Abstract:
A multi-core optical fiber includes: a plurality of core portions; and a cladding portion positioned around the plurality of core portions and including a marker for identifying a position of a specific one of the plurality of core portions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT International Application No. PCT/JP2009/068908 filed on Nov. 5, 2009 which claims the benefit of priority from Japanese Patent Application No. 2008-327924 filed on Dec. 24, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multi-core optical fiber having a plurality of core portions. 
     2. Description of the Related Art 
     As means for remarkably increasing communication capacity, multi-core optical fibers, each having a plurality of core portions, have been disclosed. 
     As one type of the multi-core optical fibers, a multi-core holey fiber is disclosed (see Japanese National Publication of International Patent Application No. 2008-534995). This holey fiber is an optical fiber that has core portions and a cladding portion positioned around the core portions and having a plurality of holes arranged around the core portions. In the holey fiber, an average refractive index of the cladding portion is reduced by the holes, and the principle of total reflection of light is utilized to confine the light to the core portions and to propagate the light (see International Publication Pamphlet No. WO2008/093870). Because the refractive index is controlled by the holes, the holey fiber is able to realize endlessly single mode (ESM) characteristics realizing single mode transmission at all wavelengths and unique properties such as anomalous dispersion at shorter wavelengths. A multi-core holey fiber is a holey fiber having a plurality of core portions, and is thought of as being possible to realize SDM transmission in addition to the ESM characteristics. 
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     When the multi-core optical fiber is connected to another multi-core optical fiber or to an optical device by fusion splicing or the like, it is necessary to connect a specific core portion of the multi-core optical fiber and a specific core portion of the another multi-core optical fiber or the optical device. 
     However, if the plurality of core portions is symmetrically arranged with respect to a central axis in the multi-core optical fiber, there is a problem that it is difficult to identify the specific core portion by the appearance. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, a multi-core optical fiber includes: a plurality of core portions; and a cladding portion positioned around the plurality of core portions and including a marker for identifying a position of a specific one of the plurality of core portions. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a multi-core HF according to a first embodiment; 
         FIG. 2  is a schematic cross-sectional view of a multi-core HF according to a first modification; 
         FIG. 3  is a cross-sectional view of a cross section opposed to the cross section illustrated in  FIG. 2  when the multi-core HF illustrated in  FIG. 2  is cut; 
         FIG. 4  is a schematic cross-sectional view of a multi-core HF according to a second modification; 
         FIG. 5  is a schematic cross-sectional view of a multi-core HF according to a third modification; 
         FIG. 6  is a schematic cross-sectional view of a multi-core HF according to a fourth modification; 
         FIG. 7  is a schematic cross-sectional view of a multi-core HF according to a fifth modification; 
         FIG. 8  is a schematic cross-sectional view of a multi-core HF according to a sixth modification; 
         FIG. 9  is a schematic cross-sectional view of a multi-core optical fiber according to a second embodiment; 
         FIG. 10  is a schematic cross-sectional view of a multi-core HF according to a seventh modification; 
         FIG. 11  is a schematic cross-sectional view of a multi-core optical fiber according to a third embodiment; 
         FIG. 12  is a schematic cross-sectional view of a multi-core optical fiber according to a fourth embodiment; 
         FIG. 13  is a schematic cross-sectional view of a multi-core optical fiber according to a fifth embodiment; 
         FIG. 14  is a cross-sectional photograph of a multi-core optical fiber according to a first example; and 
         FIG. 15  is a cross-sectional photograph of a multi-core optical fiber according to a second example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a multi-core optical fiber according to the present invention will be explained in detail below with reference to the drawings. The present invention is not limited by the embodiments. In addition, those terms, which are not particularly defined in this specification, follow the definitions and measurement methods defined by ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.650.1. 
     First Embodiment 
       FIG. 1  is a schematic cross-sectional view of a multi-core holey fiber (multi-core HF) according to a first embodiment of the present invention. As illustrated in  FIG. 1 , a multi-core HF  1   a  includes core portions  111  to  117  arranged separated from each other and a cladding portion  12  positioned around the core portions  111  to  117 . The core portions  111  to  117  and the cladding portion  12  are made of silica-based glass such as pure silica glass not containing any dopant for adjusting the refractive index. 
