GB1559768A - Optical fibre preform manufacture - Google Patents

Description

(54) OPTICAL FIBRE PREFORM MANUFACTURE (71) We, INTERNATIONAL STAN DARD ELECTRIC CORPORATION, a Corporation organised and existing under the Laws of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the manufacture of optical fibre preforms, which after manufacture are drawn down to make optical fibres.

The increasing use of optical fibres for communication applications has resulted in the development of very high purity glass materials. Since the publication by C.K.

Kao and G.A. Hockham in the Proceedings of the IEE Volume 13, July 1966 suggesting that a dielectric waveguide could be used for the guided transmission of energy at optical frequencies and discussing the mechanisms for the loss of energy passing through the waveguide, various attempts have been made to decrease this loss.

The discovery that cation impurities and OH radicals in the glass structure contribute significantly to the light loss when the transmitted light is in the infrared region of the spectrum prompted research into methods for forming high purity glasses with low metal cation and hydroxyl ion content.

To prevent the OH radicals from forming within the glass structure, the method of chemical vapour deposition (CVD) of the glass-forming materials in the absence of hydrogen and hydrogen containing compounds was developed. The CVD technique produced highly purified glass by distilling the glass-forming materials on the inner surface of a high purity manufactured silica tube. Since the CVD process occurred in the absence of hydrogen and hydrogen containing compounds, the principal remaining source of OH ions was the silica tube itself.

Various in situ methods such as outgassing and thermally treating the silica tube prior to CVD to remove residual moisture from the tube reduce the amount of OH radicals remaining in the glass to some extent.

An object of this invention is to provide materials for making optical communication glasses that are substantially free from cation impurities and OH ions by chemically distilling high purity materials without using a manufactured silica tube.

According to the present invention there is provided a method for depositing high purity optical fibre materials, including the steps of: (a) providing a tubular deposition substrate from a first material; (b) depositing a high purity second material on the inner surface of the substrate to form at least one layer of the second material of high purity; and (c) removing the tubular deposition substrate from the high purity material to provide a high purity cylinder consisting essentially of said second high purity material.

This method preferably uses a highly purified carbon deposition substrate for chemically distilling high purity materials on to its inner surface. The carbon is in the form of a cylinder and provides an environmental enclosure for the material as it is deposited. After a quantity of highly purified material is deposited on the inner surface of the carbon cylinder, the carbon is removed, e.g. by grinding. The material can also be separated by deposition upon a split mold and removing the material from the mold.

In another embodiment, the carbon cylinder is a split section mold wherein the high purity materials are deposited within the mold by the CVD method. The carbon mold is then opened to remove the materials from the mold in cylindrical form.

Surrounding the carbon cylinder with an RF coil and translating the RF coil relative to the linear axis of the cylinder provides for localized heating of the cylinder and forces the chemical vapour deposition reaction to deposit uniform layers along the substrate.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a water cooled cylinder for use with the apparatus of this invention; Figure 2 is a perspective view of a disposable cylinder for use with the cylinder of Figure 1 Figure 3 is a perspective view of the cylinders of Figures 1 and 2 including a concentric RF coil; Figure 4 is a perspective view of a split cylinder mold containing material deposited on its interior surface; Figure 5 is a perspective view of the mold of Figure 4 in an open position; Figure 6 is a cross-sectional view of a cylinder of high purity material produced by the method of the invention; Figure 6A is a cross-sectional view of the material of Figure 6 in a consolidated form;; Figure 6B is a cross-sectional view of the material of 6A after being drawn and coated to form an optical fibre; Figure 7 is a further embodiment of the material of Figure 6; Figure 7A is a further embodiment of the material of Figure 6A; Figure 7B is the material of Figure 7A after drawing into an optical fibre; and Figure 8 is a sectional view of a glassforming lathe for use with the method of this invention; Figure 9 is a sectional view of glass deposition apparatus for use with the method of this invention Figure 10 is a sectional view of an alternative glass deposition apparatus for use with the method of this invention with glasses which tend to slump; Figure 11 is a perspective view of an alternative apparatus used for electrically heating the substrate; Figure 12 is a perspective view of an alternative to apparatus of Figuree 11; Figure 13 is a perspective view of the substrate with at least one layer of material deposited on its inner surface; Figure 14 is a perspective view of the substrate of Figure 13 being treated for removal from the deposited material by the cracking method;; Figure 15 is the deposited material of Figure 14 in solid form separate from the deposition substrate; Figure 16 is a cross-section of the collapsed material of Figure 15; Figure 16a is a cross-section of the material of Figure 16 after drawing into an optical fibre and being coated with a plastic outer layer; Figure 17 is a perspective view of the substrate with two layers of material deposited on the inner surface; Figure 18 is a cross-section of the deposited material of Figure 17 in solid form separate from the deposition substrate; and Figure 19 is a cross-section of the material of Figure 18 after drawing into an optical fibre.

