GB1596869A - Directional coupler for optical fibre systems - Google Patents

Description

(54) DIRECTIONAL COUPLER FOR OPTICAL FIBRE SYSTEMS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, WC2R 1DU, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to directional couplers for optical fibre communication systems.

Directional couplers are used where it is desired to segregate optical signals transmitted in opposite directions in an optical fibre, e.g. to segregate the transmit and receive signals at a terminal. An optical fibre is a structure consisting of a transparent core having a first refractive index surrounded by a transparent cladding having a second, lower refractive index. There are two types of fibre which may be considered here. One, hereinafter termed a 3-layer fibre, has its core and cladding both made from the same basic material suitably treated to provide the two refractive indices required, e.g. doped and undoped silica, and is further clad in an outer protective cladding, e.g. of plastic. In such a fibre transmission of light normally only occurs in the core. However, if the outer cladding is made of a transparent material having a refractive index even lower than that of the inner cladding then it is possible to launch and transmit light within the inner cladding. If this transmission is in the opposite direction to that in the core the two transmissions are kept substantially separate. The other type of fibre, hereinafter termed a 2-layer fibre, has the core directly covered by an outer protective cladding without any inner cladding. In this case the core and cladding may be of two different materials, e.g. silica for the core and plastics for the cladding which, again, is transparent and has lower refractive index than the core.

According to the present invention there is provided a directional coupler for an optical fibre transmission system, the coupler including a single 3-layer optical fibre from a portion of which the outer cladding has been removed and at least one length of another fibre from a portion of which the outer cladding has been removed, both said portions being of substantially the same material and being laid side-by-side and fused wholly or partly together.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig 1 illustrates the basic arrangement of a directional coupler, Fig. 2 illustrates one form of directional coupler in cross-section, Fig. 3 illustrates a second form of directional coupler in cross-section, and Fig. 4 illustrates a third form of directional coupler in cross-section.

In the coupler of Fig. 1 a 3-layer fibre 10 has its outer protective plastics cladding 11 stripped from a portion 12 of its length. A 2-layer fibre 13 has its outer cladding 14 stripped from a similar portion 15 of its length, e.g. at one end of the fibre. The two stripped portions are then laid side by side and fused together, as indicated by the dotted line 16. Finally the fused lengths are encased in a protective plastics sheath 17.

The cross-section through the line X-X is shown in Fig. 2.

Consider now the transmission of light in the arrangement of Fig. 1. Light is launched in the core 18 of fibre 10 by a narrow stripe laser (not shown) and is transmitted solely within the core, emerging at the far end of the fibre, where it is coupled into a 2-layer fibre, 19 the core 20 of which is considerably larger than that of core 18. Light transmitted in the opposite direction from the far end of fibre 19 is coupled into both the core and the inner cladding 21 of fibre 10, with most of the light entering the inner cladding in proportion to the cross-section areas of the core and inner cladding respectively. This light will travel in the inner cladding until it reaches the fused region when some of the light is coupled into the core of fibre 13. The proportion of light so coupled is dependent on the ratio of the cross-sectional areas of the two fibres in the fused region. If the fibre 13 is larger than the fibre 10 then most of the light from fibre 19 will be transferred to fibre 13.

To enhance this effect the arrangement of Fig. 3 may be adopted. Here the 3-layer fibre 30 is surrounded by a number of larger 2-layer fibres 31, as shown in Fig. 3(a). When they are fused together the ratio of the area of the single 3-layer fibre to the sum of the areas of the 2-layer fibres is considerably increased. Fig. 3(b), as compared with the two fibre arrangement of Fig. 2.

The fibres 31 are themselves fused together after they leave the fibre 30 to provide a single outlet for the light coupled out of the fibre 30. All the preceding examples have been where dissimilar fibres have been fused together. It is also possible to use 3-layer fibres throughout, as shown in Fig. 4 (a). This ensures that if all the fibres are from the same source the materials will have the same thermal and optical characteristics. The fused area then has a cross-section as shown in Fig. 4 (b). There will be little or no light coupled into the cores surrounding the central core so these do not significantly affect the performance of the coupler. Again the surrounding fibres are fused together after they leave the central fibre to provide a single outlet for the coupled out light.

WHAT WE CLAIM IS: 1. A directional coupler for an optical fibre transmission system, the coupler including a single 3-layer optical fibre from a portion of which the outer cladding has been removed and at least one length of another fibre from a portion of which the outer cladding has been removed, both said portions being of substantially the same material and being laid side-by-side and fused wholly or partly together.

2. A coupler according to claim 1 wherein the other fibre is a 2-layer fibre.

3. A coupler according to claim 2 wherein the 3-layer fibre is fused between two or more 2-layer fibres.

4. A coupler according to claim 1 wherein the other fibre is a 3-layer fibre.

5. A coupler according to claim 4 wherein the 3-layer fibre is fused between two or more 3-layer fibres.

6. A coupler according to any preceding claim including an outer protective cladding applied to the fused fibres, the outer cladding being transparent and having a refractive index lower than that of the fused fibres.

7. A directional coupler for an optical fibre transmission system substantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. Fig. 3 may be adopted. Here the 3-layer fibre 30 is surrounded by a number of larger 2-layer fibres 31, as shown in Fig. 3(a). When they are fused together the ratio of the area of the single 3-layer fibre to the sum of the areas of the 2-layer fibres is considerably increased. Fig. 3(b), as compared with the two fibre arrangement of Fig. 2. The fibres 31 are themselves fused together after they leave the fibre 30 to provide a single outlet for the light coupled out of the fibre 30. All the preceding examples have been where dissimilar fibres have been fused together. It is also possible to use 3-layer fibres throughout, as shown in Fig. 4 (a). This ensures that if all the fibres are from the same source the materials will have the same thermal and optical characteristics. The fused area then has a cross-section as shown in Fig. 4 (b). There will be little or no light coupled into the cores surrounding the central core so these do not significantly affect the performance of the coupler. Again the surrounding fibres are fused together after they leave the central fibre to provide a single outlet for the coupled out light. WHAT WE CLAIM IS:

1. A directional coupler for an optical fibre transmission system, the coupler including a single 3-layer optical fibre from a portion of which the outer cladding has been removed and at least one length of another fibre from a portion of which the outer cladding has been removed, both said portions being of substantially the same material and being laid side-by-side and fused wholly or partly together.

2. A coupler according to claim 1 wherein the other fibre is a 2-layer fibre.

3. A coupler according to claim 2 wherein the 3-layer fibre is fused between two or more 2-layer fibres.

4. A coupler according to claim 1 wherein the other fibre is a 3-layer fibre.

5. A coupler according to claim 4 wherein the 3-layer fibre is fused between two or more 3-layer fibres.

6. A coupler according to any preceding claim including an outer protective cladding applied to the fused fibres, the outer cladding being transparent and having a refractive index lower than that of the fused fibres.

7. A directional coupler for an optical fibre transmission system substantially as described with reference to the accompanying drawings.