Abstract:
A ferrule includes a ferrule body having a first opening formed in a side face thereof, and having a second opening formed in a top face thereof, wherein the first opening is configured to receive an optical waveguide inserted into the ferrule body at the side face, and wherein the second opening is formed over and in communication with the first opening, such that the second opening connects an inner space of the first opening to an outside of the ferrule body.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0001]    The disclosures herein relate to a ferrule. 
       2. Description of the Related Art 
       [0002]    High-speed interface for supercomputers and high-end servers has been increasingly employing optical communications because of its capacity to achieve high-speed signal transmission and to increase transmission distance. 
         [0003]    Next generation interface studied for use in such standards as 100G Ethernet (registered trademark) and IBTA EDR (registered trademark) providing a long transmission distance such as a few dozen meters employs optical communications, and also uses optical modules for converting electrical signals into optical signals. Optical modules, which couple optical cables to servers or the like, convert optical signals from optical cables into electrical signals for outputting to servers, and also convert electrical signals from servers into optical signals for outputting to optical cables. 
         [0004]    Optical modules have a light emitting device for converting an electrical signal into an optical signal, a light receiving device for converting an optical signal into an electrical signal, a driver IC (integrated circuit) for driving the light emitting device, and a TIA (trans impedance amplifier) for converting electric current into voltage. The light emitting device, the light receiving device, the driver IC, and the TIA are mounted on a printed circuit board inside the housing. Optical waveguides which are formed as a flexible sheet provide coupling between the light emitting device, the light receiving device, and a ferrule such as a lens ferrule. 
         [0005]    The optical waveguide sheet is inserted into the slit of a lens ferrule, and is bounded therein with adhesive. The thickness of the slit is designed to be slightly greater than the thickness of the optical waveguide in consideration of tolerance. Warpage of the optical waveguide inside the slit may thus hinder the mounting of the optical waveguide at desired position inside the slit. The lens ferrule and the optical waveguide are required to be fastened with each other at the desired position such that light from the optical guide enters the lens of the lens ferrule, and such that light condensed by the lens enters the core of the optical guide. The optical waveguide fastened to the lens ferrule away from the desired position gives rise to light loss, thereby hindering optical communication. 
         [0006]    It may be desired to provide a ferrule that allows an optical waveguide to be fastened at desired position. 
       RELATED-ART DOCUMENTS 
     Patent Document 
     [Patent Document 1] Japanese Patent Application Publication No. 2015-23143 
     [Patent Document 2] Japanese Patent Application Publication No. 2015-22130 
     [Patent Document 3] Japanese Patent Application Publication No. 2015-22125 
     [Patent Document 4] Japanese Patent Application Publication No. 2013-20027 
     SUMMARY OF THE INVENTION 
       [0007]    It is a general object of the present invention to provide a ferrule that substantially obviates one or more problems caused by the limitations and disadvantages of the related art. 
         [0008]    According to one embodiment, a ferrule includes a ferrule body having a first opening formed in a side face thereof, and having a second opening formed in a top face thereof, wherein the first opening is configured to receive an optical waveguide inserted into the ferrule body at the side face, and wherein the second opening is formed over and in communication with the first opening, such that the second opening connects an inner space of the first opening to an outside of the ferrule body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
           [0010]      FIGS. 1A and 1B  are drawings illustrating a lens ferrule; 
           [0011]      FIGS. 2A and 2B  are drawings illustrating a lens ferrule; 
           [0012]      FIGS. 3A and 3B  are drawings illustrating a lens ferrule of a first embodiment; 
           [0013]      FIGS. 4A and 4B  are drawings illustrating the lens ferrule of the first embodiment; 
           [0014]      FIGS. 5A and 5B  are drawings illustrating the lens ferrule of the first embodiment; 
           [0015]      FIGS. 6A through 6C  are drawings illustrating a method of making the lens ferrule according to the first embodiment; 
           [0016]      FIGS. 7A and 7B  are drawings illustrating a lens ferrule of a second embodiment; 
           [0017]      FIGS. 8A and 8B  are drawings illustrating a method of making the lens ferrule according to the second embodiment; 
           [0018]      FIGS. 9A through 9C  are drawings illustrating the coupling of an optical waveguide to the lens ferrule of the second embodiment; 
           [0019]      FIGS. 10A and 10B  are drawings illustrating a lens ferrule of a third embodiment; 
           [0020]      FIG. 11  is a drawing illustrating molds for making the lens ferrule according to the third embodiment; 
           [0021]      FIGS. 12A and 12B  are drawings illustrating a lens ferrule of a fourth embodiment; 
           [0022]      FIG. 13  is a drawing illustrating a method of making the lens ferrule according to the fourth embodiment; 
           [0023]      FIGS. 14A and 14B  are drawings illustrating a lens ferrule of a fifth embodiment; and 
           [0024]      FIG. 15  is a drawing illustrating a mold for making the lens ferrule according to the fifth embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    In the following, embodiments for implementing the invention will be described. The same members or the like are referred to by the same numerals, and a description thereof will be omitted. 
