Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OPTIC~L FIBER CABLE WITH HYDROGE~ COMBINING MArrERIAL
The present invention relates to an optical fiber
cable utilized for telecommunication in which the optical fiber,
or fibers, is protected against the absorption of gaseous
hydrogen.
The absorption of hydroge~ has adverse effects on the
properties of an optical fiber, amongst which effects are the
increased attenuation which results following the exposure of the
fibers themselves to gaseous hydrogen and a degradation in the
mechanical properties of the fiber.
In cables which comprise one or more optical fibers,
the transmission properties of the fibers sometimes deteriorates
in cases where the fibers are subjected to the action of hydrogen
which originates from members which are either outside or inside
the cable.
In actual fact, even the mechanical characteris~ics of
the fiber are altered a~though by such hydrogen, as a rule, the
effects of the macroscopical increased attenuation which are the
first to become manifest. In fact, the fibers affected by the
hydrogen are found -to have an increased attenuation, especially
for wavelenghts of over 1 micron, i.e. at the wavelengths utilized
for transmitting the signals.
Tests which have been carried out have demonstrated
that a first source of an increased attenuation arises because
of the hydrogen itself which, once diffused inside the fiber, is
capable of absorbing energy in a spectrum comprising the wave-
lengths utilized for the optical signals.
Under particular conditions, this phenomenom is re-
versible, and the attenuation due to it becomes reduced, even
appreciably, if the hydrogen is allowed to be diffused outside
the fiber, for example, due to a lowering of the external hydrogen
concentration which originated the phenomenon.
In other cases, it was possible to establish that a
second source of attenuation is to be associated with chemical
reactions taking place between the main constituents of the fiber,
for example, SiO2 and/or its dopants, for example ~eO2, P2O5,
etc., and the hydrogen diffused 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 utilized for the
transmission of signals. These latter reactions are irreversible
and hence, the corresponding deterioration in the fiber properties
can be expected under all operating aonditions.
The parameters which control this phenomenon are,
apart from the chemical composition of the fiber, the partial
hydrogen pressure to which the fiber is exposed, the temperature,
and of course, time.
The fiber can come into contact with the hydrogen
generated either during the cable manufacturing process, or else
during the operation of the cable itself. In fact, the hydrogen
can be generated by the metallic or non-metallic members which
are present in the cable which have absorbed said gas during the
manufacturing, treating or finishing processes for the materials
forming the cable.
Moreover, the hydrogen can 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 (either in a liquid state or as vapor~ which is
eventually present in the cable, with metallic members forming
the cable itself.
Moreover, certain organic materials that are sometimes
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used in the fiber coating, are capable of producing hydrogen
owing to chemical reactions of various natures. The diffusion of
the hydrogen through the various materials varies in rate and
increases from a low for metals with passing from higher values,
successively, for polymers, liquids and gases.
Therefore, depending upon the type of cable and upon the
environment wherein it is utilized, various emission rates will
be had for the hydrogen produced by the members constituting the
cable and the cable will have various absorption rates for the
hydrogen which is eventually produced outside the cable and which
permeates the cable operating environment. The value of the
partial hydrogen pressures inside the cable depends upon these
various rates and is a function of time, i.e., the greater the
pressure and the duratio~ is, the greater will be the risk level
for the fibers.
~ n general, it is necessary, in each case, to take into
consideration a detailed balance of the production rate of the
hydrogen (either originating inside or outside of the cable), the
diffusion rate of the hydrogen through the cable sheath and
finally, the spreading rate of the hydrogen through environmental
means, for the purpose of establishing what partial hydrogen
pressure will be, during a transient period and eventually in a
steady state condition, in proximity to the cable fibers.
For example, given the service lifetime of an optical
fibers cable, under foreseeable temperature and pressure con- -
ditions, the diffusion rate of the hydrogen through the metals is
so low that the metallic sheaths of a normal thickness can be
considered as being practically impermeable for the hydrogen. In
particular, cables having metallic sheaths, especially if they
also have a small inner space, are cables which, within a short
time and at high levels, can show an increase of attenuation due
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to the hydrogen liberated by the elements found inside the
sheath.
The object of the present invention is to provide an
optical fiber cable provided with a protection against the
absorption of gaseous hydrogen by the optical fibers found in
the cable.
This protection is obtained, according to the inven-
tion by introducing, in a suitable form into the cable, at least
one metallic element that is capable of absorbing the hydrogen
and coinbining with it.
According to one aspect, the present invention pro-
vides an optical fiber cable comprising a plurality of optical
fibers surrounded by a sheath and at least one metal wire with-
in said sheath which extends longitudinally of said optical
fibers, and a protective layer around said fibers, said protec-
tive layer being formed, at least in part, of at least one
gaseous hydrogen absorbing metal selected from Group VIII and
subgroup b of Groups III, IV, V of the periodic system for
protecting the optical fibers with respect to the absorption
of gaseous hydrogen.
