Abstract:
The present invention relates to an optical fiber connector, and in particular to a Subscriber Connector (SC)-type push/pull optical fiber connector and to a method of forming and using such a connector. The SC-type optical fiber connector ( 101 ) is made up of an optical fiber ( 8 ), a cylindrical ferrule ( 14 ), a ferrule holder ( 16 ), a spring biasing means ( 17 ), a ferrule holder carrier ( 50 ), an inner housing ( 120 ), and an outer housing ( 40 ). The optical fiber ( 8 ) is held within the ferrule ( 14 ) and the ferrule ( 14 ) is held by the ferrule holder ( 16 ). The ferrule holder ( 16 ) is engaged within a receiving portion ( 54 ) of the ferrule holder carrier ( 50 ), said holder and carrier being relatively moveable with respect to each other along the ferrule axis ( 5 ) between limits defined by an interaction between the ferrule carrier ( 50 ) and the ferrule holder ( 16 ).

Description:
BACKGROUND 
     a. Field of the Invention 
     The present invention relates to an optical fibre connector, for example an SC-style connector, and to a method of forming and using such a connector. 
     b. Related Art 
     The mechanical tolerances involved in terminating single mode optical fibre are much tighter than those for multimode optical fibre. Therefore, while it is quite common for multimode optical fibre be terminated at the point of use, for example at a user&#39;s premises or at an outside junction box, in most product applications, single mode optical fibre is not terminated in the field. When single mode fibre must be terminated in the field, then it can take a skilled technician between about 15 to 20 minutes to splice fibres together either by using a V-groove clamp or expensive fusion welding equipment. 
     Single mode fibre is therefore often provided in a range of different lengths, pre-terminated at both ends with a connector plug ready to plug into a matching receptacle. Commonly, eight or twelve single mode optical fibres may be bundled together in an optical fibre cable having an outer protective tube inside of which the optical fibres run. 
     An example of such a connector is the “Subscriber Connector”, or SC connector, originally developed by NTT®. SC connectors have convenient push/pull style mating, and are approximately square in cross-section and with a 2.5 mm diameter ferule at the termination of the optical fibre, surrounded by a plastic housing for protection. SC connectors are available in single or duplex configurations. The SC connector latches into a matching socket in a simple push motion. The push-pull design includes a spring against which the ferrule slides within a plastic inner housing. This arrangement provides a reliable contact pressure at the ferrule end and resists fibre end face contact damage of the optical fibre during connection. The connector can be quickly disconnected by first pulling back an outer housing, which is slidable relative to the inner housing, to disengage a latch inside socket between the socket and the inner housing, before pulling the optical fibre connector from the socket. Until the latch is thus disengaged, the latch prevents withdrawal of the connector when the optical fibre cable is pulled in a direction away from the socket. 
     A different type of connector is the ST type connector has a key that prevents rotation of the ceramic ferrule and which connects with a socket using a bayonet lock with an outer shell. All such bayonet type optical fibre connectors are referred to herein for convenience as “ST-type” optical fibre connectors. 
     An advantage of the SC connector over this type of bayonet connector is that the connector sockets can be more closely packed, as there is no need to get fingers fully around the connector to disengage the connector from the socket. 
     Other examples of push/pull type connectors are LC connectors or MU connectors. Often, the fibre-end is angled to reduce back reflections and this is usually described by adding APC (Angled Physical Contact) to the name. All such push/pull type optical fibre connectors are for convenience referred to herein as “SC-type” optical fibre connectors. SC-type LC or MU connectors are also known as small form factor connectors, by virtue of having a 1.5 mm diameter ferrule and a plastic housing. 
     It is important to avoid bending optical fibre around curves having too sharp a bend radius, as this will increase optical losses and can permanently damage the optical fibre. Optical fibre cables are therefore often routed inside a protective outer tube or conduit, which can have minimum bend-limiting properties. Protective bend limiting conduits normally have an outer diameter of 8 mm or 10 mm tubes. The cross-section of a standard SC connector has dimensions of about 7 mm×9 mm, and even a small form factor SC connector is too large to fit inside the inner diameter of a typical protective conduit. 
