Investigation of EDFA power transients in circuit-switched and packet-switched optical networks

Erbium-doped fibre amplifiers (EDFA’s) are a key technology for the design of all optical communication systems and networks. The superiority of EDFAs lies in their negligible intermodulation distortion across high speed multichannel signals, low intrinsic losses, slow gain dynamics, and gain in a wide range of optical wavelengths. Due to long lifetime in excited states, EDFAs do not oppose the effect of cross-gain saturation. The time characteristics of the gain saturation and recovery effects are between a few hundred microseconds and 10 milliseconds. However, in wavelength division multiplexed (WDM) optical networks with EDFAs, the number of channels traversing an EDFA can change due to the faulty link of the network or the system reconfiguration. It has been found that, due to the variation in channel number in the EDFAs chain, the output system powers of surviving channels can change in a very short time. Thus, the power transient is one of the problems deteriorating system performance. In this thesis, the transient phenomenon in wavelength routed WDM optical networks with EDFA chains was investigated. The task was performed using different input signal powers for circuit switched networks. A simulator for the EDFA gain dynamicmodel was developed to compute the magnitude and speed of the power transients in the non-self-saturated EDFA both single and chained. The dynamic model of the self-saturated EDFAs chain and its simulator were also developed to compute the magnitude and speed of the power transients and the Optical signal-to-noise ratio (OSNR). We found that the OSNR transient magnitude and speed are a function of both the output power transient and the number of EDFAs in the chain. The OSNR value predicts the level of the quality of service in the related network. It was found that the power transients for both self-saturated and non-self-saturated EDFAs are close in magnitude in the case of gain saturated EDFAs networks. Moreover, the cross-gain saturation also degrades the performance of the packet switching networks due to varying traffic characteristics. The magnitude and the speed of output power transients increase along the EDFAs chain. An investigation was done on the asynchronous transfer mode (ATM) or the WDM Internet protocol (WDM-IP) traffic networks using different traffic patterns based on the Pareto and Poisson distribution. The simulator is used to examine the amount and speed of the power transients in Pareto and Poisson distributed traffic at different bit rates, with specific focus on 2.5 Gb/s. It was found from numerical and statistical analysis that the power swing increases if the time interval of theburst-ON/burst-OFF is long in the packet bursts. This is because the gain dynamics is fast during strong signal pulse or with long duration pulses, which is due to the stimulatedemission avalanche depletion of the excited ions. Thus, an increase in output power levelcould lead to error burst which affects the system performance

Ultrawideband Systems and Networks: Beyond C + L-Band

In the evolution of optical networks, enhancement of spectral efficiency (SE) enhancement has been the most cost-efficient and thus the main driver for capacity enhancementincrease for decades. As a result, the development of optical transport systems has been focused on the C -and L -bands, where silica optical fiber exhibits the lowest attenuation, and erbium-doped fiber amplifiers provide an efficient solution forto compensatinge for the optical loss. With a gradual maturity in the SE growth, however, the extension of the optical bandwidth beyond the C + L -band is expected to play a significant role in the future capacity upgrades of optical networks and, thus, attracting increasing research interests. In this article, we discuss the merits and challenges of ultrawideband optical transport systems and networks beyond conventional bands

Advances in Optical Amplifiers

