WO2003014036A1 - Furnace for stretching a glass rod - Google Patents

Furnace for stretching a glass rod Download PDF

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Publication number
WO2003014036A1
WO2003014036A1 PCT/GB2002/003630 GB0203630W WO03014036A1 WO 2003014036 A1 WO2003014036 A1 WO 2003014036A1 GB 0203630 W GB0203630 W GB 0203630W WO 03014036 A1 WO03014036 A1 WO 03014036A1
Authority
WO
WIPO (PCT)
Prior art keywords
furnace
glass rod
tube
ducts
diameter
Prior art date
Application number
PCT/GB2002/003630
Other languages
French (fr)
Inventor
John Wainwright Baker
Original Assignee
Stanelco Fibre Optics Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Stanelco Fibre Optics Ltd filed Critical Stanelco Fibre Optics Ltd
Publication of WO2003014036A1 publication Critical patent/WO2003014036A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/01205—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments
    • C03B37/01225—Means for changing or stabilising the shape, e.g. diameter, of tubes or rods in general, e.g. collapsing
    • C03B37/0124—Means for reducing the diameter of rods or tubes by drawing, e.g. for preform draw-down
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B23/00—Re-forming shaped glass
    • C03B23/04—Re-forming tubes or rods
    • C03B23/043—Heating devices specially adapted for re-forming tubes or rods in general, e.g. burners

