AU2066800A - Method of fabricating a preform - Google Patents

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): THE UNIVERSITY OF SYDNEY Invention Title: METHOD OF FABRICATING A PREFORM The following statement is a full description of this invention, including the best method of performing it known to me/us: -2- METHOD OF FABRICATING A PREFORM Field of the Invention This invention relates to a method of fabricating a preform from which single mode optical fibre may be drawn.

The invention has particular application to the fabrication of preforms from which dispersion shifted (including nonzero dispersion shifted) and rare-earth doped optical fibre may be drawn, but it will be understood that the invention does have broader application.

Background of the Invention Preforms conventionally are fabricated using one of two different processes: 1i. An internal process, involving modified chemical vapour deposition (MCVD), plasma activated chemical o:oo vapour deposition (PCVD) or, in the case of rare earth doped preforms, solution doping.

2. An external process, involving outside chemical vapour .'.deposition (OVD) or axial chemical vapour deposition

(AVD).

The process that is employed in any given case will be dependent largely upon the length and characteristics required of fibre that is to be drawn from the preform. In the context of the invention to be described in this specification, it may be stated generally that the external deposition process (OVD or AVD) provides for relatively inexpensive fabrication of large preforms, whilst the internal deposition process (MCVD, PCVD or solution doping) typically has application in fabrication of relatively expensive, smaller preforms.

Preforms that are fabricated by the internal process are sometimes sleeved with glass tubes so as to achieve a final preform diameter that is sufficiently large that long lengths of fibre can be drawn. However, sleeving is an expensive process insofar as the demanding specifications on the concentricity of the core and the outer diameter of S:36172 -3the drawn fibre require that the sleeving should have very low tolerances on its geometry.

Various special-application single mode optical fibres, have index profiles that require minimal diffusion of dopants during fabrication of preforms from which the fibres are to be drawn.

In the case of non-zero dispersion shifted fibre the core must have a complex profile relative to that of standard fibre. Generally, the core comprises a composite structure composed of an inner region and a number of annuli having alternating high and low refractive index.

Designs include structures in which the inner region has a high refractive index and the adjacent annulus has a low index, and structures in which the inner region has a low index and the adjacent region has a high index. These designs optimise the dispersion, the spot size, the bending o• radius and other properties such as non-linear interactions to meet desired applications. In a preferred design, the central region of the fibre core has a relatively high refractive index, the central region of the core is surrounded (ie, "ringed") within the core by an annulus having a lower refractive index, and the annulus is separated from the central region by an intermediate core region having a refractive index that is lower than that of both the central region and the surrounding annulus.

Ideally, the intermediate core region will have a refractive index that is lower than that of the fibre cladding, and this dictates the use of dopants (eg, fluorine) that have a high potential to diffuse between layers.

In the case of rare earth doped fibres, it is required that deposition of the dopant be finely controlled to create regions having different dopant concentrations.

However, it is recognised that rare earth ions have a tendency to aggregate through diffusion if individual S:36172 -4deposited layers containing the dopant are not consolidated before subsequent layers are deposited.

Summary of the Invention The present invention is based on an understanding that, in contrast with the external (OVD or AVD)deposition process, the internal (MCVD, PCVD or solution doping) deposition process effectively enables control to be exercised over the diffusion of dopants between deposited layers. As a consequence, the internal deposition process facilitates fine control of index profile or, in the case of rare earth doped fibre, fine control of the dopant profile. Also, the invention embodies the concept of hybridising internal and external deposition processes.

This permits the fabrication of a preform that has a finely o ~.controlled index or dopant profile but which is less .i expensive than a comparatively sized preform that might conventionally be produced wholly by an internal process and sleeving.

Broadly defined, the present invention provides a method of fabricating a preform from which a single mode optical fibre is to be drawn and which comprises the steps of: a) depositing layers of doped silica on the inside of a silica tube by an internal deposition process, b) consolidating each layer of the doped silica before depositing succeeding layers, so as to minimise diffusion of dopant between the layers, c) thermally collapsing the tube and the deposited material to form a solid rod, d) cladding the outside of the rod with a substantially homogeneous silica material by an external vapour deposition process, and e) completing fabrication of the preform by exposing the clad rod to consolidation heating.

S:36172 The invention may also be defined in terms of a preform when fabricated by the above method, and in terms of an optical fibre when drawn from the preform.

The present invention effectively involves a hybrid fabrication process. It provides for minimal inter-layer diffusion of dopants during formation of the preform core whilst, at the same time, permitting relatively economical fabrication of large diameter preforms.

