Time Magnification Used to Measure Chaotic Pulses in Real Time

A technique that provides the ability to expand timescales in optics has been used to measure ultrafast, intense light pulses directly. Observations from the experiments confirm theoretical predictions that were made decades ago, and could play a role in the prediction of very high, sudden and rare rogue waves on the surface of the oceans or the appearance of other extreme events in nature. Waves similar to rogue waves exist in optics in the form of short and intense light pulses.

Natural random phenomena such as rogue waves are usually sensitive to fluctuations in initial conditions, no matter how small.

An unstable modulation instability optical field consisting of picosecond pulses that are normally too fast to be detected. The use of the technique of time magnification allows these chaotic pulses to be measured. Courtesy of Benjamin Wetzel.

To understand these natural phenomena, a research team from Canada, France, Finland and Ireland propagated light waves in optical fibers, to form pulses in the order of picoseconds (PS). The ultrafast PS timescale was too fast for current detector technology to measure, preventing researchers from being able to observe the chaotic behavior in real time. To overcome this limitation, they used a novel technique known as a time lens. Exploiting advances in ultrafast optical metrology, they performed real-time measurements in an optical fiber system of the unstable breakup of a continuous wave field, simultaneously characterizing emerging modulation instability breather pulses and their associated statistics. The “butterfly effect,” in optics known as modulation instability, amplifies microscopic noise present in the laser beam.

"In a similar way as a stroboscope can resolve the evolution of a bouncing ball in the dark or the movements of dancers in a night club, this time lens technique can take one million snapshots of the optical field every second, while additionally increasing the temporal resolution by a factor of 100. This approach allowed us to efficiently measure the chaotic dynamics of the light pulses and their temporal characteristics via available electronic detectors," said Benjamin Wetzel, INRS researcher.

The researchers were able to observe in real time the chaotic dynamics and the formation of giant light pulses with an intensity of more than 1,000 times greater than that of the initial fluctuations of the laser. The experimental results could contribute to the understanding of noise-amplifying instabilities in different areas of physics, from the physics behind giant rogue waves on the ocean to the dynamics of plasma in the early universe.

"There are many systems in nature where it is very difficult to study rapid fluctuations associated with instabilities, but the ability to magnify ultrafast dynamics in optics now opens a new window into performing more experiments in this field," said FEMTO-ST Institute researcher John Dudley.

The team was comprised of researchers from the Tampere University of Technology (Finland), Institut national de la recherche scientifique (INRS) (Canada), University College Dublin (Ireland) and Institut FEMTO-ST, CNRS (France).

The research was published in Nature Communications (doi:10.1038/ncomms13675).