Estimating the Performance of Direct-Detection DPSK in Optical Networking Environments Using Eigenfunction Expansion Techniques

WOS:000281971100002 (Nº de Acesso Web of Science)“Prémio Científico ISCTE-IUL 2011”In-band crosstalk, due to multiple interferers, has been identified as one of the most severe impairments in optical transparent networks, especially in the ones with a large number of nodes and a high wavelength density. Due to its robustness to in-band crosstalk differential phase-shift keying (DPSK) emerges as an attractive modulation scheme to be used in such environments. This paper proposes a rigorous formulation to estimate the performance of direct-detection DPSK receivers using an eigenfunction expansion technique in the time domain. The method takes into account both the in-band crosstalk, due to an arbitrary number of interfering terms, and the amplified spontaneous emission noise and is able to deal with any combination of optical and electrical filter shapes. Using this method the accuracy of an approximation based on the wideband optical filtering assumption was evaluated and shown that the approximation, although not providing reliable results for the error probabilities, can be used with confidence to compute power penalties due to in-band crosstalk. Furthermore, the crosstalk tolerance of DPSK over on-off keying was quantified and shown that this tolerance is reduced when the number of interferers increases

Hybrid fibre and free-space optical solutions in optical access networks

This thesis evaluates the potentials of hybrid fibre and free space optical (FSO) communications access networks in providing a possible solution to an all optical access network. In such network architectures, the FSO link can extend the system to areas where an optical fibre link is not feasible, and/or provide limited mobility for indoor coverage. The performance of hybrid fibre and FSO (HFFSO) networks based on digital pulse position modulation (DPPM), for both the indoor and outdoor environments of the optical access network, are compared with the performance of such a network that is based on conventional on-off keying non-return-to-zero (OOK NRZ) modulation using results obtained through computational and analytical modelling. Wavelength division multiplexing (WDM) and/or code division multiple access (CDMA) are incorporated into the network for high speed transmission and/or network scalability. The impacts of optical scintillation, beam spreading and coupling losses, multiple access interference (MAI), linear optical crosstalk and amplified spontaneous emission noise (ASE) on the performance of hybrid fibre and FSO (HFFSO) access networks are analysed, using performance evaluation methods based on simple Gaussian approximation (GA) and more complex techniques based on moment generating function (MGF), including the Chernoff bound (CB), modified Chernoff bound (MCB) and saddlepoint approximation (SPA). Results in the form of bit error rate (BER), power penalty, required optical power and outage probability are presented, and both the CB and MCB, which are upper bounds, are suggested as safer methods of assessing the performance of practical systems. The possibility of using a CDMA-based HFFSO network to provide high speed optical transmission coverage in an indoor environment is investigated. The results show a reduction in transmit power of mobile devices of about 9 – 20 dB (depending on number of active users) when an optical amplifier is used in the system compared to a non-amplified system, and up to 2.8 dB improvement over OOK NRZ receiver sensitivity is provided by a DPPM system using integrate and compare circuitry for maximum likelihood detection, and at coding level of two, for minimum bandwidth utilization. Outdoor HFFSO networks using only WDM, and incorporating CDMA with WDM, are also investigated. In the presence of atmospheric scintillations, an OOK system is required (for optimum performance) to continuously adapt its decision threshold to the fluctuating instantaneous irradiance. This challenge is overcome by using the maximum likelihood detection DPPM system, and necessitated the derivation of an interchannel crosstalk model for WDM DPPM systems. It is found that optical scintillation worsens the effect of interchannel crosstalk in outdoor HFFSO WDM systems, and results in error floors particularly in the upstream transmission, which are raised when CDMA is incorporated into the system, because of MAI. In both outdoor HFFSO networks (with WDM only and with WDM incorporating CDMA), the optical amplifier is found necessary in achieving acceptable BER, and with a feeder fibre of 20 km and distributive FSO link length of 1500 m, high speed broadband services can be provided to users at safe transmit power at all turbulence levels in clear air atmosphere

