Page 218 - Chalcogenide Glasses for Infrared Optics

194 Cha pte r Ei g h t

spacing until 150 µm was exceeded. We decided that 100-µm core was
our practical limit.

8.4.2 AMI Glass Clad Fibers
In the AMI drawing of glass clad fiber, too much of the space at the
bundle ends is occupied by cladding. One approach taken was to
have a special tool designed in which the fused bundle section could
be placed and heated and pressure could be applied by the tool to
force excess clad glass out, to increase the packing density of the
fibers. The goal was to reach 70 percent active area. Unfortunately,
the fibers moved as well, destroying the spatial arrangement between
the fibers. We tried a special ordered split die for applying glass clad-
ding to small-diameter fibers. We ordered a split die with a 125-µm
hole to use on the cladding of fiber with a core diameter of 100 µm or
less. The glass cladding is a viscous liquid when applied, even when
the die hole is much larger than the fiber diameter. When the hole
diameters are close, the drag increases. The fiber began to break even
at low draw speeds of 0.5 m/min. A draw speed of even 1 m/min
would be impossibly slow considering we were to make a 70 × 70 ribbon
bundle which would require 49,000 m of fiber. At 1 m/min, it would
take thirty-four 24-h days of continuous fiber drawing, if it were
possible, with no interruptions. The highest active area achieved with
cladding for a 1-m bundle was only 33 percent. The conclusion drawn
was that clad fiber was out and unclad fiber was the only possibility
of reaching 70 percent and producing a 70 × 70 bundle in a reasonable
time frame. A comparison of FTIR results for internal transmission
for 10 m of C2 clad compared to 10 m of unclad differs by only a few
percent.

8.4.3 AMI Production of High-Purity Arsenic Trisulfide Glass
AMI developed a method to produce high-purity arsenic trisulfide
glass. We were able to achieve our goals of attenuation levels for
water at 2.9 µm of < 1 dB/m and a level for hydrogen sulfide at 4 µm
of < 5 dB/m. Measurement of transmission at 5.25 µm using the CO
laser for lengths of 10, 8, 4, and 1.5 m confirmed our level of attenua-
tion was reproducible at 0.3 dB/m when measured for lengths > 4 m.
Extrapolation to the 2.44-µm wavelength of the FTIR plot indicated
an attenuation level of 0.16 dB/m, close to the program goal of
0.1 dB/m. Figure 8.10 shows an FTIR internal transmission scan for a
100-cm length of 1000-µm core glass clad C2 fiber. AMI developed a
process for consistently producing 4- to 6-kg plates of striae-free, low-
absorption As S glass plates 20 cm in diameter, which was described
2 3
7
at the SPIE meeting held at Orlando, Florida, in April 1997.
Figure 8.11 shows a diagram in which, under vacuum, the very
high purity sulfur vapor is passed over heated high-purity arsenic,
and the resulting As-S molecules are collected on a cool quartz