Optical amplifiers play a central role in all categories of fibre communications systems and networks. By compensating for the losses exerted by the transmission medium and the components through which the signals pass, they reduce the need for expensive and slow optical-electrical-optical conversion. The photonic gain media, which are normally based on glass- or semiconductor-based waveguides, can amplify many high speed wavelength division multiplexed channels simultaneously. Recent research has also concentrated on wavelength conversion, switching, demultiplexing in the time domain and other enhanced functions. Advances in Optical Amplifiers presents up to date results on amplifier performance, along with explanations of their relevance, from leading researchers in the field. Its chapters cover amplifiers based on rare earth doped fibres and waveguides, stimulated Raman scattering, nonlinear parametric processes and semiconductor media. Wavelength conversion and other enhanced signal processing functions are also considered in depth. This book is targeted at research, development and design engineers from teams in manufacturing industry, academia and telecommunications service operators

Photonic Vector Processing Techniques for Radiofrequency Signals

[EN] The processing of radiofrequency signals using photonics means is a discipline that appeared almost at the same time as the laser and the optical fibre. Photonics offers the capability of managing broadband radiofrequency (RF) signals thanks to its low transmission attenuation, a variety of linear and non-linear phenomena and, recently, the potential to implement integrated photonic subsystems. These features open the door for the implementation of multiple functionalities including optical transportation, up and down frequency conversion, optical RF filtering, signal multiplexing, de-multiplexing, routing and switching, optical sampling, tone generation, delay control, beamforming and photonic generation of digital modulations, and even a combination of several of these functionalities. This thesis is focused on the application of vector processing in the optical domain to radiofrequency signals in two fields of application: optical beamforming, and photonic vector modulation and demodulation of digital quadrature amplitude modulations. The photonic vector control enables to adjust the amplitude and phase of the radiofrequency signals in the optical domain, which is the fundamental processing that is required in different applications such as beamforming networks for direct radiating array (DRA) antennas and multilevel quadrature modulation. The work described in this thesis include different techniques for implementing a photonic version of beamforming networks for direct radiating arrays (DRA) known as optical beamforming networks (OBFN), with the objectives of providing a precise control in terrestrial applications of broadband signals at very high frequencies above 40 GHz in communication antennas, optimizing the size and mass when compared with the electrical counterparts in space application, and presenting new photonic-based OBFN functionalities. Thus, two families of OBFNs are studied: fibre-based true time delay architectures and integrated networks. The first allow the control of broadband signals using dispersive optical fibres with wavelength division multiplexing techniques and advanced functionalities such as direction of arrival estimation in receiving architectures. In the second, passive OBFNs based on monolithically-integrated Optical Butler Matrices are studied, including an ultra-compact solution using optical heterodyne techniques in silicon-on-insulator (SOI) material, and an alternative implementing a homodyne counterpart in germanium doped silica material. In this thesis, the application of photonic vector processing to the generation of quadrature digital modulations has also been investigated. Multilevel modulations are based on encoding digital information in discrete states of phase and amplitude of an electrical signal to enhance spectral efficiency, as for instance, in quadrature modulation. The signal process required for generating and demodulating this kind of signals involves vector processing (phase and amplitude control) and frequency conversion. Unlike the common electronic or digital implementation, in this thesis, different photonic based signal processing techniques are studied to produce digital modulation (photonic vector modulation, PVM) and demodulation (PVdM). These techniques are of particular interest in the case of broadband signals where the data rate required to be managed is in the order of gigabit per second, for applications like wireless backhauling of metro optical networks (known as fibre-to-the-air). The techniques described use optical dispersion in optical fibres, wavelength division multiplexing and photonic up/down conversion. Additionally, an optical heterodyne solution implemented monolithically in a photonic integrated circuit (PIC) is also described.