     In one product application involving the provision of a single mode optical connection all the way to a service subscriber, the protective conduit may run for tens or hundreds of meters from a residential home to a curb-side connection box. While it is relatively straight forward to blow or push a non-terminated length of optical fibre cable down the conduit, it is not possible to do this with cable pre-terminated with SC-type connectors owing to the mismatch in the size of the connector with the inner diameter of the conduit. 
     While, in principle, it would be possible to provide a length of optical cable pre-terminated at one end only, and then after blowing the cable down the conduit either splice or fuse the optical fibre to a terminated optical fibre stub, this adds significant time and cost to the job of providing a single mode optical fibre connection, particularly in product applications such as the provision of single mode optical fibre connection to a user&#39;s home. 
     It is an objection of the present invention to provide a more convenient optical fibre connector and optical fibre communication system and method for providing such a system. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided an optical fibre connector sub-assembly for an optical fibre connector, comprising an optical fibre, a cylindrical ferrule, a ferrule holder, a spring biasing means, and a ferrule holder carrier; wherein:
         the optical fibre is held within the ferrule along an axis of the ferrule, the optical fibre being terminated at an end face of the ferrule;   the ferrule is held by the ferrule holder;   the ferrule holder is engaged within a receiving portion of the ferrule holder carrier, said holder and carrier being relatively moveable with respect to each other along the ferrule axis between limits defined by an interaction between the ferrule holder carrier and the ferrule holder, a portion of the ferrule leading to said end face of the ferrule projecting along the ferrule axis away from the ferrule holder carrier;   the spring biasing means is provided between the ferrule holder and the ferrule holder carrier so that the projecting portion of the ferrule is spring biased along the ferrule axis away from the ferrule holder carrier;   the ferrule holder includes a base in which the ferrule is seated;   the optical fibre base has a plurality of crenellations that extend circumferentially around the ferrule, the ferrule holder carrier having at least two separate arms that extend in an axial direction through the crenellations towards the ferrule, said arms including a movement limitation feature that interacts with the ferrule holder base to limit the relative movement of the ferrule holder and the ferrule holder carrier along one direction of movement along the ferrule axis.       

     The optical fibre connector sub-assembly may be suitable for inclusion in any type of optical fibre connection, for example, an ST-type optical fibre connector. In preferred embodiments of the invention the optical fibre connector sub-assembly is for an SC-type optical fibre connector. 
     In a preferred embodiment of the invention, the movement limitation feature is one or more projecting fingers at an end of the arms. The arms may terminate with circumferentially extending features that engage with the base to limit the relative movement along the ferrule axis of the ferrule holder away from the receiving portion of the ferrule holder. The movement limitation feature may be one or more projecting fingers that extend in a circumferential direction relative to the axis of the ferrule. 
     Preferably, the movement limitation feature engages with a side of the crenellations nearest the end face of the ferrule. The crenellations may have alternating radially high and low regions, the movement limitation feature engaging with a radially high region of the crenellations. 
     The spring biasing means is preferably a coil spring provided between the ferrule holder base and the ferrule holder receiving portion, one end of said spring coil being engaged with a side of the crenellations furthest the end face of the ferrule. 
     Most preferably, the outermost surfaces of the ferrule holder carrier and ferrule holder, relative to the ferrule axis, lie on a common cylindrical envelope. This help to minimise the size of the sub-assembly in a radial direction, making it easier to insert the sub-assembly down a conduit or to pass the sub-assembly through a restricted space, prior to completing the assembly to create an SC-type optical fibre connector assembly. 
