Key Signal Processing Technologies for High-speed Passive Optical Networks

With emerging technologies such as high-definition video, virtual reality, and cloud computing, bandwidth demand in the access networks is ever-increasing. Passive optical network (PON) has become a promising architecture thanks to its low cost and easy management. IEEE and ITU-T standard organizations have been standardizing the next-generation PON, targeting on increasing the single-channel capacity from 10 Gb/s to 25, 50, and 100 Gb/s as the solution to address the dramatic increase of bandwidth demand. However, since the access network is extremely cost-sensitive, many research problems imposed in the physical layer of PON need to be addressed in a cost-efficient way, which is the primary focus of this thesis. Utilizing the low-cost 10G optics to build up high-speed PON systems is a promising approach, where signal processing techniques are key of importance. Two categories of signal processing techniques have been extensively investigated, namely optical signal processing (OSP) and digital signal processing (DSP). Dispersion-supported equalization (DSE) as a novel OSP scheme is proposed to achieve bit-rate enhancement from 10 Gb/s to 25 Gb/s based on 10G class of optics. Thanks to the bandwidth improved by DSE, the non-return-zero on-off keying which is the simplest modulation format is able to be adopted in the PON system without complex modulation or DSP. Meanwhile, OSP is also proposed to work together with DSP enabling 50G PON while simplifying the DSP complexity. Using both DSE and simple feed-forward equalizer is able to support 50 Gb/s PAM-4 transmission with 10G optics. For C-band 50 Gb/s transmission, injection locking techniques as another OSP approach is proposed to compress the directly modulated laser chirp and increase system bandwidth in the optical domain where a doubled capacity from 25 Gb/s to 50 Gb/s over 20 km fiber can be built on top of 10G optics. For DSP, we investigated the advantages of neural network (NN) on the mitigation of the time-varying nonlinear semiconductor optical amplifier pattern effect. In order to reduce the expense caused by the high computation complexity of NN, a pre- equalizer is introduced at the central office that allows cost sharing for all connected access users. In order to push the PON system line rate to 100 Gb/s, a joint nonlinear Tomlinson- Harashima precoding-Volterra algorithm is proposed to compensate for both linear and nonlinear distortions where 100 Gb/s PAM-4 transmission over 20 km fiber with 15 GHz system bandwidth can be achieved

Advances in local area optical data communication systems

This paper reviews optical fibre technology for local area optical communications systems. Technologies used in local systems include single and multimode fibre, single and multimode lasers, optical modulators, photodetectors, wavelength division multiplexing, multilevel modulation formats, electronic packet switching, electronic equalization and error correction. These methods have enabled the local area optical link data rate to increase from 0.1 Gb/s in 1990 to nearly a Tb/s in 2019. The challenges to increasing link data rates further, whilst reducing the transmitted power per bit, at reduced cost are discussed. Potential technical solutions and newly proposed methods which might address these challenges are highlighted

Advanced Digital Signal Processing Techniques for High-Speed Optical Links

L'abstract è presente nell'allegato / the abstract is in the attachmen

Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

The rapid proliferation of the Internet has been driving communication networks closer and closer to their limits, while available bandwidth is disappearing due to an ever-increasing network load. Over the past decade, optical fiber communication technology has increased per fiber data rate from 10 Tb/s to exceeding 10 Pb/s. The major explosion came after the maturity of coherent detection and advanced digital signal processing (DSP). DSP has played a critical role in accommodating channel impairments mitigation, enabling advanced modulation formats for spectral efficiency transmission and realizing flexible bandwidth. This book aims to explore novel, advanced DSP techniques to enable multi-Tb/s/channel optical transmission to address pressing bandwidth and power-efficiency demands. It provides state-of-the-art advances and future perspectives of DSP as well

Advanced high speed data and clock transmission over optical fibre for square kilometre telescope array