         [0026]    By referring to  FIGS. 1A and 1B  and  FIGS. 2A and 2B , a description will be given below with respect to the case in which an optical waveguide inserted into the slit of a lens ferrule fails to be fastened at desired position.  FIG. 1A  is an axonometric view of a lens ferrule.  FIG. 1B  is an axonometric cross-sectional view of the lens ferrule.  FIG. 2A  is an axonometric view of a lens ferrule having an optical guide placed therein.  FIG. 2B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 2A . 
         [0027]    A lens ferrule  910  is made of a resin material such as COP (i.e., cycloolefin polymer) resin by resin molding using a mold. An optical waveguide sheet  950 , which is made of a resin material, has one or more cores transmitting light and covered with a clad. The same arrangement is used for the examples that will be described herein. 
         [0028]    The lens ferrule  910  has an opening  911  for receiving the optical waveguide sheet  950 , and also has, at the back end of the opening  911 , a slit  912  into which the end of the optical waveguide sheet  950  is inserted. The optical waveguide sheet  950  is secured with adhesive inside the slit  912  with the end thereof abutting the back end of the slit  912 . 
         [0029]    The thickness of the optical waveguide sheet  950  exhibits some variation. In consideration of this, the thickness (i.e., height) of the slit  912  is designed to be slightly greater than the thickness of the optical waveguide sheet  950  to allow the optical waveguide sheet  950  to be reliably inserted into the slit  912 . For example, the thickness of the slit  912  is 108 micrometers, and the thickness of the optical waveguide sheet  950  is 105 micrometers. The thickness of the slit refers to the dimension of the slit measured in the same direction as the thickness direction of the optical waveguide. 
         [0030]    A mold is used for molding the lens ferrule  910 . Thinning the portion of the mold corresponding to the slit for the purpose of thinning the slit  912  may result in this portion of the mold being undesirably warped at the time of molding the ferrule. This may result in a failure to form the slit  912  with high precision. This portion of the mold thus needs to have a certain thickness, which prevents the slit  912  from being sufficiently thin. 
         [0031]    Since the thickness of the optical waveguide sheet  950  is 3 micrometers thinner than the thickness of the slit  912 , the optical waveguide sheet  950  may shift vertically inside the slit  912 , causing warpage of the optical waveguide sheet  950 . Warpage of the optical waveguide sheet  950  inside the slit  912  causes the optical waveguide sheet  950  to be fastened inside the slit  912  away from the desired position, thereby causing part or all of the light exiting from the optical waveguide sheet  950  to fail to enter the lenses of the lens ferrule  910 , or causing part or all of the light condensed by the lenses of the lens ferrule  910  to fail to enter the cores of the optical waveguide sheet  950 . As a result, light loss occurs, which undermines optical communication. 
         [0032]    As was previously noted, the lens ferrule  910  is made by resin molding using a mold. The portion of the mold for making the slit  912  is 108 micrometers in thickness, and is thus easy to bend. Warpage of the portion of the mold for making the slit  912  at the time of resin molding results in the slit  912  being formed at an incorrect position in the lens ferrule  910 . Displacement of the slit  912  from the desired position causes the contact points of the optical waveguide sheet  950  to be also displaced, resulting in light loss, lowered reliability, and lowered yield. 
