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

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Abstract

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 net-work capacity, in an efficient and cost-effective way (as mandatory for access netnet-works 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

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• A Wavelength Division Multiplexing (WDM)-PON reference architecture compati-ble 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 Passive Optical Network (PON) archi-tectures and standards, and presents the reference Ultra-Dense WDM PON architecture for the system solution presented in this work.

Chapter 2 ”8×1.25 Gb/s Ultra-Dense WDM-PON based on simplified co-herent 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 po-larization independent operation without any popo-larization diversity scheme or automatic polarization control tecniques.

Chapter 3 ”A dynamic wavelength allocation algorithm for filterless coher-ent Wavelength Division Multiplexing PON (WDM-PON)” prescoher-ents 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 ”4×10 Gb/s Long-Reach WDM-PON based on Directly Modu-lated Laser (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, λ-to-the-user WDM-PON” reports the results of a Field-Trial held in Pisa, in which the two systems described in chapters 2 and 4 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.

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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.

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4.1 Chirp Managed Laser - overview . . . 54 4.2 Receiver scheme for chirp management by means of electrical filtering . . . 56 4.3 4×10 Long-Reach WDM-PON Experimental Setup . . . 57 4.4 4×10 Long-Reach WDM-PON experimental results . . . 59 4.5 Preliminary analysis for real-time implementation solution of the receiver . 61 4.6 Polarization independent upgrade of the receiver . . . 65 4.6.1 Working principle and experimental setup . . . 65 4.6.2 Experimental results . . . 67

5 25 Gb/s duobinary PON 71

5.1 Optical and Electrical Duobinary . . . 72 5.2 Experimental Setup . . . 76 5.3 Experimental Results . . . 77 6 Field Trial of a high budget, filterless, λ-to-the-user WDM-PON 79 6.1 Field-Trial Description . . . 80 6.2 Field trial experimental results . . . 82

7 Conclusions 88

List of Publications 91

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List of Figures

1.1 Global Annual IP traffic forecast 2016-2021 [1] . . . 16

1.2 Percentage of Internet traffic in the different section of the network infras-tructure [1] . . . 16

1.3 Different access networks architectures . . . 17

1.4 Time Division Multiplexing PON (TDM-PON) architecture . . . 20

1.5 TWDM-PON architecture . . . 22

1.6 Evolution of access technologies . . . 23

1.7 Diagram of the COCONUT baseline architecture . . . 26

1.8 Migration strategy increasing the reach of the PON . . . 28

1.9 Migration strategy allowing Node Consolidation . . . 28

2.1 ASK pol-ind receiver scheme . . . 31

2.2 ASK receiver: experimental setup . . . 33

2.3 RF Analog multipliers . . . 34

2.4 Traces after a) photodiode b) RF multiplier c) combiner . . . 34

2.5 1.25 Gb/s receiver: electrical spectra and eye diagrams . . . 36

2.6 RX performance for different operating conditions . . . 37

2.7 1.25 Gb/s ASK: BER vs detuning . . . 38

2.8 ASK crosstalk measurements: experimental setup . . . 38

2.9 Power penalty (FEC level) versus crosstalk level . . . 39

2.10 Power penalty (FEC level) versus signal-interferer detuning . . . 40

2.11 1.25 Gb/s receiver dynamic range . . . 40

2.12 1.25 Gb/s receiver prototype . . . 41

2.13 UD-WDM PON experimental setup . . . 42

2.14 Optical spectrum of the UD-WDM PON signal at the AWG output . . . . 42

2.15 UD-WDM BER measurements . . . 43

2.16 UD-WDM: measured sensitivities of the 8 UD-WDM channels . . . 44

3.1 Dynamic wavelenght allocation algorith reference architecture . . . 46

3.2 Hitless wavelength allocation and reconfiguration . . . 47

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3.4 Hitless wavelength allocation: laser control operation . . . 50

3.5 Hitless wavelength allocation: experimental results . . . 50

4.1 LR-PON: Network simplification . . . 53

4.2 Transient and adiabatic chirp . . . 54

4.3 CML schematic . . . 55

4.4 adiabatic chirp and dispersion . . . 56

4.5 Receiver scheme for chirp-management in electrical domain . . . 57

4.6 Chirp management by means of coherent detection and electrical filtering . 58 4.7 4x10 Gb/s: experimental setup . . . 58

4.8 4x10 Gb/s: experimental setup in the laboratory . . . 59

4.9 (a) Optical Spectra in back-to-back and after transmission (b) Eye dia-grams of the four 10 Gbit/s channels . . . 59

4.10 BER curves with 17 dBm/ch (b) for the four WDM channels . . . 60

4.11 4x10Gb/s: power budget . . . 61

4.12 10 Gb/s real time circuit setup . . . 62

4.13 Behaviour of the XOR chip . . . 62

4.14 10 Gb/s real-time circuit: power combiner design . . . 63

4.15 10 Gb/s real-time circuit: low-pass Bessel filter design . . . 64

4.16 10 Gb/s real-time circuit: eye diagrams . . . 64

4.17 10 Gb/s PI receiver system setup . . . 65

4.18 10 Gb/s PI receiver working principle . . . 66

4.19 10 Gb/s PI receiver: single channel B2B results . . . 68

4.20 10 Gb/s PI receiver: single channel results . . . 69

4.21 10 Gb/s PI receiver: WDM results . . . 69

5.1 Coding and decoding of a duobinary signal . . . 73

5.2 Mach-Zender Bias-point setting for ODB and EDB . . . 73

5.3 Comparison of signal constellations of NRZ, EDB and ODB signals . . . . 75

5.4 25 Gb/s PON: Experimental setup . . . 75

5.5 Optical spectra of 25 Gb/s NRZ signal, EDB and ODB . . . 76

5.6 Chirp management by means of coherent detection and electrical filtering . 77 6.1 Schematic representation of field trial Setup. . . 81

6.2 Field-trial: fiber deployment . . . 81

6.3 Field-trial: Optical spectrum of downstream channels . . . 83

6.4 Field-trial: UD-WDM 8×1.25 Gb/s downstream performance . . . 83

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6.6 Field-trial: Optical spectra of the upstream Amplitude Shift Keying (ASK) signal . . . 85 6.7 Field-trial: (a)ASK Intradyne prototype operating during the field-trial

(b) demonstration of 4K quality video streaming . . . 85 6.8 Field-trial: 4×10 Gb/s performace . . . 86

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Acronyms

10G-EPON 10-Gigabit Ethernet Passive Optical Network ADC Analog-to-Digital Converters

APON Asynchronous Transfer Mode PON APD Avalanche Photo-Diode

ASK Amplitude Shift Keying AWG Array Waveguide Grating BER Bit Error Rate

BPON Broadband Passive Optical Network CO Central Office

CDR Clock and Data Recovery CML Chirp Managed Laser

CWDM Coarse Wavelength Division Multiplexing CDM Code Division Multiplexing

CDMA Code Division Multiple Access CPE Customer Premises Equipment

DWDM Dense Wavelength Division Multiplexing DAC Digital-to-Analog Converter

DFB Distributed Feedback Laser DML Directly Modulated Laser DSL Digital Subscriber Line DS Downstream

DSP Digital Signal Processing

DWA Dynamic Wavelength Allocation GVD Group Velocity Dispersion ECL External Cavity Laser

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EPON Ethernet Passive Optical Network EDFA Erbium-Doped Fiber Amplifier F-P Fabry-Perot

