GB2109367A

GB2109367A - Manufacture of a preform for optical fibres by the rod in tube method

Info

Publication number: GB2109367A
Application number: GB08134638A
Authority: GB
Inventors: Stephen Roy Norman
Original assignee: Prysmian Cables and Systems Ltd
Current assignee: Prysmian Cables and Systems Ltd
Priority date: 1981-11-17
Filing date: 1981-11-17
Publication date: 1983-06-02
Also published as: GB2109367B

Abstract

A method of manufacturing a preform to be drawn down in the production of monomode optical fibre comprises the steps of forming a rod core blank from a first material of predetermined optical character, machining said rod core blank to define a circular cylindrical rod core for the preform having a predetermined high degree of axial rectilinearity and radial circularity and having an optically smooth cylindrical surface, forming a generally circular cylindrical tube of a second material of predetermined optical character different to that of said first material and with a bore sized to accept said rod core, inserting said rod core into the bore of said tube, collapsing the tube onto the rod core such that the inner surface of the tube makes intimate contact with the outer surface of the rod core, and machining the outer surface of the tube with a reference datum established by the geometry of the rod core to obtain a circular cylindrical preform wherein the rod core and the surrounding tubular cladding display a high degree of concentricity and a constant ratio of core to preform diameter throughout the axial length of the preform.

Description

SPECIFICATION Improvements in or relating to the production of optical fibres This invention concerns improvements in or relating to the production of optical fibres or waveguides for information transmission utilization, and more particularly concerns optical fibres adapted for monomode light propagation in the fibre and having a stepped refractive index characterized by a high refractive index core material clad with a lower refractive index material.

One method of manufacturing such monomode fibres is to draw the fibre from a preform comprising a central cylindrical rod or core introduced into the bore of a hollow tubular cladding member, the core either being fused with the cladding prior to drawing down of the preform or alternatively being fused with the cladding during the drawing down process; this method is sometimes referred to as the "rod and tube" method of manufacturing optical fibres. The rod and tube method is susceptible to eccentricity of the rod relative to the tube which gives rise to an eccentric optical fibre configuration unsuited to monomode propagation applications particularly in the case where large diameter preforms, that is to say preforms of overall diameter say of the order of 1 50 millimetres for example, are desired to be utilized.

A typical monomode fibre might for example comprise a central core of say 8 to 10 ym (metres-6) diameter within a cladding of the order of 1 25 ym over diameter. Monomode fibres are characterized by the extreme small size of their central core relative to the cladding layer. For monomode operation of an optical fibre, the normalized frequency V, given by the relationship:1/2 V = 2na (n21-n22) A where n, = refraction index of core n2 = refractive index of cladding a = core radius A = operating wavelength must be less than the second-order mode outoff va;ue of 2.405.On account of the extreme small size of the core of a monomode fibre, jointing of fibres requires a high degree of core to cladding concentricity if attenuation of a transmitted signal across the joint is not to be unacceptably high; an offset from the core centre to the centre of the cladding of typically less than 0.25 pm is required.

From the foregoing it will be appreciated that in the manufacture of monomode optical fibres by the rod and tube method, the concentricity of the core relative to the cladding in the.preform is of paramount significance.

The present invention provides a method of manufacturing a preform to be drawn down in the production of monomode optical fibre which comprises the steps of forming a rod core blank from a first material of predetermined optical character, working said rod core blank to define a circular cylindrical rod core for the preform having a predetermined high degree of axial rectilinearity and radial circularity having an optically smooth cylindical surface, forming a generally circular cylindrical tube of a second material of predetermined optical character different to that of said first material and with a bore sized to accept said rod core, inserting said rod core into the bore of said tube, collapsing the tube onto the rod core such that the inner surface of the tube makes intimate contact with the outer surface of the rod core, and working the outer surface of the tube with a reference datum established by the geometry of the rod core to obtain a circular cylindrical preform wherein the rod core and the surrounding tubular cladding display the requisite high degree of concentricity.

The working of the rod core blank to define the rod core can be achieved by means of techniques which are in themselves commonplace in the glass-working arts such as cutting, grinding, or machining for example, and the required optically smooth outer surface of the rod core can be obtained by other conventional techniques such as acid etching, mechanical polishing, or flame or plasma polishing for example. However since the rod core must be free of contaminants such as water (absorbed or diffused) present in acid etches and removable only by employment of sophisticated drying techniques or surface impurities such as particulate debris from mechanical cutting or polishing techniques, it is preferred to employ a laser for machining and fire polishing the core.A laser such as a high power CO2 laser can be controlled for precise cutting of the rod core blank to size and simultaneouis flame polishing of the cut surface and need give rise to no impurity depositions upon the core.

