Compensation of Laser Phase Noise Using DSP in Multichannel Fiber-Optic Communications

One of the main impairments that limit the throughput of fiber-optic communication systems is laser phase noise, where the phase of the laser output drifts with time. This impairment can be highly correlated across channels that share lasers in multichannel fiber-optic systems based on, e.g., wavelength-division multiplexing using frequency combs or space-division multiplexing. In this thesis, potential improvements in the system tolerance to laser phase noise that are obtained through the use of joint-channel digital signal processing are investigated. To accomplish this, a simple multichannel phase-noise model is proposed, in which the phase noise is arbitrarily correlated across the channels. Using this model, high-performance pilot-aided phase-noise compensation and data-detection algorithms are designed for multichannel fiber-optic systems using Bayesian-inference frameworks. Through Monte Carlo simulations of coded transmission in the presence of moderate laser phase noise, it is shown that joint-channel processing can yield close to a 1 dB improvement in power efficiency. It is further shown that the algorithms are highly dependent on the positions of pilots across time and channels. Hence, the problem of identifying effective pilot distributions is studied.The proposed phase-noise model and algorithms are validated using experimental data based on uncoded space-division multiplexed transmission through a weakly-coupled, homogeneous, single-mode, 3-core fiber. It is found that the performance improvements predicted by simulations based on the model are reasonably close to the experimental results. Moreover, joint-channel processing is found to increase the maximum tolerable transmission distance by up to 10% for practical pilot rates.Various phenomena decorrelate the laser phase noise between channels in multichannel transmission, reducing the potency of schemes that exploit this correlation. One such phenomenon is intercore skew, where the spatial channels experience different propagation velocities. The effect of intercore skew on the performance of joint-core phase-noise compensation is studied. Assuming that the channels are aligned in the receiver, joint-core processing is found to be beneficial in the presence of skew if the linewidth of the local oscillator is lower than the light-source laser linewidth.In the case that the laser phase noise is completely uncorrelated across channels in multichannel transmission, it is shown that the system performance can be improved by applying transmitter-side multidimensional signal rotations. This is found by numerically optimizing rotations of four-dimensional signals that are transmitted through two channels. Structured four-dimensional rotations based on Hadamard matrices are found to be near-optimal. Moreover, in the case of high signal-to-noise ratios and high signal dimensionalities, Hadamard-based rotations are found to increase the achievable information rate by up to 0.25 bits per complex symbol for transmission of higher-order modulations

Digital signal processing for fiber nonlinearities [Invited]

This paper reviews digital signal processing techniques that compensate, mitigate, and exploit fiber nonlinearities in coherent optical fiber transmission systems

Overhead-optimization of pilot-based digital signal processing for flexible high spectral efficiency transmission

We present a low-complexity fully pilot-based digital signal processing (DSP) chain designed for high spectral efficiency optical transmission systems. We study the performance of the individual pilot algorithms in simulations before demonstrating transmission of a 51 724 Gbaud PM-64QAM superchannel over distances reaching 1000 km. We present an overhead optimization technique using the system achievable information rate to find the optimal balance between increased performance and throughput reduction from adding additional DSP pilots. Using the optimal overhead of 2.4%, we report 9.3 (8.3) bits/s/Hz spectral efficiency, or equivalently 11.9 (10.6) Tb/s superchannel throughput, after 480 (960) km of transmission over 80 km spans with EDFA-only amplification. Moreover, we show that the optimum overhead depends only weakly on transmission distance, concluding that back-to-back optimization is sufficient for all studied distances. Our results show that pilot-based DSP combined with overhead optimization can increase the robustness and performance of systems using advanced modulation formats while still maintaining state-of-the-art spectral efficiency and multi-Tb/s throughput

Phase-sensitive amplifiers for nonlinearity impairment mitigation in optical fiber transmission links