There is an ever present need from Internet users for more bandwidth. This is manifested by continuous increase in bandwidth demanding applications such as 5G wireless, new end user consumer links like thunderbolt, video conferencing, high definition video-on-demand transmitted over the Internet and massive data transfers required with and within data centres for backup, storage and data processing in cloud computing. Fibre optic communications technologies are playing a pivotal role in communication, being a major enabling technology in our increasingly Internet-centric society. As network services continue to become more dynamic and diverse, Internet service providers are faced with a challenge of cost reduction in the transmission network, power and spectral efficiency as well as scalability of the optical network infrastructure to support incremental expansions and virtual machines. Intelligent design of terrestrial optical networks to allow for simultaneous signal transmission through shared network infrastructure, and the use of low cost, power efficient, high bandwidth transmitters such as vertical surface emitting lasers (VCSELs) as well as exploitation of spectral efficient in-complex advanced modulation formats is a viable approach to this situation. In this study, techniques for spectral efficiency upgrade and simultaneous transmission of data signal, reference frequency (RF) clock signal and pulse-per-second (PPS) over shared infrastructure have experimentally been optimized in a laboratory environment for adoption in next-generation telescope array networks such as the Square Kilometre Array (SKA), time keeping systems such as banking systems, Coordinated Universal Time(UTC) timing and Global Positioning Systems (GPS), as well as high capacity spectral efficient short reach optical fibre networks such as data centres. This work starts by experimentally optimizing VCSEL technology for simultaneous transmission of 10 Gbps data and 1.712 GHz RF clock signal over a single G. 655 optical fibre of length 24.75 Km at different channel spacing and different propagation direction for implementation in a cost effective next-generation telescope array network. The wavelength tuneability property of VCSEL transmitters allows for wavelength adjustment, a key requirement for simultaneous data and RF clock signal transmission over a single optical fibre. A receiver sensitivity of -19.19 dBm was experimentally achieved at back-to-back analysis. A 24.75 Km of simultaneous data and RF clock signal transmission performed at 0.4 nm channel spacing introduced a transmission penalty of 1.07 dB and 1.63 dB for counter and co-propagation scheme respectively. This work mainly utilized direct modulation and direct detection using a positive intrinsic negative (PIN) due to its simplicity and cost effectiveness. A novel modulation technique for simultaneous data and polarization-based pulse-per-second timing clock signal transmission using a single VCSEL carrier is experimentally demonstrated. Two signal types, a directly modulated 10 Gbps data signal and a polarization-based pulse per second (PPS) clock signal are modulated onto a single mode 10 GHz bandwidth VCSEL carrier at 1310 nm. Spectral efficiency is maximized by exploiting the inherent orthogonal polarization switching of the xiv VCSEL with changing bias in transmission of the PPS signal. A 10 Gbps VCSEL transmission with PPS over 11 Km of G.652 fibre introduced a transmission penalty of 0.52 dB. The contribution of PPS clock signal to this penalty was found to be 0.08 dB. A technique for simultaneous directly modulated data and phase modulated reference clock signal transmission over a signal channel in wavelength division multiplexing (WDM) solutions is experimentally demonstrated. This is to prepare solutions to the ever-increasing demand over gigabit/s, terabit/s and gigahertz capacities in WDM-based terrestrial optical fibre transmission systems such as telescope array networks. a total capacity of 30 Gbps (310 Gbps) data and 12 GHz ( 4 3 GHz) reference clock signal are multiplexed at a channel spacing of 100 GHz and simultaneously transmitted over a single mode G.655 fibre of length 24.73 Km. The recovery of the phase modulated RF clock signal using a differential delay line interferometry technique is experimentally demonstrated. A 625 Gbps (2525 Gbps) DWDM data transmission system is further implemented in simulation by multiplexing 25 channels at 25 Gbps per channel using 50 GHz channel spacing. A four level pulse amplitude modulation (4-PAM) data modulation format employing VCSELs is experimentally demonstrated for adoption in high bitrate networks such as big data science projects and data centre networks. 4-PAM offers a good trade-off between complexity, efficiency, reach, and sensitivity. A software defined digital signal processing (DSP) receiver is designed and implemented in MATLAB to recover the transmitted 4-PAM data signal cost effectively without the necessity of costly receiver hardware. A novel technique for maximizing carrier spectral efficiency through simultaneous 20 Gbps 4-PAM data and phase modulated 2 GHz RF clock signal transmission on a single mode 10 GHz bandwidth VCSEL carrier at 1310 nm is experimentally demonstrated for the first time to the best of our knowledge. Data transmission and clock stability performance of the designed high spectral efficient VCSEL-based link network is evaluated through BER curve plots, phase noise measurements and Allan variance analysis respectively. VCSEL-based Raman amplification is experimentally demonstrated as a viable approach for RF clock signal distribution in extended reach astronomical telescope array networks and other extended reach terrestrial optical fibre network application. This is achieved by adopting two pumping techniques namely forward pumping and backward pumping. A maximum on off gain of 5.7 dB and 1.5 dB was experimentally attained for forward pumping and backward pumping at 24 dBm pump power respectively, while a maximum 100.8 Km fibre transmission achieved experimentally. In summary, this study has successfully demonstrated in-complex, spectral efficient, low cost and power efficient simultaneous data signal, reference frequency (RF) clock signal and pulse-per-second (PPS) transmission techniques over shared network infrastructure. Simultaneous transmission of data, RF clock and PPS timing signal is relevant in nextgeneration telescope array networks such as the Square Kilometre Array (SKA), time keeping systems such as banking systems, Coordinated Universal Time (UTC) timing and Global Positioning Systems (GPS), as well as high capacity spectral efficient short reach optical fibre networks such as data centres

