Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Staple length fiber
Reexamination Certificate
2001-02-14
2003-11-18
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Staple length fiber
C428S364000, C428S373000, C385S124000
Reexamination Certificate
active
06649261
ABSTRACT:
A rod-shaped preform for manufacturing an optical fibre therefrom, a method for manufacturing such a rod-shaped preform as well as a method for manufacturing an optical fibre, using such a rod-shaped preform.
The present invention relates to a rod-shaped preform for manufacturing an optical fibre therefrom, as well as to a method for manufacturing such a rod-shaped preform. The present invention furthermore relates to a method for manufacturing an optical fibre, wherein one end of a rod-shaped preform is subjected to a heat treatment, after which the thus softened end of the rod-shaped preform is subjected to a pulling force for the purpose of drawing an optical fibre therefrom.
Such a preform is known from Japanese patent publication JP-A-11079772, and according to said document said preform can be regarded as a plurality of separate grooves of different depths which are disposed at the end of the preform, perpendicularly to the axial direction of the preform. Such a stepped diameter reduction is disadvantageous, because in the furnace the glass of the preform softens at the location of such a step, to such an extent that the material sags from the furnace from the location of said step. Since the cone geometry is heated by the furnace at the circumference thereof, there is a considerable risk of softening of the zones beside the centre of the preform, as a result of which drawing will not take place from the centre of the cone, which is undesirable.
Such a preform is also known per se from European patent application no. 0 530 917 filed in the name of the present applicants. The preform known therefrom can be regarded as a rod-shaped preform having a uniform diameter along its entire length. When an optical fibre is to be drawn from such a rod-shaped preform, said rod-shaped preform is heated in a furnace at one end, so that a softening zone is formed. During said heating the zone thus softened is subjected to a pulling force, after which the plastic condition of said softened zone makes it possible to draw the optical fibre therefrom. According to said patent publication the direction of the heating zone will be opposed to that of the pulling force being exerted on the softened end.
The current developments in the glass fibre industry show a tendency towards increasing the capacity of the rod-shaped preform. Such a capacity increase is effected mainly by increasing the diameter of the rod-shaped preforms. It would also be possible, however, to increase the length of the rod-shaped preforms, but such a measure would lead to problems in practice, since the current equipment on which the preforms are manufactured is not suitable therefor. In addition, long preforms are difficult to handle.
The term “rod-shaped preform” as used in the present introduction to the disclosure is understood to include both a massive rod and a tube, in particular a rod surrounded by one or more concentric tubes. Generally, such a rod-shaped preform is drawn out in a draw tower to form an optical fibre having a diameter of about 125 &mgr;m. During said drawing the rod-shaped preform is slowly moved through the furnace, whereupon it will melt at one end thereof, thus forming a cone. From said cone, which has been obtained by heating, the aforesaid glass fibre is carried away at a high velocity. Thus the starting up of the drawing process, in particular the forming of the cone, will take up a considerable amount of time when using the current generation of rod-shaped preforms, in particular preforms having a large diameter. It should be understood that this start-up time is substantially longer when rod-shaped preforms having a larger diameter are used. Owing to the above-described tendency, wherein rod-shaped preforms having a larger diameter are to be processed, it is thus desirable that the start-up time of the drawing process be influenced advantageously, thus making it possible to shorten the drawing process, in particular the start-up phase thereof.
The object of the present invention is thus to provide a rod-shaped preform, which rod-shaped preform has a geometry that makes it possible to reduce the start-up phase of the drawing process to a significant degree.
Another object of the present invention is to provide a method for manufacturing a rod-shaped preform, which rod-shaped preform effects a significant reduction of the start-up phase of the drawing process, in particular in the case of preforms having a large diameter.
Another object of the present invention is to provide a rod-shaped preform for manufacturing an optical fibre therefrom, as well as a method for manufacturing said optical fibre, wherein a precisely defined geometry of the end of the rod-shaped preform can be effected.
According to the present invention, the rod-shaped preform for manufacturing an optical fibre as referred to in the introduction is characterized in that one end of a rod-shaped preform has a cone-like geometry, which cone-like geometry complies with the following equations:
D
d
0
=
2
-
25
,
D
d
1
2
=
1
,
2
-
5
,
L
C
D
=
0
,
8
-
3
,
5
en
D
>
d
1
2
>
d
0
wherein
L
c
=the cone length (mm)
D=the diameter of the rod-shaped preform (mm)
d
O
=the diameter of the rod-shaped preform at the cone end (mm)
d
1/2
=the diameter of the rod-shaped preform halfway the cone (mm).
Although European patent application no. 0 999 189 (not pre-published) discloses a preform having a reduced diameter, said publication does not provide any indication as to the aforesaid combination of requirements, in particular the ratio between d
1/2
and D, which the present preform is to meet in order to reduce the start-up phase of the drawing process to a significant degree.
By giving the rod-shaped preform a cone-like geometry already before the eventual drawing process, the time required for the drawing process, in particular the start-up phase thereof, is considerably reduced. Thus it is in particular desirable for the rod-shaped preform to comprise an end having a cone-like geometry, the shape of which cone-like geometry substantially corresponds to that of the cone that would automatically form upon continuous drawing of the optical fibre from the rod-shaped preform. Thus also the time needed to achieve the desired end velocity can be reduced considerably in such an embodiment. It should be apparent, therefore, that the present preform can be regarded as a preform from which the optical fibre is yet to be drawn, which rod-shaped preform should not be mixed up with the rod-shaped preform that is already known from the prior art, from which an optical fibre has already been drawn. The cone geometry starts at the tip of the preform, where diameter D starts to decrease. The present cone geometry can be regarded as a flowing, continuous reduction from diameter D to diameter d
O
which geometry essentially differs from the discontinuous, stepped geometry in accordance with JP-A-11079772.
The aforesaid ratio D/d
O
is important in particular in order to generate a specific diameter reduction so as to provide the desired effect, in particular a shortened start-up phase of the eventual drawing process. It should be understood that the lower limit of the ratio D/d
O
especially applies to rod-shaped preforms having a small diameter, in particular rod-shaped preforms having a diameter upwards of about 30 mm. The upper limit of the ratio D/d
O
especially applies to rod-shaped preforms having a larger diameter, in particular rod-shaped preforms having a diameter of about 150 mm. It should be noted, however, that the present rod-shaped preform is not restricted to a specific diameter in any embodiment thereof, but only to the ratios between the cone-like geometry and the rod-shaped preform as described before.
The aforesaid ratio D/d
1/2
relates to the specific geometry of the softened end of the rod-shaped preform, in particular the cone. If this ratio is larger than 5, the end of the cone will exit the heated zone too quickly, which means that too large a glass mass will exit the
de Fouw Marinus Jacob
Jensen Henrikus Lambertus Maria
de Guzman Dennis M.
Draka Fibre Technology B.V.
Gray J. M.
Kelly Cynthia H.
Seed IP Law Group PLLC
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