Tech Briefs Magazine - March 2022 - PIT-5

500°C-Rated
Optical Fiber for
High Temperature
Applications
S
ilica-based glass optical fibers
without coating can withstand
temperatures greater than 600°C.
However, glass fibers need to be
protected from the environment. Standard
telecom fibers are typically coated
with acrylate that allow their use
in temperatures up to 85°C. Specialty
optical fibers can be produced with a
polyimide coating, which allows these
fibers to be used in environments up to
300°C. This type of fiber has been used
extensively in the oil and gas industry
to provide important communications
and sensing functions for reservoir
management.
For temperatures above 300°C, metal
coatings would be attractive. Those
produced to date have been deemed
unsuitable for geothermal well deployment
due to high attenuation values
at low temperatures1
. Downstream oil
processing can also benefit from high
temperature measurements requiring
low attenuation fibers that perform beyond
300°C. This attenuation, as well as
significant attenuation changes during
cycling, is generally attributed to micro-bending
and the large mismatch of
Photonics & Imaging Technology, March 2022
the coefficients of thermal expansion
between the metal coating and the glass
fiber2
. Among other things, thinner metal
coatings could help to mitigate these
issues; however, the production of long
lengths of high-quality metal-coated fiber
with controlled thickness of the coating
is non-trivial2
.
In this article, a metal-coated fiber
capable of withstanding temperatures
up to 500°C will be demonstrated, and
it will be shown that this fiber can be
cycled between room temperature and
500°C, while maintaining low attenuation,
even at low temperatures.
Fiber Design
It has been demonstrated since the
early 1980s that hydrogen ingression in
silica-based glass induces losses in optical
fibers at specific wavelengths due to
the absorption of a variety of hydrogen
related species3
are typically doped with refractive index
increasing elements such as germanium
and phosphorus. Depending on
the temperature and H2
concentration,
once hydrogen diffuses in the fiber core,
it can migrate to interstitial sites of the
structure and/or bond with existing defects
in the glass such as SiO, GeO and
P-O. The overall fiber loss reaches hundreds
of decibels per kilometer, which
makes it unusable for any light transmission
applications.
. Common silica fibers
used in communications such as standard
single-mode (SM) and standard
graded-index multimode (MM) suffer
a dramatic optical degradation in the
presence of hydrogen even at room
temperature. The cores of these fibers
AFL took an innovative approach
to prevent the optical degradation of
optical fibers immersed in a harsh environment
by modifying and optimizing
the design of the glass component
of the fiber itself. In particular, the
approach consists of eliminating the
dopants that create more defects in
the glass structure such as germanium,
phosphorus, and boron. The fiber is
designed with only silica in the core,
along with fluorine doping to achieve
the graded index profile of the multimode
fiber4
. This fiber is produced
by AFL and is branded as Verrillon®
VHM5000; it is a 0.2 NA 50/125µm GIMMF.
5
PIT
Fiber Optics Feature 0322_1.indd 5
Cov
ToC
2/15/22 6:40 PM
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http://info.hotims.com/82319-780
Tech Briefs Magazine - March 2022

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