METHOD FOR MANUFACTURING GLASS PREFORMS TO BE USED IN THE MELT DRAWING OF OPTICAL FIBERS

Abstract: A succession of layers of solutions of metal alkoxides (preferably based on alkyl silicate) is applied to a substrate, each layer being hydrolytically cured to a stiff gel and heat densified before the next layer is applied. The compositions of the solution corresponding to the said layers vary from a layer to the next according to a predetermined pattern. The lamina thus obtained by the superimposition of the successive layers is then heat treated and sintered to a glass preform the refractive index radial variation profile thereof being in conformity with said pattern.

Description: MEIHCD FOR MENUFACTURING GLASS PREFORMS TO BE USED IN THE MELT DRPHING OF CPTICXL FIBERS The present invention concerns fiber optics for telecommunications and, more particularly, a method for manufacturing an elongated glass preform suitable for the melt drawing of optical fibers. As is well known, optical fibers in general consist of a glass or doped silica core surrounded by a cladding made of a lower refractive index medium for ensuring total reflection transmission of the light pulses travelling through the core. Such cladding can have a stepped refractive index relative to the core or, otherwise, the refractive index change frcm the core to the cladding can follow a gradient profile, this being to the extent that no distinct boundary further exists between the so called core and cladding. A profile of such kind having a parabolic shape is often desirable as it decreases the pulse dispersion in multimode fibers. Many methods have been proposed in the art to provide fibers with gradient index profiles among which one of the most favored methods consist in subjecting an elongated preform or mother-rod to melt drawing, said preform consisting of an elongated piece of glass or silica of diameter for instance, 1 to several cm, i.e. many times that of the fiber to be drawn therefrcm and composed of layers of glasses with indices of refraction slightly varying from one to the next, thus roughly providing the same general pattern corresponding to the desired graded index profile to be given to the fiber. Thus, under melt drawing conditions, both the core and the cladding layers will fuse together, the general optical geometry of the mother-rod being thus preserved in the fiber although with a much reduced diameter (a few tens of microns) and no further radial refractive in- dex step transition being then observable in the glass. The preforms suitable for the melt-drawing of graded index fibers as rentoned above can be obtained by many methods including the progressive deposition (on the outside of a rod or the inside wall of a tube) of successive layers of glasses or glass precursor materials with compositions that vary from one layer to the next one according to a desired pattern, such pattern being defined for pro viding ultimately in the enseitble (after final glassification if re quired) the desired gradient index profile. It should be clear to the reader that the terms "glasses" or "glassy material" used here rather refer to the generic meaning of these terms, i.e. to the vi trifled state of such materials (as opposed to the crystallized state). Thus, in the present disclosure a glass will include pure silica as well as metal oxides or mixtures thereof in the glassy state whatever the method used for their preparation. For applying successive layers of glasses or glass precursors on a substrate e.g. a preform web (a rod or the inside of a tube) several methods are cccmonly being used including the classical vapor deposition (CVD) of reagents the reaction of which will provide the required metal oxides on the surface of said substrate, flame or plasma spraying of metal oxides, vapor phase hydrolysis and other related methods. For instance, in FR-A-2 429 040 and 2 473 497, there is disclosed a method according to which a rotating pure silica preform web (i.e. a rod or a tube) is sprayed with a solution of one or several oxides, the solvent is evaporated and the oxide layer thus formed is sintered to - a glass by heat, the sequence of operations being repeated as many times as needed with successively varying solutions compositions for giving the desired ensemble of layers with the desired gradient index pattern. For varying the solutions in the successive layers, the base silica solution is doped with progressively varying amounts of dopants such as P2o5, B203 and camDour.ds of germanium, titanium, aluminium, arsenic, niobium, antimony and others. In this method which has unquestionable merits, the spraying of the silica or other metal oxides and densification by heat of the layer thus obtained are performed practically simultaneously, i.e. the spraying means and the heating means are so arranged that densification heat is applied to the coated area practically as scon as that area has been coated with the metal oxide solution. This is a seemingly very attractive approach but it is not economical as part of the metal oxides will not are tn lhe substrate and bD driven off with