GB2184258A - Wavelength division multiplex fused optical monomode fibre coupler - Google Patents

Abstract

A coupler for use in wavelength division multiplex having a pair of monomode optical fibers (10, 11) with adjacent portions fused together (12) and a biconical taper in the fused portion. The fused portion (12) has a near-circular cross-section such that it has substantially zero birefringence.

Description

SPECIFICATION Optical coupler The present invention concerns single mode couplers and in particular, though not exclusively, Wavelength Division Multiplex (WDM) couplers.

Such a coupler can be manufactured by fusing together two single mode optical fibres, this being done by placing two portions of the fibres in contact, and subjecting the contiguous portions to both heat and elongation. This biconical fusion technique produces couplers with natural wavelength dependence. Such couplers should be highly suitable for low loss and cheap wavelength division in multiplexing/demultiplexing.

In orderfor a couplerto be useful in WDM it hasto be able to provide high wavelength isolation ()30 dB) and maintain itthrough a broad temperature range and for all possible input polarisation states.

The present invention has for an object two provide a coupler which affords high isolation, is capable of maintaining this through a wide temperature range and which is relatively insensitive to the polarisation of input light.

Accordingly the present invention consists in a coupler for use in wavelength division multiplex, the coupler comprising a pairofmonomodeoptical fibres having had adjacent portions fused together, and a biconical taperfabricated in the fused portion, the fused portion having a near-circular cross-section such that it has substantiallyzero birefringence.

In order that the present invention may be more readily understood an embodimentthereofwil I now be described by way of example and with reference to the accompanying drawings, in which Figure lisa refractive index profile across a monomode optical fibre, Figure2 is a diagrammatieviewofa wave coupler utilising two fused monomodefibres, and Figure 3 shows some cross-sections of fused mono-mode optical fibres.

Referring nowto Figure 1 ofthedrawingsthis showsthe refractive index profile of a matched, monomode optical fibre. In orderto fabricate a WDM couplertwo lengths of optical fibre profiles are placed and held in juxtaposition. One fibre may be twisted around the other but this is not normal. The juxtaposed portions are then heated,forexample by an oxy-butaneflame. The heating causes fusion between the two fibres. After a predetermined period the fused fibres are subjected to extrusion so as to generate a biconical taper. The heating period determines the degree effusion between the two fibres.

Whilstthefused fibres are being extended light at selected wavelength is launched down one ofthe fibres and the power transmitted through the fibre monitored. The extension of the fibre causes a biconical taper to be formed in the extended portion, and this taper becomes a multi-mode section. In this now multi-mode section interference of the local LP01 and LP11 modes causes powertransfer along the taper. The taper length could be such that for a particulartapershape all power emerges at the taper end in one fibre orthe other produced during the fabrication of the taper, the actual tapering process can be controlled to give a desired degree of ta per.

In the embodiment being described, the extension of the coupled fibres is stopped after 2 power oscillations have been detected.

The resultant pair of fused fibres are shown in Figure 2 at 10 and 11 The fibres are fused together at the portion generally indicated at 12 which also includes a biconical taper which, as mentioned, has been stopped at two power oscillations during the fabrication process.

When operating as a WDM coupler light at two differentwavelengthsA1 and A2is launched at13 down fibre 10 and the resultant outputs detected at 14 and 15.Thefused portion is surrounded bya suitable potting agent. Forthe couplerto function ideally there should be for example approximately 90% of the A1 light appearing at the output of fibre 10 and thesame proportion of A2 lightatthe outputof fibre 11. This degree of wavelength or channel isolation should be maintained through a broad temperature range, and also should be independent ofthe degrees of polarisation of A1 and A2.

It has been discovered that these factors are difficultto obtain and are in fact largely dependent on the nature of the cross-section of the fused portion ofthetwo optical fibres making upthe coupling.

Referring nowto Figure3a-cthis shows some possible fusion cross-sectional geometrics obtainable with fused couplers ofthe kind shown in Figure 2.

In the structure shown in Figure 3a fusion between fibres 10 and 11 is far from total and the contours of the two fibres are easily discernible. This structure has poor field confinement so that the coupling ratio is very sensitive to the outside index of the potting material . This i n tu rn makesthecouplervery sensitive to temperature variations and these change the refractive index of the potting material.

The other two structures, 3b and 3cshow successively greater degrees effusion between the two fibres. The amount effusion is dependent on the intensity and duration ofthe heat applied. In fact, the circular nature of Figure 3c shows that total fusion has occurred. The structures in Figures3b and 3care progressively less sensitive to index changes of the potting material. However, this sensitivity to refractive index changes of the potting material is not the onlyfactor in the fabrication ofsatisfactory WDM couplers.

All three structures shown in Figure 3 are birefringent in nature. This birefringence comprises offormed birefringence, BF, (due to shape) and stress birefringence, Bs, such thatthetotal birefringence, BTliS: BT= BF+ (-Bs) Stress birefringence usually opposes formed birefringence. The structure in Figure 3a displays a very small total birefringence because the formed birefringence and stress birefringence are very small and comparable.

However, since it is highly sensitive to index changes, it is notsuitablefor making stable WDMs in an environment of changing temperature.

The next structure in Figure 3b is well fused and has very good field confinement and thus it is insensitive to index changes. However, the formed birefringence due to its elliptical shape is much greater than its stress birefringence giving an overall birefringencewhich cannot maintain a constant coupling ratio with different input polarisation states. This makes it unsuitable again for application toWDM.

As the degree effusion from Figure 3b is increased to make the cross-sectional geometry approach a circle then the formed birefringence begins to decrease reaching zero in the structure of Figure 3c.

The only birefringence that remains is that due to stress.

Just before the cross-sectional geometry becomes a circle the formed birefringence has the same value as that ofthe stress birefringence and therefore gives a total birefringence of zero. Since the coupler is well fused it is not very sensitive to the index change of its surrounding potting material and therefore can be operated over a broad temperature range. Its birefringence of zero also means that it can be operated with all possible polarisation input states.

It will be appreciated that the wavelength period of WDM couplers is also related to the fusion cross-section so that if a zero birefringence structure does not exactly have the required channel wavelengths at the maxima and minima of its period, then it has to be adjusted very slightly to make them fall on the maxima and minima. This minor adjustment to morethan zero birefringence only sacrifices an equally small degree of insensitivity to polarisation. It is thus possible to compromise betweenthethreemajorrequirementsofhigh isolation, and maintaining high isolation over a broad temperature range and for all polarisation input states.

Claims (3)

1. A coupler for use in wavelength division multiplex,the coupler comprising a pairof monomode optical fibres having adjacent portions fused together and a biconical taper fabricated in the fused portion, and characterised in thatthe fused portion has a near-circularcross-section and has substantially zero birefringence.

2. Amethod of manufacturing an optical coupler forwavelength division multiplex comprising fusing portions of two monomode optical fibres together by heating the portions and applying tension to the two fibres so as to produce a biconical tapered portion at the point of fusion, and wherein light of two different selected wavelengths is launched down one ofthe fibres, the light emitted is detected at the other ends ofthe two fibres, and the transmitted light is monitored during the fusion process until the formed birefringence in the fused fibres is equal to the stress birefringence and that the cross-section of thefushed portion is substantially circular.

3. A method as claimed in Claim 2 wherein the extension of the fibres being fused is stopped when two power oscillations have been monitored.