By consolidating each layer of the doped silica during the internal deposition process, diffusion of dopant is essentially limited to the respective layers. The diffusion length (ie, distance over which diffusion occurs) depends upon the porosity of deposited soot, being large initially and becoming smaller as consolidation proceeds, and the time during which a given layer is held at high temperature. Thus, diffusion is minimised during the internal deposited process by consolidating those deposited layers that are thinner than the diffusion length of dopant species that are present during consolidation, and the achievement of this objective is ensured by consolidating each layer before depositing each succeeding layer.

Following consolidation of the respective layers, the diffusion length is very small and subsequent reheating of the preform will have negligible effect. Therefore, a large diameter preform may be fabricated economically as a hybrid structure. The soot that is deposited by the external vapour deposition process to produce the homogeneous silica cladding may be consolidated over a period of time that, but for the two-stage process, would result in unacceptable diffusion within the doped silica layers.

Also, by creating a hybrid structure, the two-stage fabrication procedure permits optical measurement of the preform core profile before the outer cladding is deposited.

S:36172 -6- The dopant may comprise rare earth dopants such as erbium and ytterbium, dopants that facilitate high rare earth concentrations, such as aluminium, and/or dopants that influence the refractive index of fibre that is drawn from the preform, such as germanium lead boron and fluorine.

Preferred Features of the Invention The outside of the rod preferably is clad with silica by the outside vapour deposition (OVD) process.

Also, the rod preferably is formed to incorporate both a core structure, which forms the core of optical fibre drawn from the preform, and an outer region that forms a radially inner portion of the cladding of the optical fibre.

15 The internal vapour deposition process may be controlled to provide the core structure with any desired refractive index profile. However, the deposition process preferably is controlled to establish three notionally *distinguishable co-axial regions within the core structure a central region of relatively high refractive index nl; an outer annulus having a lower refractive index n 2 and an inner annulus having a refractive index n 3 that is lower than that of the outer annulus.

The refractive index n 3 of the inner annulus most preferably is lower than that of the refractive index n 4 of the cladding. However, as indicated previously, other preform designs may be adopted for manufacture of non-zero dispersion shifted fibre, such as those that have a central region that has a refractive index that is low relative to that of the inner annulus.

Also, the internal vapour deposition process may be controlled to deposit successive layers of material in a manner to provide a graded refractive index profile in the S:36172 central, inner annular and/or outer annular regions of the core structure.

Furthermore, as indicated previously, preform designs may be adopted with coaxial regions that are notionally distinguishable by virtue of their rare earth or other dopant concentrations.

The invention will be more fully understood from the following description of a preferred embodiment of the invention. The description is provided with reference to the accompanying drawings.

Brief Description of the Drawings In the drawings: ~Figure 1 shows a diagrammatic representation of the 15 cross-section of a complete preform of a type that may be oooo fabricated by the method of the present invention, Figure 2 shows a schematic representation of a first stage of fabricating the preform, Figures 3 and 4 show schematic representations of procedures that are employed in a second stage of fabricating the preform.

Figure 5 shows a graphical representation of refractive index in the core of optical fibre that is drawn from the preform, and Figures 6 to 10 show graphical representations of optical fibre characteristics that are derived from varying parameters of the optical fibre and, hence, the preform from which the optical fibre is drawn.

Detailed Description of the Invention In making reference to the drawings, Figure 1 shows a diagrammatic representation of the cross-section of a preform that might be fabricated by the two-stage process of the present invention. However, it will be understood that the various concentric regions that are shown in Figure 1 are not drawn to scale. The outside diameter of the preform would typically have a dimension in the order S:36172 -8of 10 to 15 times that of the core (as defined by the inner full circle) of the preform.

The preform as illustrated has notionally distinguishable concentric regions composed of a core structure 10, a radially inner cladding portion 11 that is deposited by an internal vapour deposition process at the time of forming the core structure 10, an intermediate cladding portion 12 that is formed by collapsing a silica tube 14 in a first stage of fabricating the preform, and a radially outer cladding portion 13 that is deposited during a second fabricating stage. The various (notionally distinguishable) concentric portions of the preform might typically have the following dimensions.

"DIAMETER (mm) Core structure (10) 4.60 5.20 oooo oo Cladding portion (11) 12.42 14.04 Cladding portion (12) 21.45 22.42 Cladding portion (13) 62.10 70.70 The first fabrication stage, as illustrated in Figure 2, is effected by depositing layers of silica and silica- S. .doped material within a rotating silica tube 14 by a moreor-less conventional modified chemical vapour deposition (MCVD) process, and then collapsing the tube and deposited material to form a solid rod 15. The rod 15 is then used as a mandrel upon which cladding material is deposited in the second fabrication stage, as shown in Figure 3.

