Method for the fabrication of fluoride glass single mode optical fibres

Abstract: Fluoride glass optical fibres are fabricated by minimizing the number of high-temperature operations. A tube comprising an external layer of a first fluoride glass of composition suitable to form the cladding of the fibre and an internal layer of a second fluoride glass of composition suitable to form the core of the fibre is prepared from a melt by a conventional rotational casting technique. Then, the internal layer is thinned by means of chemical etching at ambient temperature until the attainment of a ratio between the volumes of the two layers that corresponds to the ratio between the core and cladding diameters required for a single mode fibre, and the resulting tube is drawn.

Description: The invention described herein relates to optical fibres for telecommunications, and in particular it concerns a method for the fabrication of fluoride glass single mode optical fibres, comprising the steps of fabricating a tube comprising an external layer of a first fluoride glass of a composition suitable to form the cladding of the fibre and an internal layer of a second fluoride glass of a composition suitable to form the core of the fibre; thinning the internal layer by means of chemical etching until the attainment of a ratio between the volumes of the internal layer and the external layer of the tube corresponding to the ratio between the core and cladding diameters required for a single mode fibre; and drawing the tube obtained as a result of the preceding step. Such method is known from JP-A-62 265140 ("Patent Abstracts of Japan", Vol. 12 (C-493)), disclosing the chemical etching of a hollow core of a silica optical fibre preform by dipping the core part in an etching solution to thin it and to obtain a cladding/core diameter ratio suitable for a monomode fibre. The process used is a static process. Tube-shaped preforms, i.e. having a hollow core, can be manufactured according the technique disclosed in US-A-5 055 120 which describes a process of distributing the core material within the cladding tube by spinning at a high rotational speed or by pouring a fluoride glass melt into the cladding tube. It is well known that single mode optical fibres in fluoride glass intended for manufacturing high-efficiency optical amplifiers require a ratio between core and cladding diameters of the order of 1:100. Taking into account that the diameter of the cladding of these fibres must correspond with that of a conventional optical fibre for telecommunications (typically 125 µm), for obvious reasons of compatibility, this means that the diameter of the core must be slightly larger than 1 µm (typically, about 1.5 µm). Commonly adopted methods for the production of preforms for fluoride glass fibres (e.g. those known in the art as "rotational casting" or "build in casting" and others) do not allow obtaining diameter ratios in the aforementioned order of magnitude, but only ratios of the order of 2-3 :10. Actually, on the one hand it is not possible to form cores whose diameter is less than a few millimetres and, on the other hand, the maximum diameter of the cladding must not exceed about ten millimeters, as larger diameters cause glass stability problems and do not allow drawing the preform. For this reason, besides the above not generally utilized technique, it is usual to produce single mode optical fibres in fluoride glass by "streching" the preform before drawing it, and subsequently coating the stretched preform with a tube of the same composition as the cladding. The stretching causes a reduction of the overall diameter of the initial preform (and thus of the core); coating with the tube, which comes to be part of the cladding, allows increasing the ratio of the cladding diameter to the core diameter. It may be necessary to repeat the stretching and coating operations several times before obtaining the desired diameter ratio. The final preform is then drawn, generally after having been collapsed. An example of this technique is described in the paper "Fabrication of single mode ZBLAN optical fibres", by W. Andrews, D. Coulson and G. Rosman, Journal of Non-Crystalline Solids 140 (1992), pages 281-284. This technique requires repeated operations at a temperature higher than the glass transition temperature of the material (more particularly, in addition to the manufacturing and drawing of the preform, a heating is required for each stretching of the preform and each coating with a tube), and this gives rise to processes of crystallization or devitrification of the glass matrix, thereby worsening the mechanical and optical characteristics of the fibre. The presence of additional interfaces, created by the coating of the stretched preform, also contributes to worsen the mechanical and optical characteristics of the fibre. According to the invention, the above method is improved by providing a "dynamic" etching, i.e. flowing etchant through the preform tube, and by etching at ambient temperature so that the working speed is enhanced, the number of heating phases required to attain the final fibre is reduced, and the formation of additional interfaces is not necessary. The method according to the invention comprises the following additional steps: selectively connecting the two ends of the tube to a source of a chemical etching solution, a source of water and a source of an alcohol, for introducing into the