Method of fabricating optical fibres by the solution-doping technique

Abstract: Silica intended to form the core of an optical-fibre preform is deposited as an unvitrified soot inside a reaction tube, where a non-aqueous solution of dopant precursor(s) is then introduced without removing the tube from the lathe.

Description: The present invention refers to the industrial manufacture of physical carriers for optical telecommunications, and more particularly it concerns a method of doping the core of silica optical-fibre preforms by the solution-doping technique, to be carried out in the course of fabricating optical-fibre preforms and optical fibres. Optical fibres for telecommunications comprise an outer part (cladding) of vitreous silica and an inner part (core) of silica doped with metal oxides having higher refractive index. Doping is generally performed by deposition techniques, such as CVD (Chemical Vapour Deposition) technique. Among the dopants, metal oxides which considerably raise the refractive index even in limited concentration and which, unlike germania generally used for this kind of fibres, do not give rise to the central refractive-index depression (dip) typical of fibres fabricated by deposition techniques, are of particular interest. An example of such dopants is alumina. Other dopants, such as rare earths, which modify the fibre emission or absorption characteristics so as to permit the fibres to be used as sensors, amplifiers or lasers in particular spectral regions, in particular in the infrared region, are often desiderably added to the dopants increasing the core refractive index. The use of deposition techniques for manufacturing optical fibres of this kind gives rise to problems in the choice of reactants yielding the dopant(s), chiefly when alumina is dealt with, since no inorganic salts of aluminium exist which are liquid at ambient temperature and are easy to handle like SiCl₄ or GeCl₄. It would be necessary to use compounds with a melting point higher than 100°C and heated lines which present a certain complexity degree. As an alternative, organometallic salts of Al exist which are gaseous or liquid under normal conditions, but the chemical stability thereof is rather reduced. The same problem of unavailability of compounds with the desired characteristics is encountered when dealing with rare earths. To solve said problems, it has been proposed to dope silica by using solutions of compounds of the dopant precursors. Said technique has been described by J. E. Townsend, S. B. Poole and D. N. Payne in the article entitled "Solution-doping technique for fabrication of rare-earth doped optical fibres", Electronics Letters, 26th March 1987, Vol.23, No. 7, pages 329 - 331. According to the technique described in the cited paper, a number of layers of vitreous silica and other oxides are deposited in soot form inside a support tube, in an ambient isolated from the outside. The tube is subsequently removed from the lathe and the unvitrified layers are impregnated with aqueous solutions of compounds (in particular halides) of the rare earths or other required metals. The tube with the impregnated silica is rinsed with acetone to remove excess water, replaced in the lathe and submitted to a high-temperature treatment with Cl₂. Core vitrification and preform collapsing take then place in conventional manner. This technique gives rise to a number of problems due to the use of aqueous solutions and to the tube removal from the lathe. The use of aqueous solutions renders the vitreous matrix highly polluted with OH-groups, which are not completely removed by the elimination of excess water and dehydration with Cl₂: due to the presence of said OH-groups, the fibres have high attenuation. Besides, aqueous solutions are also unsuitable when the doping metal hydrolyzes yielding corresponding oxyacids or hydroxides. This is the case of aluminium, which is a very suitable dopant for the core of silica optical fibres for telecommunications, but whose compounds generally used as dopant precursors (e.g. AlCl₃) violently react in H₂O. Tube removal from the lathe causes contamination of the deposited material due to contact with the atmosphere of the production environment, and this reduces or even annuls the advantages of deposition in a closed environment. Moreover, the removal of the tube from and its subsequent replacement on the lathe are operations of a certain complexity, since they require cutting and soldering again the glass tube; furthermore, these operations entail further pollutions and are quite time-consuming, thus