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Illuminating telecoms

The reach and scope of the telecommunications industry is expanding at a dizzying rate, with photonics an integral part of developments. Tim Gillett reports

Parents of teenage children will be familiar with their doubts that there was, in fact, a life before the internet.Even those who have witnessed the move from dial-up technology to super-fast connections can find it difficult to imagine how we communicated effectively just 15 or 20 years ago.

Telecommunications continue to evolve apace – with a slew of massive projects around the world aiming to ensure that people can communicate faster, more reliably and more efficiently.

Huge projects in Western countries bear testament to this, with optical fibres and photonics increasingly important. Data shows a fast increase in broadband adoption in OECD countries – with steep acceleration in the use of fibre.

Figures for the international economic organisation of 34 countries, for the year to December 2012, show a steady increase in fibre adoption – with the share of fibre subscriptions in fixed broadband increasing to 14.9 per cent (up to 48.7 million fibre broadband lines).

Luxembourg (324 per cent), Austria (193.9 per cent), United Kingdom (169.9 per cent) and Switzerland (149.6 per cent) had the strongest annual growth in fibre, while seven countries had growth rates above 100 per cent year-on-year and 11 countries more than 50 per cent. Broadband providers are investing in fibre networks to differentiate themselves against their competitors.

The rush towards a world connected by fibre is not restricted to western, urbanised areas. Huawei, the Chinese ICT giant, is involved in plans to link Pakistan and China fibre-optically – including laying an 800-kilometre stretch of cable alongside the world-famous Karakoram Highway.

The proposed project is part of an $18 billion plan to better link the two countries with a new road and rail track aimed at creating an efficient Pakistan-China ‘economic corridor’. The cable would link Rawalpindi, in Pakistan, to the Trans-Asia Cable in China – giving both countries different options for international telecom traffic.

Efficiency and reliability

Maintaining the march towards a world connected by photonics largely relies on two essentials – speed and reliability – and organisations worldwide are developing products dedicated to this.

With fibres covering ever-longer distances and transmitting exponentially greater bandwidths, efficiency and reliability are paramount. A huge contributory factor to both of these qualities is in the linking of the fibres themselves, and the extent to which the cores of the fibres are aligned with each other at connector interfaces.

Neil Ashby, UK sales manager at Diamond SA, explains: ‘Very long fibre transmission lengths have been in operation for a long time but, critically, bandwidth demand is increasing dramatically, and so are the requirements for first-class fibre optic connectors. Diamond has developed and successfully established on the market the “0,1dB class”, virtually the “Grade A” connector anticipating international standardisation.

There are, inevitably, optical losses along the length of any fibre when dealing with long distances, but once the fibre has been installed this is taken as read to a certain extent. But Ashby says the most critical part in terms of efficiency and reliability remains the connection between fibres, where most power and optical performance is typically compromised.

He explains: Although fibres are manufactured to very high tolerances – we are talking about microns – all the different intolerances can add up.’ With the core of a singlemode fibre being typically 9μm diameter, tolerances on the fibre core and cladding diameters and concentricities, as well as in the fibre alignment ferrule and split sleeve, mean that mis-alignment of the fibre cores is inevitable, leading to variable optical performance and repeatability across the connector interface.

Line of least resistance

Diamond manufactures a two component ferrule composed of a hard outer tube (Zirconia ceramic) and a malleable, titanium insert. The ferrules are precision-ground and polished to a dimensional tolerance of 0.5μm or less. This ferrule construction allows Diamond’s unique active core alignment process, which involves a two-stage crimping process:

First crimping process: the manufacturing process is initially similar to that used with conventional connectors: the fibre is stripped, cleaned and then inserted into the ferrule with epoxy. Before the epoxy cures, a circular crimping tool with a wedge profile is used to deform the metal surrounding the fibre. The tool closes the hole to match the fibre cladding and leaves a circular impression on the ferrule end face. This process moves the fibre into the centre of the ferrule with an eccentricity of 2μm or less.

 Second crimping process: after the epoxy cures, the assemblers cleave and polish the fibre, then mount the ferrule in the Diamond crimping instrument. Visible light from an incoherent source is launched into the opposite end of the fibre.

A charge-coupled-device (CCD) camera captures the light exiting the fibre, displaying it on a video monitor as a point image. Two calibrated marker lines on the monitor permit the operator to quantify any misalignment. Eccentricities greater than 0.125μm can be corrected in a second crimping step using an arc-shaped tool with a wedge profile. Furthermore the fibre tilt angle is also taken under control.

By insuring that each parameter is controlled during termination, the ‘0.1dB Class’ can be guaranteed for randomly mated connectors.

Lightest of the light

While tweaking systems for performance is still a focus of attention, researchers at the Universities of Bath and Exeter say the use of graphene in telecoms could increase internet speeds 100 times.

Graphene is just one atom thick, but remarkably strong. Already dubbed a miracle material due to its strength, lightness, flexibility, conductivity and low cost, it could now enter the telecommunications market. Researchers from the Centre for Graphene Science at the two universities have shown ultra-short optical response rates using graphene, which they say could create a revolution in telecommunications.

Ordinarily, optical switches respond at rate of a few picoseconds; around a trillionth of a second. Physicists have observed the response rate of an switch using ‘few layer graphene’ to be around 100 femtoseconds – nearly 100 times faster.

Researcher Enrico Da Como explained: ‘We’ve seen an ultrafast optical response rate, using few-layer graphene, which has exciting applications for the development of high speed optoelectronic components based on graphene. This fast response is in the infrared part of the electromagnetic spectrum, where many applications in telecommunications, security and also medicine are currently developing and affecting our society.’

Co-director of the Centre for Graphene Science at Bath, Simon Bending, added: ‘The more we find out about graphene, the more remarkable its properties seem to be. This research shows that it also has unique optical properties which could find important new applications.’

The need for speed

The USA’s Defence Advanced Research Projects Agency (DARPA) has developed a hollow optical fibre that it says can transmit light – and data – 30 per cent faster than conventional broadband fibre.

DARPA says the design, using a hollow, air-filled core, improves performance by forcing light through channels of air, instead of the glass around it. It says the fibre (pictured opposite) is the first to demonstrate single-spatial-mode, low-loss and polarisation control – key properties for advanced military applications such as high-precision fibre-optic gyroscopes for inertial navigation.

DARPA-funded researchers, led by Honeywell International, developed the technology. ‘Previous instantiations of hollow-core fibre have shown these high propagation speeds, but they weren’t able to do so in combination with the properties that make it useful for military applications,’ said Josh Conway, DARPA programme manager.

‘The real breakthrough with Cougar fibre is that it can achieve a single-spatial-mode, maintain polarisation and provide low loss, while keeping more than 99 per cent of the beam in the air.

‘While we are still working on integrating this new technology into a gyroscope, the fibre itself is revolutionary,’ added Conway. ‘This type of technology may also lend itself to other types of high-power sensors and additional applications where intense optical beams are required.’

About the author

Tim Gillett is the editor for Research Information and contributor to Fibre Systems.

You can contact him at tim.gillett@europascience.com or on +44 (0) 1223 275 469.

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