Page 250 - Chalcogenide Glasses for Infrared Optics

226 Cha pte r Ni ne

absorption at a 4-µm wavelength around 200°C due to thermally
generated intrinsic and extrinsic carriers.
The value of silicon as an infrared optical material was recognized
in the 1960s. Processes were developed at TI to spin-cast silicon domes
for heat-seeking 3- to 5-µm missiles. A spinning graphite convex-
shaped dome mandrel was dipped into a crucible containing molten
silicon that stuck to the surface, slowly forming a dome. For this
application, the absorption level was not critical because the dome
thickness was not great. Silicon was selected because of its outstand-
ing physical properties. Silicon is characterized by a high melting
point, high thermal conductivity, and small thermal expansion, indi-
cating resistance to thermal shock. The inherent strength of the mate-
rial is indicated by its surface hardness and Young’s modulus. Table 9.3
compares pertinent physical parameters for commonly used infrared
optical materials (3 to 12 µm) including silicon. Examination of the
numbers makes one conclude silicon is harder, stronger, and more
resistant to thermal shock than any other material listed except
sapphire. In fact, silicon, sapphire, and glassy quartz are in a class all
by themselves. Considering cost, availability in high-quality form,
and physical properties, silicon would have become the universally
used infrared optical material (1 to 12 µm) except for two reasons:
low use temperature and a lattice absorption band at 9 µm, right in
the middle of the passive infrared night vision band at 8 to 12 µm. For
that reason, in spite of higher cost and inferior physical properties
including lower use temperature, germanium became the FLIR night
vision standard optical material. Silicon was favored for 3- to 5-µm
applications. During the 1970s efforts to find materials capable of
withstanding very high intensity laser radiation, the very low absorp-
tion level of silicon from 3 to 5 µm, were reported. 9
There was great interest in materials that could be used with
high-intensity CW gaseous lasers such as the HF emitting at 2.8 µm,
the DF emitting at 3.8 µm, and the CO emitting at 5.25 µm. Another
laser of interest was the pulsed Er:Yag laser emitting at 2.94 µm. At
this point, the selection of the lasers for weapon systems had not
been made. Great interest still remained in the CO laser emitting from
2
9.3 to 10.8 µm. Although at that time AMI did not have one of these
lasers, the very low absorption of vacuum float zoned silicon was
confirmed in a conventional manner. Taking a P-type single crystal
12 cm in length and 2.5 cm in diameter, the infrared transmission was
carefully measured and the absorption coefficient calculated as a
function of wavelength as far out as 8 µm. The results are shown in
Fig. 9.12.
A value below 0.001 cm was found out to be 5.5 µm. An absorp-
−1
−1
tion level of 0.001 cm corresponds to 0.5 dB/m in fiber language.
The low value and resistance to radiation damage was confirmed at
Quantronix by Z. Drozowicz and M. Cohen using a pulsed Er:Yag