Abstract:
A clamp ring for connecting a tensile strength member to the periphery of a rear end portion of a ferrule retaining part for holding a ferrule, in which an optical fiber of an optical fiber cable incorporating a coated optical fiber and the tensile strength member surrounding the coated optical fiber is inserted and fixed with an outer covering of the optical fiber cable stripped off from its terminal portion, the clamp ring comprising a first annular portion which fits on the periphery of the rear end portion of the ferrule retaining part, a second annular portion located in the back of the first annular portion, the second annular portion having a smaller diameter than the first annular portion, a step portion interconnecting the second annular portion and the first annular portion, and a clamping portion extending inward from the rear end of the second annular portion in such a way that the extreme end of the clamping portion secures the periphery of the optical fiber cable, in which the first annular portion, the second annular portion, the step portion and the clamping portion are formed as a single piece of metal having approximately the same wall thickness.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to clamp rings such as those used in optical connectors which are located between optical fiber transmission lines for switching, connecting or disconnecting the transmission lines and those directly fitted to ferrules, as well as to optical fiber terminating structures.  
           [0002]    Presently, detachable optical connectors are used for connecting optical fiber cables and optical fiber cords intended for short-distance applications such as in-building wiring and wiring to equipment. Since optical fibers are so thin and flexible that they are usually secured by using ferrules. An optical connector is constructed of a plug incorporating a ferrule and an adaptor incorporating a sleeve.  
           [0003]    [0003]FIG. 6 shows a cross section of an optical connector employing a conventional crimp ring. As shown in FIG. 6, a ferrule  101  in which an optical fiber  201  is inserted and fixed has a flange  102 , and a stopper  104  is fitted behind a rear end portion of the ferrule  101  in the back of the flange  102  with a compression spring  103 , which is mounted on the outer periphery of the rear end portion of the ferrule  101 , placed in between. This means that the ferrule  102  is fitted in such a way that it can move relative to the stopper  104  in its axial direction with the aid of the compression spring  103 . A coated optical fiber  202  is inserted into the compression spring  103  and the stopper  104  with a tensile strength member  204  of an optical fiber cable  203  located at the rear end of the stopper  104 , and the tensile strength member  204  is secured around the outer periphery of a rear end portion of the stopper  104  by crimping a crimp ring  105 .  
           [0004]    The crimp-on ring  105  is formed of a first cylindrical portion  111  whose inside diameter fits the outer periphery of the rear end portion of the stopper  104 , a second cylindrical portion  112  which fits on the outer periphery of the optical fiber cable  203 , and a connecting part  113  interconnecting the first and second cylindrical portion  111 ,  113 , as shown in FIG. 7. The first cylindrical portion  111  is crimped to secure the tensile strength member  204  between an inside curved surface  111   a  of the first cylindrical portion  111  and the stopper  104  and, then, the second cylindrical portion  112  is crimped to firmly hold the optical fiber cable  203  in position with a compressive effect provided by an inside curved surface  112   a  of the second cylindrical portion  112 .  
           [0005]    Since the optical fiber cable  203  is held in position with compressive and frictional forces exerted by the inside curved surface  112   a  of the second cylindrical portion  112  of the crimp-on ring  105  as described above, its total securing force varies as a result of changes in the outside diameter of the optical fiber cable  203 . It is therefore necessary to design the crimp-on ring  105  with varying inside diameters of the second cylindrical portion  112  to cater for such changes in the outside diameter of the optical fiber cable  203 . In addition, the outside diameter of the second cylindrical portion  112  should be made suitable for diameter rating of a crimping tool to be used. Thus, one problem is that the crimp-on ring  105  is remarkably expensive, because it is conventionally produced by cutting an aluminum material, for instance. Another problem is that the second cylindrical portion  112  becomes relatively thick as a result of a relationship between the outside diameter of the optical fiber cable  203  and the diameter rating of the crimping tool, thereby requiring a great force in crimping operation.  
           [0006]    This invention is intended to solve these problems and has as an object the provision of clamp rings which are easy to crimp and manufacture.  