     The core portion  111  is arranged at an approximate center portion of the cladding portion  12 , and the core portions  112  to  117  are arranged at respective apexes of an equilateral hexagon around the core portion  111 . The cladding portion  12  includes a plurality of holes  13  periodically arranged around the core portions  111  to  117 . The holes  13  are arranged so as to form a triangular lattice L, and form equilateral hexagonal layers so as to surround each of the core portions  112  to  117 . Moreover, in the multi-core HF  1   a , the core portions  111  to  117  are surrounded by the holes  13  of at least five-layers, and four holes  13  are each present between the core portions  111  to  117 . The cladding portion  12  includes a marker M 1  being a hole formed outside an area in which the holes  13  are formed and on an outer side of the core portion  112  and the core portion  117 . The marker M 1  is located at a position at approximately equal distances from the core portion  112  and the core portion  117 . 
     The multi-core HF  1   a  is configured to confine light to each of the core portions  111  to  117  by the holes  13  to propagate the light. A diameter d [μm] of the holes  13  and a lattice constant Λ [μm] of the triangular lattice L are not particularly limited and, thus, are set appropriately according to desired optical characteristics. For example, if d/Λ is 0.35 to 0.65 and Λ is 10 μm, then similarly to the HF disclosed in International Publication Pamphlet No. WO2008/093870, ESM characteristics are realized for each of the core portions  111  to  117  at least at a wavelength of 1260 nm to 1610 nm. Furthermore, at a wavelength of 1550 nm, an effective core area Aeff becomes large at approximately 114.6 μm 2 , and a bending loss upon bending with a diameter of 20 mm becomes small at approximately 1.6 dB/m, and thus, an optical fiber with reduced optical nonlinearity and suitable for cabling is realized. 
     In the conventional multi-core HF, because the core portions are symmetrically arranged with respect to a central axis thereof, it is difficult to identify a specific core portion by the appearance. However, as explained above, because the cladding portion  12  of the multi-core HF  1   a  has the marker M 1 , a specific core portion is easily identified on the basis of the location of the marker M 1 . The marker M 1  is a hole, the diameter thereof is not particularly limited, and the diameter may be, for example, approximately 1.0 μm to 10.0 μm. Furthermore, if present outside the area in which the holes  13  are formed, like the marker M 1 , the marker M 1  does not affect confinement of the light to the core portions  111  to  117  by the holes  13 , and thus the optical characteristics of the multi-core HF  1   a  is not changed. 
     As explained above, because the cladding portion  12  of the multi-core HF  1   a  includes the marker M 1 , identification of a specific core portion of the core portions  111  to  117  becomes easy, and thus connection to another multi-core optical fiber becomes easy. 
     If, for example, the multi-core HFs  1   a  are to be connected to each other using fusion splicing, connector connection, or mechanical splice connection, the following method, for example, is used. Specifically, in a state in which end faces of, for example, two multi-core HFs  1   a  are placed opposite to each other, a mirror or a prism is inserted between the end faces of the two multi-core HFs  1   a . Then, at least one of the two multi-core HFs  1   a  is rotated around the central axis while the end faces of the two multi-core HFs  1   a  which have become observable from the outside by the mirror or the prism are being observed, and on the basis of the positions of the markers M 1 , a rotational position is determined so that, for example, the core portions  112  are connected to each other. Thereafter, the two multi-core HFs  1   a  are connected to each other. 
     In the connection method, light may be passed through a specific core portion, for example, a core portion  112  of one of the multi-core HFs  1   a , and a light receiver may be connected to a core portion  112  at an end face on an opposite side of a to-be-connected-to end face of the other multi-core HG  1   a , and then connection may be performed while intensity of the light received by the light receiver is being monitored. In this case, by using the markers M 1  for coarse adjustment of rotational positions of the two multi-core HFs  1   a , and performing fine adjustment of the rotational positions by a light intensity monitor, it is possible to achieve quick and easy coarse adjustment and accurate fine adjustment. 
