Optical Add-drop Multiplexer Research Articles

(INVITED)Reconfigurable topology testbeds: A new approach to optical system experiments

Optical transmission systems provide high capacity, low latency and jitter, and high reliability for city-scale networks. Recirculating loop experiments have facilitated the study of signal propagation in long-haul optical transmission systems. However, they are unsuited for developing control and management software for city-scale optical networks with dozens or hundreds of reconfigurable optical add drop multiplexer (ROADM) units, diverse interconnection topologies, and dynamic traffic patterns. Large-scale testbeds can help, but may be inflexible and time- or cost-prohibitive. Reconfigurable testbeds such as COSMOS enable piece-wise emulation of a city-scale network by applying space and wavelength switching, dual-use software-defined networking (SDN) controllers, and comb sources, while digital twin models enable software emulation. Results from the development of a digital twin for COSMOS are presented for optical amplifiers and stimulated Raman scattering (SRS) including both analytical and machine learning (ML) models.

Study of Radio over Fiber Transmission with a Single Line in Ring Topology for Deployment of MIMO Antenna Access Points in a Cell-less Network

The massive growth of wireless traffic goes hand in hand with the deployment of advanced radio interfaces as well as network densification. This growth has a direct impact on the radio access architecture, which today is moving from centralized to distributed deployments through the use of a large number of access points (APs). This paper verifies the feasibility of deploying multiple APs in series on a single line in a ring topology in a cell-less network. On the one hand, this technique will further improve the communication capacity and flexibility of a Radio-over-Fiber (RoF) based mobile communication system and will reduce its construction cost. And on the other hand, this deployment topology is a solution to achieve a massive cell-free Multiple-Input Multiple-Output (MIMO) architecture and a cost-effective fronthaul solution. First, a passive optical add/drop multiplexer (OADM) is used to extract and add downlink and uplink signals from the remote access points of one kilometer. Then, a deployment model is developed with version 17 Optisystem software. The results obtained showed that the quadrature amplitude modulation (QAM) does not adapt to this multi-carrier transmission to deploy several AP in series on a single line. Thus, the performance degradation increases when the number of APs integrated on the line increases.

Optical Add-Drop Multiplexer for Dense Wavelength Division Multiplexing System Based on Photonic Crystals

Optical Add-Drop Multiplexer for Dense Wavelength Division Multiplexing System Based on Photonic Crystals

Scaling Optical Interconnects for Hyperscale Data Center Networks

Due to widespread applications and rapid development of cloud computing and Internet services, hyperscale data centers have experienced an unprecedented demand for networking. Data center networks are required not only to handle fast-growing traffic, which doubles almost every one to two years, but also to provide high flexibility and availability to support rapidly changing businesses as well. Therefore, hyperscale data center networks have become one of the main drivers for optical interconnect technologies in recent years. In this article, technologies for scaling optical interconnects for inter-data center interconnects (DCIs) are presented. We first describe hyperscale data center network architectures and requirements, as well as the differences between carrier’s optical transport networks and DCI optical networks, with the main focus on metro-DCI and campus-DCI networks. The scale and fast growth rates of DCI networks require innovations not only in data plane technologies but also in control and management planes as well. In data planes, high-capacity flexible coherent technology, pluggable wavelength-division-multiplexing (WDM) optical transponders, including 100G direct-detection four-level pulse-amplitude modulation (PAM4) and coherent 400ZR/800ZR, and flex-grid and disaggregated reconfigurable optical add-drop multiplexer (ROADM) optical networks are described. Control and management planes are an integral part of optical transport networks and crucial for effectively operating DCI networks. We show that standard protocols, data models, and modular design of a software platform are essential to building a scalable open and disaggregated optical network, which is fundamental to enabling high automation and intelligence in an optical network. An example control and management platform is discussed in detail. With current fiber capacity approaching the nonlinear Shannon limit, challenges to further scale DCI networks and some potential technologies to support ever-increasing traffic growth are discussed at the end.

Packet switching in quantum networks: A path to the quantum Internet

Large-scale quantum networks with thousands of nodes require scalable network protocols and physical hardware to realize. In this paper, we introduce packet switching as a paradigm for quantum data transmission in both future and near-term quantum networks. We propose a classical-quantum data frame structure and explore methods of frame generation and processing. Further, we present conceptual designs for a quantum reconfigurable optical add-drop multiplexer to realize the proposed transmission scheme. Packet switching allows for a universal design for a next-generation Internet where classical and quantum data share the same network protocols and infrastructure. In this quantum networking paradigm, entanglement distribution, as with quantum key distribution, is an application built on top of the quantum network rather than as a network designed especially for those purposes. For analysis of the network model, we simulate the feasibility of quantum packet switching for some preliminary models of quantum key and entanglement distribution. Finally, we discuss how our model can be integrated with other network models toward a realization of the quantum Internet.