FEC Forward Error Correction FTTB Fiber to the Building FTTC Fiber to the Curb FTTH Fiber to the Home FTTP Fiber to the Premises

FPGA Field Programmable Gate Array FSAN Full Service Access Network FWM Four-Wave Mixing

G-PON Gigabit-capable Passive Optical Network GEM G-PON Encapsulation Method

ICT Information Communication Technology

IEEE Institute of Electrical and Electronics Engineers IPTV Internet protocol television

ITU-T Telecommunication Standardization Sector of the International Telecom- muni-cation Union

LAN Local Area Network

LR-PON Long Reach Passive Optical Network LO Local Oscillator

LPF Low Pass Filter

MAC Media Access Control

MAN Metropolitan Area Networks MUX Multiplexer

MZM Mach-Zehnder Modulator

NGANs Next Generation Access Networks

NG-PON1 Next-Generation Passive Optical Network 1 NG-PON2 40-Gigabit-capable Next Generation PON 2

NG-WBAN Next-Generation Wireless Broadband Access Network NRZ Non-Return to Zero

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OAM Operations, Administration and Maintenance OAN Optical Access Network

OCDMA Optical Code Division Multiple Access ODN Optical Distribution Network

OFDM Orthogonal Frequency Division Multiplexing OLT Optical Line Terminal

ONT Optical Network Terminal ONU Optical Network Unit OPEX Operating Expenditure

PLOAM Physical Layer Operation, Administration, and Management PMD Physical Media Dependent

PBS Polarizing Beam Splitter

PRBS Pseudo-Random Bit Sequence PSM PLOAM Power Save Mode POP Point of Presence

P2P Point to Point

P2MP Point to Multipoint PtP Point to Point

PON Passive Optical Network QoS Quality of Service

RN Remote Node Rx Receiver

SCM Sub-carrier Multiplexing SCMA Sub-carrier Multiple Access SLA Service Level Agreement SMF Single Mode Fiber SoP State of Polarization

TWDM Time and Wavelength Division Multiplexing

TWDM-PON Time and Wavelength Division Multiplexing PON TDM Time Division Multiplexing

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TDM-PON Time Division Multiplexing PON TEC Thermo-Electric Cooler

TIA Trans-Impedance Amplifier TRX Transceiver

Tx Transmitter

UD-WDM PON Ultra-Dense WDM PON US Upstream

VDSL Very-high-bit-rate Digital Subscriber Line VOA Variable Optical Attenuator

WDM Wavelength Division Multiplexing WDMA Wavelength Division Multiple Access

WDM-PON Wavelength Division Multiplexing PON WAN Wide Area Network

WLAN Wireless Local Area Network

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Chapter 1 Introduction

The aim of this chapter is to progressively introduce the research topics of this thesis work, namely, the development of solutions for next-generation Fiber-to-the-Home access networks using coherent detection in low-cost transceivers. In the first section 1.1 a brief description about the general scenario of access traffic evolution provides a rationale to the whole research. Then, section 1.2 introduces the access network architectures, with particular focus on Passive Optical Network (PON) and the standards follwed by current legacy systems. Section 1.3 provides a brief overview of Wavelength Division Multiplexing PONs (WDM-PONs) and the evolution of coherent detection in optical systems and access networks. Section 1.4 presents a novel architecture for coherent access networks, that is the real core of this thesis work. Finally, section 1.5 gives an overview of the thesis and of its key contributions.

1.1 Rationale

The development of the Information and Communication Technologies (ICT), and, in particular, of high-speed networks is having nowadays the same structural impact on society as the development of electricity and transportation networks one century ago. Political institutions at any level are progressively assimilating its importance and impact on the improvement of the quality of life in sectors like, for example, healthcare, transport, environment, instruction, culture.

At a European level, the Digital Agenda for Europe was approved in 2010, as one of the flagships of the more general Europe 2020 strategy [2]. One of the action areas outlined in the document is the provision of ”fast” and ”ultrafast” internet access, with the goal, by 2020, of ensuring internet speeds to all Europeans above 30 Mbps and 50% or more household connections above 100 Mbps.

The actual trends in terms of network traffic increase track someway those ambitious goals. According to a white paper from Cisco [1], by the end of 2021 the overall global IP traffic will reach 3.3 zettabites (ZB) per year by 2021, or 278 exabytes (EB) per month

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Figure 1.1: Global Annual IP traffic forecast 2016-2021 [1]

(Fig 1.1), with a Compound Annual Growth Rate (CAGR) of 24% from 2016 to 2021; in western Europe, traffic will increase from 14 EB per month of 2016 to the 37 EB of 2021 (22% CAGR), with a percentage of connections with speeds higher than 25 Mbps of around 74%.

Another interesting aspect is how the volume of traffic over the network infrastructure will progressively shift from the ”backbone” or ”core” to the ”periphery” or ”access” segment of the network, as shown in Fig. 1.2: by 2021 the traffic delivered at metro/local level will represent the 35% of the overall internet traffic (from the 22% of the 2016), and this trend is doomed to continue in the future, as higher access speeds will increase the volume of user generated traffic.

Figure 1.2: Percentage of Internet traffic in the different section of the network infrastructure [1]

In such a scenario, it is clear how the performances of the access segment of the infrastructure have a crucial importance to keep up with the rising demand of bandwidth from residential users. The most direct way to meet these requirements is pushing the

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deployment of the optical fiber from the backbone progressively up to the user-end, in the so called Fiber-to-the-x (FTTx) networks.

1.2 State of the art and evolution on access PON and

standards

Despite a significant effort has been made into squeezing the ultimate capacity of legacy access networks based on copper, when their capacity limits were reached, they were progressively replaced by fiber-based network architectures [3]. The optical fiber, due to its low losses and inherent huge bandwidth, was the natural candidate for meeting the incresing capacity demands, also taking into account that had already been adopted in core and metropilitan networks. Among the different architectures, one that gathered a relevant success is the PON. In the last years, different standards have been set for this type of architecture, that is nowaday the most adopted solution in operating fiber access networks.

1.2.1 Fiber access architectures

Access networks using fiber connection can be classified by the label Fiber-to-the-X (FTTx), with the ”x” representing the end point of the fiber link

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For example, in the Fiber to the Curb (FTTC) or Fiber to the Building (FTTB) ar-chitectures, the fiber runs up to a street or building cabinet, from where on an ADSL line on twisted copper pairs or a wireless local area network goes to the home. These solution have the advantage of maximising the return on investments made on the old copper net-works, and are giving, alongside the latest most advanced copper line tecniques in the last link to the end user, the possibility of a progressive migration from copper to pure fiber networks. Fiber to the Home (FTTH) networks have been in the last years, however, the dominant solution, especially in green-field situations, and are the most ”future-proof” or ”forecast-tolerant”. FTTH networks are already a relevant percentage of the access network architectures in countries like Japan or South Korea [4], and it is already widely predicted that they will be the dominant access technology is the next decades, progres-sively replacing the different generation of Digital-Subscriber-Lines (DSLs) connections, working on copper-based or hybrid fiber-copper access architectures [5].

From the point of view of the network topology, there are three possible architectures that have been taken into consideration for access networks:

• Point to Point (P2P) architecture: individual fibers run from the local exchange to each home/building. This solution offers the highest capacity and flexibility for individual service upgrade, but requires many fiber connection, with increase of manteinance costs.

