Performance limits in optical communications due to fiber nonlinearity

In this paper, we review the historical evolution of predictions of the performance of optical communication systems. We will describe how such predictions were made from the outset of research in laser based optical communications and how they have evolved to their present form, accurately predicting the performance of coherently detected communication systems

All-optical nonlinearity mitigation in fiber-optic communications

The two main factors limiting the data throughput in modern fiber-optic communication links are the noise added by amplifiers and the nonlinear response of the optical fiber due to the Kerr effect. Today there are communication systems operating remarkably close to the limits set by these two factors. In order to increase the data throughput in a single fiber one can attempt reducing the noise added by the amplifiers or mitigate the nonlinear distortion which is dominated by deterministic effects. The work presented in this thesis is focused on reducing the negative impact of the Kerr nonlinearity through the use of all-optical signal processing by transmission of a phase-conjugated copy alongside the signal. A concept where the phase-conjugated data is repeated in time domain is investigated and it was found that it performs comparably to the conventional phase-conjugated twin waves concept. The rest of the thesis is dedicated to studying various aspects of the two-mode copier-phase-sensitive amplifier (PSA) scheme. These studies focus both on improving the understanding and on optimizing the performance of the nonlinearity mitigation in copier-PSA links. We study the optimization of the dispersion map on a single- and two-span basis and it is shown in simulations that significant improvements can be achieved by optimization over two spans. In a numerical study it was found that there is potential for improving the efficiency of the nonlinearity mitigation by addition of dsitributed Raman amplification (DRA) in a copier-PSA link. Long-haul transmission using the copier-PSA scheme is demonstrated experimentally both with and without DRA. Without DRA at 10 Gbaud, it is shown that it is possible to improve the transmission reach by up to a factor of 5.6 with the addition of PSA. With DRA at 28 Gbaud, the improvement in transmission reach is smaller but we observe an increase in the optimum launch power when enabling the PSA indicating improved nonlinearity mitigation

Wavelength tunable transmitters for future reconfigurable agile optical networks

Wavelength tuneable transmission is a requirement for future reconfigurable agile optical networks as it enables cost efficient bandwidth distribution and a greater degree of transparency. This thesis focuses on the development and characterisation of wavelength tuneable transmitters for the core, metro and access based WDM networks. The wavelength tuneable RZ transmitter is a fundamental component for the core network as the RZ coding scheme is favoured over the conventional NRZ format as the line rate increases. The combination of a widely tuneable SG DBR laser and an EAM is a propitious technique employed to generate wavelength tuneable pulses at high repetition rates (40 GHz). As the EAM is inherently wavelength dependant an accurate characterisation of the generated pulses is carried out using the linear spectrogram measurement technique. Performance issues associated with the transmitter are investigated by employing the generated pulses in a 1500 km 42.7 Gb/s circulating loop system. It is demonstrated that non-optimisation of the EAM drive conditions at each operating wavelength can lead to a 33 % degradation in system performance. To achieve consistent operation over a wide waveband the drive conditions of the EAM must be altered at each operating wavelength. The metro network spans relatively small distances in comparison to the core and therefore must utilise more cost efficient solutions to transmit data, while also maintaining high reconfigurable functionality. Due to the shorter transmission distances, directly modulated sources can be utilised, as less precise wavelength and chirp control can be tolerated. Therefore a gain-switched FP laser provides an ideal source for wavelength tuneable pulse generation at high data rates (10 Gb/s). A self-seeding scheme that generates single mode pulses with high SMSR (> 30 dB) and small pulse duration is demonstrated. A FBG with a very large group delay disperses the generated pulses and subsequently uses this CW like signal to re-inject the laser diode negating the need to tune the repetition rate for optimum gain-switching operation. The access network provides the last communication link between the customer’s premises and the first switching node in the network. FTTH systems should take advantage of directly modulated sources; therefore the direct modulation of a SG DBR tuneable laser is investigated. Although a directly modulated TL is ideal for reconfigurable access based networks, the modulation itself leads to a drift in operating frequency which may result in cross channel interference in a WDM network. This effect is investigated and also a possible solution to compensate the frequency drift through simultaneous modulation of the lasers phase section is examined

Optics for AI and AI for Optics

Artificial intelligence is deeply involved in our daily lives via reinforcing the digital transformation of modern economies and infrastructure. It relies on powerful computing clusters, which face bottlenecks of power consumption for both data transmission and intensive computing. Meanwhile, optics (especially optical communications, which underpin today’s telecommunications) is penetrating short-reach connections down to the chip level, thus meeting with AI technology and creating numerous opportunities. This book is about the marriage of optics and AI and how each part can benefit from the other. Optics facilitates on-chip neural networks based on fast optical computing and energy-efficient interconnects and communications. On the other hand, AI enables efficient tools to address the challenges of today’s optical communication networks, which behave in an increasingly complex manner. The book collects contributions from pioneering researchers from both academy and industry to discuss the challenges and solutions in each of the respective fields

