Abstract:
An optical connector is structured so as to include a fiber connection structure therein. A multi-core fiber is included inside a ferrule, and affixed to the ferrule substrate. One end surface of the multi-core fiber is exposed to an end surface of the ferrule. The other end of the multi-core fiber passes through and is affixed to a capillary. A plurality of optical fiber pass through a capillary that faces the capillary, and are affixed thereto the capillary in the same manner. Seven optical fiber cores of the same diameter are joined in a close-packed arrangement in the fiber connection structure.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a continuation of International Application Number PCT/JP2012/055926, filed Mar. 8, 2012, and claims priority from, Japanese Application Number 2011-051265, filed Mar. 9, 2011. The above listed applications are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an optical connector including a connection structure connecting a multi-core fiber having plural cores to a bundle structure in which plural optical fibers are bundled, to a method for aligning the multi-core fiber with the bundle structure, and to the others. 
       BACKGROUND OF THE INVENTION 
       [0003]    Due to rapid increase of traffic in optical communications in recent years, data-transmission capacity through a single-core optical fiber presently utilized is approaching the limit. Then, as a means to expand communication capacity furthermore, a multi-core fiber in which plural cores are formed in one fiber has been proposed. 
         [0004]    As such a multi-core fiber, for example, there is one having plural core parts within a cladding part and having a flat part perpendicular to the longitudinal direction in one part of the outer circumference of the cladding part (See Patent Document 1). 
         [0005]    When the multi-core fiber is utilized as a transmission line and receives/sends transmission signals, each core part of the multi-core fiber needs to be connected to a corresponding core part of another multi-core fiber, to an individual optical fiber and to an individual optical element or the like. For connecting such a multi-core fiber to single-core fibers to receive/send transmission signals, a method has been proposed, in which the multi-core fiber is connected to a bundle fiber having single-core optical fibers arranged at the corresponding positions of the respective core parts of the multi-core fiber (See Patent Document 2). Also, as a method for producing such bundled optical fibers, a method to bundle plural single-core fibers at the predetermined intervals by binding them has been proposed (See Patent Document 3). 
       PRIOR ART DOCUMENTS 
     Patent Documents 
       [0000]    
       
         [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2010-152163 (JP-A-2010-152163) 
         [Patent Document 2] Japanese Unexamined Patent Application Publication No. S62-47604 (JP-A-S62-47604) 
         [Patent Document 3] Japanese Unexamined Patent Application Publication No. H03-12607 (JP-A-H03-12607) 
       