the exhaust gases. Also it is not readily applicable when using metal alkoxide solutions as in the present invention. Indeed, the present inventors have found that when subjecting a freshly deposited metal alkoxide layer to high temDerature (i.e. to simple eva poration) withbut first allowing it to cure hydrolytically, only defective sintered glass layers are obtained. Hence, in the present invention, after one layer of the alkoxide solution has been applied, this is first hydrolytically cured to a polysiloxane backbone and then heat densified to a teliperature of preferably 200 - 6000C, although these limits can be exceeded in some cases, before applying the next layer. This method provides a much more regular distributed multilayer composite than the methods of the prior art. Briefly summarized, the method of the invention comprises coating the side or lateral surrounding surface of a generally cylindrically shaped web of circular or elliptical cross-section, i.e. that of a rod or of the inside and/or the outside of a tube, by a plurality of layers of solution of metal aThoxides, each layer being hydrolytically cured to a stiff gel under a te7rperature and humidity controlled atmosphere and then heat dens if ied before applying the sub sequent layer, until a cylindrically laminated composite is obtained, then either removing the web OE not removing it, (this step being optional when the web is made of glass or of a glass precursor mar terial), and finally drying, heat sintering and optionally collapsing said composite to a glass preform, the composition of the metal alkoxides solutions corresponding to each successive layer of the coating Deing suitably varied from one to the next layer to cause the refractive index of the glass in said preform to radially vary according to a predetermined pattern. Such pattern can correspond to any desired gradient index profile in the mother-form (and in the melt-spun fiber derived from said mother-form) including desirable parabolic shaped profiles. In the method summarized above, it has been stated that in so me cases the web can be removed from the composite and in other cases the web is not removed. Obviously, if the web is made of a material different from glass or a glass precursor then the web will be removed at scze stage, ct-rwise it should remains a an undesirable foreign hody in the desired preform. If the web is made of glass, or silica or any glass precursor material then its removal becomes optional and said removal ie either desirable or not desirable but not harmful as a foreign body in the ultimate preform since under final heat treatment the web will get fused with the coating. Obvious ly also, when the web is a removable rod, the cortposite will be a tube and when the web is a removable tube the inside of which has been coated, the composite will be a rod. However, in both cases the end product will serve the same purpose as, under the final high temperature treatment to be described hereinafter, the tube may be collapsed to a rod if temperatures are high enough to cause the glass to soften. However, it is not even necessary to have such a tube be converted into a rod for the making of fibers as the direct melt drawing of the tube according to the present invention will also provide a desirable optical fiber, the collapsing and vanishing of the central void of said tube cccurring simultaneously with the drawing thereof to a fiber. Thus, in the present invention, the achieved optical preforms include tube-like as well as roo-like preforms. Ninen performing the method of the invention with removable webs, the material for the latter can be any, provided it is stable tow'ard metal alkoxide solutions and provided it can be removed from the hy drolytically gelled composite without difficulty either by pulling off if the affinity of the web and the coating is not significant or by other means. Thus, metal or organic webs can be contemplated such as platinum, silver, zinc, copper, steel and the like for the metals and resins for the organics, preferably teflon which can be removed by pulling or thermofusible or soluble resins such as acry lic, polyester and polyolefin resins. Indeed, when the web is of metal, and cannot be simply removed mechanically, it is dissolved in a suitable aqueous acid such as HCl, HIND3, H2SO4 or the like and when the web is of an organic resin and either sticks to the gel, it is melted away.by heat or dissolved in a solvent like acetone, DES, chloroform, methylene chloride, methyl cellosolve or the like. Or, in some cases the web (if made of an insoluble coirbustible material for instance) can even be removed by burning whereby the separate step normally provided for removing the web before final heat sintering can be omitted, said removal occurring simultaneously with said sintering by reason of the relatively high tenperatue nvoIve. ever, in many cases the web can be removed from the composite by simply heating to a temperature where physical separation occurs due to the heat expansion coefficients being different in the web and the deposited lamina. Then performing the present