• Point to Multipoint (P2MP) with active node architecture: a single fiber carries all the traffic to an active node from where individual fibers run to each premise. Only a single feeder fiber is needed, but the active node requires power and manteinance • Passive P2MP: the active node is replaced by a passive optical power splitter/combiner

that feeds individual branching fibers to the users, avoiding the manteinance costs of an active node. This topology has therefore become very popular for introduc-tion of optical fiber into access network, and it is widely known as Passive Optical Network (PON)

1.2.2 Multiple access tecniques in PONs

In PON the common fiber feeder is shared by all the so-called ONUs terminating the branch fibers. It is necessary, therefore, to demultiplex the Downstream (DS) traffic among the different ONUs, and to aggregate the Upstream (US) traffic towards the Optical Line Terminal (OLT), in a collision free way, towards appropriate multiple access tecniques. Four major catagories of demultiplexing/multiple access tecniques for fiber access networks have been developed:

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• Time Division Multiplexing (TDM) - Time Division Multiple Access (TDMA), where each user (i.e. ONU) is distinguished by a pre-assigned time slot. Care-ful sinchronization of the upstream packets from different users towards the OLT is needed.

• WDM - Wavelength Division Multiple Access (WDMA), where each ONU receives and transmits over a dedicated wavelength channel.

• Sub-carrier Multiplexing (SCM) - Sub-carrier Multiple Access (SCMA). In this technique each ONU transmits at the same wavelength but has an assigned RF frequency for encoding his data.

• Code Division Multiplexing (CDM) - Code Division Multiple Access (CDMA), where each ONU is characterized by a unique and orthogonal code for transmis-sion at any time regardless of when the others are transmitting; then at the OLT all the overlapping codes are received and distinguished by sets of matching codes. Among these tecniques, TDM-TDMA networks were the first ones that have been completely standardised, since they allowed for the most simple and cost-effective solution, and are still nowadays the most deployed systems in fiber access network. They will be described with major detail in section 1.2.3.

Of course, multiple tecniques can be implemented jointly in order to exploit more than one multiplexing dimension at the same time. In particular, a Time and Wavelength Division Multiplexing PON (TWDM-PON) has been recently standardised, and will be described in major detail in section 1.2.4.

1.2.3 Standard TDM-PONs

TDM-PON systems have initially received the major attention for broadband access net-works, mainly because they allow for high-speed data transmission at relatively moderate complexity, with the required digital signal processing that can be realised in a cost-effective fashion in electronic integrated circuits.

Standards for these networks have been introduced by different important standardiza-tion bodies. The Telecommunicastandardiza-tion Standardizastandardiza-tion Sector of the Internastandardiza-tional Telecom-munication Union (ITU-T), together with the Full Service Access Network (FSAN) intro-duced the Asynchronous Transfer Mode PON (APON) (also named Broadband Passive Optical Network (BPON)) standard back in 1995 (recommendation G.983 [6]), followed by standards for Gigabit-capable Passive Optical Network (G-PON) and 10-Gigabit-capable Passive Optical Network (XG-PON), with recommendations, respectively, G.984 [7] and G.987 [8]. At the same time, Institute of Electrical and Electronics Engineers (IEEE)

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developed standards for Ethernet Passive Optical Network (EPON) and 10-Gigabit Eth-ernet Passive Optical Network (10G-EPON), in IEEE 802.3ah [9] and 802.3av [10]. The general reference architecture in all of these standards is sketched in 1.4.

Backbone4 network Central4office OLT2 OLTN Switch OLT1 Power splitter 41:32 (1:64) 204km

Figure 1.4: TDM-PON architecture

An OLT, located at the Central Office (CO) serves several ONUs at the same time, connected through a passive star Optical Distribution Network (ODN) with a splitting ratio of 1:32 or 1:64.

Historically, the first PON standards were the APON and BPON, developed for busi-ness purposes, and providing a downstream bitrate of 155 Mb/s (622 Mb/s in case of the BPON), an upstream bitrate of 155 Mb/s, a maximum optical splitting ratio of 32, and the maximun fiber connection length ranging from 0 to 20 km.

G-PON and EPON were developed in order to extend the capacity of PONs into the Gbit/s range. The foreseen bitrates for G-PON were 1.25 Gb/s or 2.5 Gb/s in downstream, and 155 Mb/s, 622 Mb/s, 1.25 or 2.5 Gb/s, with a maximum splitting ratio of 1:128. This standard provides recommendations on data framing format as well, with an encapsulation method that allows transmission of Ethernet packets, as well as native ATM and/or native TDM ones. On the other side, EPON was proposed specifically for Ethernet transport, and focuses on a symmetrical 1.25 Gb/s network. Table 1.1 summarizes the main features of these two standards, that constitutes nowadays the majority of the deployed fiber access networks.

Finally, XG-PON and 10G-EPON are the natural 10 Gb/s evolution of G-PON and EPON, respectively. XG-PON, in particular, provides an aggregate 10 Gb/s downstream capacity, and 2.5 Gb/s in upstream.

1.2.4 Next-Generation TWDM PONs (NG-PON2)

In 2011, FSAN initiated a working group in order to investigate on upcoming technologies enabling next-generation PONs. The target requirements identified by the operators for these networks were [11]

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G-PON EPON

Service Full Services (Ethernet, TDM, POTS)

Ethernet Data

Frame GEM frame Ethernet Frame

US Bit-Rate 155 Mb/s, 622 Mb/s, 1.25 Gb/s 1.25 Gb/s DS Bit-Rate 1.25 Gb/s, 2.5 Gb/s 1.25 Gb/s Distance up to 20 km up to 20 km Split Ratio up to 64 up to 64 Optical Loss 15/20/25 dB 15/20 dB

Table 1.1: Comparison between legacy G-PON and EPON

• 40 Gb/s aggregate downstream capacity, 10 Gb/s upstream • Maximum passive fiber reach of at least 40 km

• Splitting ratio of at least 1:64

These requirements had to be fulfilled while having an architecture compatible with the legacy standards, in order to facilitate coexistence and migration from one technology to the other. This meaning, in first place, the possibility of re-using the already widely deployed passive power-splitter based ODNs.

The solution that was finally adopted by FSAN in April 2012 as primarly solution standard was the TWDM-PON, and defined the so-called 40-Gigabit-capable Next Gen-eration PON 2 (NG-PON2), that is currently the latest PON standard introduced [12]. The main motivation behind this choice was that some operators and large vendors, con-sidered TWDM-PON as a less risky, less disruptive and less expensive solution than other approaches, because it reused existing components and technologies [11]. However, other operators looked at a P2P WDM for (big cell) backhauling applications. The NG-PON2 standard defines, then, both TWDM-PON and an optional P2P WDM architecture which can be deployed in two different scenarios, a shared wavelength scenario in combination with Time and Wavelength Division Multiplexing (TWDM), and an expanded scenario occupying all the wavelength range with spectrum flexibility [13].

The architecture, shown in Fig. 1.5, relies on four (optionally eight) pairs of wave-lengths (e.g. {λ1, λ5}, {λ2, λ6}, {λ3, λ7}, {λ4, λ8} and is able to provide 40 Gb/s and 10

Gb/s in downstream and upstream, respectively.

This solution allows for a extensive re-use of XG-PON development, since each of the four λ carries the bit-rate of 10 Gb/s in downstream and 2.5 Gb/s in upstream. Moreover, since the individual line-rate is not changed from XG-PON and 10G-EPON, the system upgrade is not a burden for the electronics involved in the equipments.