The tube of second optical material with a lower refractive index than the first material can be formed by machining from a solid cylindrical bar of the material or by building up the material on a removable mandrel for example, and must have an inner surface at least which is impurity free and preferably is optically smooth, as the outer surface of the rod core. The tube can be collapsed into intimate contact with the core rod by thermally softening the material of the tube and, with or without the assistance of drawing down the tube, allowing the tube to constrict radially onto the rod core. It must of course be ensured that the void between the rod core and the tube is free of contaminants during the collapsing process.

Having collapsed the tube onto the rod core, the preform is finished by machining the outer surface of the tube for a high degree of concentricity with the rod core. The rod core itself is used as a reference datum for this purpose, the machining of the preform outer surface being carried out with reference to the previously effected precise dimensioning of the rod core and in order to obtain precisely the dimensional relationship required between the core and cladding diameters. A laser again is the preferred tool for machining the outer surface of the preform, through more conventional techniques as aforementioned could be employed. Precise optical techniques could be employed for controlling the machining of the outer preform surface according to the sensed position of the underlying rod core surface.

The method according to the invention is thus capable of accommodating variations in the diameter of the rod core at different axial positions in situations where, for one reason or another, it is not possible or not convenient to obtain a constant diameter rod core. By continuously monitoring the core diameter at the cutting location on the outer preform surface it is readily possible to vary the overall preform diameter with the rod core diameter so as to obtain a constant core: cladding diameter ratio. The resulting preform can be used in the production of constant diameter optical fibre by varying the drawing speed in proportion to the preform diameter.It is conceivable that the collapsing of the tubular cladding onto the rod core might, at least to some small degree, alter the dimensions of the rod core, but it is believed unlikely that any such alterations would disturb the circularity of the rod core at any position. Rather it is believed that any such alterations would be to the axial constancy of the diameter of the rod core and, as has been above mentioned, any such variations can be accommodated during the machining of the outer preform surface so that a constant ratio of core to preform diameter is obtained throughout the axial length of the preform.

It is of course to be appreciated that the machining of the preform outer surface will be performed so as to obtain a finished preform of the correct dimensions having regard to the optical properties of the materials and the intended application of the monomode fibre to be manufactured from the preform. Having collapsed the outer cladding onto the rod core, it may be desirable to determine the geometrical, dimensional and optical properties of the embryo preform thus obtained as a preliminary to the machining of the outer preform surface, and these may be determined by well known optical and other measuring techniques.

The invention as described herein is believed to provide the basis for the production of optical fibre preforms of the quality required for production of monomode optical fibres.

Claims

1. A method of manufacturing a preform to be drawn down in the production of monomode optical fibre which comprises the steps of forming a rod core blank from a first material of predetermined optical character, working said rod core blank to define a circular cylindrical rod core for the preform having a predetermined high degree of axial rectilinearity and radial circularity and having an optically smooth cylindrical surface, forming a generally circular cylindrical tube of a second material of predetermined optical character different to that of said first material and with a bore sized to accept said rod core, inserting said rod core into the bore of said tube, collapsing the tube onto the rod core such that the inner surface of the tube makes intimate contact with the outer surface of the rod core, and working the outer surface of the tube with a reference datum established by the geometry of the rod core to obtain a circular cylindrical preform wherein the rod core and the surrounding tubular cladding display a high degree of concentricity.

2. A method as claimed in claim 1 wherein the working of the rod core blank to define the rod core is achieved by employing a laser for machining and polishing the core.

3. A method as claimed in claim 2 wherein said laser is controlled for precise cutting of the rod core blank to size and simultaneous flame polishing of the cut surface without giving rise to impurity depositions upon the core.

4. A method as claimed in any of the preceding claims wherein the tube of second optical material with a lower refractive index than the first material is formed either by machining from a solid cylindrical bar of the material or by building up the material on a removable mandrel and so as to have an inner surface at least which is as impurity free and preferably is as optically smooth as the outer surface of the rod core.

5. A method as claimed in any of the preceding claims wherein the tube is collapsed into intimate contact with the core rod by thermally softening the material of the tube and, with or without the assistance of drawing down the tube, allowing the tube to constrict radially onto the rod core.

6. A method as claimed in any of the preceding claims wherein, having collapsed the tube onto the rod core, the preform is finished by machining the outer surface of the tube for a high degree of concentricity with the rod core, the rod core itself being used as a reference datum for this purpose and the machining of the preform outer surface being carried out with reference to the previously effected precise dimensioning of the rod core and in order to obtain a predetermined precise dimensional relationship between the core and cladding diameters.

7. A method as claimed in claim 6 wherein the machining of the outer preform surface is controlled according to the sensed position of the underlying rod core surface by continuously monitoring the core diameter at the working location on the outer preform surface and varying the overall preform diameter with the rod core diameter so as to obtain a constant ratio of core to preform diameter throughout the axial length of the preform.

8. A method as claimed in any of the preceding claims wherein the machining of the outer surface of the preform is effected by means of a laser.

9. A method of manufacturing an optical fibre preform substantially as herein described.

1 0. An optical fibre preform, or optical fibres formed by drawing down such a preform, formed by a method as claimed in any of the preceding claims.