increasing manufacturing costs. The present invention aims to provide a solution-doping method, which does not use aqueous solutions of the dopant precursor(s), and hence solves the problems deriving from the presence of hydroxyl groups and from the hydrolysis of the used compounds, and which does not require tube removal from the lathe. The present invention provides a method of doping the core of silica-based optical fibre preforms, wherein soot layers intended to form the preform core and contained inside a reaction tube supported by a lathe, are at first deposited by chemical vapour deposition inside of the reaction tube by the introduction of respective reactants via an inlet conduit, reaction volatile products being expelled through an exhaust end of the reaction tube and an exhaust conduit, and are then im-pregnated with a solution of a precursor of at least a first dopant intended to raise the refractive index of said layers without giving rise to the central dip of said index, charac-terized in that said solution of the precursor of said first dopant is a solution in an anhydrous organic solvent and in that, after said layer deposition, the reaction tube is disconnected from the reactant inlet conduit as well as from the exhaust conduit for the reaction volatile products; at least the reaction product exhaust end of the reaction tube is sealed, while maintaining an inert and anhydrous gas flow in said reaction tube during the sealing opera-tion; said solution is introduced into the reaction tube, without removing it from the lathe, so that the solution covers the whole surface of the deposited layers; and, once the impregnation is over, the solution is made to slowly flow out, at uniform rate, from the reaction tube, still mounted on the lathe. To make the invention clearer, reference is made to the annexed drawings in which: Fig. 1 is a schematic representation of a deposition tube to be used to perform the present invention; Fig. 2 is a cross-sectional view of a tube sealing system; Figs. 3 and 4 are schematic representations of a second embodiment of the invention, under two different working conditions; Fig. 5 is a partial view of a variant of the second embodiment of the invention; and Fig. 6 shows the absorption spectrum of a fibre produced according to the invention. Fig. 1 shows a conventional vitreous silica tube 1 for chemical vapour deposition of the materials designed to form the optical-fibre preform. Said tube is mounted on a conventional lathe of a chemical vapour deposition apparatus, whose chucking system is schematized by mandrels 3. At the exhaust end, the reaction tube 1 is joined to a tube element 2 of greater diameter, in order to allow reaction tube rotation around its axis. An exhaust conduit for volatile reaction products ends into such element 2. A conduit for the reactant introduction ends at the opposite end of reaction tube 1. These conduits are not shown for sake of simplicity of the drawings, and arrows E, U schematize such inlet and such exhaust, respectively. A heat source 4 can be moved along the axis of the tube while it rotates to originate a region 5 where the temperature is high enough to cause deposition of a silica soot layer, but not its vitrification. Tube 1 is equipped with sealing systems 6 at both ends and has a pair of radial branches 7, 8 placed at opposite ends of the deposition region 5 and provided with respective sealing means 9, 10. Branch 7 serves for introducing into and removing from tube 1 the dopant precursor solution; branch 8 serves for connection to a vacuum pump for tube drying. Sealing systems 6 can be rigid systems, as shown in Fig. 1, associated with rods 6a allowing them to be brought into or out of contact with the internal walls of the tube end portions. As an alternative, as shown in Fig. 2, each sealing system can consist of an inflatable elastomeric membrane 11 mounted at the end of a tube 12 connected to a compressed air source (not shown). Membrane 11 is fastened to a sleeve 13 which presents a threaded part designed to engage the corresponding threaded part of a second sleeve 14 solidary of the tube. A gasket 15 ensures sealing between the two sleeves. During use, tubes 12 will be introduced through ends E, U of reaction tube 1 and a certain overpressure is created inside them to inflate membranes 11 and obtain sealing of tube 1. The dopant solution used according to the present invention is a solution in an anhydrous organic solvent of a precursor of at least a first dopant suitable for raising the refractive index of the core without