Controlling Crystals of Light

Illustration of light pulses in an optical microresonator forming a perfect soliton crystal. (Source: EPFL / Second Bay Studios)

Optical micro­resonators convert laser light into ultrashort pulses travelling around the resonator’s circum­ference. These dissi­pative Kerr solitons can propagate in the micro­resonator maintaining their shape. When solitons exit the micro­resonator, the output light takes the form of a pulse train – a series of repeating pulses with fixed intervals. In this case, the repe­tition rate of the pulses is determined by the micro­resonator size. Smaller sizes enable pulse trains with high repetition rates, reaching hundreds of gigahertz in frequency. These can be used to boost the performance of optical communi­cation links or become a core tech­nology for ultrafast lidar with submicron precision.

Exciting though it is, this tech­nology suffers from light-bending losses – loss of light caused by structural bends in its path. A well-known problem in fiber optics, light-bending loss also means that the size of micro­resonators cannot drop below a few tens of microns. This therefore limits the maximum repetition rates we can achieve for pulses. Now, researchers from the lab of Tobias Kippen­berg at École Poly­technique Fédérale de Lausanne EPFL have found a way to bypass this limitation and uncouple the pulse repetition rate from the micro­resonator size by gene­rating multiple solitons in a single micro­resonator.

The scientists discovered a way of seeding the micro­resonator with the maximum possible number of dissi­pative Kerr solitons with precisely equal spacing between them. This new formation of light can be thought of as an optical analogue to atomic chains in crys­talline solids – perfect soliton crystals (PSCs). Due to inter­ferometric enhance­ment and the high number of optical pulses, PSCs coherently multiply the perfor­mance of the resulting pulse train, not just its repe­tition rate, but also its power.

The researchers also inves­tigated the dynamics of PSC formations. Despite their highly organized structure, they seem to be closely linked to optical chaos, a phenomenon caused by light instabilities in optical micro­resonators, which is also common for semi­conductor-based and fiber laser systems. “Our findings allow the generation of optical pulse trains with ultra-high repe­tition rates with several terahertz, using regular micro­resonators,” says researcher Maxim Karpov. “These can be used for multiple appli­cations in spectro­scopy, distance measure­ments, and as a source of low-noise terahertz radiation with a chip-size footprint.” Meanwhile, the new under­standing of soliton dynamics in optical micro­resonators and the behavior of PSCs opens up new avenues into the funda­mental physics of soliton ensembles in nonlinear systems. (Source: EPFL)

Reference: M. Karpov et al.: Dynamics of soliton crystals in optical microresonators, Nat. Phys., online 9 September 2019; DOI: 10.1038/s41567-019-0635-0

Link: Laboratory of Photonics and Quantum Measurements,École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

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