In the first fabrication stage, the tube 14 is supported by lathe chucks 16 and rotated about its own axis. Vapour is delivered to the interior of the tube from a chemical delivery system 17, and the tube is heated (whilst rotating) by a traversing heat source 18 which typically comprises an oxy-hydrogen torch. The torch derives fuel from a source 19.

Although not shown in the drawings, the downstream end of the tube 14 would normally be fused to a larger diameter glass tube which is employed to collect unexpended material S:36172 -9from the deposition process and to direct gaseous unexpended material to an exhaust system The deposition process may be described simply as involving high temperature gas phase oxidation of volatile vapour-carrying components that are deposited as submicro particles and fused to the silica tube 14. As vapour is delivered into the tube 14 from the chemical delivery system 17 and the torch 18 is traversed along the length of the tube, oxygen reactions and particle nucleation occur within the tube. The particles grow as they are carried through the tube and a portion of the particles are deposited by thermophoresis at the cool wall of the tube downstream from the region that is heated directly by the torch 18. During each pass of the torch 18 the particles 15 that previously have been deposited downstream are oo* *consolidated by viscous sintering as the torch passes by.

High purity cladding is first deposited to form the radially inner cladding portion 11, and subsequent layers of material that form the core 10 are then deposited. The compositions of the core layers are controlled to create the required refractive index profile. For this purpose, germanium and phosphorous dopants are used typically to raise the refractive index above that of pure silica, and "fluorine and boron dopants typically are used to lower the refractive index below that of silica.

When fabricating a preform that includes rare earth dopants, the core structure 10 may be doped with rare earth and other dopants. These may include aluminium and other elements that facilitate the incorporation of rare earth ions, as well as rare earth ions and mixtures of rare earth ions. The latter include erbium and ytterbium.

Figure 2 illustrates the various chemicals that are provided as the sources of vapour components that are to be delivered under controlled conditions into the tube 14.

Numeral 30 in figure 2 indicates a volatile rare earth that S:36172 may be sublimed from a solid to a gaseous phase in the presence of oxygen.

The processes for controlling operation of the lathe and the torch and for controlling delivery of vapour to the interior of tube are well known to persons involved in vapour deposition processes and are not further described in this specification.

When the required number of layers of silica and doped-silica have been deposited within the tube 14, the composite structure is collapsed to form the solid rod This procedure also is well known in the art and, typically, a number of very high temperature passes are *made by the torch 18 to induce collapsing of the structure comprising the core 10 and the cladding portions 11 and 12.

15 In the second fabrication stage, as shown e00e oe schematically in Figures 3 and 4, the radially outer .e cladding portion 13 of the eventual preform is deposited onto the rod 15 by a process involving flame hydrolysis.

0000 00 This is referred to as external chemical vapour deposition 0000 or, more usually, as outside vapour deposition.

The rod 15 functions as a mandrel and is supported 0 00 between lathe chucks 21 and rotated about its own axis 0000 whilst a torch 22 is traversed back and forth along the length of the rod. Vapour deposition is effected by directing silica-forming vapour to the torch from a chemical delivery system 23 and by oxidising the vapour components in the torch flame. Fuel for the torch is supplied from a fuel source 24. The resultant particles are deposited layer-upon-layer about the rotating rod until a required mass is accumulated, following which the composite structure is removed from the lathe and moved axially in a vertical direction through a consolidation furnace 25 as shown schematically in Figure 4.

The deposited powder-form particles or soot are consolidated within the furnace 25 by exposing the composite structure in an appropriate atmosphere to a S:36172 -11temperature sufficient to effect bulk viscous sintering within the deposited cladding. This procedure is to be contrasted with the internal vapour deposition process, in which sintering occurs layer-by-layer during deposition of the material by the MCVD process.

The outer cladding layer 13 would normally be deposited to form pure silica but, as indicated in Figure 3, selected dopants may be added during the deposition process in order to met specific cladding criteria.

The above described two-stage method of fabricating the preform may be employed for establishing various refractive index profiles in the core region of the preform eand, hence, in the core of the optical fibre that is drawn *from the preform. One such profile is indicated by the 15 notionally distinguishable dotted regions in Figure 1 and, in the context of a optical fibre, graphically in Figure As illustrated, vapour deposition of the core structure controlled to establish a central region 26 of Cc.• relatively high refractive index nl, an outer annular region 27 having a refractive index n 2 which is lower than the refractive index nl but higher than that (n 4 of pure :"*silica, and an inner annular region 28 having a refractive index n 3 that is lower than that of the outer annulus.