[ES] El procesamiento de señales de radiofrecuencia (RF) utilizando medios fotónicos es una disciplina que apareció casi al mismo tiempo que el láser y la fibra óptica. La fotónica ofrece la capacidad de manipular señales de radiofrecuencia de banda ancha, una baja atenuación, procesados basados en una amplia variedad de fenómenos lineales y no lineales y, recientemente, el potencial para implementar subsistemas fotónicos integrados. Estas características ofrecen un gran potencial para la implementación de múltiples funcionalidades incluyendo transporte óptico, conversión de frecuencia, filtrado óptico de RF, multiplexación y demultiplexación de señales, encaminamiento y conmutación, muestreo óptico, generación de tonos, líneas de retardo, conformación de haz en agrupaciones de antenas o generación fotónica de modulaciones digitales, e incluso una combinación de varias de estas funcionalidades. Esta tesis se centra en la aplicación del procesamiento vectorial en el dominio óptico de señales de radiofrecuencia en dos campos de aplicación: la conformación óptica de haces y la modulación y demodulación vectorial fotónica de señales digitales en cuadratura. El control fotónico vectorial permite manipular la amplitud y fase de las señales de radiofrecuencia en el dominio óptico, que es el procesamiento fundamental que se requiere en diferentes aplicaciones tales como las redes de conformación de haces para agrupaciones de antenas y en la modulación en cuadratura. El trabajo descrito en esta tesis incluye diferentes técnicas para implementar una versión fotónica de las redes de conformación de haces de en agrupaciones de antenas, conocidas como redes ópticas de conformación de haces (OBFN). Se estudian dos familias de redes: arquitecturas de retardo en fibra óptica y arquitecturas integradas. Las primeras permiten el control de señales de banda ancha utilizando fibras ópticas dispersivas con técnicas de multiplexado por división de longitud de onda y funcionalidades avanzadas tales como la estimación del ángulo de llegada de la señal en la antena receptora. En la segunda, se estudian redes de conformación pasivas basadas en Matrices de Butler ópticas integradas, incluyendo una solución ultra-compacta utilizando técnicas ópticas heterodinas en silicio sobre aislante (SOI), y una alternativa homodina en sílice dopado con germanio. En esta tesis, también se han investigado técnicas de procesado vectorial fotónico para la generación de modulaciones digitales en cuadratura. Las modulaciones multinivel codifican la información digital en estados discretos de fase y amplitud de una señal eléctrica para aumentar su eficiencia espectral, como por ejemplo la modulación en cuadratura. El procesado necesario para generar y demodular este tipo de señales implica el procesamiento vectorial (control de amplitud y fase) y la conversión de frecuencia. A diferencia de la implementación electrónica o digital convencional, en esta tesis se estudian diferentes técnicas de procesado fotónico tanto para la generación de modulaciones digitales (modulación vectorial fotónica, PVM) como para su demodulación (PVdM). Esto es de particular interés en el caso de señales de banda ancha, donde la velocidad de datos requerida es del orden de gigabits por segundo, para aplicaciones como backhaul inalámbrico de redes ópticas metropolitanas (conocida como fibra hasta el aire). Las técnicas descritas se basan en explotar la dispersión cromática de la fibra óptica, la multiplexación por división de longitud de onda y la conversión en frecuencia. Además, se presenta una solución heterodina implementada monolíticamente en un circuito integrado fotónico (PIC).