Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OPTICAL FIBER AND cAsLE WITH HYDROGEN COMBI~IING LAYER
The present invention relates to the protection of an
optical fiber with respect to the absorption of gaseous hydrogen,
particularly, when the optical fi~er is incorporated within a
cable.
In cables comprising one or more opticalfibers, there
is found, at times, a deterioration in the transmission properties
of the fibers if the fibers are subjected to the action of
hydrogen however such gas is generated, e.g. by members which are
either outside or inside the cable.
In actual fact, even the mechanical characteristics of
the fiber are modified by such hydrogen although, as a rule, the
macroscopic effects of increased attenuation are the first to
become apparent. In fact, the fibers affected by hydrogen show
an increase of attenuation for the wavelengths higher than 1
micron, i.e. at the wavelengths utilized for transmitting the
signals.
Generally, the optical fibers comprise a glass
structure formed with a cladding and a core of the "step index",
of "graded index" type,, but they may have even other structures,
and a primary coating is applied to the fiher immediately after
its formation, for the purpose of preventing the fiber from
having any direct contact with the outside environment. Over
said primary coating, there are applied other protective coatings,
for example, constituted by a layer of silicone rubber and by a
more rigid layer or tube made, for example, of nylon.
An optical fibers cable generally comprises one or more
optical fibers, enclosed in a sheath, together with one or more
traction-resistant members. Said sheath, which can either be
metallic or not, is, in its turn, surrounded by other mechanical
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members such as armorings, coverings, etc.
Tests which have been carried out have demonstrated
that a primary cause of attenuation in the optical fibers in-
corporated in a cable is constituted by the hydrogen which, once
it becomes diffused inside the fiber, is capable of absorbing
energy in a spectrum comprising the wavelengths utilized for the
optical signals.
Under particular conditions, this phenomenon is
reversible, and the attenuation can even be considerably reduced
if it is possible for the hydrogen to diffuse towards outside the
fiber, for example, due to a lowering of the outside concentration
of hydrogen which caused the phenomenon.
On the other hand, in other cases, it has been possible
to establish that a second cause of attenuation must be attributed
to chemical reactions taking place between the main constituents
of the fiber, for example, Si02 and/or its dopants Ge02, P205,
etc., and the hydrogen which are contained inside the fiber itself.
The result of these reactions is the formation of
groups containing the hydroxyl radical ~OH) which are responsible
for the absorption at the wavelengths which are also used for the
transmission of signals. These latter reactions are irreversible,
and hence, there is a ~orxesponding deterioration of the fiber
properties which can be expected under all conditions of use.
The parameters which control these phenomena are, apart
from the chemical composition of the fiber, the partial pressure
of the hydrogen to which the fiber is exposed, the temperature
and, of course, time.
The fiber can come into contact with the hydrogen
generated inside the cable, either during the cable manufacturing
process, or else during the operation of the cable itself. As a
matter of fact, the hydrogen can be generated by metallic or
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non-metallic members present in the cable which have absorbed said
gas during the manufacturin~, treating or finishing processes for
the materials forming the cable.
The hydrogen can also be generated because of the
eventual chemical degradation, through the oxidation, of the
organic materials forming the cable, or else through the reaction
of the water, ei-ther in a liquid state or as vapor and eventually
present in the cable, with the metallic members of the cable in-
cluded in the cable structure.
Certain organic materials used in the fiber cladding,
are capable of producing hydrogen due to chemical reactions of
various natures. It has been found that one hydrogen source is
constituted by the protective coatings themselves, and in paxti-
cular, when the protective coating is the silicone rubber. As a
theory, it is assumed that when the cross-linking process is pro-
longed in duration, there is a liberation of hydrogen at the
fiber surface. The spreading of the hydrogen takes place towards
the fiber, as well as towards outside of the protective coating
but does not cause any appreciable phenomena on the outside of
the coating because, in this case, the hydrogen becomes dispersed
in the surrounding environment.
Nevertheless, when the fiber is situated within a closed
cable and without there being sufficient free space around the
fiber, the hydrogen concentration can achieve relatively high
values which cause its appreciable diffusion, even towards the
fiber itself, aided by the fact that the cladding, from which the
hydrogen is developed, is very near to the fiber.
The diffusion of the hydrogen through the varlous
materials varies with the material and is the lowest with metals
but increases, successively with polymers, liquids and gases.
