Method of making an optical fiber communication system

Fusion splicing of optical fiber generally requires removal of a polymer coating from the end portions of two lengths of fiber. A conventional removal method involves immersion of the fiber ends in an appropriate polymer stripping liquid, e.g., in hot, concentrated sulfuric acid. It is known that the strength of fusion splices generally is statistically distributed, such that there exists a probability that a given splice will pass at a given proof test level. We have found that the splice strength distribution can be shifted towards higher strength if the polymer stripping liquid comprises means for insuring that the liquid is essentially free of strength-reducing particles. Preferred means are concentrated nitric acid. Exemplarily, the liquid is at a temperature in the range 170.degree.-200.degree. C. and comprises concentrated (about 95%) sulfuric acid and about 5% b.v. concentrated (about 70%) nitric acid.

FIELD OF THE INVENTION 
This invention pertains to methods of making an optical fiber communication 
system. More particularly, it pertains to methods of signal-transmissively 
joining two lengths of optical fiber. 
BACKGROUND OF THE INVENTION 
In order to be able to make a typical optical fiber communication system, 
it is necessary to be able to abuttingly join two optical fibers such that 
optical signals can pass from one fiber to the other with, desirably, only 
small loss. A well known technique for so joining two lengths of optical 
fiber is fusion splicing. See, for instance, C. M. Miller, "Optical Fiber 
Splices and Connectors", M. Dekker, Inc., 1986 especially pp. 282-293. 
It is not only highly desirable that such fiber splices not introduce 
significant loss into the optical transmission path, but also that the 
splices have high mechanical strength. These requirements are particularly 
severe for, e.g., submarine optical fiber communication systems. 
Exemplarily, some systems require that splices pass a 200 ksi 
(13.8.times.10.sup.8 N/m.sup.2) tensile test, and some undersea systems 
require a 300 ksi (20.7.times.10.sup.8 N/m.sup.2) proof test. 
As is well known, optical fiber conventionally comprises a polymer coating. 
This coating has to be removed from the relevant end portions of the 
fibers that are to be joined by fusion splicing. This is conventionally 
accomplished by dipping in hot (e.g., 180.degree.-200.degree. C.) 
concentrated sulfuric acid, whereby the coating is etched away. See, for 
instance, C. M. Miller, op. cit., page 292. Subsequent to fusion splicing 
the bare fiber portion typically is re-coated. By "etching" of the polymer 
coating we mean a chemical removal process that can result in a relatively 
sharp boundary of the remaining polymer, and does not damage the fiber. 
This is to be contrasted with a removal process, (e.g., one involving a 
swelling process) which does not result in a sharp boundary. 
It has been observed that typically only a relatively small fraction 
(frequently &lt;50%) of prior art fusion splices can pass the 300 ksi 
strength test, frequently requiring re-splicing. This is obviously costly 
and thus undesirable. It will be understood that associated with a batch 
of fusion spliced optical fibers is a strength distribution, with a 
certain percentage of the fibers passing at a given proof test level. 
In view of the importance of increasing the fraction of fusion splices that 
passes an appropriate strength test, a method of splicing optical fiber 
that can yield splices of improved strength would be desirable. This 
application discloses such a method.

