Light Swell: Optical rogue waves resemble oceanic ones

By Davide Castelvecchi
From Science News, December 15th, 2007; Vol.172 #24 (p. 374)

Slightly noisy signals can turn into rare large spikes in an optical fiber’s output, in much the same way as unpredictable weather conditions occasionally create monstrous, isolated oceanic waves, researchers have found.

The new technique for creating such “rogue waves” in the lab might help physicists understand them as a general phenomenon, in the hope of predicting the risks for vessels at sea.

Rogue waves—waves significantly higher than the local average at a given time—belonged to seafarer lore long before scientists conclusively demonstrated their existence in the mid-1990s.

Theoretical studies and computer simulations have shown that rogue waves can originate from what physicists call nonlinearity. Two waves crossing each other’s path ordinarily add up in height so that the new peaks’ heights equal the sum of the original waves’ heights. After this linear interaction, the waves will then proceed unperturbed, each on its way. Occasionally, however, the overlap will result in waves that are shorter in length but have heights larger than the sum of the original waves. Some scientists believe that in the ocean, such nonlinearity can create the waves of 30 meters or more that have sometimes been observed.

These waves will be very short-lived, although winds may increase their lifetime, as Christian Kharif of Aix-Marseille University in France and his team recently observed by creating artificial rogue waves in a 30-m-long water tank. Their results will appear in the Journal of Fluid Mechanics.

A rogue wave will appear “at a random location, at a random time,” says Bahram Jalali, an electrical engineer at the University of California, Los Angeles (UCLA), who developed an interest in rogue waves while spending time on his 36-foot sailboat.

Jalali and his collaborators examined laser light propagating in an optical fiber using a so-called time-stretch digitizer. This instrument allowed the researchers to analyze the waveforms of light and resolve spikes that lasted less than a trillionth of a second. Some of the random spikes, which occurred thousands of times per second, were up to 30 times more intense than average, says team member Daniel Solli of UCLA. The report appears in the Dec. 13 Nature.

The team’s own computer simulations showed that noise in the input laser waves should indeed result in rogue light waves occurring about as often as they do in the lab. Although the physics is different, this nonlinear amplification is analogous to the idea that a butterfly flapping its wings in one continent can trigger a storm in another continent.

Although the optical system is essentially one-dimensional, it “shows the growth of freak waves very similar to what you see in the ocean,” says Mattias Marklund, a theoretical physicist at Umeå University in Sweden.

Marklund adds that optical experiments will allow physicists to test their theories with greater control over the variables.

Jalali says that he himself has never seen a rogue wave while sailing, which is probably a good thing. But the possibility tickles his scientist’s curiosity. “To be honest, I do hope I see one,” he says.

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