Patent Publication Number: US-2023141449-A1

Title: Optical connection structure, ferrule, and optical connector

Description:
TECHNICAL FIELD 
     The present disclosure relates to an optical connection structure, a ferrule, and an optical connector. 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-058258 filed on Mar. 27, 2020, and the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     Patent Literature 1 discloses a technique for aligning multi-core optical fibers with each other using a guide pin. In this technique, end portions of a pair of guide pins are respectively inserted into a pair of guide pin insertion holes provided in the tip surface of a ferrule, and the other end portions of the pair of guide pins are respectively inserted into a pair of guide pin insertion holes provided in the tip surface of a connection counterpart ferrule. As a result, the alignment of the multi-core optical fibers (that is, the alignment of the multi-core optical fiber and the multi-core optical fiber of the connection counterpart) is performed. 
     Citation List 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2019-90974 
     SUMMARY OF INVENTION 
     Technical Problem 
     An optical connection structure according to one embodiment of the present disclosure includes: a plurality of optical fibers; a ferrule holding the plurality of optical fibers; and a tubular adapter where the ferrule is inserted and fitted such that the ferrule and another ferrule face each other in the tubular shape. The ferrule has a first side surface and a second side surface facing each other. The first side surface is provided with a first recess portion or a first protrusion portion extending along a first direction in which the ferrule is inserted into the adapter. The second side surface is provided with a second recess portion or a second protrusion portion extending along the first direction. An inner surface of the adapter is provided with a third protrusion portion or a third recess portion fittable with the first recess portion or the first protrusion portion and a fourth protrusion portion or a fourth recess portion fittable with the second recess portion or the second protrusion portion. 
     A ferrule according to one embodiment of the present disclosure includes: a plurality of optical fiber holding portions for respectively holding a plurality of optical fibers; and a first side surface and a second side surface facing each other. The first side surface is provided with a first recess portion or a first protrusion portion extending along a first direction in which the plurality of optical fiber holding portions extend. The second side surface is provided with a second recess portion or a second protrusion portion extending along the first direction. 
     An optical connector according to one embodiment of the present disclosure includes: the ferrule described above; and the plurality of optical fibers respectively held in the plurality of optical fiber holding portions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a perspective view illustrating a state where an optical connector is attached to an adapter in an optical connection structure according to one embodiment. 
         FIG.  1 B  is a perspective view illustrating a state where the optical connector is removed from the adapter in the optical connection structure according to one embodiment. 
         FIG.  2    is a perspective view illustrating a ferrule according to one embodiment. 
         FIG.  3    is a front view illustrating the ferrule according to one embodiment. 
         FIG.  4    is a cross-sectional view of the ferrule along the IV-IV line of  FIG.  3   . 
         FIG.  5    is a cross-sectional view illustrating the adapter in a state where the ferrule is inserted and fitted. 
         FIG.  6    is a cross-sectional view illustrating an adapter in a state where a ferrule is inserted and fitted in an optical connection structure according to a first modification example. 
         FIG.  7    is a cross-sectional view illustrating an adapter in a state where a ferrule is inserted and fitted in an optical connection structure according to a second modification example. 
         FIG.  8    is a cross-sectional view of the adapter along the VIII-VIII line of  FIG.  7   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problems to Be Solved by Present Disclosure 
     The following problems can arise in positioning a plurality of optical fibers and the plurality of optical fibers of a connection counterpart using a guide pin and a ferrule provided with a guide pin insertion hole as in the technique disclosed in Patent Literature 1. For example, in order to position the plurality of optical fibers and the plurality of optical fibers of the connection counterpart with high accuracy, a guide pin with high dimensional accuracy is required such that the clearance between the guide pin insertion hole and the guide pin is as small as possible. Further, in cleaning the ferrule with the guide pin inserted in the guide pin insertion hole, foreign matter such as dust near the guide pin may not be completely removable. In this case, the foreign matter may become a hindrance, the positioning accuracy of the plurality of optical fibers and the plurality of optical fibers of the connection counterpart may decline, and an increase in connection loss may arise. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     Effect of Present Disclosure 
     According to the optical connection structure, the ferrule, and the optical connector according to the present disclosure, it is possible to position a plurality of optical fibers with a simple configuration. 
     Description of Embodiment of Present Disclosure 
     First, the content of an embodiment of the present disclosure will be listed and described. An optical connection structure according to one embodiment of the present disclosure includes: a plurality of optical fibers; a ferrule holding the plurality of optical fibers; and a tubular adapter where the ferrule is inserted and fitted such that the ferrule and another ferrule face each other in the tubular shape. The ferrule has a first side surface and a second side surface facing each other. The first side surface is provided with a first recess portion or a first protrusion portion extending along a first direction in which the ferrule is inserted into the adapter. The second side surface is provided with a second recess portion or a second protrusion portion extending along the first direction. An inner surface of the adapter is provided with a third protrusion portion or a third recess portion fittable with the first recess portion or the first protrusion portion and a fourth protrusion portion or a fourth recess portion fittable with the second recess portion or the second protrusion portion. 
     In this optical connection structure, when the ferrule is inserted into the adapter and fitted, the first recess portion or the first protrusion portion is fitted to the third protrusion portion or the third recess portion and the second recess portion or the second protrusion portion is fitted to the fourth protrusion portion or the fourth recess portion. As a result, the position of the ferrule with respect to the adapter (that is, the position of the plurality of optical fibers held by the ferrule) can be defined in a plane perpendicular to the first direction. In other words, by using the adapter where the ferrule is inserted and fitted as a positioning member at the time of positioning the plurality of optical fibers, the plurality of optical fibers can be positioned without providing a guide pin insertion hole in the ferrule. As a result, it is not necessary to use a high-dimensional accuracy guide pin for positioning between the plurality of optical fibers (that is, positioning between the plurality of optical fibers and the plurality of optical fibers of the connection counterpart). Further, it is possible to avoid a situation in which the use of a foreign matter-attached guide pin leads to a decline in the positioning accuracy of the plurality of optical fibers. As a result, it is possible to suppress a decline in connection loss between the plurality of optical fibers and the plurality of optical fibers of the connection counterpart. Accordingly, according to the optical connection structure described above, the plurality of optical fibers can be positioned with a simple configuration. 
     The ferrule may further have a plurality of optical fiber holding portions respectively holding the plurality of optical fibers. The plurality of optical fiber holding portions may be disposed side by side along a second direction intersecting the first direction. In this case, it is possible to suitably realize a configuration in which the plurality of optical fibers are positioned by inserting the ferrule into the adapter and fitting the ferrule. 