Optimization of flexible spectrum in optical transport networks

The ever-increasing demand for broadband services by end-user devices utilising 3G/4G/LTE and the projected 5G in the last mile will require sustaining broadband supply from fibre-linked terminals. The eventual outcome of the high demand for broadband is strained optical and electronic devices. The backbone optical fibre transport systems and techniques such as dense wavelength division multiplexing (DWDM), higher modulation formats, coherent detection and signal amplification have increased both fibre capacity and spectrum efficiency. A major challenge to fibre capacity and spectrum efficiency is fibre-faults and optical impairments, network management, routing and wavelength assignment (RWA). In this study, DWDM and flexible spectrum techniques such as wavelength assignment and adjustment, wavelength conversion and switching, optical add and drop multiplexing (OADM) and bitrate variable transmission have been experimentally optimized in a laboratory testbed for short- and long-haul optical fibre networks. This work starts by experimentally optimising different transmitters, fibre-types and receivers suitable for implementing cost effective and energy efficient flexible spectrum networks. Vertical cavity surface-emitting lasers (VCSELs) and distributed feedback (DFB) lasers have been studied to provide up to 10 Gb/s per channel in 1310 nm and 1550 nm transmission windows. VCSELs provide wavelength assignment and adjustment. This work utilises the non-return-to-zero (NRZ) on-off keying (OOK) modulation technique and direct detection due to their cost and simplicity. By using positive intrinsic negative (PIN) photo-receivers with error-free BER sensitivity of -18±1 dBm at the acceptable 10-9-bit error rate (BER) threshold level, unamplified transmission distances between 6 km and 76 km have been demonstrated using G.652 and G.655 single mode fibres (SMFs). For the first time, an all optical VCSEL to VCSEL wavelength conversion, switching, transmission at the 1550 nm window and BER evaluation of a NRZ data signal is experimentally demonstrated. With VCSEL wavelength conversion and switching, wavelength adjustments to a spectrum width of 4.8 nm (600 GHz) can be achieved to provide alternative routes to signals when fibre-cuts and wavelength collision occurs therefore enhancing signal continuity. This work also demonstrates a technique of removing and adding a wavelength in a bundle of DWDM and flexible channels using an OADM. This has been implemented using a VCSEL and a fibre Bragg grating (FBG) providing a wavelength isolation ratio of 31.4 dB and ~0.3 add/drop penalty of 8.5 Gb/s signal. As a result, an OADM improves spectrum efficiency by offering wavelength re-use. Optical impairments such as crosstalk, chromatic dispersion (CD) and effects of polarization mode dispersion (PMD) have been experimentally investigated and mitigated. This work showed that crosstalk penalty increased with fibre-length, bitrate, interfering signal power and reduced channel spacing and as a result, a crosstalk-penalty trade-off is required. Effects of CD on a transmitted 10 Gb/s signal were also investigated and its mitigation techniques used to increase the fibre-reach. This work uses the negative dispersion fibres to mitigate the accumulated dispersion over the distance of transmission. A 5 dB sensitivity improvement is reported for an unamplified 76 km using DFB transmitters and combination of NZDSF true-wave reduced slope (TW-RS) and submarine reduced slope (TW-SRS) with + and – dispersion coefficients respectively. We have also demonstrated up to 52 km 10 Gb/s per channel VCSEL-based transmission and reduced net dispersion. Experimental demonstration of forward Raman amplification has achieved a 4.7 dB on-off gain distributed over a 4.8 nm spectral width and a 1.7 dB improvement of receiver sensitivity in Raman-aided 10 Gb/s per wavelength VCSEL transmission. Finally, 4.25-10 Gb/s PON-based point to point (P2P) and point to multipoint (P2MP) broadcast transmission have been experimentally demonstrated. A 10 Gb/s with a 1:8 passive splitter incurred a 3.7 dB penalty for a 24.7 km fibre-link. In summary, this work has demonstrated cost effective and energy efficient potential flexible spectrum techniques for high speed signal transmission. With the optimized network parameters, flexible spectrum is therefore relevant in short-reach, metro-access and long-haul applications for national broadband networks and the Square Kilometre Array (SKA) fibre-based signal and data transmission

