BOUCOUVALAS ANTHONY CHRISTOS (GB)
IEE Proceedings section A-I, Volume 132, No. 5, part J, October 1985, (Stevenage, Herts., GB), G. GEORGIOU et al.: "Low Loss Single-Mode Optical Couplers", pages 297-302, see figures 3,8,12; Abstract; page 298, left-hand column, lines 30-47; page 299, right-hand column, lines 1-4; page 300, right-hand column, lines 28-30; page 301, left-hand column, lines 1-28; page 302, right-hand column, lines 1-3
Electronics Letters, Volume 20, No. 23, 8 November 1984, (Hitchin, Herts., GB), C.M. LAWSON et al.: "In-line Single-Mode Wavelength Division Multiplexer-Demultiplexer", page 963, see page 963, left-hand column, lines 58-65 and right-hand column, lines 1-19, 40-46
Electronics Letters, Volume 21, No. 6, 14 March 1985, (Hitchin, Herts., GB), M.S. YATAKI et al.: "All Fibre Polarising Beam Sputter", pages 249-250, see Abstract; page 249, right-hand column, lines 33-38; page 250, left-hand column, lines 1-8, 35-52; figures 1,2
| 1. | A coupler for use in wavelength division multiplex, the coupler comprising a pair of monomode optical fibres having adjacent portions fused together and a biconical taper fabricated in the fused portion, and characterised in that the fused portion has a nearcircular crosssection and has substantially zero birefringence. |
| 2. | A method of manufacturing an optical coupler for wavelength 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 of the fi res, the light emitted is detected at the other ends of the 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 crosssection of the fused 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. |
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 order for a coupler to be useful in WDM it has to be able to provide high wavelength isolation (>30 dB) and maintain it through a broad temperature range and for all possible input polarisation states.
The present invention has for an object to 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 pair of monomode optical fibres having had adjacent portions fused together, and a biconical taper fabricated in the fused portion, the fused portion
having a near-circular cross-section such that it has substantially zero birefringence.
In order that the present invention may be more readily understood an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which
Figure 1 is a refractive index profile across a monomode optical fibre,
Figure 2 is a diagrammatic view of a wave coupler utilising two fused monomode fibres, and
Figure 3 shows some cross-sections of fused mono- mode optical fibres.
Referring now to Figure 1 of the drawings this shows the refractive index profile of a matched, monomode optical fibre. In order to fabricate a WDM coupler two 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, for example by an oxy-butane flame. 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 of fusion between the two fibres. Whilst the fused fibres are being extended light at selected wavelength is launched down one of the 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 LP ni and LP-.. modes causes power transfer along the taper. The taper length could be such that for a particular taper shape all power emerges at the taper end in one fibre or the other." '"
produced during the fabrication of the taper, the actual tapering process can be controlled to give a desired degree of taper. 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 different wavelengths λ 1 and Λ 2 is launched at 13 down fibre 10 and the resultant outputs detected at 14 and 15. The fused portion is surrounded by a suitable potting agent. For the coupler to function ideally there should be for example approximately 90% of the A- light appearing at the output of fibre 10 and the same proportion of ? light at the output of fibre 11. This deg ' ree of wavelength or channel isolation should be maintained through a broad temperature range, and also should be independent of the degrees of polarisation of A 1 and Λ . It has been discovered that these factors are difficult to obtain and are in fact largely dependent on the nature of the cross-section of the fused portion of the two optical fibres making up the coupling.
Referring now to Figure 3 a-c this shows some possible fusion cross-sectional geometries obtainable with fused couplers of the 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
-it- very sensitive to the outside index of the potting material. This in turn makes the coupler very sensitive to temperature variations and these change the refractive index of the potting material. The other two structures, 3_b and 3£ show successively greater degrees of fusion between the two fibres. The amount of fusion is dependent on the intensit and duration of the heat applied. In fact, the circular nature of Figure 3£ shows that total fusion has occurred. The structures in Figures 3b_ and 3 are progressively less sensitive to index changes of the potting material. However, this sensitivity to refractive index changes of the potting material is not the only factor in the fabrication of satisfactory WDM couplers. All three structures shown in Figure 3 are birefringent in nature. This birefringence comprises of formed birefringence, B„, (due to shape) * and stress birefringence, B„, such that the total birefringence, B τ , is: B τ = B F + (-B s )
Stress birefringence usually opposes formed birefringence. The structure in Figure 3a_ displays a very small total birefringence because the formed bire¬ fringence and stress birefringence are very small and comparable.
However, since it is highly sensitive to index changes, it is not suitable for 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 bire¬ fringence due to its elliptical shape is much greater than its stress biref ingence giving an overall bire¬ fringence which cannot maintain a constant coupling ratio with different input polarisation states. This makes it unsuitable again for application to WDM.
As the degree of fusion 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 of the 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, fehen it has to be adjusted very slightly to make them fall on the maxima and minima. This minor adjustment to more than zero birefringence only sacrifices an equally small degree of insensitivity to polarisation. It is thus possible to compromise between the three major requirements of high isolation, and maintaining high isolation over a broad temperature range and for all polarisation input states.