method with non-removable webs, the material for the latter becomes part of the desired preforms. Thus glasses with suitable refraction indices will be selected. Of course, if the web is a rod, it will form the core of the preform; therefore a material with a relatively high refractive index will be selected. If the web is a tube, it will form the outside of the cladding of the preform, therefore, a material with a relatively low refractive index will be selected. As examples of such high and low refractivity glasses, it can be mentioned that the addition of P205, TiO2, GeO2 and other metal oxides will raise the refractive index of high silica content glasses. To lower the refractive index of high silica glasses, B203 can advantageously be added. Further, multiccm- ponent glasses (such as for instance Na2o-Al203- SiO2 glasses) are also useful in the present invention. :A material which is particularly favored for the nonremovable types webs in the present invention is the stiff hydrolytically cured gel which is obtained from the same metal alkoxide solutions used for providing the layers around said web. Thus, such webs can be obtained from doped or undoped alkyl silicate and/or alkoxide solutions by techniques similar to that to be described hereinbelow in connection with the detailed description of the invention and need not be further discussed. Suffice to say here that when metal alkoxide gelled substrates are used as the present webs, the full composite (web plus laminated ensemble of gelled layers) will be subjected to the final heat treatment in bulk, drying and heat-sintering occurring simultaneously in the core and in the cladding. The metal alkoxide solutions usable in the invention for making the successive layers (and also the web on some occasion as recited above) are extremely varied in kinds and fundamentally depend on the availability in the form of alkoxides or similar compounds of the metals normally present as oxides in glasses. Alkoxides here mean lower alkoxides, e.g. metho:1 e,ho^-y; propcry, isoprcr xxy, Futoxy and the like although other alkoxides can be contemplated in case of availability. Briefly, t'ne preferred alkoxides are those soluble in, mainly, alcohols so as to be able to provide solutions having total alkoxide concentration ranging from about 5% to 70%. For Si, Ge, P and Ti alkoxides, it is convenient to use the corresponding ethoxides; for K, Na and Li the methoxides, for Al and Zr the isopropoxide and butoxide, respectively, and for other elements like Y, B, Ba-, Ca, etc, any lower soluble alkoxides. special chelated compounds can be occasionally used i.e. those chelated Ti compounds based on Ti-iso-propoxide. In general, the basic ingredient of the solutions used in the present invention is tetraethoxy silane to which variable amounts of dopant metal alkoxides are added according to a predetermined con centration that varies from one layer to the next one. This predetermined variation follows the same general pattern as that pertaining to the graded index profile of the preform which will ultimately result after the application of the plurality of layers of metal alkoxides is completed. For instance in the case of using a solution containing basically silicon ethoxide and germanium ethoxide, a variation by weight of the germanium dopant can be 1 to 3% approximately with reference to the coirposition of the previous layer solution. In addition to the metal aLkoxides, the present solutions may also contain other metal compounds such as oxalates, acetylacetonates, chlorides, nitrates, carbonates and the like. The solvents which are preferred in the present solutions are water compatible solvents and are for instance ethanol, methanol, butanol, glycols, acetone, tetrahydrofurane, dioxane and the like. Water penetration into the solution and compatibility therewith is important with regard to the hydrolytic gelation cure that occurs in each successive layer of solution. Thus, the solution must be permeable to the humidity from the atmosphere to which the web is sub jected for building the successive layers of gelled alkoxides and the above solvents are convenient in this regard. Generally the ran ge of concentrations of the metal alkoxides in the above solvents is from about 5% to 70% by weight for the tetraethoxy silane. For the dopants, the concentration is adjusted such that, in the final sintered glass, the dopant percent will range from about 0.5 to 50%. It may be sometimes advantageous to directly add a proportion of water to the alkoxide solution to induce in situ hydrolysis and polycondensation in the liquid state that will increase its viscosity and its adhesion on the web. The quantity of water, expressed as the ratio of moles of H2O per moles of alkoxides present can range from about 0.1 to 20. The e mechanism according to which gelation is taking place in the alkoxide solutions is probably linked with the progressive hydrolysis of the alkoxide groups and self polymerization of the ME-OH functions (E being the symbol for metals). However, the exact definition of this mechanism is immaterial for the working of