     Each of the first recess portion or the first protrusion portion and the second recess portion or the second protrusion portion may be V-shaped in a cross section perpendicular to the first direction. In this case, the ferrule can be accurately positioned with respect to the adapter. In other words, the positioning of the plurality of optical fibers can be performed with high accuracy. 
     Each of the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion may be V-shaped in a cross section perpendicular to the first direction. In this case, the ferrule can be accurately positioned with respect to the adapter. In other words, the positioning of the plurality of optical fibers can be performed with high accuracy. 
     The first side surface may be provided with the first recess portion, and the second side surface may be provided with the second recess portion. The inner surface of the adapter may be provided with the third protrusion portion fittable with the first recess portion and the fourth protrusion portion fittable with the second recess portion. In this case, an increase in the width of the ferrule can be suppressed as compared with a case where the first side surface and the second side surface are provided with the first protrusion portion and the second protrusion portion, respectively. In other words, an increase in the size of the ferrule can be suppressed. 
     The first recess portion or the first protrusion portion may be capable of coming into contact with the third protrusion portion or the third recess portion and the second recess portion or the second protrusion portion may be capable of coming into contact with the fourth protrusion portion or the fourth recess portion in a plane perpendicular to the first direction. In this case, a positional deviation of the ferrule with respect to the adapter can be suppressed, and thus the positioning of the plurality of optical fibers can be performed with high accuracy. 
     At least one of the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion may be configured to be elastically deformable in a second direction intersecting the first direction. In this case, the ferrule can be easily inserted into the adapter, and thus the workability in inserting the ferrule into the adapter is improved. Further, when the ferrule is inserted into the adapter, in a case where the first recess portion or the first protrusion portion and the second recess portion or the second protrusion portion respectively abut against the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion, a force that causes at least one of the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion to return to the original position is applied to the ferrule. As a result, the ferrule is held and fixed by the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion, and thus a positional deviation of the ferrule with respect to the adapter is suppressed. As a result, the positioning of the plurality of optical fibers can be performed with high accuracy. 
     The adapter may have a pair of regions positioned on both sides in the second direction with the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion interposed between the regions and respectively provided with the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion. A hollow portion may be provided in the region as one of the pair of regions where at least one of the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion is provided. In this case, at least one of the third protrusion portion or the third recess portion and the fourth protrusion portion or the fourth recess portion can be elastically deformed with ease in the second direction. As a result, the ferrule can be more easily inserted into the adapter, and thus the workability in inserting the ferrule into the adapter is further improved. 
     A ferrule according to one embodiment of the present disclosure includes: a plurality of optical fiber holding portions for respectively holding a plurality of optical fibers; and a first side surface and a second side surface facing each other. The first side surface is provided with a first recess portion or a first protrusion portion extending along a first direction in which the plurality of optical fiber holding portions extend. The second side surface is provided with a second recess portion or a second protrusion portion extending along the first direction. 
     When this ferrule is inserted into the adapter and fitted, by using the first recess portion or the first protrusion portion and the second recess portion or the second protrusion portion as positioning guides with respect to the adapter, the position of the ferrule with respect to the adapter (that is, the position of the plurality of optical fibers held by the ferrule) can be defined in a plane perpendicular to the first direction. In other words, by using the adapter where the ferrule is inserted and fitted as a positioning member at the time of positioning the plurality of optical fibers, the plurality of optical fibers can be positioned without providing a guide pin insertion hole in the ferrule. As a result, it is not necessary to use a high-dimensional accuracy guide pin for positioning between the plurality of optical fibers. Further, it is possible to avoid a situation in which the use of a foreign matter-attached guide pin leads to a decline in the positioning accuracy of the plurality of optical fibers. As a result, it is possible to suppress a decline in connection loss between the plurality of optical fibers and the plurality of optical fibers of the connection counterpart. Accordingly, according to the ferrule described above, the plurality of optical fibers can be positioned with a simple configuration. 
     The plurality of optical fiber holding portions may be disposed side by side along a second direction intersecting the first direction. In this case, it is possible to suitably realize a configuration in which the plurality of optical fibers are positioned by inserting the ferrule into the adapter and fitting the ferrule. 
     Each of the first recess portion or the first protrusion portion and the second recess portion or the second protrusion portion may be V-shaped in a cross section perpendicular to the first direction. In this case, the ferrule can be accurately positioned with respect to the adapter. In other words, the positioning of the plurality of optical fibers can be performed with high accuracy. 
     An optical connector according to one embodiment of the present disclosure includes: the ferrule according to any of the above; and the plurality of optical fibers respectively held in the plurality of optical fiber holding portions. Since this optical connector includes the ferrule according to any of the above, the plurality of optical fibers can be positioned with a simple configuration as described above. 
     Details of Embodiment of Present Disclosure 
     Hereinafter, one embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same reference numerals will be used for the same or functionally identical elements with redundant description omitted. 
       FIG.  1 A  is a perspective view illustrating a state where an optical connector  2  is attached to an adapter  40  in an optical connection structure  1  according to the present embodiment.  FIG.  1 B  is a perspective view illustrating a state where the optical connector  2  is removed from the adapter  40  in the optical connection structure  1  according to the present embodiment. As illustrated in  FIGS.  1 A and  1 B , the optical connection structure  1  includes the optical connector  2  and the adapter  40  into which the optical connector  2  is inserted. The optical connector  2  has an optical fiber tape core wire  5  accommodating a plurality of optical fibers  10  and a ferrule  20  attached to the tip portion of the optical fiber tape core wire  5  via a boot  15 . 
     The ferrule  20  has, for example, a substantially rectangular parallelepiped appearance. The ferrule  20  is configured by, for example, a material such as polyphenylene sulfide (PPS), polyetherimide (PEI), polycarbonate (PC), polymethylmethacrylate (PMMA), and polyethersulfone (PES). The ferrule  20  is inserted into the adapter  40  along, for example, a direction D1 and fitted to the adapter  40 . 
     The adapter  40  has a tubular shape capable of accommodating the ferrule  20 . The adapter  40  is fitted with the ferrule  20  such that a tip surface  21  of the ferrule  20  and the tip surface of a connection counterpart ferrule (not illustrated) face each other in the adapter  40 . The adapter  40  is configured by, for example, an elastic material having elasticity such as polyetherimide (PEI), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethersulfone (PES), and polyamide (PA). In order to reduce the difference between the coefficient of linear expansion of the material of the adapter  40  and the coefficient of linear expansion of the material of the ferrule  20 , it is preferable to use the same material as the ferrule  20  as the material of the adapter  40 . The material of the adapter  40  may contain a filler or an additive for slidability improvement. In the adapter  40 , the tip surface of the ferrule  20  and the tip surface of the connection counterpart ferrule may be in contact with each other by abutting against each other or may be separated from each other with a predetermined distance. 