According to another aspect, the present invention
provides an optical fiber cable structure comprising at least
one optical fiber surrounded by at least one protective layer,
wherein the improvement comprises forming said proteckive layer
with a tape including at least one gaseous hydrogen absorbing
powder of a metal selected from Groups III, IV, V and VIII of
the periodic system for protecting said fiber or fibers with
respect -to the absorption of gaseous hydrogen.
Among these metals, those that have proved to be
particularly suitable are lanthanum for the Group III, titanium,
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zirconium and hafnium for the Group IV, vanadium, niobium for
the Group V, and palladium for -the Group VIII, in the form of
pure metals, their alloys and/or intermetallic compounds.
In the presence of hydrogen, the above-indicated
elements tend to form solid interstitial solutions that are
similar to hydrides, having a good stability, and this permits
the reduction of the partial hydrogen pressure in the cable to
values which balance with the hydrogen solubility in the
elements themselves.
By utilizing appropriate quantities of these ele-
ments, one can succeed in limiting the residual pressure values
of hydrogen in the cable, in such a way as to render negligible
the
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adverse effects of said hydrogen pressure on the fiber properties
and in particular, upon their increases of attenuation throughout
the entire foreseen service life of the cable.
Preferably, the above-stated elements are subjected to
a thermal treatment under vacuum, at a temperature of over 1600C.
In fact, it has been verified that after said treatment, the
described elements become more active in absorbing hydrogen,
particularly at low partial pressure values.
It is assumed that these elements can, in some cases,
already contain a certain quantity of hydrogen and/or other gases
that were absorbed during the manufacturing, purification and
finishing processes of the elements themselves and that they have
a certain level of superficial oxidation. Both these phenomena
could reduce the efficacy of the protection against the hydrogen,
and the thermal treatment at temperatures that are approximate to,
but less than the melting temperature, provide a degasification
and/or the elimination of the superficial oxidation through
sublimation.
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 con~unction with the accompanying drawings in
which:
Fig. l is a schematic, perspective view il-
lustrating the structure of part of an optical
fiber cable which may include the invention; and
Figs. 2 to 6, schematically show cross-sections
of the inside of optical fiber cables including various
embodiments of the invention.
The optical fiber cablel0 shown schematically in Fig. l comprises
an optical unit 12 formed by six optical fibers 22 laid on a
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traction-resistant member 12 and covered by one or more tapes 14.
The optical unit 22 is contained inside a sheath 16, over which
there are provided other layers, coverings and various structures,
depending upon the type of cable, which is schematically il-
lustrated by the layer 20.
The sheath 16 can be an impermeable meta-lic sheath, for
example, of a submarine cable, or else a sheath of plastic
material. Inside, the sheath 16 there can be contained a filler
having a mechanical function, e.g., a non-vulcanized thermoplastic
compound of ethy]ene-propylene or polyvinylchloride or a water-
blocking filler, such as petroleum ~elly or a silicone grease
which may include a swelling agent, such as carboxymethylcellulose,
etc.
The optical unit 22 can comprise longitudinal supporting
traction-resistant members different from the member 12, and the
fibers can either be of the "loose" type or the "tight" type,
i.e., loosely enclosed by a covering or covered with a layer
tightly engaging the fiber. In view of this, the illustration
given in Fig. 1 is to be understood as being only general and
schematic and is given only for the purpose of facilitating the
understanding of the invention.
According to a first embodiment,illustrated in Fig. 2
and which is particularly suited for protecting a cable already
containing a filler 21 for the purpose of limiting any eventual
penetration of water in a submarine cable, the filler material 21,
which occupies the spaces within the outer sheath 16 (that can be
in the order of about 5 cm3 per meter of cable~ which are not
occupied by fibers and other element~s, contains a dispersion of
powders of one or more elements of the Groups III, IV, V and
VIII of the perioclic system, amongst which lanthanum, titanium,
zirconium, hafniumr niobium, tantalum and palladium, their
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alloys and/or intermetallic compounds, are preferred.
The quantity of powders introduced into the filler
material 21 depends upon the type of cable, upon its geometry
and upon the element (or elements) selected from those described
and of which these powders are constituted, upon their shape and
upon the size of the granules.
In the case of a cable hav:ing a water-blocking filler
underneath a metallic sheath of normal dimensions, e.g. 5-20 mm.
depending upon the number of optical fibers enclosed by the
sheath, it has been found, for example, that a quantity of between
10 and 100 mg of palladium in powder form per meter of cable and
having particles with dimensions of between, for example, 10 and
lO0 microns, preferably, 30-50 microns, is sufficient for pro-
tecting the fibers against the hydrogen quantities and pressures
which develop in this type of cable.
