Page 158 - Chalcogenide Glasses for Infrared Optics

134 Cha pte r S i x

that molded millions of aspheric camera lenses from silicate-based
glasses and plastics. He presented a paper at the SPIE OE/LASE
Technical Symposium in January 1988 entitled, “Survey of Present
Lens Molding Techniques.” After Pollicove left Kodak, he joined the
American Precision Optics Manufacturers Association (APOMA) at
the University of Rochester. Many companies were members along
with the U.S. Army. Because of the proprietary nature of the AMI effort,
a direct inclusion of Pollicove in the AMI program was not possible.
Shortly after AMI began the molding program, the author met
Harvey Pollicove at a NVL (Night Vision Laboratory) meeting held
at the Army Picatinny Arsenal. Later, Pollicove visited at AMI and spent
some time with the author. He provided a copy of his paper and many
glass molding patents. On return from a trip to Japan, he furnished copies
of a technical nature describing automatic systems produced by Toshiba.
The information he furnished was very helpful to AMI.
The Toshiba units at that time cost $350,000 each, much too expen-
sive for AMI. The units used up to 3500 kg (about 8000 lb) of force,
much too severe, we believed, for chalcogenide glasses. The lenses
are stamped out. Also, the glass blanks were a sphere of glass heated
rapidly in the open. Chalcogenide glasses in contrast to oxide glasses
are volatile when heated. Besides being expensive, the method did
not seem suited to chalcogenide glasses.
Another approach to be considered was injection molding. Mira-
cles have been produced in the injection molding of plastics. The
organic thermal plastics lend themselves quite well to the process.
Raw materials are powdered or supplied in small pieces, easily heated
to form a flowing liquid. However, they are not volatile and not
oxidized when exposed to air. Most contain pigments and do not
require optical homogeneity. A few attempts were made at AMI to use
the glass extruder unit as a source of glass flowing into a mold with-
out any success. The approach was abandoned.
AMI decided to develop glasses well suited for the molding pro-
cess. Low-softening-point glasses were chosen. Lower molding tem-
peratures would increase mold lifetime, a cost factor. Selecting glasses
not containing germanium would lower the cost of the glass.
Molding at low temperature and pressure should minimize the stress
of the molding process which in turn would improve lens quality. An
arsenic-selenium glass composition was selected, produced, and
characterized optically and physically. The glass, designated Amtir 4,
was suitable for use in both the 3- to 5-µm band and the 8- to 12-µm
band. Because of its very low dispersion, Amtir 4 can be used as a
replacement for germanium parts. Amtir 4 has a small negative thermal
1
−6
change in index (−24 × 10 /°C) in both bands due to a large thermal
expansion coefficient. The index change should be contrasted to
−6
the over +400 × 10 /°C for germanium. As a bonus, it was also found
that Amtir 4 could be used to draw small-diameter (50-µm core)
flexible, unclad bare fibers that improved the quality of IR imaging