Glass manufacturing – Processes – Forming product or preform from molten glass
Patent
1992-02-28
1994-02-22
Lindsay, Robert L.
Glass manufacturing
Processes
Forming product or preform from molten glass
65 29, 65 322, 65 12, 219494, C03B 3723
Patent
active
052883019
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus for, and a method of, fabricating fused optical fibre devices, for example for the purpose of making fused fibre couplers.
The standard method of fabricating fused fibre devices is to heat twisted fibres in a bunsen flame while stretching the fibres. An important fundamental step in the fabrication of fused fibre devices is, therefore, the fibre heating process which is required to raise the temperature of the fibres sufficiently for the glass to flow in response to an applied force. For silica fibres, this temperature is generally greater than 1500.degree. C. To achieve repeatable low loss devices this heating source must be both clean and well controlled. For example, to fabricate fused fibre couplers, the temperature profile should be uniform along the heated length of the fibres, so that a gradual adiabatic taper is obtained, and the temperature must be carefully adjusted to stop bending and general distortion of the fibres due to excessive temperature in combination with the applied tension. The heat source normally employed for this purpose is a gas (such as a mixture of hydrogen, nitrogen and oxygen) flame. By adjusting the relative proportions of these gases, it is possible to vary the flame temperature. However, it is difficult to obtain the required temperature uniformity, both temporal and spatial, and the fibres can be easily distorted by the pressure of the flowing gas striking the softened glass.
With the introduction of wavelength-flattened couplers and wavelength multiplexers, the heating difficulties increase, and much better control of the heating source is required if high performance devices are to be fabricated with high yield. In the fabrication of standard fused couplers, the fibres are elongated until the required coupling ratio is obtained at the specified wavelength. So long as the taper is sufficiently gentle, the actual taper shape and degree of fusion of the fibres is relatively unimportant, and the desired coupling ratio will always be achieved. The performance of wavelength-flattened couplers and wavelength multiplexers, on the other hand, depends critically on the actual taper shape and degree of fusion of the fibres. As the viscosity of silica follows an exponential law with temperature, the temperature control must be very precise in order to obtain repeatedly identical components to a given specification.
The present invention provides apparatus for fabricating fused optical fibre devices, the apparatus comprising an electric heating element, a heat-distributing insert within the heating element, an elongate heating chamber for an optical fibre arrangement being defined within the heating element and the insert, means for passing electric current through the heating element, and means for supplying and maintaining an inert atmosphere within the heating chamber, wherein the insert extends substantially the entire length of the heating chamber so that, in use, the heating chamber attains a substantially uniform temperature, and wherein the combined thermal capacity of the heating element and the insert is such that an optical fibre device placed within the heating chamber can be heated to fusion temperature in less than 30 seconds.
In a preferred embodiment, said combined thermal capacity is such that fusion temperature is reached in less than 10 seconds, and preferably after at least 1 second.
Advantageously, the heating element has a generally C-shaped portion which defines the heating chamber, the C-shaped portion having a diameter of between 4.5 mm and 5 mm, and preferably of 4.7 mm. Where the apparatus is used to make single fibre tapers, the C-shaped portion may have a length of between 2 and 5 mm. Alternatively, where the apparatus is used to make devices such as fibre couplers, it is preferable for the C-shaped portion to have a length of between 9 and 15 mm. Preferably, the heating element is made of tungsten or tantalum.
Conveniently, the insert is made of ceramic material, and preferably is a slotted alumina tube, the slot in t
REFERENCES:
patent: 4295033 (1981-10-01), Lindgren et al.
patent: 4584464 (1986-04-01), Myer
patent: 4879454 (1989-11-01), Gerdt
patent: 5141545 (1992-08-01), Grigsby
Achurch Nicholas E.
Mortimore David B.
Rowe Christopher J.
Wilkinson Iain J.
BT&D Technologies Limited
Lindsay Robert L.
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