Smarter multiplexer pushes optical closer to the edge

Light waves, left unchecked for too long as they travel through the miniscule glass fibres in our optical networks, have a tendency to go on wildly rambunctious rides. The unruly wavelengths scatter apart and break up until their energy fizzles out.

The light wave’s troublesome, self-divisive behaviour is curbed by a multiplexer, which simply means the wavelength is knocked back in line to ensure our packets of data arrive safely at the other end.

The process, dense wavelength division multiplexing (DWDM), is reasonably uncomplicated over a point-to-point lightpath, even over a simple ring DWDM fibre optic network typical of a carrier backbone.

Photonics scientists are growing more confident that intelligent optical wavelength switches will be deployed beyond the long-haul backbone in the next few years.

The more the behaviour of light waves can be curbed, the more of our metro and regional networks will become all-optical. This means DWDM switches will have to cope with grid networks, requiring more than the ability to add and drop wavelengths to realign (multiplex) the light waves.

Professor David Plant of Montreal sees a bright optical future in a multiplexer known as a ROADM (reconfigurable optical add-drop multiplexer). Being reconfigurable, the device can act rapidly to either add or drop wavelengths on the scattering, dispersing light waves.

Agility is the important progress marker for Plant and his team of scientists at the Agile All-Photonic Network (AAPN) research program based at McGill University.

“ROADM technology is bringing optical connectivity closer to the end-user,” said Plant, scientific director at AAPN and a professor of electrical and computer engineering.

“What we’re aiming for, ultimately, is an all-optical network that eliminates, or at least minimises, the electrical-optical-electro conversions. The continual conversion of data packets, back and forth between binary electrical impulses and photonic light waves, adds cost to the operation of the network and reduces performance,” said Plant.

“The goal is a dynamically reconfigurable, or agile, photonic network that supports high-bandwidth, interactive services. To this end, predictions are that the deployment of ROADM technology will proliferate.”

Data packets initially depart on their passage as electrical impulses, traversing the copper wires of cable networks as stable binary combinations. To enter the long-haul fibre optic channel, which offers less resistance, data is converted to light waves that are periodically multiplexed en route to reduce the jitter along the journey.

“The further the data travels, the more optical impairments there are, resulting in a dirty signal. The electrical conversions reshape, retime and regenerate the signal,” said Kevin Drury, senior marketing manager, optical networks, at Nortel Networks Corp. in Canada.

“A multiplexer is analogous with a prism, adding and dropping the colours (wavelengths) of light. This is how we’re able to redress the attenuation, the intensity of the signal as it runs out of energy, and the dispersion of the light waves, by compensating either under or over, depending on how the wavelength changes.

“With ROADM technology, combined with automation tools, we can remotely make the necessary changes in power equalisation and dispersion compensation. The ROADM also allows us to reroute the data around multiple branches, as apposed to DWDM ring architecture.

“ROADM technology is basically a combination of wavelength selective switching and wavelength blocking. It’s like a tuneable prism: we can adjust the physical structure of the prism to disperse the colours.

“Nortel provides an automatic switched optical network by using our own technology, called electronic dynamically compensating optics, in combination with ROADM technology. The ROADM is used for branching, while our compensating optics technology allows the transmitter and receiver to talk to each other, dirtying and cleaning the signal on its way.”

“The ROADM effectively makes it easier to deploy and operate a DWDM topology,” said Ron Johnson, product manager, optical networking, at Cisco Systems Inc. “Instead of a DWDM ring, we can go end-to-end without electro-optical-electrical conversions in a mesh network. The ROADM is helping the evolution of DWDM.”

The drivers for ROADM are triple-play demands on ever-bulging bandwidth availability, stimulated by interactive and broadcast video, packetized voice and broadband data.

Johnson has seen a steady ramp-up since Cisco launched their first-generation ROADM last October, with service providers and large enterprises filling the DWDM space. “But adoption has been slow…the technology has its limitations and is not quite ready,” said Johnson.

“ROADM technology is seen as relatively new to the industry,” said Nortel’s Drury. “For agility, we have to overcome the need for dispersion compensation. And in order to pre-engineer which wavelengths you want, you first have to know the distance the data will be travelling and what the compensation demands will be.”

Drury said not much of a ramp in ROADM was being anticipated until 2008, with the technology not likely to be accepted as mainstream before 2010.

“There’s a spectrum of possibilities and varying degrees to an all-photonic network,” said Plant. “But we do know optical-electronic conversions are bottlenecks in our networks that add great cost and consume large amounts of power.”

Plant and his team of photonics scientists are backed by government funding from the Natural Sciences and Engineering Research Council, as well as some major industry players that include Telus Corp. and Nortel. The AAPN consortium is spread over five universities in Ontario and Quebec.

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