Waves of light: you can get freak peaks in optical systems.Getty

Gigantic 'rogue' waves that swallow ships have been brought into the laboratory. No one knows exactly how they form in the oceans, but a team at the University of California, Los Angeles, has made similar freak events in light¹. The researchers say that their work should help to explain this terrifying maritime phenomenon.

Rogue waves were originally thought to be a myth. Lone waves tens of metres high had been described in sailors' anecdotes, but it wasn’t until the mid-1990s that they were documented convincingly. One such wave struck an oil platform in the North Sea in 1995, another was photographed in 1993 in the Bay of Biscay.

There have been more sightings since. In 2001, for example, a passenger ship in the South Atlantic was hit by a wave about 30 metres high. Yet conventional theories of ocean waves predict that these monsters should be so rare that they would effectively never be seen.

It’s now recognized that the oceans defy those theories. Most waves are small; the bigger the wave, the rarer it is. But there is still an anomalously high number of extremely big waves. It isn’t clear why.

Various explanations have been proposed, invoking unusual wave-focusing effects or freak winds. A key feature of rogue waves is that they stay as sharp peaks as they move, making them look like ‘walls of water’. This is a similar to ‘self-reinforcing solitary waves’ or solitons, which can sweep over large distances without dispersing, keeping the same size and shape.

Solitons were first observed in a Scottish canal in the nineteenth century. They are now well studied in light as well as in water. So Daniel Solli and his colleagues wondered whether they could use the techniques known to generate optical solitons to make rogue waves in light.

Drowning in noise

They added a little bit of noise to a series of light pulses in a ‘nonlinear’ optical fibre, in which the shape and frequencies of light coming out doesn’t depend in a simple way on those of the light going in.

The noise turns the pulses into a dim signal made up of many frequencies. But within this ‘broadband’ output there are some extremely bright and narrow spikes — even narrower and more intense than the peaks going in. These are the optical equivalent of rogue ocean waves, they report in Nature.

The spikes could happen purely by chance, but the team found that they saw ‘rogue solitons’ much more often than simple random statistics would predict. “The noise is a seed that acts to focus the energy of the pulse into an intense rogue pulse,” Solli says.

“This is the first observation of man-made rogue waves,” says team leader Bahram Jalali. And they are not so different from the water-going versions. “Optical rogue waves bear a close connection to their oceanic cousins,” says Solli.

Whether the mechanism for rogue formation is the same at sea as it is in the lab remains unclear. But the ingredients all seem to be there: waves, nonlinearity and a little noise. ”Water-wave propagation is already known to be nonlinear,” says Solli. And 'wave noise' could come from factors such as changing wind direction.

So Solli says that experiments such as these “may help to resolve the mystery of oceanic rogue waves.” Jalali adds that their observations could be used to develop mathematical models to identify the conditions that lead to such events. Perhaps it might even be possible one day to forecast a danger of rogue waves just as we can now issue storm warnings — a help to those at risk from these whopping events. 

• References

  1. Solli, D. R. , Ropers, C. , Koonath, P. & Jalali, B. Nature 450, 1054-1057 (2007).