Estimating Network Components PDL Using Performance Statistical Measurements

In this paper, we propose a novel technique to estimate the polarization-dependent loss (PDL) of reconfigurable optical add-drop multiplexers (ROADMs) in a terrestrial optical network. The idea is to monitor the signal-to-noise ratio (SNR) distribution induced by the PDL and infer the corresponding PDL values through linear regression. The proposed method relies on only the mean and the variance of the SNR distribution, which can be collected by transceivers and sent to a central controller. The PDL regression algorithm and the methodology to acquire the statistical information are presented. We check the method accuracy for different link configurations. We show that our technique well predicts the ROADMs PDL obtaining an 80% uncertainty cut compared to the worst-case values provided by the datasheet for a ROADM after the transmitter and 40% for a ROADM in transit. We investigate two realistic light paths (LPs) of a network in the linear regime showing that the proposed PDL estimation reduces network SNR margins by more than 0.2 dB on these links. We also show that the nonlinear Kerr effect does not reduce the effectiveness of the method.

96-Channel on-chip reconfigurable optical add-drop multiplexer for multidimensional multiplexing systems

Abstract The multi-dimensional multiplexing technology is very promising for further increasing the link capacity of optical interconnects. A 96-channel silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is proposed and demonstrated for the first time to satisfy the demands in hybrid mode/polarization/wavelengthdivision-multiplexing systems. The present ROADM consists of a six-channel mode/polarization de-multiplexer, a 6 × 16 array of microring-resonator (MRR)-based wavelength-selective switches, and a six-channel mode/polarization multiplexer. With such a ROADM, one can add/drop optical signals to/from any channels of the multimode bus waveguide arbitrarily. For the designed and fabricated ROADM chip, there are more than 1000 elements integrated monolithically, including 96 MRRs, 576 waveguide crossings, 192 grating couplers, 96 micro-heaters, 112 pads, six polarization-splitter-rotators (PSRs), four asymmetric adiabatic couplers and four asymmetric directional couplers. For any channel added/dropped with the fabricated ROADM, the on-chip excess loss is about 5–20 dB, the inter-mode crosstalk is <−12 dB, and the inter-wavelength crosstalk is <−24 dB. The system experiments are demonstrated by using 10-GBaud quadrature phase shift keying (QPSK) signals, showing that the observed optical signal noise ratio (OSNR) power penalties induced by the ROADM are less than 2 dB at a BER of 3.8 × 10−3.

Supervised graph convolution networks for OSNR and power estimation in optical mesh networks

The optical signal-to-noise ratio (OSNR) and received optical channel power are critical parameters in determining the quality of transmission. The OSNR and received optical channel power are influenced by network impairments such as fiber loss, amplified stimulated emission noise, and nonlinear impairments. Furthermore, environmental effects and routing, modulation, and spectrum assignment schemes influence the OSNR and thus the reach of the optical channels. These impairments and effects vary with the spectral loads that are hard to predict in brownfield networks. Several deep neural network (DNN)-based methods have been explored to estimate the OSNR and nonlinear noise. However, these methods ignore the network topology. This paper bridges this gap by leveraging supervised graph convolution neural networks (GCNs), which operate directly on graphs for OSNR and received power estimation in an optical mesh network. We also develop and implement a novel graph windowed neural network (GWinN) to reduce the over-smoothing effects of a GCN and thus learn localized behaviors like fiber cuts. We apply a DNN, GCN, and GWinN in practice to a testbed of 8 reconfigurable optical add-drop multiplexers and 22 amplifiers. Our procedure accurately estimates the OSNR with a prediction error mean and a standard deviation of ( − 0.02 d B , 0.35 dB) for a reference OSNR ranging from (16 dB) to (24 dB).