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Figure 1.5: TWDM-PON architecture for NG-PON2

The key element of the architecture is the Wavelength Multiplexer (WM) present at the OLT, that has the role of multiplexing the downstream signals (MUX) and dempul-tiplexing the upstream signals from the ONUs (DEMUX). Both of these operation need to be carried out at the OLT, since this architecture is based on a power-splitter ODN (as the one of the previous standards). Standard recommendations provide guidelines for design of these elements, for 100 GHz and optionally 50 GHz channel spacing [14].

Of course, the challenge is the proper managing of the wavelength domain. In order to have the system working on classical passive-splitter PONs, the receiver should be able to select their own wavelength channel. This means either using optical filters just before detection stage, or implementing tunable transceivers. The second solution is preferable, since it would allow a dynamic allocation of the wavelengths, and a more flexible management of the network bandwidth. As a matter of fact, in literature different solutions for low-cost tunable transmitters [15–17] and receivers [18–21] were proposed.

Moreover, in order to allow coexistence with other legacy standards over the same ODN, the use of a Coexistence Element (CE) is needed. CE is a wavelength multi-plexing/demultiplexing device used to provide inter-PON connectivity for multiple PON systems (technical parameters are contained in [22]).

1.3 WDM-PONs and coherent detection in access

networks

During the benchmarking process of different solutions that led to the definition of NG-PON2 standard, also pure WDM-PON solutions were proposed and analysed [23,24],

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also using coherent detection [25].

Using wavelength as the multiplexing domain potentially offers several advantages • the optical fiber bandwidth is better exploited

• the total capacity can be increased without need for hardware upgrade in terms of speed

• connection of different users can be asyncronous, therefore strict sincronization of TDMA can be avoided, as the use of burst-mode electronics

• the different channels are totally independent, therefore they can support different signals format and Quality of Service (QoS)

These advantages are obtained at the cost of more expensive equipment, since a source transmitter for every ONU is needed at the OLT side. Moreover, in case of the system needs to operate on an already existing passive-splitter, the need of tunable transceivers, or optical filters at the ONU, immediately before detection, arises (as seen in the case of NG-PON2 architecture).

However, another interesting solution can be provided by the use of coherent detection. Thanks to its inherent frequency selectivity, coherent systems open the way to a better expolitation of the fiber bandwidth, and to UD-WDM PON systems, that are being investigated as one of the possible next step in access network scenario, in order to meet the increase of capacity demand expected in the future [26] (Fig. 1.6)

Figure 1.6: Evolution of access technologies. Picture taken from [26]

1.3.1 WDM-PON based on coherent detection

Coherent detection in optical communication systems is a topic that raised interest since the beginning of the 1980s, when solutions to the problem of frequency stabilization of

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semiconductor lasers, with the object of heterodyne detection, were firstly presented [27]. However, the introduction of Erbium-Doped Fiber Amplifier (EDFA) in the 1990s made the quantum-noise-limited receiver sensitivity of the coherent receiver less significant.

It was only after 2000, with the increase in the transmission capacity of WDM sys-tems, that coherent technologies attracted a renewal of widespread interest. This lead to the possibility to meet the even increasing bandwidth demands. This time, high-speed Digital Signal Processing (DSP), Digital-to-Analog Converter (DAC)s and Analog-to-Digital Converters (ADC)s were already mature technologies, and this opened the way to a new class of DSP-based coherent schemes and the so called ”digital coherent re-ceivers” [28]. This receivers allowed to tackle in the digital domain a series of issues, like carrier-phase estimation in homodyne detection, that were really difficult to face in a pure analog way 20 years before.

Since then, it did not take much time to start investigating possibilities of using coher-ent detection not only on the high-capacity long-haul links, but also in access networks.

The main feature of coherent detection that are of beneficial use in PON are

• coherent detection boosts the receiver sensitivity (potentially up to the quantum limit [28]; therefore system reach, split ratio and number of users can be improved. • coherent detection provides channel/wavelength selection through the tuning of local oscillator (LO); these feature makes the WDM-PON architecture compatible with a power splitter based network and optical fi

lter at the receiver side might be avoided.

• coherent detection might enable higher spectral efficiency, leading the way to UD-WDM PONs. Relevant seminal works in this topic are the ones from H. Rhode et al. [29] and K.

Cho et al. [30], that demonstrated WDM-PONs with line rate of 1.25 Gb/s and 2.5 Gb/s, respectively, employing the possibility given by digital signal processing and phase-modulation formats. In 2012, Lavery et al. firstly demonstrated a coherent PON with a line rate up to 10 Gb/s [31].

First demonstrations of UD-WDM PON date back to 2011, with solutions showing channel spacing down to 3 GHz for a line rate of 1.25 Gb/s [32]. By 2014, several experiments already demonstrated WDM-PON with a total network capacity over the Terabit, thanks to the employment of hybrid WDM and Orthogonal Frequency Division Multiplexing (OFDM) [33, 34] and/or spectral-efficient pulse shaping tecniques [35]

The common feature of all these experiment is that the equipment employed resemble the structure of the classical digital coherent receiver, making intense use of digital signal processing, and/or costly optical components (like 90 degrees hybrids), making them

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costly and not immediately suitable for the access scenario. In order to tackle this issue, The EU FP7 COCONUT project [36, 37] proposed an innovative coherent access network architecture, that will be presented in the next section.

1.4 COCONUT WDM-PON architecture

The EU FP7 project COCONUT (Cost-Effective coherent ultra-dense-WDM-PON for lambda-to-the-user access) [36, 37], active between November 2012 and Febraury 2016 aimed at developing a UD-WDM PON, approaching the paradigm of the so-called ”λ-to-the-user”, with increased bandwidth and power budgets that could be used to increase the reach and/or support higher number of users.

The enabling technology was a new class of low cost, fully analogue coherent re-ceiver [38–42] that still benefit of the enhanced sensitivity and high frequency selectivity peculiar of coherent detection, while showing low latency, low power consumption level and moderate cost per bit, if compared to other competitive coherent technologies [43,44].

The system are designed to operate in two possible kinds of scenarios

• Green-Field Scenarios, in which no previous installed fiber infrastructure is consid-ered. This is the case in which maximum ODN design freedom is provided, and therefore the optimum type of elements in the architecture, in order to maximise performance can be used.

• Brown-Field Scenarios, in which the system should operate in and already installed ODN with the purpose to support legacy system. Moreover, the coexistence with these legacy systems is assumed. This poses limitations both on the ODN design, as well as on the available bandwidth.

The diagram of the COCONUT baseline architecture is shown in Fig. 1.7. The physical configuration is based on a multiwavelength PON over single working feeder and drop fibers. These are its main features:

• The OLT channel multiplexing operation (represented simbolically by the MUX el-ement in Fig. 1.7) can be carried out, depending on the considered scenario, by wavelength selective devices, like an Array Waveguide Grating (AWG), or by pure passive couplers, or a combination of both. In the first case, the spectral efficiency is limited by the fact that the AWG guard bands cannot be used, but multiplexing losses are limited. Moreover, using an AWG guarantees in a really effective way the possibility of coexistence of COCONUT systems with one or more NG-PON2 chan-nels, without the need of additional coexistence-elements. A combination of both devices, finally, provide the possibility of multiplex multiple COCONUT channels

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in a single port of an AWG, obtaining a compromise between multiplexing losses and spectral efficiency.

• Depending on the power budget requirements, an EDFA booster can be optionally considered at the OLT (as in the case on NG-PON2 architecture, see Fig. 1.5) • The branching elements of the ODN can be based, again, depending on the

sce-nario, on optical power splitting, wavelength splitter, or a combination of both. In the green-field case, using a wavelength splitter is better in terms of split losses. However, legacy infrastructures are usually based on pure passive split elements, therefore this kind of architecture should be considered in the case of a brown-field scenario.