In order to provide ultra-high purity materials for manufacturing glass having very low metal cation content, high reagent grade chemicals are deposited by CVD on the inner surface of a carbon cylinder. The materials comprise one or more of the oxides of silicon, germanium, phosphorus, zinc, boron and arsenic and are transported in the vapour phase onto the inner surface on the carbon cylinder, using suitable carrier gasses. The vapour phase reaction takes place in the hot zone. The oxides are formed by intermixing with high purity oxygen and heating under very carefully controlled conditions.

The vapours of the materials are generated by entraining the materials in a highly purified oxygen stream, or other suitable gas, wherein the oxygen is bubbled through the halides of the respective materials and directed into the interior of the carbon cylinder. To avoid the formation of any hydrogen or hydroxyl ions within the glass material, the reaction is caused to occur on the inner surface of the carbon cylinder in the absence of hydrogen. The inner surface of the carbon cylinder is carefully heated and outgassed under a constant flow of a dry inert gas to remove any traces of moisture present. Once the inner surface of the carbon mold has been carefully outgassed, the inert gas stream is shut off, and a flow of oxygen mixed with chemical vapours is allowed to proceed through the cylinder in a continuous process and form oxides on the inner surface of the carbon mold.

This carbon cylinder serves as an airtight hydrogen-free container within which the reaction is caused to occur. The oxygen flow is continued throughout the entire deposition reaction to cause a continuous flow of pure chemicals in the inlet of the carbon cylinder and out of the exhaust to provide a slightly positive pressure within the container at all times. The slight positive pressure of oxygen ensures that no hydrogen or hydrogen-containing compounds can enter the interior walls of the cylinder.

To ensure that the atmosphere surrounding the carbon cylinder is relatively oxygen and hydrogen free, an RF coil is used in lieu of the hydrogen-oxygen flame usually used for CVD purposes. The RF excitation coil surrounds the carbon cylinder and is moved along the cylinder to cause the carbon to be heated to incandescence and to cause the chemical vapours to react and become deposited upon the heated section of the cylinder. The gas flow rates and concentrations, and the translational motion of the coil can be carefully programmed to result in a uniform deposition of materials upon the inner surface of the carbon cylinder.

Preferably a carbon cylinder is used as a deposition substrate for the CVD process and the carbon material acts as a susceptor for receiving RF energy.

A method used for removing the carbon cylinder from the deposited materials is to use a split carbon cylinder as a glass deposition mold. The split cylinder is held together within the jaws of a glass forming lathe and the materials are deposited as described.

After the materials have been completely deposited upon the inner surface of the cylinder, the mold is simply opened and the solid tube material is removed.

Figure 1 shows a double-walled quartz cylinder 10 with an inlet 11 and an outlet 12 and sealed at both ends. The cylinder 10 acts as a water cooled enclosure for housing the deposition cylinder during the CVD process. The material for the cylinder 10 is quartz or silica when visible access is desired to the deposition process. However, other materials can be used, providing they do not interfere with the coupling efficiency of RF energy to the deposition cylinder 13.

The deposition cylinder 13, Figure 2, is a thin-walled carbon tube although other materials that form a deposition substrate without reacting with the glass-forming materials can also be used.

Figure 3 shows the complete deposition assembly 14, which includes the doublewalled cylinder 10 encompassing the thinwalled carbon cylinder 13 and includes an RF coil 15 having a number of turns 16.

Since the carbon cylinder 13 is a susceptor for the RF energy from the coil 15, the carbon cylinder 13 is within the outer water cooled quartz jacket 10, and the space between the outer jacket 10 and the outside surface on the carbon cylinder 13 is kept under a positive flow of inert gas to prevent the carbon from becoming oxidized when heated during the CVD process.

To heat the cylinder of high purity material to cause the cylinder to collapse into a solid preform, various methods of heating can be used to avoid any hydrogen or hydrogen-forming compounds from being absorbed in the heating process. One method is to focus a high intensity carbon dioxide laser onto the exterior surface of the high purity material while rotating the material upon a glass-forming lathe. The use of the high power CO2 laser permits the material to be heated to a softening temperature in the absence of hydrogen and hydrogen containing compounds. Another method for heating the cylinder of high purity material to its collapsing temperature is to place the RF coil 15 and a susceptor around the high purity cylinder and to heat the material to a temperature such that the surface tension forces the cylinder to collapse into a solid preform.To avoid the diffusion of hydrogen and hydrogen compounds into the material during the fibre drawing process, the carbon dioxide laser can also be used to heat the preform to the fibre drawing temperature.

The water inlet 17 and water outlet 18 for the RF coil 15 ensure that the RF coil 15 does not become heated under the high power requirements.