     
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0009]    As described above, when each of the core parts of the multi-core fiber is connected to the individual optical fiber, the core parts of the multi-core fiber and the cores of the optical fibers at their connected end surfaces have to be optically precisely connected together, respectively. However, usually, the interval between the core parts of the multi-core fiber is narrow (40 to 50 μm, for example), and thus the usual optical fiber (having the external diameter of 125 μm) cannot be utilized. That is, the optical fiber having the size of the external diameter not exceeding the interval between the core parts of the multi-core fiber must be utilized. 
         [0010]    However, such an optical fiber is extremely thin and is difficult to be handled. Also, especially when a single mode fiber is used, the amount of the axis misalignment at the connected part must be restricted to 1 to 2 μm or smaller and so a very high positional accuracy is required for alignment. 
         [0011]    Thus, if a bundle of fibers is mechanically formed by pressing force from outside or the like as described in the conventional Patent Document 3, the positions of the respective cores of the single-core fibers are not precisely arranged at the intended positions and the intervals between the cores vary a little. As a result, misalignment between the respective cores of the connection target multi-core fiber and the respective cores of the bundle of fibers is caused, and the optical loss arises as a results. That is, at present any satisfactory method for precisely aligning the connection structure between a multi-core fiber and respective optical fibers with the minimum optical loss have not been proposed. 
         [0012]    The present invention was achieved in view of such problems. Its object is: to provide an optical connector, comprising a connection structure in its inside between a multi-core fiber having core parts arranged with a narrow pitch and a bundle structure in which plural optical fibers are bundled, and thus having a function to convert a multi-core fiber into a bundle structure and vice versa; and to provide a method for aligning the multi-core fiber with the bundle structure, and others. 
       Means for Solving Problems 
       [0013]    To achieve the above object, the first invention provides an optical connector containing a fiber connection structure: which optically connects a multi-core fiber having plural cores at predetermined intervals to a bundle structure having plural optical fibers bundled in close-packed arrangement, and which is accommodated in the optical connector. 
         [0014]    An end of the bundle structure may be connected to the multi-core fiber, and the end of the multi-core fiber may be exposed on an end surface of the optical connector. The optical connector comprises a first capillary on the front side of a ferrule, a second capillary on the rear side of the ferrule and a connector flange part; the multi-core fiber is inserted into the first capillary; the bundle structure is inserted into the second capillary; and the first capillary and the second capillary may be joined together rearward the connector flange part. 
         [0015]    Also, the optical connector may comprise a first capillary on the front side of a ferrule, a second capillary on the rear side of the ferrule and an optical connector flange part; the multi-core fiber is inserted into the first capillary; the bundle structure is inserted into the second capillary; the first capillary and the second capillary are joined together frontward the connector flange part; and the external diameter of the second capillary may be smaller than that of the first capillary. 
         [0016]    The first capillary consists of a zirconia capillary and a glass capillary, and the glass capillary on the rear end side of the first capillary and the second capillary may be joined together with an ultraviolet hardening adhesive. 
         [0017]    An end of the multi-core fiber is connected to the bundle structure, and the ends of the respective optical fibers forming the bundle structure may be exposed on an end surface of the optical connector. 
         [0018]    According to the first invention, a connection structure of a multi-core fiber and plural optical fibers is embedded inside the optical connector. Thereby, if the multi-core fiber is exposed on an end surface of the optical connector, it can be easily connected to an identical multi-core fiber and thus, a multi-core fiber can be separated into the plural optical fibers through the optical connector. Similarly, if the ends of plural optical fibers are exposed on the end surface of the optical connector, they can be connected to identical optical fibers, respectively. Namely, it is possible to convert from the optical fibers into the multi-core fiber and vice versa inside the optical connector. 
         [0019]    Also, in the connection structure within the optical connector, if an end part of the multi-core fiber and the bundle structure are inserted into capillaries, respectively, and the capillaries are connected together such that they face each other, the connection work of this connection part becomes easy. 
         [0020]    The second invention is a method for aligning a multi-core fiber having plural cores with a bundle structure in which plural optical fibers are bundled. Core parts in the multi-core fiber are formed at a predetermined interval in its cross-section; and plural optical fibers are joined together and bundled in a state in which they are arranged close-packed in the cross-section of the bundle structure at an interval approximately equal to the interval between the core parts of the multi-core fiber. The method comprises: a step of aligning the central core of the multi-core fiber with the central fiber of the bundle structure through the following operation process: keeping the multi-core fiber and the bundle structure in a way that the central core of the multi-core fiber and the central fiber of the bundle structure face each other; and changing the relative positions of the central core and the central fiber within the facing plane until the detected light intensity is maximized while inputting light from the central core or from the central fiber and detecting the light at the central fiber or at the central core on the other side; a step of making parallel a pair of cores of the multi-core fiber which are placed axisymmetrically about the central core with a pair of fibers of the bundle structure by: rotating the multi-core fiber relatively with respect to the bundle structure around the position of the central core of the multi-core fiber, while inputting light from above described pair of cores of the multi-core fiber or a pair of fibers of the bundle structure and detecting the light at the pair of fibers or cores on the other side; and a step of joining and fixing together the multi-core fiber and the bundle structure in that state. 
         [0021]    After the arrangement direction of a pair of cores or fibers on one side of the facing plane are made parallel with that of a pair of fibers or cores on the other side, the amount of axis misalignment with the connection target core of the core having the maximum amount of axis misalignment may be minimized by slightly moving the multi-core fiber relative to the bundle structure in two mutually vertical directions until reaching the above intended position, and the multi-core fiber and the bundle structure may be joined and fixed together at that position. 
         [0022]    According to the second invention, the alignment of a multi-core fiber with a bundle structure in which plural optical fibers are bundled can be reliably performed. Accordingly, the multi-core fiber and the bundle structure can be certainly optically connected together. 
         [0023]    The third invention provides a fiber arrangement conversion member for converting arrangements of plural optical fibers. The fiber arrangement conversion member comprises a main body and plural optical fibers; a first fixing part with approximately hexagonal shape is formed on one end of the main body; plural optical fibers are fixed in a close-packed arrangement at the first fixing part; a second fixing part in the shape of plural grooves in a row is formed on the other end of the main body; and the optical fibers are provided in a row and are fixed in the second fixing part, respectively. 
         [0024]    According to the third invention, it is possible to connect the multi-core fiber and an optical fiber ribbon easily. 
       Effects of the Invention 
       [0025]    The present invention can provide: an optical connector, comprising in its inside a connection structure connecting a multi-core fiber having narrow-spaced core parts to a bundle structure having bundled plural optical fibers, and thus having a function to convert the multi-core fiber into the bundle structure and vice versa; a method for aligning the multi-core fiber with the bundle structure, and others. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows an optical connector  10 . 
           [0027]      FIG. 2  show a fiber connection structure  1   a .  FIG. 2  ( a ) is its front view and  FIG. 2  ( b ) its cross-sectional view at the A-A line shown in the  FIG. 2  ( a ). 
           [0028]      FIG. 3  ( a ) shows a bundle structure  5   a  and is a cross-sectional view at the B-B line shown in the  FIG. 2  ( a ).  FIG. 3  ( b ) shows a bundle structure  5   b , and  FIG. 3  ( c ) shows a bundle structure  5   c.    
           [0029]      FIG. 4  shows a manufacturing procedure of a bundle structure. 