CAlthough not so shown in Figure 5, the refractive index n 3 of the inner annular region 28 most preferably is the same as or lower than the refractive index n 4 of pure silica.

Figures 6 to 10 of the drawings show self-explanatory graphical representations of optical fibre characteristics that are derived from varying parameters of optical fibre drawn from the preform that has been described with reference to Figures 1 and 5 of the drawings. However, the word "Ring" should be understood as referring to the outer annular region 27 of the core structure 10 as shown in Figure 1 of the drawings.

S:36172

Claims (6)

12- The claims defining the invention are as follows:- 1. A method of fabricating a preform from which a single mode optical fibre is to be drawn and which comprises the steps of: a) depositing layers of doped silica on the inside of a silica tube by an internal deposition process, b) consolidating each layer of the doped silica before depositing succeeding layers, so as to minimise diffusion of dopant between the layers, 10 c) thermally collapsing the tube and the deposited material to form a solid rod, d) cladding the outside of the rod with a substantially coo* homogeneous silica material by an external vapour S"deposition process, and e) completing fabrication of the preform by exposing the clad rod to consolidation heating. 2. The method of fabricating a preform as claimed in claim 1 wherein layers of silica are deposited on the inside of the silica tube prior to depositing the layers of doped silica. 3. The method of fabricating a preform as claimed in claim 2 wherein the layers of silica and doped silica are deposited on the inside of the silica tube by the modified chemical vapour deposition (MCVD) process. 4. The method of fabricating a preform as claimed in claim 3 wherein the vapour deposition is controlled in a manner such that viscous sintering of deposited particles occurs within each deposited layer prior to deposition of each succeeding layer. 5. The method of fabricating a preform as claimed in any one of claims 1-4 wherein the cladding material is deposited on the outside of the rod by the outside vapour deposition (OVD) process and wherein the deposited material is consolidated in a manner to effect bulk viscous sintering of particles within the deposited cladding. S:36172 -13- 6. The method of fabricating a preform as claimed in any one of claims 1-5 wherein the layers of doped silica are deposited on the inside of the silica tube in a manner to provide at least three co-axial regions having differing refractive indexes in the finished preform. 7. The method of fabricating a preform as claimed in claim 1 wherein the solid rod is formed to incorporate both a core region and an outer region that forms a radially inner portion of the preform cladding. 8. The method of fabricating a preform as claimed in "claim 7 wherein layers of silica are deposited on the silica tube prior to deposition of the doped silica, and wherein the radially inner portion of the preform cladding S"is formed from the collapsed silica tube and the layer of silica that is deposited within the tube by the internal *deposition process. 9. The method of forming a preform as claimed in claim 7 wherein the layers of silica and doped silica are deposited on the inside of the silica tube in a manner such that the rod is formed in the core region with three co-axial regions comprising: a) a central region having a relatively higher refractive index nl, b) an outer annulus having a lower refractive index n 2 and c) an inner annulus having a refractive index n 3 that is lower than that of the outer annulus. The method of fabricating a preform as claimed in claim 9 wherein dopants are incorporated within the deposited material in a manner to provide the inner annulus with a refractive index n 3 that is lower than the refractive index of undoped silica. 11. The method of fabricating a preform as claimed in claim 9 or claim 10 wherein vapour deposition of the doped silica is effected in a manner to achieve grading of the S:36172 -14- refractive index within at least one of the central region, the inner annulus and the outer annulus of the core region. 12. A method of fabricating a preform as claimed in any one of the proceeding claims wherein the outside of the rod is clad by oxidising the vapour components of dopant-free silica-forming vapour in the flame of a torch that is controlled to traverse substantially the full length of the rod.

13. The method of fabricating a preform as claimed in claim 1 wherein the doped silica contains dopants selected from germanium, boron, lead and fluorine.

14. The method as claimed in claim 1 wherein the doped silica contains rare earth dopant.

15. The method of fabricating a preform as claimed in claim 15 wherein the dopant comprises erbium and/or ytterbium.

16. A preform when fabricated in accordance with the method as claimed in any one of the proceeding claims.

17. An optical fibre when drawn from the preform as claimed in claim 16. Dated this 6th day of MARCH 2000 THE UNIVERSITY OF SYDNEY By their Patent Attorneys GRIFFITH HACK S:36172