producing the central dip in the refractive index profile. Advantageously, also a precursor of a second dopant apt to affect the emission and absorption characteristics of the material is dissolved in said anhydrous organic solvent. Preferably the first dopant is alumina, and the second dopant is a rare-earth oxide or a mixture of rare-earth oxides. In this case, the dopant precursor(s) can be selected from chlorides, bromides, iodides or nitrates of aluminium or rare earths. The solvent choice obviously depends on the dopant precursor used. More particularly, all the above salts of aluminium and rare earths present a certain degree of solubility in one or more of the following organic solvents: anhydrous alcohols, ketones, ethers. The following table reports the solubility data for a number of said salts in a solvent of each of the specified groups. Quantitative data are expressed in grams per 100cc of solvent. As to qualitative data "very soluble" indicates values of the order of 50g/100cc or more; "slightly soluble" indicates values of the order of 5g/100cc or less; "soluble"' indicates intermediate values. Compound Solvent absolute ethanol ethyl ether acetone AlCl₃ 100 soluble slightly sol. AlCl₃·6H₂O 50 soluble Al(NO₃)₃·9H₂O 100 soluble AlBr₃ soluble ErCl₃ soluble Er(NO₃)₃·5H₂O soluble soluble soluble NdCl₃ 44.5 NdCl₃·6H₂O very sol. Nd(NO₃)₃·6H₂O soluble soluble CeBr₃(H₂O) very sol. CeCl₃ 30 soluble CeI₃ very sol. Ce(NO₃)₃ 50 soluble TmCl₃·7H₂O very sol. SmCl₃·6H₂O very sol. Gd(NO₃)₃·6H₂O soluble PrCl₃ very sol. YbCl₃ soluble To achieve a satisfactory refractive index difference between core and cladding (in particular, a difference higher than 6·10⁻³), the first dopant precursor concentration should range from 0.8N to the solubility limit in the chosen solvent, and preferably it should not be lower than 1N. The concentration of the possible second dopant precursor should be lower by at least one order of magnitude than that of the first dopant precursor. When fabricating preforms doped with alumina only, if AlCl₃ is used as dopant precursor, suitable solvents are also carbon tetrachloride and alkyl and acyl halides. laid halides react with AlCl₃ to produce solutions wherein complex AlCl₄⁻ present. An example of such reactions is the following: RCOCl + AlCl₃ ---> RCO⁺ + AlCl₄⁻ If ALBr₃ is used as dopant precursor for manufacturing preforms doped with alumina only, CS₂ could also be used as the solvent. To better illustrate the invention, the manufacture of a fibre doped with Al₂O₃ and Er₂O₃ by using the tube of Fig. 1 will be disclosed by way of example. The materials for the preform core are deposited in a quite conventional way, by introducing the reactants (more particularly SiCl₄ and O₂) into tube 1, heated at a temperature of about 1500 - 1600°C by burner 4, so as to obtain an unvitrified silica layer some ten micrometres thick. Once the deposition is over, the tube is disconnected from the conduits for the reactant inlet and the gaseous reaction product exhaust, and the ends of tube 1 are to be closed by sealing systems 6 or by inflated membranes 11, while an inert and dehydrated gas (e. g. N₂ or Ar) is made to flow through the tube. To this aim, branch 7 is opened and the gas is introduced through the reactant inlet, while the opposite tube end is closed; then, branch 8 is opened and the gas is introduced through branch 7, thus establishing a flow between branches 7 and 8. The reactant inlet end is then closed. Subsequently, the non aqueous solution of dopant precursor is slowly introduced, e. g. a solution in ethyl alcohol of AlCl₃ and ErCl₃. The amount of solution introduced is such as to fill the whole tube. In an exemplary embodiment of the invention, the concentrations of the two dopant precursors were 1 N and 0.1 N, respectively. The solution is let to impregnate the soot for about 1 hour, whereafter it is let to flow out, e. g. by gravity, through the same branch 7 through which it had been introduced. After possible rinsing with suitable solvents (e. g. acetone), branch 8 of tube 1 is connected to a vacuum pump and the residual solvent in the tube is made to evaporate so as to obtain dry impregnated soot. Branches 7, 8 are then closed, sealing systems 6 are removed and the soot is washed with a mixture of O₂ and Cl₂ at about 700°C to obtain a more complete dehydration of the soot and formation of Al₂O₃ and Er₂O₃. Vitrification and tube collapsing take then place according to the conventional techniques of preform manufacturing, and the preform is then drawn into a fibre in conventional manner. A fibre obtained