           [0030]      FIG. 5  show how closely gathered structure between the neighboring optical fibers is formed. 
           [0031]      FIG. 6  show how the tip of a capillary is polished. 
           [0032]      FIG. 7  show a method for aligning a multi-core fiber  3  with the bundle structure  5   a.    
           [0033]      FIG. 8  show a method for aligning the multi-core fiber  3  with the bundle structure  5   a.    
           [0034]      FIG. 9  shows an optical connector  10   a.    
           [0035]      FIG. 10  shows an optical connector  30 . 
           [0036]      FIG. 11  shows an optical connector  40 . 
           [0037]      FIG. 12  shows an optical connector  50 . 
           [0038]      FIG. 13  shows an optical connector  55 . 
           [0039]      FIG. 14  shows a connector structure  60 . 
           [0040]      FIG. 15  shows a fiber arrangement conversion member  70 . 
           [0041]      FIG. 16  ( a ) is a view from the I arrow direction shown in the  FIG. 15 , and  FIG. 16  ( b ) is a view from the J arrow direction shown in the  FIG. 15 . 
           [0042]      FIG. 17  show a jig  83  and a jig  89 . 
           [0043]      FIG. 18  show manufacturing procedures of a bundle structure utilizing the jig  83 . 
           [0044]      FIG. 19  show a multi-core fiber  90  and a bundle structure  91 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0045]    Hereinafter, an optical connector  10  according to an embodiment of the present invention will be described.  FIG. 1  is a front cross-sectional view of the optical connector  10 . The optical connector  10  contains a fiber connection structure  1   a  in its inside. 
         [0046]    A multi-core fiber  3  is embedded within a ferrule  12  and is fixed to a ferrule substrate. One end of the multi-core fiber  3  exposes on an end surface of the ferrule  12 . Namely, the multi-core fiber  3  can be connected to other multi-core fibers or the like embedded in other optical connectors. 
         [0047]    The other end of the multi-core fiber  3  is inserted into and fixed to a capillary  21   a . In a capillary  21   b  facing the capillary  21   a , plural optical fibers  7  are inserted into and fixed to the capillary  21   b  in a similar way. In the fiber connection structure  1   a , the capillary  21   a  and the capillary  21   b  are joined with adhesives, etc. That is, the multi-core fiber  3  and the plural optical fibers  7  are connected together. Here, a structure in which the plural optical fibers  7  are inserted into the capillary  21   b  and bundled is referred to as a bundle structure  5   a . That is, the capillary  21   a  in which the multi-core fiber  3  is fixed is joined with the bundle structure  5   a.    
         [0048]      FIG. 2  ( a ) is an enlarged view of the fiber connection structure  1   a ,  FIG. 2  ( b ) is a cross-sectional view at the A-A line in the  FIG. 2  ( a ), and  FIG. 3  ( a ) is a cross-sectional view at the B-B line in the  FIG. 2(   a ). 
         [0049]    The multi-core fiber  3  includes plural cores  11  arranged at predetermined intervals and a cladding  13  covering their surroundings, as shown in the  FIG. 2  ( b ). Seven cores  11  in total are placed at the center of the multi-core fiber  3  and at each vertex position of a regular hexagon around the center of the multi-core fiber  3 . That is, all the intervals between a central core  11  and the six cores  11  in its periphery are equal. Also, the intervals between the mutually neighboring cores  11  in the six peripheral cores are equal. Here, the pitch of the cores  11  is approximately 40 to 50 μm, for example. 
         [0050]    As described above, the multi-core fiber  3  is inserted into the capillary  21   a . The capillary  21   a  is a cylindrical member which has a hole in its inside. The hole in the capillary  21   a  is slightly larger than the external diameter of the multi-core fiber  3 . The multi-core fiber  3  and the capillary  21   a  are bonded together with an adhesive, for example. In this case, it is desirable that the refractive index of the used adhesive is smaller than that of the cladding  13  of the multi-core fiber  3 . In this way, leak of light from the cladding can be prevented. 
         [0051]    Also, as shown in the  FIG. 3  ( a ), in the bundle structure  5   a , seven optical fibers of the same diameter are joined in the close-packed arrangement. That is, an optical fiber  7  is placed in the center and six optical fibers  7  are placed on its periphery. Therefore, all the cores  15  of the optical fibers  7  are placed at the same interval. In addition, the optical fibers  7  are mutually bonded together with the adhesive  19   a . Thus, all claddings  17  of the neighboring optical fibers  7  contact mutually directly or through the adhesive  19   a . The gap between the neighboring optical fibers  7  is also filled with the adhesive  19   a.    
         [0052]    The multi-core fiber  3  and the optical fibers  7  are made of quartz glass, for example. In this embodiment, although an example of a close-packed arrangement which is formed by seven cores in total, having six cores on the outer circumference of one central core, will be explained, but twelve cores can further be added on its outer circumference in the close-packed arrangement. That is, in the present invention, as long as the cores are placed close-packed, their number is not limited. 
         [0053]    However, the present invention intends to arrange the fibers in a close-packed structure by self-alignment process due to the balance of surface tension of the adhesive or the like which infiltrates into the gap between the fibers. Thus, the bundle structure having seven fibers is formed most accurately, and next, the bundle structure further equipped with twelve cores on its outer circumference can be formed with sufficient accuracy. Although the present invention is applicable to the bundle structure having more fibers, the accuracy of alignment of cores (especially on the outer circumference side) decreases. However, when the number of fibers is increased, degree of the misalignment of cores can be decreased by forming a bundle structure step by step, for example, in a way that: a bundle structure having seven fibers is formed; and after the seven fibers are bonded together, twelve fibers are bonded on their outer circumference through the surface tension. 
         [0054]    The optical fibers  7  are inserted into the capillary  21   b  in the close-packed state. The capillary  21   b  is a cylindrical member having a hole inside, and the cross-section of the hole of the capillary  21   b  is a circle having slightly larger diameter than the external diameter of the circumscribed circle of the optical fibers  7  in the close-packed arrangement. Also, the optical fibers  7  and the capillary  21   b  are bonded together by the adhesives  19   b . Here, it is desirable that the refractive index of adhesives  19   b  is smaller than that of the cladding  17  of the optical fiber  7 . In this way, leak of light from the cladding can be prevented. In addition, the adhesives  19   b  may be identical with the adhesives  19   a.    
         [0055]    An end surface of the multi-core fiber  3  (an end surface of the capillary  21   a ) and the end surface of the bundle structure  5  (an end surface of the capillary  21   b ) are polished and placed face to face. In this state, respective cores  11  face respective cores  15  with each other at the position where they are optically connected. That is, the pitch of the cores  11  is approximately equal to the external diameter of the optical fibers  7  (the diameter of the cladding  17 ). Taking into consideration that the adhesive layer is formed by the adhesives  19   a  in the gaps between the neighboring optical fibers, the external diameter of the optical fiber  7  (the cladding  17 ) may be set to be 0.1 to 0.3 μm smaller than the pitch of the cores  11  of the multi-core fiber  3 . Also in this case, the interval between individual cores  15  in the bundle structure in which the optical fibers  7  are bonded together is equal to the pitch of the cores  11 . 
         [0056]    End surfaces of the capillary  21   a  and  21   b  are fixed together with an adhesive or the like in a state where they are placed facing each other and the cores  11  and the cores  15  are optically connected respectively. Details of the position adjustment will be described later but is performed as follows in short: the capillaries  21   a  and  21   b  are arranged such that their facing end surfaces face each other; at least one capillary is fixed by a jig having a rotational mechanism; while signal light is inputted in each core of the multi-core fiber  3  from the end opposite to the facing end surface and the signal light which is outputted from the end of the bundle fiber opposite to the facing end surface is received, position adjustment and rotation adjustment of the bundled fibers (or the multi-core fiber) is performed; the jig is fixed at the position where the light signal output is maximized; and both fibers are bonded (or fused) to be connected with each other. 
         [0057]    Here, since the close-packed bundled fibers of the present invention have a very high positional accuracy of the cores, adjustment has only to be performed on at least two cores. If position adjustment is performed first on the central core and next on circumferential one or two cores, the task will be simple and the accuracy will be improved. Naturally, it is also possible to measure the amount of axis misalignment for all the cores and perform position adjustment on the most optimal position in order to perform more accurate position adjustment. 