Cognition procedures for optical network design and optimization

Telecom carriers have to adapt their networks to accommodate a growing volume of users, services and traffic. Thus, they have to search a continuous maximization of efficiency and reduction in costs. This thesis identifies an opportunity to accomplish this aim by reducing operation margins applied in the optical link power budgets, in optical transport networks. From an operational perspective, margin reduction will lead to a fall of the required investments on transceivers in the whole transport network. Based on how human learn, a cognitive approach is introduced and evaluated to reduce the System Margin. This operation margin takes into account, among other constraints, the long-term ageing process of the network infrastructure. Telecom operators normally apply a conservative and fixed value established during the design and commissioning phases. The cognitive approach proposes a flexible and variable value, adapted to the network conditions. It is based on the case-based reasoning machine learning technique, which has been further developped. Novel learning schemes are presented and evaluated. The cognition solution proposes a new lower launched power guaranteeing the quality of service of the new incoming lightpath. It will lead to provide transmission power savings with appropiate success rates when applying the cognitive approach. To this end, it relies on transmission values applied in past and successful similar network situations. They are stored in a knowledge base or memory of the system. Moreover, regarding the knowledge base, a static and a dynamic approaches have been developped and presented. In the last case, five new dynamic learning algorithms are presented and evaluated. In the static context, savings in transmission power up to 48% are achieved and the resulting System Margin reduction. Furthermore, the dynamic renewal of the knowledge base improves mean savings in launched power up to 7% or 18% with respect to the static approach, depending on the path. Thus, the cognitive approach appears as useful to be applied in commercial optical transport networks with the aim of reducing the operational System Margin.Los operadores de telecomunicaciones tienen que adaptar constantemente sus redes para acoger el volumen creciente de usuarios, servicios y tráfico asociado. Han de buscar constantemente una maximización de la eficiencia en la operación, así como una reducción continua de costes. Esta tesis identifica una oportunidad para alcanzar este objetivo por medio de la reducción de los márgenes operacionales aplicados en los balances de potencia en una red óptica de transporte. Desde un punto de vista operacional, la reducción de márgenes operativos conlleva una optimización de las inversiones requeridas en transceivers, entre otros puntos. Así, basándonos en cómo aprendemos los humanos, se introduce y evalúa una aproximación cognitiva para reducir el System Margin. Este margen operativo se introduce en el balance de potencia, entre otros puntos, para compensar el proceso de envejecimiento a largo plazo de la infraestrcutura de red. Los operadores emplean normalmente un valor fijo y conservador, que se establece durante el diseño y comisionado de la red. Nuestra aproximación cognitiva propone en su lugar un valor flexible y variable, que se adapta a las condiciones de red actuales. Se basa en la técnica de machine learning conocida como case-based reasoning, que se desarrolla más profundamente. Se han propuesto y evaluado nuevos esquemas de aprendizaje. La solución cognitiva propone un nuevo valor más bajo de potencia transmitida, que garantiza la calidad de servicio requerida por el nuevo lighpath entrante. La propuesta logra ahorros en la potencia transmitida, a la vez que garantiza una tasa de éxito correcta cuando aplicamos esta solución cognitiva. Para ello, se apoya en la potencia transmitida en situaciones pasadas y similares a la actual, donde se transmitió una potencia que aseguró el correcto establecimiento del lighpath. Esta información se almacena en una base de conocimiento. En este sentido, se han desarrollado y presentado dos aproximaciones: una base de conocimiento estática y otra dinámica. En el caso del contexto dinámico, se han desarrollado y evaluado cinco nuevos algoritmos de aprendizaje. En el contexto estático, se consigue un ahorro en potencia de hasta un 48%, con la correspondiente reducción del System Margin. En el contexto dinámico, la actualización online de la base de conocimiento proporciona adicionalmente una ganancia en potencia transmitida con respecto a la aproximación estática de hasta un 7% o un 18%, dependiendo de la ruta. De esta forma se comprueba que la propuesta cognitiva se revela como útil y aplicable sobre una red óptica de transporte comercial con el objetivo de reducir el margen operativo conocido como System Margin

