GB2193956A - Optical fibre preform manufacture - Google Patents

Abstract

In the manufacture of an optical fibre preform of solid cross-section from a tubular precursor the rate of collapse of the bore is speeded up by stretching the tube during the initial stage of bore collapse.

Description

SPECIFICATION Optical fibre preform manufacture This invention relates to the manufacture of glass optical fibre preform rod of solid cross-section from which optical fibre can be produced by a drawing operation, and in particular is concerned with the manufacture of such preform by a method involving producing the preform from a tube by effecting the collapse of the bore of that tube by heat-softening it with a localised hot zone traversed along its axis.

United Kingdon Patent Specification No. 1322993 describes a method of manufacturing an optical fibre in which material to form the core of the fibre is deposited as a fibre upon the bore of a substrate tube of cladding glass. The internally coated tube is subsequently heat-softened and drawn to reduce the diameter of the tube until the film of core glass collapses and seals the bore. Thereafter continued drawing further reduces the diameter to form the requisite optical fibre.

United Kingdom Patent Specification No. 1475496 describes an alternative manufacturing method in which the bore of the coated tube is collapsed without axial extension. A feature of this alternative method is that it separates the operations of drawing and of bore collapse. This is significant because in order to preserve circular symmetry of bore collapse the dominating agency promoting that collapse should be surface tension, whereas in a drawing operation, without attendant collapse, it is desirable that it is the tensile forces that dominate. Thus the optimum temperature for bore collapse is in general significantly higher, in order to provide the lower visocity desirable for bore collapse, than that appropriate for drawing.

The time taken to collapse the coated tube into preform rod is a not insignificant proportion of the toal processing time from commencement of deposition to production of the preform rod, and moreover the collapse time is significantly increased with changes towards the production of more massive preforms required for drawing longer continuous lengths of optical fibre. The present invention is concerned with shortening the time required for collapse.

According to the present invention there is provided a method of manufacturing an optical fibre preform of solid cross-section from which optical fibre can be drawn, which preform is produced from a glass tube by effecting the collapse of the bore of that tube by heat-softening it with a localised hot zone traversed at least twice along its axis, in which method the tube is axially stretched as its bore is shrunk during at least one traversal of the hot zone.

There follows a description of a comparative test program investigating the effect upon collapse time of stretching the substrate tube during the collapsing process. The description refers to the accompanying drawing in which: Figure 1 schematically depicts the apparatus employed in performance of the collapse process, and Figure 2 schematically depicts a typical cross-section obtained when attempting a single traverse collapse with stretching.

Referring to Fig. 1, a silica substrate tube 10 is mounted between synchronously driven headand tail- stocks 11 and 12 of a special-purpose lathe provided with an axially traversable burner 13. This lathe is typically used not only for the tube bore collapse processing, but also for the internal deposition processing that precedes bore collapse.

Using the collapse method of the previously referenced Patent Specification No. 1475496, the substrate tube 10 is rotated about its axis by means of the head- and tail-stocks 11 and 12 which are kept at constant separation while the burner 13 is traversed along the tube. The heat from the burner causes localised softening of the glass which shrinks in diameter due to the action of surface tension and gas pressure forces. Generally, it is arranged that the first traverse of the burner does not bring about a complete collapse of the bore, but only a shrinkage of its diameter, this shrinkage being accompanied by a corresponding increase in wall thickness. In a subsequent traverse the shrinkage causes complete collapse, eliminating the bore to produce a preform rod with a solid cross-section.Typically, this subsequent traverse which eliminates the bore is the second traverse, though in some instances it is the third.

One of the factors governing the rate of traversal, and hence the total time taken to achieve full collapse, is the wall thickness of the tube. The thicker the wall becomes, the harder it is to collapse. If the head- and tail-stocks are kept at constant separation during the whole collapse process it will be evident that a progressive shrinkage of the bore of a substrate tube is accompanied by a progressive increase in its wall thickness. If, however, the head- and tailstocks are moved apart while the collapse is taking place, the tube is caused to stretch.

Accompanying this stretching is a reduction in wall thickness compared with that of an unstretched tube of the same diameter. Thus subsequent collapsing of the stretched tube is easier, and may therefore be carried out more quickly. Additionally, while the stretching is taking place the tube may still be caused to collapse, and the rate of this collapse is greater than for a collapse without stretching. Thus collapse rates can be improved both during the stretch and subsequently.

