GB2108102A - Optical fibre preform manufacture - Google Patents

Abstract

In the manufacture of relatively long lengths of optical fibre preform by forming a coating on the bore of a tube (12) and then collapsing its bore to form a preform of solid cross- section, the two ends of the tube are rotated in synchronism, and, while a hot zone (11) is traversed along the tube, one end of the tube is moved with respect to the other in such a way as to reduce or eliminate the tendency of the tube to form a swan- neck at the heat-softened region. For instance, tail-stock 16a may be height-adjusted relative to head-stock 14 during traversal of hot zone 11, and/or either or both stocks may be tilted about horizontal axes at right angles to their rotational axes. <IMAGE>

Description

SPECIFICATION Optical fibre preform manufacture This invention relates to the manufacture of optical fibre preforms and in particular to those methods that involve the deposition of material upon the bore of a tube in a localised zone traversed along the length of the tube.

According to the present invention there is provided a method of optical fibre preform manufacture including the step of providing a coating of material upon the bore of a glass tube, which material is deposited by chemical vapour reaction in a localised hot zone traversed along the tube is rotated about its axis by synchronously driven head- and tail-stocks gripping the tube in regions near its two ends, and including the subsequent step of collapsing the bore of the internally coated tube by traversing along the tube a localised hot zone to soften the material of the tube and its coating wherein during said coating and said collapse steps the tube is held substantially horizontal and is rotated about its axis by synchronously driven head- and tail-stocks gripping the tube in regions near its two ends, wherein the axes of the head- and tail-stocks are constrained to lie in a common vertical plane and during at least part of said coating and collapse steps are moved relatively to each other as a function of position of the hot zone along the tube so that in the region of the tube in the hot zone when in a heat softened condition the forces tending to produce a swan-neck distortion of the tube in this region are eliminated or at least reduced in magnitude compared with those prevailing if the axes were maintained co-axial.

The manufacture of optical fibre preform by methods embodying the invention in preferred forms will now be described, but this description will be prefaced with an explanation of the background to the invention. The description and explanation make reference to the accompanying drawings, in which Figures 1, 2 and 3 depict apparatus used in optical fibre preform manufacture and show how a swan-neck is liable to be introduced into a tube supported only at its ends as a hot zone is traversed along its length.

Figures 4, 5 and 6 depict the apparatus of Figures 1, 2 and 3 modified by the inclusion of intermediate supports.

Figures 7, 8 and 9 depict an apparatus used in the performance of the present invention as a hot zone is traversed along the length of a tube held between a headstock and a synchronously driven adjustable height tailstock.

Figure 10 depicts a drive arrangement for the tailstock.

Figures 11, 12 and 13 are diagrams showing how the tube may alternatively be held by tilting head- and tail-stocks in the performance of the invention, and Figures 14, 15 and 16 are diagrams showing a third way in which the tube may be held in the performance of the invention using a fixed axis headstock and a tilting adjustable height tailstock.

In a typical process for making optical fibre preform by a method involving deposition of material upon the bore of a tube, the vapour reaction used for deposition involves reacting one or more halides or oxyhalides with oxygen within a localised hot zone that traversed along the length of the tube. Such a reaction is preferably arranged in such a way that hydrogen and its compounds are excluded from the reaction. This is advantageous because it precludes the formation of water vapour as a reaction product.

The production of water vapour is preferably avoided because it is liable to become incorporated in the deposit where it will provide an undesirable absorption spectrum attributed to hydroxyl absorption peak overtones.

The hot zone, which may be provided by an oxyhydrogen burner, is required because the reaction does not proceed spontaneously at room temperatures. Its temperature should in general be as high as possible, as this tends to provide a high deposition rate. In practice for any particular deposition the choice of deposition is a compromise, for it should be high enough to promote the required reaction at a reasonable rate and fuse the resulting deposit to a clear glass, but on the other hand it should not be so high as to cause tube collapse or excessive loss of material byvolatilisation. During the deposition process the tube is held substantially horizontal and is rotated about its axis so as to maintain circular symmetry in the growth of the deposit.