     The plurality of optical fibers  10  of the optical fiber tape core wire  5  extend along the direction D1 and are disposed side by side along a direction D2 intersecting (for example, orthogonal to) the direction D1. In the optical fiber tape core wire  5 , the plurality of optical fibers  10  are disposed so as to overlap in a plurality of stages. The plurality of optical fibers  10  are inserted along the direction D1 into a plurality of optical fiber holes H (see  FIG.  4    to be described later) formed in the ferrule  20 . 
     Next, the shape of the ferrule  20  will be described in detail with reference to  FIGS.  2 ,  3 , and  4   .  FIG.  2    is a perspective view illustrating the ferrule  20 .  FIG.  3    is a front view illustrating the ferrule  20 .  FIG.  4    is a cross-sectional view of the ferrule  20  along the IV-IV line of  FIG.  3   . As illustrated in  FIGS.  2 ,  3 , and  4   , the longitudinal direction of the ferrule  20  is along the direction D1, the lateral direction of the ferrule  20  is along the direction D2, and the thickness direction of the ferrule  20  (that is, up-down direction) is along a direction D3 intersecting (for example, orthogonal to) the directions D1 and D2. The ferrule  20  has the tip surface  21  positioned at the tip in the direction D1, a rear end surface  22  positioned at the rear end in the direction D1, and four side surfaces  23 ,  24 ,  25 , and  26  connecting the tip surface  21  and the rear end surface  22  and extending along the direction D1. 
     As illustrated in  FIGS.  2  and  4   , the tip surface  21  and the rear end surface  22  extend along the directions D2 and D3. A light transmitting surface  21   a  is provided in the middle portion of the tip surface  21 . The light transmitting surface  21   a  is slightly recessed to the rear end surface  22  side in the direction D1 with respect to the tip surface  21 . The light transmitting surface  21   a  is provided with a plurality of lenses  21   b . The optical axes of the lenses  21   b  are disposed so as to respectively overlap the central axes of the optical fibers  10  when viewed from the direction D1. The optical fibers  10  respectively abut against back surfaces  21   c  of the lenses  21   b  (that is, surfaces positioned on the side opposite to each lens  21   b  in the direction D1). The light emitted from the optical fibers  10  is collimated by the respective lenses  21   b  and then incident on the respective optical fibers of the connection counterpart. The central axis of the optical fiber  10  and the optical axis of the lens  21   b  may be deviated from each other in order to suppress the reflected return light to the end surface of the optical fiber  10 . Likewise, the end surface of the optical fiber  10  or the surface of the lens  21   b  may be inclined by, for example, 8° with respect to the direction D3 for the purpose of suppressing reflected return light. 
     The side surfaces  23  and  24  face each other in the direction D3 and extend along the directions D1 and D2. In one example, the side surfaces  23  and  24  extend parallel to each other. The side surface  23  is provided with openings  23   a  and  23   b . The opening  23   a  is positioned on the tip surface  21  side in the direction D1 with respect to the opening  23   b . As illustrated in  FIG.  4   , the plurality of optical fiber holes H for respectively holding the plurality of optical fibers  10  are provided in the ferrule  20 . The plurality of optical fiber holes H extend along the direction D1 and are disposed side by side along the direction D2. The plurality of optical fibers  10  are respectively inserted into and fixed to the plurality of optical fiber holes H. The plurality of optical fiber holes H penetrate the ferrule  20  between the openings  23   a  and  23   b . The plurality of optical fiber holes H are disposed so as to correspond to the plurality of optical fibers  10 . The plurality of optical fibers  10  are respectively inserted into the plurality of optical fiber holes H. An adhesive is injected into the ferrule  20  from the openings  23   a  and  23   b . As a result, the space formed in the ferrule  20  is filled with the adhesive, and the positions of the optical fibers  10  respectively inserted in the optical fiber holes H are fixed. Although a structure in which the optical fibers  10  are inserted into and fixed to the optical fiber holes H have been described here, V-grooves may be formed in the ferrule  20  as an optical fiber holding portion. In this case and structure, the optical fibers  10  may be placed on the V-grooves and the optical fibers  10  may be pressed by a lid member from above the V-grooves. 
     As illustrated in  FIGS.  2  and  3   , the side surfaces  25  and  26  face each other in the direction D2 and extend along the directions D1 and D3. In one example, the side surfaces  25  and  26  extend parallel to each other. The side surface  25  is provided with a V-groove  31   extending along the direction D1. The side surface  26  is provided with a V-groove  32  extending along the direction D1. The V-grooves  31  and  32  are V-shaped in a cross section perpendicular to the direction D1. The V-grooves  31  and  32  are, for example, provided so as to continuously extend from the tip surface  21  to the rear end surface  22  along the direction D1. In other words, the V-grooves  31  and  32  extend over the entire length of the ferrule  20  in the direction D1. The V-grooves  31  and  32  are provided at positions facing each other in the direction D2. In other words, when viewed from the direction D2, the position of the V-groove  31  in the side surface  25  coincides with the position of the V-groove  32  in the side surface  26 . The V-groove  31  is positioned in, for example, the middle portion of the side surface  25  in the direction D3, and the V-groove  32  is positioned in, for example, the middle portion of the side surface  26  in the direction D3. 
     In a cross section perpendicular to the direction D1, the opening angle of the V-groove  31  (that is, the angle formed by the pair of surfaces configuring the V-groove  31 ) is, for example, 45° or more and 120° or less. The opening angle of the V-groove  31  may be, for example, 60° or more and 100° or less, or may be 90°. In the present embodiment, in a cross section perpendicular to the direction D1, the bottom portion of the V-groove  31  is, for example, rounded and the diameter of the inscribed circle in contact with the roundness is set to, for example, 0.7 mm. The V-groove  32  has, for example, the same shape as the V-groove  31 . 
       FIG.  3    illustrates a shortest distance W1 between the V-groove  31  and the V-groove  32  facing each other along the direction D2. The shortest distance W1 can be defined as the shortest distance between the bottom portion of the V-groove  31  and the bottom portion of the V-groove  32  in the direction D2. 