           [0007]    Also known in the prior art are structures for terminating optical fiber ends by directly fitting a ferrule to each end of an optical fiber cord. An example of such structures is shown in FIGS.  8 A- 8 B. As shown in FIGS.  8 A- 8 B, after inserting a coated optical fiber into a ferrule, a tensile strength member  204  of an optical fiber cable  203  is placed around the periphery  302  of a rear end portion of the ferrule  301 , a ring member  303  is fitted, and both ends of the ring member  303  is fixed in position by an adhesive  304  to thereby secure the individual components in integral form.  
           [0008]    Such conventional structure has such a problem that a sufficient tensile strength (about 10 kgf) can not be achieved since the tensile strength member  204  is fixed by using the adhesive  304 .  
           [0009]    Another problem of this conventional structure is that it increases the chance of fiber breakage. This is because the adhesive  304  is sucked into the tensile strength member  204  and that portion of the tensile strength member  204  where the adhesive  304  has been sucked looses flexibility.  
           [0010]    The structure has yet another problem in that it requires a good deal of man hours for assembly work, because the adhesive  304  needs a long time to cure.  
           [0011]    The invention aims at solving these problems. Accordingly, it is also an object of the invention to provide a structure which makes it possible to directly secure an optical fiber cable to a ferrule without using an adhesive, as well as an optical fiber terminating structure which provides a sufficient tensile strength without causing a loss of optical fiber cable flexibility, yet permitting ease of production.  
         SUMMARY OF THE INVENTION  
         [0012]    In a first mode of carrying out the invention, a clamp ring for connecting a tensile strength member to the periphery of a rear end portion of a ferrule retaining part for holding a ferrule, in which an optical fiber of an optical fiber cable incorporating a coated optical fiber and the tensile strength member surrounding the coated optical fiber is inserted and fixed with an outer covering of the optical fiber cable stripped off from its terminal portion, is characterized in that it comprises a first annular portion which fits on the periphery of the rear end portion of the ferrule retaining part, a second annular portion located in the back of the first annular portion, the second annular portion having a smaller diameter than the first annular portion, a step portion interconnecting the second annular portion and the first annular portion, and a clamping portion extending inward from the rear end of the second annular portion in such a way that the extreme end of the clamping portion secures the periphery of the optical fiber cable, and that the first annular portion, the second annular portion, the step portion and the clamping portion are formed as a single piece of metal having approximately the same wall thickness.  
           [0013]    The aforementioned clamp ring of the invention is formed by press-forming operation, for example.  
           [0014]    In a second mode of carrying out the invention, an optical fiber terminating structure for connecting a tensile strength member to the periphery of a rear end portion of a ferrule, in which an optical fiber of an optical fiber cable incorporating a coated optical fiber and the tensile strength member surrounding the coated optical fiber is inserted and fixed with an outer covering of the optical fiber cable stripped off from its terminal portion, is characterized in that the tensile strength member is connected to the periphery of a rear end portion of the ferrule by crimping a clamp ring with its one end fitted on the periphery of the rear end portion of the ferrule and the other end fitted on the periphery of the optical fiber cable.  
           [0015]    In this structure, it is preferable that the clamp ring comprises a first annular portion which fits on the periphery of the rear end portion of the ferrule, a second annular portion located in the back of the first annular portion, the second annular portion having a smaller diameter than the first annular portion, a step portion interconnecting the second annular portion and the first annular portion, and a clamping portion extending inward from the rear end of the second annular portion in such a way that the extreme end of the clamping portion secures the periphery of the optical fiber cable, and that the first annular portion, the second annular portion, the step portion and the clamping portion are formed as a single piece of metal having approximately the same wall thickness.  