     (First Modification) 
     Next, first to sixth modifications of the multi-core HF  1   a  according to the first embodiment will be explained below.  FIG. 2  is a schematic cross-sectional view of a multi-core HF  1   b  according to the first modification. As illustrated in  FIG. 2 , the multi-core HF  1   b  has a configuration in which the marker M 1  of the multi-core HF  1   a  illustrated in  FIG. 1  is replaced with a marker M 2 . The marker M 2  is a hole similar to the marker M 1 , but is located closer to the core portion  112  than to the core portion  117 . As a result, in the multi-core HF  1   b , it is possible to identify a specific direction around a periphery of the cladding portion  12 , for example, a direction from the core portion  117  toward the core portion  112  by the marker M 2 . 
     More specifically, if the multi-core HF  1   b  is cut, one of the cross section is as illustrated in  FIG. 2 , while the other cross section opposite thereto becomes, as illustrated in  FIG. 3 , mirror-symmetrical with respect to the cross section in  FIG. 2 , and thus the positions of the core portion  112  and the core portion  117  are interchanged. However, as illustrated in  FIG. 3 , the marker M 2  is disposed so that its position changes from one cross section to a cross section opposite thereto and being its mirror image. As a result, in the multi-core HF  1   b , the direction from the core portion  117  toward the core portion  112  is identifiable by the marker M 2 , and thus positions of the core portion  112  and the core portion  117  become more infallibly identifiable and positions of the other core portions also become more infallibly identifiable, facilitating the connection further. 
     (Second Modification) 
       FIG. 4  is a schematic cross-sectional view of a multi-core HF  1   c  according to a second modification. As illustrated in  FIG. 4 , the multi-core HF  1   c  has a configuration in which the marker M 1  of the multi-core HF  1   a  illustrated in  FIG. 1  is replaced with a marker M 3 . The marker M 3  has three holes formed into a triangle, and is formed so as to specify a direction from the core portion  117  to the core portion  112 . Therefore, in the multi-core HF  1   c , similarly to the marker M 2  of the multi-core HF  1   b , a specific direction around the periphery of the cladding portion  12  becomes identifiable and the position of each core portion becomes more infallibly identifiable by the marker M 3 , facilitating the connection further. 
     (Third Modification) 
       FIG. 5  is a schematic cross-sectional view of a multi-core HF  1   d  according to a third modification. As illustrated in  FIG. 5 , the multi-core HF  1   d  has a configuration in which the marker M 1  of the multi-core HF  1   a  illustrated in  FIG. 1  is replaced with a marker M 4 . Similarly to the marker M 3  of the multi-core HF  1   c , the marker M 4  also has three holes formed into a triangle so as to specify a direction from the core portion  117  to the core portion  112 , but this triangle is elongated than that of the marker M 3 . Therefore, in the multi-core HF  1   d , the position of each core portion is more infallibly identifiable by the marker M 4 , and the connection is facilitated even further. 
     (Fourth Modification) 
       FIG. 6  is a schematic cross-sectional view of a multi-core HF  1   e  according to a fourth modification. As illustrated in  FIG. 6 , the multi-core HF  1   e  has a configuration in which the marker M 1  of the multi-core HF  1   a  illustrated in  FIG. 1  is replaced with markers M 5  and M 6 , which are holes. The marker M 5  is located, similarly to the marker M 1 , at a position that is approximately equally distant from the core portion  112  and the core portion  117 , while the marker M 6  is located on the outer side of the core portion  112  and the core portion  113 . As explained above, in this multi-core HF  1   e , the markers M 5  and M 6  are arranged to face different sides of an equilateral-hexagonal area in which the holes  13  are formed, and thus the position of each core portion is more infallibly identifiable, and the connection is facilitated even further. 
     (Fifth Modification) 
       FIG. 7  is a schematic cross-sectional view of a multi-core HF  1   f  according to a fifth modification. As illustrated in  FIG. 7 , the multi-core HF  1   f  has a configuration in which the marker M 6  of the multi-core HF  1   e  illustrated in  FIG. 6  is replaced with a marker M 7 , which is holes. The marker M 7  is located on the outer side of the core portion  113  and the core portion  114 . In the multi-core HF  1   f , similarly to the multi-core HF  1   e , the markers M 5  and M 7  are arranged to face different sides of an equilateral-hexagonal area in which the holes  13  are formed, and thus the position of each core portion is more infallibly identifiable, and the connection is facilitated even further. 