tube the materials required for treating the tube itself and recycling the used materials to the respective sources; circulating through the tube: a) in a first operating phase, the chemical etching solution at ambient temperature for the time required to reduce the internal layer thickness to the required value; b) in a second operating phase, the water at ambient temperature for rinsing the tube; and c) in a third operating phase, the alcohol for eliminating residual water from the tube. The invention concerns also the device for carrying out the method of the invention, as characterized in claim 5; that device is new as concerns means for connecting to the preform tube sources of treatment liquids, and means for sending them into the tube. The invention will now be described in greater detail with reference to the enclosed drawings, where: Figure 1 is an outline of the fabrication process of a tube with two layers made of different fluoride glasses; Figure 2 is a cross-section of the tube; and Figure 3 is a simplified representation of a plant that can be used for thinning the internal layer of the tube in Figure 2 by means of chemical etching. The method according to the invention comprises, as first step, the fabrication of a tube comprising two coaxial layers, of the material required respectively for the cladding and the core of a fluoride glass fibre. By way of purely indicative example, the cladding may be made up of a ZHBLAN glass (i. e., a glass composed of fluorides of Zr, Hf, Ba, La, Al, Na), whilst the core may be made up of a ZBLAYNP glass (i.e. a glass composed of fluorides of Zr, Ba, La, Al, Y, Na, Pb). Again by way of non limiting example, it shall be assumed that the tube be prepared by means of the rotational casting technique, though other known techniques conventionally utilized to realize fluoride glass fibre preforms may be used as well. A detailed description of this process is given, for instance, by D. C. Tran et al. in "Fluoride glass preforms prepared by a rotational casting process", Electronics Letters, Vol. 18 (1982), pages 657 and following. This being stated, to form the tube, the mixture of constituents of the glass of the cladding is poured into a mould 1, preheated to a temperature close to the glass transition temperature. Mould 1 should have an internal diameter such that the external diameter of the resulting tube be compatible with the drawing requirements, thus a diameter of the order of about 10 millimetres (e.g. 8-12 mm). The mould is rapidly rotated (e.g. at 3000 rpm or more) in a crucible so as to obtain a tube 2 (phases a, b in Figure 1), which shall make up the external layer of the final tube. The amount of material should be such as to form a layer 3.5-4.5 mm thick: in this way there are no problems for the subsequent formation of the layer intended to originate the core of the fibre, taking into account that generally the diameter of the axial hole of a tube obtainable with this technique is not smaller than a few millimetres (e.g. 2-3 mm). Once layer 2 is solidified, it is heated to the glass transition temperature, the molten mixture of the constituents of the glass of the core is poured into axial hole 3, and mould 1 is rotated, as before, to originate internal layer 4 of the tube (phases c, d). The amount of material poured for the core mixture shall be such as to form for instance a layer that is, at most, 0.5-1 mm thick. A cross-sectional representation of final tube 5 is shown in Figure 2. The subsequent step consists in thinning internal layer 4 to reduce its thickness to a value equal to about 1/100 that of the external layer (thus, to about 30 - 40 µm), so that, in the tube to be drawn, the ratio between core and cladding diameters be the one required for the realization of a single mode fibre. The plant that allows to carry out this thinning is represented in an extremely simplified form in Figure 3. The two ends of tube 5 are mounted in teflon joints 6, 7 to be connected to intake duct 8 for the materials necessary for treatment of the tube and to exhaust duct 9 for the used material. Duct 8 is connected to a pump 10, specifically a peristaltic pump, which, depending on the work phases, takes via selecting means 15 from respective tanks 11, 12, 13 and sends into tube 5 a chemical etching solution, or water, or yet an alcohol, as shall be better explained below. Duct 9 takes back into the respective tanks, via distributing means 16, the chemical etching solution, containing the material getting removed, the water and the alcohol. For an understanding of the invention it is not necessary to describe in further detail the structure of the plant. Suitable chemical etching solutions are, for instance, solutions of HCl and ZrOCl 2 or of HNO 3 , H 3 B0 3 and HCl. In exemplary embodiments of the invention, the concentrations of the components of the first solution were 1 M for HCl and 0.4 M for ZrOCl 2 , and those of the components of the second solution were 1 M for HCl, 0.5 M for H 3 B0 3 , and 1 M for HNO 3 . With the above concentrations, the first solution has an etching rate on the core material of the order of 400 µm/h in static conditions (i.e. in case of mere immersion in the solution), and the second one has an etching rate of the order of 1200 µm/h, again in the same conditions. Given the initial thicknesses indicated for internal layer 4, it can be seen that the times required to achieve the final thickness are