Definitions

  • the invention relates to a furnace for stretching a glass rod to form a rod of smaller diameter, particularly but not exclusively for stretching a core rod.
  • Optical fibres are typically of diameter about 120 ⁇ m, with a core of diameter about 10 ⁇ m; the core is of higher refractive index than the surrounding cladding glass.
  • Such optical fibres may be made by drawing out a preform with the same proportions but on a much larger scale, for example a rod of glass of diameter say 84 mm with a core of diameter 7 mm.
  • the core must be accurately concentric within the surrounding glass, and there must not be any bubbles or inclusions in the preform.
  • One way to make such a preform is to make a tube of the cladding glass; and to make a core rod of the core glass material
  • the core rod being initially of larger diameter than the bore of the tube.
  • the core rod is then heated and stretched in a furnace so that it fits closely within the tube.
  • the core rod can then be inserted into the tube and subsequently fused together in a furnace, using a vacuum, to form the preform.
  • a furnace for stretching a glass rod comprising a graphite tube and means for heating the graphite tube to a temperature at which the glass becomes plastic, and means to supply an inert gas to the bore of the graphite tube, at least two pairs of slots being defined through the wall of the tube along a circumferential path at a position along the tube at which, in use, the glass rod has decreased in diameter but is still plastic, viewing ducts extending radially outward from each such slot, the ducts being such that an optical measuring instrument can monitor the diameter of the glass rod by observation through at least one such pair of ducts.
  • the ducts are of a heat resistant material such as ceramic or graphite.
  • the ducts of at least one pair terminate, at their outer ends, at optically transparent windows through which the measuring instrument can observe the glass rod.
  • one such window may be a clear glass window, and the other window may be a narrow band filter transparent only to radiation of the frequency emitted by the instrument .
  • each duct is provided at its outer end with an inlet for the inert gas, preferably the inert gas inlet being arranged so a gas jet sweeps any contamination away from the transparent window.
  • each duct is wider than the expected diameter of the glass rod at that position along the furnace, to simplify the measurement of diameter. Consequently a substantial proportion of the said circumferential path is constituted by the slots .
  • the slots inevitably have a cooling effect on the glass rod, but the symmetry of the slots and the provision of slots occupying almost all the circumferential path ensures that the circular cross-section does not become distorted.
  • the ducts are preferably of the same cross- sectional shape as the corresponding slot, but may be wider, and indeed may be defined between annular plates separated by the axial height of the slot .
  • the furnace is provided with means for pulling the glass rod through the furnace, and the measuring instrument is linked to the control means for the pulling means, so that the pulling speed can be adjusted to ensure the final diameter of the rod has the desired value .
  • Figure 1 shows a longitudinal sectional view of a glass rod stretching furnace
  • Figure 2 shows a cross-sectional view on the line 2- 2 of figure 1.
  • an induction furnace 10 for stretching a glass rod 12 (shown in broken lines) includes an upper cylindrical housing 14 of copper with a stainless steel cover plate 16, and a lower cylindrical housing 18 of stainless steel with a bottom stainless- steel cover plate 20, each cover plate 16 and 20 defining a central aperture through which the glass rod 12 passes.
  • the housings 14 and 18 and the cover plates 16 and 20 are water-cooled.
  • a graphite tube 24 that acts as a susceptor. It is of internal diameter 97 mm, and is of high purity graphite of density
  • the copper housing 14 also encloses a water-cooled copper coil 32 that, in use, is connected to a high frequency electrical generator (not shown) . This induces currents in the susceptor tube 24, so in use it may reach a temperature that may be in the range 2000-2500°C.
  • a graphite muffle tube 34 of tapering bore aligned with the tube 24, and surrounded by additional half-tubes 36 of low-density graphite fibre material.
  • Feed equipment supplies the glass rod 12 at a steady rate through the upper aperture (which may be provided with a felt seal) .
  • the upper aperture which may be provided with a felt seal
  • two laser sensors monitor the position of the glass rod 12 through apertures 40 to ensure it remains concentric with the tube 24.
  • Feed equipment (not shown) pulls the glass rod 12 out of the lower aperture at a faster rate, so that the glass is stretched while it is hot and plastic.
  • the glass rod consequently necks down to a narrower diameter in the lower part of the tube 24, and by the time it reaches the bottom of the muffle tube 34 it is again solid.
  • nitrogen gas is supplied to the bore of the furnace 10 around the glass rod 12, and is also supplied within the copper housing 14 and the lower housing 18.
  • the lower aperture is provided with pneumatically operated graphite block shutters to restrict loss of nitrogen gas.
  • the bottom end of the tube 24 and the top of the muffle tube 34 are castellated to fit together, and are shaped so as to define four radially-extending coplanar slots 44 each 2 mm high and 50 mm wide extending along two orthogonal diameters.
  • Rectangular-section graphite ducts 46 are shaped to fit onto the outside of the tube 24 and the muffle tube 34, and extend radially outward to the periphery to locate in corresponding slot-shaped apertures in the housing 18.
  • To the outside of these apertures are attached rectangular frames 48 (in one opposed pair) or blanking plates 50 (in the other opposed pair) , in each case provided with tangential inlets 52 for nitrogen.
  • the frames 48 are closed by glass windows 54, 55, one of which is transparent and the other is a narrow band filter.
  • the glass rod 12 will have significantly decreased in diameter by the time it reaches the plane in which the slots 44 are defined.
  • the laser beam scanning to and fro parallel to the longitudinal axis of the duct 46, and observing the radiation through the window 55 (which is transparent to the radiation emitted by the laser)
  • the diameter of the glass rod 12 can be accurately measured. This measurement may be used in controlling the speed at which the glass rod 12 is drawn out of the furnace 10, so as to ensure that once it has set solid it has the desired diameter .
  • the bore of the tube 24 containing the glass rod 12 is at a very high temperature, typically in the range 2000 to 3000°C, more typically between 2000 and 2500°C, and contains pressurised gas.
  • the walls of the ducts 46 prevent the consequential intense radiation from reaching the low-density insulation 26 and 36 or the housing 14 and the lower housing 18.
  • the gas flow introduced through the ducts 46 prevents the hot gas from the bore of the furnace 10 flowing out through the slots 44.
  • the furnace 10 may be modified in various ways while remaining within the scope of the invention.
  • there might be three equally spaced pairs of opposed ducts i.e. six radial ducts) , so that substantially the entire circumferential path is occupied by such slots.
  • the tube 24 and the muffle tube 34 might be spaced apart by narrow studs projecting from the lower end of the tube 24, or alternatively the rectangular graphite ducts 46 might locate in recesses defined in the ends of the tube 24 and muffle tube 34, so that the tube 24 and muffle tube 34 are spaced and held apart by the ducts 46; in this case the entire circumferential path would be occupied by the slots .
  • the optical equipment used to observe and monitor the glass rod 12 may be of a different type.
  • it might comprise a digital or TV camera to provide an image of the glass rod 12, enabling its concentricity to be monitored, as well as measuring its diameter.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geochemistry & Mineralogy (AREA)
  • Manufacturing & Machinery (AREA)
  • Manufacture, Treatment Of Glass Fibers (AREA)

Abstract

A furnace (10) for stretching a glass rod (12) includes a graphite tube (24) that can be heated by an induction coil (32) to a temperature at which the glass becomes plastic. The glass rod (12) is passed through the bore of the tube (24) while being stretched. At least two pairs of slots (44) are defined through the wall of the tube along a circumferential path at a position along the tube at which, in use, the glass rod (12) has decreased in diameter but is still plastic. Viewing ducts (46) extend radially outward from each such slot, arranged so an optical measuring instrument can monitor the diameter of the glass rod by observation through at least one such pair of ducts. Using the claimed furnace, The stretching process can be adjusted in accordance with this measured diameter to provide a rod (12) of a desired diameter.