     In the present embodiment, no guide pin insertion hole is provided between the side surface  25  and the plurality of lenses  21   b . Accordingly, a distance L1 between the side surface  25  and the plurality of lenses  21   b  can be set without considering the outer diameter of a guide pin insertion hole. As a result, the distance L1 between the side surface  25  and the plurality of lenses  21   b  can be set smaller than the shortest distance between the side surface  25  and the plurality of lenses  21   b  in a case where a guide pin insertion hole is provided. The distance L1 between the side surface  26  and the plurality of lenses  21   b  (specifically, the lens  21   b  closest to the side surface  26  in the direction D2) in the direction D2 can be set in the same manner as the distance L1 between the side surface  25  and the plurality of lenses  21   b  in the direction D2. As a result, the maximum width of the ferrule  20  in the direction D2 (that is, the maximum distance between the side surface  25  and the side surface  26  in the direction D2) can be smaller than the maximum width of the ferrule in the direction D2 in a case where a guide pin insertion hole is provided. Accordingly, the ferrule  20  can be reduced in size. 
     As illustrated in  FIGS.  2  and  3   , a chamfered portion C 1  is provided at the part where the tip surface  21  and the V-groove  31  intersect. The chamfered portion C 1  is formed so as to have a reverse taper shape from the tip surface  21  to the V-groove  31  and is continuously connected to the V-groove  31 . The chamfered portion C 1   may be smoothly connected to the V-groove  31 . A chamfered portion C 2  is provided at the part where the tip surface  21  and the V-groove  32  intersect. The chamfered portion C 2  has the same shape as the chamfered portion C 1 . The chamfered portion C 2  is continuously connected to the V-groove  32 . The chamfered portion C 2  may be smoothly connected to the V-groove  32 . 
     Next, the shape of the adapter  40  will be described in detail with reference to  FIG.  5   .  FIG.  5    is a cross-sectional view illustrating the adapter  40  in a state where the ferrule  20  is inserted and fitted. As illustrated in  FIG.  5   , the adapter  40  has, for example, a rectangular tube shape extending along the direction D1. The total length of the adapter  40  in the direction D1 is, for example, longer than the total length of the ferrule  20  in the direction D1. 
     As illustrated in  FIG.  5   , the adapter  40  has an insertion hole  41  configuring the inside of the rectangular tube shape. The insertion hole  41  penetrates the adapter  40  along the direction D1. The insertion hole  41  has a rectangular shape when viewed from the direction D1 and is configured by four inner surfaces  43 ,  44 ,  45 , and  46 . The inner surfaces  43  and  44  face each other in the direction D3 and extend along the directions D1 and D2. The inner surface  43  faces the side surface  23  of the ferrule  20  in the direction D3. In one example, the inner surface  43  extends parallel to the side surface  23 . The inner surface  44  faces the side surface  24  of the ferrule  20  in the direction D3. In one example, the inner surface  44  extends parallel to the side surface  24 . 
     The inner surfaces  45  and  46  face each other in the direction D2 and extend along the directions D1 and D3. The inner surface  45  faces the side surface  25  of the ferrule  20  in the direction D2. The inner surface  45  may extend parallel to the side surface  25 . The inner surface  46  faces the side surface  26  of the ferrule  20  in the direction D2. The inner surface  46  may extend parallel to the side surface  26 . The adapter  40  has four outer surfaces  47 ,  48 ,  49 , and  50  configuring the outer shape of the rectangular tube shape. The outer surfaces  49  and  50  do not necessarily have to configure the outer shape of the rectangular tube shape. For example, the outer surfaces of V-protrusions  51  and  52  may be exposed to the outside of the adapter  40 . 
     The inner surface  45  is provided with the V-protrusion  51  extending along the direction D1. The inner surface  46  is provided with the V-protrusion  52  extending along the direction D1. The V-protrusions  51  and  52  are V-shaped in a cross section perpendicular to the direction D1. The V-protrusion  51  is, for example, provided on the inner surface  45  so as to continuously extend over the direction D1. In other words, the V-protrusion  51  extends over the entire length of the adapter  40  in the direction D1. The V-protrusion  52  is, for example, provided on the inner surface  46  so as to continuously extend over the direction D1. In other words, the V-protrusion  52  extends over the entire length of the adapter  40  in the direction D1. The length of the V-protrusion  51  in the direction D1 is longer than, for example, the length of the V-groove  31  in the direction D1. The length of the V-protrusion  52  in the direction D1 is longer than, for example, the length of the V-groove  32  in the direction D1. The V-protrusions  51  and  52  are provided at positions facing each other in the direction D2. In other words, when viewed from the direction D2, the position of the V-protrusion  51  on the inner surface  45  coincides with the position of the V-protrusion  52  on the inner surface  46 . 
     The V-protrusions  51  and  52  are provided so as to guide the V-grooves  31  and  32  of the ferrule  20 , respectively. The V-protrusion  51  is provided so as to be fittable with the V-groove  31  of the ferrule  20 . In other words, the V-protrusion  51  is provided at a position facing the V-groove  31  in the direction D2 and has a shape corresponding to the V-groove  31 . In a cross section perpendicular to the direction D1, the angle of the V-protrusion  51  (that is, the angle formed by the pair of surfaces configuring the V-protrusion  51 ) is, for example, 45° or more and 120° or less. The angle of the V-protrusion  51  may be, for example, 60° or more and 100° or less, or may be 90°. In the present embodiment, in a cross section perpendicular to the direction D1, the top portion of the V-protrusion  51  is, for example, rounded and the diameter of the inscribed circle in contact with the roundness is set to, for example, 0.7 mm. 
     The V-protrusion  52  is provided so as to be fittable with the V-groove  32  of the ferrule  20 . In other words, the V-protrusion  52  is provided at a position facing the V-groove  32  in the direction D2 and has a shape corresponding to the V-groove  32 . The V-protrusion  52  has, for example, the same shape as the V-protrusion  51 .  FIG.  5    illustrates a shortest distance W2 between the V-protrusion  51  and the V-protrusion  52  facing each other along the direction D2. The shortest distance W2 can be defined as the shortest distance between the top portion of the V-protrusion  51  and the top portion of the V-protrusion  52   in the direction D2 in a state where the ferrule  20  is not inserted in the adapter  40 . 
     The adapter  40  has a pair of regions R 1  and R 2  outside the insertion hole  41  in the direction D2. The pair of regions R 1  and R 2  are positioned on both sides in the direction D2 with the V-protrusions  51  and  52  interposed therebetween. One region R 1  is at a position sandwiched between the inner surface  45  and the outer surface  49  in the direction D2. The other region R 2  is at a position sandwiched between the inner surface  46  and the outer surface  50  in the direction D2. The pair of regions R 1  and R 2  are provided with a pair of hollow holes  61  and  62 , respectively. The hollow holes  61  and  62  extend along, for example, the V-protrusions  51  and  52 , respectively. In other words, the hollow holes  61  and  62  extend over, for example, the entire length of the adapter  40  in the direction D1. Each of the hollow holes  61  and  62  has, for example, a substantially rectangular shape when viewed from the direction D1. 
