GB2206705A

GB2206705A - Non-intrusive optical tap

Info

Publication number: GB2206705A
Application number: GB08716361A
Authority: GB
Inventors: Melvin Murray Ramsay; John Lees
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1987-07-10
Filing date: 1987-07-10
Publication date: 1989-01-11
Anticipated expiration: 2007-07-10
Also published as: GB8716361D0; GB2206705B

Abstract

A non-intrusive optical tap comprises a mating pair of transparent plastics members (11a, 11b) of substantially wedge- shaped cross-section and together defining a cylindrical cavity in which the fibre (13) is located. The tap applies pressure to the fibre 13 to induce microbending whereby coupling between bound and radiating modes is effected. This causes leakage of light from the fibre into the tap. The tap is inserted through a split in the sheath of a cable carrying the fibre and is self aligned to the fibre. <IMAGE>

Description

OPTICAL TAP This invention relates to an optical tap for providing non-intrusive coupling between an optical fibre and e.g. a terminal station.

Fibre optic transmission systems are finding increasing use for the handling of large volumes of data or voice traffic. The capability of optical systems to handle such large volumes of traffic is of particular value in applications such as the connection of a plurality of data terminals to a computer or the interconnection of a plurality of active systems. The full potential of optical transmission systems in such applications has however not yet been exploited.

Conventional systems comprise a number of stations to which optical cables are connected. These systems cannot be expanded or modified without considerable recalling of the optical connections. In an attempt to overcome this disadvantage it has been suggested that, e.g. in an office environment, a system of permanent optical 'wiring' may be provided and that terminal stations may be coupled at will to various points of this wiring system. However, difficulties have been experienced in coupling the terminal stations to the optical fibre systems. Present couplers are both costly and involve intrusion into (i.e. cutting and reconnecting) the optical fibre system.

The object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a non-intrusive tap for coupling to an optical fibre, the tap having means for providing optical alignment with the fibre, and means for inducing local microbending of the fibre whereby coupling between bound and radiating modes in the fibre is effected so as to provide optical coupling between the fibre and the tap.

According to a further aspect of the invention there is provided a non-intrusive tap for coupling to an optical fibre, the tap including two mating similarly shaped members having means for locating a fibre therebetween in optical alignment with the tap, said locating means having a surface in contact with the fibre when in use and so profiled as to induce local microbending of the fibre whereby coupling between bound and radiating modes in the fibre is effected so as to provide optical coupling between the fibre and the tap.

An embodiment of the invention will now be described with reference to the accompanying drawings in which Fig. 1 is a cross-sectional view of a non-intrusive optical tap; Fig. 2 shows in detail the fibre engaging portion of the tap of Fig. 1; Fig. 3 is a longitudinal view of the tap of Fig. 1; and Fig. 4 shows, in cross-sectional view, the tap of Figs. 1 and 2 fitted to an optical fibre cable.

Referring to Figs. 1 and 2, the non-intrusive tap 10 includes a mating pair of similarly shaped transparent members 11, lla typically formed from polymethylmethacrylate (Perspex), and each of generally triangular cross section. Each member has a longitudinal channel 12 of semicircular cross-section and of such dimensions that, when the two members 11, lla are mated together, the fibre 13 of a cable to which the tap is applied, is a snug fit within the cylindrical cavity found by the two semi-circular channels, Each channel 12 has a number of radial protrusions or corrugations 14 which, when the tap is in position around the fibre 13, exert pressure on the fibre and cause microbending. This induces coupling between board and radiating modes within the fibre 13 resulting in optical coupling between the fibre 13 and the tap.The tap has a refractive index, e.g. about 1.5, similar to that of the acrylate coating commonly applied to silicon optical fibres. Under these conditions light tapped from the fibre 13 is confined to a cone where semi-angle is about 150. The semicircular channels 12 are so positioned with respect to the members 11, lla that, when the tap is in position with the fibre 13 located in the cylindrical cavity defined by the channel 12, optimum coupling between the fibre and tap is provided. The channels 13 thus provide a means of self alignment between the tap and the fibre.