     (Sixth Modification) 
       FIG. 8  is a schematic cross-sectional view of a multi-core HF  1   g  according to a sixth modification. As illustrated in  FIG. 8 , the multi-core HF  1   g  has a configuration in which the marker M 3  of the multi-core HF  1   c  illustrated in  FIG. 4  is replaced with a marker M 8 . The marker M 8  is formed of three silica-based glass portions made of silica-based glass doped with a material having a refractive index different from that of the cladding portion  12 , for example, doped with germanium, which increases the refractive index, or fluorine, which decreases the refractive index, and is formed into an approximate isosceles triangle so as to specify a direction from the core portion  117  to the core portion  112 . As a result, in the multi-core HF  1   g , by the marker M 8 , a specific direction around the periphery of the cladding portion  12  is identifiable, the position of each core portion is more infallibly identifiable, and the connection is facilitated even further. 
     When ordinary optical fibers are fusion spliced, an end face of an optical fiber is irradiated with light from a lateral side, the light transmitted through the optical fiber is imaged, and a position of a core portion is identified based on shading produced by a refractive-index difference between the core portion and the cladding portion on the picked-up image. In the multi-core HF  1   g  also, similarly to the ordinary optical fibers, a position of the marker M 8  is identified based on shading produced by a refractive-index difference between the marker M 8  and the cladding portion  12  on an image imaged by irradiating light from a lateral side, and the position of each core portion is identifiable based on the identified position. 
     Next, a method of manufacturing multi-core HFs  1   a  to  1   g  according to the first embodiment and the first to sixth modifications will be explained below. The multi-core HFs  1   a  to  1   g  may be manufactured using a well-known drill method, sol-gel method, or stack-and-draw method. When the stack-and-draw method is used, the following is performed. In, for example, a pure silica-based glass tube, solid pure silica-based glass rods to become core portions are placed, hollow pure silica-based glass capillaries to form holes are placed around the glass rods, and solid pure silica-based glass rods are filled into space between the glass tube and the glass capillaries, to produce a glass preform. When this is performed, one or more of the glass rods to be filled therein, which corresponds/correspond to a position where a marker is to be formed is/are replaced with a hollow pure silica-based glass capillary/capillaries, or with a silica-based glass rod/rods doped with germanium, fluorine, or the like. Thereafter, by drawing the glass preform, the multi-core HFs  1   a  to  1   g  is able to be manufactured. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be explained below. A multi-core optical fiber according to the second embodiment is of a type having a refractive-index difference between core portions and a cladding portion and light is confined to the core portions by this refractive-index difference. 
       FIG. 9  is a schematic cross-sectional view of a multi-core optical fiber according to the second embodiment. As illustrated in  FIG. 9 , a multi-core optical fiber  2  includes core portions  211  to  217  which are disposed separated from each other and a cladding portion  22  around the core portions  211  to  217 . The core portion  211  is arranged at an approximate center portion of the cladding portion  22 , and the core portions  212  to  217  are arranged at respective apexes of an equilateral hexagon around the core portion  211 . Each separated distance between the core portions  212  to  217  and the core diameters of the core portions  211  to  217  are not particularly limited. Each separated distance may be, for example, approximately 60 μm and the core diameter may be about 5.0 μm to 10.0 μm. Each of the core portions  211  to  217  is made of silica-based glass doped with germanium, and the cladding portion  12  is made of pure silica glass. As a result, the cladding portion  22  has a refractive index lower than a refractive index of each of the core portions  211  to  217 , and a relative refractive-index difference of each of the core portions  211  to  217  with respect to the cladding portion  22  is approximately 0.3% to 1.5%. The multi-core optical fiber  2  confines the light to each of the core portions  211  to  217  by this refractive-index difference to propagate the light. 
     Moreover, in the multi-core optical fiber  2 , the cladding portion  22  includes a marker M 9  formed on an outer side of the core portion  212  and the core portion  217 . The marker M 9  has three holes formed into an approximate isosceles triangle, similarly to the marker M 3  in the multi-core HF  1   c , so as to specify a direction from the core portion  217  to the core portion  212 . As a result, also in the multi-core optical fiber  2 , by the marker M 9 , a specific direction around the periphery of the cladding portion  22  is identifiable and the position of each of the core portions  212  to  217  is more infallibly identifiable, facilitating the connection even further. 