     The ferrule holder may include a base having a socket in which the ferrule is seated, and a hollow stem that projects axially from the base in a direction away from said projecting portion of the ferrule. The ferrule holder carrier receiving portion then extends around at least a portion of the stem and includes a feature that interacts with the ferrule holder base to limit the relative movement of the ferrule holder and the ferrule holder carrier along one direction of movement along the ferrule axis. 
     The optical fibre will normally lead to the ferrule through the stem. 
     The spring biasing means may be provided between the ferrule holder base and the ferrule holder receiving portion. 
     The spring biasing means is preferably a coil spring that is concentrically seated around the stem of the ferrule holder. 
     The optical fibre base may have a plurality of crenellations that extend circumferentially around the ferrule. The ferrule holder carrier can have at least two arms that slideably engage with the crenellations. 
     The arms may terminate with circumferentially extending features that engage with the base to limit the relative movement along the ferrule axis of the ferrule holder away from the receiving portion of the ferrule holder. 
     The ferrule holder and ferrule holder carrier may be adapted to be assembled by a press-fitting operation in which the ferrule holder moves along the ferrule axis into the receiving portion of the ferrule holder. 
     The optical fibre will usually include a length of optical fibre extending away from the ferrule, the ferrule holder and the ferrule holder carrier. This length can be surrounded by a material for protection and/or reinforcement, in which case the ferrule holder carrier advantageously includes means for attaching said cladding to this material. 
     Also according to the invention, there is provided a Subscriber Connection (SC)-type optical fibre connector comprising an optical fibre, an optical fibre connector sub-assembly and assembled about the sub-assembly an inner and an outer housing, the inner and outer housings when assembled about the sub-assembly serving to provide SC-style push/pull engagement/disengagement with a mating optical fibre socket, the sub-assembly comprising:
         a ferrule having an end face at which said optical fibre is terminated;   a ferrule holder, the ferrule being held within the ferrule holder and extending away from the ferrule holder towards said end face;   a ferrule holder carrier, the ferrule holder being held within the ferrule holder carrier and said holder and carrier being relatively moveable with respect to each other along the ferrule axis between limits defined by an interaction between the ferrule holder carrier and the ferrule holder;   a spring biasing means acting between the ferrule holder carrier and the ferrule holder and biasing the ferrule holder along the ferrule axis towards one of said limits;
 
wherein:
   the sub-assembly is mechanically integral prior to assembly of the inner and outer housing about the sub-assembly; and   radially outermost surfaces of the sub-assembly define a cylindrical envelope of the sub-assembly, the ferrule holder carrier having a groove in said envelope and the inner housing having a radially inwardly projecting feature for locating with said groove when the inner housing is assembled to the sub-assembly       

     This SC-type optical fibre connector may include an optical fibre connector sub-assembly according to the invention. 
     The sub-assembly may be mechanically integral in the sense of being a physically complete unit, or a whole, which is secure enough to be inserted inside a length of conduit, for example being blown or pushed down a length of bend-limiting conduit, without coming apart in the process. The sub-assembly can be formed with an outer diameter or other maximum external cross-sectional dimension significantly smaller than a corresponding dimension of the completed SC-type optical fibre connector, and can therefore be small enough to fit inside a wide range of commercially available conduits, for example 8 mm or 10 mm external diameter bend-limiting conduits. Then after insertion of the sub-assembly down the length of conduit, the inner and outer housings can be assembled about the sub-assembly to complete the SC-type optical fibre connector. 
     In a preferred embodiment of the invention, the inner housing and then the outer housing in turn are inserted over the projecting ferrule and sub-assembly by pres-fitting along an axial direction. 
     The invention additionally provides a Subscriber Connection (SC)-type optical fibre connector comprising an optical fibre connector sub-assembly, an inner housing, and an outer housing, the connector sub-assembly being according to the invention, wherein:
         the inner housing is fixedly connected to the ferrule holder carrier, the inner housing comprising a portion which coaxially extends around said projecting portion of the ferrule to define a receptacle for a socket to receive the projecting ferrule portion;   the spring biasing means is provided between the ferrule holder and the ferrule holder carrier so that the projecting portion of the ferrule is spring biased along the ferrule axis towards the receptacle portion of the inner housing; and   the inner housing is engaged within the outer housing, the inner housing and the outer housing being relatively moveable with respect to each other along the ferrule axis between limits defined by an interaction between the inner housing and the outer housing in order to provide SC-style push/pull engagement/disengagement with a mating optical fibre socket.       

