39 research outputs found

    Bandwidth Extension in a Mid-Link Optical Phase Conjugation

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    In this paper, we investigate various designs of distributed Raman amplifier (DRA) to extend amplification bandwidth in mid-link optical phase conjugation (OPC) systems and compare bands 191−197 THz and 192−198 THz giving a total bandwidth of 6 THz using a single wavelength pump. We demonstrate the use of highly reflective fiber Bragg grating (FBG) to minimize gain variation across a WDM grid by optimizing forward and backward pump powers as well as the wavelength of FBGs for original and conjugated channels. Finally, we also simulate OSNR and Kerr nonlinear reduction as a product of signals asymmetry and nonlinear phase shift (NPS) for all channels

    Symmetry & nonlinear compensation in fiber-optic transmissions

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    This thesis presents methods and practical implementations for compensating or suppressing signal distortions induced by fiber nonlinearity in long-distance transmissions. Our methods take advantage of the availability and already wide deployment of dispersion-compensating fibers with various choices of dispersions and dispersion slopes. The basic principle behind the methods is to choose suitable fibers and to arrange them properly into transmission lines manifesting scaled symmetries. Based on the nonlinear Schrodinger equation which describes the nonlinear and dispersive signal propagation in optical fibers, we have shown analytically that a scaled symmetry renders the nonlinear signal distortion by the first part of a transmission line to be largely undone by the second part, when an optical phase conjugator is installed in the middle of the line. Without a phase conjugator, the most detrimental nonlinear interactions among pulses within a wavelength channel may be significantly suppressed in a scaled symmetric line. We have identified two types of scaled symmetries: mirror and translation. Although mirror-symmetric systems have been discussed by other authors before, our own proposals and designs using high-dispersion fibers in conjunction with distributive Raman or erbium-doped amplification could make practical transmission systems manifesting nearly perfect mirror symmetries in the scaled sense and hence excellent nonlinear compensations. Firstly noted and investigated thoroughly by us, the concept of scaled translation symmetries in transmission lines may well spur the adoption of nonlinear compensation methods in practical transmission systems, since distributive amplifiers are no longer necessary for translation symmetries. To support our mathematical analyses, extensive computer simulations have been carried out to validate the effectiveness of our proposed systems, most of which assume practical system setups and parameters and could therefore serve as paradigms for real system designs

    Optical Phase Conjugation in Installed Optical Networks

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    We demonstrate a record throughput of 5.7 Tbit/s employing an optical phase conjugator to jointly compensate chromatic dispersion and increase the nonlinear threshold in an installed optical network using commercially available lumped amplifiers

    Signal processing techniques for reducing the impact of fiber nonlinearities on system performance

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    Signal processing techniques for mitigating intra-channel and inter-channel fiber nonlinearities are reviewed. More detailed descriptions of three specific examples highlight the diversity of the electronic and optical approaches that have been investigated

    Phase-sensitively amplified wavelength-division multiplexed optical transmission systems

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    The throughput and reach in fiber-optic communication links are limited by in-line optical amplifier noise and the Kerr nonlinearity in the optical transmission fiber. Phase-sensitive amplifiers (PSAs) are capable of amplifying signals without adding excess noise and mitigating the impairments caused by the Kerr nonlinearity. However, the effectiveness of Kerr nonlinearity mitigation depends on the dispersion pre-compensation in each span. This paper investigates dense wavelength-division multiplexed PSA-amplified links using joint processing with a less complex digital domain Volterra nonlinear equalizer at the receiver. Both numerically and with experiments, it is shown that this significantly reduces the impact of the dispersion pre-compensation in each span. Also, with simulations, a substantial improvement in transmission reach is demonstrated for PSA links

    Nonlinearity compensation using optical phase conjugation deployed in discretely amplified transmission systems

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    We introduce a closed form equation, validated by simulations and experimental results, that predicts the residual nonlinear noise ratio in mid-link OPC assisted discretely amplified systems. The model anticipates the reduction in performance enhancement achieved by mid-link OPC as the bandwidth of the modulated signals increases. The numerical analysis shows that uncompensated signal-signal interactions limit the performance improvement achieved by the introduction of additional OPCs. The numerical analysis predicts that the deployment of shorter amplifier spacing will lead to a greater performance enhancement. The numerical results are validated by experimentally testing of 2x, 4x, and 8x28Gbaud PM-QPSK systems with mid-link OPC compensation in a discretely amplified system with 100km amplifier spacing. The experimentally obtained reach enhancement (43%, 32%, and 24% for 2x28Gbaud, 4x28Gbaud, and 8x28Gbaud, respectively) confirms that the compensation efficiency of mid-link OPC is highly dependent on the number of channels (bandwidth) propagating along the system

    Asymmetry Optimization for 10 THz OPC Transmission over the C + L Bands Using Distributed Raman Amplification

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    An optimized design for a broadband Raman optical amplifier in standard single-mode fiber covering the C and L bands is presented, to be used in combination with wideband optical phase conjugation (OPC) nonlinearity compensation. The use of two Raman pumps and fiber Bragg grating reflectors at different wavelengths for the transmitted (C band) and conjugated (L band) WDM channels is proposed to extend bandwidth beyond the limits imposed by single-wavelength pumping, for a total 10 THz. Optimization of pump and reflector wavelength, as well as pump powers, allows us to achieve low asymmetry across the whole transmission band for optimal nonlinearity compensation. System performance is simulated to estimate OSNR, gain flatness and nonlinear Kerr distortion

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

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    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

    Raman fibre laser based amplification in coherent transmission systems

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    The thesis presents a detailed study of different Raman fibre laser (RFL) based amplification techniques and their applications in long-haul/unrepeatered coherent transmission systems. RFL based amplifications techniques were characterised from different aspects, including signal/noise power distributions, relative intensity noise (RIN), mode structures of induced Raman fibre lasers, and so on. It was found for the first time that RFL based amplification techniques could be divided into three categories in terms of the fibre laser regime, which were Fabry-Perot fibre laser with two FBGs, weak Fabry-Perot fibre laser with one FBG and very low reflection near the input, and random distributed feedback (DFB) fibre laser with one FBG. It was also found that lowering the reflection near the input could mitigate the RIN of the signal significantly, thanks to the reduced efficiency of the Stokes shift from the FW-propagated pump. In order to evaluate the transmission performance, different RFL based amplifiers were evaluated and optimised in long-haul coherent transmission systems. The results showed that Fabry-Perot fibre laser based amplifier with two FBGs gave >4.15 dB Q factor penalty using symmetrical bidirectional pumping, as the RIN of the signal was increased significantly. However, random distributed feedback fibre laser based amplifier with one FBG could mitigate the RIN of the signal, which enabled the use of bidirectional second order pumping and consequently give the best transmission performance up to 7915 km. Furthermore, using random DFB fibre laser based amplifier was proved to be effective to combat the nonlinear impairment, and the maximum reach was enhanced by >28% in mid-link single/dual band optical phase conjugator (OPC) transmission systems. In addition, unrepeatered transmission over >350 km fibre length using RFL based amplification technique were presented experimentally using DP-QPSK and DP-16QAM transmitter
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