Cognition procedures for optical network design and optimization

Telecom carriers have to adapt their networks to accommodate a growing volume of users, services and traffic. Thus, they have to search a continuous maximization of efficiency and reduction in costs. This thesis identifies an opportunity to accomplish this aim by reducing operation margins applied in the optical link power budgets, in optical transport networks. From an operational perspective, margin reduction will lead to a fall of the required investments on transceivers in the whole transport network. Based on how human learn, a cognitive approach is introduced and evaluated to reduce the System Margin. This operation margin takes into account, among other constraints, the long-term ageing process of the network infrastructure. Telecom operators normally apply a conservative and fixed value established during the design and commissioning phases. The cognitive approach proposes a flexible and variable value, adapted to the network conditions. It is based on the case-based reasoning machine learning technique, which has been further developped. Novel learning schemes are presented and evaluated. The cognition solution proposes a new lower launched power guaranteeing the quality of service of the new incoming lightpath. It will lead to provide transmission power savings with appropiate success rates when applying the cognitive approach. To this end, it relies on transmission values applied in past and successful similar network situations. They are stored in a knowledge base or memory of the system. Moreover, regarding the knowledge base, a static and a dynamic approaches have been developped and presented. In the last case, five new dynamic learning algorithms are presented and evaluated. In the static context, savings in transmission power up to 48% are achieved and the resulting System Margin reduction. Furthermore, the dynamic renewal of the knowledge base improves mean savings in launched power up to 7% or 18% with respect to the static approach, depending on the path. Thus, the cognitive approach appears as useful to be applied in commercial optical transport networks with the aim of reducing the operational System Margin.Los operadores de telecomunicaciones tienen que adaptar constantemente sus redes para acoger el volumen creciente de usuarios, servicios y tráfico asociado. Han de buscar constantemente una maximización de la eficiencia en la operación, así como una reducción continua de costes. Esta tesis identifica una oportunidad para alcanzar este objetivo por medio de la reducción de los márgenes operacionales aplicados en los balances de potencia en una red óptica de transporte. Desde un punto de vista operacional, la reducción de márgenes operativos conlleva una optimización de las inversiones requeridas en transceivers, entre otros puntos. Así, basándonos en cómo aprendemos los humanos, se introduce y evalúa una aproximación cognitiva para reducir el System Margin. Este margen operativo se introduce en el balance de potencia, entre otros puntos, para compensar el proceso de envejecimiento a largo plazo de la infraestrcutura de red. Los operadores emplean normalmente un valor fijo y conservador, que se establece durante el diseño y comisionado de la red. Nuestra aproximación cognitiva propone en su lugar un valor flexible y variable, que se adapta a las condiciones de red actuales. Se basa en la técnica de machine learning conocida como case-based reasoning, que se desarrolla más profundamente. Se han propuesto y evaluado nuevos esquemas de aprendizaje. La solución cognitiva propone un nuevo valor más bajo de potencia transmitida, que garantiza la calidad de servicio requerida por el nuevo lighpath entrante. La propuesta logra ahorros en la potencia transmitida, a la vez que garantiza una tasa de éxito correcta cuando aplicamos esta solución cognitiva. Para ello, se apoya en la potencia transmitida en situaciones pasadas y similares a la actual, donde se transmitió una potencia que aseguró el correcto establecimiento del lighpath. Esta información se almacena en una base de conocimiento. En este sentido, se han desarrollado y presentado dos aproximaciones: una base de conocimiento estática y otra dinámica. En el caso del contexto dinámico, se han desarrollado y evaluado cinco nuevos algoritmos de aprendizaje. En el contexto estático, se consigue un ahorro en potencia de hasta un 48%, con la correspondiente reducción del System Margin. En el contexto dinámico, la actualización online de la base de conocimiento proporciona adicionalmente una ganancia en potencia transmitida con respecto a la aproximación estática de hasta un 7% o un 18%, dependiendo de la ruta. De esta forma se comprueba que la propuesta cognitiva se revela como útil y aplicable sobre una red óptica de transporte comercial con el objetivo de reducir el margen operativo conocido como System Margin.Postprint (published version

Investigation of performance issues affecting optical circuit and packet switched WDM networks

Optical switching represents the next step in the evolution of optical networks. This thesis describes work that was carried out to examine performance issues which can occur in two distinct varieties of optical switching networks. Slow optical switching in which lightpaths are requested, provisioned and torn down when no longer required is known as optical circuit switching (OCS). Services enabled by OCS include wavelength routing, dynamic bandwidth allocation and protection switching. With network elements such as reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connects (OXCs) now being deployed along with the generalized multiprotocol label switching (GMPLS) control plane this represents the current state of the art in commercial networks. These networks often employ erbium doped fiber amplifiers (EDFAs) to boost the optical signal to noise ratio of the WDM channels and as channel configurations change, wavelength dependent gain variations in the EDFAs can lead to channel power divergence that can result in significant performance degradation. This issue is examined in detail using a reconfigurable wavelength division multiplexed (WDM) network testbed and results show the severe impact that channel reconfiguration can have on transmission performance. Following the slow switching work the focus shifts to one of the key enabling technologies for fast optical switching, namely the tunable laser. Tunable lasers which can switch on the nanosecond timescale will be required in the transmitters and wavelength converters of optical packet switching networks. The switching times and frequency drifts, both of commercially available lasers, and of novel devices are investigated and performance issues which can arise due to this frequency drift are examined. An optical packet switching transmitter based on a novel label switching technique and employing one of the fast tunable lasers is designed and employed in a dual channel WDM packet switching system. In depth performance evaluations of this labelling scheme and packet switching system show the detrimental impact that wavelength drift can have on such systems