     The ferrule housing assembly serves as an inner housing in relation to the outer housing in the provision of the SC-style push/pull engagement/disengagement. 
     The ferrule is preferably rotationally fixed with respect to the ferrule holder about the ferule axis, with the ferrule holder bring rotationally aligned by means of one or more rotational keys with respect to the outer housing so that the rotational orientation of the ferrule with respect to the outer housing can be set during assembly of the connector at one of one or more predefined rotational orientations. 
     A first rotational key may be provided between the ferrule holder and the ferrule holder carrier. A second rotational key may then be provided between the ferrule holder carrier and the inner housing. The ferrule holder and the ferrule holder carrier may have aligned features which provide a combined rotational key between, on the one hand, the ferrule holder and the ferrule holder carrier, and, on the other hand, the inner housing. 
     A further rotational key may be provided between the ferrule holder sub-assembly and the outer housing. 
     The ferrule holder and ferrule holder carrier may be adapted to be assembled by a press-fitting operation in which the ferrule holder moves along the ferrule axis into the receiving portion of the ferrule holder. 
     The inner housing is adapted to be assembled to the ferrule holder and the ferrule holder carrier by a press-fitting operation in which the inner housing moves along the ferrule axis over the projection portion of the ferrule towards the ferrule holder carrier. 
     Also according to the invention, there is provided a method of making an optical fibre connection using a Subscriber Connector (SC)-type optical fibre connector, comprising the steps of:
         forming an optical fibre connector sub-assembly comprising a spring-biased ferrule that terminates a length of optical fibre;   inserting the optical fibre sub-assembly down a length of protective conduit;   after said inserting, forming an SC-type optical fibre connector from the optical fibre connector sub-assembly by engaging around said sub-assembly an inner housing and an outer housing, said inner and outer housing being movable relative to each other to provide SC-style push/pull engagement/disengagement functionality and   mating said SC-type optical fibre connector with a matching optical fibre socket.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view of a prior art SC connector, comprising an optical fibre cable, optical fibre connector sub-assembly (including an inner housing) and an outer housing to provide SC-style push/pull engagement/disengagement with a mating optical fibre socket; 
         FIG. 2  is a an exploded perspective view of the optical fibre connector sub-assembly of  FIG. 1 ; 
         FIG. 3  is a perspective view of the optical fibre connector sub-assembly of  FIG. 1  when assembled; 
         FIG. 4  is a perspective view of the SC connector of  FIG. 1  when assembled; 
         FIG. 5  is an exploded perspective view of an SC connector according to the invention, comprising an optical fibre cable, an optical fibre connector sub-assembly, and an inner housing and an outer housing to provide SC-style push/pull engagement/disengagement with a mating optical fibre socket; 
         FIG. 6  is a perspective view of the optical fibre connector sub-assembly of  FIG. 5  when assembled; 
         FIG. 7  is a perspective view of the SC connector of  FIG. 5  when assembled; 
         FIGS. 8 and 9  show in perspective the components forming the fibre connector sub-assembly of  FIG. 5 , namely a ferrule holder carrier, an optical ferrule held by a ferrule holder, and a helical spring; 
         FIG. 10  is another perspective view of the optical fibre connector sub-assembly of  FIG. 5  when assembled; 
         FIG. 11  is a perspective view of the inner housing of  FIG. 5   
         FIG. 12  is a perspective view of a ferrule housing sub-assembly formed when the inner housing is press-fitted over the ferrule holder carrier; 
         FIG. 13  is another perspective view of the ferrule holder carrier of  FIG. 8 , showing how the holder has a number of cylindrical or concentric surfaces; 
         FIG. 14  is a cross-sectional view along a central axis of the ferrule holder carrier of  FIG. 13 ; 
         FIG. 15  is another cross-sectional view along the central axis of the ferrule holder carrier taken along line XV-XV of  FIG. 14 . 
