Soliton water waves can travel several kilometers without any significant change in their shape or amplitude, as opposed to normal waves, which widen as they travel, and eventually disappear.

Discovered over 150 years ago in water canals, solitons represent a surprising phenomenon of wave propagation and have been observed in natural phenomena including moving sand dunes and space plasmas.

A unique aspect of solitons is that they can retain their shape because of non-linear and dispersive effects that stabilize the wave. Solitons can even occur as pulses of light that can propagate through a suitable transparent medium, e.g.

an optical telecommunication fiber. Publishing in Nature Photonics, EPFL scientists collaborating with the Russian Quantum Center and the M.V. Lomonosov Moscow State University have now discovered that so-called optical dissipative solitons can also exist in small millimeter-size optical resonators.

The optical resonators are crystals shaped to form a resonator that can guide a soliton light pulse on an endless circular path. When such a soliton light pulse circulates inside the resonator, a small fraction of it can be extracted every time the pulse completes one roundtrip.

The scientists at EPFL's Laboratory for Photonics and Quantum Measurement analyzed the extracted light pulses from the resonator and found them to be surprisingly short in duration; much shorter in fact than one millionth of one millionth of a second. Due to the small size of the optical resonator, the time between two extracted pulses is extremely short and the pulse rate very high.

Besides being of large scientific interest, the high rate of repeated ultra-short light pulses is important for many applications: In astronomy, it can be used to search for Earth-like planets, chemists can identify unknown substances, and the capacity of today's telecommunication networks can be boosted by orders of magnitude.

Moreover, the solitons can be used for low-noise microwave generation or in future space-based optical clocks, significantly improving today's geo-navigation. Together with EPFL's Tech-Transfer Office, the scientists have applied for a patent and hope that their discovery will soon prove itself in one of its many applications.

Reference: Herr T, Brasch V, Jost JD, Wang CY, Kondratiev NM, Gorodetsky ML and T. J. Kippenberg. 2013 Temporal solitons in optical microresonators. Nature Photonics, 22 December 2013.