Ultrashort flashes of only four trillionths of a second apart are precisely and quickly coupled together

Flashes shorter than 1/400 billionth of a second are also known as femtosecond pulses. Today, they are used to study energy materials, for the 3D fabrication of components, or as precision scalpels in medicine. In lasers, these flashes are produced as solitons, “packets” of stable light waves.

Ultrashort solitons superimpose and generate spectral interference modes: real-time spectroscopy resolves their fast dynamics and tracks the switching of soliton molecules in femtosecond fiber lasers.Image showing successive experimental spectra recorded during switching

The newly published findings on the coupling of ultrashort flashes were obtained on a laser resonator. It contains a fiberglass ring that allows the solitons to circulate endlessly. In such systems, one can often observe coupled femtosecond flashes, so-called soliton molecules. Using high-resolution real-time spectroscopy, the team managed to track the dynamics of the two coupled flashes in real time over hundreds of thousands of orbits.

Based on these data, the scientists were able to show that it is the optical reflections within the laser resonator that couple the single lone solitons in time and space. The binding distance can be predicted from the transit time difference within the resonator and can be precisely adjusted by moving the optics.

Furthermore, the new study shows that the bond between two flashes can quickly loosen and create a new bond. For example, it is now possible to exclusively switch back and forth between flashes of light that occur in pairs and have different time intervals.

“Based on our findings, it is now possible to switch soliton molecules at the push of a button. This opens up new perspectives for the technical application of femtosecond pulses, especially in spectroscopy and materials processing,” said the first of the study. author Luca Nimmesgern, a master student in physics at the University of Bayreuth. The findings obtained on laser resonators can be transferred to various ultrashort pulse laser sources. Therefore, it is possible to generate coupled flashes in other laser systems, switching their distances without much effort.

“Since pulse pairs in fiber lasers were first reported more than 20 years ago, different explanations have been proposed for the stability of soliton molecules in lasers. The usual model has been disproved by many observations, but is still used today. Our new The research now provides, for the first time, a precise interpretation that fits the measured data. In a way, it provides a piece of the puzzle that makes the numerous early data understandable. The complex laser physics can now be specialized for high-speed generation Soliton sequences,” says Georg Herink, Junior Professor of Ultrafast Dynamics and coordinator of the research effort at the University of Bayreuth.

Co-author Prof. Dr. Alfred Leitenstorfer from the University of Konstanz, whose research group has been developing fiber lasers as a tool for spectroscopy for many years, added. “Based on our new findings, we can expect to realize multifunctional technical applications”.

At the University of Bayreuth, a DFG research project was launched with the aim of understanding the interactions between ultrashort solitons in laser sources in detail and making them available for future laser applications.