by the method described exhibited a refractive index difference between core and cladding of 7.5·10⁻³. A fibre fabricated in a similar way, using an alcoholic solution of AlCl₃ only, still with a 1 N concentration, exhibited a refractive index difference of 6.5·10⁻³. Fibres fabricated by using AlCl₃ solutions with 0.2 N and 0.05 N concentrations presented an index difference of 2.2·10⁻³ and 1.6·10⁻³, respectively: said data allow evaluation of the influence of aluminium concentration in the attainment of good results. Fig. 6 shows the absorption spectrum, in the visible and in the near-infrared, of the fibre obtained by drawing the Al₂O₃-Er₂O₃ doped preform. Very low attenuations in correspondence with transmission windows and the low amplitude of the absorption peak of hydroxyl groups at 1380nm can be clearly appreciated. The method of the invention can be carried out also by using a perfectly conventional CVD tube, which however is to be mounted on a lathe where the mandrel support zone is movable in such a manner that the tube can be disposed either horizontal or inclined by an angle which can reach 90°. This embodiment is represented in Figs. 3 and 4, where the elements corresponding to those of Fig. 1 are denoted by the same references. The reaction tube 16 is, as said, a conventional CVD tube, i.e. a tube without branches 7, 8, and is mounted on a lathe presenting an upper portion 17, with mandrels 3, which is hinged on a base member 18 according to a horizontal axis 19, perpendicular to the axis of tube 16. The membrane sealing system of Fig. 2 is associated with the tube exhaust end. Referring again to the fabrication of a fibre like that mentioned above, the deposition of the layers to be impregnated takes place also in this case in a quite conventional manner, with horizontal tube, as shown in Fig. 3. At this point part 17 of lathe is rotated around axis 19, so as to arrange the tube inclined (more particularly vertical, Fig. 4) with the exhaust end down; then the reactant inlet and gas exhaust conduits are disconnected and the exhaust end is closed by membrane 11, while maintaining a flow of anhydrous and inert gas in tube 16. The impregnating solution is then introduced through the upper end of tube 16, by using another tube 20 which penetrates into tube 16 and is arranged so that the impregnating solution level slowly raises from the bottom. After impregnation, the pressure is reduced in membrane 11, so as to open again the tube exhaust end and to allow the solution to flow out. The outlet orifice must ensure that the tube is slowly emptied at uniform speed, to avoid damaging the impregnated soot layer. As an alternative, the solution can be sucked through feeding tube 20. The tube is then slightly heated (at 50 - 70°C), while maintaining a flow of N₂ or Ar, to make the solvent evaporate from the impregnated material, and a mixture of O₂ and Cl₂ at about 700°C is made to pass in the tube to obtain a more complete dehydration of the soot and formation of Al₂O₃ and Er₂O₃. Vitrification and tube collapsing take then place according to the conventional techniques of preform manufacturing, and the preform is then drawn into a fibre in conventional manner. In the variant shown in Fig. 5, relevant to the use of a tilting lathe like that of Figs. 3, 4, introduction and removal of the dopant precursor solution take place through a tube 21 which is introduced into tube 16 from gas exhaust end U. For sealing end U of tube 16, a toroidal element 22 is provided, which surrounds the end of tube 21, is made by an elastomer (e. g. a silicone or polyurethane elastomer) and is connected through a pipe 23 to a compressed air source (not shown). By this arrangement, after silica soot deposition, element 22 is inflated so as to adhere to the internal surface of reaction tube 16, thus sealing exhaust end U. During sealing, the inert gas flow is maintained as in the preceding cases. Then reaction tube 16 is slightly inclined and the solution is introduced through tube 21. After impregnation, the solution will flow out by gravity or will be sucked through the same tube 21. It is clear that even though in the example reference was made to a solution of AlCl₃ as precursor of the first dopant, that precursor could consist of salt of another element, such as Zr, Mg and the like, apt to give the fibre the desired properties, such salt presenting sufficient solubility at ambient temperature in anhydrous organic solvents such as ethanol or other anhydrous alcohols, ketones, ethers, alkyl or acyl halides, carbon tetrachloride or carbon sulphide.