     This multi-core optical fiber  2 , similarly to the multi-core HFs  1   a  to  1   g , may be manufactured using a well-known drill method, sol-gel method, or stack-and-draw method. When the stack-and-draw method is used, in the manufacturing method, the pure silica-based glass capillaries are replaced with solid glass rods, and the glass rods to become core portions are made of silica-based glass doped with germanium of a predetermined amount. 
     The multi-core HFs  1   a  to  1   f  according to the first embodiment or the modifications thereof include the markers M 1  to M 7 , which are holes, in addition to the holes  13  for confining the light to the core portions  111  to  117 , but part of the holes  13  may serve also as the marker. In this case, the hole  13  to be the marker shall be a hole separated from the core portions  111  to  117  as much as possible so as not to affect confinement of the light to the core portions  111  to  117 , and a radius of this hole or a disposed position of the hole may be changed so that the hole is identifiable from the other holes  13 . 
     In the multi-core HFs  1   a  to  1   f  according to the first embodiment or the modifications thereof, or in the multi-core optical fiber  2  according to the second embodiment, the markers, which are holes, may be replaced with a marker made of a material having a refractive index different from that of the cladding portions. 
     Each of the first or the second embodiments, or the modifications thereof is just an example, and thus, the number and arrangement of the core portions, the holes for confining the light to the core portions, and the markers are not particularly limited. Furthermore, the material of the multi-core optical fiber is not particularly limited. Hereinafter, further modifications and embodiments will be explained. 
     (Seventh Modification) 
       FIG. 10  is a schematic cross-sectional view of a multi-core HF  1   h  according to a seventh modification. As illustrated in  FIG. 10 , the multi-core HF  1   h  has a configuration in which the number of holes  13 , in the multi-core HF  1   a  illustrated in  FIG. 1 , between the core portion  111  and each of the core portions  112  to  117  is increased from 4 to 9 and the marker M 1  is replaced with a marker M 10 . In the multi-core HF  1   h , each separated distance between the core portions  111  to  117  is longer than that in the multi-core HF  1   a  or the like, and thus, degradation by crosstalk of optical signals transmitted through the core portions  111  to  117  is suppressed, and a specific direction around the periphery of the cladding portion  12  is identifiable by the marker M 10 , facilitating the connection further. 
     Third Embodiment 
       FIG. 11  is a schematic cross-sectional view of a multi-core HF  3  according to a third embodiment of the present invention. As illustrated in  FIG. 11 , the multi-core HF  3  includes two core portions  311  and  312  and a cladding portion  32  provided around the core portions  311  and  312 . The core portions  311  and  312  and the cladding portion  32  are made of silica-based glass such as pure silica glass. 
     The cladding portion  32  includes a plurality of holes  33  arranged around the core portions  311  and  312 . The holes  33  are arranged, similarly to the holes  13  in the multi-core HF  1   a  or the like, to form a triangular lattice, and form an equilateral-hexagonal layer so as to surround each of the core portions  311  and  312 . Moreover, each of the core portions  311  and  312  is surrounded by holes  33  of at least five layers. Although diameters of the holes  33  and a lattice constant of the triangular lattice are not particularly limited, they may be the same as those of, for example, the multi-core HF  1   a . Furthermore, the cladding portion  32  includes a marker M 11 , which is a hole located outside an area in which the holes  33  are formed and at a position closer to the core portion  312  than to the core portion  311 . In the multi-core HF  3  also, a specific direction around the periphery of the cladding portion  32  is identifiable by the marker M 11 , and thus the connection is further facilitated. 
     Fourth Embodiment 
       FIG. 12  is a schematic cross-sectional view of a multi-core HF  4  according to a fourth embodiment of the present invention. As illustrated in  FIG. 12 , the multi-core HF  4  includes three core portions  411  to  413  and a cladding portion  42  around the core portions  411  to  413 . The core portions  411  to  413  and the cladding portion  42  are made of silica-based glass such as pure silica glass. 