       First Embodiment 
       [0033]    In the following, a lens ferrule of the first embodiment will be described by referring to  FIGS. 3A and 3B  through  FIGS. 5A and 5B .  FIG. 3A  is an axonometric view of a lens ferrule according to the present embodiment.  FIG. 3B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 3A .  FIG. 4A  is an axonometric view of the lens ferrule into which an optical waveguide is inserted halfway through.  FIG. 4B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 4A .  FIG. 5A  is an axonometric view of the lens ferrule to which an optical waveguide is coupled.  FIG. 5B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 5A . 
         [0034]    The lens ferrule of the present embodiment includes a ferrule body  10  and a pressing member  30 . 
         [0035]    The ferrule body  10  has an opening  11  for receiving an optical waveguide  50 , and also has a slit  12  at the back end of the opening  11 . Further, the back face of the slit  12  has an upper opening  13  opening upwardly. The opening  11  and the upper opening  13  communicate with each other. Although not illustrated, lenses are disposed on the back face of the ferrule body  10  at the position corresponding to the cores of an optical waveguide  50 . The pressing member  30  serves to press the optical waveguide  50  from above in the ferrule body  10 . 
         [0036]    The slit  12  has a thickness (i.e., height) equal to, or slightly smaller than, the thickness of the optical waveguide  50 , such that the optical waveguide  50  inserted into the slit  12  does not disengage (i.e., fall off). The ferrule body  10  and the pressing member  30  are made of a resin material such as the COP resin by resin molding using a mold. An optical waveguide sheet  50 , which is made of a resin material, has one or more cores transmitting light and covered with a clad. 
         [0037]    The face of the upper opening  13  with which an end  50   a  of the optical waveguide  50  comes in contact has a step  15  formed thereon. The step  15  has a step lateral face  16  perpendicular to a bottom face  14  of the upper opening  13  and a step upper face  17  parallel to the bottom face  14  of the upper opening  13 . As the optical waveguide  50  is inserted into the opening  11 , the end  50   a  of the optical waveguide  50  comes in contact with the step lateral face  16 . In this state, the optical waveguide  50  is in its intended position, and the step  15  is formed to achieve such positioning. The step lateral face serves as a contact face that comes in contact with the end  50   a  of the optical waveguide  50 . The height of the step  15 , i.e., the height of the step lateral face  16 , is substantially the same as the thickness of the optical waveguide  50 . 
         [0038]    The face of the upper opening  13  with which the end  50   a  of the optical waveguide  50  comes in contact has at the center thereof a bulging face  18  bulging toward the inside of the upper opening  13 . The face of the bulging face  18  coming in contact with the end  50   a  of the optical waveguide  50  is flush with the step lateral face  16 . This serves to prevent the end  50   a  of the optical waveguide  50  from exceeding the step lateral face  16  when the optical waveguide  50  is inserted into the opening  11 . 
         [0039]    The provision of the bulging face  18  ensures that the optical waveguide  50  inserted into the opening  11  stops when the end  50   a  of the optical waveguide  50  comes in contact with the step lateral face  16 . 
         [0040]    The opening  11  has a slope part  19  whose vertical gap (i.e., height) gradually decreases toward the slit  12  away from the entrance of the opening  11 . The slope part  19  has an upper slope face  19   a  and a lower slope face  19   b . The upper slope face  19   a  and the lower slope face  19   b  are symmetric with each other with respect to the center line defined by the optical waveguide  50  inserted into the opening  11 . The symmetric structure of the upper slope face  19   a  and the lower slope face  19   b  is provided for the purpose of easy insertion of the optical waveguide  50  into the slit  12 . An asymmetric structure of the upper slope face  19   a  and the lower slope face  19   b  may lead to the occurrence of thickness variation when the ferrule body  10  is formed by resin molding. The symmetric structure of the upper slope face  19   a  and the lower slope face  19   b  serves to prevent the occurrence of thickness variation, thereby providing a lens ferrule with high evenness. 