Description

FURNACE FOR STRETCHING A GLASS ROD
The invention relates to a furnace for stretching a glass rod to form a rod of smaller diameter, particularly but not exclusively for stretching a core rod.
Optical fibres are typically of diameter about 120 μm, with a core of diameter about 10 μm; the core is of higher refractive index than the surrounding cladding glass. Such optical fibres may be made by drawing out a preform with the same proportions but on a much larger scale, for example a rod of glass of diameter say 84 mm with a core of diameter 7 mm. The core must be accurately concentric within the surrounding glass, and there must not be any bubbles or inclusions in the preform. One way to make such a preform is to make a tube of the cladding glass; and to make a core rod of the core glass material
(typically with an outer coating of the cladding glass) , the core rod being initially of larger diameter than the bore of the tube. The core rod is then heated and stretched in a furnace so that it fits closely within the tube. The core rod can then be inserted into the tube and subsequently fused together in a furnace, using a vacuum, to form the preform.
According to the present invention there is provided a furnace for stretching a glass rod, the furnace comprising a graphite tube and means for heating the graphite tube to a temperature at which the glass becomes plastic, and means to supply an inert gas to the bore of the graphite tube, at least two pairs of slots being defined through the wall of the tube along a circumferential path at a position along the tube at which, in use, the glass rod has decreased in diameter but is still plastic, viewing ducts extending radially outward from each such slot, the ducts being such that an optical measuring instrument can monitor the diameter of the glass rod by observation through at least one such pair of ducts.
In the preferred embodiment the ducts are of a heat resistant material such as ceramic or graphite. Preferably the ducts of at least one pair terminate, at their outer ends, at optically transparent windows through which the measuring instrument can observe the glass rod. For example one such window may be a clear glass window, and the other window may be a narrow band filter transparent only to radiation of the frequency emitted by the instrument .
Preferably each duct is provided at its outer end with an inlet for the inert gas, preferably the inert gas inlet being arranged so a gas jet sweeps any contamination away from the transparent window.
Preferably each duct is wider than the expected diameter of the glass rod at that position along the furnace, to simplify the measurement of diameter. Consequently a substantial proportion of the said circumferential path is constituted by the slots . The slots inevitably have a cooling effect on the glass rod, but the symmetry of the slots and the provision of slots occupying almost all the circumferential path ensures that the circular cross-section does not become distorted. The ducts are preferably of the same cross- sectional shape as the corresponding slot, but may be wider, and indeed may be defined between annular plates separated by the axial height of the slot .
Desirably the furnace is provided with means for pulling the glass rod through the furnace, and the measuring instrument is linked to the control means for the pulling means, so that the pulling speed can be adjusted to ensure the final diameter of the rod has the desired value .
The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:
Figure 1 shows a longitudinal sectional view of a glass rod stretching furnace; and
Figure 2 shows a cross-sectional view on the line 2- 2 of figure 1.
Referring to figure 1, an induction furnace 10 for stretching a glass rod 12 (shown in broken lines) includes an upper cylindrical housing 14 of copper with a stainless steel cover plate 16, and a lower cylindrical housing 18 of stainless steel with a bottom stainless- steel cover plate 20, each cover plate 16 and 20 defining a central aperture through which the glass rod 12 passes. The housings 14 and 18 and the cover plates 16 and 20 are water-cooled. Within the copper housing 14 is a graphite tube 24 that acts as a susceptor. It is of internal diameter 97 mm, and is of high purity graphite of density
-3 1780 kg m and porosity less than 10%. This is surrounded by a layer of thermal insulation formed of two half-tubes 26 of low-density graphite fibre material; as shown in figure 2 these half-tubes meet along lines that are tangential to the tube 24, and are additionally located by alumina pins 28. The copper housing 14 also encloses a water-cooled copper coil 32 that, in use, is connected to a high frequency electrical generator (not shown) . This induces currents in the susceptor tube 24, so in use it may reach a temperature that may be in the range 2000-2500°C. Within the lower housing 18 is a graphite muffle tube 34 of tapering bore, aligned with the tube 24, and surrounded by additional half-tubes 36 of low-density graphite fibre material.
Feed equipment supplies the glass rod 12 at a steady rate through the upper aperture (which may be provided with a felt seal) . Below the upper aperture two laser sensors monitor the position of the glass rod 12 through apertures 40 to ensure it remains concentric with the tube 24. Feed equipment (not shown) pulls the glass rod 12 out of the lower aperture at a faster rate, so that the glass is stretched while it is hot and plastic. The glass rod consequently necks down to a narrower diameter in the lower part of the tube 24, and by the time it reaches the bottom of the muffle tube 34 it is again solid. To prevent oxidation of the graphite, nitrogen gas is supplied to the bore of the furnace 10 around the glass rod 12, and is also supplied within the copper housing 14 and the lower housing 18. The lower aperture is provided with pneumatically operated graphite block shutters to restrict loss of nitrogen gas.
Referring now also to figure 2 , the bottom end of the tube 24 and the top of the muffle tube 34 are castellated to fit together, and are shaped so as to define four radially-extending coplanar slots 44 each 2 mm high and 50 mm wide extending along two orthogonal diameters. Rectangular-section graphite ducts 46 are shaped to fit onto the outside of the tube 24 and the muffle tube 34, and extend radially outward to the periphery to locate in corresponding slot-shaped apertures in the housing 18. To the outside of these apertures are attached rectangular frames 48 (in one opposed pair) or blanking plates 50 (in the other opposed pair) , in each case provided with tangential inlets 52 for nitrogen. The frames 48 are closed by glass windows 54, 55, one of which is transparent and the other is a narrow band filter.
It will be appreciated from figure 1 that the glass rod 12 will have significantly decreased in diameter by the time it reaches the plane in which the slots 44 are defined. By scanning a laser (not shown) through the window 54, the laser beam scanning to and fro parallel to the longitudinal axis of the duct 46, and observing the radiation through the window 55 (which is transparent to the radiation emitted by the laser) , the diameter of the glass rod 12 can be accurately measured. This measurement may be used in controlling the speed at which the glass rod 12 is drawn out of the furnace 10, so as to ensure that once it has set solid it has the desired diameter .
In use the bore of the tube 24 containing the glass rod 12 is at a very high temperature, typically in the range 2000 to 3000°C, more typically between 2000 and 2500°C, and contains pressurised gas. The walls of the ducts 46 prevent the consequential intense radiation from reaching the low-density insulation 26 and 36 or the housing 14 and the lower housing 18. The gas flow introduced through the ducts 46 prevents the hot gas from the bore of the furnace 10 flowing out through the slots 44.
It will be understood that the furnace 10 may be modified in various ways while remaining within the scope of the invention. For example there might be three equally spaced pairs of opposed ducts (i.e. six radial ducts) , so that substantially the entire circumferential path is occupied by such slots. In such an arrangement the tube 24 and the muffle tube 34 might be spaced apart by narrow studs projecting from the lower end of the tube 24, or alternatively the rectangular graphite ducts 46 might locate in recesses defined in the ends of the tube 24 and muffle tube 34, so that the tube 24 and muffle tube 34 are spaced and held apart by the ducts 46; in this case the entire circumferential path would be occupied by the slots .
It will be appreciated that the optical equipment used to observe and monitor the glass rod 12 may be of a different type. For example it might comprise a digital or TV camera to provide an image of the glass rod 12, enabling its concentricity to be monitored, as well as measuring its diameter.