Next generation optical access networks and coexistence with legacy PONs

Nowadays, Fiber-to-the-Home is one of the most promising solutions to provide broadband services in access networks. However, the fiber is inefficiently used as most of the deployed systems are still based on Time Division Multiplexing Passive Optical Networks (TDM-PONs) providing shared transmission capacities up to 2.5 Gb/s down and 1.25 Gb/s up, among multiple users. Research on high-speed electronics and Wavelength Division Multiplexing (WDM) has allowed the emergence of what is known as the second generation PON (NG-PON2), which specify aggregated capacities up to 40 Gb/s, stacking four channels at symmetric data rates of 10 Gb/s each, for residential scenarios. Nevertheless, the capacity per channel is still shared between multiple users due to the use of TDM. Moreover, the optical spectrum efficiency is low because channels are widely spaced (50 to 100 GHz). In addition, the sensitivity, reach and number of users is limited as consequence of using direct detection (DD) systems. In consequence, and due to the increase in bandwidth demands of new multimedia applications, it is necessary to propose solutions that cope with this tendency and, even more important, that can coexist with legacy systems, being one of the major requirements of network operators to guarantee a smooth and non-disruptive technology migration. In this thesis, a breakthrough technology such as Ultra-Dense WDM (UDWDM) that allows to allocate a large number of channels spaced only by a few GHz is used. This approach consent to envision the concept of Wavelength-to-the-User, where each costumer can be served with dedicated bandwidth links. The key technologies are based on coherent systems, with inherent wavelength selectivity and improved sensitivity compared to DD systems, thanks to the booster action of a tunable local oscillator (LO) laser. Because of cost is the main constraint in access networks, especially at the customer premises equipment (Optical Network Unit - ONU), in this thesis, a new class of coherent transceivers, based on low-cost direct modulated lasers and simplified receiver schemes, are proposed and experimentally tested. Moreover, the issue of coexistence is investigated through theoretical studies and real-time implementations, demonstrating full compatibility with legacy systems. Between the proposed solutions, a simple technique to adjust digitally the direct phase modulation of a distributed feedback (DFB) laser is presented to support flexible transmission rates. Next, several multilevel phase modulation formats for achieving higher transmission rates and better spectral efficiency are experimentally compared. Subsequently, the topic of photonic integration is addressed, demonstrating for the first time an 8-ary hybrid amplitude and phase modulated transmitter (Tx), by using a low-cost, small-footprint and energy efficient dual electro-absorption modulated laser (DEML). Finally, two novel proposals, to reduce the complexity of heterodyne and intradyne detection, are provided to face the typical issue of complexity and high-cost of coherent systems. The former explores the possibility of using only one DFB laser as LO and Tx at the ONU. The later demonstrates for the first time, a novel phase time diversity technique alternating phase modulation at each complex component (in-phase - I and quadrature - Q) achieving a 10 Gb/s' transmission with polarization independence.En la actualidad, la Fibra hasta el Hogar es una de las soluciones más prometedoras para proporcionar servicios de banda ancha en las redes de acceso. Sin embargo, la fibra se usa de manera poco eficiente, ya que la mayoría de los sistemas implementados todavía están basados en redes ópticas pasivas de multiplexación por división en el tiempo (TDM-PON) que brindan capacidades de transmisión compartidas entre múltiples usuarios de hasta 2.5 Gb/s y 1.25 Gb/s. La investigación en electrónica de alta velocidad y la multiplexación por división de longitud de onda (WDM) ha permitido el surgimiento de lo hoy se conoce como PON de segunda generación (NG-PON2), que especifica capacidades agregadas de hasta 40 Gb/s, apilando cuatro canales a velocidades de datos simétricas de 10 Gb/s cada uno, para escenarios residenciales. Sin embargo, la capacidad por canal todavía se comparte entre múltiples usuarios debido al uso de TDM. Además, la eficiencia en el uso del espectro óptico es baja porque los canales están muy separados (50 a 100 GHz). Asimismo, la sensibilidad, el alcance y el número de usuarios están limitados debido al uso de sistemas de detección directa. En consecuencia, y debido al aumento de las demandas de ancho de banda de las nuevas aplicaciones multimedia, es necesario proponer soluciones que respondan a esta tendencia y, lo que es más importante, que puedan coexistir con sistemas heredados, siendo uno de los principales requisitos de los operadores de red para garantizar una migración de tecnología fluida y sin interrupciones. En esta tesis, se utiliza una tecnología de vanguardia, como la multiplexación por división ultra densa de longitud de onda (UDWDM) que permite distribuir un gran número de canales espaciados solo por unos pocos GHz. Este enfoque permite vislumbrar el concepto de longitud de onda para el usuario, donde cada cliente puede usar enlaces de ancho de banda dedicados. Las tecnologías clave están basadas en los sistemas coherentes, con selectividad de longitud de onda inherente y sensibilidad mejorada en comparación con los sistemas de detección directa, gracias al efecto de amplificación óptica de un láser oscilador local (LO) sintonizable. Debido a que el costo es la principal restricción en las redes de acceso, especialmente del equipo en las instalaciones del cliente (unidad de red óptica - ONU), en ésta tesis, una nueva clase de transceptores coherentes, basados en láseres de bajo coste modulados directamente y esquemas de recepción simplificados, son propuestos y probados experimentalmente. Además, el problema de la coexistencia es investigado a través de estudios teóricos y experimentos en tiempo real, demostrando compatibilidad total con los sistemas heredados. Entre las soluciones propuestas, se presenta una técnica simple para ajustar digitalmente la modulación de fase directa de un láser de retroalimentación distribuida (DFB), y admitir velocidades de transmisión flexibles. Acto seguido, se comparan experimentalmente varios formatos multinivel de modulación de fase, para lograr tasas de transmisión más altas y una mejor eficiencia espectral. Posteriormente, se aborda el tema de la integración fotónica, demostrando por primera vez un transmisor (Tx) con modulación híbrida de fase y amplitud de ocho puntos, mediante el uso de un dispositivo pequeño, de bajo coste y eficiente energéticamente, como lo es el láser dual de electro-absorción modulada (DEML). Finalmente, se presentan dos propuestas novedosas para reducir la complejidad de la detección heterodina e intradina, afrontando el problema típico de la complejidad y alto coste de los sistemas coherentes. La primera explora la posibilidad de usar solo un láser DFB en la ONU, como LO y Tx. La segunda, demuestra por primera vez, una nueva técnica de diversidad fase en el tiempo, que alterna la modulación de fase en cada componente del plano complejo (fase-I y cuadratura-Q) logrando una transmisión de 10 Gb / s / λ con independencia de polarizació