     The cladding portion  42  includes a plurality of holes  43 , similar to the holes  13  in the multi-core HF  1   a  or the like, which are arranged to form a triangular lattice around the core portions  411  to  413 . Furthermore, the cladding portion  42  includes markers M 12  and M 13  arranged to face different sides of a polygonal area in which the holes  43  are formed. In the multi-core HF  4  also, a specific direction around the periphery of the cladding portion  42  is identifiable by the markers M 12  and M 13 , and thus the connection is further facilitated. 
     Fifth Embodiment 
       FIG. 13  is a schematic cross-sectional view of a multi-core HF  5  according to a fifth embodiment of the present invention. As illustrated in  FIG. 13 , the multi-core HF  5  includes four core portions  511  to  514  and a cladding portion  52  positioned around core portions  511  to  514 . The core portions  511  to  514  and the cladding portion  52  are made of silica-based glass such as pure silica glass. 
     The cladding portion  52  includes a plurality of holes  53 , similar to the holes  13  in the multi-core HF  1   a  or the like, which are arranged to form a triangular lattice around the core portions  511  to  514 . Furthermore, the cladding portion  52  includes a marker M 14  having three holes formed into a triangle, which is arranged outside an area in which the holes  53  are formed and on an outer side of the core portion  511 . In the multi-core HF  5  also, a specific direction around the periphery of the cladding portion  52  is identifiable by the marker M 14 , and thus the connection is further facilitated. 
     The present invention may be applied to a multi-core optical fiber configured as illustrated in  FIG. 9 , in which core portions are arranged similarly to the multi-core HFs illustrated in  FIGS. 11 to 13 , to arrange a marker as appropriate. 
     First and Second Examples 
     As first and second examples of the present invention, multi-core optical fibers were manufactured, which confine light to core portions by a refractive-index difference between the core portions and a cladding portion, by using the stack-and-draw method, similarly to the multi-core optical fiber according to the second embodiment. 
     Specifically, first, in a pure silica-based glass tube, core portions having a diameter of 7 mm and which have been doped with germanium so that a relative refractive-index difference with respect to pure silica becomes 0.3%, and solid silica-based glass rods forming a cladding portion about four times each core portion around the core portions, are placed in an equilateral hexagonal shape and at the center of this equilateral hexagon, and a space between the glass tube and the glass rods is filled with a large number of solid pure silica-based glass rods, to produce a glass preform. The diameter of the glass preform (that is, the diameter of the pure silica-based glass tube) was 24 mm in the first example and 36 mm in the second example. 
     When the glass rods were filled in, glass rods at positions where markers were to be formed were replaced with hollow pure silica-based glass capillaries. The positions where the markers were to be formed, similarly to the multi-core optical fiber according to the fifth modification illustrated in  FIG. 7 , faced two different sides of the equilateral hexagon formed by the core portions, and one or two holes were arranged at each position. Thereafter, the glass preform was drawn to manufacture the multi-core optical fibers according to the first and the second examples. 
       FIG. 14  is a cross-sectional photograph of the multi-core optical fiber according to the first example.  FIG. 15  is a cross-sectional photograph of the multi-core optical fiber according to the second example. In  FIGS. 14 and 15 , circular areas surrounded by dotted lines represent positions of the core portions. Solid circles represent the markers. Furthermore, in both the first and the second examples, the core diameter is approximately 10 μm, and a separated distance between the cores is approximately 40 μm. An outer diameter of the multi-core optical fiber is 141 μm in the first example, and 215 μm in the second example. 
     As illustrated in  FIGS. 14 and 15 , in the multi-core optical fibers according to the first and the second examples, a specific direction around the periphery of the cladding portion is identifiable and the position of each core portion is more infallibly identifiable, further facilitating the connection. Moreover, like the multi-core optical fibers according to the first and the second examples, even if the number of cores, the diameter of the cores, and the separated distance between the cores are the same, by changing the diameter of the glass tube, multi-core optical fibers of different diameters are easily obtainable. 
     According to an embodiment of the present invention, it is possible to easily identify a position of a specific core portion, and thus there is an effect that it is possible to realize a multi-core optical fiber that is easily connected. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.