Optical fiber insensitive to temperature variations

Metal-coated optical fibers are often employed in high temperature optical communications applications. However, such optical fibers have been found to evidence a substantial decrease in optical transmission as a function of increased temperature. Optical fibers having a temperature-insensitive optical transmission are obtained by annealing a metal-coated optical fiber at a temperature at which optical transmission is substantially the same as that observed at room temperature. Aluminum-coated optical fibers annealed at 560.degree. C. evidence an optical transmission independent of temperature between about -200.degree. C. and at least about 560.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical fibers of the type commonly used for 
transmitting information by optical signals, and more particularly, to 
improving transmission in such fibers. 
2. Description of the Prior Art 
Optical fibers comprising at least a light-conducting core and a glass 
optical cladding thereon are generally coated with plastic materials in 
order to protect them against the detrimental effects of water vapor, 
which lead to stress corrosion cracking. However, plastic materials tend 
to limit the operation of optical fibers between about -50.degree. C. and 
+150.degree. C. The lower temperature boundary is dictated by 
considerations of flexibility; organic coatings tend to become brittle at 
lower temperatures. The upper temperature boundary is dictated by 
considerations of temperature stability; organic coatings tend to 
decompose at higher temperatures. 
The use of metal coatings has enabled optical fibers to be employed in 
temperature applications which are limited only by the melting point of 
the metal coating. However, it has been observed that metallic-clad 
optical fibers show a substantial decrease in optical transmission as a 
function of increased temperature. Such transmission variations of optical 
transmission are, of course, unacceptable for optical fibers employed at 
elevated temperatures. As used herein, optical transmission includes at 
least the wavelength range from about 0.41 to 0.85 .mu.m. 
SUMMARY OF THE INVENTION 
In accordance with the invention, metal-coated optical fibers evidencing a 
loss in optical transmission as a function of temperature are annealed by 
heating the fibers to at least a temperature at which transmission is 
substantially the same as that observed at room temperature, such 
temperature being higher than the temperature range over which the 
metal-clad optical fiber exhibits a loss in optical transmission. The 
annealing process results in an optical fiber having a optical 
transmission which is substantially independent of temperature up to close 
to the melting point of the metal coating.

DETAILED DESCRIPTION OF THE INVENTION 
The optical fibers beneficially processed in accordance with the invention 
typically comprise a core of about 50 .mu.m in diameter, primarily of 
silica, with other additives such as boron oxide (B.sub.2 O.sub.3), 
germanium dioxide (GeO.sub.2), phosphorus pentoxide (P.sub.2 O.sub.5) and 
the like, with a cladding of vitreous SiO.sub.2 about 35 .mu.m in 
thickness. As is conventional, a barrier of about 1 to 2 .mu.m of a 
borosilicate glass is generally provided between the core and the 
cladding. The optical fibers are drawn from precursor rods, employing 
processes conventional in the art of fabricating optical fibers. 
The optical fibers additionally include a metal coating about 12 to 30 
.mu.m in thickness. The composition of the metal coating may be any metal 
or alloy, such as aluminum, copper, nickel, and the like. The metal or 
alloy is advantageously applied to the fiber as it emerges from a drawing 
furnace by passing the fiber through a pool of molten metal. The 
particular composition of the metal coating and the process of applying it 
to the optical fiber form no part of this invention. 
FIG. 1 depicts the effects of heating (Curve 1) on optical transmission (at 
a wavelength of 0.82 .mu.m) of an aluminum-clad optical fiber up to about 
560.degree. C. As can be seen, there is a substantial decrease in optical 
transmission, beginning at about 225.degree. C., which reaches a maximum 
at about 350.degree. C. The optical transmission returns to approximately 
the observed room temperature value at about 500.degree. C. 
In accordance with the invention, this temperature variation of 
transmission is removed by heating the metal-coated optical fiber to at 
least about 500.degree. C. Below this temperature, there is little 
beneficial effect, and hysteresis of optical transmission as a function of 
temperature is still observed. Above this temperature, the optical 
transmission remains at substantially the room temperature value as the 
optical fiber is cooled (Curve 2). 