         [0058]    Thus, a connection structure in which each core  11  of the multi-core fiber  3  and corresponding core  15  of the optical fiber  7  are optically connected, can be obtained. Here, in the fiber connection structure  1   a , since the optical fibers  7  are bundled in a close-packed arrangement, the intervals between the cores  15  can be kept equal with sufficient accuracy. Also, since an end of the multi-core fiber  3  and that of the bundle structure  5   a  are accommodated in the capillaries  21   a  and  21   b  respectively, both the multi-core fiber and the bundled fibers are easily handled. Also, since the surfaces of the capillaries are joined together and so the joint area is wide, they can be certainly joined. 
         [0059]    The connection structure of the present invention between the seven-core bundle structure and the seven-core multi-core fiber as above described have shown improvement in signal energy loss by 1 dB on the average among the seven cores compared with the conventional connection structure between the seven-core bundle structure and the seven-core multi-core fiber. 
         [0060]    The bundle structure in which the optical fibers  7  are inserted into the capillary  21   b  can be a bundle structure  5   b  shown in  FIG. 3  ( b ). The hole within the capillary  21   b  of the bundle structure  5   b  does not have a round shape but has an approximately regular hexagonal shape. Namely, the hole is an approximately regular hexagon which circumscribes to the close-packed arrangement of the optical fibers  7 , and the optical fibers  7  are placed at its vertexes, respectively. Accordingly, the arrangement of the optical fibers  7  is restricted, and the optical fibers  7  can be always arranged at specific circumferential positions with respect to the capillary  21   b.    
         [0061]    Also, the bundle structure may be a bundle structure  5   c  as shown in  FIG. 3  ( c ). The bundle structure  5   c  has a projection  23  in at least a part of the inner surface of the approximately circular hole within the capillary  21   b . That is, on the inner surface of the circle which circumscribes the close-packed arrangement of the optical fibers  7 , the projection  23  is formed such that it fits in the concave part formed in the gap between the neighboring optical fibers  7  in the close-packed arrangement. Accordingly, the arrangement of the optical fibers  7  is restricted and the optical fibers  7  can always be arranged at the specific circumferential position with respect to the capillary  21   b . Only one or plural projections  23  may be formed. 
         [0062]    Next, a manufacturing method of the bundle structure will be explained, especially targeting the bundle structures  5   a  to  5   c  in which the optical fibers  7  are bonded close-packed together. First, as shown in  FIG. 4 , coats of the predetermined number of the optical fibers  7  are removed and the stripped parts of the optical fibers Tare inserted into the capillary  21   b . On this occasion, the optical fibers  7  are inserted into the capillary  21   b  such that the ends of the optical fibers  7  protrude by the same length, respectively (about 10 mm, for example) from an end of the capillary  21   b . The capillary  21   b  is temporarily fixed to the optical fibers  7 , for example. 
         [0063]    The ends of the optical fibers  7  protruded from the end of the capillary  21   b  are soaked in an adhesive  25  placed in a container in advance. The adhesive  25  is a liquid adhesive, for example, and is a liquid containing solid state polymers such as synthetic resin dissolved in solvents such as water, alcohol, and organic solvents. When such a liquid adhesive is utilized to bond the optical fibers  7 , the solute remained after the solvent evaporated, hardens and bonds the optical fibers  7  together. 
         [0064]    The adhesives  25  with lower solute concentration than the normally utilized one are desirable. In that way, viscosity of the adhesive can be lowered and solute quantity which remains can be reduced. Thereby, the thickness of the adhesive layer between the optical fibers is reduced and the intervals between the optical fibers  7  can be kept uniform with higher accuracy. That is, although adhesive strength may become weak, an adhesive with very low viscosity of 100 cps or less, for example, is desirable. An effect of gathering the optical fibers more closely together is obtained by the adhesive contraction at the time of hardening. The adhasive with lower refractive index than that of the cladding of the optical fibers is desirable. 
         [0065]    As such an adhesive, for example, we can use the followings, for example: as a solution system adhasive, “Cemedine C” (a trade name) made by Cemedine Co., Ltd., diluted utilizing a thinner (it is desirable to add fluoride for adjustment of the refractive index); as a very low viscosity adhesive (acrylate type), a refractive index control resin (UV hardening) made by NTT-Advanced Technology corporation, and as an adhesive with a very low viscosity (epoxy type), a heat hardening type adhesive made by Epoxy Technology Inc. Since viscosity of the adhasive can be lowered more by heating the adhesive, the gaps between the optical fibers can be made smaller after adhesion. 
         [0066]    The optical fibers  7  are inserted into the capillary  21   b  in the approximately close-packed state, but before the ends of the optical fibers  7  are soaked in the adhesive  25 , the optical fibers are hard to be kept perfectly close-packed (at the uniform core intervals), and gaps can be formed between the optical fibers at a part and a too closely gathered part can be formed at the other parts. 
         [0067]      FIG. 5  are schematic diagrams showing the states of the optical fiber  7  before and after they are gathered closely by the surface tension of the adhesive  25 .  FIG. 5  ( a ) are front views (for simplification, only two optical fibers  7  are shown) and  FIG. 5  ( b ) are cross-sectional views. 
         [0068]    As described above, gaps may be formed in some cases between the optical fibers  7 . Also in such case, the adhesive  25  is sucked into the gaps between the optical fibers  7  by surface tension (by capillarity) because viscosity of the adhesive  25  is low. At this time, the optical fibers  7  are gathered closely together by the surface tension (to the direction of the arrows C in the figure). 
         [0069]    That is, as shown in the  FIG. 5(   b ), even if some uneven gaps are formed between the optical fibers  7 , the adhesive  25  is sucked up into the gaps and the optical fibers  7  are closely gathered together. On this occasion, the optical fibers are arranged in a configuration in which the surface tension of the adhesive sucked up into and existing at the gap between respective fibers stabilizes; namely, the optical fibers  7  are certainly arranged close-packed. And at the same time, the optical fibers  7  can be mutually bonded together by the hardened adhesive  25  in this state. Such an effect is especially effective for the very minute optical fibers  7  (the diameter Φ is 50 μm or less, for example) as in the present invention. Since the adhesive  25  is diluted solution type, gaps may be created after hardening in the bundle of fibers at the parts where the neighboring fibers were not closely gathered by the contraction of the adhesive. 
         [0070]    Next, as shown in  FIG. 6 , after the optical fibers  7  are bonded together in a close-packed state, the close-packed part is bonded to the capillary  21   b . As an adhesive (the adhesive  19   a ) utilized on this occasion, heat hardening epoxy type adhesive or a UV hardening acrylate type adhesive may be used. The adhesive  19   a  bonds the fiber bundle and the capillary together such that the adhesive  19   a  fills up the gaps between the capillary  21   b  and the fiber bundle and the gaps between the optical fibers (between the neighboring adhesives  25 ). Although the capillary and the optical fibers are bonded together here, only the optical fiber bundles may be connected to the multi-core fiber, removing the capillary. 
         [0071]    Next, the optical fibers  7  protruded from the capillary  21   b  and a part of the capillary  21   b  are ground at the polished surface  27 . The bundle structure  5   a  is formed in this way. Also, a uniform surface may be obtained, not by polishing the end surface of the bundle structure, but by cutting it off with a dicing saw, etc., for example. 
         [0072]    Although the adhesive  19   a  ( 19   b ) desirably has low viscosity, it can have viscosity higher than that of the adhesive  25  (5000 cps or less, for example). Also, the contraction percentage at the time of hardening is desirably low and the hardness is high (60 or more in Shore D scale). Although the hardness of the adhesive  25  is desirably high after hardening, the adhesive layer after hardening is fairly thin, and so effects that the hardness gives to the characteristics important at the time of polishing is small. 
         [0073]    As such adhesives, for example, “Epo-tek 353-ND” (a trade name) made by EPOXY TECHNOLOGY, Inc. which is an epoxy type heat hardening adhesive, “OP-40Z” (a trade name) made by DIC corporation which is an acrylate type UV hardening adhesive, or a refractive-index control resin (UV hardening) made by NTT-Advanced Technology corporation can be utilized. 
