NZ211369A - Coating optical fibre to protect against hydrogen absorption - Google Patents

Description

211369 * Priori?/ Da:e(s): . .^.7." " Complete.Specification Filed: ciesk ..v.P.?-6G|iO, t6 ,'aal C.o.^ca5/oz- Publication Date- 3 0 MAR *1988 P.O. Journal, No: ...

"•! NEW ZEALAND PATENTS ACT. 1953 s: No.: Date: COMPLETE SPECIFICATION «»<■ ■J Pi.

OPTICAL FIBRE PROTECTED AGAINST THE ABSORPTION OF GASEOUS HYDROGEN Socleia. ^er Ajiom SOCIETA1 CAVI PIRELLI an Italian company of Piazzale Cadorna, 5 Milan, Italy I I O hereby declare the invention for which ji / we pray that a patentjmay be granted to ilW/us, and the method by which it is to be perfoiimed^f T ^ ''' L'' ~smi9s$ (followod by pago la) -v - , •• .

The present invention concerns an optical fibre protected against the absorption of gaseous hydrogen, particuarly when the optical fibre is incorporated within a cable.

In cables comprising one or more optical fibres, at times a deterioration is found in the transmissive properties of the fibres where these are subjected to the action of hydrogen that is in any way generated (by members that are either outside or inside the cable).

In actual fact, even the mechanical characteristics of the fibre result in being modified - although, as a rule, it is above all the macroscopical effects of increased attenuation that are the first to become apparent.

In fact, the fibres found under these conditions, show an increase in attenuation for the wavelengths highej than 1 micron i.e. in the-interval of the^wavelengths utilized for transmitting the signal.

Generally, the optical fibres comprise a glass structure formed by a cladding and a core of the "step index", "graded index" type, or of other types of structures and a primary coating applied to the fibre immediately after its formation, for the purpose of preventing the fibre from having any direct contact with the outside environment. Above said primary coating there are applied other protective coatings, for example, comprised of a layer of silicone rubber and of a more rigid layer or tube made, for example, of nylon.

An optical fibres cable generally comprises one or more optical fibres housed inside a sheath, together with one or more traction-resistant members. Said sheath, which can either a- ? f■ be metallic or not, is, in its turn, surrounded by other mechanical members such as armourings, coverings etc.

Tests carried out by the Applicant have established that a primary cause of attenuation in the optical fibres incorporated in a cable is constituted by the hydrogen which, once it becomes diffused inside the fibre, is capable of absorbing energy with an absorption spectrum comprising the wavelengths utilized for the optical signal.

Under particular conditions this phenomenon can be reversed and the attenuation can even be considerably reduced if the hydrogen has the chance of diffusing towards the outside of the fibre (for example by lowering 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 put down to chemical reactions taking place between the main constituents of the fibre (for example Si02) and/or its dopants (Ge02, P20^, etc.) and the hydrogen that is contained inside the fibre itself.

The result of these reactions is the formation of groups containing the hydroxyl radical (OH), that are responsible for the absorption at other wavelengths that are also used for the transmission. These latter reactions are irreversible and hence, the corresponding worsening of the fibre properties can be expected under all conditions of use.

The parameters that control these phenomena are, apart from the chemical composition of the fibre, the partial pressure 211369 of the hydrogen to which the fibre is exposed, the temperature and, of course, the time.

The fibre can come into contact with the hydrogen generated inside the cable, either during the manufacturing process of the cable, or else during the functioning of the cable itself. As a matter of fact, the hydrogen can be generated by metallic or non-metallic members present in the cable, that have absorbed said gas during the manufacturing, refining or finishing processes of the constituent materials. ^ The hydrogen can also be generated by the eventual •j chemical degradation through the oxidation of the organic materials constituting the cable, or else through the reaction 'A | of the water (either in a liquid state or as vapour) eventually | ; present in the cable, yith the metallic members forming the 1 . ' ; cable itself.

Certain .organic materials used in the fibre cladding, are capable of-producing hydrogen due to various chemical reactions. It has been found that one hydrogen source is the protective coatings themselves, and in particular the silicone rubber - for which it is assumed, the cross-linking process is prolonged in duration, leading to the liberation of hydrogen precisely near the fibre surface. The spreading of the hydrogen takes place towards the fibre, as well as towards outside of it, but it does not cause any appreciable effect on the insulated fibre because in this instance the hydrogen • becomes dispersed in the surrounding environment.

