Page 178 - Chalcogenide Glasses for Infrared Optics

154 Cha pte r Se v e n

In 1984 AMI was approached by Codman Fiber Optics, a com-
pany owned by Johnson & Johnson, to join in a research effort to
develop a chalcogenide glass to serve as a source of fibers capable of
transmitting substantial energy from a CO laser emitting at 10.6 µm.
2
Codman is the surgical division of Johnson & Johnson, and its desire
was to be the first with a laser scalpel. Also involved in the project
was Kathy Laakman of Laakman Electro-Optics (later known as
Synrad) which was to supply the carbon dioxide lasers required for
the scalpel. It turned out that Tom Loretz, now a senior scientist at
Codman, originated the program. Our chances for success improved
because Loretz was not only well versed in fiber technology but had
a background in glass science, a Master’s in Glass Science from Alfred
University. The Codman program manager was Mr. Igino Lombardo.
At this time, the FDA had approved the CO laser emitting at 10.6 µm
2
for use in surgery. The original goal of the program was an absorp-
tion level at 10.6 µm for the fiber of 0.5 dB/m which corresponds to
−1
a bulk absorption of 0.001 cm and at 4 µm a goal of 1 dB/m or
−1
0.002 cm . One should realize that such low absorption levels in
infrared optics were very unusual. Lenses and windows in infrared
systems were built using materials with absorption levels of, at best,
–1
0.01 to 0.03 cm . Lenses and plates were only 1 to 2 cm thick. Fibers
could be 100 cm or meters in length. Silicate fiber people spoke in
terms of decibels per kilometer. For this application, the low absorption
was important to avoid the fiber heating up and failing while trans-
mitting the laser energy. The chalcogenide glasses have low thermal
conductivity. Absorption of the laser energy raises the temperature
along the axis of the fiber. The sign of the change in refractive index
for the chalcogenide glasses is positive. The result is the formation of
a positive lens or thermal lensing. As a result, the fiber melts either at
the ends or in the middle.
During the program it became apparent that an absorption level of
−1
0.001cm at 10.6 µm was below the intrinsic level for selenium-based
−1
glasses. Codman agreed to change its required level to 0.003 cm or to
1.3 dB/m. Still later in 1985 after much effort had been expended to
lower the 10.6-µm level, Codman agreed to accept core material with
−1
measured values less than or equal to 0.005 cm or 2.3 dB/m. At
the same time, AMI pointed out to Lombardo that the lower level for
the glasses at 9.27 µm, the wavelength of another strong emission line
from the carbon dioxide laser, had already been achieved. Switching
to the other line should be just as useful in surgery as the 10.6 µm.
Neither the doctor nor the patient could tell the difference between the
two wavelengths. The suggestion was refused by Lombardo on the
grounds that there was no FDA approval for the use of the 9.27-µm
emission for surgery.
Both core glass and clad glass were to be developed as proprietary
and exclusive to Codman & Shurtleff of Johnson & Johnson. The clad