Optical microresonators turn laser light into ultrashort pulses moving around the resonator’s circumference. These pulses, known as “dissipative Kerr solitons,” can propagate in the microresonator keeping their shape.
When solitons leave the microresonator, the output light takes the shape of a pulse train—a sequence of repeating pulses with fixed intervals. Here, the repetition rate of the pulses is established by the size of the microresonator. Smaller sizes allow pulse trains having high repetition rates, reaching hundreds of gigahertz in frequency. These can be used to improve the performance of optical communication links or become an essential technology for ultrafast LiDAR with sub-micron precision.
Stimulating though it is, this technology endures from what researchers call “light-bending losses”—loss of light resulting from structural bends in its path. A recognized issue in fiber optics, light-bending loss also means that the size of microresonators cannot fall below a few tens of microns. This, thus, restrict the maximum repetition rates one can realize for pulses.
Reporting in Nature Physics, scientists from the lab of Tobias J. Kippenberg at EPFL have presently discovered a way to overcome this drawback and uncouple the pulse repetition rate from the microresonator size by producing numerous solitons in a single microresonator.
The researchers found a way of seeding the microresonator with the highest possible number of dissipative Kerr solitons with precisely equal spacing between them. This novel formation of light can be said to be an optical analog to atomic chains in crystalline solids, and so the scientists named them “perfect soliton crystals” (PSCs).
Owing to interferometric improvement and the high number of optical pulses, PSCs coherently increase the performance of the resulting pulse train—not merely its repetition rate, but also its power.
The scientists also examined the dynamics of PSC formations. Regardless of their highly organized structure, they appear to be closely connected to optical chaos, an occurrence caused by light inconsistencies in optical microresonators, which is also typical 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. 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.
Maxim Karpov, Researcher, EPFL
In the meantime, the new insight into soliton dynamics in optical microresonators and the behavior of PSCs paves the way to new opportunities into the fundamental physics of soliton ensembles in nonlinear systems.