This paper presents the high-level COCONUT architecture of an optical access network based on coherent technology, supporting ultra-dense Wavelength Division Multiplexing (WDM). The COCONUT network should allow for seamless evolution from present PON architectures, but also support new emerging applications such as mobile back-haul and front-haul. Coherent techniques will hence allow serving a higher number of users, thanks to ultra-dense WDM, and reaching higher power budget, thanks to higher sensitivity. These features should allow for economy of scale and network consolidation. Although coherent solutions are well-developed for core networks, the prohibitive cost of their optical and electronic parts makes them unsuitable for the massive deployment of access networks. To this aim, COCONUT will address realization of coherent transmitter/receivers exploiting low-cost components and simple electronics, so that the cost of typical line terminals would be affordable to the end-users. The paper presents an overview of the target results and key issues that are addressed by the COCONUT partners.

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Abstract-New technologies for ultra-dense WDM-PON (udWDM-PON), enabled by coherent techniques and low cost devices, are developed for an efficient utilization of the optical spectrum, revealing that the "Wavelength-to-the-User" concept can be feasible. In this work, an udWDM-PON with only 6.25 GHz channel spacing is implemented with conventional DFB lasers, for a splitter-based PON infrastructure with 256 ONUs. The results of the analysis of udWDM access network architecture with respect to their associated complexity, cost and migration scenarios, exhibit the potential for higher aggregate throughput, higher split ratios and node consolidation, when compared to competing technologies.Index Terms-Fiber optical communications, fiber-to-thehome, access networks, PON, WDM.

Wider deployment of fiber in the last mile is driven by increased customer needs for broadband communication services. This deployment requires solutions that reduce operational expenditures for the operator. A costefficient fully reliable and accurate monitoring solution supporting fault detection, identification, and localization in different fiber access architectures will be essential. In this article, we present a fast, automatic, and precise monitoring method applicable to both power-splitter-and wavelength-router-based passive optical networks through the combined techniques of optical time domain reflectometry and optical transceiver monitoring. The description of the architecture, components, and process flow is followed by tests on setups with live data transmission.

We experimentally demonstrate an innovative Ultra Dense Wavelength Division Multiplexing (UDWDM) Passive Optical Networks (PON) that implements the full λ-to-the-user concept in a filterless distribution network. Key element of the proposed system is a novel class of coherent transceivers, purposely developed with a non-conventional technical approach. Indeed, they are designed and realized to avoid D/A-A/D converter stages and Digital Signal Processing (DSP) in favor of simple analog processing so that they match system, cost and power consumption requirements of the access networks without sacrificing the overall performance. These coherent transceivers target different use case scenarios (residential, business, fixed, wireless) still keeping perfect compatibility and coexistence with legacy infrastructures installed to support gray, Time Division Multiplexed (TDM) PON systems. Moreover, the availability of coherent transceivers of different cost/performance ratios allows for deployments of different quality service grades. In this paper, we report the successful field trial of the proposed systems in a testbed where 14 UDWDM channels (and one legacy E-PON system) are transmitted simultaneously in a dark-fiber network deployed in the city of Pisa (Italy), delivering real-time and/or test traffic. The trial demonstrated filterless operations (each remote node selects individually its own UDWDM channel on a fine 6.25 GHz grid), real-time GbE transmissions (by using either fully analog or light digital signal processing), multirate transmission (1.25 and 10 Gb/s/), high Optical Distribution Network loss (18 ÷ 40 dB) as well as a bidirectional channel monitoring system.