     The hollow hole  61  is adjacent to the inner surface  45  of the insertion hole  41  at a predetermined interval in the direction D2. In the region R 1 , the region sandwiched between the hollow hole  61  and the insertion hole  41  in the direction D2 is configured as a wall portion  71  separating the hollow hole  61  and the insertion hole  41 . The thickness of the wall portion  71  is, for example, constant. The wall portion  71  extends along the direction D3 between the hollow hole  61  and the insertion hole  41 . Specifically, the wall portion  71  extends along the shape of the inner surface  45  provided with the V-protrusion  51 . The part of the wall portion  71  where the V-protrusion  51  is provided protrudes toward the ferrule  20  side in the direction D2 so as to follow the shape of the V-protrusion  51 . 
     The wall portion  71  includes a pair of parts P 1  and P 2  connected to the part provided with the V-protrusion  51  at the positions where the part provided with the V-protrusion  51  is sandwiched in the direction D3. The parts P 1  and P 2  extend along a direction slightly inclined from the direction D3 in the cross section illustrated in  FIG.  5   . In the cross section illustrated in  FIG.  5   , each of the inclination angles of the part P 1  and the part P 2  with respect to the direction D3 is, for example, 5° or more and 15° or less. The part P 1  is inclined so as to be positioned on the side opposite to the ferrule  20  in the direction D2 from the inner surface  43  toward the V-protrusion  51  in the direction D3. The part P 2  is inclined so as to be positioned on the side opposite to the ferrule  20  in the direction D2 from the inner surface  44  toward the V-protrusion  51  in the direction D3. 
     The hollow hole  62  is adjacent to the inner surface  46  of the insertion hole  41  at a predetermined interval in the direction D2. In the region R 2 , the region sandwiched between the hollow hole  62  and the insertion hole  41  in the direction D2 is configured as a wall portion  72  separating the hollow hole  62  and the insertion hole  41 . The thickness of the wall portion  72  is, for example, constant. The wall portion  72  extends along the direction D3 between the hollow hole  62  and the insertion hole  41 . Specifically, the wall portion  72  extends along the shape of the inner surface  46  provided with the V-protrusion  52 . The part of the wall portion  72  where the V-protrusion  52  is provided protrudes toward the ferrule  20  side in the direction D2 so as to follow the shape of the V-protrusion  52 . 
     The wall portion  72  includes a pair of parts P 3  and P 4  connected to the part provided with the V-protrusion  52  at the positions where the part provided with the V-protrusion  52  is sandwiched in the direction D3. The parts P 3  and P 4  extend along a direction slightly inclined from the direction D3 in the cross section illustrated in  FIG.  5   . In the cross section illustrated in  FIG.  5   , each of the inclination angles of the part P 3  and the part P 4  with respect to the direction D3 is, for example, 5° or more and 15° or less. The part P 3  is inclined so as to be positioned on the side opposite to the ferrule  20  in the direction D2 from the inner surface  43  toward the V-protrusion  52  in the direction D3. The part P 4  is inclined so as to be positioned on the side opposite to the ferrule  20  in the direction D2 from the inner surface  44  toward the V-protrusion  52  in the direction D3. 
     By each of the wall portion  71  and the wall portion  72  having a part inclined from the direction D3 as described above, stress concentration on the base portion of the wall portion  71  (that is, the connection parts between the inner surfaces  43  and  44  and the wall portion  71 ) and the base portion of the wall portion  72  (that is, the connection parts between the inner surfaces  43  and  44  and the wall portion  72 ) can be suppressed as compared with a case where each of the wall portion  71  and the wall portion  72  is provided in parallel with the direction D3. As a result, damage to each of the wall portion  71  and the wall portion  72  can be suppressed. 
     When the ferrule  20  described above is inserted into the adapter  40  and fitted, the ferrule  20  and the adapter  40  are disposed such that the tip surface  21  of the ferrule  20  is first inserted into the adapter  40  as illustrated in  FIG.  1 A . Then, the ferrule  20  is inserted into the adapter  40  by moving the ferrule  20  along the direction D1 with respect to the adapter  40 . When the ferrule  20  is inserted into the adapter  40 , the V-grooves  31  and  32  of the ferrule  20  are fitted to the V-protrusions  51  and  52  of the adapter  40 , respectively. At this time, the V-protrusion  51  enters the V-groove  31  and abuts and the V-protrusion  52  enters the V-groove  32  and abuts. 
     Here, in a case where the shortest distance W1 between the V-groove  31  and the V-groove  32  of the ferrule  20  is larger than the shortest distance W2 between the V-protrusion  51  and the V-protrusion  52  of the adapter  40  as in the present embodiment, the V-protrusions  51  and  52  of the adapter  40  enter the V-grooves  31  and  32  of the ferrule  20  in a state of being compressed in the direction D2. In other words, the V-protrusions  51  and  52  of the adapter  40  receive a reaction force from the V-grooves  31  and  32  of the ferrule  20  and are elastically deformed to the side opposite to the ferrule  20  in the direction D2 (that is, the outside of the adapter  40 ). Then, a force that causes the V-protrusions  51  and  52  facing each other to return to the original positions is applied to the ferrule  20 , and the ferrule  20  is held and fixed by the V-protrusions  51  and  52 . 
     As a result, the V-protrusions  51  and  52  respectively come into contact with the V-grooves  31  and  32 , and each of the gap between the V-protrusion  51  and the V-groove  31  in the direction D2 and the gap between the V-protrusion  52  and the V-groove  32  in the direction D2 becomes zero. As a result, the position of the ferrule  20  with respect to the adapter  40  is defined in the directions D2 and D3, and the rotation-direction position of the ferrule  20  with respect to the adapter  40  is defined. Subsequently, a spring (not illustrated) attached to the rear of the ferrule  20  urges the ferrule  20  to the connection counterpart ferrule side in the direction D1. Accordingly, the position of the ferrule  20  in the direction D1 with respect to the adapter  40  is defined. The plurality of optical fibers  10  are positioned in this manner. 
