Ultrashort flashes of light lasting less than a quadrillionth of a second
are fast growing in technological importance. In laser sources, pairs and
groups of light flashes can be created instead of individual flashes.
Similar to the chemically bonded atoms in a molecule, they are coupled with
each other and their short temporal intervals can possess remarkable
stability. Researchers at the Universities of Bayreuth and Constance have
now revealed a cause for the stable coupling of ultrashort light flashes and
found a way to control their spacing both very precisely and rapidly. They
present their research results in the journal Optica.
Light flashes shorter than a quadrillionth of a second are also called
femtosecond pulses. Today, they are used for researching energy materials,
in the 3D manufacturing of components, or as precision scalpels in medicine.
In lasers, these flashes are created as solitons, stable packets of light
waves. The findings about their coupling that have now been published were
obtained on a laser resonator. This contains a ring of glass fibers that
allows the solitons to circulate endlessly. In such systems, one often
observes coupled femtosecond flashes, so-called soliton molecules. By using
high-resolution real-time spectroscopy, the research team succeeded in
tracking the dynamics of two coupled flashes in real time during many
hundreds of thousands of orbits. Based on this data, the scientists were
able to show that it is optical reflections within the laser resonator that
couple the individual solitons in time and space. The binding distances
could be predicted on the basis of transit time differences within the
resonator and could finally be precisely adjusted by shifting optical
elements.
In addition, the new study shows how the bond between two flashes can be
quickly loosened and a new bond created. It is now possible, for example, to
specifically switch back and forth between light flashes that occur in pairs
and have different temporal intervals. "Based on our research results, 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," says Luca Nimmesgern
B.Sc., first author of the study and physics master's student at the
University of Bayreuth.
The findings obtained at the laser resonator can be transferred to a variety
of ultrashort pulse laser sources. Consequently, it is possible to generate
coupled light flashes in other laser systems and switch their distances
without much effort. "Since the first reports of pulse pairs in fiber lasers
more than 20 years ago, different explanations have been proposed for the
stability of soliton molecules in lasers. The usual models have been
contradicted by numerous observations, but are still used today. Our new
study now offers a precise explanation compatible with the measured data for
the first time. In a way, it provides a piece of the puzzle that makes a
multitude of earlier data understandable. Now, complex laser physics can be
used specifically to generate soliton sequences at high speed," says Georg
Herink, Junior Professor for Ultrafast Dynamics at the University of
Bayreuth and coordinator of the research work. 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 years, adds: "Based
on our new findings, we can look forward to the realization of versatile
technological applications."
At the University of Bayreuth, a DFG research project was recently kicked
off with the aim of understanding the interactions between ultrashort
solitons in laser sources in detail, and making them usable for future laser
applications.
Reference:
Luca Nimmesgern et al, Soliton molecules in femtosecond fiber lasers:
universal binding mechanism and direct electronic control, Optica (2021).
DOI: 10.1364/OPTICA.439905
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