THE INVENTION 
The invention is based on our discovery that the presence of particles in 
the liquid that is used to remove the polymer coating of the fibers is 
frequently correlated with relatively low splice strength. 
Generally speaking, the invention is an improved method of making an 
optical fiber communication system that comprises an end-to-end fusion 
splice between two lengths of optical fiber. More particularly, the 
invention is embodied in an improved method of fusion splicing of optical 
fiber than can result in a significantly greater percentage of fiber 
splices passing a given proof test level than prior art splices. 
The method of making the optical fiber communication system typically 
comprises providing two lengths of polymer-coated optical fiber, etching 
the polymer coating from an end portion of each of the two lengths of 
optical fiber, joining the two lengths by abuttingly fusing the 
polymer-free ends, and coating the polymer-free portion of the joined 
fibers. Etching the polymer coating comprises contacting the respective 
end portions with a liquid that attacks the polymer, in a controllable 
fashion without damaging the fiber. Exemplary of such a liquid is 
concentrated (about 95%) sulfuric acid. Significantly, the liquid also 
comprises means for insuring that the liquid is, during the etching step, 
essentially free of particles that are effective in reducing the strength 
of the resulting fusion splice, exemplarily particles who can adhere to 
the fiber surface and interact with the fiber. Such particles will be 
referred to as "strength-reducing" particles. In a currently preferred 
embodiment the liquid comprises concentrated sulfuric acid and nitric 
acid, typically at a temperature in the range 170.degree.-200.degree. C. 
It is known that concentrated sulfuric acid is a moderate oxidizing agent 
whose potency depends on temperature. It is also known to be a powerful 
dehydrating agent and can readily carbonize a variety of organic 
materials, including polymers of the type used for optical fiber coatings 
(e.g., aliphatic acrylate-based polymers). 
In an exemplary embodiment of the invention the percentage of fiber splices 
that pass a 200 ksi proof test exceeds by at least 5%, preferably at least 
10%, the percentage of, otherwise identically prepared, prior art splices. 
Equivalently, the probability that a splice according to the invention 
passes a 200 ksi proof test is at least 5%, preferably at least 10% 
greater than the probability that an, otherwise identical, prior art 
splice passes that test. Those skilled in the art will appreciate that 
statements about splice strength are inherently statistical in nature, 
requiring consideration of a sample of splices. Typically such a sample 
will contain more than 10, preferably more than 20 splices. 
We have observed that the prior art hot sulfuric acid used to etch polymer 
coatings typically turns orange-brown after just a few etching operations, 
and have observed a correlation between increasing color and decreasing 
splice strength. This typically requires replacement of the sulfuric acid 
after a relatively few etching operations. We currently believe that the 
observed color change is due to an accumulation of colloidal particles, 
most likely carbonaceous particles, in the liquid. Addition of 
concentrated (about 70%) nitric acid (typically 2-10% by volume, e.g., 
about 5%) can prevent or at least substantially delay the color change. We 
currently believe that the nitric acid serves to substantially completely 
oxidize the polymer, exemplarily to CO or CO.sub.2, thus avoiding 
formation of carbonaceous particles. Thus, at least in principle, any 
other compatible strong oxidizing agent (e.g., perchloric acid) that can 
completely oxidize the polymer could be used instead of nitric acid. 
Nitrate salts could in principle be used but are not preferred because it 
is desirable to keep the fiber surface free of metal ions. Nitric acid is 
the currently preferred additive, offering the advantages of 
effectiveness, convenience and safety, in addition to low cost, as 
compared to other, more powerful oxidants. 
The presence of strength-reducing particles of any kind in the 
coating-removing liquid being potentially detrimental to fiber strength, 
it is desirable to insure the absence of such particles in the virgin 
liquid, as well as to prevent contamination of the liquid with such 
particles. The former indicates use of commercially available "low 
particulate" reagents, and the latter indicates provision of a 
substantially particle-free environment. 
Subsequent to polymer removal, the bare fibers advantageously are rinsed in 
a "particle-free" organic solvent such as methanol to remove residual 
acid. Solvents that do not attack the remaining polymer are preferred. 
Alternatively, rinsing in "particle-free" pure water is possible. 
FIG. 1 shows data on arc fusion splice strength for fiber whose polymer was 
etched away in prior art hot sulfuric acid. FIG. 2 shows analogous data 
for identical fiber whose polymer was etched away in 185.degree. C. 
sulfuric and (5% by volume) nitric acid according to the invention, 
followed by a water rinse. The improvement is clear and striking, with the 
percentage of splices according to the invention that pass the 200 ksi 
proof test exceeding the corresponding percentage of prior art splices by 
more than 10%. The data are presented in form of conventional Weibull 
plots. 
An optical fiber communication system according to the invention comprises, 
in addition to at least two lengths of optical fiber that are spliced in 
accordance with the invention, typically also such conventional means as 
optical signal generating means, means for coupling the signal into the 
optical fiber, means for regenerating or amplifying the optical signal, 
and signal detection means. The optical fiber typically is single mode 
silica-based fiber. Such fiber is well known.