The deposition assembly 14 of Figure 3 can be attached to glass-forming lathe 19, Figure 8. Although other methods may be used for connecting the apparatus of Figure 3 to the glass working lathe 19, the method used in Figure 8 conveniently allows the carbon cylinder 13 to rotate while the double-walled outer cylinder 10 remains stationary throughout the deposition process. While the cylinder 13 is rotating, one end 20 of the cylinder 13 acts as the chemical vapour inlet for nozzle 21, and the other end 22 of the cylinder 13 acts as the chemical vapour exhaust for nozzle 23. During deposition, the various chemical vapours are introduced through the inlet 20 whereupon they become heated upon contact with the heated inner carbon cylinder 13 which is enclosed within the moving RF coil 15.The temperature of the cylinder 13 can be so adjusted that the materials deposit either in a powder form or as a vitrified glass-like material.

During the deposition process an inert gas is introduced between the outer doublewalled silica cylinder 10 and the inner carbon cylinder 13 by inert gas inlets 24, 24' and the inert gas is kept at a slightly positive pressure to ensure that no air reaches the carbon cylinder 13 during deposition.

When high purity silica is required to make plastics clad silica optical fibre wherein the silica material provides the optical fibre core and the plastics material provides the low index cladding, the starting material for the silica is highly purified reagent grade silicon tetrachloride. Highly purified oxygen gas is bubbled through the silicon tetrachloride, and the silicon tetrachloride vapours are reacted via inlet 21 within the heated carbon cylinder 13 to deposit as highly purified silica either in a powdered or vitreous form.

The best way is to deposit the core and cladding on one substrate to avoid the core/cladding imperfection, which leads to an increase in loss. When a glass-on-glass optical fibre preform is to be generated on separate substrates, a source of highly purified silicon tetrachloride is used to generate a silica cylinder as described earlier and that cylinder is removed without collapsing. A fresh carbon substrate cylinder is then inserted within the lathe of Figure 8 and a cylinder of core material is generated by the use of silicon tetrachloride and germanium tetrachloride when the core is to consist of a germania-silicate glass or phosphorus trichloride and silicon tetrachloride when the core is to consist of a phosphorus-silicate glass. Other materials can be used such as boron trichloride and zinc chloride when other properties are to be imparted to the glass material.Once the core glass material has been deposited upon the inner surface of the carbon cylinder 13, that cylinder 13 is removed as described earlier and the resultant core material is removed in solid form.

To provide for a close fit between the inner cylinder of core material formed by the higher index germania silicate or phosphorus silicate glass within the cladding cylinder, formed by pure silica or borosilicate glass, the carbon deposition cylinder are of different diameters. The cylinder for the cladding substrate is larger than that for the core deposition substrate so that the resulant core and cladding cylinders can be inserted one within the other prior to collapsing to form a preform.

When this method is used to produce ultra-high purity materials without the need for destroying the carbon cylinder a split cylinder carbon mold is employed. Figure 4 shows such a mold 25 having a first section 26 and a separate section 27.

The core materials, with high index of refraction, can be deposited on the cladding materials within the same deposition cylinder. As described earlier, an optical preform can be deposited in one step. This method prevents the formation of any imperfections at the core-cladding interface.

The split mold 25 is used in place of the thin-walled carbon cylinder 13 described earlier and the deposition reaction is caused to occur in the same manner wherein the deposition material 28 is deposited on the inner surface of the split mold 25. Once the deposition process is completed the split mold 25 is removed, by separating the first section 26 from the second section 27 to provide a solid core cylinder 28 of the deposited material. The split cylinder mold 25 can be used when ultra-high purity materials are required and damage to the carbon is to be avoided.

Figure 6 shows the cylinder 28 of high purity material when the material is silica and is to be used for a core of a plastics clad silica optical fibre.

Figure 6A shows the high purity silica 28 after collapsing to form a solid preform for drawing into the fibre 29 of Figure 6B, which is covered with a cladding layer 30 of a plastics material of low index material.

Figure 7 shows a cylinder of high purity silica 28 of low index material and an inner cylinder 31 of materials consisting essentially of germania and silica to result in the collapsed preform 32 shown in Figure 7A.

Figure 7B shows the resulting optical fibre 33 formed from the preform 32 of Figure 7A where the germania silicate material 31 forms the core and the pure silica material 28 forms the cladding.

Figure 9 shows another glass deposition apparatus 19 usable with this method. It has a stand carrying a water-cooled electrically insulating or poorly conducting jacket 10 and a hollow carbon cylinder 13 within the jacket 10, the carbon cylinder being held in water-cooled mounts at each end. Between the jacket and the cylinder is a water-cooled HF induction coil 15 forming the primary of an induction furnace, the secondary or receptor being the carbon cylinder 13. This is so arranged as to heat the carbon cylinder 13 during material deposition. Highly purified chemical reagents are introduced by oxygen or other gas entrainment through the inlet end 20 of the carbon cylinder mount, and the excess is removed at the exhaust end 22 of the carbon cylinder mount.Alternatively the reagents may be introduced at the upper end 22 and the excess removed at the lower end 20 of the carbon cylinder mount.

The spacing of the turns 11 of the induction coil 15 may be varied along its length to provide for proper heat distribution along the length of the cylinder 13. Alternatively the coil 15 may be sectionalized as shown in Figure 10 and each section power fed separately to control heat distribution along the length of the carbon cylinder 13.