Next generation optical access networks and coexistence with legacy PONs

Nowadays, Fiber-to-the-Home is one of the most promising solutions to provide broadband services in access networks. However, the fiber is inefficiently used as most of the deployed systems are still based on Time Division Multiplexing Passive Optical Networks (TDM-PONs) providing shared transmission capacities up to 2.5 Gb/s down and 1.25 Gb/s up, among multiple users. Research on high-speed electronics and Wavelength Division Multiplexing (WDM) has allowed the emergence of what is known as the second generation PON (NG-PON2), which specify aggregated capacities up to 40 Gb/s, stacking four channels at symmetric data rates of 10 Gb/s each, for residential scenarios. Nevertheless, the capacity per channel is still shared between multiple users due to the use of TDM. Moreover, the optical spectrum efficiency is low because channels are widely spaced (50 to 100 GHz). In addition, the sensitivity, reach and number of users is limited as consequence of using direct detection (DD) systems. In consequence, and due to the increase in bandwidth demands of new multimedia applications, it is necessary to propose solutions that cope with this tendency and, even more important, that can coexist with legacy systems, being one of the major requirements of network operators to guarantee a smooth and non-disruptive technology migration. In this thesis, a breakthrough technology such as Ultra-Dense WDM (UDWDM) that allows to allocate a large number of channels spaced only by a few GHz is used. This approach consent to envision the concept of Wavelength-to-the-User, where each costumer can be served with dedicated bandwidth links. The key technologies are based on coherent systems, with inherent wavelength selectivity and improved sensitivity compared to DD systems, thanks to the booster action of a tunable local oscillator (LO) laser. Because of cost is the main constraint in access networks, especially at the customer premises equipment (Optical Network Unit - ONU), in this thesis, a new class of coherent transceivers, based on low-cost direct modulated lasers and simplified receiver schemes, are proposed and experimentally tested. Moreover, the issue of coexistence is investigated through theoretical studies and real-time implementations, demonstrating full compatibility with legacy systems. Between the proposed solutions, a simple technique to adjust digitally the direct phase modulation of a distributed feedback (DFB) laser is presented to support flexible transmission rates. Next, several multilevel phase modulation formats for achieving higher transmission rates and better spectral efficiency are experimentally compared. Subsequently, the topic of photonic integration is addressed, demonstrating for the first time an 8-ary hybrid amplitude and phase modulated transmitter (Tx), by using a low-cost, small-footprint and energy efficient dual electro-absorption modulated laser (DEML). Finally, two novel proposals, to reduce the complexity of heterodyne and intradyne detection, are provided to face the typical issue of complexity and high-cost of coherent systems. The former explores the possibility of using only one DFB laser as LO and Tx at the ONU. The later demonstrates for the first time, a novel phase time diversity technique alternating phase modulation at each complex component (in-phase - I and quadrature - Q) achieving a 10 Gb/s' transmission with polarization independence.En la actualidad, la Fibra hasta el Hogar es una de las soluciones más prometedoras para proporcionar servicios de banda ancha en las redes de acceso. Sin embargo, la fibra se usa de manera poco eficiente, ya que la mayoría de los sistemas implementados todavía