Giuseppe Rizzelli

As integrated sensing and communication (ISAC) has become a topic of considerable interest, state of polarization (SOP) monitoring is increasingly being investigated across all optical network segments. In this work, we propose a solution for simultaneous SOP sensing in upstream passive optical networks (PONs) across all active optical network units (ONUs) using a single polarimeter placed in parallel with the optical line terminal (OLT) receiver. The proposal is experimentally demonstrated using commercial GPON OLT and ONU hardware, showing that the system can detect whether the abnormal vibrations occurred in the feeder fiber or in a specific distribution fiber, thus providing coarse event localization. Since our solution exploits the inherent TDMA burst-mode operation of upstream GPON, we discuss the requirements in terms of polarimeter sampling rate and synchronization. Moreover, we experimentally characterize the ultimate limitation of the proposed SOP-sensing system when operating at very low average received power, as required by an implementation using an asymmetric tap that directs most of the power to the OLT receiver (for a minimal additional insertion loss on the data path) and only a small fraction to the polarimeter. To this end, we discuss the (post-)processing algorithm employed to extract and enhance the SOP estimate from the polarimeter, based on proper averaging of Stokes vector samples related to the same ONU burst. A quantitative metric based on the angular deviation of the Stokes vector on the Poincaré sphere is used to characterize polarization noise and estimation accuracy under heterogeneous traffic conditions. Finally, we measure and discuss the resulting SNR when applying a mechanical vibration to the feeder fiber.

We present a comprehensive and computationally efficient analytical framework to estimate the performance of linear dual-polarization coherent optical systems employing finite-length multiple input multiple output (MIMO) equalizers. By adopting a formulation based on Block-Toeplitz matrices, our model extends prior art by uniquely capturing the complex interplay between optical filtering, chromatic dispersion (CD), distributed polarization impairments, namely polarization mode dispersion (PMD) and polarization dependent loss (PDL), distributed and colored noise, and fractional sampling regimes (e.g., 3/2, 4/3 samples-per-symbol (SpS)) in coherent optical transmission systems. Unlike traditional asymptotic models, we explicitly quantify the impact of limited equalizer memory and colored noise statistics on the post-equalization signal-to-noise-ratio (SNR), which can directly lead to bit error ratio (BER) degradation. Extensive validation against time-domain decision-directed (DD)-least mean square (LMS) simulations, using probabilistic shaping (PS)-16-QAM format, demonstrates the model’s accuracy, showing an average estimation error up to 0.3 dB even under severe impairment scenarios with an SNR penalty ≲15 dB. Furthermore, we leverage the model to highlight the advantages of fractional sampling, showing that 3/2 SpS and 4/3 SpS architectures yield superior performance compared to standard 2 SpS solutions given the same equalizer complexity (i.e., same number of taps) once converging. Notably, this framework offers a dramatic reduction in computational time, by orders of magnitude, compared to traditional time-domain simulations, making it a pivotal tool for the design and optimization of future reconfigurable optical networks.

This tutorial paper presents a survey on several current ongoing research activities about the expected evolution of passive optical networks, focusing on the physical layer (i.e., on the physical media dependent or PMD layer). After the recent public release of the ITU-T G.9804.3 50G-PON standard, there is today ongoing extensive scientific brainstorming on what can be the best optical transmission solutions for the PON “next step,” which will likely be targeting 100G or 200G per wavelength per direction. Elaborating on our OFC2025 tutorial on these topics, we start with a brief overview of the possible IM-DD solutions and their actual limitations for 200G-PON, while the core of the paper discusses the pros and cons of the introduction of advanced modulation formats and coherent detection also in PON, including some hybrid solutions that are currently under investigation in the ITU-T Very High Speed PON initiative. We show that coherent PON can actually reach the 200G per wavelength target, including some spare link budget margin, which can potentially be used for new access architectures. As an example, we present a summary of our recent results on hybrid solutions and on the all-optical convergence between the metro and access segments.