Prospects and Challenges of Photonic Switching in Data Centers and Computing Systems

This Tutorial will discuss the motivation, benefits, and challenges of photonic switching in data centers and cover prospects of future data centers involving emerging new technologies and cross-layer solutions. The primary motivation for considering photonic switching in data centers rises from the need for energy-efficient and scalable intra-data center networks to meet rapid increases in data traffic driven by emerging applications, including machine learning. The data traffic inside the data centers (East-West traffic) is typically significantly greater than that of the traffic coming in and out of the data centers (North-South traffic) (Benson <i>et al.</i> 2010). To accommodate such traffic, today&#x0027;s large-scale data centers employ cascaded stages of many power-hungry electronic packet switches interconnected across the data center network in fixed hierarchical communication topologies. These electronic switches add significant latency and energy consumption while limiting the communication bandwidth. On the other hand, photonic switches can, in principle, support interconnections at very high data rates on many parallel wavelengths while keeping their energy consumption nearly independent of the switch port bandwidth. Numerous research papers have predicted significant benefits in scalability, throughput, and power efficiency from deploying photonic switches in data centers. However, photonic switching is not yet widely deployed in commercial warehouse-scale data centers at the time of writing this Tutorial due to significant challenges. They are related to 1) cross-layer issues involving control and management planes together with data integrity during switching, 2) scalability to &gt;5000 racks (&gt;a quarter-million servers), 3) performance monitoring required for reliable operation, 4) currently existing standards allowing limited power margin (3 dB), and 5) other practical (technology-dependent) issues relating to polarization sensitivity, temperature sensitivity, cost, etc. In telecom, commercial deployments of reconfigurable optical add-drop multiplexers (ROADMs) (Lightwave, 2003)&#x2013;(Perrin, 2015) also had faced similar challenges and took place nearly ten years after the first research network testbed demonstrations in 1997 (Garrett, 2015) with fully implemented network control and management planes. In data centers, the challenges are far more significant due to the scale of the network and the dynamicity of the traffic. We will discuss possible solutions for future data centers involving cross-layer methods, new topologies, and innovative photonic switching technologies. In particular, the Tutorial broadly surveys state-of-the-art photonic switching technologies, architectures, and experimental results, and further covers the details of arrayed-waveguide-grating-router-based switch fabrics offering hybrid switching methods with distributed control planes towards scalable data center networking.

Benefits of quasi-continuous symbol rate tunability in links constrained by ROADM filtering

As modern optical networks continue evolving toward higher symbol rates while supporting tight channel spacing, penalties due to optical filtering incurred by transmission through reconfigurable optical add-drop multiplexers (ROADMs) can become significant within the overall link budget. For this reason, filtering impairments must be realistically assessed and accurately modeled, particularly in terrestrial networks where signals may traverse many ROADMs. Given that modern transceivers can provide flexible modulation options and most ROADMs in use today support flexible grid operation, there remains the challenge of optimizing performance and/or capacity while managing these multiple degrees of freedom. In this work, we use real-time transceivers with a tunable symbol rate up to 72 GBaud and variable modulation up to 64QAM to experimentally investigate the impact of ROADM filtering on the optimum operating condition required to maximize channel capacity or margin. We also present a simplified model for considering the filtering impairment as an additive noise term and use it to predict performance in other scenarios. It is shown that symbol rate tunability in steps below 5 GBd is required to maximize the margin for a given link passband. Finally, we explore the difference in performance for filtering conditions with passband narrowing on both sides of the signal, assuming individual wavelength routing, or only on one side of the signal, assuming multi-carrier routing.

Disaggregation and cloudification of metropolitan area networks: enabling technologies and impact on architecture, cost, and power consumption [Invited

The deployment of the new 5G system and ultra-broadband access networks, on one side, and the advent of network disaggregation and cloudification paradigms, on the other, are producing disruptive changes in metropolitan area networks (MANs). The new central offices will be more like small data centers than traditional Telco-type central offices. The presence of general-purpose servers, in partial or total replacement of specialized equipment or cards, and also hosting new functions and services, significantly changes the way the central office is organized and how the equipment internetworks. After having defined a reference architecture named cloudified-MAN, the paper briefly introduces the enabling technologies that include information technology (IT) systems, packet switches, and optical equipment. A disaggregated model is applied to all layers and systems. In the optics field, special insight on high data rate (up to 1.6 Tb/s) transceiver technology developed by the TERIPHIC project is provided as an example of low-cost and low-power technology. Through a quantitative techno-economic case study on a realistic network scenario aimed at short-term and medium-term time horizons, the paper provides indications on what could be the impact of the new architecture on costs and energy consumption. The main outcome of the study is that the IT and optical parts are those with the highest fractions of overall cost, and specifically for the optics, the cost of wavelength division multiplexing transceivers increases significantly in the medium term due to the demand for high-capacity connections ( &gt; 400 G b / s ), while the line system cost (reconfigurable optical add-drop multiplexer) decreases in percentage. Another significant result is that in terms of energy consumption, the IT component has been found to be the dominant one (70%–75% of the total), followed by packet devices (about 20%–25%) and optical systems, the latter with significantly lower percentage values (less than 5%) in all cases analyzed.