The inner surface of the turns 11 of the induction coil 15 can be polished and treated to increase their infra-red reflectance, to reduce radiative heat transfer away from the carbon cylinder 13. Also, see Figure 1, the coil 15 may be made of rectangular rather than round section tubing to improve further the reflectance back to the carbon cylinder. Further, the coil 15 should be protected from oxidation by suitable treatment to reduce damage during subsequent oxidation of the carbon cylinder 13 as described below. One such treatment which would serve both purposes is a plating of metallic gold on the induction coil 15.

During the deposition step in the process the region between the water-cooled jacket 10 and the carbon cylinder 13 is kept under a continuous purge of an inert gas by means of the gas inlets 24 and outlets 25.

The vertical orientation of the carbon cylinder 13 as shown in Figure 9 is chosen to permit radially symmetrical deposition without the complication of rotating the deposition substrate. However, when depositing glasses which tend to slump in the gravitational field, the apparatus shown in Figure 10 may be used. In this case the glass deposition apparatus is a glass-forming lathe 19 carrying a water-cooled jacket 10 and the carbon cylinder 13 within the jacket 10.

Between the water jacket 10 and the rotatable cylinder 13 is the induction coil 15, similar to that of Figure 9. Highly purified chemical reagents are introduced by gas entrainment through the inlet 20 of the carbon cylinder mount, and excess is removed at the opposite end 22. To further assure uniformity of glass deposition within the cylinder 3 it is desirable with certain concentrations or flow of the entrained chemicals to reverse the direction of gas flow for part of the deposition time, the entraining gas entering at end 22 and leaving at end 20.

Because of the high temperature to which the cylinder 13 must be heated to fuse into a glass such a deposited material as silica, and as carbon (and especially graphite) is a fair conductor of heat, it is necessary to cool the bearings of the lathe 19. One arrangement for this is shown in Figure 10. A tubular extension 26 of the holder for the carbon cylinder 13 has fixed to it a ring 27 in the periphery of which are a series of radial holes 9 leading to a circular recess cut inside the ring 27. A fixed bearing 28, provided with a circular recess surrounding the radial holes 9 in the rotating ring 27, is fed with cooling air through the pipe 29, the air exhausting through a series of openings 30 between the ring 27 and the carbon tube holder 26 to which the ring 27 is fixed.Thus the bearing and the holder 11 for the carbon tube 13 can be maintained at a safe operating temperature in spite of the heat conducted to them from the hot portions of the carbon tube 13.

In both the apparatus of Figure 9 and that of Figure 10 the induction coil 15 may be mounted exterior to the water jacket 10 and give similar results. However, this introduces an added complication in that not only must the water jacket 10 be made of good insulating material such as fused silica, but also the water that is circulated must be of good purity and de-ionized to reduce its electrical conductivity to the point where the water will not seriously rob electrical power from the induction coil 15.

The graphitic form of the carbon can have two different crystal structures, the alpha which is hexagonal and the beta which is rhombohedral form. Natural graphite is a mixture of the two forms while artificial graphite is purely alpha. The beta form is normally converted to the alpha by heating, so the alpha form is the important one.

Single crystals of this material have a low thermal expansion coefficient along an "a" axis and a relatively high coefficient of expansion along the "c" axis. Tubes of polycrystalline graphite are commercially available where the crystals are oriented at random, or nearly at random. In such material the expansion coefficient is between that along the "a" axis and "c" axis, and is far greater than that of fused silica.

The present method contemplates, among other glasses, the deposition of pure fused silica, and silica-containing compounds introduced to alter the refractive index but with resulting expansion coefficient not very different from that of pure fused silica. By keeping the temperature of the graphite at or not far from that which the deposited glasses begin to soften, it is possible to avoid serious thermal stresses both during and after the deposition process, and until the steps are taken to remove the graphite substrate as described below.

Once the deposition process is complete and a sufficient quantity of the glasses have been deposited on the inner surface of the graphite cylinder, the assembly of graphite and hollow glass cylinder can be cooled rapidly by a stream of gas or other convenient means. If the graphite cylinder or tube is polycrystalline and the crystals are randomly oriented, and if the graphite tube wall thickness is of the same order of magnitude as the wall thickness of the glass tube within, the graphite, which is weak in tension will crack without injuring the glass tube which is relatively strong in compression. The graphite fragments can then be separated from the glass tube.

Alternatively the inert gas which has been flowing through the space between the graphite cylinder and the water-cooled jacket during the deposition of glass inside the graphite tube can be replaced, at the conclusion of the deposition process with oxygencontaining gas while the graphite is maintained above its combustion temperature.

