GB2158264A

GB2158264A - Optical fibre cable

Info

Publication number: GB2158264A
Application number: GB08510657A
Authority: GB
Inventors: Pietro Anelli; Marco Santini
Original assignee: Pirelli Cavi SpA; Cavi Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1984-04-27
Filing date: 1985-04-26
Publication date: 1985-11-06
Also published as: GR851014B; FR2563635B1; ES8608182A1; AU575123B2; IT1176134B; BR8501644A; ES543174A0; FR2563635A1; NO168208B; NL8501206A; NZ211335A; NO851685L; SE8502046L; SE8502046D0; SE459049B; MX158233A; NO168208C; GB2158264B; AU3927385A; DE3515227A1

Abstract

An optical fibre cable for telecommunications comprises a sheath 16 within which there is disposed at least one optical unit having at least one optical fibre 10. The or each optical fibre is protected against the absorption of hydrogen which will cause increased attenuation of the signal being transmitted by the presence in the cable of at least one metallic element of Group III, IV, V or VIII of the periodic system. In one embodiment the or each metallic element is present in a powder which is dispersed within a filler 21 disposed within the sheath. Particular metals referred to are lanthanum or lanthanides, titanium, zirconium, hafnium, vanadium, niobium, tantalum and palladium. <IMAGE>

Description

SPECIFICATION Telecommunications optical fibre cable The present invention relates to optical fibre cables for telecommunications, which comprise one or more optical fibres.

It has been found that exposure of an optical fibre to gaseous hydrogen causes a deterioration in its properties, for example the attenuation of the signal being transmitted is increased and the mechanical properties of the fibre are lowered. The hydrogen which acts on the fibres to cause this deterioration may be generated from parts of the cable itself or from external sources. Generally, the first effect of a fibre's exposure to hydrogen to become apparent is the increase in the attenuation of the signal being transmitted. This increased attenuation occurs particularly in signals being transmitted with wavelengths of over 1 micron, that is in signals having wavelengths typically used in telecommunications optical fibre cables.

From the tests we have carried out we have established that the increase in attenuation may be caused in two ways. firstly, by hydrogen itself which when diffused in the optical fibre is capable of absorbing energy with an absorption spectrum comprising the wavelengths used for the optical signal. Under certain circumstances, this phenomenon is reversible by causing or allowing the hydrogen in the fibre causing the increased attenuation to diffuse out of the fibre (for example by lowering the hydrogen concentration external to the fibre which originated the phenomenon).The second way in which hydrogen may cause the increase in attenuation is by the hydrogen, when diffused in the fibre, chemically reacing with the constituents of the fibre (for example (Si02) and/or its dopants (for example GeO2, P205 etc) to form groups containing the hydroxyl radical (OH) that are able to absorb signals of other wavelengths used.

These chemical reactions are not reversible and hence increased attenuation of the signals caused thereby can be expected to be permanent under any conditions in which hydrogen is present. The factors which govern both causes of increased attenuation are, in addition to the chemical composition of the fibre, the partial pressure of the hydrogen to which the fibre is exposed, temperature and time.

The cable fibres may be exposed to hydrogen which is generated during the manufacture of the cable and/or during the operation of the cable. Hydrogen can be generated by the presence of metallic or non-metallic parts of the cable which have absorbed the hydrogen during production, refining and finishing of the materials of the cable. Hydrogen can be generated due to the eventual chemical degradation through oxidation of the organic materials present in the cable, or through the oxidation of metallic materials of the cable by water (either in its liquid state or as a vapour).

Furthermore, certain organic materials which are sometimes used to coat the fibres themselves are capable of chemically reacting to produce hydrogen. The rate of diffusion of hydrogen through the materials of the cable varies. It is lowest through metals, greater through polymers, greater still through liquids and greatest through gases. Hence depending upon the type of cable and the environment in which it is situated there will be various emission rates for the hydrogen generated by the parts of the cable and also various rates at which the hydrogen is absorbed into the environment. The partial pressure of the hydrogen within the cable depends on these rates and is therefore a function of time. The greater the partial pressure and the longer the fibre is exposed to hydrogen the greater the risk of it deteriorating due to it absorbing hydrogen.

In general it is necessary for each case to take into consideration the production rate of hydrogen (originating either inside or outside the cable), the diffusion rate of hydrogen through the cable sheath and the rate at which hydrogen is dispersed through the environment in order to establish what the partial pressure of the hydrogen will be in the proximity of the optical fibres both in an initial transient period and thereafter in steady state conditions. For example given the typical service lifetime for an optical fibre cable the diffusion rate of hydrogen through metals is so low under typical service temperatures and pressures that a metallic sheath of the cable can be considered to be practically impermeable to hydrogen.Thus the optical fibres of cables having metallic sheaths especially of small inner diameter, are particularly susceptible to deterioration due to absorption of hydrogen generated by the cable parts inside the sheath, and significant increases in the attenuation of signals being transmitted thereby occur within a relatively short time.

An object of the present invention is to provide an optical fibre cable with means of protecting the optical fibre or fibres thereof against the absorption of hydrogen, This is broadly achieved by having present in the cable at least one metallic element which is capable of absorbing and combining with any hydrogen gas within the cable.