     In a case where there are a gap in the direction D3 between the V-protrusion  51  and the V-groove  31  (that is, difference between the width of the V-protrusion  51  and the width of the V-groove  31 ) and a gap in the direction D3 between the V-protrusion  52  and the V-groove  32  (that is, difference between the width of the V-protrusion  52  and the width of the V-groove  32 ), the sizes of these gaps may result in a positional deviation or an angular deviation between the ferrule  20  and the connection counterpart ferrule. Therefore, it is desirable that these gaps are set to be as small as possible. 
     In the present embodiment, the V-protrusions  51  and  52  configure a part of the adapter  40  configured by an elastic material. Accordingly, in the present embodiment, both the V-protrusions  51  and  52  are configured to be elastically deformable. In an alternative configuration, only one of the V-protrusions  51  and  52  may be elastically deformable. In this case, a hollow hole may be provided only in the region that is one of the regions R 1  and R 2  and provided with either the V-protrusion  51  or the V-protrusion  52 . In other words, a hollow hole may be provided in one region provided with one V-protrusion that is elastically deformed and no hollow hole may be provided in the other region provided with the other V-protrusion that is not elastically deformed. 
     For example, in a case where only the V-protrusion  51  is configured to be elastically deformable, the hollow hole  61  may be provided in the region R 1  provided with the V-protrusion  51  with the hollow hole  62  not provided in the region R 2  provided with the V-protrusion  52 . In this case, when the ferrule  20  is inserted into the adapter  40  and fitted, the V-groove  32  of the ferrule  20  is disposed so as to abut against the V-protrusion  52  that is not elastically deformed and the V-groove  31  of the ferrule  20  is caused to abut against the V-protrusion  51  that is elastically deformed. At this time, the V-protrusion  51  receives a reaction force from the V-groove  31  and is elastically deformed. Then, by a force that causes the V-protrusion  51  to return to the original position being applied to the ferrule  20 , the ferrule  20  is held and fixed by the V-protrusions  51  and  52 . As a result, the position of the ferrule  20  with respect to the adapter  40  is defined as in a case where both the V-protrusions  51  and  52  are configured to be elastically deformable. Likewise, in a case where only the V-protrusion  52  is configured to be elastically deformable, the hollow hole  62  may be provided in the region R 2  provided with the V-protrusion  52  with the hollow hole  61  not provided in the region R 1  provided with the V-protrusion  51 . Also in this case, the position of the ferrule  20  with respect to the adapter  40  is defined by the V-protrusion  52  being elastically deformed. 
     The effects of the optical connection structure  1 , the ferrule  20 , and the optical connector  2  according to the present embodiment described above will be described. In the optical connection structure  1 , the ferrule  20 , and the optical connector  2  according to the present embodiment, when the ferrule  20  is inserted into the adapter  40  and fitted, the V-groove  31  is fitted to the V-protrusion  51  and the V-groove  32  is fitted to the V-protrusion  52 . As a result, the position of the ferrule  20  with respect to the adapter  40  (that is, the position of the plurality of optical fibers  10  held by the ferrule  20 ) can be defined in a plane perpendicular to the direction D1. In other words, by using the adapter  40  where the ferrule  20  is inserted and fitted as a positioning member at the time of positioning of the plurality of optical fibers  10 , the plurality of optical fibers  10  can be positioned without providing a guide pin insertion hole in the ferrule  20 . As a result, it is not necessary to use a high-dimensional accuracy guide pin for positioning between the plurality of optical fibers  10  (that is, positioning between the plurality of optical fibers  10  and the plurality of optical fibers of the connection counterpart). Further, it is possible to avoid a situation in which the use of a foreign matter-attached guide pin leads to a decline in the positioning accuracy of the plurality of optical fibers  10 . As a result, it is possible to suppress a decline in connection loss between the plurality of optical fibers  10  and the plurality of optical fibers of the connection counterpart. Accordingly, according to the optical connection structure  1 , the ferrule  20 , and the optical connector  2  according to the present embodiment, the plurality of optical fibers  10  can be positioned with a simple configuration. 
     In a case where the optical connection of the plurality of optical fibers  10  is performed using the plurality of lenses  21   b  provided on the tip surface  21  of the ferrule  20  as in the present embodiment, it is desirable to make as small as possible the angular deviation between the plurality of optical fibers  10  and the plurality of optical fibers of the connection counterpart. In the optical connection structure  1 , by performing the optical connection of the plurality of optical fibers  10  using the V-grooves  31  and  32  provided over the entire length of the ferrule  20  (for example, 8 mm), it is possible to accurately regulate the rotation direction of the plurality of optical fibers  10  with respect to the plurality of optical fibers of the connection counterpart as compared with a case where the optical connection of a plurality of optical fibers is performed using the protrusion length of a guide pin (for example, 2 mm) that slightly protrudes from the insertion hole of a ferrule. Accordingly, according to the present embodiment, the angular deviation between the plurality of optical fibers  10  and the plurality of optical fibers of the connection counterpart can be suppressed to be smaller, and thus it is suitable for suppressing a decline in the connection loss between the plurality of optical fibers  10  and the plurality of optical fibers of the connection counterpart. 
     In the present embodiment, the plurality of optical fiber holes H are disposed side by side along the direction D2. According to this configuration, it is possible to suitably realize a configuration in which the plurality of optical fibers  10  are positioned by inserting the ferrule  20  into the adapter  40  and fitting the ferrule  20 . 
     In the present embodiment, each of the V-groove  31  and the V-groove  32  is V-shaped in a cross section perpendicular to the direction D1. Each of the V-protrusion  51  and the V-protrusion  52  is V-shaped in a cross section perpendicular to the direction D1. In this configuration, the ferrule  20  can be accurately positioned with respect to the adapter  40  by fitting the V-groove  31  and the V-groove  32  to the V-protrusion  51  and the V-protrusion  52 , respectively. In other words, the positioning of the plurality of optical fibers  10  can be performed with high accuracy. 
     In the present embodiment, the side surface  25  is provided with the V-groove  31 . The side surface  26  is provided with the V-groove  32 . The inner surfaces  45  and  46  of the adapter  40  are provided with the V-protrusion  51  that can be fitted to the V-groove  31  and the V-protrusion  52  that can be fitted to the V-groove  32 . As a result, an increase in the width of the ferrule  20  in the direction D2 can be suppressed as compared with a case where a V-protrusion is provided on each of the side surface  25  and the side surface  26 . In other words, an increase in the size of the ferrule  20  can be suppressed. 
     In the present embodiment, the V-groove  31  is in contact with the V-protrusion  51  and the V-groove  32  is in contact with the V-protrusion  52  in a plane perpendicular to the direction D1. As a result, a positional deviation of the ferrule  20  with respect to the adapter  40  can be suppressed, and thus the positioning of the plurality of optical fibers  10  can be performed with high accuracy. 