[CA] El processament de senyals de radiofreqüència (RF) utilitzant mitjans fotònics és una disciplina que va aparèixer gairebé al mateix temps que el làser i la fibra òptica. La fotònica ofereix la capacitat de manipular senyals de radiofreqüència de banda ampla, una baixa atenuació, processats basats en una àmplia varietat de fenòmens lineals i no lineals i, recentment, el potencial per implementar subsistemes fotònics integrats. Aquestes característiques ofereixen un gran potencial per a la implementació de múltiples funcionalitats incloent transport òptic, conversió de freqüència, filtrat òptic de RF, multiplexació i demultiplexació de senyals, encaminament i commutació, mostreig òptic, generació de tons, línies de retard, conformació de feix en agrupacions d'antenes i la generació fotònica de modulacions digitals, i fins i tot una combinació de diverses d'aquestes funcionalitats. Aquesta tesi es centra en l'aplicació del processament vectorial en el domini òptic de senyals de radiofreqüència en dos camps d'aplicació: la conformació òptica de feixos i la modulació i demodulació vectorial fotònica de senyals digitals en quadratura. El control fotònic vectorial permet manipular l'amplitud i la fase dels senyals de radiofreqüència en el domini òptic, que és el processament fonamental que es requereix en diferents aplicacions com ara les xarxes de conformació de feixos per agrupacions d'antenes i en modulació multinivell. El treball descrit en aquesta tesi inclou diferents tècniques per implementar una versió fotònica de les xarxes de conformació de feixos en agrupacions d'antenes, conegudes com a xarxes òptiques de conformació de feixos (OBFN), amb els objectius de proporcionar un control precís en aplicacions terrestres de senyals de banda ampla a freqüències molt altes per sobre de 40 GHz en antenes de comunicacions, optimitzant la mida i el pes quan es compara amb els homòlegs elèctrics en aplicacions espacials, i la presentació de noves funcionalitats fotòniques per agrupacions d'antenes. Per tant, s'estudien dues famílies de OBFNs: arquitectures de retard en fibra òptica i arquitectures integrades. Les primeres permeten el control de senyals de banda ampla utilitzant fibres òptiques dispersives amb tècniques de multiplexació per divisió en longitud d'ona i funcionalitats avançades com ara l'estimació de l'angle d'arribada del senyal a l'antena receptora. A la segona, s'estudien xarxes de conformació passives basades en Matrius de Butler òptiques en fotònica integrada, incloent una solució ultra-compacta utilitzant tècniques òptiques heterodinas en silici sobre aïllant (SOI), i una alternativa homodina en sílice dopat amb germani. D'altra banda, també s'ha investigat en aquesta tesi tècniques de processament vectorial fotònic per a la generació de modulacions digitals en quadratura. Les modulacions multinivell codifiquen la informació digital en estats discrets de fase i amplitud d'un senyal elèctric per augmentar la seva eficiència espectral, com ara la modulació en quadratura. El processat necessari per generar i desmodular aquest tipus de senyals implica el processament vectorial (control d'amplitud i fase) i la conversió de freqüència. A diferència de la implementació electrònica o digital convencional, en aquesta tesi s'estudien diferents tècniques de processament fotònic tant per a la generació de modulacions digitals (modulació vectorial fotònica, PVM) com per la seva demodulació (PVdM). Això és de particular interès en el cas de senyals de banda ampla, on la velocitat de dades requerida és de l'ordre de gigabits per segon, per a aplicacions com backhaul sense fils de xarxes òptiques metropolitanes (coneguda com fibra fins l'aire). Les tècniques descrites es basen en explotar la dispersió cromàtica de la fibra òptica, la multiplexació per divisió en longitud d'ona i la conversió en freqüència. A més, es presePiqueras Ruipérez, MÁ. (2016). Photonic Vector Processing Techniques for Radiofrequency Signals [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/63264TESI

Investigation of high bit rate optical transmission systems employing a channel data rate of 40 Gb/s

