A METHOD OF AND AN APPARATUS FOR COOLING AN OPTICAL FIBRE

Abstract: A method of and an apparatus for cooling an optical fibre drawn from a heated glass blank by passing the fibre (3) through a cooling pipe (9) in which a cooling gas (A) flows in the axial direction of the fibre around it. In order to reduce the consumption of cooling gas, the cooling gas is caused to flow turbulently (B) around the fibre by means of a throttle point (20) formed by a partition wall (19) in the cooling pipe, so that a laminar flow of cooling gas around the fibre is prevented and the transfer of heat between the cooling gas and the fibre is improved.

Description: A method of and an apparatus for cooling an optical fibre This invention relates to a method of cooling an optical fibre when the fibre is being drawn from a heated glass blank, in which method - the drawn fibre is passed through a cooling space, and - a gaseous cooling medium is supplied in the axial direction of the fibre around the fibre through the cooling space. In the manufacture of optical fibres, a fibre drawn from a glass blank (preform) has to be coated with a primary coating after the heating apparatus so as to protect the surface of the fibre and to in¬ crease the strength of the fibre. In order to cool a hot fibre to a suitable temperature prior to coating, the fibre is passed through a cooling apparatus, in which the fibre is brought into heat transfer contact with a cooling medium. U.S. Patent Specification 4,388,093 suggests cooling a fibre by passing it through a cooling vessel filled with a liquid cooling medium. The cooling liquid, however, causes considerable sealing problems at the high drawing rates aimed at in modern fibre drawing processes. According to the teachings of Swedish Patent Specification 433,605, a fibre is cooled by passing it through a cooling apparatus in which a gaseous cooling medium, preferably dry nitrogen gas, is blown towards the fibre through a tubular wall of a porous material. In this way the cooling gas is distributed evenly and the fibre is not exposed to the effects of disadvanta¬ geous transverse forces. The use of this kind of cool- ing arrangement, however, makes it difficult to circulate the cooling medium axially through the cool¬ ing space against the direction of movement of the fibre, which would be preferable in view of the trans¬ fer of heat. In an article "Method for cooling and bubble- free coating of optical fibres at high drawing rates", C. Jochem and I. Van der Ligt, Electronics Letters, 29th August 1985, Vol. 21, No. 18, p. 786, it is sug¬ gested to cool a fibre by passing it through a water- cooled cooling pipe which is filled with helium gas circulated axially through a cooling space formed by the cooling pipe. Helium gas has a good coefficient of heat transfer, which ensures that the fibre is cooled sufficiently even at high fibre drawing rates. A dis- advantage, however, is that large quantities of expen¬ sive helium gas are required for cooling the fibre. The object of the present invention is to pro¬ vide a method of cooling an optical fibre, which method avoids the above-mentioned disadvantages and enables the achievement of a high cooling efficiency with a relatively small cooling medium flow. This ob¬ ject is achieved by means of a method according to the invention, which is characterized in that the cooling gas is caused to flow turbulently around the fibre at least at one point in the cooling space. The invention is based on the idea that the turbulence of the cooling medium breaks up the laminar flow of the cooling medium occurring around the fibre to be cooled; consequently, the heat transfer contact between the cooling medium and the surface of the fibre is improved. In addition, the flow of the cool¬ ing medium through throttle points provided therefor causes pressure losses, so that the cooling space is divided into compartments in which the medium is pressurized, which improves the heat transfer. The invention thus provides a high cooling efficiency with a smaller consumption of cooling medium, without any risk that the axial flow of the cooling medium causes a disadvantageous laminar flow around the fibre. The invention is also concerned with a cooling apparatus for effecting the method according to the invention. The apparatus comprises a cooling pipe de¬ fining a tubular cooling space, and an inlet and an outlet for passing the fibre axially through the cool- ing space, and means for supplying a gaseous cooling medium axially through the cooling space. The cooling apparatus is characterized in that the cooling space comprises at least one partition wall positioned be¬ tween the fibre inlet and the fibre outlet, the parti- tion wall comprising a common opening for the fibre and the cooling gas to pass through, which opening is such in size that the cooling gas flows turbulently therethrough. According to the invention the means to be add- ed to the cooling pipe for achieving turbulence are very simple in structure, because the partition wall forming the throttle point may be formed by a ring- shaped plate comprising a central opening through which the fibre to be cooled and the gaseous cooling medium are passed. The number of partition walls and their mutual spacing in the cooling space are chosen according to the requirements in each particular case so that the turbulent points prevent the formation of disadvantageous laminar flow around the fibre during the cooling step. In a cooling apparatus in which the cooling pipe is surrounded by a jacket defining a cooling water space between the jacket and the cooling pipe, it is preferable that the ring-shaped partition plate comprises not only an opening common to the fibre and the gas but also a number of flow-through holes for the cooling gas between the compartments divided by the partition plates. This kind of flow-through holes provide more contact area between the gaseous medium and the partition plate cooled by the cooling water space, which further improves the cooling of the fibre. Tests carried out with a cooling apparatus ac¬ cording to the invention have shown that the gas con- ouraption of the apparatus according to the invention is only 1/15 of the gas consumption " of a cooling pipe having no partition plates but with the same cooling efficiency. In the following the invention will be de- scribed in more detail with reference to the attached drawing, wherein Figure 1 is a schematical view of the drawing step of an optical fibre; Figure 2 is a schematical view of the operating principle of a cooling apparatus according to the invention; and Figure 3 is a detailed axial sectional view of a partition plate of the cooling apparatus. In Figure 1 of the drawing, the reference n - meral 1 indicates a glass blank which is heated in a furnace 2 and which forms a fibre 3 which is drawn continuously from the furnace by means of a drawing means 4. The reference numeral 5 indicates a device for measuring the diameter of the fibre. After the furnace the fibre passes through a cooling apparatus 6 into a coating apparatus 7 for a primary layer and f rther into a hardening unit 8. The cooling apparatus 6 comprises a metal cool¬ ing pipe 9 which defines a cooling space 10 through which the fibre to be cooled passes. An inlet 11 and an outlet 12 for the fibre are provided at the top and at the bottom of the cooling pipe, respectively. The inlet 11 and the outlet 12 are sealed with seals 11a and 12a. At the bottom of the cooling pipe is provided an inlet 13 and at the top an outlet 14 for a cooling gas, such as helium. The cooling pipe is surrounded by a jacket 15, so that a cooling water space 16 is de¬ fined between the jacket and the pipe. A water inlet is indicated with the reference numeral 17 and a water outlet with the reference numeral 18. A number of transverse ring-shaped partition plates 19 axially spaced from each other are mounted within the cooling pipe. Each partition plate com¬ prises a central opening 20 and a number of parallel flow-through holes 21. The openings of the partition plates are positioned concentrically with each other and with the inlet and the outlet for the fibre. The cooling gas A flows upwards in the cooling space 10, through which the fibre to be cooled passes at a high rate. The openings 20 of the partition plates are so dimensioned that the cooling gas flows turbulently through the opening from a compartment 22 separated by the partition plate to the following com¬ partment 23, Figure 3. The turbulence B of the cooling gas efficiently breaks up any laminar cooling gas flow, which otherwise tends to be formed around the fibre. This, in turn, improves the transfer of heat between the cooling gas and the fibre, even with a small flow of the cooling gas, thus reducing the con- sumption of gas. Part of the cooling gas flows through the flow-through holes 21 of the partition plates, which increases the contact area between the gas and the cooling water space. The partition plates cause pressure losses in the cooling gas flow, which increases the pressure difference between the inlet and the outlet of the cooling gas, so that the heat transfer capacity of the cooling gas contained in the cooling pipe is im¬ proved. The drawing and the description related thereto are only intended to illustrate the idea of the inven¬ tion. In its details, the method and the apparatus ac¬ cording to the invention may vary within the scope of the claims. Therefore the cooling apparatus is shown only schematically in the drawings. If, for instance, a cooling gas is used which can be- released into the atmosphere, the seal 11a and the outlet 14 can be omitted, whereby the cooling gas is discharged through the fibre inlet 11 into the surrounding space.