For this purpose the two ends of the tube are mounted in synchronously driven head- and tailstocks of a lathe. Generally it is preferred to arrange for the deposition to take place only when the flame traversal is in the same direction as the flow of the reacting gases. In order to build up a sufficient thickness of deposit it will be necessary to deposit material over a succession of traverses of the hot zone. Deposition of a layer of material to form the core of the fibre may be preceded by the deposition of a layer of material to form the optical cladding for the core. Deposition of the cladding material layer may itself be preceded by deposition of a layer of material to form a barrier layer inhibiting the diffusion of impurities from the substrate material into the deposited material that is to form the core or optical cladding.

Uniformly of deposition in the axial direction is ensured by careful control of the rate of traversal of the hot zone along the length of the tube during the deposition process. The entire length of the tube can not be used for deposition because the hot zone can not be allowed to approach too close to the lathe chucks gripping the tube at its two ends. Furthermore it is found that, at the commencement of each deposition, the deposition reaction takes a finite period to stabilise. This is liable to produce a region at the commencement end of the deposition traversals where the pattern of growth differs from that further along the tube.The length of this region may, in suitable circumstances, be shortened by arranging for, the hot zone to be stationary for a set 'dwell time' period at the beginning of each deposition cycle before proceeding to traverse at a carefully controlled uniform rate.

When the deposition has been completed, the temperature of the hot zone is increased so that the tube softens to the extent that its bore begins to collapse under the effects of surface tension.

Usually several traversals of the hot zone are used to effect complete collapse of the bore to produce a solid cross-section rod known in the art as an optical fibre preform. A small excess pressure may be maintained within the bore of the coated tube during collapse to facilitate the preservation of circular symmetry during the collapse process.

The resulting optical fibre preform of solid cross section is then removed from the deposition lathe and mounted vertically in a drawing tower.

In the drawing tower its end is lowered into a furnace at a controlled rate and fibre of uniform diameter, is drawn from its tip.

The above description process is essentially a batch process in which the amount of fibre that can be drawn from a single preform is limited by the length and diameter of that preform. Either or both of these parameters need to be increased if more fibre is to be obtained from a single preform.

If it is desired to increase the diameter of the preform, it is necessary to increase the crosssectional area of the deposited material in the same proportions as the increase in the cross-sectional area of the substrate tube in order to preserve the same proportions of deposited material to substrate-derived material in the drawn fibre.

Therefore, although the use of a thicker crosssection tube allows a longer length of fibre to be drawn from a single preform, the deposition time increases in the same proportion. If, on the other hand, it is desired to increase the length of fibre obtainable from a single preform merely by increasing the length of tubing over which deposition is arranged to occur, it is found that the yield of fibre increases at a faster rate than the deposition time. This is because the length of the region of the tube where the deposition is nonuniform due to the stabilisation effect referred to above is constant, and thus the proportion of the deposit that is unusable becomes smaller with increasing deposition length.Therefore, if 20 cm of the deposition length are unusable, this represents a 33% wastage of deposition time in the case of a tube whose total deposition length is only 60 cm, whereas in the case of deposition over a distance of one metre it represents only 20% wastage. Thus to provide more fibre per preform it appears preferable to increase tube length rather than to increase tube cross-section.

Using a lathe whose head- and tail-stocks lie on a common axis it is found that a practical limit to the length of substrate tubing that can be processed is set by the effects of gravity. During the deposition process the temperature in the hot zone is typically in the region of 1 6000C, at which the silica is softened although it is still quite viscous. (Deposition can be performed at lower temperatures, but in general this will result in a reduced deposition rate). Then, during the collapse process, the temperature of the hot zone is raised to about 20000C so that the silica becomes quite fluid. It is found that good quality short tubes of about 16 mm bore and 2 mm wall thickness can be run for many tens of hot zone traverses without any major distortion occurring.