The user-cases considered range from the classical FTTH or FTTC wired connection, both for residential and business customers, Wi-fi connectivity, and macro/small cells for radio backhaul/fronthaul.

Figure 1.7: Diagram of the COCONUT baseline architecture

Considering the wavelength bands defined for each PON, COCONUT system were thought to re-use the spectrum defined for one or more of the legacy systems due to the lack of available spectrum and because guard bands between each PON generation need to be considered to allow coexistence in the same system. In any case, the systems re-use the NG-PON2/P2P WDM concept of spectrum flexibility, meaning that the wavelength channels can be located anywhere in the full C and L bands (depending on the tunability of the sources in use).

Table 1.2 summarizes the main characteristics of two of the COCONUT transceivers and architectures presented in this thesis work, in comparison with the NG-PON2. When

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referring to the total network capacity, green-field scenario is considered, that is, the spectrum region considered is fully available to the allocation of COCONUT channels.

The first is an Ultra-Dense WDM PON (UD-WDM PON), with line 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.

The second system is a Long-Reach WDM PON, with line 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.

NG-PON2 COCONUT @1.25 Gb/s COCONUT @10 Gb/s Channel spacing (GHz) 50/100 6.25 25 # Channels 4 64 16 Total capacity (Gb/s) 40 80 160 Sensitivity @ BER 10−3(dBm) -28 -48 -35.5

Power budget (dB) 35 (E2 class) 45 >40

Table 1.2: NG-PON2 vs COCONUT

The numbers indicated in Table 1.2 consider a green-field scenario, in which CO-CONUT apparatus doesn’t need to coexist with any other legacy systems, and the spec-trum is completely available for COCONUT channel allocation. In the case of a brown-field scenario, instead, the enhanced sensitivity and power budget can be exploited in a variety of ways:

• Extending the reach of the distribution network, reaching far away customers, even-tually adding further splitting stages to the ODN, as shown in Fig. 1.8

• Consolidating the OLTs into a bigger network such as a Metro Network (Fig. 1.9). By hosting COCONUT OLT in a bigger node, will eventually allow for the node consolidation once all the customers on the PON have been migrated to the CO-CONUT system. Then the legacy OLT can be removed and the small and local node can be bypassed and therefore closed.

1.5 Thesis overview

The work presented in this thesis shows, in a comprehensive manner, some of the results obtained during COCONUT project, and some of their further developments, obtained after its conclusion. Key contribution of this work, with an highlight on the personal contribution of the author, are listed below:

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Figure 1.8: Migration strategy increasing the reach of the PON. L-PON: Legacy PON, C-PON: CO-CONUT PON

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• Realization and evaluation of a real time and polarization independent coherent receiver suitable for 1.25 Gb/s/λ UD-WDM PON. The considered architecture has been fully developed and prototyped in order to be used in a network test-bed. Personal contribution of the author is the design, practical realization and testing of the prototype. This part of the work has been carried out in the framework of COCONUT, and the prototype operation has been shown also during the final demonstration of the project.

• Experimental demonstration of a 100 km-reach WDM-PON-based architecture which can provide a power budget > 50dB. A polarization dependent version of the re-ceiver employed in this system, based on offline processing, has been realized in the framework of COCONUT project and of ROAD-NGN, a PRIN project running from Febraury 2013 to January 2016. Personal contribution of the author is a preliminary investigation for a real-time implementation of the receiver, and the demonstration of a polarization-independent upgrade of the receiver. Both this contributions have been developed after the conclusion of the two aforementioned projects.

• Experimental demonstration of a novel algorithm for real-time ONU activation and reconfiguration in coherent WDM-PONs, that prevents service interruption. This part of the work has been also carried out during COCONUT project.

• Demonstration of a 25 Gb/s/λ solution for WDM-PON using duobinary coding and 10G electrical front end.

The rest of the thesis is organised as follows. The Real-time polarization-independent 1.25 Gb/s receiver is presented in chapter 2, together with the Ultra-Dense WDM PON de-montration. The algorithm for ONU activation is demonstrated in chapter 3, and demon-strated employing the receiver described previously. The Long-Reach PON is presented in chapter 4, while the 25 Gb/s extension is described in chapter 5. Chapter 6 reports the results of the final Field-Trial of the COCONUT project, held in Pisa at Sant’Anna school facilities, in which the two systems described in chapters 2 and 4 have been tested, in coexistence with other coherent PON solutions developed during the project, and a legacy EPON system. Finally, chapter 7 draws the conclusions of the whole work and gives some suggestions to continue the research.

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Chapter 2 8×1.25 Gb/s Ultra-Dense

WDM-PON based on simplified

coherent detection

In the previous chapter it has been stated how coherent detection, thanks to its intrinsic frequency selectivity, opens the way, in the scenario of access networks, to the realization of UD-WDM PONs, with increased reach and bandwidth efficiency. At the same time, it has been stated the importance of compatibility of new solutions with existing PON systems and deployed infrastructures, in order to offer a smooth, seamless upgrade.

This chapter describes the realization and demonstration of an UD-WDM PON, op-erating on a PON infrastructure as the one described in section 1.4, with a 1.25 Gb/s line rate and channel spacing of 6.25 GHz. As already stated in section 1.4, this solution, demonstrated on an architecture tailored to the specifics of the NG-PON2 standard, is not presented as an alternative to it, but it is rather targeted to be used as a stand-alone system (in green-field deployment scenarios) or to be used to enhance standard PON or eventually NG-PON2 operating systems.

The PON makes use of a low-cost, coherent 1.25 Gb/s ASK transceiver, that works in a polarization-independent way, without the need of any polarization diversity schemes, or automatic polarization control. The envelope detection of the signal is based on simple analogue processing, without the need of DSP (nor ADC).

The chapter is organised as follows: section 2.1 firstly provides a description of the the-oretical operation of the receiver scheme, and how it achieves a polarization-independent operation. Section 2.2 describes in detail the 1.25 Gb/s real-time receiver implementation and the setup for its characterization, then in section 2.3 the relative experimental results are reported. Finally, in section 2.4, the UD-WDM PON architecture is described, using two prototypes based on the proposed scheme, and the bidirectional transmission with a DS grid of 8 channels spaced at 6.25 GHz is demonstrated.

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The architecture has been also validated on a field-trial, in simultaneous operation with a legacy EPON system and other coherent WDM access architectures. This field trial, that was the final public demontration of the COCONUT project, is described separately in chapter 6.

The results reported in section 2.3 have been published in [40, 41], while results of section 2.4 have been reported in [45].

2.1 Polarization-independent receiver scheme

opera-tion

In this section we summarize the operating principles of the polarization-independent scheme that was largely used in the experiments reported in this thesis. One of the advantages of coherent detection is the possibility of increasing spectral efficiency by the use of phase-modulation or higher-order modulation formats. Infact, great part of the research work on PONs using coherent receivers are based on this kind of modulation [29, 30, 33, 34]. The main drawbacks with phase-modulation are the stringent requirement on phase/frequency locking between the Local Oscillator (LO) and the signal, and the need of using low-phase noise optical sources [46], which might be expensive.