están basados en redes ópticas pasivas de multiplexación por división en el tiempo (TDM-PON) que brindan capacidades de transmisión compartidas entre múltiples usuarios de hasta 2.5 Gb/s y 1.25 Gb/s. La investigación en electrónica de alta velocidad y la multiplexación por división de longitud de onda (WDM) ha permitido el surgimiento de lo hoy se conoce como PON de segunda generación (NG-PON2), que especifica capacidades agregadas de hasta 40 Gb/s, apilando cuatro canales a velocidades de datos simétricas de 10 Gb/s cada uno, para escenarios residenciales. Sin embargo, la capacidad por canal todavía se comparte entre múltiples usuarios debido al uso de TDM. Además, la eficiencia en el uso del espectro óptico es baja porque los canales están muy separados (50 a 100 GHz). Asimismo, la sensibilidad, el alcance y el número de usuarios están limitados debido al uso de sistemas de detección directa. En consecuencia, y debido al aumento de las demandas de ancho de banda de las nuevas aplicaciones multimedia, es necesario proponer soluciones que respondan a esta tendencia y, lo que es más importante, que puedan coexistir con sistemas heredados, siendo uno de los principales requisitos de los operadores de red para garantizar una migración de tecnología fluida y sin interrupciones. En esta tesis, se utiliza una tecnología de vanguardia, como la multiplexación por división ultra densa de longitud de onda (UDWDM) que permite distribuir un gran número de canales espaciados solo por unos pocos GHz. Este enfoque permite vislumbrar el concepto de longitud de onda para el usuario, donde cada cliente puede usar enlaces de ancho de banda dedicados. Las tecnologías clave están basadas en los sistemas coherentes, con selectividad de longitud de onda inherente y sensibilidad mejorada en comparación con los sistemas de detección directa, gracias al efecto de amplificación óptica de un láser oscilador local (LO) sintonizable. Debido a que el costo es la principal restricción en las redes de acceso, especialmente del equipo en las instalaciones del cliente (unidad de red óptica - ONU), en ésta tesis, una nueva clase de transceptores coherentes, basados en láseres de bajo coste modulados directamente y esquemas de recepción simplificados, son propuestos y probados experimentalmente. Además, el problema de la coexistencia es investigado a través de estudios teóricos y experimentos en tiempo real, demostrando compatibilidad total con los sistemas heredados. Entre las soluciones propuestas, se presenta una técnica simple para ajustar digitalmente la modulación de fase directa de un láser de retroalimentación distribuida (DFB), y admitir velocidades de transmisión flexibles. Acto seguido, se comparan experimentalmente varios formatos multinivel de modulación de fase, para lograr tasas de transmisión más altas y una mejor eficiencia espectral. Posteriormente, se aborda el tema de la integración fotónica, demostrando por primera vez un transmisor (Tx) con modulación híbrida de fase y amplitud de ocho puntos, mediante el uso de un dispositivo pequeño, de bajo coste y eficiente energéticamente, como lo es el láser dual de electro-absorción modulada (DEML). Finalmente, se presentan dos propuestas novedosas para reducir la complejidad de la detección heterodina e intradina, afrontando el problema típico de la complejidad y alto coste de los sistemas coherentes. La primera explora la posibilidad de usar solo un láser DFB en la ONU, como LO y Tx. La segunda, demuestra por primera vez, una nueva técnica de diversidad fase en el tiempo, que alterna la modulación de fase en cada componente del plano complejo (fase-I y cuadratura-Q) logrando una transmisión de 10 Gb / s / λ con independencia de polarizaciónPostprint (published version

