Tackling the coming network crunch

A European Union research project is looking at ways to expand by one-hundredfold the capacity of an optical fibre. The project will need to develop techniques to exploit multimode and specialist fibre, and if successful, will require new fibre deployments in the network.

"In the end you run out of the ability to transmit more information along a single-mode fibre"

Ian Giles, Phoenix Photonics

The project, dubbed MODE-GAP, is part of the EC's Seventh Framework programme, and includes system vendor Nokia Siemens Networks (NSN), as well as optical component, fibre firms and several universities.

Current 100 Gigabit-per-second (Gbps) dense wavelength division multiplexing (DWDM) systems are able to transmit a total of 10 terabits-per-second of data across a fibre (100 channels, each at 100Gbps). System vendors have said that with further technology development, 25Tbps will be transported across fibre.

But IP traffic in the network is growing at over 30% each year. And while techniques are helping to improve overall transmission, the rate of progress is slowing down. A view is growing in the industry that without some radical technological breakthrough, new transmission media will be needed in the next two decades to avoid an inevitable capacity bottleneck.

"The Shannon Limit - the amount of information that can be transmitted - depends on the signal-to-noise and the amount of power you can put down a fibre," says Ian Giles, CEO of Phoenix Photonics, a fibre component specialist and one of the companies taking part in the project. "You can enhance transmission capacity by modulation techniques to increase bit rate, WDM and polarisation multiplexing but in the end you run out of the ability to transmit more information along a single-mode fibre."

This 'network crunch' is what the MODE-GAP project is looking to tackle.

Project details

One of the approaches that will be investigated is exploiting the multiple paths light travels down a multimode fibre to enable the parallel transmission of more than one channel.

These multiple paths light takes traveling in a multimode fibre disperses the signal. "The proposal we are making is that we take a low-moded fibre and select specific modes for each channel, or a high-moded fibre and select modal groups that are very similar," says Giles. The idea is that by identifying such modes in the multimode fibre, the dispersion for each mode or model groups will be limited.

But implementing such a spatially modulated system is tricky as the modes need to be identified and then have light launched into them. In turn, the modes must be kept apart along the fibre's span.

The project will tackle these challenges as well as use digital signal processing at the output to separate the transmitted channels. The project consortium believes that up to 10 channels could be used per fibre.

The second approach the MODE-GAP project will explore involves using specialist or photonic bandgap fibre. "The problem with solid core fibre is that the core will scatter light, and with higher intensity, you start to see non-linear scattering," says Giles. "So there is a limit to how much power you can put down a fibre without introducing these non-linear effects."

Photonic bandgap fibre has an air core that doesn't create scattering. As a result the non-linear threshold is some 100x higher, meaning that more power can be put into the fibre.

What next?

The MODE-GAP project is still in its infancy. The goal is to develop a system that allows the multiplexing and demultiplexing of the spatially-separated channels on the fibre. That will be done using multimode fibre but Giles stresses that it could eventually be done using photonic bandgap fibre. "You then enhance capacity: you increase the number of channels, and decrease the non-linearities which means you can increase the amount of information sent per channel," says Giles.

"Up till the spatial modulation part, the system is the same as you have now," he adds. "It is only the spatial modulation part that needs new components." NSN will use any prototype developed within its test-bed where it will be trailed. "They don't want to reinvent their equipment at each end," says Giles.

The project will also look to develop a fibre-amplifier that will boost all the fibre's spatial separated channels.

The project's goal is to demonstrate a working system. "The ultimate is to show the hundredfold improvement," says Giles. "We will do that with multiple channel transmission along a single photonic bandgap fibre and higher capacity [data transmission] per channel."

Project partners

In addition to NSN's systems expertise and test-bed, Eblana Photonics will be developing lasers for the project while Phoenix will address the passive components needed to launch and detect specific modes. OFS Fitel is providing the fibre expertise, while the University of Southampton's Optoelectronics Research Centre is leading the project. 

The other universities include the COBRA Institute at the Technische Universiteit Eindhoven which has expertise in the processing and transmission of spatial division multiplexed signals, while the Tyndall National Institute of University College Cork is providing system expertise, detectors, transmitters and some of the passive optics and planar waveguide work.

ESPCI ParisTech, working with the University of Southampton, will provide expertise in surface finishes. "The key here is that for the fibres to be low loss, and to maintain the modes in the fibre, they have to have very good inside surfaces," says Giles.