Distributed acoustic sensing (DAS), implemented through phase-sensitive optical time-domain reflectometry ($\phi$OTDR), has gained attention in geophysical and telecommunications environments. By utilizing the existing telecom infrastructure, DAS facilitates self-monitoring of networks and external events observation, from human activities to large-scale geophysical phenomena. However, integrating it with data signals on the same fiber is challenging, as DAS employs higher power signals that can interact with data channels. Although increasing DAS performance with higher bandwidths is possible, practical applications have mostly been limited to 1 GHz due to cost and complexity constraints. This paper investigates the coexistence conditions of DAS, specifically direct detection (DD) chirped-pulse-$\phi$OTDR (CP-$\phi$OTDR), with intensity-modulated and DD (IM-DD) telecommunication systems, both operating at comparable complexities (5 GHz chirp and 10 Gbit/s), particularly interesting for metro/access contexts. We varied the frequency spacing of data and DAS channels, the DAS pulse peak power, and the receiver optical filter bandwidth, identifying the conditions for feasible coexistence. Our analysis shows that nonlinear Kerr-induced modulation instability (MI) is the main impairment affecting IM-DD channels co-propagating with DAS. In this scenario, MI cannot be approximated as originating from a single frequency source, and the observed nonlinear impairments vary along the pulse with its sweeping central frequency. Numerical simulations validate the experimental results, showing close alignment.

We present a fully experimental investigation on the fundamental physical-layer capabilities of future converged metro + access architectures using commercial coherent transceivers and reconfigurable optical add-drop multiplexers (ROADMs), which are placed at the boundary between the two network segments to implement all-optical routing of upstream and downstream wavelengths. For the access part, we focus on passive optical network architectures, as they are the most commonly deployed and also the most demanding at the physical layer due to their very high loss and single fiber operation. In our experimental demonstration, we target very high bit rates (200G and 400G net data rates) and key physical-layer scalability, such as the maximum achievable optical distribution network loss as a function of available optical signal-to-noise ratio in the metro segments, ROADM internal insertion loss, and other possible impairments, like tight optical filtering or power level unbalances.

Given the ubiquity of optical fiber networks in both terrestrial and submarine environments, leveraging these facilities for sensing anomalous conditions alongside telecommunications can provide significant added value. In this context, distributed acoustic sensing (DAS) systems have been widely employed and discussed due to their sensitivity and ability to locate events. However, integrating them within existing networks is complex and expensive. On the other hand, the received state of polarization (SOP) is also sensitive to external factors, and it can be used for sensing: in this case, no extra hardware would be required since the SOP is already estimated in coherent receivers for data demodulation. The sensing information is provided “for free” by the already installed hardware, potentially requiring only a software upgrade. In this work, we analyze the feasibility of using polarization-based sensing to detect anomalous conditions in metropolitan environments. A polarimeter was used to evaluate SOP noise induced by urban factors, while a commercial coherent transceiver was employed to assess SOP estimation noise. We propose two algorithms for processing polarization data: a time-based method called SOP angular speed (SOPAS) and an adaptive, frequency-based approach named SOP-power spectral density gap (SOP-PSDG). These algorithms were compared by processing Stokes vector samples from the polarimeter when different sinusoidal vibrations are applied to the fiber through a mechanical shaker. Results demonstrate that a sampling rate of just a few tens of Hz is sufficient to effectively identify various hazardous conditions, with SOP-PSDG consistently outperforming SOPAS. Additionally, preliminary findings on the performances of these algorithms using SOP samples from a commercial coherent receiver are discussed.

We present in this paper a detailed brainstorming on the future option of merging the metro and the passive optical network (PON) access network segments, enabled by the introduction of end-to-end coherent transmission. We begin by reporting the experimental results presented by our group at OFC2024 (for which this paper is an invited extension). Starting from these preliminary but very promising results, we elaborate on two different possible schematics for metro+PON convergence using edge reconfigurable optical add-drop multiplexers (ROADMs) at the boundary of the two segments, and then we study their physical layer scalability by a mix of experimental characterization and numerical modeling. We show that coherent transceivers enable excellent performance in this scenario, allowing at least 200G per wavelength and even 400G in most cases when traversing all-optically two ROADMs before being routed towards a high splitting ratio PON in the access part of the network. We study several realistic conditions analyzing different bit rates, modulation formats, and network architectures, showing the physical layer conditions that would enable the PON optical distribution network loss to be in the range from 29 to 35 dB, as required by current international standards. The scalability analysis is first based on link budget and optical signal-to-noise ratio (OSNR) fundamental limitations, and it is then extended considering other physical layer issues, such as tight optical filtering in the ROADMs.

We present the experimental demonstration of high-speed transmission in downstream and upstream direction using off-the-shelf coherent transceivers in a 40-channel DWDM metro+access network configuration based on ROADMs for future all-optical metro-access converged solutions.

We experimentally investigate on the coexistence over field-deployed metro fibers between pulse-based Distributed Acoustic Sensing (DAS) and 400G per wavelength coherent transmission on a 100GHz DWDM grid using commercially available instruments.