The invention includes an optical fibre cable comprising, within a sheath, at least one optical unit having at least one optical fibre, wherein at least one metallic element of Group Ill, IV, V, or VIII of the periodic system is present in the cable for protecting the or each optical fibre against the absorption of hydrogen.

Among these metallic elements the following have been found particularly suitable: Lanthanum or any one of the Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niohium, Tantalum, and Palladium, and any one may be present in the cable, as a pure metal, in an alloy on in an intermetallic compound.

In the presence of hydrogen, the aboveindicated elements tend to form solid interstitial solutions which are comparable to hydrides and have a good stability, thus reducing the partial pressure of the hydrogen in the cable to values that balance with the hydrogen solubility in the elements themselves.

By utilising appropriate quantities of one or more of these elements, one can succeed in limiting the residual pressure of hydrogen in the cable, to values at which the hydrogen's effect on the fibre's properties and in particular, upon the increase in attenuation caused thereby, is negligible throughout the entire design service life of the cable.

Preferably the above-stated elements before being used in the manufacture of the cable are subjected to thermal treatment under vacuum, at a temperature of a few hundred degrees Celsius, e.g. over 1600'C, since it has been found that after such a thermal treatment, the above-indicated elements are better able to absorb hydrogen, particularly at low partial pressures. It is assumed that these elements may, in some cases, already contain a certain amount of hydrogen and/or other gases which are absorbed during the manufacturing, purification and finishing processes of the elements in the form used in the cable, and that they have a certain level of surface oxidation.Both these phenomena could reduce the ability of the elements to absorb hydrogen, and the thermal treatment referred to above which is carried out at temperatures which are approximate to, but less than the melting temperature of the elements, perform degasification and/or the elimination of the surface oxidation through sublimation.

In order that the invention may be better understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings in which: Figure 1 schematically shows the structure of a typical optical fibre cable; and Figures 2 to 6 are respective schematic crosssections of the cores of five optical fibre cables embodying the invention.

The optical fibre cable 1 shown schematically in Figure 1, comprises an optical unit 2 formed by a plurality of optical fibres 10 laid up on a central tensile resistant member 1 2 and lapped by one or more tapes 1 4.

The optical unit is contained within a sheath t6, over which there are successively provided other layers, coverings and various structures depending upon the type of cable, and schematically shown by portion 20.

The sheath 1 6 may be a water impermeable metallic sheath (for example: when the cable is intended to be used underwater as a submarine cable). Alternatively the sheath 1 6 may be of a plastics material. A filler for example having a mechanical function and/or forming a water barrier may be disposed within the sheath 1 6.

The optical unit 2 may comprise other longitudinal supporting tensile-resistant members in addition to the central member 1 2 and the fibres may be provided with either loose or tight jackets. More than one optical unit 2 may be provided in the sheath 1 6. It is therefore to be understood that the sketch given in Figure 1 is to be taken as generally indicating a typical construction of an optical fibre cable in order that the embodiments of the invention may be more readily understood.

In each of the embodiments shown in Figures 2 to 6 the cables have present therein at least one metallic element of Group ill, IV, V or VIII of the periodic system and preferably at least one of Lanthanum or any one of the Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalum and Palladium, which are hereafter referred to as 'the cited metallic elements'. These cited metallic elements may be present in the embodiments as pure metals, in alloys or in intermetallic compounds.

A first embodiment, illustrated in Figure 2, comprises a submarine cable which has a filler material 21 which occupies the otherwise empty spaces of the cable inwardly of the sheath 1 6 (which may be of the order of about 5 cm3 per meter length of cable) to form a water barrier therein should water penetrate the sheath 16. A powder, in which at least one of the cited metallic elements is present is dispersed within the filler material 21 to protect the optical fibres against the absorption of hydrogen.

The quantity of powder in the filler material depends upon the type of cable, its geometry, which of the cited metallic elements is present in the powder, and the sizes and form of the particles of the powder.

In the case of a cable having a water-barrier filler inwardly of a metallic sheath of normal dimensions, it has been found, for example, that dispersion of between 10 and 100 mg per meter length of cable of Palladium in a powder, whose particles have diameters of between 10 and 100 microns, is sufficient for protecting the fibres against hydrogen in the quantities and pressures thereof which develope in this type of cable.

It is to be understood that the filler material in which the powder is dispersed does not necessarily have to be a water-barrier filler for a submarine cable. Instead, it could be a filler material in the cable for another purpose (for example, for making the structure more com pact), or alternatively, it could be a filler which is present in that cable expressly for carrying the powder.

In a second embodiment, as shown in Figure 3, the sheath 1 6 of the cable is formed of a polymeric material, and a powder, in which at least one of the cited metallic elements is present, is dispersed within the sheath.