However, with tubes appreciably longer than about one metre, the number of traverses that can be executed before bow and run-out become a limiting influence is considerably less. Part of the reason for this is that the effect of alignment errors between the chucks is amplified over the longer distance that separates them. Another factor is the cumulative effect of flame pressure on the tube. It is believed that a significant factor, which becomes predominant at elevated temperatures, is the bowing of the tube due to the influence of gravity.

A cold tube clamped between 2 chucks can be considered as a uniformly loaded beam clamped at both ends. The deflection at the centre is described by the formula: D=f14/384 El where D=deflection f=force per unit length E=Young's modulus l=moment of inertie, and I=length It can be seen that the deflection increases with the fourth power of the tube length. However the maximum deflection is approximately a fiftieth of that occurring at the end of a similar beam clamped at only one end for which the equivalent formula is: D=f14/8 EL Particuiarly under collapse conditions the portion of the tube (or preform) within the hot zone is sufficiently fluid for it to provide only a very weak mechanical link between the two ends of the tube.The two portions of the tubing on either side of the hot zone therefore behave like independent single cantilevers, each portion being effectively supported at only one end. With the hot zone situated mid-way between the two chucks the vertical deflection of the 'free' ends of the two cantilevers will be equal. Therefore if the chucks are co-axial the two 'free' ends will be level. However, with the hot zone displaced towards one end, the vertical deflections will be different; and therefore, if the chucks are co-axial, the region in the hot zone will be swan-necked.

The heat-softened silica in the hot zone will act as a kind of universal joint, and will accommodate the continuous bending consequent upon the rotation of the tube provided that the difference in levels of the two 'free' ends does not become excessive. However if the length of the tube is increased this difference in level will be increased whenever the hot zone is displaced further from the mid-point between the two chucks. A similar effect is also apparent during deposition, but is less pronounced because the moment of inertia of the tube before collapse is larger. It will also be noted that the greater viscosity of the glass in the hot zone means that the residual mechanical coupling between the 'free' ends is stronger.Both these factors make differential deflection less important for a single traverse of hot zone during deposition than one during collapse; but on the other hand the number of traverses is generally higher during deposition, and hence cumulative effects during deposition can become significant.

The main consequences of an excessive swanneck are the disruption of the cross-sectional geometry of the resultant preform and bow. Both these features degrade the quality of the product and if sufficiently severe will make it quite unusable.

The effects of differential deflection are illustrated schematically in Figures 1, 2 and 3 which respectively show a burner 10 at the beginning of a traverse when it is producing a hot zone 11 in the part of the tube 12 near the chuck 1 3 of the head stock 14 of the lathe, at the midpoint between the chuck 13 and the chuck 1 5 of the tail-stock 16, and at the end of the traverse when the burner is near the chuck 1 5.

One way of attempting to overcome this problem of differential deflection is by means of supports that are traversed with the hot zone.

This is illustrated in Figures 4, 5 and 6. A leading support 40 and a trailing support 41 are mounted respectively ahead of and behind the burner (not shown). These supports may consist of carbon blocks, each with a Vee groove in its top surface, and the two supports are mounted on the carriage (not shown) used for traversing the burner (not shown) that is used to provide the hot zone. At the beginning of each traverse the leading support is raised so that the sagging tube settles into its Vee groove, vertical adjustment of the support will either reduce or eliminate the 's' bend in the tube. At about the mid-point of the traverse the trailing support 41 is raised to support the tube and then the leading support may be lowered to clear the tail-stock chuck. Although this use of supports can be made to work, it suffers from a number of disadvantages.First it requires a degree of skill on the part of the operator to retain uniformity of diameter of the tube along its length during the whole deposition period and to avoid introducing unacceptably large bow or diameter fluctuations along the length of the tube during the deposition process. Second it is found that if the tube starts off with any bow or diameter fluctuation, the effect of restraining the tube at the supports is liable to cause these irregularities to be propagated along the tube and amplified in the process. Third there is some reason to believe that the supports can affect the surface of the tube in such a way as to reduce the strength of fibres drawn from the preform produced by collapse of its bore.