Das Ziel dieser Doktorarbeit war eine detaillierte Untersuchung von hoch bit ratigen optischen Übertragungssystemen mit einer Kanaldatenrate von 40 Gbit/s, die als wavelength division multiplexing (WDM) Systeme realisiert sind. Die Erkenntnisse, die durch umfangreiche numerische Untersuchungen gewonnen worden sind, wurden für die Erarbeitung von Designkriterien für die Übertragungssysteme der nächsten Generation verwendet. Der Schwerpunkt der Arbeit liegt dabei an 40 Gbit/s basierten WDM Systemen mit amplitudenmodulierten optischen Signalen. Nach einer umfangreichen Beschreibung der Funktionsweise und des Standes der Technik von Systemkomponenten, die in optischen Übertragungssystemen zum Einsatz kommen, wurden die Übertragungseffekte (z.B. chromatische Dispersion, Kerr-Effekt) erklärt und beschrieben, die eine störungsfreie Übertragung von optischen Pulsen in Übertragungsstrecken beeinträchtigen. Wegen der Fokussierung der Arbeit auf amplitudenmodulierte Systeme, wurden Erzeugungsmethoden und Spektraleneigenschaften von zahlreichen amplitude-shift-keying (ASK) basierten Modulationsformaten erklärt. Die untersuchten Modulationsformate wurden in drei Gruppen unterteilt: Non-return-to-zero (NRZ) basierende Formate, Return-to-zero (RZ) basierende Formate und neue Modulationsformate. Zu der Gruppe von NRZ basierten Modulationsformaten gehören konventionelles NRZ und Duobinary Modulation. In der Gruppe von RZ basierten Formaten wurden konventionelles RZ, Carrier-suppressed RZ (CSRZ) und Single-side-band RZ (SSB-RZ) eingeführt. Die Gruppe der neuen Formate beinhaltet Modulationsformate, die vom Autor im Rahmen der Arbeit vorgeschlagen und weiterentwickelt worden sind: Alternate-chirped NRZ (alCNRZ), Novel-chirped RZ (nCRZ), Alternate-polarized NRZ (alPNRZ) und Alternate-polarized RZ (alPRZ). Die Anforderungen, die bei der Entwicklung von neuen Modulationsformaten berücksichtigt worden sind, waren die Verbesserung der nichtlinearen Übertragungseigenschaften (z.B. nichtlineare Toleranz) der Übertragungsstrecke und eine effizientere Ausnutzung der zur Verfügung stehenden Systembandbreite (z.B. Erhöhung der spektralen Effizienz), wobei die vorgeschlagenen Modulationsformate kompatibel mit herkömmlichen Systemkonfigurationen (z.B. Empfänger) sein sollten. Aufgrund numerischer Natur der Arbeit wurden diverse Auswertekriterien eingeführt, die eine genaue Evaluierung der Übertragungsqualität ermöglichen und im Rahmen der Arbeit verwendet worden sind. Die Vor- und Nachteile der Auswertekriterien wie Bitfehlerrate (BER), Q-Faktor, optischer Signalrauschabstand (OSNR) und Augendiagramme wurden erläutert, und ein Vergleich zwischen allen Kriterien ist gemacht worden. Die 40 Gbit/s basierten numerischen Untersuchungen wurden für Einkanal- und Mehrkanalübertragungssysteme durchgeführt. Dabei wurde im Mehrkanalfall zwischen WDM-Systemen mit einer spektralen Effizienz von 0.4 bit/s/Hz und effizienteren dense WDM (DWDM) Systemen mit einer spektralen Effizienz von 0.8 bit/s/Hz unterschieden. Das Ziel dieser Untersuchungen war eine 40 Gbit/s Systemoptimierung durch Bestimmung von optimalen Übertragungsfasern, optimalen Dispersionskompensationsschemen und optimalen Leistungsbereichen, in denen die zukünftigen Systeme betrieben werden sollen. Dabei wurden alle Untersuchungen unter Berücksichtigung von unterschiedlichen Modulationsformaten durchgeführt, um einen Vergleich zwischen den Modulationsformaten gewährleisten zu können. Die Ergebnisse der Einkanaluntersuchungen haben gezeigt, dass NRZ basierten Modulationsformate durch eine hohe Dispersionstoleranz (ca. ±50 ps/nm) und eine niedrige nichtlineare Toleranz charakterisiert sind, was eine Beschränkung der maximaler Übertragungslänge verursacht. Die wichtigsten Störeffekte stellen in diesem Fall Selbstphasenmodulation (SPM) und die Interaktion zwischen SPM und chromatischer Dispersion dar. Die RZ basierten Verfahren zeichnen sich durch eine reduzierte Dispersionstoleranz (ca. ±25 ps/nm) aus, aber ermöglichen wegen erhöhter nichtlinearer Toleranz eine Verbesserung der maximalen Übertragungslänge verglichen zu NRZ Formaten. Die limitierenden Effekte in einer RZ basierten Übertragung sind Intrakanaleffekte (z.B. Intrakanalkreuzphasenmodulation IXPM), die bei höheren Signalleistungen von SPM begleitet sind. Die wichtigste Eigenschaft der neuen Modulationsverfahren ist die große nichtlineare Toleranz, die besonders bei alternierend polarisierten Modulationsverfahren (z.B. alPNRZ, alPRZ) zur Geltung kommt. Es wurde gezeigt, dass in allen untersuchten Fällen die Übertragungsqualität von eine mittleren Faserdispersion (ca. 4-8 ps/nm·km) profitiert und dass Dispersionskompensationsschemen mit einem bestimmten Prozent (variiert von Format zu Format) der Vorkompensation das Optimum darstellen. Die Mehrkanaluntersuchungen haben gezeigt, dass solange die spektrale Effizienz eines 40 Gbit/s basierten WDM systems klein (£ 0.4 bit/s/Hz) ist, die Einkanaleffekte (z.B. SPM, IXPM) die dominierenden Effekten sind. Demzufolge haben WDM und Einkanalsysteme ähnliche optimale Systemparameter, was ein einfaches System- und Kapazitätsupgrade ermöglichen würde. Des weiteren wurde gezeigt, dass für die Realisierung von DWDM Systemen eine schmalbandige optische Filterung sowohl am Sender als auch am Empfänger notwendig ist, deren Folge die Zerstörung der RZ Pulsform ist, wodurch die untersuchten RZ und NRZ basierten Modulationsformate identische Übertragungseigenschaften in DWDM Systemen aufweisen. Eine ähnliche Tendenz wurde auch bei manchen neuen Formaten (z.B. alCNRZ) beobachtet, was mit einem breiten Signalspektrum zu erklären ist. Auf der anderen Seite zeigten alternierend polarisierte Modulationsverfahren (z.B. alPNRZ) auch in DWDM Systemen eine Verbesserung hinsichtlich Filtertoleranz und Toleranz zu Mehrkanaleffekten (z.B. XPM), und empfählen sich als optimaler Kandidat für die zukünftigen 40 Gbit/s Systeme. Es wurde gezeigt, dass der optimale Fasertyp für eine DWDM Übertragung weitgehend unabhängig vom Modulationsformat ist und dass Faser eine möglichst hohe Dispersion besitzen sollen, um eine Unterdrückung der Mehrkanaleffekte ermöglichen zu können. Um zu erkennen, wie eine weitere Verbesserung der Übertragungseigenschaften in 40 Gbit/s Systemen ermöglicht werden könnte, wurden Verfahren wie orthogonal polarisierte Kanäle sowie phase shift keying (PSK) basierte Modulationsformate (z.B. DPSK, DQPSK) untersucht. Es wurde gezeigt, dass die orthogonale Polarisation zwischen den Kanälen als eine Verbesserungsmethode auf eine Übertragungslänge von ca. 200 km begrenzt ist. PSK-Formate ermöglichen eine Verbesserung der Übertragungseigenschaften der Strecke, wobei die notwendigen komplizierten Sender- und Empfängerrealisierungen vom Nachteil sein könnten.The focus of this work was set on 40 Gb/s based optical transmission systems with a varying number of channels and various spectral efficiencies in order to investigate the potential of 40 Gb/s technologies for the implementation in the next generation optical transmission networks. The results of this work can be used as design guidelines enabling a better understanding of propagation limitations in high bit rate transmission systems and give useful insights needed for the capacity upgrade of existing transmission lines. Using conventional amplitude-shift-keying (ASK) based modulation formats and by the author proposed novel modulation formats, the optimization of the system settings is performed in 40 Gb/s based single channel, wavelength division multiplex (WDM) and dense WDM (DWDM) transmission lines, in order to enable a comparison between different modulation formats in terms of the total transmission distance and the maximum achievable spectral efficiency. The signal generation and dominant transmission characteristics of various conventional non return-to-zero (NRZ), return-to-zero (RZ), duobinary, single side band RZ (SSB-RZ), carrier suppressed RZ (CSRZ) - and novel modulation formats alternate chirped NRZ (alCNRZ), novel chirped RZ (nCRZ), alternate polarized (N)RZ (alP(N)RZ) were introduced. The idea behind the development of novel modulation formats was the performance improvement of the existing transmission lines with possibly low signal generation complexity, employing conventional ASK-based receiver configuration for the signal detection. Dividing all modulation formats in two groups NRZ- and RZ-based - their tolerances to linear and nonlinear transmission disturbances are investigated in single channel transmission, indicating that an implementation of NRZ-based modulation formats provides a better dispersion tolerance, but suffers from strong nonlinear limitations. The use of novel NRZ-based formats enables a significant improvement of nonlinear transmission characteristics at the cost of a slightly increased transmitter complexity. RZ-based formats are characterized by an increased sensitivity to residual dispersion and a significant nonlinear tolerance. It is shown that an additional phase or polarization modulation of RZ pulses enables more compact signal spectra and a further improvement of nonlinear transmission robustness, thus enlarging the maximum transmission distance. Strong intra-channel limitations were indicated as the dominant transmission limitation especially in RZ-based formats characterized