The sizes of the particles of the powder in this embodiment are less than that in the previous embodiment. The diameters of the particles are less than 10 microns and preferably of the order of a few microns. This embodiment is particularly suited for protecting optical fibre cables which are devoid of an outer metallic sheath and which are used in environments having a high hydrogen content, when, say, Palladium is present in the sheath with a concentration of say, at least 0.1 parts per hundred of resin. Of course one or more additional polymer sheaths may be provided over the sheath 1 6 and additionally or alternatively have a powder in which at least one of the cited metallic elements is present, dispersed therein.

In a third embodiment shown in Figure 4, one or more of the cited metallic elements is present in, or as a coating on, one or more wires 1 8. The wires 1 8 can be added to the central tensile-resistant member and/or laid up with the optical fibres as shown in Figure 4. Alternatively,the wire or wires canform, or partially form, the central tensile-resistant member, indicated at 1 2 in Figure 1, and in such cases the optical fibres are helically disposed around the wire or wires 1 8.

This embodiment is particularly suited for cables havinga relatively large volume of free space inwardly of the sheath 1 6 (for example, in the order of about 50 cm3 per meter length of cable). A Palladium wire with a diameter between 0.02 to 0.2 mm is able to protect the optical fibres against hydrogen in the quantities and pressures thereof which develop in this type of cable.

Since the absorption of hydrogen by the wires 1 8 occurs at the outer surface thereof, these wires can be formed out of other materials and, as indicated above, coated with a layer of appropriate thickness with a coating in which one or more of the cited metallic elements is present. In such a case the diameter of wire required is obviously different.

In a fourth and a fifth embodiment, shown in Figures 5 and 6 respectively, at least one of the cited metallic elements is present in the cable in a film or layer disposed around the optical unit or units.

In the cable shown in Figure 5, the outer surface of the sheath 1 6 is metallized with such a film or layer, as indicated at 1 9.

In the cable shown in Figure 6, such a layer or film is disposed on a tape which may be a plastics tape, a metallic tape (for example of steel, aluminium or copper), or a metal/plastics laminate tape (for example of aluminium covered with polyethylene), which is lapped around the optical cable unit, in the manner of and instead of tape 14 illustrated in Figure 1. This embodiment is particularly suitable for protecting cables which will be vulnerable to an external source of hydrogen when the film on the tape comprises Palladium and has a thickness of from 1 to 20 microns, and more than one of such tapes are successively wound with short pitch over the optical unit, preferably with the Palladium film facing outwardly.

As already indicated, one or more of the cited metallic elements may be present as pure metals, in alloys or in intermetallic compounds in the embodiments. The choice of material to be used depends upon various factors including cost, the efficacy of the material in absorbing hydrogen, availability and workability. However, in tests we have noted that a mixture of Niobium and Zirconium used in the form of a wire or as a metallized layer, has an extremely good performance with regard to absorption of hydrogen. This is probably due to the fact that, apart from both of these metals being hydrogen absorbers, Zirconium combines very easily with oxygen, thus safeguarding Niobium.

It is to be understood that each of the various means by which the cited metallic elements are present in the cable described in the embodiments may be combined with one or more of the other such means in a cable.

Claims

1. An optical fibre cable comprising, within a sheath, at least one optical unit having at least one optical fibre, wherein at least one metallic element of Group Ill, IV, V, or Vlil of the periodic system is present in the cable for protecting the or each optical fibre against the absorption of hydrogen.

2. A cable as claimed in claim 1, wherein at least one of Lanthanum or any one of the Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalum and Palladium is present in the cable as a pure metal, in an alloy or in an intermetallic compound.

3. A cable as claimed in claim 1 or 2, wherein the or each metallic element is present in a powder.

4. A cable as claimed in claim 3, wherein said powder is dispersed within a filler disposed within said sheath.

5. A cable as claimed in claim 4, wherein the filler forms a water barrier.

6. A cable as claimed in claim 5, wherein said sheath is metallic and the cable is an undewater cable.

7. A cable as claimed in claim 3, wherein said sheath is formed of a polymeric material having said powder dispersed therein.

8. A cable as claimed in claim 7, wherein the particles of said powder are less than 10 microns in diameter.

9. A cable as claimed in claim 1 or 2, wherein the or each metallic element is pre sent in, or as a coating on, at least one wire.

10. A cable as claimed in claim 9, wherein the or each wire at least partially forms a tensile resistant member of the cable.

11. A cable as claimed in claim 1 or 2, wherein the or each said metallic element is present in the cable in a film or layer.

1 2. A cable as claimed in claim 11, wherein said film or layer is disposed on tape which is lapped around the or each optical unit.

1 3. A cable as claimed in claim 12, wherein said tape comprises plastics tape.

14. A cable as claimed in claim 12, wherein said tape comprises metallic tape.

1 5. A cable as claimed in claim 12, wherein said tape comprises a metal/plastics laminate.

1 6. A cable as claimed in any one of claims 1 2 to 14, wherein said film or layer comprises Palladium and has a thickness of from 1 to 20 microns, and wherein more than one of said tapes are successively wound with a short pitch over the or each respective optical unit.

1 7. A cable as claimed in claim 11, wherein said film or layer is disposed on the outer surface of said sheath.

1 8. An optical fibre cable substantially as hereinbefore described with reference to any one of Figures 2 to 6 of the accompanying drawing.