Butt welding optical fiber preforms with a plasma torch

Details Butt welding optical fiber preforms with a plasma torch Butt welding optical fiber preforms with a plasma torch

1999-03-30

2001-01-30

Hoffmann, John (Department: 1731)

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

Process of manufacturing optical fibers, waveguides, or...

C065S407000, C065S272000, C065S057000, C065S036000

Reexamination Certificate

active

06178779

ABSTRACT:

The invention relates to a method of assembling two optical fiber preforms end-to-end, in which:
the two cylindrical preforms are placed on a common axis;
the two preforms are rotated about the axis;
the facing ends of the two preforms are heated; and
the two preforms are moved towards each other parallel to the axis so as to press them against each other, thereby forming intimate contact between the heated ends after they have cooled.
BACKGROUND OF THE INVENTION
Such a method is known in particular from German patent application DE-29 32 196. In that application, the preforms to be assembled typically have a length and a diameter of 300 mm and 10 mm, respectively. The heating means used is a blow torch, or a resistance oven, or indeed a laser. The cylindrical preforms are rotated about their axis to ensure that heating is uniform around the periphery of each end.
Nowadays, the preforms used have a diameter that lies typically in the range 100 mm to 170 mm, so as to increase the length of optical fiber that can be drawn therefrom.
Assembling such preforms together requires heating power that is much greater than that required for preforms that are 10 mm in diameter, given that the volume of material that needs to be softened varies with the square of the diameter. In addition, heat must be delivered in such a manner that the material softens in substantially uniform manner throughout the heated volume at each end; this is particularly critical since the periphery and the core of the preform do not have the same melting temperature, with the periphery of the preform generally containing no doping elements, whereas the core does contain doping elements.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is thus to develop a method of assembling end-to-end two large-diameter optical fiber preforms of non-uniform radial composition.
To this end, the invention provides a method of assembling two optical fiber preforms end-to-end, the method comprising the following operations:
the two cylindrical preforms are placed in alignment along a common longitudinal axis;
said preforms are rotated about said common longitudinal axis;
the facing ends of said preforms are heated by heater means; and
said preforms are moved towards each other parallel to said common axis to press them against each other so as to form intimate contact between said ends after cooling;
wherein said heating is performed by causing said heater means to move relative to said ends in a radial direction orthogonal to said common longitudinal axis in such a manner as to heat the peripheries and then the cores of said preforms.
By displacing the heater means radially in front of each end from the peripheries towards the centers of the preforms, heating begins with layers that have a relatively high melting temperature because of the absence of doping elements, and continues with the core layers that have a lower melting temperature because of the presence of dopants.
In this way, the volume heated at each end is softened in substantially uniform manner, thereby making it possible to butt weld the two preforms together while preventing any flow of the more meltable material while the two preforms are being pressed together. This gives rise to a uniform weld which ensures that the portion of fiber drawn from the weld zone conserves the same mechanical and optical properties as the remainder of the drawn fiber.
Advantageously, the heater means comprise a plasma torch. The plasma torch generates a soft flame which is both hot and focused. Plasma torch heating also makes it possible to vaporize a surface layer of material from each preform end. This thus eliminates a prior operation of cleaning by means of chemical agents. It is preferable to use a clean plasma-generating gas to avoid polluting each end.
The invention also applies to assembling a preform with a glass endpiece. Under such circumstances, the invention provides a method of assembling an optical fiber preform and a cylindrical endpiece of vitreous material end-to-end, the method comprising the following operations:
said cylindrical preform and said endpiece are placed in alignment along a common longitudinal axis;
said preform and said endpiece are rotated about said common longitudinal axis;
the facing ends of said preform and of said endpiece are heated by heater means; and
said preform and said endpiece are moved towards each other parallel to said common axis to press them against each other so as to form intimate contact between said ends after cooling;
wherein said heating is performed by causing said heater means to move relative to said ends in a radial direction orthogonal to said common longitudinal axis so as to heat the periphery of said preform initially and then heat the core of said preform and said endpiece.


REFERENCES:
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