Thus the graphite can be burned away at a controlled rate. At the same time a flow of air or other gas can be maintained through the inside of the glass cylinder within the graphite to keep the glass at a temperature below its softening point. Control of the burning rate of the graphite can be accomplished by control of the oxygen content or flow rate of the gas passing through the space outside of the graphite cylinder. Too high a flow rate and/or too high an oxygen content can result in an undesirable rate of combustion and consequent high temperature. Too low a flow rate of the oxygencontaining gas, especially with low oxygen content, can result in oxygen depletion and thus a slower burning rate at the down stream end of the graphite tube.Reversing the direction of flow, (alternating the inlet between the gas ports 24 and the ports 25 in Figure 10) results in symmetrical but not necessarily uniform combustion along the full length of the graphite cylinder. Additional control to achieve reasonable uniformity is conveniently accomplished by selective electrical heating along the length of the cylinder during the combustion process.

This can be accomplished either by applying power selectively to sectionalized induction coils shown in Figure 10, or to a movable coil as shown in Figure 8. A coated cylinder 13 is shown in Figure 13 with a layer of glass 23 on the inner surface.

It will be evident that a combination of the two methods of graphite removal described above can also be effective. If the burning-off process is carried out until the remaining graphite tube wall is quite thin, and this is followed by cooling the graphite and glass assembly so that the remaining graphite cracks, very effective graphite removal can be accomplished from even a relatively thin-walled glass tube without injury to the glass. In one example using the combination of the methods of graphite removal by burning the graphite tube to reduce its thickness prior to cooling the graphite and glass assembly to crack the remaining thin graphite wall, the graphite tube 13 containing the deposited glass 23 has a thinner portion 13' removed by combustion.

Figure 15 shows the material 23 of Figure 14 remaining after the graphite 13 has completely shattered.

Any number of resultant cylinders of high purity material 23 can be formed in this manner. Thus, the outer cladding layer of glass on a glass optical fibre can be provided by the deposition of high purity silica material on a first graphite cylinder of a certain size. Once the graphite has been removed from the cylinder, the resultant silica cylinder can be used as the cladding layer for an optical fibre. The core material can be provided by depositing a higher refractive index material upon a new graphite cylinder to form a resultant cylinder of high purity material slightly smaller than the cylinder of the cladding material.

A completed optical fibre preform can then be fabricated by inserting the higher index of refraction cylinder within the lower index of refraction cylinder and heating to collapse and form a solid preform.

Figure 16 shows the material 23 of Figure 15 after heating to collapse to form a solid preform and Figure 16a shows an optical fibre 25 where the material 23 of Figure 16 comprises the core and a low refractive index plastic cladding material 26 is coated over the core.

Figure 17 shows the graphite cylinder 13 with a second layer of high purity material 27 coated over the first material 23. Figure 18 shows the first material 23 and the second material 27 after removing the graphite cylinder 23 and collapsing the materials 27, 23 into a solid preform 28.

Figure 19 is an optical fibre 29 formed by heating and drawing the preform 28. The index of refraction of the first material 23 is selected to be lower than the index of refraction of the second material 27 to form a glass-on-glass type optical fibre 29 wherein the higher index material 27 forms the core and the lower index material 23 forms the cladding.

Because of the heat loss at the ends of the graphite tube due to thermal conduction to the end supports and the resulting lower temperature of the tube ends very little combustion of the graphite will occur at the ends. This is not important because at the lower temperature very little deposition of the materials forming the glass will occur at the ends. Moreover, in the case of silica or material high in silica, there will be little or no fusion into a glass, even where some deposit has occurred. The result is that the useful deposit of glass is confined to the main central body of the graphite tube, the same region which is burned away in the removal step of the process described above.

In the foregoing discussion it has been assumed that the starting graphite tube is a cylinder of uniform cross-section. This is usually convenient, but in some cases it is preferable to start with a substrate tube which does not have a uniform cross-section along its length, as an aid to heating control or removal control.

Carbon at high temperature is a powerful reducing agent and will react to some extent with the first layers of silica deposited within the graphite tube. Depending on time, temperature and other factors a thin layer of the silica may be reduced to a lower oxide or even-to elemental silicon. Also under some conditions a layer of silicon carbide may form. Any such layers are contaminants and should be removed, as by grinding or etching or a combination of both, before the glass tube is collapsed and drawn into a fibre.

In the foregoing description the heating of the graphite cylinder has been by high frequency induction. Other means for keeping the graphite cylinder at a constant uniform temperature during the deposition process can be seen by referring to Figure 11. The graphite cylinder 13 contains a pair of electrodes 20 and 21 one at either end of the cylinder 13 and electrically and mechanically attached by bands 22 and 23 to ensure good electrical contact between the electrodes 20 and 21 and the cylinder 13.

When the cylinder 13 is mounted in the glass apparatus of Figure 9 and is under a steady stream of inert gas, a source of low direct voltage of high amperage is connected across electrodes 20 and 21.

Other methods may also be used to keep the graphite cylinder 13 at a high, constant and fairly uniform temperature during the deposition process. Thus, the entire graphite cylinder 13 can be placed within a specially designed oven or the graphite cylinder per se could be wound with resistive heating wires. Figure 12 shows a cylinder 13 wound with a winding of high melting point wire 30 such as tungsten or molybdenum around the graphite tube 13 to heat the tube 13. These metals are protected from oxidation during the deposition process by the inert atmosphere used to protect the graphite, as previously described.