           [0016]    Preferably, the periphery of the rear end portion of the ferrule is formed into a surface having pits and protrusions. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a cross-sectional view of a clamp ring according to an embodiment of the invention;  
         [0018]    [0018]FIG. 2 is a cross-sectional view of an optical connector employing the clamp ring of the invention;  
         [0019]    FIGS.  3 A- 3 B are exploded cross-sectional diagrams showing an optical fiber terminating structure according to a second embodiment of the invention;  
         [0020]    [0020]FIG. 4 is a diagram showing one variation of the optical fiber terminating structure according to the second embodiment of the invention;  
         [0021]    FIGS.  5 A- 5 B are exploded cross-sectional diagrams showing an optical fiber terminating structure according to a third embodiment of the invention;  
         [0022]    [0022]FIG. 6 is a cross-sectional view of an optical connector employing a crimp-on ring of the prior art;  
         [0023]    [0023]FIG. 7 is a cross-sectional view of the clamp ring of the prior art; and  
         [0024]    FIGS.  8 A- 8 B are diagrams illustrating one example of an optical fiber fixing method using a clamp ring of the prior art. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Embodiments of the invention is now described in detail with reference to the drawings.  
         [0026]    [0026]FIG. 1 shows a cross section of a clamp ring according to an embodiment of the invention. As shown in FIG. 1, the clamp ring  10  of this embodiment comprises a first annular portion  11 , a second annular portion  12  having a smaller diameter than the first annular portion  11 , a step portion  13  connecting the first annular portion  11  and the second annular portion  12  to each other, and a clamping portion  14  which is provided immediately adjacent to the rear end of the second annular portion  12  and extends inward, all these potions being formed as a single piece. The inside diameter of an opening  14   b  formed by an end face  14   a  of the clamping portion  14  is made slightly larger than the outside diameter of the earlier mentioned optical fiber cable, and the outside diameter of the second annular portion  12  is set to fit diameter rating of a crimping tool to be used in crimping operation. The first annular portion  11  has such inside diameter that fits a rear end portion of a stopper as described earlier, and is so constructed that a tensile strength member is secured between an inside curved surface  11   a  of the first annular portion  11  and an inside surface  13   a  of the step portion  13  when the first annular portion  11  is crimped from around its outer periphery.  
         [0027]    [0027]FIG. 2 shows a general construction diagram of an optical connector which employs the clamp ring of this embodiment. As shown in FIG. 2, a ferrule  31  in which an optical fiber  21  is inserted and fixed has a flange  32 , and the stopper  34  is fitted behind a rear end portion of the ferrule  31  in the back of the flange  32  with a compression spring  33 , which is mounted on the outer periphery of the rear end portion of the ferrule  31 , placed in between. A coated optical fiber  22  is inserted into the compression spring  33  and the stopper  34  with a tensile strength member  24  of an optical fiber cable  23  located at the rear end of the stopper  34 , and the tensile strength member  24  is secured between the clamp ring  10  and an outer peripheral surface of the rear end portion of the stopper  34  by crimping.  
         [0028]    As stated above, the inside diameter of the opening  14   b  formed by the end face  14   a  of the clamping portion  14  is slightly larger than the outside diameter of the above-mentioned optical fiber cable, and the outside diameter of the second annular portion  12  is set to fit diameter rating of the crimping tool used in crimping operation. The optical fiber cable  23  is clamped by the clamping portion  14  by crimping the second annular portion  12  with the crimping tool while the tensile strength member  24  is secured between the inside curved surface  11   a  of the first annular portion  11  and the stopper  34 , and between the inside surface  13   a  of the step portion  13  and the stopper  34 , by crimping the first annular portion.  
         [0029]    The clamp ring  10  of this embodiment provides such advantageous effects that it can be produced extremely easily by press forming, for instance, and its crimping operation can be performed relatively easily.  
         [0030]    Clamp rings of the invention and optical connectors employing the clamp rings are not limited to the above-described embodiment in their basic construction. For example, an optical connector may be a resin-molded product in which a flange, a compression spring and a stopper are integrally formed.  
         [0031]    FIGS.  3 A- 3 B show an optical fiber terminating structure according to a second embodiment of the invention. A clamp ring  40  used in this embodiment comprises a main annular portion  41  which fits on the outer periphery of a rear end portion of a ferrule and a clamping portion  42  which extends inward from the rear end of the main annular portion  41  and so that its innermost end clamps the outer periphery of an optical fiber cable  23 . The main annular portion  41  and the clamping portion  42  are formed as a one-piece metallic part having approximately a uniform thickness.  