On the other side, it has been shown that phase-diversity receivers, in combination with ASK modulation formats, have phase/frequency locking requirements very relaxed. This opens the way to use lasers of wider linewidth [47] such as Distributed Feedback Laser (DFB)s and avoids the implementation of sophisticated Phase Locked Loops (PLLs). In addition, signal demodulation is largely simplified so that it can be carried out by using analog processing. This is a desirable property in access network, where the simplicity and cost-efficiency of the solution is fundamental. The proposed receiver scheme is based on a diversity configuration proposed for the first time by Kazovski [48] for phase-diversity homodyne detection. Ciaramella has shown [49] how, with some modification to the scheme, and operating in intradyne regime, the scheme can work in a polarization-independent fashion. The detection scheme is reported in Fig 2.1.

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The input signal, described by a Jones vector U (t) is split by a Polarizing Beam Splitter (PBS), and the two orthogonal State of Polarization (SoP) components enter the first two arms of a symmetric 3x3 coupler. The polarization of one of them is rotated by 90◦ so that they both enter the coupler with parallel SoP. The LO is injected in the third input port of the coupler, with the same SoP as the two signal components. The three coupler outputs are then detected by three identical photodiodes, with bandwidth Bpd (modeled in the figure by low-pass filters). The three photocurrents are then squared

(⊗) and summed together (Σ). The sum S(t) is then furtherly low-pass filtered, with a low-pass filter with bandwidth BRX.

Assuming that the signal and the LO have a frequency detuning of ∆ν and the signal can have a random polarization, its Jones vector can be described as

U (t) =

r(t)ei(2π∆νt)_{cos φ}

r(t)ei(2π∆νt+ψ)_{sin φ}

(2.1) where r(t) is the modulated amplitude, φ is the orientation of the main axis of the polarization ellipse and ψ is the SoP ellipticity angle (ψ = 0 for linear polarization).

Given the transmission matrix of the 3x3 coupler [50], and assuming that the band-width of the photodiodes is much greater than the signal rate and that the DC-block completely eliminates the continous wave component due to the LO, the expressions for the three photocurrents are simply derived. As an example, for the current i1(t)

i1(t) = 2 3Rr(t)ELO cos 2π∆νt − ψ + 2π 3 sin φ − cos 2π∆νt +2π 3 cos φ (2.2) where R is the photodiodes responsivity and ELO the LO field amplitude. Given this

expression, the sum of the squares of the three photocurrents S(t) = i12+i22+i32 assumes

the following shape

S(t) = 2 3R 2 r(t)2ELO2 h 1 − sin(2φ) sin π 6 − 4π∆ν − ψ i (2.3) S(t) includes two terms, both proportional to the square value of the modulation signal r(t). Of these two terms, only the second one, centered around the detuning frequency ∆ν, depends on the SoP of the signal (through φ and ψ). This term is in general different from 0, being equal to 0 only if the signal is exactly aligned in polarization to the LO (in that case, φ=0). The first term, centered in baseband, is instead independent on the polarization parameters, and still contain the signal information r(t). Therefore, the final low-pass filter operation (BRX) can effectively suppress the signal component

dependent on the SoP, leading to a polarization-independent operation. In Non-Return to Zero (NRZ) receivers, BRX is usually fixed at the 75% of the signal bit-rate [48]. Thus,

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suppression of the polarization-dependent component can be obtained by a careful choice of the detuning frequency ∆ν. As theoretically demonstrated in [49], intradyne operation is needed to achieve polarization independence and the value of ∆ν should be >70% of the bit-rate. In the next section, we present our experimental demonstration of the operation of the scheme is given, in a practical realization of a 1.25 Gb/s.

2.2 1.25 Gb/s Real-Time receiver description

Fig. 2.2 shows a sketch of the 1.25 Gb/s receiver scheme and of the experimental setup for its characterization.

DFB-2 MZM PBS LPF BERT PS 90° DFB-1 PPG 60nkm SMF VOA TIA TIA TIA coherentnrx

Figure 2.2: ASK system: experimental setup

The Transmitter (Tx) is made of a common DFB (λ = 1540.8 nm, < 10 MHz linewidth and <140 dB/Hz RIN) externally modulated by a Mach-Zehnder Modulator (MZM), driven at 1.25 Gb/s by a NRZ Pseudo-Random Bit Sequence (PRBS) (231_{− 1 bits long).}

External modulation is used at the transmitter in order to isolate signal-related impair-ments. The Receiver (Rx) can also work with directly modulated lasers (DML) with similar performance [51].

The considered ODN consists of a 60 km of Single Mode Fiber (SMF) (G.652, 13 dB loss), a Variable Optical Attenuator (VOA) emulating splitting loss, and a polarization scrambler (PS) producing random variations of the signal SoP, in order to emulate a typical real world operating environment.

The schematic of the real-time polarization independent RX (PI-RX), is reported on the right of Fig. 2.2 (yellow box). The receiver, implementing the detection scheme described in the previous section, has been realised employing the following components: • a PBS, with polarization mantaing fiber patchcords at the two outputs, and one of the connectors rotated by 90◦, in order to realize the rotation needed in order to align the two orthogonal signal components to the LO

• a symmetrical 3x3 fused-fiber optical coupler

• a DFB laser as LO (emitted power +3 dBm) with the same specs of the one used in the Tx. The LO wavelength is thermally tuned and controlled so as to work in intradyne regime with ∆ν = 850 MHz (as prescribed in [49]).

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• three identical PIN photodetectors followed by trans-impedance amplifiers with 2 GHz 3 dB bandwidth and differential outputs.

• three RF analogue multipliers (Fig. 2.3 (a), ⊗ in Fig. 2.2), with 2 GHz operating bandwidth (starting from DC). Since the RF multipliers inputs were the differential current pairs (i(t) and i(t), shown in Fig. 2.4(a)), the squaring operation is effec-tively obtained on each photocurrent (see Fig.2.4(a)). To this aim the electrical response of the three multipliers must be as similar as possible. This is shown in Fig. 2.3 (b), where the measured second harmonic conversion efficiencies of the three devices as a function of the input frequency are reported. Identical results were ob-tained within experimental accuracy demonstrating that operation is uniform across the used devices.

• a passive electrical power combiner (Σ in Fig. 2.2, bandwidth 18 GHz), realizing the sum of the squares of the three photocurrents

• a standard low-pass filter (LPF, Bessel fourth order, 933 MHz bandwidth)

(a) (b)

Figure 2.3: RF Analog multipliers (a) evaluation board picture and (b) second harmonic conversion efficiencies of the three devices as a function of the input frequency

(a) (b) (c)

Figure 2.4: Acquired traces after a) photodiode b) RF multiplier c) combiner

In order to show that the PI-RX is actually working, in Fig. 2.5 the electrical spectra and eye diagrams of the recovered signal before and after the final low pass filter are

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shown. Trace (a) shows the spectrum obtained when the signal SoP is aligned to the one of the LO, which is a typical NRZ spectrum. A residual tone at the frequency ∆ν, due to non idea behavior of the receiver components, is observed. However, this spurious tone has a relatively low amplitude and does not affect the performance of the receiver. Trace (b) shows the spectrum before the Low Pass Filter (LPF) when the SoP of the input signal is rotated by 45◦ with respect to the LO, which is the worst-case condition: a significant component at 2∆ν: this component, whose amplitude is dependent on the SoP of the signal, reaches its maximum amplitude in this case [49], and might impair the system. However, being at high frequency, it is effectively suppressed by the filter, and a clear, polarization-independent signal is obtained, as observed in Trace (c). Minor differences are observed in the eye diagrams in case (a) and (c), namely in case (c) the eye diagram shows thicker traces. As it will be shown into the next section, this has a very limited impact on the system performance.