by strong interactions of consecutive pulses within the bit stream, due to the fast broadening of short optical pulses at 40 Gb/s. This effect is accompanied by self-phase modulation (SPM) group velocity dispersion (GVD) interplay, which becomes evident in both format groups at larger channel powers. It is shown that the dominance of intra-channel effects requires implementation of transmission fibers with moderate dispersion values. Furthermore, it was shown, that as long as intra-channel effects dominate transmission performance, the best dispersion compensation scheme is characterized by a small amount of dispersion pre-compensation, due to suppression of interactions between adjacent pulses. Thereby, right amount of dispersion pre-compensation is dependent on the modulation format in use, because of the interplay between the pulse internal chirp induced during modulation and the local dispersion in transmission line. The importance of pre-compensation increases in long-haul transmission lines employing dispersion compensation on a span-by-span basis, because of constructive superposition of intrachannel cross-phase modulation (IXPM) contributions in each span. The modulation formats employing polarization switching between consecutive pulses were identified as best solution for the performance enhancement in 40 Gb/s single channel based transmission lines. The 40 Gb/s based WDM systems with spectral efficiency of 0.4 bit/s/Hz showed identical transmission behavior as in single channel transmission for all modulation formats, which can be explained by the dominance of single-channel effects in 40 Gb/s systems with a channel spacing of 100 GHz. This leads to the conclusion that a system upgrade from single channel to WDM at 40 Gb/s channel data rate can be made using identical transmission infrastructure. As in the single channel case, RZ-based formats indicated a significant robustness to nonlinear propagation effects, which could be further improved by the use of novel modulation formats. Basically, RZ-based modulation formats outperform the NRZ-based ones in 40 Gb/s single channel and WDM transmissions, and transmission advantages of RZ based formats become even more evident with an increased transmission distance. It was shown that an increase of spectral efficiency to 0.8 bit/s/Hz in 40 Gb/s based DWDM systems results in increased pulse distortions, because of the reduced tolerance to implemented narrow-band filtering and larger impact of multi-channel nonlinearities (particularly XPM). The differences between RZ- and NRZ-based modulation formats vanish in DWDM transmissions, because of the distortion of RZ pulse shape due to narrow-band filtering needed at the transmitter side. It was shown that transmission performance of DWDM systems could profit from implementation of transmission fibers with a large chromatic dispersion, due to suppression of multi-channel effects independently of the modulation format in use. Accordingly, already deployed fibers (e.g. G.652) can be further used in next generation of DWDM transmission systems. Furthermore, considering concatenation of identical spans in a DWDM transmission line, it was observed that XPM-induced impacts superpose constructively from span to span independently of the implemented dispersion compensation scheme, resulting in an transmission penalty, which is in high power regime proportional to number of concatenated spans. This behavior enables together with already know transmission rules (e.g. Pmax) an efficient estimation of the maximum transmission performance and maximum transmission distance in 40 Gb/s DWDM systems. This work is completed by representation of some promising technologies, e.g. polarization orthogonality between the channels or phase-shift-keying (PSK) based modulation formats, which enable a further increase of spectral efficiency (beyond 0.8 bit/s/Hz) and an enhanced maximum transmission distance. The investigations of PSK-based modulation formats showed that not all recently proposed PSK-based system could compete with ASK-based formats for implementation in DWDM systems. Differential quadrature PSK (DQPSK) based modulation formats were identified as a potential candidate for the implementation in future spectrally efficient DWDM systems

Fibre Optic Parametric Amplifiers For Transient Limited Optical Fibre Systems