         [0032]    The inside diameter of an opening  42   a  formed by the clamping portion  42  is made slightly larger than the outside diameter of the aforementioned optical fiber cable  23 , and the outside diameter of the main annular portion  41  is set to fit diameter rating of a crimping tool used in crimping operation. Further, the inside diameter of the main annular portion  41  is made slightly larger than the diameter of the outer periphery  53  of the rear end portion of the ferrule  51  so that the tensile strength member  24  is secured between an inside curved surface  41   a  of the main annular portion  41  and the outer periphery  53  of the ferrule  51  to be joined by crimping both ends of the main annular portion  41  from around its outer periphery.  
         [0033]    The ferrule  51  of this embodiment in which an optical fiber  21  is inserted and fixed has a flange  52 , and the outer periphery  53  of the rear end portion of the ferrule  51  on which the aforementioned clamp ring  40  is fitted is located in the back of the flange  52 . In this embodiment, three stepped ridges  54   a - 54   c  are integrally formed on the surface of the outer periphery  53 .  
         [0034]    It is needless to say that the flange  52  may be produced either as a discrete component of stainless steel, for instance, or as an integral part of the ferrule when producing it.  
         [0035]    The following discussion deals with a procedure for making a terminating structure in which the optical fiber cable  23  is connected to the ferrule  51  by using the aforementioned clamp ring  40 .  
         [0036]    As shown in FIG. 3A, the clamp ring  40  is passed over the optical fiber cable  23  at first, and an outer covering, or jacket, of the optical fiber cable  23  is removed near its end to expose the coated optical fiber  22 , the tensile strength member  24  and the optical fiber  21 . Then, the optical fiber  21  is inserted into an optical fiber insertion hole  25  of the ferrule  51  while the coated optical fiber  22  is inserted into a core insertion hole  26  of the ferrule  51 . The optical fiber  21  and the coated optical fiber  22  are secured in position with an adhesive readily filled in the aforementioned optical fiber insertion hole  25  and the core insertion hole  26 .  
         [0037]    The tensile strength member  24  of the optical fiber cable  23  is located around the outer periphery  53  of the rear end portion of the ferrule  51  thus attached at this point. With the tensile strength member  24  disposed between the clamp ring  40  and the outer peripheral surface of the rear end portion of the ferrule  51  in the back of the flange  52 , the clamp ring  40  is crimped and fixed by using the crimping tool as shown in FIG. 3( b ). Thus, the tensile strength member  24  can be secured in position by crimping the ferrule side of the clamp ring  40 , while the jacket (formed of PVC, for instance) of the optical fiber cable  23  can be secured by crimping the optical fiber cable side of the clamp ring  40 .  
         [0038]    Subsequently, the far end of the ferrule  51  is polished together with the optical fiber  21  to complete an optical fiber cable product already fitted with a ferrule, for instance.  
         [0039]    As shown in the foregoing discussion, the present embodiment provides such advantageous effects that it becomes unnecessary to fix with the conventionally used adhesive, the optical fiber cable  23  and the ferrule  51  can be joined together in an extremely simple and easy way, and the optical fiber cable does not loose its flexibility.  
         [0040]    Tensile tests carried out on ferrules each joined to an optical fiber cable have proved that they could withstand tensile stresses of about 15 kgf. On the other hand, a structure employing the conventional adhesive bond disjoined under tensile stresses ranging from 3 to 4 kgf.  
         [0041]    Although there are formed three stepped ridges  54  for clamping on the outer periphery  52  of the rear end portion of the ferrule  51  in this embodiment, the number of the stepped ridges  54  is not limited thereto in this invention. Moreover, the outer periphery  53  of the rear end portion of the ferrule  51  may feature small pits and protrusions on its surface (to form a so-called knurled surface) instead of forming the aforementioned stepped ridges so that the tensile strength member  24  is firmly retained by the small pits and protrusions when the clamp ring is fitted.  
         [0042]    Although there is formed the flange  52  on the ferrule  51  in this embodiment, its structure may be varied by employing another type of clamp ring as shown in FIG. 4, in which the outer periphery of the clamp ring  40  and that of a ferrule  51  have the same diameter and an outer peripheral part  73  of the rear end portion of the ferrule  71  is made slightly smaller than an inside curved surface  41   a  of the clamp ring  40  in diameter so that the optical fiber cable and the ferrule would look like a one-piece element.  
