Next-Generation FTTH Networks: Innovative System Solutions for Low-Cost Coherent WDM PONs

Information and Communication Technologies (ICT) are having a determinant impact on the evolution of society since past century, and in particular in the last two decades. Nowadays, an ever-increasing number of people and things are getting connected, and figures as bandwidth demand and number of devices connected are just doomed to raise in the future. Optical fibers are the only medium that can support such a massive demand for data rate, reliability and energy efficiency, in backbone links, and progressively more and more also into access networks. Current standard Passive Optical Network (PON) are expected to satisfy the capacity needs for the greatest part of common user connections in the short term. However, the scenarios are rapidly evolving: the bandwidth-hungry applications (high-quality video streaming, remote video-conference, online gaming etc.), the close integration of fixed access and radio mobile networks, the upcoming introduction of 5G technology and all its possible applications (from smart-cities scenarios to Internet-of-Things (IoT)), will soon ask for innovative solutions that increase networks capacity even further. In this thesis, I present my work on different system solutions opening the way to Next-Generation Optical Access Networks. These solutions show a clear improvement with respect to current systems in terms of power budget, spectral efficiency, total network capacity, in an efficient and cost-effective way (as mandatory for access networks equipments). They are, therefore, an interesting candidate for the future, and inevitable, network upgrades. The main peculiar characteristics of the systems that were investigated in this work can be shortly summarized as follows: - A detection scheme based on coherent detection that allows increased sensitivity of the receivers, extended reach and higher spectral efficiency with respect to the current legacy standards. At the same time, these benefits are obtained with a low-cost implementation of coherent detection, avoiding use of costly components and digital signal processing, which are typical of today coherent transceivers - A WDM-PON reference architecture compatible with the legacy network infrastructure based of the current and past standards. This allows reusing of currently deployed networks. - Frequency flexibility, since the coherents tranceivers can work in any region of the C band (if lasers with sufficient tunability are employed). This means that the proposed systems can work in coexistence with the current standards, allowing for a smooth and seamless migration in a network upgrade. The thesis is organised as follows. Chapter 1 "Introduction" gives the rationale to the whole work, provides an overview of current PON architectures and standards, and presents the reference Ultra-Dense WDM PON architecture for the system solution presented in this work. Chapter 2 "8x1.25 Gb/s Ultra-Dense WDM-PON based on simplified coherent detection" presents the first system solution, an Ultra-Dense WDM PON (UD-WDM PON), with bit rate of 1.25 Gb/s and channel spacing of 6.25 GHz. This system employs transceivers based on a novel low-cost coherent detection scheme, enabling polarization independent operation without any polarization diversity scheme or automatic polarization control tecniques. Chapter 3 "A dynamic wavelength allocation algorithm for filterless coherent WDM-PON" presents a dynamic wavelength allocation algorithm, enabled by the employment of coherent receivers, that allows flexible wavelength allocation without any service interruption, in the reference architecture of this work. Chapter 4 "4x10 Gb/s Long-Reach WDM-PON based on DML transmitters and simplified coherent detection" describes the second system solution: a Long-Reach WDM PON, with bit rate of 10 Gb/s. This system employs direct modulation of the transmitters and a tecnique based on chirp-managed lasers, that allows long reach (up to 110 km), without any dispersion compensation. Chapter 5 "25 Gb/s duobinary PON" demonstrates an upgrade to a 25 Gb/s bit rate PON using the same 10 GHz electrical front-end receiver and duobinary modulation. Chapter 6 "Field Trial of a high budget, filterless, lambda-to-the-user WDM-PON" reports the results of a Field-Trial held in Pisa, in which the two systems described in chapters \ref{chapter:1G_Receiver} and \ref{chapter:10G_Receiver} have been tested, in coexistence with other coherent PON solutions and a legacy EPON system. Finally, Chapter 7 "Conclusions" draws the conclusions and gives some suggestions for further development of the research. Part of the work was carried out in the framework of the EU project COCONUT, a STREP project funded by European Union within the 7th Framework Program, which run from November 2012 to Febraury 2016, and ROAD-NGN, a PRIN project running from Febraury 2013 to January 2016. The content of this thesis includes the presentation of the results of the projects, as well as some of their main developments, obtained after their conclusions.