Optics as a key to understanding rogue waves

Rogue waves are powerful waves that erupt suddenly. They are rare, but destructive. Above all, they are unpredictable. Surprisingly, researchers have been able to better understand these fascinating waves by studying similar phenomena in fiber optic lasers.

Before scientists began measuring and observing them, rogue waves had long been perceived as legends. They can reach a height of 30 meters, forming a wall of water facing ships. French explorer Dumont d’Urville faced one such wave in the southern hemisphere. More recently, in 1995, the commander of the transatlantic liner Queen Elizabeth II described a wave as a “solid wall of water”, adding that he felt he was sailing the boat “straight into the cliffs of Dover”. These waves are also a major cause of containers lost at sea.

A genius idea

But the rare and unpredictable nature of these mysterious waves makes them difficult to study and nearly impossible to predict. Tests have been conducted in specially designed pools, but the resulting waves are much smaller and do not sufficiently reflect reality. Theoretical models, on the other hand, are not accurate enough.

However, in 2007, Daniel Solli and his team from the University of California had the genius idea of comparing the propagation of waves with that of light pulses in optical fibers. Waves and light pulses are in fact both waves and are subject to the same laws of physics. And it is much easier to study light pulses, since all the parameters can be easily controlled: wavelength, intensity, the type of fiber used, etc. Furthermore, we can study thousands of pulses per second, making it possible to observe rare events.

Real time

Now, a group of researchers including Arnaud Mussot from IMT Lille Douai has published an article on this subject in the scientific journal Nature Physics, describing the research on the analogies between oceanography and optics to better understand rogue waves.

“Many experiments have been conducted in optics,” Arnaud Mussot explains. “For these experiments, we sent laser pulses into optical fibers and we analyzed the speed of these pulses at the output of the fiber. These observations were made in real time, over extremely short periods of time–a few tens of femtoseconds, which is much less than a billionth of a millisecond.”

These experiments showed that there are several ways to create rogue waves. One of the most effective methods is to make several waves crash into each other. But only wave collisions from certain angles, directions and amplitudes generate rogue wave phenomena. However, these experiments do not provide all the answers, since some of the more powerful rogue waves predicted by theorists have not yet been observed.

Predicting rogue waves

In the long term, a better understanding of rogue waves should make it possible to better predict them and prevent certain accidents. “Certain companies are currently developing radar that can map the state of the sea, which can be taken on a boat,” Arnaud Mussot explains. “This data is sent into a computing program that predicts what will happen in the sea in the next minutes. The ship can then modify its course to avoid a rogue wave or mitigate its effects. The more we improve our knowledge and calculations, the more we will succeed in predicting these waves in advance.”

This research also benefits other fields, such as optics. It offers a better understanding of the start-up of high-power lasers and certain tasks the lasers perform, for which the characteristics vary as the power of the laser increases.

Article written in French by Cécile Michaut for I’MTech.