**1. Introduction**

Fiber optic access networks are the necessary infrastructural approach for a real broadband delivery, allowing the fiber to arrive closer to the final customer, eventually up to the premises equipment; such infrastructures, depending on the depth of reach of the fiber, are usually referred to as FTTX (Fiber To The X), where X stands for H (Home), B (Building), C (Curb) or Cab (Cabinet). They are the basis for broadband access networks, enabling high-speed Internet access, digital TV broadcast (IPTV), video on demand (VOD) and other services. Comparing with copper technologies, like for example DSL (digital subscriber line), higher bandwidths (up to several 10 Gbit/s) and higher distances (up to several tens of km) are possible. FTTX infrastructures can address both residential and business access, and support mobile backhauling.

Access networks can be divided in two main categories: Active Optical Networks (AONs) and Passive Optical Networks (PONs):


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The OLT is the interface between backbone and access networks, and it is also responsible for the enforcement of any media access control (MAC) protocol for upstream bandwidth arbitration. The ONU cooperates with the OLT in order to control and monitor all PON transmission and to enforce the MAC protocol for upstream bandwidth arbitration; it also acts as the residential gateway, coupling the ODN with the in-home network.

The ODN consists of the distribution fibers and all the passive optical distribution elements, mainly optical splitters and/or wavelength-selective elements (WDM filters), that are located in sockets or cabinets. The splitting ratio in most cases is between 1:8 and 1:128, and can be performed in lumped or cascaded elements. The ODN power budget is usually taken as a reference for PON reach calculations, and is computed as the difference between the trans‐ ceiver back-to-back power budget (i.e. OLT transmitter directly coupled into ONU receiver or vice versa) and the passive optical equipment necessary inside the OLT and the ONU to perform all multiplexing of PON signals into a single fiber. Hence, the ODN power budget considers the remaining power budget that can be spent for the distribution fibers and the distribution elements in the remote nodes.

**Figure 1.** Passive Optical Network architecture

During the last 20 years, the Full Service Access Network (FSAN) and the Ethernet in the First Mile alliance (EFM) working groups, in cooperation with the International Telecommunica‐ tions Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) standardi‐ zation bodies, defined several PON standards, as summarized in Figure 2.

For the purposes of this Chapter, we will not go through all these standards, since none of these has been approached with self-coherent reflective architectures that will be presented in this Chapter, but we will mention only the most recent one, called NG-PON2 (Next Generation PON 2), in progress of standardization at the time of writing and with still open technological issues. In particular, we will address only physical layer aspects. The definition of such standard started in 2010 by FSAN.

Since about the 70% of the total investments in deploying PONs is due to the optical distribu‐ tion networks, it is crucial for the NG-PON2 evolution to be compatible with the deployed networks and to reuse the outside plant. Moreover, NG-PON2 technology must outperform existing PON technologies in terms of ODN compatibility, bandwidth, capacity and costefficiency. For this reason, the initial "wish list" for NG-PON2 was originally very demanding, requiring not only an increase of bit rates with respect to previous generations, but also in terms of total number of users per PON tree and reach.

The major original general requirements of NG-PON2 can be summarized as follows:


The OLT is the interface between backbone and access networks, and it is also responsible for the enforcement of any media access control (MAC) protocol for upstream bandwidth arbitration. The ONU cooperates with the OLT in order to control and monitor all PON transmission and to enforce the MAC protocol for upstream bandwidth arbitration; it also acts

The ODN consists of the distribution fibers and all the passive optical distribution elements, mainly optical splitters and/or wavelength-selective elements (WDM filters), that are located in sockets or cabinets. The splitting ratio in most cases is between 1:8 and 1:128, and can be performed in lumped or cascaded elements. The ODN power budget is usually taken as a reference for PON reach calculations, and is computed as the difference between the trans‐ ceiver back-to-back power budget (i.e. OLT transmitter directly coupled into ONU receiver or vice versa) and the passive optical equipment necessary inside the OLT and the ONU to perform all multiplexing of PON signals into a single fiber. Hence, the ODN power budget considers the remaining power budget that can be spent for the distribution fibers and the

During the last 20 years, the Full Service Access Network (FSAN) and the Ethernet in the First Mile alliance (EFM) working groups, in cooperation with the International Telecommunica‐ tions Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) standardi‐

zation bodies, defined several PON standards, as summarized in Figure 2.

as the residential gateway, coupling the ODN with the in-home network.

366 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

distribution elements in the remote nodes.

**Figure 1.** Passive Optical Network architecture


**Figure 2.** PON standards


**Table 1.** Classes for optical XG-PON path loss

Many PON technologies have been proposed to provide broadband optical access beyond 10 Gbit/s, like for example:


The first proposal, based on a single wavelength (per direction) at very high bit rate, was finally discarded, since it was perceived as too expensive. Indeed, it had to include a duo-binary modulation and a downstream transmission around 1300 nm to avoid dispersion compensa‐ tion. Moreover, it was not scalable, thus no future upgrades above 40 Gbit/s downstream seemed feasible on a single wavelength.