the present invention and needs not being further developed here. Catalysts have been found to speed up the gelation process. Such catalysts can be for instance NH40H, HC1, alkylpolymethacrylate, etc..., the concentration of which may range from about 1Zr4 to 1% by volume in the alkoxide solution. In practice, the method of the present invention can be carried out as follows: A web is first selected which can be a rod or a tube of glass or other material as described before or of dried metal alkoxide gel and this web is contacted with the metal alkoxide solution so its surrounding surface gets coated with said alkoxide so- lution. Said coating can be performed by usual means, i.e. by spraying or painting the solution on the substrate or immersing the lat ter in the solution and withdrawing slowly to ensure the formation of a thin film of the solution on the surface of the web. Usually, the web is allowed to drain until a film of substantially even thickness (about 0.1-5 microns) is obtained. One particular embodiment of such technique applied to the inside coating of tubes is described hereinafter in the Examples. Then the substrate thus coated is allowed to rest for a period of a few minutes to several days in a usually flowing atmosphere of about 10 to 80% humidity at a temperature from about 5 to 1000C to ensure moisture penetration into the solution layer and gelation. The te:tperature range given here is not particularly critical however, below 50C the reaction becomes too slow to be practical and, above 100"C, the rate of evaporation of the solvents that may occur simultsneously with gelation may be ¯ me too high, thus increasing the solution viscosity and concentration, whereby thicker films are formed and crack formation may occur in the gelled layer. Then, after the gel has formed, the substrate is further heated for a few minutes to several hours at higher temperatures, i.e. from about 100 to 6000C to drive off the solvents still trapped in the gel to complete the condensation reactions and, if desired, to densify to almost full density. If desired, the complete sintering of the densified layer can also be undertaken at this stage, i.e. at temperature above 6000C, for instance of 1000"C or more. In some other cases, full densifying can be omitted, a gelling step of the alkoxide layer being enough for ensuring sufficient stiffness thereto before applying the next layer. Also, other drying techniques, besides simple heat, can be applied for removing the water trapped within the gelled layer, i.e. applying high vacuum (together with heat) or heating under a chlorine atmosphere. Anyway, the exact teimperatures, heating times and reduced pressures will depend each time on the particular gel composition and will be easily determined as such by the skilled operator. At this stage, the substrate is ready to receive a new layer of coating with a new solution the composition of which is slightly different from the previous one ac cording to the criteria already discussed and the above procedure is repeated. Then further layers are applied still by the same method until a required total of individual layers is obtained which can be about a dozen to more than a hundred. The thickness of each gel layer can be a fraction of a micron to several microns, the actual thickness being actually governed by the rheological properties of the solutions used and the rate of appllcation. Then, after the last layer has been applied, the web is remo ved (or not removed if it is designed to stay there) as already described and the whole composite is put in an oven and further dried at progressively raised- temperatures, i.e. 400 to 11000C, to achieve final sintering of the slightly porous layers into the desired assembly of fully condensed glass layers bonded to the substrate. The maximum sintering temperature will strongly depend on the overall glass transition temperature and will be adapted accordingly. The heating will be effected for the required time until the cel is converted to a true glass. This completes the preform which can then be subjected to i,el.-urawillj ibr a flSPr, melt r' draf performed according to usual means and needs not be particularly discussed here. Some practical examples will be now given by way of illustration for which the appended drawing may be helpful. In this drawing: Fig. 1 is a schematic representation of a device for perfor.77e ing the coating and gelation steps in one embodiment of the manufacture of the present preform. Figs. 2a, 2b and 2c illustrate schematically the steps of preparing a gelled alkoxide web suitable for being used in an embodiment of the method of the invention. The device schematically represented qn Fig. 1 comprises an oven type enclosure 1 provided with a horizontal feed tube 2 extending to the outside through the wall of the oven and carrying a plurality of side tubings 3 to which corresponding tubes 4 can be connected by usual means (tapered or flexible connectors and the like). The side tubes comprise the valves 5 for controllably enabling a liquid to enter into the tubes 4. The tubes 4 are actually the webs