Pulses of darkness let digital data travel farther

Optical fibres can carry digital data for much longer distances without distortion if it is encoded in pulses of darkness, rather than pulses of light. To exploit this, Steven Cundiff, a physicist at the University of Colorado, Boulder, and colleagues have developed a device that has been dubbed the dark laser.

A standard laser can send out millions of picosecond-long light pulses each second, separated by longer dark intervals. The dark laser operates in reverse: it generates short pulses of darkness separated by relatively long intervals of brightness.

At the heart of Cundiff's dark laser is a chunk of material that emits light when an electric current is passed through it. This light bounces back and forth inside a mirrored chamber, in a similar manner to the way light is reflected back and forth in the resonance cavity of an ordinary laser to generate concentrated pulses of light.

The difference in the dark laser is that one of the mirrors has a light-absorbing coating. In this arrangement, with light of the right frequency, the chamber emits short pulses characterised by an absence rather than presence of light. The pulses, produced at a rate of up to 400 million per second, are 70 per cent less intense than the background light.

This is not the first laser capable of transmitting dark pulses. Han Zhang and his team at the Nanyang Technological University, Singapore, built their own last year. But Zhang notes that one obvious advantage of Cundiff's dark laser is the high rate at which it produces dark pulses.

Jeremy Baumberg at the University of Cambridge points out that it has long been possible to generate dark pulses using external devices attached to a standard laser to intermittently shut off the beam. "I showed that such dark pulses were possible while I was working at Hitachi here in Cambridge about 10 years ago," he says. What makes this work interesting is that it spontaneously generates dark pulses, he adds.

However, Baumberg is unsure of how dark lasers might be used. In the same way that lasers were initially regarded as a solution looking for a problem, the dark laser is "a curiosity looking for an application", he says.

Cundiff says the dark pulse could prove useful in long-distance optical communications. There are limits to how far a coherent light pulse can travel along a fibre-optic cable without "smearing" and distorting, as different wavelengths within the pulse travel at slightly different speeds. That's not a problem for a pulse defined by an absence of light, says Cundiff.

via Pulses of darkness let digital data travel farther - tech - 15 June 2010 - New Scientist.