         [0074]    In this embodiment, although a step for inserting the plural optical fibers  7  into the capillary  21   b  is carried out first, there is no necessity that the present invention is limited to this. For example, the plural optical fibers  7  may be closely gathered and fixed together by the same method as this embodiment, and after that, the optical fibers  7  may be inserted into the capillary  21   b  and fixed by the second adhesive. In this occasion, the plural optical fibers  7  can be fixed into a close-packed structure by soaking the optical fibers  7  in the first adhesive  25 , under the condition that they are inserted in a cylindrical temporary arranging member. 
         [0075]    In this method, since insertion of the optical fibers  7  into the capillary  21   b  becomes easier, the clearance of the inner diameter of the capillary  21   b  can be made smaller. 
         [0076]    Although the adhesive  19   a  is different from the adhesive  25  in the present embodiment, the adhesive  25  can be used also as the adhesive  19   a . That is, such an adhesive  25  that contracts only a little when it hardens, and so does not create gaps between the fibers, can be used in order to closely gather and fix the neighboring fibers. Also in this case, the neighboring fibers can be closely gathered and fixed together utilizing the surface tension of the adhesive  25 . It is desirable that the adhesive  25  has high hardness (60 or more at Shore D scale). In addition, the capillary is not indispensable and the connection structure may be formed directly within the optical connector. 
         [0077]    In the present embodiment, a diluted solution type adhesive has been utilized as the adhesive  25 , but the present invention is not limited to this and it is possible to obtain the same effect by using an adhesive having very low viscosity, without adhesive dilution. Also, it is desirable that the adhesive  25  has a low refractive index because it enhances the light confinement effect, but if the optical fibers having an enough light confinement effect are utilized, the adhesive  25  having a high refractive index can also be used. 
         [0078]    Additionally, as a means to improve the aggregation effect of the optical fibers, wettability of the surface of the optical fibers  7  may be improved. As a means for improving the wettability, a method of spreading and drying the surface treatment agent called a primer and a method of performing plasma discharge process are known. It is natural and is desirable that the optical fibers  7  are fully kept clean during the process. 
         [0079]    Next, a method for aligning the bundle structure with the multi-core fiber will be described in detail.  FIG. 7  shows the method for aligning the bundle structure  5   a  with the multi-core fiber  3  where the multi-core fiber  3  is shown by a dotted line (the core part is black) and the bundle structure  5   a  is shown by a solid line (the core part is white). In the following examples, the method will be explained about the bundle structure  5   a , but even on the bundle structures according to the other embodiments, the method can be performed similarly. 
         [0080]    First, as shown in  FIG. 7  ( a ), the positions of the central core  11   a  and the central core  15   a  are adjusted, in a situation where the multi-core fiber  3  and the bundle structure  5   a  are faced mutually (the distance between the mutually facing end surfaces is 5 μm, for example). At this time, while the light is inputted from the multi-core fiber side for example, the multi-core fiber  3  (the capillary  21   a ) is moved in the X direction and in the Y direction which is perpendicular to the X direction (D and E directions in the figures, respectively) with respect to the bundle structure  5   a  (the capillary  21   b ). 
         [0081]    When the positions of the central core  11   a  and the central core  15   a  are adjusted correctly as shown in  FIG. 7  ( b ), light intensity which is detected with the light detector connected to the core  15   a , for example, is maximized. Instead, the light may be inputted from the core  15   a  side and detected at the core  11   a  side. 
         [0082]    In this state, the multi-core fiber (the capillary  21   a ) is rotated about the cross-sectional center of the capillary  21   a  with respect to the capillary  21   b  (arrow F direction in the figure). At this time, the light is inputted, for example, from the cores  11  side and detected at the cores  15  side. 
         [0083]    As shown in  FIG. 7(   c ), when the positions of the cores  11  precisely overlap the cores  15 , the light intensity detected with the light detector connected to the cores  15 , for example, is maximized. Instead, the light may be inputted from the cores  15  side and detected at the cores  11  side. 
         [0084]    It is also possible to align the bundle structure with the multi-core fiber by moving (rotating) the capillary  21   b . However, rotation of the multi-core fiber  3  is favorable, because it is possible to make the clearance between the capillary  21   a  and the multi-core fiber  3  smaller. Thereby, the central positions of the capillary  21   a  and the multi-core fiber  3  are approximately coincided together. Therefore, if the capillary  21   a  is rotated about its cross-sectional center, the cross-sectional center of the multi-core fiber  3  serves nearly as the rotational axis. 
         [0085]    As for the capillary  21   b  side, since the plural optical fibers  7  have to be inserted into it, bigger clearance is required compared with the clearance between the capillary  21   a  and the multi-core fiber  3 . Thereby, the cross-sectional center of the capillary  21   b  and the cross-sectional center of the close-packed arrangement of the optical fibers  7  may not precisely agree with each other. Accordingly, when the cross-sectional center of the capillary  21   b  serves as a rotational axis, the cross-sectional center of the close-packed arrangement of the optical fibers  7  does not become a rotational center, and the position of the central core  15   a  itself may be moved from the position of the central core  11   a . Therefore, it is desirable to fix the capillary  21   b  and rotate the capillary  21   a.    
         [0086]    Also, by repeating alignment of the central cores and that of another core, the detected intensity of the light at respective cores can be maximized. After completion of the above alignment, the five points alignment (an alignment method to calculate the present amount of axis misalignment from the detected light intensity at five positions: the current position; the positions moved in the ±X directions; and the positions moved in the ±Y directions, and to move the multi-core fiber by the optimal distance and in the optimal directions) can be utilized. After completion of the alignment, the multi-core fiber and the fiber bundle may be joined together and fixed by the adhasive or the like in that state. 
         [0087]    Also, it is possible to utilize another alignment method. For example, it is possible to utilize a method for aligning arbitrary two cores first, and aligning the rest of cores after that. Specifically, the method is as follows: First, two cores placed on both sides of the central fiber on the X-axis are aligned by adjusting the positions in the X- and Y-direction and by rotation. Then, the amount of axis misalignment of every optical fiber (axis misalignment of the optical fiber in the X and Y directions) is measured and the multi-core fiber is moved in the optimal direction by optimal distance. 
         [0088]    “Moved in the optimal direction by optimal distance” means here to “minimize the amount of the axis misalignment of the core on which the largest amount of axis misalignment is detected”. Alternatively, a method in which the average amount of axis misalignment on all cores is minimized, or a method in which the mean square amount of the axis misalignment is minimized (the least square method) can be used. 
         [0089]    Further, other alignment methods can be utilized. For example, the central cores are aligned first ( FIG. 7  ( b )). Subsequently, a pair of cores of the optical fibers which is symmetrically placed on both sides of the central optical fiber are aligned through rotation around the center core.  FIG. 8  show how the cores  15   b  and  15   c  of the optical fibers symmetrically placed with respect to the central core  15   a  are aligned with the cores  11   b  and  11   c  of the multi-core fiber through rotation. Even if the central cores are aligned, other cores may not precisely overlap each other because of the variation in the diameter of the fibers or the like. 
         [0090]    In this case, alignment is performed so that the arrangement direction of a pair of cores  15   b  and  15   c  (G in the figures) and the arrangement direction of a pair of cores  11   b ,  11   c  (H in the figures) become parallel. In order to adjust so that the directions G and H become parallel, the light is inputted from the side of the cores  15   b  and  15   c , for example, and the rotation angle can be estimated by the light intensity detected at the side of the cores  11   b  and  11   c . Thus, when rotational alignment is performed on a pair of cores which are placed at symmetrical positions with respect to the central core as above, accurate alignment can be performed, even if the positions of the cores  15   b  and  15   c  are slightly dislocated from the symmetrical positions. 
         [0091]    After performing the rotational alignment, the dislocation as a whole may be adjusted. For example, as shown in the  FIG. 7  ( c ), after aligning the positions of central cores and performing rotational alignment, the multi-core fiber  3  may be slightly moved in two directions (X direction and Y direction perpendicular to the X direction (D and E directions in the figure) relative to the bundle structure  5   a . In this case, respective cores of the multi-core fiber  3  and respective optical fiber cores of the bundle structure  5   a  are optically connected, and intensities of the light from respective couples of cores are measured. 