The fundamental limitations in fiber-optic communication are caused by optical amplifier noise and the nonlinear response of the optical fibers. The quantum-limited noise figure of erbium-doped fiber amplifiers (EDFAs) or any phase-insensitive amplifier is 3 dB. However, the noise added by the amplification can be reduced using phase-sensitive amplifiers (PSAs), whose quantum-limited noise figure is 0 dB. PSAs can also compensate for the nonlinear distortions from the optical transmission fiber in the copier-PSA implementation. At the transmitter, a copier which is nothing but a phase-insensitive amplifier, is used to create a conjugated copy of the signal. The signal and idler are then copropagated in the fiber link, experiencing correlated nonlinear distortions. The nonlinear distortions are reduced by the all-optical coherent superposition of the signal and idler in the PSA.In this work, an investigation is made for the nonlinearity mitigation using the PSAs, by calculating the residual nonlinear distortion after the coherent superposition in a copier-PSA link. The nonlinearity mitigation efficiency in PSA links is studied with respect to modulation formats, symbol rates and number of wavelength channels. The effectiveness of nonlinearity mitigation is found to increase with higher-order modulation formats. However, the efficiency of nonlinearity mitigation decreases with increasing number of wavelength channels and increasing symbol rate resulting in larger residual nonlinear distortions. A modified Volterra nonlinear equalizer (VNLE) is implemented to reduce the residual nonlinear distortions after PSAs in single- and multi-channel PSA links. Cross-phase modulation mitigation using PSAs is also demonstrated

Photonic processing of microwave signals

La distribution par fibre optique de signaux de type « ultra-wideband (UWB)» requiert le développement de nouvelles technologies photoniques qui seront le sujet d'étude de cette thèse. Nous commençons avec un démonstration expérimentale d'une technique de sculpture d'impulsions qui offre une solution économique et à faible consommation de puissance pour les systèmes UWB . Dans cette étude, nous procédons à l'apodisation de deux réseaux Bragg identiques avec une variation de période linéaire qui sont placés aux deux entrées d'un photodétecteur balancé. L'apodisation est réalisée par l'application d'un profile de température à l'aide d'éléments résistif de petite dimensions, ce qui permet une consommation énergétique réduite et une bonne résolution spectrale. Le filtrage spectral d'une source laser puisée suivi d'une conversion fréquence-temps par propagation dans une fibre optique standard permet de générer une impulsion UWB efficace d'un point de vue énergétique pour les communications à courte portée dans la bande spectrale de 3 a 10 GHz. Dans un deuxième temps, pour générer des signaux passe-bande à haute fréquence, nous avons utilisé un laser puisé à commutation de gain. Après la conversion optique/électrique des impulsions en utilisant des filtres optiques et RF appropriés, nous réussissons à générer des signaux large bande dans des bandes spectrales ayant des fréquences centrales de 25, 35 et 45 GHz. Nous examinons diverses configurations de filtres permettant cette conversion selon qu'il y ait ou non transmission dans une fibre optique. Finalement, nous démontrons la détection de signaux RF dans le domaine optique par le design et la fabrication de filtres adaptés. Notre récepteur utilise un modulateur de Mach-Zehnder pour faire la conversion électrique-optique et des filtres à base de réseau de Bragg comme filtres adaptés. Nous examinons la performance du récepteur pour deux conditions de polarisation différentes du Mach-Zehnder. Nous avons conçu des filtres adaptés pour ces deux cas et nous discutons de la performance résultant

