École Polytechnique Fédérale de Lausanne (EPFL) scientists have bypassed light-bending losses that can occur when light pulses exit an optical microresonator, and have uncoupled the pulse repetition rate from the microresonator size by generating multiple solitons in a single microresonator.

Microresonators convert laser light into ultrashort pulses traveling around the resonator’s circumference. These pulses, called dissipative Kerr solitons, can propagate in the microresonator while maintaining their shape. When solitons exit a microresonator, the output light takes the form of a pulse train — a series of repeating pulses with fixed intervals.

The pulse train repetition rate is determined by the microresonator size. Smaller sizes enable pulse trains with high repetition rates, reaching hundreds of gigahertz in frequency. These smaller resonators can be used to boost the performance of optical communication links or to increase the speed and precision of lidar. However, the loss of light caused by structural bends in the path of a pulse train has meant that the size of microresonators cannot drop below a few tens of microns.

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Light pulses in an optical microresonator forming a perfect soliton crystal. Courtesy of Second Bay Studios.

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The EPFL scientists seeded a microresonator with the maximum possible number of dissipative Kerr solitons and put equal spacing between the solitons. They called this new light formation perfect soliton crystals (PSCs). PSCs coherently multiplied the power and repetition rate of the resulting pulse train due to interferometric enhancement and their high number of optical pulses. 

“Our findings allow the generation of optical pulse trains with ultrahigh repetition rates with several terahertz, using regular microresonators,” researcher Maxim Karpov said. “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.”

The researchers also investigated the dynamics of PSC formations. They found that despite their highly organized structure, PSCs appear to be closely linked to optical chaos, a phenomenon caused by light instabilities in microresonators.

Dissipative Kerr solitons show potential for use in applications ranging from optical coherent communications to astrophysical spectrometer calibration, and are of fundamental interest to the physical sciences. The increased understanding of soliton dynamics in optical microresonators and the behavior of PSCs could open up new avenues into the fundamental physics of soliton ensembles in nonlinear systems.

The research was published in Nature Physics (https://doi.org/10.1038/s41567-019-0635-0). 

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