Nevertheless, when the fibre is situated in a closed cable and without there being any sufficient free space, the hydrogen 21 1^-9 concentration can achieve relatively high values that cause its * appreciable diffusion - even towards the fibre itself, aided by \ the fact that the cladding, from which the hydrogen is i I developed, is very near to the fibre. » The diffusion of the hydrogen through the various 3 i i materials occurs at an increasing rate - passing from the 4 metals to the polymers, to the liquids, to the gases, hence, ? i depending upon the type of cable and upon the environment wherein it is utilized, several rates occur for the emission of the hydrogen produced by the constituting cable members and so also diverse rates of absorption, on the part of the cable, of the hydrogen eventually produced outside it and which permeates the environment in which it is used. From these diverse rates there depends the value of the partial pressure of the hydrogen j • ■ ' * i inside the cable, that will result in being a function of the ; I time the hydrogen is in the cable, for the greater the pressure j I t and the duration is, so much greater will the level of risks be ] i for the fibres.

Given the service lifetime of an optical fibres cable, under expected 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 practically impermeable to the hydrogen. j In particular, the cables endowed with metallic sheaths, j especially if they have a small space inside them, are those that can shown in a short time and at high levels, increases in attenuations owing to the hydrogen that is liberated from the elements inside of the sheath. •••• - a*®.- The aim of the present invention is to produce an optical fibre that is protected against the absorption of gaseous hydrogen which may be present in the cable containing the fibre.

Said protection is obtained, according to the invention, by providing around the outermost glass layer of the fibre, one or more coatings containing powders of metals which are capable of combining with the hydrogen and thus forming a barrier with the said coating.

The optical fibre according to the invention, has at least one protective coating, which includes, in at least one of said protective coatings, powders of one or more metals of the Groups III, IV, V and VIII of the periodic system as a protection against the absorption of gaseous hydrogen on the part of the fibre.

Among these metals that have proved to be particularly suitable are the following: Lanthanides for Group III; Titanium, Zirconium and Hafnium for Group IV; Vanadium, Niobium and Tantalum for Group V; Palladium for Group VIII, in the form of pure metals, their alloys or intermetallic compounds.

In the presence of hydrogen, the above-indicated elements tend to form solid interstitial solutions that are assimilable to hydrides having a good stability, and this allows for reducing the partial hydrogen pressure in the cable to values which balance with the solubility of the hydrogen in the members themselves.

Preferably, the abovenamed elements are subjected to a thermic treatment under vacuum at temperatures of some hundreds of degrees centigrade, prior to being utilized in cable production, for the purpose of eliminating any hydrogen that 6 could have been absorbed, and/or the combined oxygen.

The invention will now be described with reference to certain preferred but non-limiting embodiments that are illustrated in the attached drawings in which: Fig. 1 - schematically shows a cross-section of an optical fibre provided with a primary metallic coating; Fig. 2 - schematically shows a cross-section of an optical fibre provided with a primary coating; Fig. 3 - schematically shows a cross-section of an optical fibre provided with a primary and a secondary coating; Fig. 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 Fig. 5 - schematically shows a cross-section of an optical fibre of'the loose type i.e. loosely housed inside a small tube.

With reference to Fig. 1, an elementary optical fibre comprises a glass portion 1 of any type - i.e. "step index", "graded index" or other types, and a primary coating 2 adjacent to this, that has the function of protecting it from the outer environment.

According to a first embodiment of the invention, said protection is obtained with a metallizing layer formed by one or more of the materials indicated. Said layer can constitute the primary coating 2 shown in the figure in close contact with the glass structure of the optical fibre. There is thus obtained a fibre where the primary coating is of the metallic type and which, at the same time, also performs the mechanical function as well as the function of safeguarding against the 21136 absorption of hydrogen by the environment surrounding the fibre, during use.

According to a variation (not shown) the metallizing layer is applied immediately over the usual primary coating made of cross-linked resin. This construction is useful whenever it is not possible or convenient to modify the plant for producing the fibre that employs the application of the protective ' coating immediately after drawing the optical fibre to the , desired dimensions.

^ A further variation, not shown either, envisages the < application of the metallized layer around one of the « — successive coatings.

In accordance with a second embodiment, that is schematically illustrated in Fig. 2, the primary coating 2, 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 intermetallic compounds. This allows for the incorporation of the protective characteristics against the hydrogen, in a conventional manufacturing process.