It must be pointed out here that the filler to which
the powders are added does not necessarily have to be the water-
blocking filler of a submarine cable. The cable could already
have a filler for other purposes, for example, for making the
structure more compact and to which the powders are added later,
or else, as an alternatlve, the cable could originally be devoid
of a filler, and in such case, the filler would be added expressly
for including the powders.
In a second embodiment shown in Fig. 3, the cable
comprises at least one outer, elastomeric or plastomeric sheath
16a inside which are dispersed the powders of one or more elements
of the Groups III, IV, V and VIII of the periodic system, pre~
ferably, lanthanum, zirconium, hafnium, vanadium, niobium,tantalum
or palladium, or their alloys and/or intermetallic compounds.
30The size of the dispersed powders is, in this second
embodiment, reduced (on the order of a few microns) with respect
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to the previous embodiment. This second embodiment, which is
particularly suitable for protecting optical fiber cables which
are devoid of an outer metallic sheath and which are used in
environments having a high hydrogen content, requires the
adoption of mixtures having, for example, at least 0.1 phr (parts
per hundred of resin) of palladium in the production of said
outer sheath. A range of 0.1-10 phr is preferred, and the
palladium should be at least 0.01 g./m. of cable length.
In a third embodiment (Fig. 4), the cable comprises
one or more wires 18 formed, at least at the exterior, by one
or more elements of ghe Groups III, IV, V, VIII of the periodic
system, preferably, lanthanum, titanium, zirconium, hafnium,
vanadium, niobium, tantalum or palladium, or one of their alloys
and/or their intermetallic compounds. The wire or wires 1~ can
form the traction-resistant member (12 in Fig. 1) or else one of
the components of the traction-resistant member, and in such
cases the fibers are helically disposed around it. ~s an alter-
native, said wire 18 can be added to the members which are already
found present in the cable as shown in Fig. 4.
This embodiment is particularly suited for cables
having a large inner free space between the elastomeric sheath 16b
and the fibers 23, for example, on the order of about 50 cm3 per
meter of cable, and it requires, in case the metal used in
palladium, a wire having a diameter in the range of from 0.02 to
0.2 mm in order to protect the fibers against the action of the
hydrogen in the quantities and at the pressures that are developed
in this type of cable.
Since the absorption phenomenon involves only the outer
surface of these metals~ the wires can be made from other
materials and coated externally by a layer of the described metals
which is thick enough, e.g. 0.02 to 0.2 mm, to provide the desired
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results. In this case, the diameter of the wires are obviously
different.
Fig. 5 illustrates a further embodiment whereby the
protecting members are obtained by means of a coating or film
made from one or more of the described metals (or their alloys
and/or intermetallic compounds) disposed around the optical unit
or units. In the cable of Fig. 5, the outer surface of the
sheath 16b is metallized with a film or coating 19. As already
stated, such film or coating 19 may be one or more of the metals
cited andtor their alloys or intermetallic compounds.
The choice of the metallic combination utilized depends
upon various factors amongst which are the cost involved, the
efficacy of the metal in absorbing hydrogen, the availability of
the metal, its workability, etc. However, in the case of certain
combinations, the phenomena of an improved efficacy has been
noted, in particular, for a mixture of niobium and zirconium
which is used in the form of wires, or as a metallization layer.
The excellent performance of this mixture is probably due to the
fact that, apart from both of these metals being hydrogen
absorbers, zirconium combines very easily with oxygen, thereby
protecting the niobium. An alloy of niobium and zirconium in
which the zirconium content is 15-25~ by weight is preferred,
but other compositions, such as an alloy having equal parts by
weight of niobium and zirconium may be used with good results.
According to a fifth embodiment, illustrated in Fig. 6,
the cable comprises a layer of film 1~1 of at least one of the
already cited elements and/or their alloys and/or intermetallic
compounds, applied on a tape of plastic, or else a metallic tape,
for example, steel, Al, Cu, etc.,), or metal-plastic laminated
tape, for example, aluminum covered with polyethylene, which
provide a wrapping 1~ for the optical cable unit 2.
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This embodiment, which is particularly suitahle for
protecting cables which can be attacked by an external source of
hydrogen, requires a thickness for a palladitlm layer which is
in the range of from l to 20 microns for tapes which are wound,
at short pitch, over an optical unit having the usual dimensions,
e.g. 8-lO mm. in diameter, to obtain the protection of the fiber
under the normally foreseen conditions of use. With other metals,
particularly metals not in Group VIII, the tape thickness should
be greater.
In the case of external sources of hydrogen, it is
preferable for the active layer to face outwardly.
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
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 of 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 shouId not be less than
10 m~m. of cable. A preferred range is between froml5 to 150mg./m
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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 proper*ies
of the layer. The latter considerations apply when other metals
are used.
It must be understood that the various embodiments
illustrated herein are not incompatible with one another and that
they can, in fact, co-exist and be rendered advantageously
complementary in a same cable.
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.