Although carbon is normally preferred as material for the substrate tube, other refractory materials can be useful. However, to retain the important advantage of the present method, the substrate material must remain rigid at a temperature at which the deposited glass or glasses fuse. Zirconium oxide bonded with a very small quantity of kaolin or other clay, or with an organic binder such as gum tragacanth and fired at a high temperature is suitable. It can be made sufficiently rigid, yet weak in tension, by controlling the small quantity of binder and the firing temperature. Zirconium oxide is a non-conductor and must be heated by some external means. This can be electrical as shown in Figure 13 as described above.

Alternatively since it is quite stable in air and need not be surrounded by an inert gas in the deposition process, the entire length can be flame heated selectively or simultaneously, the water-cooling jacket being dispensed with. The thermal expansion of such a polycrystalline zirconia tube is, like graphite, much larger than that of fused silica, so it can be removed by cracking as described above for graphite.

If it is desired to use high frequency induction heating, there can be added to the zirconium oxide approximately 10% of yttrium oxide, this combination being bonded preferably by an organic material.

The organic material burns out when fired in a kiln, but at an appropriate firing temperature, well below the normal melting point, micro-sintering occurs and bonds the micro-crystalline mass into a rigid structure.

This will be recognisezed as the well known Nernst glower material. If heated, as by a flame, to a moderate red temperature it becomes electrically conducting, and can then serve as the secondary or receptor of a high frequency induction furnace just as in the case of graphite. Moreover this mixture, like pure zirconium oxide, is very stable in air so no protection by an inert atmosphere is required. Again as with pure zirconia or graphite, removal is accomplished by cracking as the temperature is reduced at the conclusion of the deposition process, or by mechanical grinding.

As with a carbon tube there will be some chemical reaction between the substrate and the outer layers of the deposited glass. In this case the principle compound formed is zirconium silicate which as a melting point substantially higher than the softening point of silica. Thus as with a carbon substrate, the deposited glass cylinder which is formed should have its outer surface cleaned by grinding and/or etching before being collapsed and drawn into a fibre.

For reasons of economy it may be desirable to recover for re-use the zirconia-yttria material which is substantially unaltered except for the thin layer of silicate which has formed on the inner surface of the substrate tube. This contamination can be largely removed by a suitable etch, the fragments crushed and formed again into a refractory tube.

Although the invention is directed to providing high purity materials for glass for optical fibre purposes, the invention also finds application wherever such high purity glasses may be required.

WHAT WE CLAIM IS: 1. A method for depositing high purity optical fibre materials, including the steps of: (a) providing a tubular deposition substrate from a first material; (b) depositing a high purity second material on the inner surface of the substrate to form at least one layer of the second material of high purity; and (c) removing the tubular deposition substrate frm the high purity material to provide a high purity cylinder consisting essentially of said second high purity material.

2. A method as claimed in claim 1, and including the step of depositing high purity third material on the inner surface of said second material prior to removing said deposition substrate in order to provide a tube containing layers of both said second and third high purity materials.

3. A method as claimed in claim 1 or 2, wherein said first material is carbon.

4. A method as claimed in claim 1, 2 or 3, wherein the step of depositing said high purity second material includes chemical vapour deposition.