These problems can be avoided by dispersing with the supports and instead arranging for the tail-stock to be movable with respect to the headstock in a vertical direction using a drive mechanically or otherwise linked to the drive controlling the traversal of the burner. The arrangement is depicted in the diagrams of Figures 7, 8 and 9.

Figure 7 depicts the situation at the beginning of a traversal. The axis of head-stock and tailstock chucks 13 and 1 5 are constrainted to lie at all times in the same vertical plane. The tail-stock 1 6a is movable in a vertical direction on guides 70. At the beginning of the traversal the tail-stock is elevated with respect to the headrstock 14 so that the tail-stock chuck axis is a predetermined height above the head-stock axis. This difference in height is equal to the calculated difference in the vertical single cantilever deflection of the tip of an unloaded length of the tube held horizontal at a distance from the tip equal to the distance of the hot zone from the tail-stock less the equivalent deflection for the tube held horizontal at a distance equal to the distance of the hot zone from the head-stock.This means that there are no shear forces acting at the hot zone that would to tend to put a swan-neck bend into the tube. This relationship is maintained while the hot zone is traversed along the tube. This involves a progressive lowering of the height of the tail stock as the burner advances towards it. Figure 8 shows that by the time the burner reaches the mid-point the tail-stock has been lowered to the same height as the head-stock, while Figure 9 shows that when the burner approaches still closer to the tail-stock the height of the tail-stock is lowered beneath that of the head-stock.

Clearly the tail-stock has to be mounted on a high quality slide so that its axis is maintained pointing in exactly the same direction as its position is raised and lowered. A further requirement is that synchronism is maintained between the rotation of the head-stock and that of the tail-stock during this raising and lowering.

Figure 10 shows one arrangement for achieving this synchronism using a belt drive 100 to drive the tail-stock spindle 101 from a layshaft spindle 1 02. The belt is tensioned by a pair of jockey wheels 103 urged outwardly by springs 104, and constrained by some mechanical linkage (not shown) to move horizontally in opposite directions by equal amounts.

The vertical movement of the tail-stock may be controlled by a cam of suitable profile which is driven from a lead-screw that drives the burner arrangement along the length of the tube, or alternatively the two drives may be mechanically separate being driven by separate motors under common control, typically the control of some form of microprocessor. The latter approach has the advantage that it is generally easier to modify the microprocessor program than to prepare a separate cam for each new set of operating conditions.

The foregoing analysis regarding the appropriate relative heights of the head- and tailstocks has assumed that the tube is soft enough to be capable of forming a swan-neck at the hot zone. It will be clear however that the head- and tail-stocks must be at the same height when the tube is cold. This means that in practice before the start of each traverse, at the beginning of the half minute or so dwell time, the head- and tail stocks will be co-axial, and then, as the temperature rises its equilibrium and the tube softens, so the tailstock is displaced vertically by the appropriate amount. At the hot zone is traversed down the tube, so the tailstock is lowered according to the predetermined profile.

At the end of the traverse, when the heat is turned down, it becomes necessary once again to restore the condition in which the tailstock is co-axial with the headstock. A second dwell period may be necessary while this is accomplished. Next there is a rewinding operation to restore the burner to its original position ready for the commencement of the next traverse. This rewinding is normally performed either with the flame turned right down or off, or at such a speed that no part of the tube is softened during this process. Therefore the tailstock remains co-axial with the headstock during this rewinding, and thus is ready for commencement of the next traverse cycle of operations.

The requirement to make the relative movements of the head- and tail-stocks dependent upon tube temperature in the hot zone can be avoided if provision is also made to tilt the tallstock, or the headstock, or both, in a controlled way, each about a horizontal axis at right angles to the rotating axis.

