GB1590082A

GB1590082A - Directional y coupler for optical fibre

Info

Publication number: GB1590082A
Application number: GB25199/78A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-05-28
Also published as: FR2427624B1; FR2427624A1

Description

(54) DIRECTIONAL Y COUPLER FOR OPTICAL FIBRE (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company, of 190 Strand, London W.C.2. 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 fibres such as are used in telecommunication systems.

In such systems there is often a requirement that light transmitted in one direction be coupled out of a fibre whilst at the same time light transmitted in the other direction is not coupled out.

According to the present invention there is provided a directional coupler for an optical fibre system comprising a length of 3-layer optical fibre, as hereinafter defined, from a portion of which the outer protective cladding is stripped, said portion being bent away from the axis of the remainder and embedded in a body of material having a refractive index substantially equal to or greater than that of the inner cladding, said body extending to at least the point at which the stripped portion emerges from the unstripped portion, and means for focussing to an external point light coupled out of the embedded fibre into the body.

A 3-layer optical fibre is defined as one having a core of transparent material of a first refractive index, an inner cladding of transparent material of a second, lower refractive index and an outer protective cladding. A suitable 3-layer optical fibre is one having a doped silica core of diameter d, = 20,u, with an inner cladding of undoped silica of diameter d2 = 200cm and an outer protective cladding of a lower refractive index material such as silicone resin. Such a fibre is illustrated in cross-section in Fig. 1 and its refractive index profile is illustrated in Fig. 2 of the accompanying drawings. Light launched solely into the core of such a 3-layer fibre is constrained within the core, even when the fibre is bent in a curve. Light launched in both the core and the inner cladding is constrained in both of them, even if the fibre is bent in a curve.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 3 illustrates a directional coupler according to the invention, and Fig. 4 illustrates a transversal equaliser incorporating a coupler of the type illustrated in Fig. 3.

The directional coupler illustrated in Fig. 3 consists of a length of 3-layer optical fibre from which the outer protective cladding 1 has been stripped for a portion of its length, leaving the core 2 and inner cladding 3. The stripped portion is bent away from the axis of the remainder and the curved part is then embedded in a body 4 of transparent material having a refractive index equal to or greater than that of the cladding 3. The body 4 should cover the fibre right up to the point at which the outer protective cladding 1 ceases, and is shaped so that a lens 5 can be attached to it, the optical axis of the lens being colinear with that of the fibre where the stripped portion emerges from the unstripped portion. A suitable material for the body 4 is a clear epoxy resin, or it may be a liquid such as glycerine suitably encapsulated.

Consider now the transmission of light in such a device. Firstly, if light is launched into the end of the fibre from a narrow semiconductor laser 6, whereby all the light is launched into the core 2, then all the light is constrained within the core by the lower refractive index material of the inner cladding 3. This light will not escape from the fibre where the latter is bent, provided of course that the radius of curvature of the bent part is not less than a critical value. This light can therefore be coupled into another fibre having a core 7 of diamter d3, where d3 is either larger or smaller than the diameter dl of the core 2. If d3 is greater than d1 the efficiency of the coupling, ignoring reflection losses, is launch = 100% where u is efficiency.

Conversely, light received in the opposite direction from the other fibre 7 can be coupled into both the core 2 and the inner cladding 3 of the 3-layer fibre. Again, ignoring reflection losses, the efficiency of the coupling will be as follows:

71receive = d 2 d 2 if d2 > d3 dsL or receive = d 2 d 2 if d3 > d2 d3 Because the body 4 has a refractive index at least equal to if not greater than that of the cladding 3 substantially all of the light in the inner cladding 3 can escape into the body 4 at the cessation of the outer cladding 1. This light can be focussed by the lens 5 onto a photodetector 8. If the refractive index of the body 4 is greater than that of the inner cladding 3 the escaping light will come out in the form of a hollow cone in the body 4.

