Tech Briefs Magazine - March 2022 - PIT-6

Fiber Optics
vents these metal-coated fibers from
bonding. This process was applied to
all the fibers in these tests.
Figure 1. SEM cross-section of fiber produced using the current process.
15 k V
X500 50 µm
VHM5000 was the base fiber used with
this metal coating. It had a gold-based
coating with a wall thickness of approximately
3 - 5 µm which is well below the
typical coating thickness of 15 - 25 µm
for commercially available metal-coated
fibers. A cross-sectional SEM image
demonstrating the good concentricity
and integrity of the coating process is
shown in Figure 1.
Metal-coated fibers can have optical
losses in as-drawn condition as high as
20-100 dB/km at room temperature2
.
Figure 2 shows the spectral attenuation
of VHM5000 with a gold-based coating
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Figure 2. Spectral attenuation of VHM5000 with gold-based coating, 88m
long, 300 mm diameter loose coil, room temperature.
6
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Figure 3. VHM5000 with gold-based coating, 40m long, 850 nm attenuation,
six temperature cycles between room temperature and 375°C.
Photonics & Imaging Technology, March 2022
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20 30 SE I
shown in Figure 1, at room temperature,
measured on 88m of fiber. Fiber was measured
in a 300 mm diameter loose coil.
The spectral attenuation of this fiber
with a gold-based coating shows attenuation
levels similar to standard acrylate or
polyimide coated multimode fibers, as
opposed to the significantly higher levels
shown by other commercially available
metal-coated multimode fibers.
Metal-coated fibers also have the tendency
to 'cold bond' to other metals,
or itself, at temperatures significantly
below their melting temperature. AFL
has a patent-pending process which pre
Results
and Discussion
Figure 3 shows six temperature
cycles of VHM5000 with gold-based
coating, between room temperature
and 375°C. Data was acquired every
5 minutes using an OTDR. The fiber
was in a 114 mm loose coil, 40 meters
in length. Each cycle consisted
of a 30°C/hour ramp to 375°C, the
temperature was held at 375°C for
24 hours, and then it was ramped
down 30°C/hour until 60°C. At that
point, the oven was allowed to return
to room temperature, and then the
next cycle was started. 850 nm was the
wavelength that was monitored.
Forty-three meters of VHM5000
gold-based coated fiber was put in a
500°C oven for 900 hours. An OTDR
was connected to the fiber at the
conclusion of the 900 hours, and a
500°C cycle was run. Figure 4 shows
this temperature cycle, between room
temperature and 500°C. Data was acquired
every 5 minutes. The fiber was in
a 114 mm loose coil. The cycle consisted
of a 30°C/hour ramp to 500°C, the temperature
was held at 500°C for 34 hours,
and the oven was stopped and allowed to
return to room temperature on its own.
The wavelength that was evaluated was
850 nm.
Conclusion
A low attenuation metal-coated optical
fiber capable of withstanding temperatures
up to 500°C was demonstrated.
Performance was validated using an
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PIT Fiber Optics Feature 0322_1.indd 6
Cov
ToC
2/15/22 11:17 AM
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
http://info.hotims.com/82319-780
Tech Briefs Magazine - March 2022

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