GB2158263A

GB2158263A - Optical fibre

Info

Publication number: GB2158263A
Application number: GB08510656A
Authority: GB
Inventors: Giuseppe Bianchi; Laura Gherardi
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: FR2563634B1; NZ211369A; SE8502047L; NO168209B; NO851686L; BR8501841A; IT8420700A1; DE3515228A1; CA1251075A; NO168209C; NL8500892A; GR851015B; GB2158263B; AU3953585A; IT8420700A0; SE8502047D0; JPS60239703A; SE462007B; FR2563634A1; GB8510656D0

Abstract

An optical fibre 1 is protected against the absorption of hydrogen by a covering (3,9), which comprises at least one metallic element of Group III, IV, V, or VIII of the periodic system (e.g. Ti, Zr, Hf, Va, Ni, Ta, Pa). In one embodiment this covering comprises a metallized layer disposed on the fibre immediately adjacent the outer surface thereof (see Fig. 1 not shown) or adjacent a further protective covering (2) - or the metallic element may be present as a powder in a resin coating (3,9). In a further embodiment, the covering may be a loose tube (9) surrounding the fibre (1,2). The metallic element being present also or alternatively as a gel or powder filling the tube. <IMAGE>

Description

SPECIFICATION Optical fibre The present invention relates to optical fibres, and particularly to such fibres for telecommunications cables.

Generally, optical fibres used for telecommunications cables comprise a glass structure formed by a cladding disposed over a core, for example of the "step index" or "graded index type, and a primary coating applied to the fibre over the cladding thereof immediately after its formation to prevent the fibres having direct contact with the environment. Over this primary coating other protective coverings are applied, for example, a layer of silicone rubber and a more rigid layer or tube made, for example, of nylon. An optical fibre telecommunications cable generally comprises one or more such optical fibres housed inside a sheath, together with one or more tensile-resistant members. The sheath, which may be metallic, is provided with one or more coverings such as armouring.

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, In cables comprising one or more optical fibres, 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 (SiO2) and/or its aopants (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 in the cladding or to coat the fibres themselves are capable of chemically reacting to produce hydrogen. One cource of hydrogen has been found to be a slicone rubber covering of the fibre, possibly due to the construction of the cross-linking process. Although the hydrogen produced by the cladding or coverings of the fibres is able to diffuse away from as well as towards the fibre, when the fibres are disposed within the cable sheath, and particularly when there is a limited amount of free space within the cable, disperson away from the fibres is limited.

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 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 with means of protection against the absorption of hydrogen which may be present in the cable in which the fibre is located, in use. This is broadly achieved by having present in at least one covering of the fibre a,t least one metallic element which is capable of absorbing and combining with any hydrogen gas within the cable.

The invention includes an optical fibre having at least one protective covering, wherein the or at least one said protective covering comprises at least one metallic element of Group Ill, IV, V, or VIII of the periodic system for protecting the fibre against the absorption of hydrogen.

Among these metallic elements the following have been found particularly suitable: any one of the Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalum, and Palladium, and any one may be present in the or each protective covering as a pure metal, in an alloy or 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 particu lar, upon the increase in attenuation caused thereby, is negligible throughout the entire design service life of the cable.

Preferably the above-stated elements, in whatever form they are present in the cable, are subjected to thermal treatment under vacuum, at a temperature of a few hundred degrees Celcius, e.g. over 1 600 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 re duce the ability of the elements to absorb hydrogen, and the thermal treatment referred to above which is carried out at temperatures with 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 schematially shows a cross-section of an optical fibre provided with a primary metallic coating; Figure 2 schematically shows a cross-section of an optical fibre provided with a primary coating; Figure 3 schematically shows a cross-section of an optical fibre provided with a primary and a secondary coating; Figure 4 schematically shows a cross-section of an optical fibre provided with a primary and a secondary coating, between which there is interposed a cushioning layer; and Figure 5 schematically shows a cross-section of an optical fibre provided with a loose covering, or jacket.

In each of the illustrated embodiments the optical fibre is illustrated at 1 and is provided with at least one protective covering which comprises at least one metallic element of Group III, IV, V, or Vlil of the periodic system and preferably at least one of the Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalium or Palladium, which are referred to hereinafter as 'the cited metallic elements'. Any one of the cited metallic ele ments may be used as a pure metal, in an alloy, or in an intermetallic compound.

In a first embodiment, shown in Fig. 1, the protective covering comprising at least one of the cited metallic elements is a metallized layer 2 which forms a primary coating 2 of the fibre and is disposed on the fibre immedi ately adjacent the outer surface of the glass structure of the optical fibre. Thus the metallized layer not only performs the me chanical function of the usual primary coating of the fibre but also protects the fibre against the absorption of hydrogen. The fibre will normally be provided with one or more further protective coverings. In a non-illustrated varia tion the metallizing layer is applied immedi ately over the usual cross-linked resin primary coating. This construction is beneficially utili zable when it is either not possible or not convenient to modify existing fibre producing plant which applies a resin primary coating to the fibre immediately after the fibre is drawn.

It will also be appreciated that the metallized layer may be disposed on one or more of any of the other protective coverings of the fibre.

In second, third and fourth embodiments shown in Figs. 2 to 4 at least one of the cited metallic elements is present in a powder in a resin coating of the fibre as a pure metal, in an alloy or in an intermetallic compound. The particles of the powder are preferably less than 10 microns in diameter and are present in the resin coating with a concentration of from 0.1 to 10 parts per hundred of resin.