Design and evaluation of a reconfigurable optical add-drop multiplexer with flexible wave-band routing in SDM networks

Space-division multiplexing (SDM) is expected to increase the capacity of photonic networks. Reconfigurable optical add-drop multiplexers (ROADMs) for SDM-based networks must have high scalability in terms of port count. However, the ROADM architecture adopted in present networks cannot support large numbers of ports due to the limited port count of wavelength-selective switches. In this paper, we propose a ROADM architecture composed of space switches and wavelength-routing switches. Space switches have lower per-port cost than wavelength-routing switches. However, space switches cannot route optical paths on a wavelength basis, unlike wavelength-routing switches. By combining these two types of switches, the ROADM port count can be cost-effectively expanded virtually unlimitedly. Numerical simulations show the routing performance of our ROADM; they elucidate that the maximum fiber increment penalty is about 5% compared to the reference scheme. Experiments detail the transmission performance of the ROADM, where 32 Gbaud DP-4QAM/8QAM/16QAM signals are aligned on a 50 GHz grid, are examined.

Optimized IP-over-WDM core networks using ZR+ and flexible muxponders for 400 Gb/s and beyond

We compare IP core networks in three scenarios—those using point-to-point optical links between adjacent nodes with ZR/ZR + based pluggables (called hop-by-hop routing), leveraging reconfigurable optical add–drop multiplexers (ROADMs) with ZR + pluggables, and ROADMs using performance-optimized flex-coherent muxponders—from hardware count, wavelength utilization, and latency perspectives. Specifically, we compare the number of components/router ports used to build an IP-over-WDM core network and attempt to understand if off-the-shelf pluggables are indeed suitable candidates for replacing custom-designed coherent muxponders (which we conveniently term as flexponders) in core networks. To this end, we build a constrained optimization model whose goal is to minimize the number of components used by optimally placed regenerators—the single biggest variable metric in core IP-over-WDM network design that eventually impacts total hardware count. Results show a 75% decrease in transceiver count for flexponders developed from coherent transponders/muxponders when compared to ZR + pluggables in routers with hop-by-hop routing and a 51% decrease in flexponders when compared to using ZR + pluggables in routers across ROADMs in a U.S. core network and a pan-India core network with 5 and 3 years of increasing demands, respectively.

Planning of Optical Connections in 5G Packet-Optical xHaul Access Network

One of the main challenges in dense 5G radio access networks (RANs) is provisioning of low-cost connectivity between a large number of antennas, located at remote sites, and a central site (hub) in which baseband processing functions are performed. Packet-switched Ethernet and wavelength division multiplexing (WDM) are two principal transport network technologies enabling the reduction of the demand for direct optical fiber connections between the antennas and the hub. Whereas Ethernet allows for statistical multiplexing of multiple xHaul (fronthaul/midhaul/backhaul) flows and their aggregation in a high-capacity transmission link, WDM makes it possible to establish a number of such links (using different wavelengths) in a single optical fiber. Additional savings in the amount of fibers required can be achieved by means of optical add-drop multiplexers (OADMs) that allow for obtaining access to unused wavelengths by intermediate remote nodes, whenever the capacity on the WDM system is not fully utilized by the end remote node. In this work, we focus on the problem of planning optimal fiber connections, including the placement of OADMs for a set of wavelength demands at remote sites, with the aim of minimizing the amount of fibers used in a packet-optical xHaul access network carrying 5G traffic. We consider a passive WDM system in which the maximum transmission distance, estimated using an optical power-budget model, depends on the number of OADMs that are present on the transmission path. To formulate and solve the optimization problem, we make use of integer linear programming (ILP). We apply the ILP model in network analysis. In particular, by means of numerical experiments performed for two different network topologies, we study the impact of traffic load (in terms of the number of requested wavelengths) and optical multiplexer loss on the number of transmission paths that have to be established in the network. Obtained results show that the savings in fiber connections of up to 65% can be achieved in a packet-optical xHaul network if OADMs are used when compared to the scenario without OADMs.