Hybrid fibre and free-space optical solutions in optical access networks

This thesis evaluates the potentials of hybrid fibre and free space optical (FSO) communications access networks in providing a possible solution to an all optical access network. In such network architectures, the FSO link can extend the system to areas where an optical fibre link is not feasible, and/or provide limited mobility for indoor coverage. The performance of hybrid fibre and FSO (HFFSO) networks based on digital pulse position modulation (DPPM), for both the indoor and outdoor environments of the optical access network, are compared with the performance of such a network that is based on conventional on-off keying non-return-to-zero (OOK NRZ) modulation using results obtained through computational and analytical modelling. Wavelength division multiplexing (WDM) and/or code division multiple access (CDMA) are incorporated into the network for high speed transmission and/or network scalability. The impacts of optical scintillation, beam spreading and coupling losses, multiple access interference (MAI), linear optical crosstalk and amplified spontaneous emission noise (ASE) on the performance of hybrid fibre and FSO (HFFSO) access networks are analysed, using performance evaluation methods based on simple Gaussian approximation (GA) and more complex techniques based on moment generating function (MGF), including the Chernoff bound (CB), modified Chernoff bound (MCB) and saddlepoint approximation (SPA). Results in the form of bit error rate (BER), power penalty, required optical power and outage probability are presented, and both the CB and MCB, which are upper bounds, are suggested as safer methods of assessing the performance of practical systems. The possibility of using a CDMA-based HFFSO network to provide high speed optical transmission coverage in an indoor environment is investigated. The results show a reduction in transmit power of mobile devices of about 9 – 20 dB (depending on number of active users) when an optical amplifier is used in the system compared to a non-amplified system, and up to 2.8 dB improvement over OOK NRZ receiver sensitivity is provided by a DPPM system using integrate and compare circuitry for maximum likelihood detection, and at coding level of two, for minimum bandwidth utilization. Outdoor HFFSO networks using only WDM, and incorporating CDMA with WDM, are also investigated. In the presence of atmospheric scintillations, an OOK system is required (for optimum performance) to continuously adapt its decision threshold to the fluctuating instantaneous irradiance. This challenge is overcome by using the maximum likelihood detection DPPM system, and necessitated the derivation of an interchannel crosstalk model for WDM DPPM systems. It is found that optical scintillation worsens the effect of interchannel crosstalk in outdoor HFFSO WDM systems, and results in error floors particularly in the upstream transmission, which are raised when CDMA is incorporated into the system, because of MAI. In both outdoor HFFSO networks (with WDM only and with WDM incorporating CDMA), the optical amplifier is found necessary in achieving acceptable BER, and with a feeder fibre of 20 km and distributive FSO link length of 1500 m, high speed broadband services can be provided to users at safe transmit power at all turbulence levels in clear air atmosphere

Digital electronic predistortion for optical communications

The distortion of optical signals has long been an issue limiting the performance of communication systems. With the increase of transmission speeds the effects of distortion are becoming more prominent. Because of this, the use of methods known from digital signal processing (DSP) are being introduced to compensate for them. Applying DSP to improve optical signals has been limited by a discrepancy in digital signal processing speeds and optical transmission speeds. However high speed Field Programmable Gate Arrays (FPGA) which are sufficiently fast have now become available making DSP experiments without costly ASIC implementation possible for optical transmission experiments. This thesis focuses on Look Up Table (LUT) based digital Electronic Predistortion (EPD) for optical transmission. Because it is only one out of many possible implementations of EPD, it has to be placed in context with other EPD techniques and other distortion combating techniques in general, especially since it is possible to combine the different techniques. Building an actual transmitter means that compromises and decisions have to be made in the design and implementation of an EPD based system. These are based on balancing the desire to achieve optimal performance with technological and economic limitations. This is partly done using optical simulations to asses the performance. This thesis describes a novel experimental transmitter that has been built as part of this research applying LUT based EPD to an optical signal. The experimental transmitter consists of a digital design (using a hardware description language) for a pair of FPGAs and an analogue optical/electronic setup including two standard DAC integrated circuits. The DSP in the transmitter compensated for both chromatic dispersion and self phase modulation. We achieved transmission of 10.7 Gb/s non-return-to-zero (NRZ) signals with a +4 dBm launch power over 450 km keeping the required optical-signal-to-noise-ratio (OSNR) for a bit-error-rate of 2x10^{-3} below 11 dB. In doing so we showed experimentally, for the first time, that nonlinear effects can be compensated with this approach and that the combination of FPGA-DAC is a viable approach for an experimental setup

