Page 222 - Chalcogenide Glasses for Infrared Optics

198 Cha pte r Ei g h t

To put these numbers in perspective, the absorption level nor-
mally acceptable in infrared optical materials used for lens materials
−1
in infrared imaging cameras is 0.01 to 0.02 cm . For example, germa-
nium, long the industry standard, is commonly used in the 8- to
12-µm range at 0.03 cm . Thus, the absorption level required for this
−1
application is 2 orders of magnitude lower than normally required
for infrared optical materials, no small order. Fresnel reflection loss is
the next factor to consider. With a refractive index value of 2.407, the
reflectivity is calculated from

2
2
R = (n − 1) /(n + 1) = 0.171 for As S
2 3
Assuming no internal reflections for fiber, meters in length, the
maximum transmission for absorption-free fiber can be calculated as
T = (1− R) = 0.69
2
The importance of these facts for a 10-m length of fiber is shown in
the following:

Absorption ¥ 70% Active
(dB/m) Uncoated %T Coated %T Area
0.1 54.9 79.5 55.7
0.2 43.5 63.1 44.2
0.3 34.6 50.1 35.1
0.4 27.5 39.9 27.9
0.5 21.9 31.7 22.2
1.0 6.9 10.0 7.0

Obviously, the attenuation level must remain between 0.1 and
0.2 dB/m throughout the band for the goal of 50 percent energy to be
met. As we shall see, broadening of the impurity bands for long
lengths is a serious problem which makes attainment of our goal
much more difficult.
Earlier, we indicated FTIR results and laser transmission results
agreed if the fiber length was > 4 m. Figure 8.14 shows internal trans-
mission for 10 m of C2 fiber, unclad, which has been marked for
absorption levels of 0.1, 0.2, 0.3, 0.4, 0.5, and 1.0 dB/m. Also for refer-
ence, the absorption value as determined by CO laser energy trans-
mission for 11 m at 5.25 µm has been added. Note the value of
50 percent, or 0.3 dB/m, falls almost on the FTIR curve, validating
the accuracy. Notice the broadening of the water and hydrogen
sulfide absorption. In the 3- to 5-µm band, only a small portion meets
our criterion of 0.1 to 0.2 dB/m. Also, squares have been marked and