         [0092]    When the multi-core fiber  3  is slightly moved in the X and Y directions relative to the bundle structure  5   a , the position where the detected light intensity is maximized (the positions where the amount of axis misalignment is minimized) may differ among the cores. In such a case, after the multi-core fiber is moved relatively in various directions, a suitable position is chosen where the amount of misalignment (the decrease percent of detected intensity relative to the maximum detected intensity, for example) of the core with largest amount of misalignment is minimum. Axis misalignment mentioned here is observed through transmission loss (the difference between the detected light intensity at the current position and the maximum detected light intensity), namely through axis misalignment loss. Namely, if the axis misalignment loss is measured, the amount of axis misalignment can be calculated. In general, the axis misalignment loss is proportional to the square of the amount of axis misalignment. 
         [0093]    For example, we assume the following situation where: at a certain position, the average of the axis misalignment loss over the whole cores (the average of decreased amount of detected intensity compared to the detected maximum intensity at respective cores) is 3 dB and the axis misalignment loss of the core with the maximum axis misalignment is 5 dB. Then, the multi-core fiber  3  is slightly moved relatively to the bundle structure  5   a  from the above situation, and the average of the axis misalignment loss becomes 3.1 dB, and the axis misalignment loss of the core with the maximum axis misalignment becomes 4.5 dB. Then, this situation with smaller maximum axis misalignment loss can be judged to be appropriate. Further, the multi-core fiber  3  may be slightly moved relatively to the bundle structure  5   a  in the X and Y directions repeatedly, and may be fixed at the position where the maximum amount of axis misalignment is minimized. 
         [0094]    The alignment may be performed such that not the maximum axis misalignment loss, but the average of axis misalignment loss over the whole cores is minimized. Moreover, the order of procedures in the foregoing alignment methods may be changed and each of the alignment methods may be combined. 
         [0095]    According to the present invention, since the optical fibers  7  are combined in the close-packed arrangement, the intervals between the neighboring optical fibers  7  can be easily set equal. Accordingly, each of the cores  11  in the multi-core fiber  3  and each of the cores  15  in the optical fibers  7  can be optically connected with certainty. 
         [0096]    Especially, since the optical fibers  7  are bonded together in the close-packed arrangement and they are held by a capillary or a holding member, connection work is easy. The circumferential direction of the close-packed arrangement of the optical fibers  7  with respect to the capillary  21   b  can be controlled in such a way that the hole of the capillary  21   b  is made to have a hexagonal shape or a projection  23  is formed inside the hole. Thereby, for example, if a mark, which can tell the arrangement of the internal optical fibers  7 , is provided on the outer circumference of the capillary  21 , the circumferential positions of the cores can easily be grasped in alignment work. 
         [0097]    Also, as for the method for arranging optical fibers close-packed, the optical fibers  7  can be certainly and easily arranged close-packed and can be bonded together by utilizing the surface tension of the diluted adhesive  25 . Then, the close-packed optical fibers  7  are bonded to the capillary  21   b  with an adhesive which has higher viscosity and higher hardness than the adhasive  25 , and their end surfaces are ground. Thus, the capillary  21   b  and the optical fibers  7  can be reliably joined together, and at the same time, the ends of the optical fibers  7  are not damaged when they are ground. 
         [0098]    Since the optical connector  10  of the present invention contains such a fiber connection structure  1   a  in its inside, a multi-core fiber having the same size as the embedded multi-core fiber  3  (or an optical connector which contains such multi-core fiber) can be easily connected to it. Also, the optical connector  10  can separate the multi-core fiber connected to it into plural optical fibers. That is, the optical fibers can be converted to the multi-core fiber and vice versa, within the optical connector  10 . 
         [0099]    Although the embodiments of the present invention have been described referring to the attached drawings, the technical scope of the present invention is not limited to the embodiments described above. It is clear that persons skilled in the art can think out various examples of changes or modifications within the scope of the technical idea disclosed in the claims, and it will be understood that they naturally belong to the technical scope of the present invention. 
         [0100]    For example, although the end surface of the multi-core fiber  3  is exposed on the connector end surface of the optical connector  10 , the present invention is not limited to this.  FIG. 9  shows an optical connector  10   a  in which an end surface of the optical fibers  7  are exposed on the connector end surface. 
         [0101]    In the optical connector  10   a , similarly to the optical connector  10 , the fiber connection structure  1   a  is contained inside and the optical fibers and the multi-core fiber can be converted to each other within the optical connector. In this case, the optical fibers  7  may be arranged such that the intervals between them are extended as they approach the end surface of the optical connector  10   a.    
         [0102]    For example, as described above, the pitch between the optical fibers  7  is equal to the core pitch of the multi-core fiber  3  (40 to 50 μm, for example) in the fiber connection structure  1   a . On the other hand, at the end surface of the optical connector  10   a , the intervals between the respective optical fibers  7  can be extended to a 125 μm pitch which is the normal diameter of an optical fiber, and thus the optical connector  10   a  can be easily connected to optical fibers of normal size. In this case, for example, it is desirable to place the optical fibers at the connector end surface in the arrangement that can be connected to optical fibers having the diameter of 125 μm arranged close-packed (arranged such that all the intervals between the optical fibers  7  are the same). 
         [0103]    The intervals between the optical fibers may be 250 μm and in this case, the optical fibers can be connected to an MT connector which is frequently used generally. On this occasion, holes may be provided for guide pins to be connected to the MT connector. Also, the optical fibers do not have to be arranged in one row but it may be arranged in plural rows or in a round shape, etc. 
         [0104]    That is, not only the optical fibers can be converted into the multi-core fiber or vice versa, but also the core intervals can be changed by utilizing the optical connector  10   a.    
         [0105]      FIG. 10  further shows a connector  30  according to another embodiment. In the connector  30 , the multi-core fiber  3  is inserted into a capillary  31   a  which is a first capillary. Similarly, the plural optical fibers  7  in the close-packed arrangement are inserted into the capillary  31   b  which is a second capillary, and the bundle structure  5   a  is formed. A connector flange part  33  is provided on the outer circumference of the capillary  31   a . The connector flange part  33  is utilized when the connector is fixed to a housing of the connector, etc. 
         [0106]    The capillary  31   a  placed on the front end side of the connector  30  and a capillary  31   b  placed on the rear end side of the connector  30  are joined together with an adhesive or by fusion splicing. The capillary  31   a  and the capillary  31   b  are joined together rearward the connector flange  33 . Also, in the joint between the capillary  31   a  and the capillary  31   b , the multi-core fiber  3  and the optical fibers  7  are optically connected. The effects of the present invention can be obtained also by the connector  30 . 
         [0107]      FIG. 11  shows a connector  40  according to further another embodiment. Although the connector  40  is approximately the same as the connector  30 , the capillary  31   a  consists of a zirconia capillary  41  and a glass capillary  43 . The front side of the capillary  31   a  is the zirconia capillary  41  and its rear side is the glass capillary  43 . 
         [0108]    In the connector  40 , the capillary  31   a  and the capillary  31   b  are bonded together with an ultraviolet hardening adhesive. On this occasion, since the capillary  31   a  is the glass capillary  43  in the vicinity of the joint with the capillary  31   b , ultraviolet rays can penetrate through the capillary  43  to its border planes. Thereby, by applying the ultraviolet hardening adhesive to the joined area in advance and irradiating the joint area with the ultraviolet rays from the exterior, the capillary  31   a  and the capillary  31   b  can be easily bonded together. In this case, it is desirable that the capillary  31   b  is also transparent and made of glass or the like. 