used in the manufacture of the preforms according to the invention. They can be made of any of the materials already disclosed hereinbefore. In a particular application they are made of very pure silica. The oven 1 is also provided with an inlet 6 for gases and a-valve controlled exhaust duct 7. Each tube is protected on top with a removable porous plug 4a, e.g. a fritted glass disk. The present device further comprises a stand 8 provided with a threaded rotatable drive-bar 9 pcwered by a motor 10 engaging in a threaded portion lla of an upwardly and downwardly displaceahle horizontal bracket 11 (see the arrows 12). Said bracket 11 is supporting a reservoir 13 of a solution 14 of alkoxide according to the invention, which reservoir is connected to the feed tube 2 by a flexi ble hose 15. The e reservoir 13 is closed with a porous plug 13a. This hose 15 is made of a very pure flexible silicone resin to prevent contamination by possible impurities of the liquid 14. For the sake of simplicity the representation shown on Fig. 1 is oversLrnli fied, only the essential features thereof being pictured. For instance, all retaining, reinforcing and fixing means for the tubes, the stand and the reservoir have bl, Citted as thoy d^ no tu- cularly contribute to the comprehensicn of the invention. The operation of the present device is described as follows: A series of web tubes 4 being tightly connected to the side arm tubes 3, the reservoir 13 is first lowered to a position below the le- vel of feed tube 2 and in such position, it is filled with an alko xide solution 14. Then, by rotating the screw bar 9 by means of the motor in the right direction (clockwise), the supporting bracket 11 and reservoir 13 is raised until the liquid flows into the tubes 4 through the hose 15, the feed tube 2 and the side tubes 3 under control from the valves 5. Thus, the liquid slowly raises in the tubes 4 and stops finally at the same level as of the liquid 14 in reservoir 13. This level can be controlled by stopping the motor 10 when the reservoir has reached a desired height. Then, an atmosphere of suitable temperature and humidity is in traduced into the oven 1 through inlet 6, the initial air being allowed to escape through the outlet 7. Then, still by acting on the motor rotating in the reverse direction, the reservoir 13 is lowered at a predetermined rate which causes the tubes 4 to progressively empty (the rate of emptying being also controlled by the valves 5) thus leaving a film of solution against the inside walls of the tubes 4. Simultaneously the humidity and tewperature controlled at atmosphere will enter the tubes 4 thrbugh the top porous plugs 4a and effect the hydrolytic gelation cure of the remaining film of alkoxide. After said cure is completed i.e. when the gel has sufficient- ly stiffened, warm air or gas is introduced in the enclosure 1 whereby further evaporation and drying of the gelled layer occurs. Then, such warm air or gas is again replaced by a regular atmosphere, the solution 14 is changed according to the criteria disclosed heretofore and the process is repeated as many times as the desired total number of gelled layers. In this regard, it is to be noted that rather than emptying reservoir 13 and replacing each time with a fresh solution, a basic solution (e.g. tetraethyl silicate) can be continuously left in the reservoir, the required change in composition being provided by adding further quantities of dopants in said solution for the building of successive new layers. Such docar.ts can by timely added dro2wi- se fror supplies 16a, 1Sb and 15 for forming new layers, after removing the porous plug 13a. When the inside of the tubes has been coated with the desired number of layers, the tubes are disconnected and removed from oven 1. Then the wseb may be separated from the semi-condensed preform if the nature of the tube and the conditions of coating permit and said preform is further heat treated and finally sintered and, optionnally, collapsed to a bar under the conditions already outlined hereinbefore. Alternatively, when the web is made of glass or a glass precursor, e.g. a processed gel web as we shall see in the Examples, the whole composite consisting of the web plus the deposited lami- na is subjected in bulk to heat treatments as said above. It should be mentioned that for the treatment of rc & ike webs a device comparable to that represented on Fig. 1 is used. In such device, not shown, the rod type webs are held over a batch of alko- xide solution in an atmosphere controlled oven by a claming element adapted to move up and down exactly as with the bracket 11 of Fig. 1, thus enabling the rcds to be dipped and withdrawn at controlled rate in said batch of alkoxide solution. After alloying the rods to fully drain until a layer solution of substantially even thickness is obtained, the batch is removed from the oven and the film or solution is allowed to gel. Then the full sequence of events is renea.