The directional coupler of Fig. 3 can form part of an optical transversal equaliser for low mode fibres as shown in Fig. 4. The refractive index of the body 4 is chosen to be between those of the core and the inner cladding, thus coupling out only the higher modes. The remaining light is taken via a length 10 of the 3-layer fibre, which acts as a delay line. The light issuing from the end of the fibre 10 is combined at the photodetector 8 with the light coupled out of the fibre by the coupler, thus equalising the longer delay of the higher order modes in the fibre. This is only a single element transversal equaliser but successive stages could be used with increasing values of ,.

WHAT WE CLAIM IS: 1. A directional coupler for an optical fibre telecommunication system comprising a length of 3-layer optical fibre, as hereinbefore defined, from a portion of which the outer protective cladding is stripped, said portion being bent away from the axis of the remainder and embedded in a body of material having a refractive index substantially equal to or greater than that of the inner cladding, said body extending to at least the point at which the stripped portion emerges from the unstripped portion, and means for focussing to an external point light coupled out of the embedded fibre into the body.

2. A directional coupler according to claim 1 wherein the portion of fibre from which the outer protective cladding has been stripped is bent through substantially 90" and the means for focussing comprises a lens affixed to the body of material, the optical axis of the lens being colinear with the axis of the fibre where the stipped portion emerges from the unstripped portion.

3. A directional coupler for an optical fibre telecommunication system substantially as described with reference to Fig. 3 of the accompanying drawings.

4. A transversal equaliser for an optical fibre telecommunication system including a directional coupler as claimed in any preceding claim and a photodetector device upon which light coupled out of the fibre by the coupler is focussed, the fibre extending beyond the body of material whereby light is emitted from the end of the fibre out to form an optical delay line the output of which is combined at the photodetector device with the focussed light from the coupler.

5. A transversal equaliser for an optical fibre telecommunication system as claimed in claim 4 and substantially as described with reference to Fig. 4 of the accompanying drawings.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. fibre having a core 7 of diamter d3, where d3 is either larger or smaller than the diameter dl of the core 2. If d3 is greater than d1 the efficiency of the coupling, ignoring reflection losses, is launch = 100% where u is efficiency. Conversely, light received in the opposite direction from the other fibre 7 can be coupled into both the core 2 and the inner cladding 3 of the 3-layer fibre. Again, ignoring reflection losses, the efficiency of the coupling will be as follows: 71receive = d 2 d 2 if d2 > d3 dsL or receive = d 2 d 2 if d3 > d2 d3 Because the body 4 has a refractive index at least equal to if not greater than that of the cladding 3 substantially all of the light in the inner cladding 3 can escape into the body 4 at the cessation of the outer cladding 1. This light can be focussed by the lens 5 onto a photodetector 8. If the refractive index of the body 4 is greater than that of the inner cladding 3 the escaping light will come out in the form of a hollow cone in the body 4. The directional coupler of Fig. 3 can form part of an optical transversal equaliser for low mode fibres as shown in Fig. 4. The refractive index of the body 4 is chosen to be between those of the core and the inner cladding, thus coupling out only the higher modes. The remaining light is taken via a length 10 of the 3-layer fibre, which acts as a delay line. The light issuing from the end of the fibre 10 is combined at the photodetector 8 with the light coupled out of the fibre by the coupler, thus equalising the longer delay of the higher order modes in the fibre. This is only a single element transversal equaliser but successive stages could be used with increasing values of ,. WHAT WE CLAIM IS:

1. A directional coupler for an optical fibre telecommunication system comprising a length of 3-layer optical fibre, as hereinbefore defined, from a portion of which the outer protective cladding is stripped, said portion being bent away from the axis of the remainder and embedded in a body of material having a refractive index substantially equal to or greater than that of the inner cladding, said body extending to at least the point at which the stripped portion emerges from the unstripped portion, and means for focussing to an external point light coupled out of the embedded fibre into the body.