The effects of stretching upon collapse time are evident from the following set of test results performed on a set of identical silica tubes 25 mm in external diameter, 500mm long, and having a 3 mm wall thickness. In order to provide a proper basis for comparison the same burner gas flow rate (c.85 1/min of hydrogen) in each test run so as to give the same temperature at the surface of the substrate tube (c.2100 C, as indicated by an optical pyrometer). (The choice of particular gas flow rate was dictated by the limitations of the particular apparatus employed.Had it been possible to employ apparatus with a burner power more typical of commerical optical fibre production apparatus, having a flow rate in the range 100 to 140 1/min, the collapse times would have been considerably shorter in every example.) In each case the inside of the substrate was maintained slightly above (0.5+0.1 mbar) ambient atmospheric pressure, as is conventional, in order to preserve circularity during the collapse process. The results are set out in tabular form in Tables 1 and 2.

The first test, CS5, was a collapse performed without stretching, and was made to provide a reference against which the performance of the other tests can be gauged. It will be noted that the collapse in CS5 proceeded very slowly, and required three traverses of the burner, rather than two, on account of the limited burner power available.

The second and third tests, CS6 and CS7, also proved to be rather slow. This was because in both instances the first traverse was performed without stretching, thus leaving most of the collapsing to be effected in the second traverse. Results for CS6 have not been quoted in Table 2 because the achieving of a full collapse to a solid cross-section rod while stretching proved such a problem that the tube was not properly collapsed in tests CS6, and fully collapsed for only a part of CS7, and then only after a range of conditions had been tried.

In CS9 attempts were made to achieve a full collapse in a single traverse of the burner, and hence is a further example of collapse not in accordance with the principles of the present invention. It did prove possible to achieve full collapse over short portions of length, but the process was not stable, and hence was not satisfactory. It was found that there was a tendency for a modulated profile to be produced with a tendency for a tubular portion of the substrate tube located in the hottest part of the flame to stretch less, on account of its relatively greater bulk and hence lower temperature, than a fully collapsed portion that hwjust passed through the hottest part of the flame. In the worst case the collapsed portion became stretched to such an extent that it parted.At other times it produced a damped modulated profile of the general configuration depicted in Fig. 2.

Tests CS8, 10, 11 and 12 are further examples of collapse effected in accordance with the principles of the present invention. In each case the tube was stretched upon the first traverse which produced a shrinkage of its bore without effecting full collapse, whereas full collapse was produced on the second traverse during which there was no stretching. In each case the total collapse time was reduced to about one third of that of CS5 where no stretching was employed at all.

Although the invention has been described in particular relation to the production of optical fibre preform of solid cross-section derived from internally coated tube, it will be apparent that the invention is applicable also to the production of optical fibre preform of solid cross-section derived from otherwise-produced tubular preform precursor, for instance the sort of tubular preform precursor that is produced by deposition upon a rod-shaped mandrel that is subsequently removed from the material deposited upon it.

TABLE 1. Stretch and Collapse Test Regimes Test No. Traverse 1 Traverse 2 Burner Burner Traverse Stretch? Traverse Stretch? Direction Direction CS 5* Forward No Forward No CS 6 Forward No Reverse Yes CS 7 Forward No Reverse Yes CS 8 Forward Yes Reverse No CS 9 Reverse Yes CS 10 Reverse Yes Reverse No CS 11 Forward Yes Reverse No CS 12 Forward Yes Reverse No * CS 5 required three traverses to effect full collapse of its bore, the third traverse being in the reverse direction, without stretching.

TARTs 2. Test Conditions and Results Test Traverse Burner Tail-stock Computed total time No. No. Traverse Speed required for collapsing Speed (mm/sec) Im of starting tube (hrs) (mm/sec) CS 5 1 + 0.095 0 2 + 0.055 0 21.9 3 - 0.020 0 CS 7 1 + 0.092 0 17.6 2 - 0.038 0.042 CS 8 1 - 0.092 0.078 6.7 2 - 0.138 0 CSlO(A) 1 - 0.115 0.115 6.6 2 - 0.134 0 (B) 1 - 0.092 0.092 7.2 2 - 0.134 0 (c) 1 - 0.162 0.165 6.5 2 - 0.124 0 CS 11 1 + 0.335 0.165 6.9 2 - 0.092 0 CS 12 1 + 0.280 0.140 7.5 2 - 0.038 0

Claims (6)

1. A method of manufacturing an optical fibre preform of solid cross-section from which optical fibre can be drawn, which preform is produced from a glass tube by effecting the collapse of the bore of that tube by heat softening it with a localised hot zone traversed at least twice along its axis, in which method the tube is axially stretched as its bore is shrunk during at least one traversal of the hot zone.

2. A method as claimed in claim 1, wherein there is no axial stretching of the tube during the traversal in which the bore is caused to vanish.

3. A method as claimed in claim 2 in which said collapse is completed by two traversals of the hot zone.

4. A method as claimed in claim 2, which method is performed substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

5. An optical fibre preform produced by the method claimed in any preceding claim.

6. An optical fibre drawn from preform as claimed in claim 5.