Claims: Claims : 1. A method of cooling an optical fibre (3) when the fibre is being drawn from a heated glass blank (1), in which method - the drawn fibre is passed through a cooling space (10); and - a gaseous cooling medium (A) is supplied in the axial direction of the fibre around the fibre through the cooling space, c h a r a c t e r i z e d in that the cooling gas (A) is caused to flow turbu¬ lently (B) around the fibre (3) at least at one point in the cooling space (10). 2. A method according to claim 1, c h a r a c- t e r i z e d in that the cooling (A) gas is passed through a flow throttle point (20) in the cooling space (10), the fibre (3) moving through the same point. 3. A method according to claim 2, c h a r a c- t e r i z e d in that the cooling gas (A) is passed through the cooling space (10) in a direction opposite to the direction of movement of the fibre (3) . 4. An apparatus for cooling an optical fibre (3) drawn from a heated glass blank (1) prior to applying a primary coating to the fibre, comprising - a cooling pipe (9) defining a tubular cooling space (10) and an inlet (11) and an outlet (12) for passing the fibre axially through the cooling space; and - means (11, 13, 14) for supplying a gaseous cooling medium (A) axially through the cooling space, c h a r a c t e r i z e d in that the cooling space (10) comprises at least one partition wall (19) posi¬ tioned between the inlet (11) and the outlet (12) of the fibre (3), the partition wall comprising a common opening (20) for the fibre and the cooling gas to pass through, whereby the size of the opening is such that the cooling gas flows turbulently (B) therethrough. 5. An apparatus according to claim 4, c h a r- a c t e r i z e d in that the cooling space (10) com¬ prises several partition walls (19) axially spaced from each other, each partition wall comprising a sep¬ arate opening (20) positioned concentrically with the other openings and with the inlet and outlet (11, 12) of the fibre. 6. An apparatus according to claim 4 or 5, com¬ prising a cooling pipe (9) surrounded with a jacket . (15) defining a cooling water space (16) between the jacket and the cooling pipe, c h a r a c t e r i z e d in that the partition wall (19) is formed by a ring- shaped partition plate attached to the cooling pipe and dividing the cooling space (10) into separate com¬ partments (22, 23). 7. An apparatus according to claim 6, c h a r- a c t e r i z e d in that in addition to an opening (20) for the fibre the ring-shaped partition plate comprises a number of flow-through holes (21) for the cooling gas (A) between the compartments (22, 23). 8. An apparatus according to claim 4 or 5, c h a r a c t e r i z e d in that an inlet and an outlet (13, 14) for the cooling gas (A) are positioned in the cooling pipe (9) so that the cooling gas flows through the opening (20) against the direction of movement of the fibre (3) . 9. An apparatus according to claim 4 or 5, c h a r a c t e r i z e d in that the fibre inlet (11) acts as an outlet for the cooling gas (A).

Inventors: LINDEN ANTTI, PENNANEN RISTO, STORMBOM LENA, LINDEN, ANTTI, PENNANEN, RISTO, STORMBOM, LENA
Applicants: NOKIA OY AB, OY NOKIA AB
Application year: 1988
Main classification: C03B37
IPC codes: C03B37, C03C25, G02B6, G02B27