Hence, depending upon the type of cable and upon the environment
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wherein it is utilized, there may be several rates for the
emission of the hydrogen produced by the cable members. There-
fore, there also are diverse rates of absorption, on the part
of the cable, of the hydrogen eventually produced outside it
and which permeates the operating ambient. The value of the
partial pressure of the hydrogen inside the cable depends on
these various rates and is a function of the time, because the
greater the pressure and the duration are, the greater the level
of risk for the fibers will be.
Given the service lifetime of an optical fibers
cable, under foreseeable temperature conditions, the diffusion
rate of the hydrogen through the metals is so low -that metallic
sheaths of a normal thickness can be considered as being prac-
tically impermeable to the hydrogen. In particular, the cables
having metallic sheaths, especially if they have a small space
inside them, are cables which have, in a short time and at high
levels, increases in attenuation due to the hydrogen which is
liberated from the elements inside of the sheath.
The object of the present invention is to provide an
optical fiber which is protected against the absorption of gas-
eous hydrogen which may be present in the cable containing the
fiber. Such protectiqn is obtained, according to the invention,
by providing around the outermost layer of the fiber itself,
one or more coatings containing metals which are capable of
combining with the hydrogen and which form a barrier in corres-
pondence to said coating.
According to the invention, an optical fiber, having
at least one protective coating, is characteri~ed by the fact
of including, in at least one of said protective coatings, at
least one powder of a metal selected ~rom the ~roups III, IV,
V, VIII of the periodic system as a protection against the ab-
sorption of gaseous hydrogen on the
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part of the fiber.
The metals which have proved to be particularly suitable
are lanthanides for Group III; titanium, zirconium and hanium
for Group IV; vanadium, niobium and tantalum for Group V; and
palladium for Group VIII, in the form of pure metals, their
alloys or intermetallic compounds.
In the presence of hydrogen, the identified elements
tend to form solid interstitial solutions which are similar to
hydrides having a good stability, and this allows Eor a reduction
in the partial hydrogen pressure in the cables to values which
counterbalance the solubility of the hydrogen in the members
themselves.
Preferably, the identified elements are suhjected to ~
thermal treatment, under vacuum, at temperatures of some hundreds
of degrees centigrade, prior to being utili~ed in cable pro-
duction, for the purpose of eliminating any hydrogen which may
have been absorbed, and/or the combined oxygen.
Other objects and advantages of the present invention
will be apparent from the following detailed description of the
presently preferred embodiments thereof, which description should
be considered in conjunction with the accompanying drawings in
which:
Fig. 1 is a schematic cross-section of an optical
fiber provided with a primary metallic coating of the
invention;
Fig. la is a schematic cross-section of an
optical fiber provided with a metallic coating of the
invention around a primary coating on the fiber;
Fig. 2 is a schematic cross-section of an optical
fiber provided with a primary insulation-metal powder
coating of the invention;
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Fig. 3 is a schematic cross-section of an
optical fiber provided with a primary coating and a
secondary coating of the invention;
Fig. 4 is a schematic cross-section of an optical
fiber provided with a primary coating and a secondary
coating of the invention, between which there is inter-
posed a cushioning layer; and
Fig. 5 is a schemat:ic cross-section of a coated
optical fiber loosely enc:Losed by a small tube, the
space between the tube and the fiber being filled with
a gel containing metal powder according to the invention.
With reference to Fig. 1, an elementary optical fiber comprises a
glass portion 1 of any type whatsoever, i.e. 'tstep index",
"graded index" or other types, and a primary coating 2 adjacent
and contacting the portion 1, the coating 2 having the function
of protecting the fiber from the outer environment.
According to a first embodiment of the invention, the
glass portion 1 is protected by a metallizing layer formed by one
or more of the materials described. Said layer can constitute
the primary coating 2 shown in Fig. 1 in close contact with the
glass structure of the optical fiber. Thus, there is obtained a
fiber where the primary coating is of the metallic type and which,
at the same time, performs the mechanical function as well as the
function of safeguarding against the absorption of hydrogen by
the environment surrounding the fiber, during operation.