2.3 1.25 Gb/s Real-Time Receiver characterization

The proposed receiver performance was first characterized as a function of the signal received power (see Fig. 2.6). The back-to-back (B2B) measurements when the signal has a parallel (black squares) or orthogonal (white circles) SoP with respect to the LO clearly shows that the receiver is actually polarization-independent. The very small different performances in the two cases can be attributed to a small difference of insertion loss of the PBS (and connectors) at its two inputs. The pre-Forward Error Correction (FEC) sensitivity obtained, i.e. -51 dBm, corresponds to an improvement of about 8/9 dB with respect to the specifications of commercially available direct-detection Avalanche Photo-Diode (APD) receivers (typical performance is described for example in [52]). No error floor is observed down to Bit Error Rate (BER)=10−6. This is a feature that can be very useful for specialized applications having strict latency requirements, such as mobile front-hauling, where the use of the FEC can be problematic.

Moreover, the measured BER curves shown here do actually match those taken with offline processing within the experimental error (±0.5 dB) down to around BER= 10−6 [53].

The polarization independent operation has been tested by turning on the PS in front of it (see Fig. 2.2). The scrambler randomly changes the signal SoP (at a frequency of 6 kHz) in a way to uniformly cover the Poincar`e sphere. The measured BER curve is the one with black dots in Fig. 2.6. In this case, no penalty is observed at FEC level, but a penalty of about 1 dB arises at BER = 10−9. This penalty can be ascribed to the combined effect of the small differences in the insertion loss in the polarization paths (including connectors), small residual differences in the optical paths, non uniformity of

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(a)

(b)

(c)

Figure 2.5: electrical spectra and eye diagrams taken when the signal SoP is exactly aligned to that of the LO (a) and when the SoP of the signal is set in the worst condition, before Bessel filter (b) and after (c)

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Figure 2.6: Receiver performance for different operating conditions and input signal SoP.

the splitting ratio of the 3x3 coupler and to the excess jitter of the PI-RX in the worst signal SoP case (see eye diagram in Fig. 2.5 (c)), as theoretically predicted in [49].

Then, the transmission performance over a 60 km strand of SMF was tested (BER curve with white squares in Fig. 2.6(a)); a penalty of about 2 dB at BER = 10−9 is observed, that can be possibly reduced by further optimization of the components. In any case, the tested distance is considerably higher than the maximum reach of G-PON or EPON standards (see Table 1.1).

Besides demonstration of the receiver functionality, it is also important to investigate the receiver resilience to system parameter variations and its robustness under the oper-ation conditions that can be found in an actual UD-WDM PON. These are important features in view of the engineering of the receiver in possible pre-commercial prototypes. As already discussed, the intradyne frequency detuning ∆ν is a critical paramenter for proper operation of the receiver. Therefore, the PI-RX tolerance to deviations with respect to the ideal frequency setting has been measured. The results in terms of BER measurement at -44 dBm are shown in Fig. 2.7.

The receiver performance is maximum at ∆ν = 900 MHz (BER = 2 · 10−9) and a deviation of about ±100 MHz around the optimal setting can be tolerated without significant degradations even at such low BER levels. For lower detuning values the baseband signal is corrupted by spurious terms introduced by squaring (the lower the detuning the higher the impairment) whereas for higher detuning values the performance is bound by the electronic components bandwidth (the higher the detuning the higher

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Figure 2.7: 1.25 Gb/s ASK RX: BER vs detuning

the impairment). Clearly, in this case the penalty could be eliminated by adopting higher bandwidth components; however, this would limit WDM operation in terms of minimal channel spacing allowed.

Another important feature is the robustness of the coherent receiver against back-reflections and crosstalk from adjacent UD-WDM channels. To perform reflection and crosstalk measurements the set-up of Fig. 2.2 was modified as shown in Fig. 2.8, where a second transmitter was added to provide for an interfering channel.

Figure 2.8: ASK crosstalk measurements: experimental setup

The second transmitter consists of a DFB laser (DFB-X), with the same specs of the other involved lasers, tuned thermally to the desired frequency spacing. The light of DFB-S (signal source) and DFB-X was modulated at 1.25 Gb/s by two different MZMs driven by a PPG with two outputs. The PPG provides two independent data patterns of different length (231− 1 for the signal modulation, and 215_{− 1 for the interferer). The}

power of each transmitter was set by means of a VOA and then the two signals were coupled to the receiver input arm by means of a 50:50 coupler. To further de-correlate the data patterns of the two signals, a fiber strand of about a hundred meters was added in the interferer path. All the measurements were taken in a B2B configuration. A PS was placed in each transmitter arm to average over polarization. The signal to interferer level and frequency spacing was controlled online by means of a optical spectrum analyser

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connected to the second output arm of the 50:50 coupler (frequency spacing was also checked in parallel using an electrical spectrum analyzer). In all measurements discussed below, a BER of 2 · 10−3 (attained at the received power of -50.5 dBm) has been assumed as a reference and the power penalty with respect to this was measured to give a direct insight of the system impact.

Back-reflections were emulated by tuning the interferer to the frequency of the channel (in upstream or downstream) closest to the signal. The results are shown in Fig. 2.9 where the measured power penalty is plotted against the crosstalk level.

Figure 2.9: Power penalty (FEC level) versus crosstalk level at 1 GHz signal interferer detuning (LO wavelength reuse back-reflection) and at 6.25 GHz, as considered in this work

As can be seen, at 6.25 GHz no penalty is induced by the interferer at all considered crosstalk levels.

Then the effect of crosstalk from adjacent channels was investigated. In this case the interferer frequency was scanned across a 12 GHz range centered on the frequency of the signal under test and power penalty measurements were taken for various interferer power levels. The obtained results are shown in Fig. 2.10.

The penalty curve appears to be centered around the LO frequency (at about -1 GHz) and the worst situation is observed when the interferer frequency coincides with that of the signal. Due to the coherent nature of the crosstalk, the penalty grows significantly with the crosstalk level [54]. Instead, no significant impairment is observed when the signal-interferer detuning is outside the -3 GHz ÷ +2 GHz detuning interval, for all considered crosstalk levels.

These results prove that the proposed PI-RX is fully compatible with UD-WDM PON systems with 6.25 GHz spacing. The use of tighter spacing is precluded by the intradyne operation of the receiver and by the use of simple ASK modulation with his low spectral efficiency.

Finally, the dynamic range of the system was measured to prove the suitability of the solution in a real PON environment. In an actual PON the ODN loss can largely

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Figure 2.10: Power penalty (FEC level) versus signal-interferer detuning for various crosstalk levels

vary among the different ONUs and the OLT receivers, therefore it is critical that the Rx can work across a wide range of different received power levels. We thus measured the BER values as function of input power, thus simulating a variable ODN loss. Results are reported in Fig. 2.11.

Figure 2.11: Measured dynamic range (in B2B) of the proposed PI-RX receiver

They show that (at FEC level) a dynamic range of about 52 dB is achieved. This value is by far higher than the expected variations in practical environments (between 15 and 23 dB in current standardized PON systems [7, 8] and opens to the possibility of implementing PON with much higher differential losses.

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2.4 Prototype assembly and UD-WDM PON

demon-stration

The proposed detection scheme was the core of the realization of two prototype of transceivers that were used to implement the demonstration of a realistic UD-WDM PON. The structure of the transceiver is shown in Fig. 2.12. The receiver scheme reproduced the one described in the previous section, while the transmitter section consist of a sim-ple temperature controled DFB laser (λ=1551 nm, <10 MHz linewidth and <140 dB/Hz RIN), similar to the one used into the setup of Fig. 2.2, and an electro-optical MZM. For simplicity of realization, the laser source and the local oscillator are not included as part of the prototype, but the optical power can be externally injected trough two fiber patchcords.