5. A method as claimed in claim 4,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (57)

**WARNING** start of CLMS field may overlap end of DESC **. 11. The graphite cylinder 13 contains a pair of electrodes 20 and 21 one at either end of the cylinder 13 and electrically and mechanically attached by bands 22 and 23 to ensure good electrical contact between the electrodes 20 and 21 and the cylinder 13. When the cylinder 13 is mounted in the glass apparatus of Figure 9 and is under a steady stream of inert gas, a source of low direct voltage of high amperage is connected across electrodes 20 and 21. Other methods may also be used to keep the graphite cylinder 13 at a high, constant and fairly uniform temperature during the deposition process. Thus, the entire graphite cylinder 13 can be placed within a specially designed oven or the graphite cylinder per se could be wound with resistive heating wires. Figure 12 shows a cylinder 13 wound with a winding of high melting point wire 30 such as tungsten or molybdenum around the graphite tube 13 to heat the tube 13. These metals are protected from oxidation during the deposition process by the inert atmosphere used to protect the graphite, as previously described. Although carbon is normally preferred as material for the substrate tube, other refractory materials can be useful. However, to retain the important advantage of the present method, the substrate material must remain rigid at a temperature at which the deposited glass or glasses fuse. Zirconium oxide bonded with a very small quantity of kaolin or other clay, or with an organic binder such as gum tragacanth and fired at a high temperature is suitable. It can be made sufficiently rigid, yet weak in tension, by controlling the small quantity of binder and the firing temperature. Zirconium oxide is a non-conductor and must be heated by some external means. This can be electrical as shown in Figure 13 as described above. Alternatively since it is quite stable in air and need not be surrounded by an inert gas in the deposition process, the entire length can be flame heated selectively or simultaneously, the water-cooling jacket being dispensed with. The thermal expansion of such a polycrystalline zirconia tube is, like graphite, much larger than that of fused silica, so it can be removed by cracking as described above for graphite. If it is desired to use high frequency induction heating, there can be added to the zirconium oxide approximately 10% of yttrium oxide, this combination being bonded preferably by an organic material. The organic material burns out when fired in a kiln, but at an appropriate firing temperature, well below the normal melting point, micro-sintering occurs and bonds the micro-crystalline mass into a rigid structure. This will be recognisezed as the well known Nernst glower material. If heated, as by a flame, to a moderate red temperature it becomes electrically conducting, and can then serve as the secondary or receptor of a high frequency induction furnace just as in the case of graphite. Moreover this mixture, like pure zirconium oxide, is very stable in air so no protection by an inert atmosphere is required. Again as with pure zirconia or graphite, removal is accomplished by cracking as the temperature is reduced at the conclusion of the deposition process, or by mechanical grinding. As with a carbon tube there will be some chemical reaction between the substrate and the outer layers of the deposited glass. In this case the principle compound formed is zirconium silicate which as a melting point substantially higher than the softening point of silica. Thus as with a carbon substrate, the deposited glass cylinder which is formed should have its outer surface cleaned by grinding and/or etching before being collapsed and drawn into a fibre. For reasons of economy it may be desirable to recover for re-use the zirconia-yttria material which is substantially unaltered except for the thin layer of silicate which has formed on the inner surface of the substrate tube. This contamination can be largely removed by a suitable etch, the fragments crushed and formed again into a refractory tube. Although the invention is directed to providing high purity materials for glass for optical fibre purposes, the invention also finds application wherever such high purity glasses may be required. WHAT WE CLAIM IS:

1. A method for depositing high purity optical fibre materials, including the steps of: (a) providing a tubular deposition substrate from a first material; (b) depositing a high purity second material on the inner surface of the substrate to form at least one layer of the second material of high purity; and (c) removing the tubular deposition substrate frm the high purity material to provide a high purity cylinder consisting essentially of said second high purity material.

2. A method as claimed in claim 1, and including the step of depositing high purity third material on the inner surface of said second material prior to removing said deposition substrate in order to provide a tube containing layers of both said second and third high purity materials.

3. A method as claimed in claim 1 or 2, wherein said first material is carbon.

4. A method as claimed in claim 1, 2 or 3, wherein the step of depositing said high purity second material includes chemical vapour deposition.

5. A method as claimed in claim 4,

wherein the chemical vapour deposition includes thermally dissociating and oxidizing the materials from the vapour by means of RF heating.

6. A method as claimed in claim 1, 2, 3, 4, or 5, wherein the second material is selected from the group consisting of the oxides of boron, silicon and phosphorus.

7. A method as claimed in claim 2 or any claim appendant thereto, wherein the third material is selected from the group consisting of the oxide of germanium, silicon, aluminium, arsenic and phosphorus.

8. A method of depositing high purity optical fibre materials, including the steps of: (a) providing a tubular deposition substrate from carbon; (b) depositing a high purity optical fibre material on the inner surface of the carbon substrate to form thereon at least one layer of the optical fibre material of high purity, said deposition being effected by a chemical vapour deposition process involving the application of heat by radio frequency induction so that the carbon substrate acts as a susceptor, and (c) removing the tubular deposition substrate from the high purity material to provide a high purity cylinder consisting essentially of said optical fibre material.

9. A method as claimed in any one of claims 1 to 8, wherein the tubular substrate is a split mold having at least two sections, and wherein the step of removing the substrate comprises separating the two mold sections.

10. A method as claimed in claim 8 or 9, including the steps of collapsing the high purity cylinder to form a fibre optic preform.

11. A method as claimed in claim 10, including the step of heat treating the preform to create an index gradient with low index on the outside.

12. A method as claimed in claim 7, 8, 9 or 10, wherein the second material or the high purity material is doped with boron oxide.

13. A method as claimed in claim 10, 11 or 12, including the steps of drawing the preform into a fibre and coating the fibre in-line with an optical quality plastics material having a lower index of refraction to serve as a cladding.

14. A method as claimed in claim 1, wherein the deposition of high purity material is substantially simultaneous along all parts of the length of the substrate.

15. A method as claimed in claim 14, wherein the high purity material deposits as a fused glass.

16. A method as claimed in claim 14, wherein the high purity material deposits as a soot that is subsequently fused by raising the temperature of the substrate.

17. A method as claimed in claim 1, wherein the deposition rate at different points along the length of the substrate is varied in a controlled manner.

18. A method as claimed in claim 17, wherein the deposition rate is controlled by temperature variations.

19. A method as claimed in claim 17, wherein the deposition rate is controlled by chemical flow and concentration.

20. A method as claimed in claim 1, wherein the step of heating the substrate comprises enclosing the substrate as a receptor inside a primary high frequency induction coil, having a plurality of turns.