         [0109]      FIG. 12  shows an optical connector  50  according to further other embodiment. Although, the optical connector  50  is approximately the same as the connector  30 , the joint between the capillary  31   a  and the capillary  31   b  is placed on the front side of the connector flange part  33 . That is, the connector flange part  33  is placed on the outer circumference of the capillary  31   b.    
         [0110]    In the optical connector  50 , the external diameter of the capillary  31   a  is larger than the external diameter of the capillary  31   b . Thereby, the outer circumference of the capillary  31   b  does not spread out of the outer circumference of the capillary  31   a  at the time of connection with another connector. Therefore, at the time of connection with another connector, the capillary  31   b  does not interfere with another connector. Also in the optical connector  50 , the capillary  31   a  may be made of the zirconia capillary  41  and the glass capillary  43 . 
         [0111]      FIG. 13  shows an optical connector  55  according to further other embodiment. Although the optical connector  55  is approximately the same as the connector  30  and the like, it differs in that the joint between the capillary  31   a  and the capillary  31   b  is held within a connector flange part  33   a . The connector flange part  33   a  is for an MU connector. In this case, the connector flange part  33   a  may be attached after the multi-core fiber  3  and the bundle structure  5   a  are aligned and connected together. 
         [0112]    On this occasion, if the connector flange part  33   a  and the multi-core fiber are matched under some rule in the circumference direction, respective core parts of the multi-core fiber can be placed at the predetermined circumferential positions with respect to the connector flange part  33   a . Also, an Oldham-coupling-compatible flange may be utilized for the connector flange  33   a . In addition, although a structure of an MU connector is shown in the present example, the structure of the connector may be other than that of the MU connector (an SC connector, for example). 
         [0113]      FIG. 14  shows a connector structure  60  utilizing each of the foregoing connectors. In the connector structure  60 , one of the optical connectors  30  to  50  (the connector  30  is shown in the figure as an example) is accommodated in a housing  61 . Boot  65  is provided behind the housing  61 . Within the boot  65 , the optical fibers  7  which are pulled out of the capillary  31   b  are configured into an optical fiber ribbon. That is, an optical fiber ribbon  63  is pulled out from the connector part on one end of the connector structure  60 . 
         [0114]    A common MT connector  67  is connected to the other end of the optical fiber ribbon  63 . The MT connector  67  can be connected to other optical fibers. That is, the multi-core fiber  3  is exposed from the connector part on one end of the connector structure  60  and the optical fibers are protruded out in a row from the connector part on the other end. Accordingly, by using the connector structure  60 , a multi-core fiber (or a bundle structure) and optical fibers (or the optical fiber ribbons) can be connected together easily. 
         [0115]      FIG. 15  shows a fiber arrangement conversion member  70 . The fiber arrangement conversion member  70  is composed of a main body  71  and optical fibers  7 . One end of the main body  71  is a fixing part  73   a  and is a first fixing part. The other end of the main body  71  is a fixing part  73   b  and is a second fixing part. A hole  75  is provided in the fixing part  73   a.    
         [0116]      FIG. 16  ( a ) is a view from the arrow I shown in the  FIG. 15  and a side view of the fiber arrangement conversion member  70 . The hole  75  formed in the fixing part  73   a  is a through hole having an approximately hexagonal shape. In the hole  75 , the optical fibers  7  are arranged close-packed and are bonded to the hole  75  with an adhesive, etc. Such a structure may be formed as shown in the  FIG. 4  to  FIG. 6 . End surfaces of the optical fibers  7  are exposed on an end surface of the fixing part  73   a . Thereby, the optical fibers  7  can be optically connected to close-packed cores of a multi-core fiber, etc. 
         [0117]    As shown in the  FIG. 15 , a lid  77 , V grooves  81 , guide holes  79 , etc. are formed in the fixing part  73   b .  FIG. 16  ( b ) is a view from the arrow J shown in the  FIG. 15  and is a side view of the fiber arrangement conversion member  70 . The plural V grooves  81  are provided at predetermined intervals in one row in the fixing part  73   b . The optical fibers  7  are accommodated in the V grooves  81 , respectively, and are pressed down by the lid  77  from above. Thereby, the optical fibers  7  are fixed being arranged in parallel in one row. The optical fibers  7  may be bonded to the V grooves  81  with an adhesive or the like. Also, the lid  77  and the main body  71  may be fixed together with an adhesive. 
         [0118]    A pair of the guide holes  79  are formed on both sides of the row of the optical fibers  7 . The guide holes  79  are sites where the guide pins are inserted into when the member is connected to another connector. The position of the optical fibers can be adjusted utilizing the guide pins. The guide holes may be formed also on the fixing part  73   a.    
         [0119]    End surfaces of the optical fibers  7  are exposed on the end surface of the fixing part  73   b . Accordingly, the optical fibers  7  can be connected to another optical fiber ribbon or the like in which optical fibers are arranged in one row. That is, by using the fiber arrangement conversion member  70 , the multi-core fiber and the optical fiber ribbon or the like can be connected easily to each other. The fiber arrangement conversion member  70  may be embedded within the connector. 
         [0120]    Although, in the forgoing embodiments, examples of the bundle structure suited for the multi-core fiber having seven cores are shown, the present invention is not limited to these. For example, the present invention is applicable to a multi-core fiber having nineteen cores equipped with one more core layer outside the seven cores. In this case, the same effects as the above examples can be obtained by using a bundle structure having nineteen optical fibers manufactured by utilizing the same method. 
         [0121]      FIG. 17  ( a ) shows a jig  83  for manufacturing the bundle structure having nineteen cores. A hole  85  is formed in the center of the jig  83  and twelve holes  87  are formed on the approximately hexagonal line around the hole  85 . The optical fibers  7  which are bundled in advance are inserted into the hole  85 . Namely, the seven optical fibers  7  joined (temporarily) in advance in the close-packed arrangement in the cross-section are inserted into the hole  85 . The optical fibers  7  are inserted into the holes  87 , respectively. 
         [0122]      FIG. 18  ( a ) shows a longitudinal cross-section at the K-K line in the  FIG. 17 . The bundled seven optical fibers  7  in the center and the optical fibers  7  around them are soaked in the adhesive  25  such that their neighboring ends are made contact with each other. This will allow the additional twelve optical fibers  7  to be closely gathered together by the surface tension on the circumference of the close-packed seven optical fibers  7 . 
         [0123]    As shown in  FIG. 18  ( b ), the holes  87  may be obliquely formed along the direction of insertion of the optical fibers  7 . Also, the placement or the size of the holes  85  and  87  of the jig  89  can be set suitably according to the number of the optical fibers to be bundled. 
         [0124]    The intervals between the cores in the multi-core fiber may not necessarily be uniform. In such a case, the external diameters (all the optical fibers do not have the same external diameter) of the optical fibers which will be bundled may be selected suitably according to the core pitches of the multi-core fiber. 
         [0125]    On this occasion, a jig such as the jig  89  shown in  FIG. 17  ( b ) may be utilized. For example, the central optical fiber  7  having a large diameter may be inserted into the hole  85  and the peripheral optical fibers having smaller diameter may be inserted into the holes  87 , respectively. A bundle structure comprising closely gathered optical fibers can be obtained by a procedure in which the ends of the optical fibers are soaked in an adhesive, etc. 
         [0126]      FIG. 19  ( a ) shows a multi-core fiber  90  having 10 cores and  FIG. 19  ( b ) shows a bundle structure  91  manufactured by the method shown in the  FIG. 17  ( b ). As shown in the  FIG. 19  ( a ), ten cores  11  are placed in the cladding  13  of the multi-core fiber  90 . That is, nine cores  11  are placed around the central core  11  with the central angle of forty degrees for each. 
         [0127]    As shown in the  FIG. 19  ( b ), the respective optical fibers  7  in the bundle structure  91  are arranged so that the bundle structure can be connected to the above multi-core fiber  90 . By the arrangement of the optical fibers  7  in a closely gathered state, the connection of the bundle structure  91  to the multi-core fiber  90  can be realized. Here, for a bundle structure corresponding to the multi-core fiber having one central core and n cores around it arranged at the same interval, the following Equation 1 holds, where R is the radius of the cladding  17  of the central optical fiber  7  and r is the radius of the optical fibers  7  placed around it. 
         [0000]    
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       
                         1 
                         - 
                         