The free end of a single cantilever can be located at a predetermined position and orientation by appropriately positioning and orienting its fixed end. The head- and tail-stocks are positioned and oriented so that, if the tube were severed in the region of the hot zone, the free ends of the resulting two single cantilevers would remain in registry with each other as regards both position and orientation. The arrangement may be as depicted in Figures 11, 12 and 13 in which the headstock (H.S.) and the tailstock (T.S.) both tilt, or as depicted in Figures 14, 1 5 and 16 in which the headstock remains horizontal and only the tailstock tilts.

It will be appreciated that in the Figures 11, 12, and 13 arrangement the head- and tail-stocks may be arranged to be at the same height, which will always be at a lower level than the centre of the tube in the hot zone by an amount that is a function of position of the hot zone along the length of the tube. Depending upon the size and shape of the hot zone, the variation may be small enough for it to be unnecessary to make any adjustment of height of the hot zone as it is traversed. In the Figures 14, 15 and 1 6 arrangement, where the headstock is horizontal at all times, it is not only the tilt but also the height of the tailstock that is a function of position of the hot zone along the length of the tube.This means that the height (depicted by a broken line) at the centre of the tube in the hot zone is a stronger function of position than in the previous example, with the result that allowance for this effect will normally be necessary, and the height of the burner, or other means used for providing the hot zone, will need to be varied as it is traversed along the tube.

It should be clearly appreciated that for the purposes of illustrating the invention the drawings greatly exaggerate the deflection of the substrate tube, and hence also the relative heights and tilts of the headstocks and their associated tailstocks. Thus is a typical substrate tube of 1 6 mm bore and 2 mm wall thickness is held horizontal in a clamp positioned 75 cm from a free end, it is found that the deflection under gravity of the tip of the tube is about 0.3 mm, and the angle to the horizontal of the tube axis at the tip is about 0.30. Increasing the single cantilever length to 1 50 cm increases these figures to 4.6 mm and 0.240 respectively. The corresponding deflection figures for a single cantilever length of 75 cm of 13 mm diameter preform rod produced by collapsing the bore of such a tube after internal coating are 1.1 mm and 0.1 1 . Increasing the single cantilever length to 1 50 cm increased these figures to 18.0 mm and 0.91 respectively.

Whereas a swan neck of 1.1 mm is easily tolerable, that of 18 mm would be highly deleterious to the preform geometry.

Claims (7)

Claims

1. A method of optical fibre preform manufacture including the step of providing a coating of material upon the bore of a glass tube, which material is deposited by chemical vapour reaction in a localised hot zone traversed along the tube is rotated about its axis by synchronously driven head- and tail-stocks gripping the tube in regions near its two ends, and including the subsequent step of collapsing the bore of the internally coated tube by traversing along the tube a localised hot zone to soften the material of the tube and its coating wherein during said coating and said collapse steps the tube is held substantially horizontal and is rotated about its axis by synchronously driven head- and tail-stocks gripping the tube in regions near its two ends, wherein the axes of the head- and tail-stocks are constrained to lie in a common vertical plane and during at least part of said coating and collapse steps are moved relatively to each other as a function of position of the hot zone along the tube so that in the region of the tube in the hot zone when in a heat softened condition the forces tending to produce a swan-neck distortion of the tube in this region are eliminated or at least reduced in magnitude compared with those prevailing if the axes were maintained co-axial.

2. A method as claimed in claim 1 wherein said relative movement of the head- and tail-stock axes is solely translational in a vertical plane.

3. A method as claimed in claim 1 wherein said relative movement of the head- and tail-stock axis is such that the headstock axis and the tailstock axis both tilt about horizontal axes at right angles to their rotation axis.

4. A method as claimed in claim 1 wherein said relative movement of the head- and tail-stock axes is such that the axis of one remains fixed.

while the axis of the other undergoes translational and rotational movement.

5. A method of optical fibre preform manufacture substantially as hereinbefore described with reference to Figures 7, 8, 9 and 10 to Figures 1 1, 12 and 13, or to Figures 14, 15 and 1 6 of the accompanying drawings.

6. An optical fibre preform made by the method claimed in any preceding claim.

7. An optical fibre drawn from a preform as claimed in claim 6.