21. A method as claimed in claim 20, including the step of varying the distance between the turns.

22. A method as claimed in claim 1, wherein the step of heating comprises passing an electric current through the substrate to heat it by means of resistive heating.

23. A method as claimed in claim 1, wheren the step of heating the substrate comprises enclosing it in a high temperature furnace.

24. A method as claimed in claim 1, wherein the substrate is a graphitic carbon cylinder.

25. A method as claimed in claim 1, wherein the substrate is a high temperature ceramic.

26. A method as claimed in claim 1, wherein the substrate is a refractory material which becomes electrically conducting at elevated temperatures.

27. A method as claimed in claim 1, wherein one high purity material is deposited upon a first deposition substrate and the first material is separated from the first substrate, and another high purity material is deposited upon a second substrate and the second substrate is removed.

28. A method as claimed in claim 27, wherein the first material is silicon oxide and the first deposition substrate is a first graphite cylinder.

29. A method as claimed in claim 27 or 28, wherein the second material contains an index of refraction modifier.

30. A method as claimed in claim 27, wherein the first graphite substrate is larger than the second graphite substrate.

31. A method as claimed in claim 27, including the step of inserting the cylinder of said second deposition material within the cylinder formed from said first deposition material and heating said first and second cylinders to collapse them and form a fibre optic preform.

32. A method as claimed in claim 31, including the step of heating the fibre optic preform to its softening temperature and drawing the preform into an optical fibre.

33. A method as claimed in claim 1, wherein the step of oxidation comprises heating the deposition substrate to incandescence in the presence of oxygencontaining gas.

34. A method as claimed in claim 20, wherein the induction coil has a form and surface which reflects a substantial amount of radiation back to the heated substrate.

35. A method for depositing high purity optical fibre material on a removable deposition substrate comprising the steps of: (a) providing a deposition substrate in the form of a hollow refractory cylinder whose thermal expansion coefficient is higher than that of the deposited materials; (b) depositing the high purity materials on the inner surface of the substrate; (c) depositing the high purity materials on the inner surface of the substrate; (d) subsequently removing the substrate by cooling the coated substrate in order to cause the substrate to break and fall away from the deposited material.

36. A method as claimed in claim 35, wherein the deposition of high purity material is substantially simultaneous along all parts of the length of the substrate, as a fused glass.

37. A method as claimed in claim 35, wherein the high purity material deposits as a soot that is subsequently fused by raising the temperature of the substrate.

38. A method as claimed in claim 35, wherein the deposition rate at different points along the length of the substrate is varied in a controlled manner.

39. A method as claimed in claim 35, wherein the step of heating the substrate comprises enclosing the substrate as a receptor inside a primary high frequency induction coil having a plurality of turns.

40. A method as claimed in claim 39, including the step of varying the spacing between the turns in order to control the heating of the substrate along its length.

41. A method as claimed in claim 35, wherein the step of heating comprises passing an electric current through the substrate to heat it by means of resistive heating.

42. A method as claimed in claim 35, wherein the step of heating the substrate comprises enclosing it in a high temperature furnace.

43. A method as claimed in claim 35, wherein the substrate comprises graphitic carbon cylinder.

44. A method as claimed in claim 35, wherein the substrate comprises a high temperature ceramic.

45. A method as claimed in claim 35, wherein the substrate comprises a refractory material which becomes electrically conducting at elevated temperatures.

46. A method as claimed in claim 35, wherein the step of cooling the coated substrate comprises the step of subjecting said coated substrate to a cooled stream of gas.

47. A method as claimed in claim 46, wherein the cooling gas is an inert gas.

48. A method as claimed in claim 35, wherein a first material is deposited upon a first deposition substrate; and the first material is separated from the first substrate; and a second material is deposited upon a second substrate and the second substrate is removed.

49. A method as claimed in claim 48, wherein the first material comprises silicon oxide and the first deposition substrate comprises a first refractory cylinder.

50. A method as claimed in claim 48, wherein the second material comprises germania silicate glass or other doped silicate glass and the second deposition substrate comprises a second refractory cylinder.

51. A method as claimed in claim 48, wherein the first refractory substrate is larger than the second refractory substrate.

52. A method as claimed in claim 48, including the step of inserting the cylinder of said second deposition material within the cylinder formed from said first deposition material and heating said first and second cylinders to collapse them and form a fibre optic preform.

53. A method as claimed in claim 52, including the step of heating the fibre optic preform to its softening temperature and drawing the preform into an optical fibre.

54. A method of making an optical fibre preform by the method of any one of claims 1 to 53.

55. A method of making an optical fibre preform, substantially as described with reference to the drawings.

56. Apparatus for making an optical fibre preform by the method of any one of claims 1 to 55.

57. An optical fibre preform made by the method of any one of claims 1 to 55, or by the apparatus of claim 56.