(a) (b)

Figure 2.12: 1.25 Gb/s receiver prototype sketch (a) and picture (b)

The setup of the 8×1.25 Gb/s UD-WDM PON experiment is shown in Fig. 2.13. The architecture in use reproduce the target-architecture of the COCONUT project, described in section 1.4, uses an AWG at the OLT with 100 GHz spacing and 50 GHz bandwidth, as recommended in NG-PON2 standard. Anyway, also in the case in which the multiplexing element is not shared with any NG-PON2 TWDM channel, the inclusion of an AWG allows to reduce the multiplexing losses of the signal, with respect to a pure power coupler. The 8 1.25 Gb/s channels are transmitted in a single port of the AWG, and individually selected at the ONU side.

The Rx side of the first transceiver prototype was used as OLT receiver, whereas its Tx side was used to modulate at 1.25 Gb/s in DS four optical carriers frequency spaced by 12.5 GHz. These carriers are generated by three External Cavity Laser (ECL) and a common DFB. A second set of ECLs generates a second group of four 12.5 GHz spaced DS optical carriers, offset by 6.25 GHz with respect to hte previous ones and modulated at 1.25 Gb/s by a second MZM. The multiplexing scheme employed both passive couplers and an AWG. This allowed using a single port of the multiplexer to be used to transmit all the 8 WDM channels. Both the subroups of channels had been

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Figure 2.13: UD-WDM PON experimental setup

multiplexed via polarization mantaining 4×1 couplers before modulation. A 2×2 coupler after the MZMs was used to form the final 6.25 GHz UD-WDM comb. To obtain the worst case Four-Wave Mixing (FWM) generation in the ODN fiber, a polarization controller was placed in one of the arms before coupling, to maximise polarization alignement of the two channel groups. The two MZMs were driven by a two outputs PPG using PRBSs of different lengths. The resulting DS spectrum of the optical channels is shown in Fig. 2.14

Figure 2.14: Optical spectrum of the UD-WDM PON signal at the AWG output

All the 8 channels were launched in the ODN through the same port of a 100 GHz AWG, that was used at the OLT side to multiplex the DS/US signals. Channel pre-emphasis was used to obtain an equalised comb at the AWG output. Maximum channel power imbalance was about 2 dB. An EDFA was used to compensate for multiplexing losses, with about -3 dBm per channel launched in the ODN.

The ODN consisted of about 40 km of G.652 fiber and a VOA to emulate splitting loss. The second prototype was used as a custome ONU. The upstream signal was received through a different port of the AWG. The channel under measurements were to a BER

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tester (BERT) for sensitivity measurements.

The summary of UD-WDM DS BER vs. received power measurements is given in Fig. 2.15(a). Here, all the measurements were taken from the DFB channel at the center of the spectrum (ch.4, red arrow in Fig. 2.14).

(a) (b)

Figure 2.15: BER vs received power measurements in UD-WDM configuration (a) Downstream (b) Upstream

The measured B2B pre-FEC sensitivity was around -47 dBm, higher than in the measurements shown in section 2.3, due to the different implementation of the PI-RX of the second prototype, where a less performing PBS had to be used. No penalty with respect to B2B single channel measurements (Fig. 2.15(a), blue squares) when the channel was selected from the complete UD-WDM comb by the action of the LO, despite the system was working in a filterless configuration (Fig. 2.15(a), white circles). Again, no penalty is observed after propagation in the ODN; the low power per channel prevents the occurence of FWM (Fig. 2.15(a), red circles).

The BER vs received power measurements on the US channel are given in Fig. 2.15(b) The pre-FEC sensitivity was around -50 dBm, in line with the measurements shown in section 2.3. Also in this case no penalty is observed after transmission through the ODN with the whole DS comb propagating.

Finally, pre-FEC sensitivity of all the channe was measured. Channel selection was done by thermally tuning the LO-DFB at the receiver side. The result in shown in Fig. 2.16

As can be seen the average sensitivity is around -48 dBm with a total fluctuation of around 2 dB. In particular, the measured sensitivities showed a dependence on the channel

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Figure 2.16: UD-WDM: measured sensitivities of the 8 UD-WDM channels

group due to imperfections in the multiplexing operation (slightly different performance of the couplers, different operating point of the lasers due to pre-emphasis, etc.). It is posible to believe that this values can be improved witha better implementation of the multiplexing procedure and of the second PI-RX prototype (aligning its performance to the first one). In any case, given the launch-power per channel (-3 dBm), a power budget of around 45 dB is obtained, which means an improvement of more than 20 dB with respect to current 1 Gb/s line rate standards (G-PON or EPON, Table 1.1), and 10 dB with respect to NG-PON2 E2 class [14](Table 1.2).

Therefore, the results show feasibility of the preposed detection scheme in a real-istic UD-WDM PON. The total capacity transmitted of 10 Gb/s equals the one of a single NG-PON2 TWDM channel, but thanks to the lower sensitivity of the proposed transceivers, the power budget of the system is increased. The additional power budget, as the absence of propagation penalties for distances up to 60 km, can be beneficially ex-ploited for increasing the network reach, or introducing additional power splitting stages, as identified in one of the migration scenarios towards COCONUT architecture described in section 1.4. Moreover, depending on the presence or not of others NG-PON2 TWDM channels using the same multiplexer, the necessity of coexistence with other legacy sys-tems on the same network, and the charachteristics of the AWG in use, the number of channels sent into a single port can be eventually increased, with the consequent increase of the total capacity. In the case of a green-field scenario, up to 16 channels can be allo-cated in place of a single NG-PON2 channel, doubling the total capacity of the network (as anticipated in table 1.2).

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Chapter 3 A dynamic wavelength allocation

algorithm for filterless coherent

WDM-PON

The recent development of coherent digital transceivers opened new perspectives to the realization of the Software Defined Networking (SDN) through the implementation of advanced functionalities (of fundamental importance in particular in core networks), such as the Dynamic Wavelength Allocation (DWA), bandwidth, and on-demand adaptation of modulation formats and resources. In general, a dynamic assignement of wavelength slots and bandwidth is necessary in all the scenarios in which the channel is shared by a variable number of active users, with diverse bandwidth demands. Allocating resources in real-time, according to the requests coming from the users, assures an efficient utilization of the physical resources of the network.

In the scenario of access networks, DWA is a particularly desirable feature, that allows to implement advanced network management features. As an example, an issue common to WDM-PON architectures based on tunable upstream transmitters is that ONU activa-tion and reconfiguraactiva-tion should occur without impairing other active upstream channels: this can be solved by using specific wavelength assignment algorithms [55]. With the employment of coherent detection in optical networks, DWA can be effectively realized by exploiting the frequency selectivity property of the receivers. However, it is worth to underline again that when targeting at the access network domain, both coherent technologies and DWA functionalities must be implemented by using cost-effective and reliable coherent transceivers [37].

This chapter describes a novel hitless laser activation and reconfiguration algorithm, suitable for coherent WDM-PONs. This algorithm is independent on the specific coherent detection implementation and thus can be in principle applied to any coherent WDM-PON architecture. The proposed algorithm exploits a tecnique known as ”push-pull” introduced in [56] and features a simple monotonic frequency control of the involved