     In the present embodiment, each of the V-protrusions  51  and  52  is configured to be elastically deformable in the direction D2. As a result, the ferrule  20  can be easily inserted into the adapter  40 , and thus the workability in inserting the ferrule  20  into the adapter  40  is improved. 
     In the present embodiment, the shortest distance W1 between the V-groove  31  and the V-groove  32  is larger than the shortest distance W2 between the V-protrusion  51  and the V-protrusion  52  in a plane perpendicular to the direction D1. In this configuration, when the ferrule  20  is inserted into the adapter  40 , the V-grooves  31  and  32  respectively abut against the V-protrusions  51  and  52  and a force that causes the V-protrusions  51  and  52  to return to the original positions is applied to the ferrule  20 . As a result, the ferrule  20  is held and fixed by the V-protrusions  51  and  52 , and thus a positional deviation of the ferrule  20  with respect to the adapter  40  is suppressed. In other words, it is possible to suppress a situation in which a gap in the direction D2 between the V-protrusion  51  and the V-groove  31  and a gap in the direction D2 between the V-protrusion  52  and the V-groove  32  are generated due to the effect of, for example, the manufacturing tolerances of the ferrule  20  and the adapter  40 . As a result, the positioning of the plurality of optical fibers  10  can be performed with high accuracy. 
     In the present embodiment, the hollow holes  61  and  62  are provided in the regions R 1  and R 2  of the adapter  40 , respectively. As a result, the V-protrusions  51  and  52  can be elastically deformed with ease in the direction D2. As a result, the ferrule  20  can be more easily inserted into the adapter  40 , and thus the workability in inserting the ferrule  20  into the adapter  40  is further improved. 
     The present disclosure is not limited to the embodiment described above and can be appropriately modified without departing from the spirit described in the claims. 
       FIG.  6    is a cross-sectional view illustrating an adapter  40 A in a state where a ferrule  20 A is inserted in an optical connection structure according to a first modification example. As illustrated in  FIG.  6   , a V-protrusion  31 A and a V-protrusion  32 A instead of the V-grooves  31  and  32  may be provided on the side surfaces  25  and  26  of the ferrule  20 A. In this case, the inner surfaces  45  and  46  of the adapter  40 A may be provided with a V-groove  51 A and a V-groove  52 A instead of the V-protrusions  51  and  52 . In this modification example, when the ferrule  20 A is inserted into the adapter  40 A and fitted, the V-protrusions  31 A and  32 A of the ferrule  20 A respectively enter the V-grooves  51 A and  52 A of the adapter  40 A and abut. Accordingly, the optical connection structure according to this modification example also has the same action and effect as the optical connection structure  1  according to the embodiment described above. 
     As in the case of the optical connection structure  1  according to the embodiment described above, in the optical connection structure according to this modification example, it is not necessary that both the V-grooves  51 A and  52 A are configured to be elastically deformable and only one of the V-grooves  51 A and  52 A may be configured to be elastically deformable. In this case, a hollow hole may be provided only in the region that is one of the pair of regions R 1  and R 2  and provided with either the V-groove  51 A or the V-groove  52 A. For example, in a case where only the V-groove  51 A is configured to be elastically deformable, the hollow hole  61  may be provided in the region R 1  provided with the V-groove  51 A with the hollow hole  62  not provided in the region R 2  provided with the V-groove  52 A. In a case where only the V-groove  52 A is configured to be elastically deformable, the hollow hole  62  may be provided in the region R 2  provided with the V-groove  52 A with the hollow hole  61  not provided in the region R 1  provided with the V-groove  51 A. Even in such a case, when the ferrule  20 A is inserted into the adapter  40 A and fitted, the position of the ferrule  20 A with respect to the adapter  40 A is defined by one of the V-grooves  51 A and  52 A that is elastically deformable being elastically deformed. 
       FIG.  7    is a cross-sectional view illustrating an adapter  40 B in a state where the ferrule  20  is inserted and fitted in an optical connection structure according to a second modification example. In the example illustrated in  FIG.  7   , the adapter  40 B is configured by a non-elastic material. In this case, examples of the material of the adapter  40 B include polyphenylene sulfide (PPS). The pair of regions R 1  and R 2  of the adapter  40 B are not provided with the hollow holes  61  and  62  in the embodiment described above. In other words, the entire regions R 1  and R 2  on both sides of the insertion hole  41  in the direction D2 are filled with the material of the adapter  40 B. In this structure, elastic deformation of the adapter  40 B is unlikely to occur even if a particularly hard material is not used as the material of the adapter  40 B. 
     As illustrated in  FIG.  7   , the inner surfaces  45  and  46  of the adapter  40 B are provided with a circular arc-shaped protrusion  51 B and a circular arc-shaped protrusion  52 B instead of the V-protrusions  51  and  52 , respectively. The circular arc-shaped protrusions  51 B and  52 B are semicircular in a cross section perpendicular to the direction D1. The circular arc-shaped protrusion  51 B extends along the direction D2 on the inner surface  45  of the adapter  40 B. The circular arc-shaped protrusion  52 B extends along the direction D2 on the inner surface  46  of the adapter  40 B.  FIG.  8    is a cross-sectional view of the adapter  40 B along the VIII-VIII line of  FIG.  7   . In  FIG.  8   , the ferrule  20  is illustrated as a side view. As illustrated in  FIG.  8   , one end portion  52   a  of the circular arc-shaped protrusion  52 B in the direction D2 is at a position slightly deviated to the other end surface  56  side from one end surface  55  of the adapter  40 B in the direction D1. The other end portion  52   b  of the circular arc-shaped protrusion in the direction D2 is at a position slightly deviated from the other end surface  56  to the one end surface  55  side in the direction D1. The one end portion  52   a  is formed so as to taper toward the one end surface  55  side in the direction D1. The other end portion  52   b  is formed so as to taper toward the other end surface  56  side in the direction D1. 
     As illustrated in  FIG.  7   , the shortest distance W1 between the V-groove  31  and the V-groove  32  in the direction D2 is smaller than the shortest distance W2 between the circular arc-shaped protrusion  51 B and the circular arc-shaped protrusion  52 B in the direction D2. Accordingly, there is a gap between the circular arc-shaped protrusion  51 B and the V-groove  31  in the direction D2 and there is a gap between the circular arc-shaped protrusion  52 B and the V-groove  32  in the direction D2. When the ferrule  20  is inserted into the adapter  40 B and fitted, the circular arc-shaped protrusion  51 B enters the V-groove  31  of the ferrule  20  and abuts and the circular arc-shaped protrusion  52 B enters the V-groove  32  of the ferrule  20  and abuts. At this time, the outer peripheral surface of the circular arc-shaped protrusion  51 B abuts against the pair of surfaces configuring the V-groove  31 , and the outer peripheral surface of the circular arc-shaped protrusion  52 B abuts against each of the pair of surfaces configuring the V-groove  32 . As a result, the ferrule  20  is held by the circular arc-shaped protrusions  51 B and  52 B of the adapter  40 B and the positioning of the plurality of optical fibers  10  is performed. 