Claims: A method of doping the core of silica-based optical fibre preforms, wherein soot layers intended to form the preform core and contained inside a reaction tube (1; 16) supported by a lathe (3, 17, 18), are at first deposited by chemical vapour deposition inside of the reaction tube (1; 16) by the introduction of respective reactants via an inlet conduit, reaction volatile products being expelled through an exhaust end (U) of the reaction tube and an exhaust conduit, and are then impregnated with a solution of a precursor of at least a first dopant intended to raise the refractive index of said layers without giving rise to the central dip of said index, characterized in that said solution of the precursor of said first dopant is a solution in an anhydrous organic solvent and in that, after said layer deposition, the reaction tube (1; 16) is disconnected from the reactant inlet conduit as well as from the exhaust conduit for the reaction volatile products; at least the reaction product exhaust end (U) of the reaction tube (1; 16) is sealed, while maintaining an inert and anhydrous gas flow in said reaction tube (1; 16) during the sealing operation; said solution is introduced into the reaction tube (1; 16), without removing it from the lathe (3, 17, 18), so that the solution covers the whole surface of the deposited layers; and, once the impregnation is over, the solution is made to slowly flow out, at uniform rate, from the reaction tube (1; 16), still mounted on the lathe (3; 17, 18). A method as claimed in claim 1, characterized in that also a precursor of a second dopant apt to modify the material absorption and emission characteristics is dissolved in said anhydrous organic solvent. A method as claimed in claim 2, characterized in that the first dopant is alumina, and the second dopant is a rare-earth oxide or a mixture of rare-earth oxides. A method as claimed in any preceding claim, characterized in that the dopant precursors are chosen from chlorides, bromides, iodides or nitrates of the elements whose oxides are to form the dopant. A method as claimed in any preceding claim, characterized in that said anhydrous organic solvent is selected from anhydrous alcohols, ketones, ethers, carbon tetrachloride, carbon sulphide, alkyl halides, acyl halides. A method as claimed in any preceding claim, characterized in that the first dopant precursor is present in the solution in a a concentration whose normality has a value ranging from 0.8 to the solubility limit in the solvent considered. A method as claimed in claim 6, characterized in that the first dopant precursor is present in the solution in a concentration whose normality is not less than 1. A method as claimed in claim 6 or 7, characterized in that the second dopant precursor is present in a concentration whose normality is lower by at least one order of magnitude than that of the first dopant precursor. A method as claimed in any of claims 1 to 8, characterized in that the solution is introduced through the same end (E) of the reaction tube (16) through which the reactants are introduced, after having placed the reaction tube (16) in an inclined position, with the exhaust end (U) down, and having sealed said exhaust end. A method as claimed in claim 9, characterized in that said solution is introduced through a second tube (20) penetrating into the reaction tube (16) and ending near the exhaust end (U) of the reaction tube (16). A method as claimed in claim 9 or 10, characterized in that the solution is made to flow out by gravity through said exhaust end (U). A method as claimed in claim 10, characterized in that the solution is removed by sucking it through said second tube (20). A method as claimed in any preceding claim, characterized in that the sealing of the exhaust end (U) of the reaction tube (16) is obtained by a resilient membrane (11) mounted on a conduit (13) which is coaxial with the reaction tube and in which an overpressure is established. A method as claimed in any of claims 1 to 8, characterized in that the solution is introduced, after having arranged the reaction tube (16) in an inclined position with the exhaust end (U) down, via a further tube (21) penetrating inside the reaction tube (16) through said exhaust end (U), and is removed through said further tube (21), by gravity or by sucking. A method as claimed in claim 14, characterized in that the exhaust end (U) of the reaction tube (16) is sealed by means of an inflatable elastomeric toroidal member (22) placed between the internal surface of the reaction tube (16) and the external surface of said further tube (21). A method as claimed in any of claims 1 to 8, characterized in that the solution introduction and removal take place, after sealing of both ends of the reaction tube (1), through a first radial branch (7) of the reaction tube (1) ending near one end of a deposition zone (5) of the layers to be impregnated. A method as claimed in claim 16, characterized in that the flow of anhydrous and inert gas is maintained during the sealing of both ends (E, U) of the reaction tube (1). A method as claimed in claim 17, characterized in that said gas flow is maintained between the inlet end (E) and said first branch (7), while the exhaust end (U) is being sealed, and is maintained between the first branch (7) and a second branch (8), ending in the reaction tube near the opposite end of said deposition zone (5), while the inlet end (E) is being sealed. A method as claimed in any of claims 16 to 18, characterized in that the sealing of the tube ends is obtained by respective resilient membranes (11) mounted on a conduit (13) which is coaxial with the reaction tube and in which an overpressure is established.

Inventors: COGNOLATO LIVIO, GNAZZO ANGELANTONIO, COGNOLATO, LIVIO, GNAZZO, ANGELANTONIO
Applicants: SIP, SIP SOCIETA ITALIANA PER L'ESERCIZIO DELLE TELECOMUNICAZIONI P.A.
Application year: 1989
Main classification: C03B37
IPC codes: C03B20, C03C13, C03B37, C03C23, G02B6, C03B8