         FIG. 16  is a perspective view of an alternative embodiment of ferrule holder carrier, showing how the holder has a number of cylindrical or concentric surfaces; 
         FIG. 17  is a cross-sectional view along a central axis of the ferrule holder carrier of  FIG. 16 ; 
         FIG. 18  is another cross-sectional view along the central axis of the ferrule holder carrier, taken along line XVIII-XVIII of  FIG. 17 . 
         FIGS. 19 to 22  illustrate the insertion of the partially assembled SC connector of  FIG. 5  through and out from a protective conduit; 
         FIG. 23  is a cross-sectional view through the ferrule holder carrier of  FIG. 10 ; 
         FIG. 24  is a cross-sectional view of the ferrule holder carrier of  FIG. 23 , including also a terminated optical fibre cable inside the ferrule and the inner housing connected about the ferrule holder carrier; and 
         FIG. 25  is a cross section at right angles to that of  FIG. 24 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 to 4  show a prior art SC connector  1 , comprising an optical fibre cable  2 , a ferrule housing sub-assembly  4 , and an outer housing  6  to provide SC-style push/pull engagement/disengagement with a mating optical fibre socket (not shown). 
     The cable  2  holds a single strand of 125 μm diameter single mode optical fibre  8 , protected by primary and secondary buffering layers  10 , about 900 μm in diameter and an outer sheath  12 , typically 3 mm to 5 mm in diameter. The optical fibre  8  is terminated by the ferrule in a manner well-known to those skilled in the art, and defines a ferrule axis  5  which extends centrally through the SC connector  1 . 
     The ferrule housing sub-assembly  4  includes a cylindrical ceramic ferrule  14 , a ferrule holder  16  having a base  15  with a socket in which the ferrule is seated, a helical spring  17 , a generally cylindrical cup  18  and an inner housing  20 . The ferrule holder  16  has a cylindrical stem  22  which extends in an axial direction away from the ferrule  14  towards the cup  18 . The spring  17  is seated around the stem  22  between an annular shoulder  24  on the ferrule holder  16  and a similar annular surface (not shown) within the cup. During assembly, the inner housing  20  is passed axially over the ferrule holder  16  and a forward part of the cup, and then rotated by 90° until a pair of slots  26  on opposite sides of the inner housing engage with a pair of ribs  28  projecting on opposite sides of the cup  18 . This engagement fixes the inner housing  20  in an axial direction, while at the same time leaving the spring  17  trapped under compression between the ferrule holder and the cup  18 . 
     The inner housing has a forwards portion  30  which coaxially extends around the axially projecting ferrule  14  to define a receptacle  32  for a socket (not shown) to receive a projecting portion of the ferrule  14 . 
     The connected cup  18  and inner housing  20  serve as a ferrule holder carrier inside of which the ferrule holder  16  is free to move backwards when an end face  34  of the ferrule  14  comes into contact with a similar end face (not shown) of another optical fibre ferrule when making an optical connection. 
     The cup  18  has a central aperture (not shown) through which the optical fibre and buffering  10  pass, and has in a rear-most portion a sleeve  36  sized to receive and be crimped to the cable sheathing  12 . 
     Unusually a strain-relief sleeve  38  is provided around the junction of the optical fibre cable  2  and the ferrule housing sub-assembly  4 . 
     An outer housing  40  is press-fitted axially over the assembled ferrule housing sub-assembly  4 . Once the inner housing  20  and outer housing  40  are engaged together, the inner and outer housings are relatively moveable with respect to each other along the ferrule axis  5  between limits defined by an interaction between the inner housing and the outer housing provided by projections  42  on a pair of opposite sides of the inner housing and a pair of apertures  44  in the outer housing. The projections  42  and apertures  44  interact with sprung features inside a matching socket (not shown) to provide SC-style push/pull  46  engagement/disengagement with a mating optical fibre socket. 