In the second embodiment, illustrated in Fig. 2, the powder is disposed in the primary coating 2 which is disposed on the fibre immediately adjacent the outer glass surface thereof and which comprises an acrylic resin, or other suitable resin. As will be appreciated the powder may be readily added to the resin material used for the primary coating and this may be applied in the conventional way.

Therefore this embodiment provides a very convenient way of protecting the fibre against the absorption of hydrogen.

In the third embodiment, shown in Fig. 3, the powder is disposed in the coating 3 immediately surrounding the primary. coating 2. This coating 3 is typically formed of silicone rubber which, as explained previously, may be a source of hydrogen which is particularly significant in view of its proximity to the fibre. The presence of the powder is this coating can effectively neutralize the hydrogen generated therein before it can diffuse towards the fibre.

In the fourth embodiment illustrated in Fig.

4, a secondary coating 4 formed, for example, of nylon or some other thermoplastic polymer, is provided on the fibre over the primary coating 2 with a cushioning layer 3 interposed therebetween. The powder in this embodiment is disposed in the coating 4.

It will be appreciated that the presence of the powder in the covering adjacent the outer glass surface of the optical fibre, primarily protects the fibre from absorption of any hydrogen generated in the protective covering of the fibre and particularly the innermost coverings thereof, while the presence of the powder in an outer protective covering (for example the silicone rubber coating 3 in Fig. 3) primarily protects the fibre from absorption of any hydrogen generated by the cable elements. It will therefore be appreciated that in certain circumstances it may be desirable to provide the powder in more than one of the protective coverings of the fibre and to this end it is to be understood that the measures in the embodiments shown in Figs. 2 to 4 for protecting the fibre against absorption of hydrogen may be combined.

A fifth embodiment of the invention, shown in Fig. 5, relates to an optical fibre provided with a loose protective covering, or jacket.

The fibre 1 is shown with a primary coating 2, but may be provided with additional protective coatings or non-adherent coverings as typically used for loose jacketed fibres. The loose protective covering, or jacket, of the fibre is a platics tube 9, whose internal diameter is greater than the external diameter of the coated fibre. A gel having dispersed therein at least one of the cited metallic elements present in a powder is disposed in the space 8 between the tube 9 and the coated fibre and thus forms a protective covering of the fibre for protecting the fibre against the absorption of hydrogen. Additionally or alternatively at least one of the cited metallic elements may be present in a powder dispersed in the tube 9. It will also be appreciated thatçany of the other protective layers of the fibre may be provided with the measures described in the embodiments shown in Figs.

2 to 4 for protecting the fibre against absorption of hydrogen. It will also be understood that the use of a metallized layer for this purpose as described in relation to the embodiment shown in Fig. 1 and the variations thereof disclosed may be combined with any of the above described arrangements in which a powder containing at least one of the cited metallic elements is dispersed in a protective covering.

Claims

1. An optical fibre having at least one protective covering, wherein the or at least one said protective covering comprises at least one metallic element of Group Ill, IV, V or VIII of the periodic system for protecting the fibre against the absorption of hydrogen.

2. An optical fibre as claimed in claim 1, wherein the or a said hydrogen absorption protective covering comprises at least one of any one of the Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalum or Palladium, as a pure metal, in an alloy or in an intermetallic compound.

3. An optical fibre as claimed in claim 1 or 2, wherein the or at least one said hydrogen absorption protective covering comprises a metallized layer.

4. An optical fibre as claimed in claim 3, wherein the or a said metallized layer is disposed on the fibre immediately adjacent the outer surface thereof.

5. An optical fibre as claimed in claim 3 or 4, wherein the or a said metallized layer is disposed on at least one other protective covering of the fibre.

6. An optical fibre as claimed in claim 1 or 2, wherein the or at least one said hydrogen absorption protective covering comprises at least one of said metallic elements present in a powder in a resin coating of the fibre.

7. An optical fibre as claimed in claim 6, wherein the particles of said powder are less than 10 microns in diameter and are present in said resin coating with a concentration of from 0.1 to 10 parts per hundred of resin.

8. An optical fibre as claimed in claim 6 or 7, wherein the or a said resin coating containing said powder is disposed on the fibre immediately adjacent the outer glass surface thereof.

9. An optical fibre as claimed in claim 6, 7 or 8, wherein the or a said resin coating containing said powder is disposed immediately adjacent a protective covering which itself is disposed immediately adjacent the other glass surface of the fibre.

10. An optical fibre as claimed in claim 9, wherein the or said resin coating referred to in claim 9 comprises a silicone rubber coating.

11. An optical fibre as claimed in claim 1 or 2, with at least one protective covering formed as a coating and provided with a loose protective covering comprising a tube whose internal diameter is greater than the external diameter of the coated fibre, wherein the or at least one said hydrogen absorption protecting covering comprises a gel between the coated fibre and the tube having dispersed therein at least one of said metallic elements present in a powder.

1 2. An optical fibre as claimed in claim 1 or 2, with at least one protective covering formed as a coating and provided with a loose protective covering comprising a tube whose internal diameter is greater than the external diameter of the coated fibre, wherein the or at least one said hydrogen absorption protective covering comprises said tube which has dispersed therein at least one of said metallic elements present in a powder.

1 3. An optical fibre substantially as herein described with reference to any one of the figures of the accompanying drawings.