The thesis explores fibre optical parametric amplifiers (FOPAs) to implement and develop the FOPA ability to provide transient free burst mode signal amplification, presenting potential applications in reach extended access networks and time-varying optical transmission systems. This document experimentally demonstrates FOPA as a potential drop-in amplifier candidate for transient limited optical systems by experimentally investigating and comparing transient effects in conventional fibre amplifiers. For example, future reach extended optical access networks. Additionally, this work provides evidence for transient free burst traffic amplification enabled by FOPA. A number of experimental techniques were implemented to demonstrate an ultra- fast response, high burst signal gain, and the ability to simultaneously amplify bi- directionally transmitted signals in a dual telecom band. Novel polarisation-insensitive FOPA employed in a 50 km reach extended access network link to achieve clean burst mode signal amplification. PI-FOPA targeted varied burst durations and burst traffic density amplification to evaluate performance compared to a commercial erbium-doped fibre amplifier (EDFA) and a discrete Raman amplifier. FOPA enhances link receiver sensitivity by >3 dB compared to EDFA and Raman amplifier for a varied burst duration amplification from 70 µs to 5 µs. For high burst traffic density amplification from 5% to 97%, FOPA allows burst traffic amplification up to 97% traffic, while EDFA and discrete Raman amplifier traffic density amplification was limited to 15% and 30%. We first presented a bi-directional non- burst and burst signal amplification by implementing a novel dual-band FOPA setup. FOPA achieved polarisation insensitive net gain of >16 dB for >50 nm apart signals in C and L bands. FOPA's ability to provide a wide broadband gain of ~10THz is utilized to amplify a non-burst and bursty signal in a dual-band transmission with a single in-line PI-FOPA amplifier simultaneously

Advanced modelling and signal processing in nonlinear coherent optical fibre systems

The importance of optical fibres in the global information society has increased significantly over the past four decades. However, the ever-growing demand for high-capacity data transmission poses a significant challenge, as the fibre channel’s nonlinear properties limit the achievable capacities, spectral efficiencies, and distances. This thesis aims to address this challenge by investigating advanced modelling and signal processing in nonlinear coherent optical fibre systems to predict and improve overall system performance. The first part of the thesis examines the effectiveness of nonlinear compensation (NLC) techniques, such as digital back-propagation (DBP) and optical phase conjugation (OPC), in enhancing achievable information rates (AIRs) in C-band systems that use both EDFA and distributed Raman amplification. Results indicate that the effectiveness of NLC techniques in enhancing AIRs depends heavily on the signal modulation formats and target transmission distances, with NLC being more effective for higher-order modulation formats at shorter system distances. The second part investigates the performance of long-haul Nyquist-spaced wavelength division multiplexing (WDM) optical communication systems with electronic dispersion compensation (EDC) and digital NLC with significant laser linewidths, and presents an analytical model based on the Gaussian noise model to predict the system performance considering the impact of equalisation enhanced phase noise (EEPN). A reduction up to 1.41 dB in SNR was observed in a 32-GBd 2000- km 5-channel system using NLC due to EEPN. This thesis also conducts a comprehensive analysis to study the performance of Kalman filter (KF) under realistic long-haul optical link conditions. The effectiveness of the KF in mitigating phase distortions has been thoroughly analysed. Numerical simulations were conducted on both dispersion-unmanaged and dispersion-managed nonlinear long-haul transmission systems. The joint application of KF and NLC significantly improved system performance, achieving approximately 4 dB higher SNRs than pilot-aided CPE. The findings of this thesis could help advance the design of nonlinear coherent optical fibre systems influenced by laser phase noise for high-capacity data transmission