The proposals based on OFDM are very interesting and technologically advanced solutions, but they do not easily guarantee a complete compatibility with existing deployed networks and devices.

Regarding the WDM-PON architectures, a key point is whether the WDM filters are inside the ODN or in the ONU.

The "pure" WDM-PON approach, based on WDM filters inside the ODN, generates some constraints to Telecom operators:


For these reasons, this solution seems to be out of question for NG-PON2 standard. Among all of the aforementioned proposals, TWDM-PON has attracted the majority support from global vendors and was selected by the FSAN community in the April 2012 meeting as a primary solution to NG-PON2. TWDM-PON increases the aggregate PON rate by stacking XG-PONs via multiple pairs of wavelengths; since an XG-PON system offers the access rates of 10 Gbit/s in downstream and 2.5 Gbit/s in upstream, a TWDM-PON system with four pairs of wavelengths will then be characterized by:


**'Nominal1' class (N1 class)**

368 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

Table taken from G.987.2

Gbit/s, like for example:

**Table 1.** Classes for optical XG-PON path loss

OFDM signals for transmission [2, 3];

multiple XG-PONs using WDM [6].

seemed feasible on a single wavelength.

and devices.

etc.);

ODN or in the ONU.

constraints to Telecom operators:

statistical multiplexing is possible.

**'Nominal2' class (N2 class)**

Many PON technologies have been proposed to provide broadband optical access beyond 10

**•** the 40 Gigabit time-division multiplexed PON (XLG-PON) proposal [1] which increases the single carrier serial downstream bit rate of a 10 Gigabit PON (XG-PON) to 40 Gbit/s;

**•** a set of orthogonal frequency-division multiplexed (OFDM)-based PON proposals which employ quadrature amplitude modulation and fast Fourier transform to generate digital

**•** a group of WDM-PON proposals which provide a dedicated wavelength channel at the rate of 1 Gbit/s to each ONU with different WDM transmit or receive technologies [4, 5];

**•** the time-and wavelength-division multiplexed PON (TWDM-PON) proposal which stacks

The first proposal, based on a single wavelength (per direction) at very high bit rate, was finally discarded, since it was perceived as too expensive. Indeed, it had to include a duo-binary modulation and a downstream transmission around 1300 nm to avoid dispersion compensa‐ tion. Moreover, it was not scalable, thus no future upgrades above 40 Gbit/s downstream

The proposals based on OFDM are very interesting and technologically advanced solutions, but they do not easily guarantee a complete compatibility with existing deployed networks

Regarding the WDM-PON architectures, a key point is whether the WDM filters are inside the

The "pure" WDM-PON approach, based on WDM filters inside the ODN, generates some

**•** all passive splitters installed in the already deployed PON (brown field scenario) need to

**•** limited backward compatibility with already deployed standards (GPON, EPON, XG-PON,

**•** a dedicated wavelength per user is a "circuit" per individual user, thus no sharing or

be substituted with Arrayed Waveguide Grating (AWG) filters;

Minimum loss 14 dB 16 dB 18 dB 20 dB Maximum loss 29 dB 31 dB 33 dB 35 dB

**'Extended1' class (E1 class)**

**'Extended2' class (E2 class)**


The basic TWDM-PON architecture is shown in Figure 3. Four XG-PONs are stacked by using four pairs of wavelengths (e.g., wavelength pairs of {*λ*1, *λ*5}, {*λ*2, *λ*6}, {*λ*3, *λ*7} and {*λ*4, *λ*8}, in Figure 3). ONUs are equipped with tunable transmitters and receivers. The tunable transmitter is tunable to any of the four upstream wavelengths; the receiver is tunable to any of the four downstream wavelengths. In order to achieve power budget higher than that of XG-PON, optical amplifiers (OAs) are used at the OLT side to boost the downstream signals as well as to pre-amplify the upstream signals. The ODN remains passive since OAs are placed at the OLT side, together with WDM Mux/DeMux.

Therefore, the TDWM-PON key features are:


Most of the TWDM-PON components are commercially available in access networks today. Comparing to previous generations of PONs (e.g., GPON, XG-PON), the only, but technically very challenging, new components in TWDM-PON are the tunable receivers and tunable transmitters at the ONU.

For what concerns the tunable receiver, it should tune its wavelength to any of the TWDM-PON downstream wavelengths by following the OLT commands. This function can be implemented by using candidate technologies such as:


Picture taken from [6]

**Figure 3.** TWDM-PON system diagram


The ONU tunable transmitter can tune its wavelength to any of the upstream wavelengths. The related implementation technologies are:


All these solutions are today under consideration for NG-PON2, but none has yet completely demonstrated to be achievable at the (very low) target prices for ONU. Purpose of this Chapter is to describe a solution for the upstream transmission that avoids the need for a tunable laser at the ONU side: it is based on self-coherent reflective PON architectures as a possible technological approach to the NG-PON2 requirements. In the following we will then first show the concept of the reflective PON, describing the key components it needs, the problems it solves and the limitations it encounters; then, we will propose the self-coherent detection enhancement and will give an overview of the latest research results available in literature.
