Page 150 - Chalcogenide Glasses for Infrared Optics

126 Cha pte r F i v e

for a small Amtir 5 molded part. The results on the sheet show a PV of
0.427 wave, an rms of 0.067 wave, and a power of –0.271 wave. The Instru-
ment uses a He-Ne laser emitting at 0.6328 µm. Also not shown in Fig. 5.7
is a mechanical device used to measure the wedge of the molded lens.
Each molded lens is evaluated in this manner, and the results are used in
improving the molding process. AMI now molds four of its glasses.

5.5 Antireflection Coatings
Previously it was pointed out that glass surfaces are generally more
inert than crystalline surfaces. Glasses are more resistant to chemical
attack. In applying antireflection coatings, inertness becomes a
handicap. Coating designers then have to revert to what they generally
call glue layers. Each develops her or his own favorite for different
materials. The purpose is to apply a very thin layer, too thin to be
optically significant, to provide a surface to which the first coating
layer will adhere. High melting oxides such as aluminum oxide or
magnesium oxide are examples that are favored by some.
AMI for many years was not involved in coatings. The situation
changed when in 1997 AMI began a SBIR Phase II program with the Navy
to fabricate an infrared imaging bundle from As S glass fibers 10 m in
2 3
length. The program goal was an overall transmission of 50 percent in the
3- to 5-µm band. With an optical path of 10 m to contend with, it seemed
obvious that an antireflection coating to reduce Fresnel reflection losses
would be required to meet the transmission goals. Ed Carr, a former col-
league of the author at TI, was recruited. Carr, although retired, had almost
40 years’ experience at TI and other companies in the area designing and
producing coatings for infrared optics. He joined the program. From a
local company, a rebuilt Temescal unit was purchased with one special
modification. Two flanges were welded on top of the chamber. The ends of
the flanges were fitted with O-ring seals such that fiber ends could be
inserted through the seals to be coated without contaminating the
chamber. All coatings designed by Carr used mixed fluorides in place of
the radioactive thorium fluoride used by many in the industry.
The results of the three-year effort to produce the 10-m bundle
1
have been reported in the literature. It turns out that the antireflection
coatings were not used on the bundle ends because they caused cross-
talk problems between the fibers with a loss in contrast of the image.
After the SBIR program ended in 1999, efforts turned to providing
coatings for all the glasses produced by AMI with special interest in the
low-temperature glasses Amtir 4 and 5 developed for use in molding.
It became important to be able to coat the lenses after they were molded.
New coating designs were developed. A new larger, computer-
controlled, more up-to-date coating chamber was purchased. The Cryo
pump was backed by a turbo pump so that ion beam assisted deposi-
tion could be used to improve coating quality. After deposition, the
coatings must be evaluated relative to reflectivity and tested for