Next generation technologies for 100 Gb/s PON systems

The worldwide explosion of Internet traffic demand is driving the research for innovative solutions in many aspects of the telecommunication world. In access systems, passive optical networks (PONs) are becoming the preferred solution towards which most providers are migrating thanks to the unrivalled bandwidth they can offer. PON systems with a capacity of 100 Gb/s are envisioned as the solution to the dramatic increase in bandwidth and will be essential to support the future fixed and mobile broadband services. However, many challenging aspects have to be addressed in order to overcome the limitations imposed by the physical layer while meeting the economical requirements for mass deployment. In this thesis a comprehensive approach is taken in order to address the most compelling problems and investigate a series of solutions to the current capacity limitations of PONs. Advanced modulation formats are used to achieve bit-rate enhancement from 10 Gb/s to 25 Gb/s re-using the same optoelectronic devices in order to provide a 2.5x increase in transmission speed without resorting to a newer, more expensive generation of higher speed devices. The management of chromatic dispersion is also addressed in order to extend the reach of the networks beyond the standard 20 km using either electronic or optical based compensation strategies. Transmission of 25 Gb/s traffic over fibre lengths of 40 and 50 km is demonstrated confirming the suitability of the proposed technologies for extended reach networks which could greatly reduce the number of existing nodes and hence the capital and operational costs of PONs. Optical amplification strategies are also discussed as a means to improve the physical reach of the networks, both in terms of distance and number of customers. Raman amplifiers and semiconductor optical amplifiers are investigated in order to extend the reach of a PON upstream channel. The results demonstrate a reach of up to 50 km which is more than double the typical fibre length of 20 km adopted in deployed systems today. A number of customers, up to 512, was also demonstrated in a 20 km network, increased from the typical 32 or 64 users of most commercial networks