FIG. 2 depicts the temperature response of transmission (at 0.82 .mu.m) for 
a metal-coated optical fiber annealed at 560.degree. C. As can be seen, 
there is substantially no temperature variation of transmission from 
25.degree. C. to over 500.degree. C. upon heating (Curve 3) or cooling 
(Curve 4). 
The metal-coated optical fiber may, of course, be heated to a temperature 
greater than about 500.degree. C. The upper limit is dictated by the 
melting point of the metal coating and thus should be at least a few 
degrees below the melting point. 
The time of heating is apparently not critical. It is merely sufficient to 
heat the fiber to at least the minimum temperature and then cool. The 
cooling rate is also apparently not critical; cooling is conveniently 
achieved by shutting off the furnace and permitting the fiber to thereby 
cool to some lower temperature before removal. 
The optical transmission of aluminum-clad optical fibers heat-treated in 
accordance with the invention is also independent of temperature from 
about -200.degree. C. to room temperature. The process of the invention 
thus provides an optical fiber evidencing temperature independence of 
optical transmission from about -200.degree. C. to at least about 
560.degree. C. 
EXAMPLES 
Example 1 
An aluminum-coated optical fiber was prepared. The optical fiber comprised 
a core 50 .mu.m in diameter of 69.6 mole % SiO.sub.2, 29.9 mole % 
GeO.sub.2 and 0.5 mole % P.sub.2 O.sub.5 and a cladding 35 .mu.m thick of 
SiO.sub.2. The thickness of the aluminum coating was 25 .mu.m. The optical 
fiber, which evidenced an optical transmission (at 0.82 .mu.m) of 90.5 
(arbitrary units) at room temperature, was heated to 410.degree. C. and 
cooled. The optical transmission decreased to a value of 55.5 at 
410.degree. C. and recovered to 77.5 at 41.degree. C. The optical fiber 
was then heated to 560.degree. C. The optical transmission was similar to 
that depicted by Curve 1 of FIG. 1; by 500.degree. C., it had increased to 
86. At 560.degree. C., the optical transmission was 89.5. Upon cooling to 
150.degree. C., the optical transmission dropped only slightly to 87. 
Subsequent heating and cooling should show no substantial temperature 
dependence of optical transmission, similar to that behavior depicted in 
FIG. 2. 
EXAMPLE 2 
An aluminum-coated optical fiber, substantially identical in dimensions and 
composition to the optical fiber of Example 1 and evidencing an optical 
transmission (at 0.82 .mu.m) of 90.5 (arbitrary units) at room 
temperature, was heated to 275.degree. C., where the optical transmission 
was 82. The optical fiber was held at 264.degree. C. for 40 hrs to 
determine whether a long term heat soak at the edge of the optical 
transmission dip could be as effective as heating the optical fiber to 
about 550.degree. C. After 40 hrs, the optical transmission decreased to 
39; upon cooling, it increased to 54.5 at 33.degree. C. The fiber was then 
heated to 550.degree. C. The optical transmission was observed to decrease 
to 34 at 320.degree. C., then gradually increase to 86.5 at 500.degree. C. 
and 89 at 550.degree. C. Upon cooling, the optical transmission remained 
substantially constant, and evidenced a value of 89.5 at 55.degree. C. 
Subsequent heating and cooling showed no substantial temperature 
dependence of transmission. 
EXAMPLE 3 
An aluminum-coated optical fiber, substantially identical in dimensions and 
composition to the optical fiber of Example 1, was heat-treated in 
accordance with the invention to 550.degree. C. and cooled to room 
temperature. The optical fiber, which evidenced an optical transmission of 
75.5, was then heated to 550.degree. C. and cooled to room temperature. 
There was substantially no change in optical transmission as a function of 
temperature. 
The fiber was then placed in liquid nitrogen (-196.degree. C.). The optical 
transmission was monitored continuously, and again, evidenced no change as 
a function of temperature.