Medical Design Briefs - October 2022 - 48

Self-Cleaning Optical Fiber Can Help in Diagnosing Cancer,
Monitoring Environment
Tampere University, Tampere, Finland
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esearchers have successfully developed
a novel optical fiber design allowing
the generation of rainbow laser light in the
molecular fingerprint electromagnetic region.
This new optical fiber with a selfcleaned
beam, developed by researchers at
Tampere University, can help in developing
applications, such as cancer diagnostics.
The finding was published in the journal
Nature Communications.
When a high-power ultrashort pulse of
light interacts with a material such as a
glass optical fiber, a range of highly nonlinear
interactions take place that cause
complex changes in both the temporal
and spectral properties of the injected
light. When taken to the extreme, such interactions
can lead to the generation of a
rainbow laser of light commonly referred
to as a supercontinuum light source. Since its
first demonstration in a special type of optical
fiber in 2000, supercontinuum laser
light has revolutionized many areas of science,
ranging from metrology to imaging
at unprecedented resolution.
The bottleneck with current supercontinuum
sources, however, is that they
are based on optical fibers that support a
single transverse intensity profile or mode,
which inherently limits their optical power.
Also, conventional optical fibers are
made of silica glass with transmission limited
to the visible and near-infrared region
of the spectrum. The extension of supercontinuum
light to other wavelength regimes,
such as the mid-infrared, requires
optical fibers made of so-called soft glasses,
but these possess a lower damage threshold
than silica, limiting even more the
power of the supercontinuum beam.
n Self-Cleaned Beam
Recently, a different type of optical fiber
with a refractive index that varies continuously
across the fiber structure has
been shown to yield a dramatic increase
in supercontinuum power, while still preserving
a smooth beam intensity profile.
" The refractive index variation of such
graded-index optical fibers leads to periodic
focusing and defocusing of the light
inside the fiber that enables coupling between
spatial and temporal nonlinear
light-matter interactions. This leads to a
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Utilizing two glasses with a different refractive index and stacked with a specific arrangement has allowed researchers
to develop for the first time a multimodal fiber with a parabolic refractive index with transmission up to the
mid-infrared and high nonlinearity. The spectrum of short pulses of light injected into the fiber massively broadens
to span from the visible to mid-infrared. Unlike in conventional multimode fibers, the light beam remains smooth as
the result of self-cleaning dynamics induced by the parabolic refractive index. (Illustration: Tampere University)
self-cleaning
mechanism
that yields
super continuum light with high power
and a clean beam profile. As well as their
many applications, they also provide a
means of studying fundamental physics
effects such as wave turbulence, " says
Professor Goëry Genty, the leader of the
research group at Tampere University.
While these fibers have recently attracted
significant attention from the research
community, their use has been,
up to now, restricted to the visible and
near-infrared. In collaboration with the
group of Profs. Buczynski and Klimczak
at the University of Warsaw (Poland) and
the group of Prof. Dudley at the University
of Burgundy France-Comté (France),
the Tampere team demonstrated for the
first time the generation of a two-octave
supercontinuum from the visible to
mid-infrared in a non-silica gradedindex
fiber with a self-cleaned beam.
" This problem has now been solved by
using a particular design that utilizes two
types of lead-bismuth-gallate glass rods
with different refractive indices drawn to
yield a nanostructured core. The result is
a graded-index fiber with an effective
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parabolic refractive index profile with
transmission up to the mid-infrared,
and, as cherry on the cake, enhanced
nonlinear light-matter interactions, " says
researcher Zahra Eslami.
n Diagnostics and Monitoring
The mid-infrared is of crucial interest as
it contains the characteristic vibrational
transitions of many important molecules.
" The novel solution will lead to more
efficient supercontinuum light sources
in the mid-infrared with many potential
applications, e.g., for pollutant tagging,
cancer diagnostics, machine vision, environmental
monitoring, quality, and food
control, " explains Genty.
The researchers anticipate that this
novel type of fiber will very soon become
an important and standard material for
the generation of broadband sources and
frequency combs. The research was carried
out at Tampere University and within
the Academy of Finland Flagship for Photonics
Research and Innovation (PREIN).
For more information, visit www.tuni.
fi/en, or contact Goëry Genty at +358 50
346 3069.
Medical Design Briefs, October 2022
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Spectrum (log)
Spectrum (log)
Refractive index
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Medical Design Briefs - October 2022

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