Claims

Claims
1. A furnace (10) for stretching a glass rod (12), the furnace comprising a graphite tube (24) and means (32) for heating the graphite tube (24) to a temperature at which the glass becomes plastic, and means to supply an inert gas to the bore of the graphite tube, and characterised by at least two pairs of slots (44) being defined through the wall of the tube along a circumferential path at a position along the tube at which, in use, the glass rod (12) has decreased in diameter but is still plastic, and by viewing ducts (46) extending radially outward from each such slot (44) , the ducts (46) being such that an optical measuring instrument can monitor the diameter of the glass rod (12) by observation through at least one such pair of ducts (46) .
2. A furnace as claimed in claim 1 wherein the ducts (46) are of graphite.
3. A furnace as claimed in claim 1 or claim 2 wherein the ducts (46) of at least one pair terminate, at their outer ends, at optically transparent windows (54, 55) through which the measuring instrument can observe the glass rod (12) , at least one such window being a narrow band filter transparent only to radiation of the frequency emitted by the instrument .
4. A furnace as claimed in any one of the preceding claims wherein each duct (46) is provided at its outer end with an inlet (52) for the inert gas.
5. A furnace as claimed in claim 4 wherein those ducts (46) through which the optical observations are made terminate at transparent windows (54, 55) , and the inert gas inlet (52) is arranged so a gas jet sweeps any contamination away from the transparent window (54, 55) .
6. A glass rod stretching apparatus incorporating a furnace as claimed in any one of the preceding claims, and means for pulling the glass rod through the furnace, wherein the measuring instrument is linked to control means for the pulling means, so that the pulling speed can be adjusted in accordance with the observed diameter at the location of the said circumferential path.
PCT/GB2002/003630 2001-08-11 2002-08-06 Furnace for stretching a glass rod WO2003014036A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0119664.1 2001-08-11
GB0119664A GB0119664D0 (en) 2001-08-11 2001-08-11 Glass stretching furnace