         [0041]    The center of the area where the slit  12  is formed has a space serving as a slit rib  20 . The position at which the slit rib  20  is formed has a greater height (i.e., vertical gap length) than the slit  12 . The slit rib  20  includes an upper slit rib  20   a  and a lower slit rib  20   b . With the optical waveguide  50  being in the inserted position, the upper slit rib  20   a  and the lower slit rib  20   b  are vertically symmetric with each other with respect to the center line defined by the optical waveguide  50 . Since the thickness of the slit  12  is small, the mold used for making the ferrule body  10  has a portion corresponding to the slit  12  that is easy to bend. The provision of the slit rib  20  in the lens ferrule of the present embodiment serves to thicken part of the portion of the mold corresponding to the slit  12 , thereby making the portion of the mold for making the slit  12  more robust against warpage. 
         [0042]    The upper opening  13  is formed on the opposite side of the ferrule body  10  from where the opening  11  is situated. The optical waveguide  50  is pressed between the pressing member  30  and the bottom face  14  at the position of the upper opening  13 . It may be noted that a slit is nonexistent in the area where the upper opening  13  is formed. In the present embodiment, thus, there is not much need to ensure the precision of slit formation at the position where the end  50   a  of the optical waveguide  50  is placed. 
         [0043]    In the following, a description will be given with respect to the process steps for coupling the optical waveguide  50  to the lens ferrule of the present embodiment. First, adhesive is applied to the internal faces of the opening  11 . Next, the optical waveguide  50  is inserted into the opening  11  as illustrated in  FIGS. 4A and 4B . As the optical waveguide  50  is inserted into the opening  11 , the optical waveguide  50  is guided by the upper slope face  19   a  and the lower slope face  19   b  situated at the back of the opening  11  to enter the slit  12 . The thickness of the slit  12  is the same as, or slightly smaller than, the thickness of the optical waveguide  50 . Since the optical waveguide  50  made of a resin material is soft, the optical waveguide  50  is able to reach deep into the slit  12 . 
         [0044]    As the optical waveguide  50  is further inserted deep into the opening  11 , the end  50   a  of the optical waveguide  50  exits from the slit  12  to travel on the bottom face  14  in the upper opening  13 , and comes to a stop upon making a contact with the step lateral face  16 . Since the bulging face  18  is present on the lateral face of the upper opening  13 , the optical waveguide  50  does not go beyond the step lateral face  16 . Further, the slit  12  having a thickness slightly smaller than the thickness of the optical waveguide  50  serves to temporarily fix the optical waveguide  50 , thereby being able to maintain the condition in which the end  50   a  of the optical waveguide  50  is in contact with the step lateral face  16 . 
         [0045]    Subsequently, the pressing member  30  is inserted into the upper opening  13  as illustrated in  FIGS. 5A and 5B . Horizontal movement of the pressing member  30  placed in the upper opening  13  illustrated in  FIGS. 5A and 5B  is restricted. The optical waveguide  50  is placed in the opening  11  of the lens ferrule to extend on the bottom face  14  and to have the end  50   a  in contact with the step lateral face  16 . Inserting the pressing member  30  in the upper opening  13  causes the optical waveguide  50  to be pressed between the pressing member  30  and the bottom face  14  to be secured at the desired position. When the pressing member  30  is inserted into the upper opening  13 , the left-hand end of the bottom face of the pressing member  30  in  FIGS. 4A and 4B  comes in with the step upper face  17 . The height of the step  15  of the present embodiment is designed to be substantially the same as the thickness of the optical waveguide  50 . As the above-noted portion of the bottom face of the pressing member  30  comes in contact with the step upper face  17 , the pressing force of the pressing member  30  is not excessively applied to the optical waveguide  50 , thereby preventing warpage of the optical waveguide  50 . In this state, the optical waveguide  50  is fastened with adhesive. 
       &lt;Production Method&gt; 
       [0046]    In the following, a description will be given of the making of the lens ferrule according to the present embodiment. The lens ferrule of the present embodiment is made by use of molds. Specifically, a mold for making the ferrule body  10  and a mold for making the pressing member  30  are used to produce the ferrule body  10  and the pressing member  30 , respectively. 
         [0047]    The mold for making the pressing member  30  has a hole having the same shape as the pressing member  30 . Resin material is poured into the mold, and is cured to make the pressing member  30 . 
         [0048]    The mold for making the ferrule body  10  includes a mold (not shown) for making the outer shape of the ferrule body  10 , a mold  60  having the same shape as the opening  11  as illustrated in  FIGS. 6A through 6C , and a mold  70  having the same shape as the upper opening  13 . The mold  70  is for making the upper opening  13  in the ferrule body  10 . 
