Method for fabrication of fused fibre devices

#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.

#An electric furnace for fabricating fused fibre devices and constructed in accordance with the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:

#FIG. 1 is a perspective view of the bottom half of the furnace;

#FIG. 2 is a perspective view of the top half of the furnace;

#Referring to the drawings, FIG. 1 shows the bottom half of the furnace, this bottom half having a pair of stainless steel blocks 1a and 1b mounted on a ceramic insulating base 2. The base 2 may be made of a ceramic material or of standard resin bonded fibre (SRBF) material. A ceramic spacer 3 separates the blocks 1a and 1b, and defines a channel 4 between the two blocks at their upper portions. This channel is 2 mm in width at its ends, and has a central, cigar-shaped, widened portion 4a.

#A heating element 5 is positioned centrally within the widened channel portion 4a. The heating element 5 is made of tungsten foil, and has a central C-shaped portion 5a and a pair of arms 5b (see FIG. 3). The diameter of the C-shaped portion 5a is 4.7 mm. The arms 5b of the heating element 5 are fixed to, and in electrical contact with, the blocks 1a and 1b by clamps 6 associated with the blocks. Each clamp 6 has a lower clamp member 6a fixed to the upper surface of the associated block 1a or 1b, and an upper clamp member 6b which can be clamped to the associated lower clamp member (with an arm 5b of the heating element 5 positioned therebetween) by means of clamping screws 6c. As shown in FIG. 3, the C-shaped portion 5a of the heating element 5 contains a slotted, tubular, ceramic insert 7, having a wall thickness of 0.95 mm and a slot width of 1.0 mm. The dimensions of the heating element 5 and the insert 7 result in a low combined thermal capacity for these items. This combined thermal capacity should be low enough to ensure rapid heating, but high enough to smooth out temperature fluctuations.

#A gas inlet pipe 8, having a diameter of 3 mm, leads through the block 1a and into the centre of the widened portion 4a of the channel 4. In use, an inert gas (such as argon) is fed into the pipe 8 at a pressure slightly greater than that of the atmosphere. The blocks 1a and 1b are also provided with water cooling channels (not shown).