Optical waveguides – With optical coupler – Particular coupling structure
Patent
1991-01-10
1992-07-07
Gonzalez, Frank
Optical waveguides
With optical coupler
Particular coupling structure
385 43, G02B 628
Patent
active
051290219
DESCRIPTION:
DESCRIPTION
The fibres may be held in their correct relative position for tapering by a glass former 2 as shown in FIG. 1. This structure, which consists of one central hole 4 surrounded by eight equally spaced holes 6 at a constant radius, may conveniently be made by drawing down a large drilled glass rod. The rod is pulled such that the holes are just large enough to accept a single fibre each. Nine lengths of fibre 10 are prepared and each comprises a single mode fibre 12 fusion spliced to a length of coreless or `dummy` fibre 14 as shown in FIG. 2. Referring to FIG. 3, the fibres 10 are inserted into the former 2 so that the splices all lie in the same plane A so that a central fibre protrudes from the former 2 from the opposite end to the outer ring of fibres, only two of which are shown for clarity in FIG. 3. The central fibre forms the one input fibre (I), the remaining eight, the output fibres O.sub.1 -O.sub.8. The assembly is then heated to a temperature sufficient for the glass former to shrink due to surface tension. Vacuum may be employed at this stage to assist the collapse and/or to remove air which may become trapped. Heating is continued until a homogeneous rod of silica is formed. The glass former is heated around the splice position and is pulled to form a taper. The heated length being sufficient to ensure small taper angles and hence a low loss device. Tapering is continued until the cores have reduced in size sufficiently such that the mode fields each side of the splice are reasonably well matched. In practice light could be launched into the central fibre and the device would be pulled until the output signal reaches a maximum.
An alternative fabrication technique involves using a glass former 20 without the central hole. In this method the prepared fibres 10 are inserted into the glass former 20, as shown in FIG. 4 (only two fibres are shown for clarity). The former 20 is collapsed as described above and it is then pulled to form a taper with a waist diameter equal to the diameter of a single fibre 10. The structure is cleaved at the waist and the resulting composite fibre end is fusion spliced to a single fibre. This fusion splice is then further tapered to equalise the mode fields on either side of the splice. The tapering is continued until maximum power transmission is obtained.
Referring now to FIG. 5, consider light propagating along the single input fibre I, towards the taper region marked 22. As light travels down the taper 22, its mode field expands and eventually becomes bounded by the cladding/external medium interface. In this central taper region 24 the fibre cores play little part in the field guidance and the output fibres O.sub.1 to O.sub.8 act as a single output fibre assembly. Therefore, the mode profiles of the single fibre I and output fibre assembly are reasonably well matched. This ensures that the optical field will traverse this critical region 24 with little loss. In the upward taper region, 26, the growing cores gradually capture equal amounts of the optical field such that when the fibres have reached their normal size, each fibre carries the same optical power.
1.times.N couplers having other N values may be formed by these methods. The output fibres need not be arranged in a symmetrical circular array. For example, four fibres can be arranged at the vertices of a rectangle and still obtain equal coupling as each fibre is positioned at the same relative distance and angular disposition from the other three, i.e. in that they are each positioned identically relative to the other output fibres.
The insertion of the fibres 10 into the former 20 may be aided by the use of a lubricating fluid that is readily evapourated before or during the application of heat to form the taper, for example, methanol, ethanol, acetone or other similar fluid.
1. An optical star coupler formed as a taper comprising: fibre assembly with each output fibre being positioned substantially identical relative to the remaining output fibres throughout the taper, wherein fundamental mode fields of th
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Electronics Letters, vol. 22, No. 6, Mar. 13, 1986, Mortimore et al., "Low loss joints between dissimilar fibers by tapering," etc. pp. 318-319.
Electronics Letters, vol. 23, No. 20, Sep. 24, 1987, Minelly et al., "Single-mode fibre Y-junction beam splitter", pp. 1087-1088.
Ainslie Benjamin J.
Mortimore David B.
Payne David B.
British Telecommunications public limited company
Gonzalez Frank
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