         [0049]    In order to make the ferrule body  10 , the mold  60  illustrated in  FIG. 6A  is arranged at a desired position relative to the mold for making the outer shape of the ferrule body  10 . The mold  70  is then arranged at a desired position relative to the mold  60  and the mold (not shown) for making the outer shape of the ferrule body  10  as illustrated in  FIG. 6B . In this state, the mold  70  is situated immediately alongside the mold  60  as illustrated in  FIG. 6B . Subsequently, resin material is poured into the mold and cured, so that the resin material covers the mold  60  and the mold  70  as illustrated in  FIG. 6C .  FIG. 6C  is an axonometric cross-sectional view of the ferrule body  10  as observed after the mold for making the outer shape of the ferrule body  10  is removed. The mold  60  and the mold  70  are then removed to produce the ferrule body  10 . 
         [0050]    The mold  60  has a slit forming portion  61  for making the slit  12  and a rib forming portion  62  for making the slit rib  20 . The rib forming portion is situated at the center of the slit forming portion  61 . The thickness of the slit forming portion  61  is as thin as approximately 105 micrometers or less, and is thus easy to warp. The mold  60  of the present embodiment has an increased thickness at the rib forming portion  62 , thereby avoiding warpage of the slit forming portion  61  at the time of pouring resin material around the mold  60 . 
       Second Embodiment 
       [0051]    In the following, a description will be given of a lens ferrule according to a second embodiment.  FIG. 7A  is an axonometric view of a lens ferrule according to the present embodiment.  FIG. 7B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 7A . 
         [0052]    As was previously described, the thickness of the slit receiving an optical waveguide in a lens ferrule is small, so that the portion of a mold for making the slit is thin and easy to warp. As a result, the thin portion of the mold for making a slit may warp at the time of making a lens ferrule, resulting in the slit being formed at a displaced position. In such a case, coupling an optical waveguide to such a produced lens ferrule may not produce desired characteristics. 
         [0053]    A lens ferrule  110  of the present embodiment includes an opening  111  and a slit  112  situated at the back of the opening  111 . A penetrating hole  121  is vertically formed to penetrate through the lens ferrule  110  at the center of the slit  112 , and penetrating holes  122  and  123  are vertically formed to penetrate through the lens ferrule  110  near the side ends of the slit  112 . 
         [0054]    In the following, a description will be given of a method of making the lens ferrule  110  according to the present embodiment. The lens ferrule  110  is made by using a mold (not shown) for making the outer shape of the lens ferrule  110 , a mold  160  having the same shape as part of the opening  111  as illustrated in  FIG. 8A , cylindrical pins  171   a  and  171   b  disposed on and beneath the mold  160 , respectively, for making the penetrating hole  121 , quadrangular prisms  172   a  and  172   b  disposed on and beneath the mold  160 , respectively, for making the penetrating hole  122 , and quadrangular prisms  173   a  and  173   b  disposed on and beneath the mold  160 , respectively, for making the penetrating hole  123 . 
         [0055]    As illustrated in  FIG. 8A , the mold  160  is arranged at a desired position relative to the mold for making the outer shape of the ferrule body  10 , followed by placing the pins  171   a  and  171   b  such that the center of a thin portion  161  of the mold  160  for making the slit  112  is placed between the pins  171   a  and  171   b . Further, the pins  172   a  and  172   b  are placed to hold therebetween one of the side ends of the portion  161  of the mold  160  for making the slit  112 , and the pins  173   a  and  173   b  are placed to hold therebetween the other one of the side ends. 
         [0056]    Subsequently, resin material is poured around the mold  160  such that the resin material covers the portion  161  of the mold  160  as illustrated in  FIG. 8B .  FIG. 8B  is an axonometric cross-sectional view of the lens ferrule  110  as observed without the mold for making the outer shape thereof. 
         [0057]    The resin material is then cured, followed by removing the mold for making the outer shape of the lens ferrule  110 , the mold  160 , the pins  171   a ,  171   b ,  172   a ,  172   b ,  173   a , and  173   b , thereby producing the lens ferrule  110 . 
