Abstract:
A fibre optic connector of the watch jewel type is provided with a transparent window adjacent the fibre end whereby the fibre is protected from dirt or damage. The window may be provided with a light guiding portion in alignment with the fibre.

Description:
This is a continuation of application Ser. No. 057,761, filed July 16, 1979, now adandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to optical fibre transmission systems and in particular to methods and arrangements for terminating the coupling fibres. 
     Optical fibres, e.g. of the plastics coated silica type, are frequently terminated and coupled by connectors of the watch jewel type. In such a connector an accurately drilled watch jewel receives and aligns the fibre for coupling to a similarly terminated fibre. A frequently encountered problem with this type of connector is the tendency of the fibre end to &#34;grow out&#34; of the jewel bore with the consequent risk of damage to the fibre. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to minimize or to overcome the above noted disadvantage. 
     According to this invention there is provided a termination for a plastics coated optical fibre, the fibre having a bared end portion, the termination including a body having a cylindrical outer reference surface, a coaxial inner bore for receiving the coated fibre, alignment means for locating the fibre end coaxial with the inner bore, and a transparent flat protection window arranged adjacent and normal to the fibre end. 
     The window causes an incremental loss due to separation of the fibre ends in a mated pair of ferrules. This loss however is acceptably low for large core small numerical aperture fibres. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of this invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 is a cross section of an optical fibre connector member provided with a protective window; 
     FIG. 2 is a cross section of a modified connector member provided with additional alignment means; and 
     FIG. 3 shows an alignment device for use, e.g. in the connector member of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the connector member 10 includes a cylindrical tubular ferrule 11 into one end of which a fibre alignment device, e.g. a bored watch jewel 12, is inserted. The bore 13 of the jewel 12 is accurately concentric with the circumference of the jewel and thus aligns an inserted fibre concentric with the outer surface of the ferrule 11. A thin flat transparent plastics glass or silica window 14 is affixed to the end of the ferrule adjacent the jewel 12 such that the plane of the window is perpendicular to the axis of the jewel bore and the assembly is then vacuum filled with a fluid whose refractive index is matched to that of a fibre to which the connector means is to be fitted. In some applications the window 14, of glass or silica, is of such a diameter to permit insertion into the end of the ferrule 11 where it is secured against the face of the jewel 12 by an adhesive or a ring of fused glass frit. 
     A plastics coated glass or silica optical fibre 15 to which the connector member is to be fitted is prepared by stripping back the plastics coating 16 to expose the end of the core fibre 17. The fibre 15 is inserted into the ferrule 11 such that the fibre and 17 enters and is located by the jewel bore 13 and the ferrule is then secured to the plastics coating 16 of the fibre e.g. by crimping the ferrule. 
     The flat window 14 prevents the fibre end from growing out and protects the fibre end from damage while the outer surface of the ferrule 11 provides a reference surface for mating with a similar connector member. 
     In a particular advantageous arrangement the center portion of the window 14 may comprise a light guiding relatively high refractive index portion 18, the remainder 19 of the window being of lower refractive index. The window may comprise a slice cut from a glass preform, the nominal aperture of which is equal to or greater than the nominal aperture of the fibre to which the connector is fitted. The preform can be made using either the rod-in-tube method or chemical vapor deposition techniques. 
     The connector member shown in FIG. 2 provides an auxiliary alignment which aids insertion of the fibre to which it is fitted. In this arrangement a tubular ferrule 21 is provided with a coaxial rear extension tube 22 which tube is crimped on to a plastics coated fibre 23 to which the connector member is fitted, thereby anchoring the connector member to the fibre. 
     First drilled watch jewel 24 and second drilled watch jewel or similar 25 are fitted into the forward &#34;plug in&#34; end of the ferrule and are enclosed by a thin flat window 26 fitted into or over the ferrule end. The interior of the ferrule is vacuum filled with a refractive index matching fluid 27. 
     A plastic coated glass or silica optical fibre 23 to which the connector member is to be fitted is prepared as previously described by stripping back a portion of the coating to expose the fibre end and is then inserted through the rearward extension tube 22 and along the ferrule until the fibre end is located in the bore of each jewel, the two jewel arrangements providing very accurate alignment of the fibre. The connector is then anchored to the fibre by crimping the extension tube 22 on to the fibre. 
     FIG. 3 shows a combined alignment and window device which may be machined or molded from a transparent plastics (or glass) material. The device comprises a cylindrical plastics body 31 having an accurately concentric axial bore 32 for receiving and locating the bared end of a plastic coated optical fibre. One end of the bore 2 is closed by a thin layer 33 of the plastics material which acts as a window for light directed along the bore 32. The other end of the bore 32 communicates with an enlarged coaxial bore 34 of a diameter suitable for accommodating the plastics coating of a fibre. In use, the device is mounted in a tubular ferrule, the outer surface of which provides a reference surface for mating with a similar arrangement. 
     In the arrangements of FIG. 2 and FIG. 3 the center of the window may comprise a light guiding portion as described with reference to the arrangement of FIG. 1. 
     Whereas this invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art, and it is intended to encompass such searches and modifications as are within the scope of the appended claims.