Harnessing machine learning for fiber-induced nonlinearity mitigation in long-haul coherent optical OFDM

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has attracted a lot of interest in optical fiber communications due to its simplified digital signal processing (DSP) units, high spectral-efficiency, flexibility, and tolerance to linear impairments. However, CO-OFDM’s high peak-to-average power ratio imposes high vulnerability to fiber-induced non-linearities. DSP-based machine learning has been considered as a promising approach for fiber non-linearity compensation without sacrificing computational complexity. In this paper, we review the existing machine learning approaches for CO-OFDM in a common framework and review the progress in this area with a focus on practical aspects and comparison with benchmark DSP solutions.Peer reviewe

Nonlinearity Mitigation in WDM Systems: Models, Strategies, and Achievable Rates

After reviewing models and mitigation strategies for interchannel nonlinear interference (NLI), we focus on the frequency-resolved logarithmic perturbation model to study the coherence properties of NLI. Based on this study, we devise an NLI mitigation strategy which exploits the synergic effect of phase and polarization noise compensation (PPN) and subcarrier multiplexing with symbol-rate optimization. This synergy persists even for high-order modulation alphabets and Gaussian symbols. A particle method for the computation of the resulting achievable information rate and spectral efficiency (SE) is presented and employed to lower-bound the channel capacity. The dependence of the SE on the link length, amplifier spacing, and presence or absence of inline dispersion compensation is studied. Single-polarization and dual-polarization scenarios with either independent or joint processing of the two polarizations are considered. Numerical results show that, in links with ideal distributed amplification, an SE gain of about 1 bit/s/Hz/polarization can be obtained (or, in alternative, the system reach can be doubled at a given SE) with respect to single-carrier systems without PPN mitigation. The gain is lower with lumped amplification, increases with the number of spans, decreases with the span length, and is further reduced by in-line dispersion compensation. For instance, considering a dispersion-unmanaged link with lumped amplification and an amplifier spacing of 60 km, the SE after 80 spans can be be increased from 4.5 to 4.8 bit/s/Hz/polarization, or the reach raised up to 100 spans (+25%) for a fixed SE.Comment: Submitted to Journal of Lightwave Technolog

Practical Quantum Key Distribution based on the BB84 protocol

[EN] This paper provides a review of the most important and widely used Quantum Key Distribution systems and then describes our recently proposed scheme based on Subcarrier Multiplexing that opens the possibility of parallel Quantum Key Distribution. We report the first-ever experimental implementation of parallel quantum key distribution using this technique showing a maximum multiplexing gain.This work was supported in part by the Spanish Government through Quantum Optical Information Technology (QOIT), a CONSOLIDER-INGENIO 2010 Project and in part by the Generalitat Valenciana through the PROMETEO 2008/092 research excellency award.Ruiz Alba Gaya, A.; Calvo Díaz-Aldagalán, D.; García Muñoz, V.; Martínez García, A.; Amaya Ocampo, WA.; Rozo Chicue, JG.; Mora Almerich, J.... (2011). Practical Quantum Key Distribution based on the BB84 protocol. Waves. 1(3):4-14. http://hdl.handle.net/10251/53967S4141

Achieving high visibility in subcarrier wave quantum key distribution system

We study influence of quantum signal polarization distortions in the optical fiber on the interference pattern visibility in a subcarrier wave quantum key distribution system. An optical scheme of the polarization compensation unit is suggested, and dynamics of the QBER depending on the unit architecture is explored

Chip-based Brillouin processing for carrier recovery in coherent optical communications

Modern fiber-optic coherent communications employ advanced spectrally-efficient modulation formats that require sophisticated narrow linewidth local oscillators (LOs) and complex digital signal processing (DSP). Here, we establish a novel approach to carrier recovery harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit/sec self-coherent optical signals, eliminating the need for a separate LO. In contrast to SBS processing on-fiber, our solution provides phase and polarization stability while the narrow SBS linewidth allows for a record-breaking small guardband of ~265 MHz, resulting in higher spectral-efficiency than benchmark self-coherent schemes. This approach reveals comparable performance to state-of-the-art coherent optical receivers without requiring advanced DSP. Our demonstration develops a low-noise and frequency-preserving filter that synchronously regenerates a low-power narrowband optical tone that could relax the requirements on very-high-order modulation signaling and be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements.Comment: Part of this work has been presented as a postdealine paper at CLEO Pacific-Rim'2017 and OSA Optic