         [0058]    In the present embodiment, the portion  161  for making the slit  112  is placed and held between the pins  171   a  and  171   b  at the center, between the pins  172   a  and  172   b  at or near one side end thereof, and between the pins  173   a  and  173   b  at or near the other side end, which prevents warpage of the portion  161 , and allows the slit  112  to be formed at the desired position with high precision. 
         [0059]    In the present embodiment, the penetrating hole  121  allows excessive adhesive and bubbles in the adhesive to escape. By referring to  FIGS. 9A through 9C , a process step of connecting the optical waveguide  50  to the lens ferrule  110  will be described. 
         [0060]    The opening  111  inclusive of the slit  112  illustrated in  FIG. 9A  is provided with adhesive  140  as illustrated in  FIG. 9B . The adhesive  140  enters the penetrating hole  121  extending over and under the slit  112 . At this time, bubbles may be formed inside the adhesive  140 . Subsequently, the optical waveguide  50  is inserted into the slit  112  as illustrated in  FIG. 9C , which causes the adhesive  140  inside the slit  112  to flow into the penetrating hole  121 . Bubbles contained in the adhesive  140  also move toward the penetrating hole  121  together with the adhesive  140 . Inserting the optical waveguide  50  into the slit  112  causes the adhesive  140  inside the slit  112  to be extruded into the penetrating hole  121 , so that insertion of the optical waveguide  50  into the slit  112  is smoothly conducted. The adhesive  140  is thereafter cured, which allows the optical waveguide  50  to be securely connected to the lens ferrule  110 . 
       Third Embodiment 
       [0061]    In the following, a lens ferrule of the third embodiment will be described by referring to  FIGS. 10A and 10B .  FIG. 10A  is an axonometric view of a lens ferrule according to the present embodiment.  FIG. 10B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 10A . The lens ferrule of the present embodiment is formed while a mold for making an opening is held between other molds placed over and under the mold. 
         [0062]    A lens ferrule  210  of the present embodiment has an opening  211  and a slit  212 . An upper opening  213  is formed over the opening  211 , and a lower opening  214  is formed under the opening  211 . 
         [0063]    The lens ferrule  210  is formed by using a mold (not shown) for making the outer shape of the lens ferrule  210 , a mold  260  having the same shape as part of the opening  211  as illustrated in  FIG. 11 , an upper mold  271  disposed on the mold  260  for making the upper opening  213 , and a lower mold  272  disposed beneath the mold  260  for making the lower opening  214 . 
         [0064]    The mold  260  is arranged at the desired position for making the opening  211  relative to the mold for making the outer shape of the lens ferrule  210 , followed by placing the upper mold  271  on the mold  260 , and placing the lower mold  272  beneath the mold  260 . As a result, the mold  260  is placed and held between the upper mold  271  and the lower mold  272 , so that the mold  260  is securely positioned relative to the outer mold. 
         [0065]    Subsequently, resin material is poured into and around the molds, and is then cured, followed by removing the mold for making the outer shape of the lens ferrule  210 , the mold  260 , the upper mold  271 , and the lower mold  272  to produce the lens ferrule  210 . In the present embodiment, resin material is poured and cured while the mold  260  is placed and held between the upper mold  271  and the lower mold  272 . The mold  260  is thus securely held between the upper mold  271  and the lower mold  272  to allow the slit  212  to be formed at the desired position with high precision. 
       Fourth Embodiment 
       [0066]    In the following, a lens ferrule of the fourth embodiment will be described by referring to  FIGS. 12A and 12B .  FIG. 12A  is an axonometric view of a lens ferrule according to the present embodiment.  FIG. 12B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 12A . 
         [0067]    A lens ferrule  310  of the present embodiment has an opening  311  and a slit  312 . The area where the slit  312  is formed has a slit rib  313  that is a space extending downwardly at the center of the slit  312  in the width direction in  FIG. 12A . The slit rib  313  extends to the back face of the slit  312  with which the end of an optical waveguide comes in contact. 