reasonably short and compatible with the needs of an industrial production: it should be noted, moreover, that etching velocity in the dynamic conditions adopted in the present invention is obviously quite higher than the one that can be achieved in static conditions. The etching solution is passed inside the tube at ambient temperature, and this is an essential characteristic of the present invention. Once the desired thickness is attained, pump 10 is disconnected from tank 11 of the etching solution and connected to tank 12 so as to pass rinsing water, also at ambient temperature, through tube 5. Subsequently, the pump is disconnected from tank 12 and it is connected to tank 13 so as to pass the alcohol (advantageously methyl or isopropyl alcohol) through tube 5 for water removal. At this point the tube is ready for the drawing, for which a light vacuum can be applied inside the tube in order to avoid possible formation of bubbles. Drawing a tube rather than a solid structure like a conventional preform presents the advantage that a higher volume of material may be drawn at a given temperature and for a given thermal gradient between the surface and the centre of the tube or, vice versa, that for drawing a given quantity of material, a lower thermal gradient is sufficient. A larger volume of material obviously allows obtaining longer fibres, whilst a lower gradient helps improve quality. As can be seen, the high temperature operations required by the process according to the invention are those inherent to forming and drawing the tube, as in the conventional processes wherein an actual preform is manufactured and drawn. The operations needed to obtain the desired dimensional ratio between cladding and core in the intermediate product to be drawn are instead carried out at ambient temperature, thus eliminating the heating phases required, according to the conventional technique, for stretching the preform and for coating it with a tube of the same material as the cladding. Moreover, the direct drawing of the tube, without collapsing it, eliminates a further heating phase of the materials. It is evident that what has been described is given by way of non-limiting example and that variations and modifications are possible without departing from the scope of the invention, as defined in the claims.

Claims: Method for the fabrication of fluoride glass single mode optical fibres, comprising the steps of: fabricating a tube (5) comprising an external layer (2) of a first fluoride glass of a composition suitable to form the cladding of the fibre and an internal layer (4) of a second fluoride glass of a composition suitable to form the core of the fibre; thinning the internal layer (4) by means of chemical etching until the attainment of a ratio between the volumes of the internal layer (4) and the external layer (2) of the tube (5) corresponding to the ratio between the core and cladding diameters required for a single mode fibre; and drawing the tube (5) obtained as a result of the preceding step; characterized in that said thinning of the internal layer (4) comprises the following steps: selectively connecting the two ends of the tube (5) to a source (11) of a chemical etching solution, a source (12) of water and a source (13) of an alcohol, for introducing into the tube the materials required for treating the tube itself and recycling the used materials to the respective sources (11, 12, 13); circulating through the tube (5): a) in a first operating phase, the chemical etching solution at ambient temperature for the time required to reduce the internal layer thickness to the required value; b) in a second operating phase, the water at ambient temperature for rinsing the tube (5); and c) in a third operating phase, the alcohol for eliminating residual water from the tube (5). Method as claimed in claim 1, characterized in that a solution of HCl and ZrClO 2 or a solution of HNO 3 , H 3 BO 3 , and HCl are used for the chemical etching. Method as claimed in claim 1 or 2, characterized in that the tube (5) resulting from the chemical etching is directly drawn, without being collapsed. Method as claimed in claim 3, characterized in that, for the drawing, a vacuum is created inside the tube (5). Apparatus for carrying out the method according to any preceding claim for processing the tube (5) consisting of: an external layer (2) of a first fluoride glass of a composition suitable to form the cladding of a fluoride glass optical fibre, and an internal layer (4) of a second fluoride glass of a composition suitable to form the core of the fibre; characterized in that it comprises: sources (11, 12, 13) of a chemical etching solution at ambient temperature, of water at ambient temperature and of an alcohol; an intake duct (8), connected at one end of the tube (5); a pump (10) and means (15) for selectively and temporarily connecting, in successive operation phases, the intake duct (8) to one of the sources (11, 12, 13); an exhaust duct (9) connected to the other end of the tube (5) and means (16) connecting, in each operation phase, the exhaust duct (9) to the same source to which the intake duct (8) is connected. Apparatus according to claim 5, characterized in that the pump is a peristaltic pump (10).

Inventors: BRAGLIA MARCO, COCITO GIUSEPPE, BRAGLIA, MARCO, COCITO, GIUSEPPE
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1995
Main classification: C03B37
IPC codes: C03B37, C03C13, G02B6