According to a variation shown in Fig la, th~ metal-
lizing layer 2 is applied immediately over the usual primary
coating la made oE cross-linked resin. This construction is
utilizable whenever it is not possible or convenient to modify
the plant ~or producing the fiber so that the protective coating
is applied immediately after drawing the optical fiber to the
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desired dimensions.
As a further variation, the coating la may be one or
more coatings described hereinafter.
In accordance with a further embodiment, illustrated in
Fig. 2, the primary coating 2a, made of acrylic resin or of some
other suitable material, contains a dispersion of the powders of
one or more of the cited metals, or their alloys or inter-
metallic compounds. I'his permits the incorporation of the pro--
tective coating in a conventional manufacturing process.
A further embodiment (Fig. 3) adds the metallic powders
to the resin coating 3 immediately surrounding the primary
covering la. This coating 3 is typically made of silicone rubber
and, as explained previously, silicone rubber can become a parti-
cularly dangerous source of hydrogen. The presence of the metals
in this coating 3 effectively neutralizes the hydrogen which is
generated, even before it can diffuse towards the fiber.
The optical fiber illustrated schematically in Fig. 4,
is a further embodiment of the invention which has a dispersion
of metallic powders in the secondary coating 4 which is con-
stituted, for example, out of nylon or some other thermoplastic
polymer.
In the embodiments described, the particles of the
powders have dimensions which are, preferably, less than 10
microns, and the quantity of the powders per length unit of the
optical fiber is selected to achieve a concentration within the
range of from 0.1 to 10 phr (parts per hundred of resin) in the
resin. The range of 0.1-10 phr is preferred, but can vary de-
pending on the thickness of the coating. The metal content of
the coating should be at least 0~01 g./m.
It must be kept in mind that the protective function,
according to the invention, is accomplished in various ways,
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depending upon the coating in which the metals are incorporated~
More precisely, the presence of a protective layer very close to
the optical fiber, protects the la-tter mainly against the hydrogen
generated in the innermost protective coatings, while an outer
protective coating, for example, around the silicone rubber,
provides mainly a protection from the hydrogen derived from the
cable elements.
In view of what has been stated, as other factors
depending upon the structure and the foreseeable conditions of
operation of the cable, it will be apparent that the several pre-
viously described different embodiments can be combined in a same
optical fiber.
A further embodiment of the invention, which is il-
lustrated in Fig. 5 r comprises an optical fiber 1 having a
primary coating la enclosed by a small tube 9 of plastic material,
the inner diameter of which is greater than the outer diameter
of the fiber 1. The fiber 1 may be provided with the usual
coatings for constituting an optical fiber of the loose type. For
this type of fiber, which may have non-adherent coverings, the
protection can be realized by having coatings such as those de-
scribed previously and/or by providing inside the small tube 9 a
gel 8, such as petroleum jelly or a silicone grease, containing
a dispersion of powders of the described metals or their alloys
or intermetallic compounds.
As an alternative, combined or not with the immediately
preceding embodiment, the material that constitutes the small
tube 9 may contain a dispersion of powders of the described metals
or of their alloys or intermetallic compounds.
In the various embodiments, the content of the metal
selected from one of -the Groups III, IV, V and VIII depends upon
the amount of hydrogen which it is expected will be released or
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generated during the life of a cable containing the fiber. There-
fore, the metal content depends on such things as cable size,
materials, treatments, environment, etc. It is desirable to keep
the hydrogen partial pressure content within the cable below
1-2 mm. Hg. The metal content should be the minimum amount
determined to be necessary plus a small additional amount for
safety reasons. The upper limit of the metal content depends
upon cost and the effect oE the metal content on the physical
properties of a coating incorporating the metal in powder form.
Palladium is,a preferred metal because it can be used
in smaller amounts. Although other metals are less expensive,
the niobium content, for example, should be of the order of ten
times, by weight, the palladium content and the zirconium content,
for example, should be of the order of one hundred times, by
weight, the palladium content.
The content of palladium should not be less than lOntg/m.
of cable. A preferred range is between from 15 to 150 mg./m. of
cable. Preferably, the palladium particle size is not greater
than 10 microns when the material in which it is admixed is nylon
to avoid significant alteration of the physical properties of the
coating. The latter considerations apply when other metals are
used.
Although preferred embodiments of the present invention
have been described and illustrated, it will be apparent to those
skilled in the art that various modifications may be made without
departing from the principles of the invention.
