Physics & Math

A long-elusive goal of physics has been reached – producing a pulse of light so short that it contains just a single oscillation of a light wave.

The flashes are almost as short as a light pulse can be, according to the laws of physics. The new super-short pulses could used as flashguns to sense very small, very fast events such as a single photon interacting with a single electron, says Alfred Leitenstorfer of the University of Konstanz in Germany. A single-cycle pulse packs in energy more densely than a pulse containing more wave peaks and troughs.

They could also show the way to boosting data transmission through fibre-optic cables, by shrinking the minimum amount of light needed to encode a single digital 1 or 0.

Leitenstorfer's group shunned the crystalline lasers typically used by physicists looking to make super-short light pulses and used optical-fibre lasers and wavelengths of light like those standard in telecommunications.

Technology milestone

"Single-cycle pulse generation with an essentially all-fibre system clearly marks a milestone in optical technology," says Martin Fermann of laser manufacturer Imra America, who was not involved with the work. He expects "the single-cycle regime will become a new standard" with applications in advanced imaging, sensing and signal processing.

The "uncertainty principle" formulated by 20th-century physicist Werner Heisenberg sets a limit on the shortest possible duration of a light pulse of any given wavelength in terms of time or number of cycles. The research team knew that at the infrared frequencies they were using the uncertainty principle meant they had to get the pulse down to a handful of femtoseconds (millionths of a billionth of a second).

The Konstanz researchers started with pulses from a single fibre laser and split them between two sets of fibres that contained atoms of the rare earth metal erbium to amplify the light waves. Each fibre then had a second stage that altered the light's wavelength, one stretching it by about 40 per cent, the other shrinking it by a similar amount. The two fibres then converged again, causing the two light beams to interfere with one another in a way that cancelled out most of the waves to leave just a single wave cycle lasting just 4.3 femtoseconds.

Pulses that are even shorter, as short as 3.9 femtoseconds, had been made before using wavelengths nearly 50 per cent shorter. But the relationship between wavelength and frequency means they weren't such pure fractions of a light wave, containing between 2 and 1.3 wave cycles.

Fibre key

The key to success, says Leitenstorfer, was using a single source to generate the two light pulses that combined to produce the short pulse. "It's because it's all-fibre technology that we can recombine these two parts," he told New Scientist. "The biggest challenge in this entire paper was to measure the pulse." A series of the short pulses were compared with each other to verify that they were each only one cycle long.

Further refinements should be possible. "We did these experiments in three weeks," Leitenstorfer says. His group say they can remove background noise to make their single cycle stand out more clearly.

Journal reference: Nature Photonics, DOI: 10.1038/nphoton.2009.258

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