Analysis of an optical SDM ring network model for applying MIMO processing

Optical multiple-input multiple-output (MIMO) technology is introduced into optical space division multiplexing (SDM) communication as an effective solution for the crosstalk issue. Unfortunately, this application is limited because all the channel information over the transmission link must be transmitted to MIMO receivers for complete crosstalk compensation, which indicates difficulty in applying MIMO to the transmission path via optical switches. We newly propose that one of the solutions to this limitation is a ring network because all channel information can be maintained in the ring. In particular, a typical ring network model comprising optical add-drop multiplexers (OADMs) and multicore fibers (MCFs) is introduced, and the ability of MIMO to compensate for capacity degradation because of the inter-core crosstalk is investigated. Moreover, the protection function, which is essential for the ring network, is also evaluated under the fiber and fiber core failures. As per the simulation results, although the protection operation affects the transmission capacity, the capacity loss is suppressed in the event of a failure to < 5%. We believe that these results provide useful information for designing SDM-based optical networks.

Cognitive and autonomous QoT-driven optical line controller

In the direction of disaggregated and cognitive optical networks, this work proposes and experimentally tests a vendor-agnostic optical line controller architecture capable of autonomously setting the working point of optical amplifiers to maximize the capacity of a ROADM-to-ROADM (reconfigurable optical add–drop multiplexer) link. From a procedural point of view, once the equipment is installed, the presented software framework performs an automatic characterization of the line, span by span, to abstract the properties of the physical layer. This process requires the exploitation of monitoring devices such as optical channel monitors and optical time domain reflectometers, available, in a future perspective, in each amplification site. On the basis of this information, an optimization algorithm determines the working point of each amplifier to maximize the quality of transmission (QoT) over the entire band. The optical line controller has been experimentally tested in the laboratory using two different control strategies, achieving in both cases a homogeneous QoT for each channel close to the maximum average and an excellent match with respect to emulation results. In this framework, the Gaussian noise simulation in Python (GNPy) open source Python library is used as the physical model for optical propagation through the fiber, and the covariance matrix adaptation evolution strategy is used as an optimization algorithm to identify properties of each fiber span and to maximize the link capacity.

Impact of System Parameters of Optical Fiber Link on Four Wave Mixing

Background: The invention of WDM technology in optical communication system has completely revolutionized the telecomm industry through its high data carrying capacity and efficiency of transmission. Advanced optical modulation formats with high spectral efficiency, advanced components like Reconfigurable Optical Add Drop Multiplexers (ROADMS), OXC, and large bandwidth requirements contributed significantly in existence of dynamic, flexible translucent and transparent networks. In these networks, it is common practice to increase the power levels as much as possible to overcome the power penalty effects and better transmission, but this introduces several non-linear impairments in the link and hence degrades the quality of signal flowing. These impairments arise when several high strength optical fields of different wavelengths interact with molecular vibrations and acoustic waves. The different non-linear impacts include Self Phase Modulation (SPM), Cross Phase Modulation (XPM), Four Wave Mixing (FWM) and scattering effects like Stimulated Raman Scattering (SRS), Stimulated Brillouin Scattering (SBS). The main cause of these impairments is variation in refractive index of fiber (also called Kerr effect) due to intensity of signal flowing through fiber. Due to the degradation impact posed by these impairments, it is crucial to analyze their cause, their influence on system performance and mitigation techniques so as to improve the overall quality of transmission. The monitoring of impairments is quite a challenging task due to their dependency on time, present state of network, signals flowing in adjoining channels and fibers. Objective: The present work aims to identify and describe the role of FWM in optical networks. The mathematical model of FWM is studied to know the parameters influencing the overall impact on system performance. The power of optical source, channel spacing, distance of transmission and presence of dispersion are considered as key factors influencing FWM power being developed. Their impact on FWM power and hence, FWM efficiency is calculated. In addition, the influence of FWM on Quality of transmission is quantified in terms of BER and Q-factor. Methods: The analysis is done through a two-channel transmitter system with varied power, channel spacing, distance of transmission and presence of other degradation factors (dispersion) is taken into account. The corresponding optical spectrums are analysed. Results: In this paper, the non-linear impairment FWM posing degradation effect on the signal quality has been discussed. The basics involved are presented along with the mathematical model. It has been found that FWM results in power transfer from one channel to generation of new waves which may lead to power depletion and interference. The new waves generated depend on the number of wavelengths travelling in the fiber and channel spacing. The influence of FWM on system performance is presented in terms of BER and Q-value. Conclusion: It has been concluded that the increased power of transmission and decreased channel spacing are the crucial factors increasing the magnitude of FWM and need to be closely monitored. On the other hand, increased distance of propagation and presence of certain level of dispersion leads to decrease in FWM power. Therefore, if selected carefully, they may act as source of FWM mitigation without requiring any external compensating device.

