Multi-core fibres to reduce per-bit costs

ADOLFO CARTAXO

Professor  Iscte Technologies and Architecture

Director and Researcher  Institute of Telecommunications – IT Iscte

A project is underway that aims to impetus the development of MCF—Multi-Core Fibre technology based on the experience and knowledge of the Optical and Photonic Communications group of the Institute of Telecommunications. How? Implementing a test bench in the Lisbon Metro network and the Iscte campus.

This is yet to be a formal research project. At what stage is the proposal?

We are at an embryonic stage. To have a test bed, we must have, in this case, the multi-core fibre in a real environment—it's the key to a test bed! There are advanced contacts with companies, especially with a European manufacturer of this fibre, Heraeus Quarzglas GmbH & Co, to make a multi-core fibre cable available.
We need this manufacturer to contact the companies that produce the cables because, in addition to the multicore fibre, a cable must be made available that ensures, among other things, the mechanical robustness to install the fibre without damaging it. We also need an institution in Portugal that provides the infrastructure, that is, the space and real environment necessary to install the cable and carry out the communication tests. We agree with Lisbon Metro to install a multi-core fibre cable on the yellow line.
Everything is quite time-consuming. The process started in July 2023. I first spoke with a fibre manufacturer who showed openness to collaborate. We conveyed this availability to the Rectory of Iscte, and the latter, through the Vice-Rector for Research and Technological Modernisation, Professor Jorge Costa, showed openness to support the project as far as possible.

In concrete, what is tested? What is your research proposal?

We propose this test bed to test the quality and transmission limitations of various types of multicore fibre in a real environment. This follows what we have been experimenting with for a few years in our laboratory at Iscte. In the test bed, we will make tests and measurements with various types of multicore fibre in a real environment, which constitutes innovation. This type of fibre makes it possible to substantially increase the transmission capacity and bandwidth because it is multi-core — instead of the fibres now installed, which only have one core. Therefore, we can multiply our capacity in each with a fibre with many cores. For example, if we have a fibre with two cores, we double the capacity, and so on.

Is there a proportional increase in transmission capacity concerning the number of cores?

Multicore fibres can be manufactured with very different numbers of cores. For example, the one we have in our lab has 19 cores; the ones of the manufacturer who is with us in the project have seven and four cores, respectively.

Our test bed will be the first demonstration to use MCF in an environment of significant stress for the transmission of information

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FIRST TELECOMMUNICATIONS SYSTEM WITH MCF

The first telecommunications system with MCF was presented for a submarine cable connection in the Pacific, with two-core optical fibre.

The Japanese company Sumitomo Electric is responsible for the world's first mass production of MCF, which began in 2023 and considers a fibre with two cores.

And why is it so important to test in a real environment?

First, we have a transmission medium, in this case, a fibre, which has different characteristics from the fibres used in current networks. Only one submarine cable was introduced last year, connecting Taiwan, the Philippines and the United States*, with a dual-core fibre. It is used for the transmission of general content. Secondly, the performance of the multicore fibre depends on the environmental conditions, particularly the mechanical stress to which the fibre is subjected. This stress is very different in the laboratory (found in coils with diameters of a few tens of centimetres) and in commercial networks where it will be installed in ducts or suspended on poles.
The bits matter in fibre transmission, as all information is encoded in bits, regardless of the source, be it movies, music, telephone conversations or Internet access. What matters to us is that fibre can transmit more bits per second.
The idea is to reduce cost per bit. If we increase the number of cores in the same fibre and space, a very small cable can have tens or hundreds of fibres! — we'll be streaming more bits. If the installation cost is the same, you will have a lower final price. That is what has made this research progress!

Is the research, therefore, aimed at increasing capacity while reducing transmission costs?

If we increased the capacity and the cost was proportional, it wouldn't be economically worth it. Currently, the fibres installed and commercially exploited only have one core. The idea is to increase complexity but with the prospect of reducing costs.
L'Aquila has a similar test bed but much smaller dimensions than the one we intend to carry out. L'Aquila was shaken by an earthquake a few years ago. They had to restructure all the urban infrastructures, take the opportunity to make new ducts and install a test bed there.
Our test bed, however, will be the first demonstration of MCF in an environment of significant stress for information transmission and with ranges that can reach hundreds of kilometres.
In the Metro tunnel, pressure is variable in time due to the air tunnel, the movement of the train, and the mechanical pressure of vibration. It is recognised that the transmission characteristics of multicore fibres are highly affected when the fibre is under mechanical stress. This research contributes to developing this type of fibre in potentially adverse transmission situations.

The importance of test beds

Which team is developing the work at this early stage?

Currently, the team comprises three professors from Iscte and IT members who are doing simulation and experimental work in the laboratory. My activities in the project are management. We have a problem: we need more people due to their teaching activities. Investing in such an infrastructure is inconsistent with part-time research actions. This team will not do the installations, but we are responsible for the transmission and quality measurement experiences. Having at least one researcher (preferably two) with experience in experimental work in Optical Communications working full-time is necessary. In other words, we may not be outdone in the installation process, but we risk being outdone in getting results.

Does the funding of a research proposal like this fall under any European funding line?

Thinking about L'Aquila's experience, as far as I know, this project also only has national funding because the infrastructure essentially serves Italy. In the face of a project like this, for example, the Innovation Agency and the Foundation for Science and Technology could provide a supportive response.

In the test bed, we will make tests and measurements with various types of multicore fibre in a real environment, which constitutes the innovation. This type of fibre makes it possible to increase the transmission capacity substantially.

Is there a cost estimate?

We're at that stage. This is a time-consuming process, although, by its nature, the project must be executed quickly. Otherwise, it loses interest and value.
The costs that we can estimate for now are related to the fibres' costs, which are added to the costs of manufacturing the compatible cable, the installation of which incurs costs that we have already estimated. In addition to the cable connecting one point to another, we will need access points to test different transmission situations. All this implies additional equipment costs, and as it is state-of-the-art technology, it is equipment with a considerably high price. The additional costs of human resources have also already been estimated.

Do you already have an execution schedule?

We would like to have the test bed installed by the end of this year. However, the partner companies propose that instead of having just one type of multicore fibre, we test several types of fibre in different situations, which is understandable. It would be great to have the test bed available in a year. The cable will connect to our (IT) laboratory in Building 2 of Iscte; from there, it will go through a duct to the Metro network and then circulate on the yellow line, which passes a hundred meters from the IT laboratory at Iscte. After circling along the yellow line, it returns to our laboratory. This is how we can do the checks without going to the Lisbon Metro facilities.