transmission systems. We show that the AIR of our system at 15 GBd was maximised using geometrically-shaped (GS) 2048 quadrature amplitude modulation (QAM), reaching 18.0 b/4D-symbol in back-to-back transmission and 16.9 b/4D-symbol after transmission through 100 km of a single-mode fibre after subtracting the pilot overhead (OH). This represents the highest-order GS format demonstrated to date, supporting the highest AIR of any standard intradyne system using conventional optics and 8-bit electronics. Detailed characterisation of the DSP, transceiver performance, and transmission modelling has also been carried out to provide insight into sources of impairments and directions for further improvement

Achievable information rates estimation for 100-nm ramana amplified optical transmission system

The achievable information rates of optical communication systems using ultra-wide bandwidth 100-nm distributed Raman amplification have been investigated for each individual subchannels, based on the first-order perturbative analysis of nonlinear distortions

Digital back-propagation for nonlinearity mitigation in distributed Raman amplified links

The performance of digital back-propagation (DBP) for distributed Raman amplified optical communication systems is evaluated through analytical models and numerical simulations, and is compared with conventional lumped amplifier solutions, such as EDFA. The complexity of the DBP algorithm including the characteristic signal power profile of distributed Raman amplifiers is assessed. The use of full-field DBP in distributed Raman amplified systems leads to 1.3 dB additional gain with respect to systems employing lumped amplification, at the cost of only a 25% increase in complexity

Experimental evaluation of impairments in unrepeatered DP-16QAM link with distributed raman amplification

The transmission impairments of a Raman amplified link using dual-polarization 16-quadrature amplitude modulation (DP-16QAM) are experimentally characterized. The impact of amplitude and phase noise on the signal due to relative intensity noise (RIN) transfer from the pump are compared for two pumping configurations: first-order backward pumping and bi-directional pumping. Experimental results indicate that with increased Raman backward pump power, though the optical signal-to-noise ratio (OSNR) is increased, so is the pump-induced amplitude and phase noise. The transmission performance is firstly improved by the enhanced OSNR at a low pump power until an optimum point is reached, and then the impairments due to pump-induced noise start to dominate. However, the introduction of a low pump power in the forward direction can further improve the system's performance

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

Calculation of Mutual Information for Partially Coherent Gaussian Channels with Applications to Fiber Optics

The mutual information between a complex-valued channel input and its complex-valued output is decomposed into four parts based on polar coordinates: an amplitude term, a phase term, and two mixed terms. Numerical results for the additive white Gaussian noise (AWGN) channel with various inputs show that, at high signal-to-noise ratio (SNR), the amplitude and phase terms dominate the mixed terms. For the AWGN channel with a Gaussian input, analytical expressions are derived for high SNR. The decomposition method is applied to partially coherent channels and a property of such channels called "spectral loss" is developed. Spectral loss occurs in nonlinear fiber-optic channels and it may be one effect that needs to be taken into account to explain the behavior of the capacity of nonlinear fiber-optic channels presented in recent studies.Comment: 30 pages, 9 figures, accepted for publication in IEEE Transactions on Information Theor

Optical Fibre Communication Systems in the Nonlinear Regime

This thesis studies solutions to increase the capacity of optical communication systems in the presence of nonlinear effects. Extending the optical bandwidth and mitigating nonlinear distortions were identified as promising ways to increase the throughput in transmission system. Raman amplification was investigated as a potential replacement of the conventional erbium-doped fibre amplifier (EDFA). In this context, the performance of discrete and distributed Raman amplifiers was studied in the linear and nonlinear regimes. Despite the bandwidth benefits, discrete Raman amplifiers were shown to exhibit an increased noise figure and nonlinear distortions, compared to EDFA. Additionally, for the first time, a thorough study of digital back-propagation for distributed Raman amplified links was performed, allowing for higher transmission rates at the expense of an increase of 25% in the algorithm complexity. A major focus of this work was to investigate the growth of nonlinear distortions in optical communication systems as the bandwidth is expanded. This work was the first to experimentally validate the Gaussian noise model (and variations accounting for inter-channel Raman scattering) in a wideband transmission regime up to 9~THz. Using these models, the merit of increasing the optical bandwidth was addressed, showing a beneficial sublinear increase in throughput despite the growth of nonlinear effects. An alternative nonlinear compensation method is optical phase conjugation (OPC). The performance of OPC was experimentally evaluated over an installed fibre link, showing limited improvements when OPC is used with practical transmission constraints. To overcome this limitation, a new method combining OPC and Volterra equalisation was developed. This method was shown to enhance the performance of two limited nonlinear compensation techniques, offering an attractive trade-off between performance and complexity. The results obtained in this research allow for higher information throughput to be transmitted, and can be used to plan and design future communication system and networks around the world