     Accordingly, the optical connection structure according to this modification example also has the same action and effect as the optical connection structure  1  according to the embodiment described above. In a case where the adapter  40 B is configured by a material that is not elastically deformed, if the V-grooves  31  and  32  are configured to be respectively fitted to the V-protrusions  51  and  52  as in the embodiment described above, gaps are likely to be generated between the V-groove  31  and the V-protrusion  51  and between the V-groove  32  and the V-protrusion  52  due to, for example, the effect of a manufacturing tolerance. In this case, it is assumed that the position of the ferrule  20  with respect to the adapter  40  significantly deviates depending on the contact position between the V-groove  31  and the V-protrusion  51  or the contact position between the V-groove  32  and the V-protrusion  52 . On the other hand, with a configuration in which the V-grooves  31  and  32  are respectively fitted to the circular arc-shaped protrusions  51 B and  52 B, it is possible to suppress a situation in which the position of the ferrule  20  with respect to the adapter  40 B deviates depending on the contact positions between the V-grooves  31  and  32  and the circular arc-shaped protrusions  51 B and  52 B. As a result, a decline in the positioning accuracy of the optical fiber  10  can be suppressed. 
     The optical connection structure, the ferrule, and the optical connector of the present disclosure are not limited to the embodiment and modification examples described above, and various other forms are possible. For example, the embodiment and modification examples described above may be mutually combined depending on the required purpose and effect. In the embodiment and modification examples described above, the shapes of the ferrule and the adapter can be changed as appropriate. For example, the ferrule may be provided with three or more recess or protrusion portions although the ferrule is provided with two recess or protrusion portions in the embodiment and modification examples described above. In this case, the adapter may be provided with three or more protrusion or recess portions respectively fitted to the three or more recess or protrusion portions of the ferrule. 
     The shape of the recess or protrusion portion of the ferrule and the shape of the recess or protrusion portion of the adapter are not limited to the embodiment and modification examples described above and can be changed as appropriate. For example, each of the recess portions of the ferrule and the adapter may be a groove having another shape, such as a circular arc-shaped groove, a rectangular groove, and a trapezoidal groove, in addition to the V-groove. Likewise, the protrusion portions of the ferrule and the adapter may be protrusions different in shape, such as rectangular and trapezoidal protrusions, in addition to the V-protrusion and the circular arc-shaped protrusion. 
     The recess or protrusion portion may not extend along the direction D2 from the tip surface to the rear end surface of the ferrule. For example, the recess or protrusion portion of the ferrule may be separated from the tip surface to the rear end surface side in the direction D2 or may be separated from the rear end surface to the tip surface side in the direction D2. The recess or protrusion portion of the adapter may not extend along the direction D2 over the entire length of the adapter. For example, the recess or protrusion portion of the adapter may be separated from one end surface of the adapter in the direction D2 to the other end surface side or may be separated from the other end surface side in the direction D2 to the one end surface side. 
     The recess or protrusion portions may be provided on the side surfaces  23  and  24  facing each other along the direction D3 in the ferrules  20  and  20 A, respectively. The position of the recess or protrusion portion of the side surface  25  and the position of the V-groove of the side surface  26  may be deviated from each other when viewed from the direction D2. The shape of the recess or protrusion portion of the side surface  25  and the shape of the recess or protrusion portion of the side surface  26  may be different from each other. For example, the side surface  25  may be provided with a V-groove with the side surface  26  provided with a circular arc-shaped groove. Alternatively, the side surface  25  may be provided with a V-groove with the side surface  26  provided with a V-protrusion. 
     The recess or protrusion portions may be provided on the inner surfaces  45  and  46  facing each other along the direction D3 in the adapters  40 ,  40 A, and  40 B, respectively. The position of the protrusion or recess portion of the inner surface  45  and the position of the protrusion or recess portion of the inner surface  46  may be deviated from each other when viewed from the direction D2. The shape of the protrusion or recess portion of the inner surface  45  and the shape of the protrusion or recess portion of the inner surface  46  may be different from each other. For example, the inner surface  45  may be provided with a V-groove with the inner surface  46  provided with a circular arc-shaped groove. Alternatively, the inner surface  45  may be provided with a V-groove with the inner surface  46  provided with a V-protrusion. 
     A plurality of lenses may not be formed on the tip surface of the ferrule. In this case, the ferrule may not be configured by a light transmitting resin. The ferrule may have a plurality of optical fiber grooves for respectively holding a plurality of optical fibers instead of the plurality of optical fiber holes for respectively holding the plurality of optical fibers. The adapter may not be configured by an elastic material in whole, and the adapter may be configured by an elastic material in part. For example, only the recess or protrusion portion of the adapter may be configured by an elastic material. 
     Reference Signs List 
       1 : optical connection structure,  2 : optical connector,  5 : optical fiber tape core wire,  10 : optical fiber,  15 : boot,  20 ,  20 A: ferrule,  21 : tip surface,  21   a : light transmitting surface,  21   b : lens,  21   c : back surface,  22 : rear end surface,  23 ,  24 : side surface,  23   a ,  23   b : opening,  25 : side surface,  26 : side surface,  31 : V-groove,  31 A: V-protrusion,  32 : V-groove,  32 A: V-protrusion,  40 ,  40 A,  40 B: adapter,  41 : insertion hole,  43 ,  44 ,  45 ,  46 : inner surface,  47 ,  48 ,  49 ,  50 : outer surface,  51 : V-protrusion,  51 A: V-groove,  51 B: circular arc-shaped protrusion,  52 : V-protrusion,  52 A: V-groove,  52   a : one end portion,  52 B: circular arc-shaped protrusion,  52   b : the other end portion,  55 : one end surface,  56 : the other end surface,  61 ,  62 : hollow hole,  71 ,  72 : wall portion, C 1 : chamfered portion, C 2 : chamfered portion, D 1 : direction, D2: direction, D3: direction, H: optical fiber hole, L 1 : distance, P1, P2, P3, P4: part, R 1 , R 2 : region, W 1 , W 2 : shortest distance.