Marconi to Telecom: ‘It’s all done with mirrors'

Light-based technology specialists are assuring their telecomms customers there's life left in the hardware and software yet.

Light-based technology specialists are assuring their telecomms customers there’s life left in the hardware and software yet.

After studiously trying to eliminate all moving parts from communications technology, the photonics champions are returning to electromechanical devices — and much of the more advanced variants will be done by mirrors.

Marconi Networks’ photonics market strategy director Steve Fergusson visited New Zealand this week with the chief aim of talking to the company’s biggest local customer here, Telecom, and reassuring it that there was a “road map” for the future of backbone technologies like dense wavelength division multiplexing (DWDM), where several signals are sent down the same fibre using different wavelengths of light and other electromagnetic radiation. DWDM is used, for example, in the Southern Cross cable.

The developers of DWDM and its less dense cousin WDM have in past years been striving for increased capacity above all, but attention has now turned to switching the signals efficiently from one fibre to another, to support more complex networks and emergency backup against failure.

Marconi has already deployed fibre rings in Europe that enable traffic to be pumped onto a common highway and picked off optically by stations at intervals on the ring, to be directed to its desired destination.

The on- and off-ramp devices are constructed with liquid crystal technology, from glass-and-fibre giant Corning. A pixel of the filter left clear lets the signal travel on, and a black pixel takes it off the ring and redirects it locally. The need for cumbersome energy-sapping electronic switching is avoided. The LCD filters are still controlled electronically, but no light-to-electricity conversion is allowed to interrupt the main trunk.

A ring, as opposed to a point-to-point link, has built-in redundancy, as a signal obstructed by a break or equipment failure can always be routed the other way round the ring.

More advanced and fail-safe communications, however, demand a branching layout, which means the traffic has to be switched from one fibre to another.

The mechanism to do this involves an array of tiny mirrors — 0.3mm to 1mm square — made of silicon, plated with gold or aluminium, fabricated with a process much like making a silicon chip, Fergusson says. The mirrors are suspended from narrow springy strips of silicon so they can twist in two directions in response to electrostatic charges.

Thus one particular mirror in a switch can take all the light on one fibre or, with the aid of dispersion elements, all the light of a particular wavelength on that fibre, and reflect it into a particular fibre on the other side.

The routing can be switched in a few tens of milliseconds, simply by turning a mirror.

A switch with 1000 ports takes up about the space of a standard mouse, Fergusson says — in comparison with an old-style backbone switch, a box almost two metres long.

However, the centralised control electronics are still rack-sized.

Lucent is also developing this technology, but no other company has it, says Fergusson.

The mirror arrays are made for Marconi by US-based Calient. Marconi was a second-round investor in Calient, with Juniper coming in on the first round.

The ring technology is reaching the price-point where it should be cost-effectively deployable in campus networks, Fergusson says.

Use for in-house data distribution, such as LANs or SAN fibre-channel, he estimates to be still about two years away.

As for use of photonics for digital circuitry within a computer, that would demand switching in nanoseconds rather than tens of milliseconds, he says. Using a different technology, with tunable lasers — one laser can be controlled to emit light of different colours — Marconi has made an experimental photonic router, “which fits on a table-top.” The switching speed is down to “tens of nanoseconds”, he says.

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