         [0068]    The lens ferrule  310  is made by resin molding using a mold (not shown) for making the outer shape of the lens ferrule  310  and a mold  360  having the same shape as part of the opening  311  as illustrated in  FIG. 13 . The mold  360  has a slit forming portion  361  corresponding to the slit  312  and a slit rib forming portion  362  corresponding to the slit rib  313 . As for the thickness of the mold  360 , the slit rib forming portion  362  is thicker than the slit forming portion  361 .  FIG. 13  is an axonometric cross-sectional view of the lens ferrule  310  as observed after pouring resin material and removing the mold for making the outer shape of the lens ferrule  310 . 
         [0069]    The mold  360  is properly arranged relative to the mold for making the outer shape of the lens ferrule  310 , and, then, resin material is poured and cured, followed by removing the mold  360  to produce the lens ferrule  310  having the opening  311 . 
         [0070]    The slit rib forming portion  362  of the mold  360  is thick and robust against warpage. The provision of the slit rib forming portion  362  under the slit forming portion  361  is thus able to reduce warpage of the thin slit forming portion  361  also. 
       Fifth Embodiment 
       [0071]    In the following, a lens ferrule of the fifth embodiment will be described by referring to  FIGS. 14A and 14B .  FIG. 14A  is an axonometric view of a lens ferrule according to the present embodiment.  FIG. 14B  is an axonometric cross-sectional view of the lens ferrule of  FIG. 14A . A lens ferrule of the present embodiment is symmetric with respect to the center line defined by an optical waveguide. 
         [0072]    A lens ferrule  410  made of the COP resin or the like has an opening  411  and a slit  412 . The opening  411  has an upper slope face  419   a  and a lower slope face  419   b  such that the height (i.e., vertical gap) of the opening  411  gradually decreases toward the slit  412  situated at the back of the opening  411  away from the entrance. The upper slope face  419   a  and the lower slope face  419   b  are vertically symmetric with each other. In the present embodiment, the upper slope face  419   a  and the lower slope face  419   b  are flat plane face. The provision of the upper slope face  419   a  and the lower slope face  419   b  allows an optical waveguide inserted into the opening  411  to be smoothly guided toward the slit  412 . 
         [0073]    In the case of the temperature for forming the lens ferrule  410  being 80 degrees Celsius, for example, the lens ferrule  410  may suffer stress caused by curing contraction or the like during the curing process in which the temperature is dropped from 80 degrees Celsius to room temperature. The symmetrical structure of the upper slope face  419   a  and the lower slope face  419   b  ensures that the distribution of applied stress at the time of forming the lens ferrule  410  is also symmetric, thereby reducing warpage of the lens ferrule  410 . 
         [0074]    Stress may concentrate on corners to cause a crack at such corners. The upper slope face  419   a  and the lower slope face  419   b  are flat plane faces without any corners at which stress would concentrate, so that cracks caused by concentration of stress may be reduced. 
         [0075]    An upper rib  420   a  is formed from halfway through the upper slope face  419   a  to the slit  412 , and a lower rib  420   b  is formed from halfway through the lower slope face  419   b  to the slit  412 . The upper rib  420   a  and the lower rib  420   b  formed as described above are vertically symmetric with each other. 
         [0076]    The lens ferrule  410  is made by using a mold (not shown) for making the outer shape thereof and a mold  460  having the same shape as the opening  411 . An end of the mold  460  on the left-hand side of  FIG. 15  has a slit forming portion for making the slit  412 . Lib forming portions for making the upper rib  420   a  and the lower rib  420   b  are provided above and below, respectively, the rib forming portion. The combined thickness of the rib forming portions is greater than the thickness of the slip forming portion, which provides robustness against warpage, thereby preventing warpage of the slip forming portion. 
         [0077]    The mold  460  is arranged relative to the outer mold (not shown) such as to form the opening  411  at the desired position, and, then, resin material is poured around and into the molds and cured, followed by removing the mold  460  to produce the lens ferrule  410 . The symmetric structure of the mold  460  of the present embodiment reduces thickness variation of resin material, thereby serving to produce a lens ferrule having a desired shape with high evenness. 
         [0078]    According to at least one embodiment, an optical guide is allowed to be fastened at desired position. 
         [0079]    Further, although a description has been given with respect to one or more embodiments of the present invention, the contents of such a description do not limit the scope of the invention. 
         [0080]    The present application is based on and claims the benefit of priority of Japanese priority application No. 2016-133579 filed on Jul. 5, 2016, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.