Design, monitoring and performance evaluation of high capacity optical networks

Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit de les TICInternet traffic is expected to keep increasing exponentially due to the emergence of a vast number of innovative online services and applications. Optical networks, which are the cornerstone of the underlying Internet infrastructure, have been continuously evolving to carry the ever-increasing traffic in a more flexible, cost-effective, and intelligent way. Having these three targets in mind, this PhD thesis focuses on two general areas for the performance improvement and the evolution of optical networks: i) introducing further cognition to the optical layer, and ii) introducing new networking solutions revolutionizing the optical transport infrastructure. In the first part, we present novel failure detection and identification solutions in the optical layer utilizing the optical spectrum traces captured by cost-effective coarse-granular Optical Spectrum Analyzers (OSA). We demonstrate the effectiveness of the developed solutions for detecting and identifying filter-related failures in the context of Spectrum-Switched Optical Networks (SSON), as well as transmitter-related laser failures in Filter-less Optical Networks (FON). In addition, at the subsystem level we propose an Autonomic Transmission Agent (ATA), which triggers local or remote transceiver reconfiguration by predicting Bit-Error-Rate (BER) degradation by monitoring State-of-Polarization (SOP) data obtained by coherent receivers. I have developed solutions to push further the performance of the currently deployed optical networks through reducing the margins and introducing intelligence to better manage their resources. However, it is expected that the spectral efficiency of the current standard Single-Mode Fiber (SMF) based optical network approaches the Shannon capacity limits in the near future, and therefore, a new paradigm is required to keep with the pace of the current huge traffic increase. In this regard, Space Division Multiplexing (SDM) is proposed as the ultimate solution to address the looming capacity crunch with a reduced cost-per-bit delivered to the end-users. I devote the second part of this thesis to investigate different flavors of SDM based optical networks with the aim of finding the best compromise for the realization of a spectrally and spatially flexible optical network. SDM-based optical networks can be deployed over various types of transmission media. Additionally, due to the extra dimension (i.e., space) introduced in SDM networks, optical switching nodes can support wavelength granularity, space granularity, or a combination of both. In this thesis, we evaluate the impact of various spectral and spatial switching granularities on the performance of SDM-based optical networks serving different profiles of traffic with the aim of understanding the impact of switching constraints on the overall network performance. In this regard, we consider two different generations of wavelength selective switches (WSS) to reflect the technology limitations on the performance of SDM networks. In addition, we present different designs of colorless direction-less, and Colorless Directionless Contention-less (CDC) Reconfigurable Optical Add/Drop Multiplexers (ROADM) realizing SDM switching schemes and compare their performance in terms of complexity and implementation cost. Furthermore, with the aim of revealing the benefits and drawbacks of SDM networks over different types of transmission media, we preset a QoT-aware network planning toolbox and perform comparative performance analysis among SDM network based on various types of transmission media. We also analyze the power consumption of Multiple-Input Multiple-Output (MIMO) Digital Signal Processing (DSP) units of transceivers operating over three different types of transmission media. The results obtained in the second part of the thesis provide a comprehensive outlook to different realizations of SDM-based optical networks and showcases the benefits and drawbacks of different SDM realizations.Se espera que el tráfico de Internet siga aumentando exponencialmente debido a la continua aparición de gran cantidad de aplicaciones innovadoras. Las redes ópticas, que son la piedra angular de la infraestructura de Internet, han evolucionado continuamente para transportar el tráfico cada vez mayor de una manera más flexible, rentable e inteligente. Teniendo en cuenta estos tres objetivos, esta tesis doctoral se centra en dos áreas cruciales para la mejora del rendimiento y la evolución de las redes ópticas: i) introducción de funcionalidades cognitivas en la capa óptica, y ii) introducción de nuevas estructuras de red que revolucionarán el transporte óptico. En la primera parte, se presentan soluciones novedosas de detección e identificación de fallos en la capa óptica que utilizan trazas de espectro óptico obtenidas mediante analizadores de espectros ópticos (OSA) de baja resolución (y por tanto de coste reducido). Se demuestra la efectividad de las soluciones desarrolladas para detectar e identificar fallos derivados del filtrado imperfecto en las redes ópticas de conmutación de espectro (SSON), así como fallos relacionados con el láser transmisor en redes ópticas sin filtro (FON). Además, a nivel de subsistema, se propone un Agente de Transmisión Autónomo (ATA), que activa la reconfiguración del transceptor local o remoto al predecir la degradación de la Tasa de Error por Bits (BER), monitorizando el Estado de Polarización (SOP) de la señal recibida en un receptor coherente. Se han desarrollado soluciones para incrementar el rendimiento de las redes ópticas mediante la reducción de los márgenes y la introducción de inteligencia en la administración de los recursos de la red. Sin embargo, se espera que la eficiencia espectral de las redes ópticas basadas en fibras monomodo (SMF) se acerque al límite de capacidad de Shannon en un futuro próximo, y por tanto, se requiere un nuevo paradigma que permita mantener el crecimiento necesario para soportar el futuro aumento del tráfico. En este sentido, se propone el Multiplexado por División Espacial (SDM) como la solución que permita la continua reducción del coste por bit transmitido ante ése esperado crecimiento del tráfico. En la segunda parte de esta tesis se investigan diferentes tipos de redes ópticas basadas en SDM con el objetivo de encontrar soluciones para la realización de redes ópticas espectral y espacialmente flexibles. Las redes ópticas basadas en SDM se pueden implementar utilizando diversos tipos de medios de transmisión. Además, debido a la dimensión adicional (el espacio) introducida en las redes SDM, los nodos de conmutación óptica pueden conmutar longitudes de onda, fibras o una combinación de ambas. Se evalúa el impacto de la conmutación espectral y espacial en el rendimiento de las redes SDM bajo diferentes perfiles de tráfico ofrecido, con el objetivo de comprender el impacto de las restricciones de conmutación en el rendimiento de la red. En este sentido, se consideran dos generaciones diferentes de conmutadores selectivos de longitud de onda (WSS) para reflejar las limitaciones de la tecnología en el rendimiento de las redes SDM. Además, se presentan diferentes diseños de ROADM, independientes de la longitud de onda, de la dirección, y sin contención (CDC) utilizados para la conmutación SDM, y se compara su rendimiento en términos de complejidad y coste. Además, con el objetivo de cuantificar los beneficios e inconvenientes de las redes SDM, se ha generado una herramienta de planificación de red que prevé la QoT usando diferentes tipos de fibras. También se analiza el consumo de energía de las unidades DSP de los transceptores MIMO operando en redes SDM con tres tipos diferentes de medios de transmisión. Los resultados obtenidos en esta segunda parte de la tesis proporcionan una perspectiva integral de las redes SDM y muestran los beneficios e inconvenientes de sus diferentes implementacionesAward-winningPostprint (published version

