CA1251076A

CA1251076A - Optical fiber cable with hydrogen combining material

Info

Publication number: CA1251076A
Application number: CA000480147A
Authority: CA
Inventors: Pietro Anelli; Marco Santini
Original assignee: Cavi Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1984-04-27
Filing date: 1985-04-26
Publication date: 1989-03-14
Also published as: NL8501206A; NZ211335A; AU575123B2; GB2158264A; FR2563635A1; SE8502046L; IT8420699A0; NO168208C; GR851014B; GB8510657D0; SE459049B; FR2563635B1; DE3515227A1; IT1176134B; GB2158264B; JPS60239702A; BR8501644A; ES543174A0; MX158233A; NO851685L

Abstract

ABSTRACT OF THE DISCLOSURE

An optical fiber cable comprising at least one optical fiber and a sheath around the optical fiber or fibers. The cable also comprises a gaseous hydrogen absorbing metal or metals from Group III, IV, V or VIII of the periodic table, or alloys or intermetallic compounds of such metals, to absorb the hydrogen and for protecting the fiber with respect to the hydrogen. The metal or metals may be in the form of metal films inside or outside the sheath, in the form of longitudinal wires within the sheath, part of tapes wound around the fiber or fibers or in the form of metal powders dispersed in the sheath or a filler with the sheath. The cable includes a central elongation resistant wire which may, at least at its surface, be made of such metal or metals.

Description

3~

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 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 ~L ~ r~

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, l~t~ .~l.0~7~

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 -4a-~Q

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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 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 ~t~

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 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 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 ~,5~

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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An optical fiber cable structure comprising at least one optical fiber surrounded by at least one protective layer, wherein the improvement comprises forming said protective 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.

2. An optical fiber structure as set forth in claim 1 wherein said powder of said metal is present in said one of the layers in an amount at least equal to 0.01 g./m. of length of the fiber.

3. An optical fiber structure as set forth in claim 1 wherein said metal is selected from the group consisting of lanthanides, titanium, zirconium, hafnium, vanadium, niobium, tantalum, palladium and mixtures, alloys and intermetallic components thereof.

4. An optical fiber structure as set forth in claim 1, 2 or 3 wherein said tape comprises a plastic material.

5. An optical fiber cable comprising a plurality of optical fibers surrounded by a sheath and at least one metal wire within said sheath which extends longitudinally of said optical fibers, and a protective layer around said fibers, said protective 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.

6. An optical fiber cable as set forth in claim 5 further comprising an elongation resistant member within said sheath and extending longitudinally of the optical fibers.

7. An optical fiber cable as set forth in claim 6 wherein said plurality of the optical fibers are disposed around an axis extending longitudinally thereof and wherein said elongation resistant member is disposed at said axis.

8. An optical fiber cable as set forth in claim 5 wherein said metal wire is an elongation resistant member and said metal selected from said Group VIII and subgroup b is at least at the surface of the wire.