                           
                             sin 
                              
                             
                               ( 
                               
                                 180 
                                 n 
                               
                               ) 
                             
                           
                           ∘ 
                         
                       
                       
                         
                           sin 
                            
                           
                             ( 
                             
                               180 
                               n 
                             
                             ) 
                           
                         
                         ∘ 
                       
                     
                      
                     r 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0128]    When the radii of the central optical fiber R and the peripheral optical fibers r satisfy the equation 1, the bundle structure possible to be connected to the multi-core fiber as described above, can be obtained. 
       EXPLANATION OF NUMERALS 
       [0000]    
       
         
           
               1   a  . . . fiber connection structure 
               3 ,  90  . . . multi-core fiber 
               5   a ,  5   b ,  5   c ,  91  . . . bundle structure 
               7  . . . optical fiber 
               10 , 10   a , 30 , 40 , 50 , 55  . . . optical connector 
               11 , 11   a  . . . core 
               12  . . . ferrule 
               13  . . . cladding 
               15 , 15   a , 15   b , 15   c  . . . core 
               17  . . . cladding 
               19   a , 19   b  . . . adhesive 
               21   a , 21   b , 31   a , 31   b  . . . capillary 
               23  . . . projection 
               25  . . . adhasive 
               27  . . . polished surface 
               33 , 33   a  . . . connector flange part 
               41  . . . zirconia capillary 
               43  . . . glass capillary 
               60  . . . connector structure 
               61  . . . housing 
               63  . . . optical fiber ribbon 
               65  . . . boot 
               67  . . . MT connector 
               70  . . . fiber arrangement conversion member 
               71  . . . main body 
               73   a , 73   b  . . . fixing part 
               75  . . . hole 
               77  . . . lid 
               79  . . . guide hole 
               81  . . . V groove 
               83 ,  89  . . . jig 
               85 ,  87  . . . hole