A third embodiment (Fig. 3) envisages the adding of the metallic powders to the coating 3 immediately surrounding the primary covering. This coating is typically made out of silicone rubber and, as explained previously, the silicone rubber can become a particularly dangerous source of hydrogen. The presence of the metals in this coating effectively w neutralizes the hydrogen that is generated, even before it can diffuse towards the fibre.

The optical fibre illustrated schematically in Fig. 4, constitutes a fourth embodiment of the invention which envisages a dispersion of metallic powders in the secondary coating 4 that is comprised, for example, of nylon or some other thermoplastic polymer. In the embodiments above, the particles constituting the powders have dimensions that are preferably lesser than 10 microns and the quantity of the powders per length unit of the optical fibre is determined with a view to achieving a concentration within the range from 0.1 to 10 phr (parts per hundred of resin) in the resin.

It must be kept in mind that the protective function, according to the invention, is accomplished in a diverse manner, according to the coating in which the metals are incorporated. More precisely, the presence of a protective layer very close to the oj^y.jal fibre, protects the latter above all* against the .ftydlro^en generated in the innermost protective coatings, while an outer protective coating (for example, above the silicone rubber) constitutes above all, a protection from the hydrogen deriving from the cable elements.

In view of what has been stated hereabove as well as other factors depending upon the structure and the expected conditions of use of the cable, the previously described diverse embodiments can be combined in the same optical fibre.

To end with, a further embodiment of the invention is illustrated in Fig. 5, wherein an optical fibre 1 having a primary coating 2, is housed inside a small tube 9 of plastic matefial, having the inner diameter greater than the outer diameter of the fibre, and provided with the usual coatings for constituting an optical fibre of the loose type.

For this type of fibre, that also uses coverings which are non-adherent, the protection can be provided by having coatings such as those illustrated previously and by providing inside the small tube 9 a gel 8 containing a dispersion of powders of the cited metals or their alloys or intermetallic compounds.

As an alternative, combined or not with the preceding embodiment, the material that constitutes the small tube may contain a dispersion of powders of the cited metals or of their alloys or intermetallic compounds.

Although the invention has been illustrated with particular reference to certain preferred embodiments, it is not be be held as being limited to these, since what is also included within the scope of the invention are all those obvious variations and/or modifications which are self-evident to a technician skilled in the art.

Claims (1)

1. 211369 N *;i i;i >;l i;s;I;O;•i;! w';I;I;WHAT WE CLAIM IS:;1. An optical fibre provided with at least one protective coating, which includes, in at least one of said protective coatings, powders of one or more metals of the groups III, IV, V and VIII of the periodic system, as a protection against the absorption of gaseous hydrogen on the part of the fibre.;2. An optical fibre, according to claim 1, wherein said metals are selected from: Lanthanides, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalum, Palladium, and their alloys and intermetallic compounds.;3. An optical fibre, according to claims 1 or 2, wherein the dimensions of the particles that constitute the said powders, are smaller than 10 microns and that the powders are contained in sufficient amounts for achieving concentrations within the range from 0.1 to 10 phr in the coating itself.;4. An optical fibre, according to any one of claims 1 to 3, wherein the powders are incorporated in the primary coating of the optical fibre and immediately adjacent to the outermost glass layer of the fibre.;5. An optical fibre, according to any one of claims 1, 2 or 3, wherein the said powders are incorporated in a coating that is immediately adjacent to the primary coating of the optical fibre.;6. An optical fibre, according to claim 5, wherein said coating is of silicone rubber.;7. An optical fibre, according to any one of claims 1, 2 or 3, wherein said coating is a secondary coating.;8. An optical fibre, according to claim 1 or 2, wl;11;211369;fibre is of the loose type, housed in a small tube having an internal diameter that is greater than the outer diameter of the fibre provided with the protective coatings, wherein said metals are in the form of powders dispersed in a gel contained inside the small tube.;9. An optical fibre, according to claim 1 or 2, whereby the fibre is of the loose type, housed in a small tube having an internal diameter that is greater than the external diameter of the fibre provided with adherent protective coatings, wherein the said metals are in the form of powders dispersed inside said small tube.;10. An optical fibre substantially as hereinbefore described with reference to the accompanying drawings.;Soue+a ' a*o,. .IVeUf B/ fcto/tlieir authorised Agents., A. J. PARK & SON. Pe'