       FIG. 5  is an exploded perspective view of an SC connector  101  according to the invention, in which features corresponding with those of  FIGS. 1 to 4  are indicated with the same reference numerals. 
     The connector  101  comprises an optical fibre cable  2 , a ferrule housing sub-assembly  104 , and an outer housing  40  to provide SC-style push/pull engagement/disengagement  46  with a mating optical fibre socket (not shown). 
     The cable  2  holds a single strand of 125 μm diameter single mode optical fibre  8 , protected by primary and secondary buffering layers  10 , about 900 μm in diameter and an outer sheath  12 , typically 3 mm to 5 mm in diameter. The optical fibre  8  is terminated by the ferrule in a manner well-known to those skilled in the art, and defines a ferrule axis  5  which extends centrally through the SC connector  101 . 
     The ferrule housing sub-assembly  104  includes a cylindrical ceramic ferrule  14 , a ferrule holder  16  in which the ferrule is seated, a helical spring  17 , a ferrule holder carrier  50  and an inner housing  120 . The ferrule holder  16  has a cylindrical stem  22  which extends in an axial direction away from the ferrule  14  towards a sleeve  136  of the ferrule holder carrier  50  used to make a crimp connection around the optical fibre cable sheathing  12 . With reference now also to  FIGS. 8 and 9 , the spring  17  is seated around the stem  22  between an annular shoulder  24  on the ferrule holder  16  and a similar annular surface  52  within the a cylindrical recess  54  of the ferrule holder carrier  50 . As shown most clearly in  FIGS. 23-25 , the stem  20  is slideably seated in a closely fitting bore  55  of the ferrule holder carrier. 
     During assembly, the ferrule holder  16  and seated ferrule  14  are inserted axially into the recess  54  of the ferule holder carrier  50 . The carrier  50  has a pair of arms  56  around a portion of the stem  22  nearest the ferrule  14  which extend axially forwards of the stem on opposite sides of the base  15  of the ferrule holder. Two pairs of curved fingers  58  are provided, each pair extending in opposite circumferential directions at the end of each arm  56 . The fingers  58  extend transversely to the length of the arms  56  partially around the circumference of a portion of the ferrule nearest the base  15 . The ferrule base  15  has four cycles of crenellations  62  spaced symmetrically around the circumference of the base and which provide four corresponding channels that extend parallel to the optical fibre axis, These crenellations are in the form of alternating radially high  65  and low  66  cylindrically shaped regions with the circumferential and axial extent of each of the high and low regions being the same. The high and low regions are separated by steps  67  that extend in a radial direction. The arms  56  are seated in opposite radially low regions  66  in a sliding fit with the adjacent high regions  65 , and reach axially forwards of the base  15  and crenellations  62  so that the fingers  58  engage with the intervening radially high regions  65  on a side  61  of the base  15  opposite the annular surface  24  against which the spring  17  is engaged. The arrangement permits a degree axial movement of the ferrule holder  50 , with movement being therefore limited in one direction by the compression of the spring  17  between the two annular surfaces  24 ,  52  and in the other axial direction by the contact of the fingers  58  with the radially high regions  65  of the crenellations  62  on the base  15  of the ferrule holder  16 . As can be seen from the drawings, the ferrule holder base  15  and ferrule holder carrier  50  also have a common cylindrical outer envelope. 
     The arrangement described above has the benefit of minimising the extent of the ferrule holder base  15  and ferrule holder carrier  50  in a radial direction so that this sub-assembly may be blown down a conduit or passed through other constricted spaces when installing an SC-type optical fibre communications link. 