Publications (1)

Publication Number Publication Date
WO2003014036A1 true WO2003014036A1 (en) 2003-02-20

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PCT/GB2002/003630 WO2003014036A1 (en) 2001-08-11 2002-08-06 Furnace for stretching a glass rod

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WO (1) WO2003014036A1 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10030961B2 (en) 2015-11-27 2018-07-24 General Electric Company Gap measuring device

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5792534A (en) * 1980-11-28 1982-06-09 Nippon Telegr & Teleph Corp <Ntt> Elongation of glass rod
JPS63195139A (en) * 1987-02-09 1988-08-12 Hitachi Cable Ltd Apparatus for drawing glass rod
JPH0891861A (en) * 1994-07-21 1996-04-09 Furukawa Electric Co Ltd:The Method for stretching glass rod
EP0795519A1 (en) * 1996-03-14 1997-09-17 Alcatel Fibres Optiques Device for injecting non-oxidising gas inside a furnace
JPH10158024A (en) * 1996-11-28 1998-06-16 Shin Etsu Chem Co Ltd Method and device for stretching optical fiber preform
EP0870736A1 (en) * 1997-04-11 1998-10-14 Sumitomo Electric Industries, Ltd. Elongating apparatus and method for glass preform and furnace body for elongating apparatus
JPH1111970A (en) * 1997-06-20 1999-01-19 Fujikura Ltd Stretching device and stretching of glass rod
JP2001048565A (en) * 1999-08-09 2001-02-20 Sumitomo Electric Ind Ltd Method for drawing optical fiber preform

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5792534A (en) * 1980-11-28 1982-06-09 Nippon Telegr & Teleph Corp <Ntt> Elongation of glass rod
JPS63195139A (en) * 1987-02-09 1988-08-12 Hitachi Cable Ltd Apparatus for drawing glass rod
JPH0891861A (en) * 1994-07-21 1996-04-09 Furukawa Electric Co Ltd:The Method for stretching glass rod
EP0795519A1 (en) * 1996-03-14 1997-09-17 Alcatel Fibres Optiques Device for injecting non-oxidising gas inside a furnace
JPH10158024A (en) * 1996-11-28 1998-06-16 Shin Etsu Chem Co Ltd Method and device for stretching optical fiber preform
EP0870736A1 (en) * 1997-04-11 1998-10-14 Sumitomo Electric Industries, Ltd. Elongating apparatus and method for glass preform and furnace body for elongating apparatus
JPH1111970A (en) * 1997-06-20 1999-01-19 Fujikura Ltd Stretching device and stretching of glass rod
JP2001048565A (en) * 1999-08-09 2001-02-20 Sumitomo Electric Ind Ltd Method for drawing optical fiber preform

Non-Patent Citations (6)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 12, no. 479 14 December 1988 (1988-12-14) *
PATENT ABSTRACTS OF JAPAN vol. 1996, no. 08 30 August 1996 (1996-08-30) *
PATENT ABSTRACTS OF JAPAN vol. 1998, no. 11 30 September 1998 (1998-09-30) *
PATENT ABSTRACTS OF JAPAN vol. 1999, no. 04 30 April 1999 (1999-04-30) *
PATENT ABSTRACTS OF JAPAN vol. 2000, no. 19 5 June 2001 (2001-06-05) *
PATENT ABSTRACTS OF JAPAN vol. 6, no. 177 11 September 1982 (1982-09-11) *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10030961B2 (en) 2015-11-27 2018-07-24 General Electric Company Gap measuring device

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