Exploring Lisbon’s LUMIRing: The Global Hub for Multicore Fibre Technology Testing

The LUMIRing project aims to advance multicore fibre technology by embedding a testbed into the subway system, addressing challenges of real-world deployment while positioning Portugal at the forefront of optical communications innovation

The global telecommunications sector is facing unprecedented demand for data transmission, driven by cloud computing, artificial intelligence (AI), high-definition video streaming, and the Internet of Things (IoT). Conventional optical networks based on single-core fibres (SCFs) are approaching their capacity limits, motivating the adoption of Space-Division Multiplexing (SDM) as a key technology for future optical infrastructures. Among SDM solutions, Multicore Fibre (MCF) enables multiple spatial channels within a single optical fibre, significantly increasing transmission capacity. However, widespread deployment requires overcoming challenges such as operation in harsh urban environments, precise fibre splicing, and minimising inter-core crosstalk.

The LUMIRing (Lisbon Underground Multicore Fibre Ring) project, coordinated by Iscte – Instituto Universitário de Lisboa, addresses these challenges through a pioneering real-world deployment of MCF technology. By integrating a state-of-the-art multicore fibre testbed into Lisbon’s underground metro infrastructure, LUMIRing bridges the gap between laboratory research and operational networks. Inaugurated in November 2025, it is the longest and most comprehensive terrestrial-deployed MCF testbed reported to date, establishing Portugal and its international partners at the vanguard of next-generation optical communications.

Project background and core mission

The LUMIRing project was created to validate, characterise, and operate a real-world MCF infrastructure under practical deployment conditions. Led by Principal Investigator Adolfo V. T. Cartaxo of the Department of Information Science and Technology, Iscte – Instituto Universitário de Lisboa, the project aims to demonstrate the feasibility of MCF technology beyond controlled laboratory environments.

To achieve this, LUMIRing deployed an experimental MCF testbed along the Yellow Line of the Lisbon Underground (Metropolitano de Lisboa). Subway tunnels provide a demanding environment for optical fibres due to continuous vibrations from train traffic, temperature variations, and high humidity. Demonstrating reliable MCF operation under these conditions provides valuable evidence for future deployment in operational optical networks worldwide.

The project followed a structured implementation strategy with three main phases. The first focused on acquiring and specifying specialised fibres, subway cables, and optical components, requiring close collaboration with international manufacturers because of the limited standardisation of MCF technologies. The second phase involved installing the MCF cable along the metro line, followed by an extensive splicing campaign to ensure operational readiness. The third phase is dedicated to the continuous characterisation of the deployed infrastructure, including measurements of attenuation, inter-core crosstalk, chromatic dispersion, polarisation mode dispersion, and high-capacity transmission experiments.

Throughout all phases, project management, dissemination, and exploitation activities have been conducted in parallel to maximise the visibility of the LUMIRing testbed, foster international collaborations, and accelerate the adoption of MCF technologies for next-generation optical communication networks.

Technical specifications of the LUMIRing testbed

The scale and engineering diversity of the LUMIRing infrastructure distinguish it from existing terrestrial optical testbeds. Its cable architecture is specifically designed to support both conventional SCF experiments and advanced MCF research across multiple optical bands.

The deployed cable contains 144 optical fibres arranged in 12 dry loose tubes, each holding 12 fibres. It combines 74 MCF strands, representing five different fibre architectures, with 70 standard single-mode SCF strands. Together, the MCF deployment provides more than 1,800 km of cumulative multicore infrastructure, while the SCFs contribute over 1,700 km, enabling direct performance comparisons between conventional and multicore technologies.

The five MCF variants (MCF-A to MCF-E) all have a standard 125 µm cladding diameter, ensuring compatibility with existing optical infrastructure, installation procedures, and handling tools. Core pitches range from 37 µm to 41 µm. MCF-A and MCF-B contain seven cores, whereas MCF-C, MCF-D, and MCF-E have four cores. Three variants use trenched core designs, while two are non-trenched, allowing systematic evaluation of crosstalk mitigation techniques. Trenched fibres confine light more effectively, significantly reducing inter-core crosstalk; for example, MCF-C achieves approximately −59 dB/km at 1550 nm.

The extensive deployment of MCF-D, comprising 28 strands and more than 700 km of cumulative fibre, enables long-haul transmission experiments by looping optical signals through multiple cores and strands. This configuration allows researchers to emulate transcontinental transmission distances within the LUMIRing, providing a unique platform for evaluating next-generation optical communication systems.

Deployment, cable engineering, and splicing operations

Deploying the LUMIRing infrastructure in the Lisbon Underground required specialised cable engineering and meticulous field operations to ensure reliable performance under demanding real-world conditions.

The multicore cable was custom-designed for the harsh subway environment through an international collaboration. Its design includes 12 dry loose tubes housing 144 optical fibres, protected by corrugated steel armouring against moisture and mechanical damage, and a Low Smoke Zero Halogen (LSZH) outer sheath with a B2ca-s1a,d1,a1 CPR fire rating to meet strict subway safety requirements.

The LUMIRing testbed forms a 25.5 km ring along Lisbon Underground’s Yellow Line, incorporating nine splicing locations and five main access points, including the Optical Communication Laboratory (LCO) at Iscte, which serves as the network’s monitoring and experimentation hub.

A major achievement was the MCF splicing campaign. Unlike conventional fibre splicing, MCF requires precise alignment of multiple cores in all spatial and rotational axes. Using specialised equipment, the project team completed nearly 1,000 MCF splices and 700 SCF splices, establishing the largest terrestrial MCF splicing campaign reported to date. The experience provides valuable benchmarks for large-scale installation, splice-loss optimisation, and the large-scale deployment of next-generation optical communication networks.

Connectivity infrastructure and experimental flexibility

The value of a testbed depends on its flexibility. To enable rapid experiment reconfiguration without modifying or risking damage to the underground cable infrastructure, LUMIRing incorporates an advanced connectivity interface at its access points.

This flexibility is provided by strategically deployed Fan-In/Fan-Out (FIFO) devices, which separate the multiple cores of an MCF into individual SCF pigtails compatible with conventional laboratory equipment.