Label-controlled optical switching nodes

Optical networks are evolving from initially static optical circuits and subsequently optical circuit switching towards optical packet switching in order to take advan- tage of the high transport capacity made available by WDM systems in a more °exible and e±cient way. Optically labeling of packets and routing the packets's payload optically under control of its label allows the network nodes to route and forward IP data without having to process the payload, thus keeping it in the optical domain; this is a promising solution to avoid electronic bottlenecks in routers. All-optical label switching can therefore be used to route and forward packets independent of their length and payload bitrate. Several optical signal labeling techniques have been proposed in previous re- search reported in literature; orthogonal labeling and time-serial labeling have been studied in this thesis. This thesis studies two orthogonal modulation label- ing techniques: one based on FSK labels with an IM payload, and another one on SCM labeling for a DPSK modulated payload. A time-serial labeling method based on IM labels with IM or DPSK payload is also presented and studied. The ¯rst two techniques assume electronic processing of the labels in the node, and hence assume that labels can be transmitted at a much lower bitrate than the payload data rate. The third technique assumes all-optical signal processing in the nodes, capable of handling a label at the same bitrate or slightly lower than the payload data. Labels at low bitrate in comparison with the payload bitrate are desirable in systems where the label processing will be conducted in the electrical domain, while labels at the same bitrate as the payload can be used in systems where the processing is conducted in the optical domain, exploiting all-optical processing techniques. These three techniques have been chosen because they are compatible with the existing networks, since the modulation format, bitrates, transmission properties, and other features of the signals are similar to the ones used for commercially available applications. Thus, they can be considered important candidates for migration scenarios from optical circuit switching towards optical burst switching networking. Orthogonal labeling based on FSK/IM is a promising scheme for implementing the labeling of optical signals, and it is the technology of choice in the STOLAS project. This technique o®ers advantageous features such as a relaxed timing de- lineation between payload and label, and ease of label erasure and re-writing of new labels. By using wavelength-agile tunable laser sources with FSK modula- tion capability, wavelength converters, and passive wavelength routing elements, a scalable modular label-controlled router featuring high reliability can be built. In this thesis, several aspects of the physical parameters of an FSK/IM labeling scheme within a routing node have been studied and presented. Optical ¯ltering requires special care, since the combined FSK/IM scheme has a broader spectrum than that of pure intensity modulated signals. The requirements on the limited extinction ratio for the IM signal can be relaxed at low bitrates of the label signal or, alternatively, by introducing data encoding. Optical labeling by using FSK/IM represents a simple and attractive way of implementing hybrid optical circuit and burst switching in optical networks. Architecturally, similar advantages can be mentioned for the second orthogo- nal labeling technique studied in this thesis, based on SCM labels and a DPSK payload. In-band subcarriers carrying low bitrate labels located at a frequency equal to half the bitrate of the payload signal can be inserted introducing only low power penalties. Wavelength conversion can be implemented by using passive highly nonlinear ¯bers and exploiting the four-wave mixing e®ect. This thesis also studies the design of two functional blocks of an all-optical core node proposed in the LASAGNE project, namely the all-optical label and payload separator and the wavelength converter unit for a time-serial labeling scheme. The label and payload processor can be realized exploiting nonlinear e®ects in SOAs. An implementation using polarization division multiplexing to transport the external control light for an IM/IM time-serial scheme was demon- strated. Label and payload processors with self-contained control signals were also demonstrated, either using a DPSK signal to simultaneously transport the payload data and the control signal or inserting a CW dummy in between the label and the payload, which were based on IM-RZ format. A study on single- and multi- wavelength conversion based on FWM in a HNLF was presented. This approach allows transparent wavelength conversion (independent of the data format used) at high bitrates (the nonlinear e®ects in a ¯ber are obtained at ultrafast speeds). The labeling techniques explored have indicated a viable way of migration towards optical burst packet switched networks while signi¯cantly improving the throughput of the routing nodes

Coherence-Multiplexed Optical RF Feeder Networks

An optical RF feeding system for wireless access is proposed, in which the radio access points are distinguished by means of coherence multiplexing (CM). CM is a rather unknown and potentially inexpensive optical code division multiple access technique, which is particularly suitable for relatively short-range applications with moderate transmission bandwidth requirements. Subcarrier multiplexing (SCM) can possibly be used on top of CM, either as single-channel or multichannel SCM. The performances of the resulting distribution networks are analyzed, incorporating the effect of chromatic dispersion, optical beat noise, shot noise, thermal noise, and—in the case of multichannel SCM—intermodulation distortion. The results of the IEEE 802.11b standard for wireless LAN.\u