         [0043]    Next, FIGS.  5 A- 5 B show an optical fiber terminating structure according to a third embodiment of the invention.  
         [0044]    As shown in FIG. 5, a clamp ring  60  used in this embodiment comprises a first annular portion  61 , a second annular portion  62  having a smaller diameter than the first annular portion  61 , a step portion  63  connecting the first annular portion  61  and the second annular portion  62  to each other, and a clamping portion  64  which is provided immediately adjacent to the rear end of the second annular portion  62  and extends inward, all these potions being formed as a single piece, in a manner similar to the first embodiment shown in FIG. 1. The inside diameter of an opening  64   b  formed by an end face  64   a  of the clamping portion  64  is made slightly larger than the outside diameter of the earlier mentioned optical fiber cable, and the outside diameter of the second annular portion  62  is set to fit diameter rating of a crimping tool to be used in crimping operation. Further, the diameter of the first annular portion  61  is made slightly larger than the diameter of the outer periphery  53  of the rear end portion of the ferrule  51  so that the tensile strength member  24  is secured between an inside curved surface  61   a  of the first annular portion  61  and the outer periphery  53  of the ferrule  51  to be joined by crimping both ends of the first annular portion  61  from around its outer periphery.  
         [0045]    The optical fiber terminating structure of this embodiment is made by joining the optical fiber cable  23  to the ferrule  51  by using the clamp ring  60  in a manner similar to the second embodiment.  
         [0046]    Specifically, the clamp ring  60  of this embodiment is passed over the optical fiber cable  23 , the optical fiber  21  is inserted into the optical fiber insertion hole  25  of the ferrule  51 , the coated optical fiber  22  is inserted into the core insertion hole  26  of the ferrule  51 , and the optical fiber and the coated optical fiber  22  are secured in position with an adhesive, as shown in FIG. 5A.  
         [0047]    The tensile strength member  24  of the optical fiber cable  23  located around the outer periphery  53  of the rear end portion of the ferrule  51  thus attached is disposed between the inside curved surface of the clamp ring  60  and the outer peripheral surface of the rear end portion of the ferrule  51  in the back of the flange  52 , and the clamp ring  60  is crimped and fixed by using the crimping tool as shown in FIG. 5B. Thus, the tensile strength member  24  can be secured between the outer periphery  53  of the rear end portion of the ferrule  51  and the inside curved surface  61   a  of the first annular portion  61  by crimping the first annular portion  61  of the clamp ring  60  from around its outer peripheral surface. The tensile strength member  24  can also be secured between the outer periphery  53  of the rear end portion of the ferrule and an inside curved surface  63   a  of the step portion  63  to provide increased fixing strength by crimping the first annular portion  61  of the clamp ring  60  from around its outer peripheral surface. Furthermore, the jacket (formed of PVC, for instance) of the optical fiber cable  23  can be secured by clamping it by the end face  64   a  at the clamping portion  64 .  
         [0048]    As shown in the foregoing discussion, the present embodiment provides such advantages, as does the earlier-described embodiment, that it becomes unnecessary to fix with the conventionally used adhesive, the optical fiber cable  23  and the ferrule  51  can be joined together in an extremely simple and easy way, and the optical fiber cable does not loose its flexibility. This embodiment makes it possible to join the optical fiber to the ferrule even more firmly.  
         [0049]    The clamp rings of the invention are easy to produce and their crimping operation can be performed relatively easily, because they are formed to have approximately a uniform thickness throughout their whole structure and the outer periphery of the optical fiber cable is clamped by the end face of the clamping portion which extends inward from the rear end of the second annular portion as described above with reference to the embodiments.  
         [0050]    On the other hand, the optical fiber terminating structures of the invention provide such advantageous effects that they facilitate fixing operation due to their construction in which the clamp rings have approximately a uniform thickness throughout and the tensile strength member is secured by crimping the clamp ring fitted over the outer periphery of the rear end portion of each ferrule, and that the number of processes and labor hours required for the fixing operation can be reduced since the individual components can be fixed together without using an adhesive unlike the conventional structure.