- ed and completed exactly as disclosed in the above case for the tubes. There is then no fundamental difference in the processing of tube-like or rod-like webs. Example 1 An alkoxide solution was prepared by mixing in an ultra-pure fused silica container the following ingredients (v/v %) according to the concentrations given in the next Table I: Si(ORt)4, ethanol, methanol, ethyl acetoacetate (AcAc) and Ce(OEt)4. Then a quantity of water or aqueous-ethanol HC1 solution was added in such quantity that the mole ratio of water to ethoxy silane be at least 4. TABLE I EMI12.1 <tb> <SEP> Coating <SEP> composition <SEP> (v/v <SEP> %) <tb> Sample <SEP> Si(OEt)4 <SEP> Ge(OEt)4 <SEP> EtOH <SEP> MeOH <SEP> AcAc <SEP> H2O <SEP> <tb> <SEP> A <SEP> 27.5 <SEP> 1.55 <SEP> 44.2 <SEP> 7.4 <SEP> 0.08 <SEP> 19.4 <tb> <SEP> (0.1 <SEP> N <tb> <SEP> HC1) <tb> <SEP> B <SEP> 21.4 <SEP> 5.3 <SEP> 48.1 <SEP> 4.5 <SEP> -- <SEP> 20.8 <tb> Then, the apparatus as described above with reference to Fig. 1 was equipped with pure silica tubes of about 20 cm long and about 20 ml liquid capacity. The prepared solution (A) was put into reservoir 13 and the mixture was raised in the tubes by acting on motor 10 at the rate of 1 mm/min, then when the tube were filled nearly to the top, an 80% humidity and 600C air was introduced into the enclosure 1 and the tubes were gradually emptied by gravity at the same rate of 1 mm/min. The tubes were left empty under the above humidity and temperature for about 1 hr after which the moist air was replaced by dry air at 150 C. This was left to dry the first gelled layer for about 10 min after which the first cycle was achieved. The thickness of the gelled layer was about 10 microns. Then, the cycle was repeated with solutions of progressively increased dopant concentration. Such concentration was increased each time about 1-5% v/v over the previous concentration and the cycles were repeated up to ten times. After the last layer was added, gelled and condensed, the tubes were removed from the enclosure and heated in a high-temperature oven for about 1 hr at 2000C, then the tem- perature was slowly raised to 100000 at the rate of about 1 C/min. After 1 hr at 1000 C the composite lamina had sintered to a parent glass and the preform was allowed to cool. The preform thus obtained was melt-drawn in a conventional apparatus at 19000C to give an optical fiber-of 140 microns diameter and with a core of 10 microns. The fiber had a graded refractive index in conformity with the invention. Similar results were obtained with solution (B). After sintering, the glass obtained from basic compo- sition (A) ccntained 91.06% SiO2 and 8.942 GeO2 by weight. The glass frcm ocosition (B) had 69.8% SiO2 and 21.35% GeO2. If, in the present Example, sucessive layers were built up using solution (A) without progressive addition of dopants (i.e. not changing the cc -csi- tion from layer to layer), a glass preform is obtained the refrac- tive index of the cladding thereof is radially constant. A step in- dex fiber is thus provided on a core made of denser a medium. Example 2 A hydrolytically gelled silica tube web was iranufactured as follows with reference to appended Figs. 2a, 2b and 2c. A tube 21 (see Fig. 2a) made of glass or of low surface adhe- siveness resin (e.g. Teflon) with a threaded opening was selected and provided with an inner tube 23 of a same or like material, a rim seal 24 and a bored screw-cap 25 through which the end of tube 23 was protruding. While leaving aside the seal and the cap, the tdE 23 was pushed right down to the bottom or tube 21 to ensure provisional rough tightness and the outer space around said tube 23 was filled with a composition containing (v/v %) Si(OEt)4 l9.27;'EtOH 43.33; -eOH 4.0; AcAc 0.06; H20 33.33; then the seal 24 and the cap 25 were installed and the assembly was upturned as shown on Fig. 2b while slightly pulling off the inside tube 23 to allow the air to penetrate the vacant space 26 provided in the bottom of tube 21 by the downward displacement of the liquid. The tube was left several hours in a humid atmosphere at 60-800C whereby gelling occurred and the tube could be put again in a straight position, the cap and the seal could be removed and the inside tube be gently pulled away, thus leaving a jacket 27 of gelled silicate. Then a mesh cap 28 was placed over the opening of the tube 21 and the latter was thermally treat- ed to further dry and finally sinter the annular jacks 27. Such treatment involved heating under air flow at the rate of 0.1 C/min to 10000C where sintering to a clear silica tube occurred. This tube was used as a web (with others) for making optical preforms according to the procedure of Example 1. It should be further noted that in using a tubular web in the method of the invention, the latter can be coated with successive metal alkoxide gels on its outside surface as well as on its inside surface or on both. For instance a non-removable tubular web of intermediate refractive index can be coated, on the inside, with layers of solutions leading to progressively increasing indices and, on the outside, with layers of solutions leading to progressively decreasing indices. Such an embodiment further demonstrates the versatility of the method relative to the prior-art.