On the benefits of phase shift keying to optical telecommunication systems

Les avantages de la modulation de phase vis-à-vis la modulation d’intensité pour les réseaux optiques sont claires et accepté par la communauté scientifique des télécommunications optiques. Surtout, la modulation de phase montre une meilleure sensibilité au bruit, ainsi qu’une plus grande tolérance aux effets non-linéaires que la modulation d’intensité. Nous présentons dans cette thése un étude qui vise à développer les avantages de la modulation de phase. Nous attaquons d’abord la complexité du récepteur en détection directe, en proposant une nouvelle configuration dont la complexité est comparable à celle du récepteur pour la modulation d’intensité traditionnel, mais avec des meilleures performances. Cette solution pourrait convenir pour les réseaux métropolitains (et même d’accès) à haut débit binaire. Nous passons ensuite à l’examen de la possibilité d’utiliser des amplificateur à semi-conducteur (SOA) au lieu des amplificateurs à fibre dopée à l’erbium pour fournir amplification optique aux signaux modulés en phase. Les non-linéarité des SOA sont étudiées, et un compensateur simple et très efficace est proposé. Les avantages des amplificateurs à semi-conducteur par rapport à ceux à fibre sont bien connus. Surtout, la méthode que nous proposons permettrait l’integrabilité des SOA avec d’autres composants de réseau (par exemple, le récepteur nommé cidessus), menant à des solutions technologiques de petite taille et efficaces d’un point de vue énergétique. Il y a deux types de systèmes pour signaux modulés en phase: basé sur la détection directe, ou sur les récepteurs cohérents. Dans le dernière partie de ce travail, nous nous concentrons sur cette dernière catégorie, et nous comparons deux solutions possibles pour la mise à niveau des réseaux terrestres actuel. Nous comparons deux configurations dont les performances sont très comparables en termes de sensibilité au bruit, mais nous montrons comment la meilleure tolérance aux effets non linéaires (en particuliers dans les systèmes à débit mixte) fait que une solution soit bien plus efficace que l’autre.The advantages of phase modulation (PM) vis-à-vis intensity modulation for optical networks are accepted by the optical telecommunication community. PM exhibits a higher noise sensitivity than intensity modulation, and it is more tolerant to the effects of fiber nonlinearity. In this thesis we examine the challenges and the benefits of working with different aspects of phase modulation. Our first contribution tackles the complexity of the direct detection noncoherent receiver for differentially encoded quadrature phase shift keying. We examine a novel configuration whose complexity is comparable to that of traditional receivers for intensity modulation, yet outperforming it. We show that under severe nonlinear impairments, our proposed receiver works almost as well as the conventional receiver, with the advantage of being much less complex. We also show that the proposed receiver is tolerant to chromatic dispersion, and to detuning of the carrier frequency. This solution might be suitable for high-bit rates metro (and even access) networks. Our second contribution deals with the challenges of using semiconductor optical amplifiers (SOAs) instead of typical erbium doped fiber amplifiers (EDFAs) to provide amplification to phase modulated signals. SOAs nonlinearities are investigated, and we propose a simple and very effective feed-forward compensator. Above all, the method we propose would permit the integrability of SOAs with other network components (for example, the aforementioned receiver) achieving small size, power efficient sub-systems. Phase modulation paves the way to high spectral efficiency, especially when paired with digital coherent receivers. With the digital coherent receiver, the degree of freedom offered by polarization can be exploited to increase the channel bit rate without increasing its spectral occupancy. In the last part of this work we focus on polarization multiplexed signaling paired with coherent reception and digital signal processing. Our third contribution provides insight on the strategies for upgrading current terrestrial core networks to high bit rates. This is a particularly challenging scenario, as phase modulation has to coexist with previously installed intensity modulated channels. We compare two configurations which have received much attention in the literature. These solutions show comparable performance in terms of back-to-back noise sensitivity, and yet are not equivalent. We show how the superior tolerance to nonlinear fiber propagation (and particularly to cross phase modulation induced by the presence of intensity modulated channels) makes one of them much more effective than the other

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

Coherent Phase-Modulated Optical Fiber Communications with Linear and Nonlinear Phase Noise

Ph.DDOCTOR OF PHILOSOPH

Terabit-Rate Transmission Using Optical Frequency Comb Sources

Energy-efficient Tbit/s optical interconnects are key elements for future communication systems. Three novel optical frequency comb sources are investigated, which have the potential of being integrated in chip-scale Tbit/s transmitters. Such frequency combs provide a large number of carriers. The equidistance of the comb lines helps to minimize spectral guard bands. For each type of comb source, coherent data transmission experiments show the potential for Tbit/s data transmission rates

Machine Learning Techniques To Mitigate Nonlinear Impairments In Optical Fiber System

The upcoming deployment of 5/6G networks, online services like 4k/8k HDTV (streamers and online games), the development of the Internet of Things concept, connecting billions of active devices, as well as the high-speed optical access networks, impose progressively higher and higher requirements on the underlying optical networks infrastructure. With current network infrastructures approaching almost unsustainable levels of bandwidth utilization/ data traffic rates, and the electrical power consumption of communications systems becoming a serious concern in view of our achieving the global carbon footprint targets, network operators and system suppliers are now looking for ways to respond to these demands while also maximizing the returns of their investments. The search for a solution to this predicted ªcapacity crunchº led to a renewed interest in alternative approaches to system design, including the usage of high-order modulation formats and high symbol rates, enabled by coherent detection, development of wideband transmission tools, new fiber types (such as multi-mode and ±core), and finally, the implementation of advanced digital signal processing (DSP) elements to mitigate optical channel nonlinearities and improve the received SNR. All aforementioned options are intended to boost the available optical systems’ capacity to fulfill the new traffic demands. This thesis focuses on the last of these possible solutions to the ªcapacity crunch," answering the question: ªHow can machine learning improve existing optical communications by minimizing quality penalties introduced by transceiver components and fiber media nonlinearity?". Ultimately, by identifying a proper machine learning solution (or a bevy of solutions) to act as a nonlinear channel equalizer for optical transmissions, we can improve the system’s throughput and even reduce the signal processing complexity, which means we can transmit more using the already built optical infrastructure. This problem was broken into four parts in this thesis: i) the development of new machine learning architectures to achieve appealing levels of performance; ii) the correct assessment of computational complexity and hardware realization; iii) the application of AI techniques to achieve fast reconfigurable solutions; iv) the creation of a theoretical foundation with studies demonstrating the caveats and pitfalls of machine learning methods used for optical channel equalization. Common measures such as bit error rate, quality factor, and mutual information are considered in scrutinizing the systems studied in this thesis. Based on simulation and experimental results, we conclude that neural network-based equalization can, in fact, improve the channel quality of transmission and at the same time have computational complexity close to other classic DSP algorithms