Roadmap of optical communications

© 2016 IOP Publishing Ltd. Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications

Storing, single photons in broadband vapor cell quantum memories

Single photons are an essential resource for realizing quantum technologies. Together with compatible quantum memories granting control over when a photon arrives, they form a foundational component both of quantum communication and quantum information processing. Quality solid-state single photon sources deliver on the high bandwidths and rates required for scalable quantum technology, but require memories that match these operational parameters. In this thesis, I report on quantum memories based on electromagnetically induced transparency and built in warm rubidium vapor, with such fast and high bandwidth interfaces in mind. I also present work on a heralded single photon source based on parametric downconversion in an optical cavity, operated in a bandwidth regime of a few 100s of megahertz. The systems are characterized on their own and together in a functional interface. As the photon generation process is spontaneous, the memory is implemented as a fully reactive device, capable of storing and retrieving photons in response to an asynchronous external trigger. The combined system is used to demonstrate the storage and retrieval of single photons in and from the quantum memory. Using polarization selection rules in the Zeeman substructure of the atoms, the read-out noise of the memory is considerably reduced from what is common in ground-state storage schemes in warm vapor. Critically, the quantum signature in the photon number statistics of the retrieved photons is successfully maintained, proving that the emission from the memory is dominated by single photons. We observe a retrieved single-photon state accuracy of $g_{c,\,\text{ret}}^{(2)}=0.177(23)$ for short storage times, which remains $g_{c,\,\text{ret}}^{(2)}<0.5$ throughout the memory lifetime of $680(50)\,$ns. The end-to-end efficiency of the memory interfaced with the photon source is $\eta_{e2e}=1.1(2)\,\%$, which will be further improved in the future by optimizing the operating regime. With its operation bandwidth of $370\,$MHz, our system opens up new possibilities for single-photon synchronization and local quantum networking experiments at high repetition rates