     The rotational orientation of the ferrule holder carrier  50  may be set at one of four orientations relative to the ferrule  14  in the ferrule holder  16  owing to the seating of the arms  56  in the crenellations. In this way, a first rotational key is provided between the ferrule holder  16  and the ferrule holder carrier  50 . 
     The optical fibre  8  is therefore terminated in a sub-assembly referred to herein as an optical fibre connector sub-assembly  60 . As can be appreciated by an inspection of  FIGS. 8 to 10 , during assembly of the optical fibre connector sub-assembly  60  the arms  56  and fingers  58 , which each have a chamfered taper  64  on an inner forwards surface, flex outwardly over the base  15  of the ferrule holder  14  until the fingers  58  snap radially inwards into engagement with the forwards surface  61  of the ferrule holder base  15 . 
       FIGS. 13 to 15  show various views of the ferrule holder carrier  50  described above. 
       FIGS. 16 to 18  show similar views of a variant design of the ferrule holder carrier  150 , which works in the same way as that described above, but which has a smaller diameter sleeve  236  over which an outer sheath  12  of the optical fibre cable may be crimped. 
     In both cases, the resulting optical fibre connector sub-assembly  60  is therefore mechanically whole or integral, both as regards the components forming the sub-assembly and as regards the mechanical connection of the sub-assembly to the optical fibre cable  2 . The sub-assembly cannot come apart without first prising the fingers  58  back over the ferule holder base  15 . The ferrule holder carrier has an outer diameter which is preferably no more than that of the widest portion of the ferrule holder  14 , i.e. the ferrule holder base  15 . 
     With reference now also to  FIGS. 19 to 22 , the benefit of this arrangement is that the optical fibre connector sub-assembly  60  is thin enough in a radial direction to be inserted down a length of protective conduit  72 , as shown in  FIG. 19 , even after first placing over the optical fibre connector sub-assembly  60  a temporary protective outer sleeve  74 , in order to prevent damage to the optical fibre connector sub-assembly  60 . The outer sleeve is preferably formed in a low friction material, such as PTFE, in order to facilitate smooth insertion of sub-assembly down the length of conduit  72 . 
     Optionally, the projecting end of the ferrule  14  may be additionally protected by a disposable end cap  76 . 
     After the insertion has been performed, the assembly of the SC-type optical fibre connector is completed as follows. With reference to  FIGS. 11 ,  12 ,  23  and  24 , an inner housing  120  is inserted in an axial direction over the projecting ferrule  14  and surrounding ferrule holder carrier  50 . The ferrule holder carrier  50 ,  150  has an annular groove  78  within the cylindrical envelope defined by the radially outermost surfaces of the ferrule holder carrier. The inner housing has a pair of arms  80  each having at an end a radially inwardly projecting detent  82 . The detents  82  are initially deflected radially outwards and then snap into engagement with the groove  78  as the inner housing is slid over the sub-assembly  60 . This engagement prevents relative axial movement of the inner housing and the sub-assembly  60  in one direction. The advantage of this arrangement is that the groove  78  provides an engagement feature for the inner housing which does not add to the radial dimensions of the connector sub-assembly, which makes it much easier to insert the sub-assembly down a narrow conduit or into other constricted spaces. 
     Relative movement in the opposite direction is prevented by abutting of surface  52  and an end surface  83  of a rib  84  that engages as a rotational key with an inner radial region of the crenellations  62 . In this way, a second rotational key is provided between the ferrule holder carrier  50 ,  150  and the inner housing  120 . 
     The external shape of the inner housing  120  where this interacts with the outer housing  6  is the same as with the prior art. The outer housing therefore is snap-fitted over the inner housing, after which the SC-type connector  101  is fully compatible with the prior art connector  1 . 
     It should be noted that although the specific example described above relate to an SC-type connector, the optical fibre connector sub-assembly can be adapted for use with other types of optical fibre connector systems, for example the ST-type connectors. 
     The invention therefore provides a convenient optical fibre connector and optical fibre communication system and method for providing such a system.