Advantageously the faces 15a, 15b and 17 of the tap are metallised to enhance their reflectivity. Also the structure may be provided with a protective surface coating 16 of a low refractive index material. Typically we employ fluorinated ethylene propylene (FEP) for this purpose. This material has a refractive index of about 1.34.

The tap 10 is shown in longitudinal sectional view in Fig. 3 and cross-sectional view in Fig. 4 as fitted to an optical cable 31. As can be seen from Fig.

3 the tap assembly has a coupling portion 32 that fits around the fibre 13 of cable 31 and provides coupling thereto, and a portion 33 of truncated cross section whereby light coupled to and from the fibre 13 is transmitted via an output port 18 to and from a terminal station (not shown). The cable 31 (Fig. 4) comprises an outer sheath 32 around which a number of strength members 33 are disposed and within which the optical fibre 13 is supported in a filling 34 of foamed elastomer. In the region of the tapping point the cable is split radially to expose a fibre core. To facilitate insertion of the tap the cable 31 may be manufactured with a longitudinal split as is described in our copending application No.

(M.M. Ramsay - J. Titchmarsh 33-10) of even date.

Under appropriate conditions of wavelength and amplitude, the introduction of microbenderturbation to the core of a single mode fibre induces coupling between the bound and first radiating modes. The radiating energy spreads through the fibre cladding in a cone whose axis lies on the fibre axis. Using the construction shown in Figs. 1 and 2, approximately half of the acrylate coating of the fibre 31 is in contact with polymethylmethacrylate (perspex) whose refractive index closely matches that of the acrylate coating. The light in this part of the radiating cone therefore passes directly into the tap. The remainder of the fibre 31 is in contact with the low refractive index coating of the tap.Because of the small angle of incidence and the large difference in refractive indices, light in this region is totally internally reflected back into the fibre.

Preferably the tap surface 15 is cylindrical, the semicircular channels 12 being coaxial with the axis of this cylinder. Thus, when the fibre 13 is located in the cylindrical cavity defined by the channels 12, it is automatically aligned along the axis of the cylindrical surface 17. Light radiated from the fibre into the tap is thus reflected from this surface as a substantially parallel beam. As the angles of incidence involved are small there is little deviation from a truly parallel beam. This parallel beam is transmitted to a terminal station (not shown) via output port 18 (Fig. 3) of the tap. In some applications a lens system may be employed between the terminal and the tap, or the output port of the tap may be provided with a lens surface.

As the tap may be applied to any part of the cable and may be removed after use, it is of particular application in systems where a plurality of terminals are to be coupled to a previously installed cable network.

Furthermore, as the tap provides automatic alignment with the fibre, it may be inserted in a cable by relatively unskilled persons and without the use of costly equipment.

Claims

CLAIMS:

1. A non-intrusive tap for coupling to an optical fibre, the tap having means for providing optical alignment with the fibre, and means for inducing local microbending of the fibre whereby coupling between bound and radiating modes in the fibre is effected so as to provide optical coupling between the fibre and the tap.

2. A non-intrusive tap for coupling to an optical fibre, the tap including two mating similarly shaped members having means for locating a fibre therebetween in optical alignment with the tap, said locating means having a surface in contact with the fibre when in use and so profiled as to induce local microbending of the fibre whereby coupling between bound and radiating modes in the fibre is effected so as to provide optical coupling between the fibre and the tap.

3. A non-intrusive tap as claimed in claim 1 or 2 and comprising a plastics body of similar refractive index to that of the optical fibre.

4. A non-intrusive tap as claimed in claim 3, wherein said plastics bo is coated with a plastics material of refractive index significantly lower than that of the plastics body.

5. A non-intrusive tap as claimed in claim 3 or , wherein said body is substantially wedge-shaped in cross section.

6. A. non-intrusive optical tap substantially as described herein with reference to and as shown in the accompanying drawings.

7. An optical transmission system incorporating a plurality of optical taps as claimed in any one of the preceding claims.