A 1×20 MEMS mirror array with large scan angle and low driving voltage for optical wavelength-selective switches

Wavelength-selective switches (WSSs) are the core devices for reconfigurable optical add-drop multiplexers (ROADMs) that are essential in all-optical networks. WSSs based on microelectromechanical (MEMS) mirror arrays exhibit high potential due to their high switching speed and large port counts, but there still exist challenges to achieve large optical scanning angle at low voltage. This paper presents a 1×20 MEMS mirror array (MMA) design that can scan over 10° in both axes at less than 5 V. The MMA is based on an unique electrothermal bimorph actuator design that can perform 2D scanning and its fill factor reaches 96 %. The 1×20 MMA design has been successfully fabricated using a combined bulk- and surface- micromachining process. The size of each mirror plate is 240 μm by 500 μm. The measured maximum optical scan angles in the x-axis and y-axis are 29.3° and 11.8°, respectively, at 3 V. The measured response time is about 3.7 ms. The thermal crosstalks between adjacent electrothermal actuators are about 10 %. Long-term stability tests show that the angular drifts of the individual mirrors are about 0.002°/h.

PERFORMANCE IMPROVEMENT OF NONLINEARITY COMPENSATION USING OPTICAL PHASE CONJUGATION FOR METRO DWDM SYSTEMS WITH OPTICAL ADD DROP MULTIPLEXERS

Wavelength Division Multiplexing (WDM) has been widely applied in optical adddrop multiplexing (OADM) fiber metro systems with an increasing number of wavelength channels and sufficiently narrow channel spacing. The problem that appears here is the signal distortion produced by dispersion and nonlinearity effects. To cope with these effects, many compensation solutions have been introduced, such as digital compensation (DC) solutions on technology of digital signal processing (DSP) or optical phase conjugation (OPC) on alloptical domain. In fact, research on using OPC to compensate for dispersion and nonlinearity in metro transmission systems are incomplete, especially with large number multiplexing systems. In this paper, we propose to use OPC for dispersion and nonlinear compensation in DWDM metro systems. Specifically, we focus on two common types of modulation signals: QPSK and 16-QAM; operations in 16- and 32-channel dense WDM system. The simulation results show that the system quality increases significantly when using OPC to compensate for dispersion and nonlinearity, e.g. when transmitting 16-QAM signals in a 32-channel system passing through 20 add/drop nodes, the gain of Q factor is greater than 2 dB.

Vibration Sensing for Deployed Metropolitan Fiber Infrastructure

The optical fiber deployed in the metropolitan infrastructure carrying high-speed WDM traffic data is used also for network surveillance, identifying and localizing the onset of vibrations and dynamic stress events, potentially dangerous for the integrity of the fiber telecom asset. A coherent approach is adopted in a counter-propagating interferometer to develop a sensing system exploiting the ring layout of the metro network. With respect to usual dual Mach-Zehnder interferometric sensors, where typically the reference arm is kept isolated, in our arrangement we use as reference arm a fiber inside the same deployed cable, minimizing the impact of the city strong environmental noise. Preliminary assessments of the proposed sensing solution have been experimented in a metro ring of 32-km standard single-mode fiber deployed in the city of Turin, Italy, devoted to telecom applications. The sensor is demonstrated to operate preserving the coexistence with the high-speed WDM telecom traffic. With respect to complex and expensive sensing solutions based on pulsed phase-OTDR techniques, in the present case continuous-wave counter-propagating optical signals are exploited and the spatial resolution in dynamic event localization depends only on the receiver sampling rate: in our implementation, by adopting off-the shelf low-cost commercial 20 MS/s sampling boards, 10-meter spatial resolution is experimentally achieved. The proposed sensing solution provides significant added value to the deployed metro fiber infrastructure and the same interferometric approach based on the counter-propagating layout can be easily exploited in metro networks even in case of presence of optical nodes equipped with reconfigurable optical add-drop multiplexers useful for WDM traffic routing.