Claims: CLAIMS 1. A method for the manufacture of a glass preform suitable for the melt drawing of optical fibers, which comprises coating a sub strate defined as the lateral surface of an elongated generally cy lindrically shaped web of circular or elliptical cross-section with a plurality of superinposed layers of solutions of metal alkoxides, each successive layer being hydrolytically cured to a stiff gel under a temperature and humidity controlled atmosphere and then heat den sified before applying the subsequent layer, until a cylindrically shaped laminated composite is obtained, then removing or not removing the web, and further condensing, drying, finally heat sintering and optiornally collapsing the resulting structure to a glass preform, the composition of the metal alkoxide solutions corresponding to each successive layer of the coating being suitably varied from one to the next to - cause the refractive index of the glass in said preform to radially vary according to a predetermined pattern. 2. The method of claim 1, wherein the web is a rod made of a plain, voidless solid glass or glass precursor material. 3. The method of claim 1, wherein the web is a tube and that said lateral surface is the inside and/or the outside surface of this tube. 4. The method of claim 3, wherein the tube is a plastic or metal tube. 5. The method of claim 3, wherein the tube is a mineral glass tu- be or a silica tube made from a hydrolytically gel-cured solution of metal alkoxides. 6. The method of claim 4, wherein the web is removed mechanically or by dissolving in a suitable acid or solvent liquid. 7. The method of claim 4, wherein the web is removed by heating to a temperature at which physical separation between the web and the gelled composite occurs due to the differences in the heat expansion coefficients. 8. The method of claim 1, wherein said coating is achieved by cotw tacting the substrate to be coated with a supply of said alkoxide solution, then removing said supply and allowing said substrate to drain until a solution layer of substantially even thickness is oh tained on said substrate. 9. The method of claim 8, wherein said contacting is effected by dipping the substrate into said solution, then slowly withdrawing therefrom 10. The method of clairrs 3 and 8, wherein said substrate is defined as the inside surface of a tube, said contacting is effected by filling said tube with said solution and said removing is effected by ezp.ying the t"e by gravity according to a predetermined rate. 11. The method of claim 10, wherein said atmosphere of controlled humidity and temperature is introduced into the tube simultaneously with said remssvir.g of the said solution. 12 The method of claim 11, wherein said solution consists of silicon ethoxide, germanium ethoxide, ethanol and methanol. 13. The method of claim 12, wherein said solution further compri- ses water, the metal alkoxides of said solution being in a partially hydrolyzed state with increased viscosity due to the presence of said water. 14. The method of claim 12, wherein said temperature is from 5 to 100 C and said humidity is from 10 to 80%. 15. The method of claim 12, wherein said variation of the ctposi- tion of said solution from one layer to the next one consists in increasing the germanium ethoxide concentration therein by 1 to 3% by weight relative to the concentration of said germanium in the solution for the previous layer.

Inventors: GONZALEZ OLIVER, CARLOS, HARMER, ALAN LEWIS, PUYANE, RAMON
Applicants: BATTELLE MEMORIAL INSTITUTE
Application year: 1981
Main classification: C03B37
IPC codes: C03B37