Patent Publication Number: US-11644632-B2

Title: Method for manufacturing optical device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority from Japanese Patent Application No. 2017-126964 filed on Jun. 29, 2017, the contents of which are incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a method for manufacturing an optical device 
     BACKGROUND ART 
     In recent years, research and development of a multicore fiber (MCF), which is an optical fiber having a structure including a plurality of cores and a common cladding covering the periphery of the plurality of cores, has been actively conducted. Multicore fibers are attracting attention as transmission media capable of increasing the transmission capacity of an optical communication system because they can transmit individual information to each core. 
     In addition, a multicore fiber may be used as an optical fiber sensor (for example, an optical fiber sensor of an FBG type). An optical fiber sensor measures various physical quantities (for example, stress, strain, temperature, and the like) using an optical fiber (including a multicore fiber) as a sensor. In the above-mentioned FBG-type optical fiber sensor, a Fiber Bragg Grating (FBG) is formed in the core of an optical fiber (including a multicore fiber). By using the reflection characteristics that the FBG changes according to the surrounding environment, distributions of various physical quantities in the length direction of the optical fiber are measured. 
     A fan-in/fan-out device is connected to such a multicore fiber as an input/output device for connecting each core and an external optical fiber. The fan-in/fan-out device is manufactured such that a plurality of holes are formed in a glass preform (capillary) where a single mode fiber is inserted into a hole, a single mode fiber is inserted in each of the plurality of holes formed in the glass preform, and an elongated portion (portion elongated while the diameter is reduced) is formed by fusing and elongating a portion of the glass preform in which the single mode fiber is inserted while heating. Each of the above-described single mode fiber is an optical fiber, a plurality of which is provided in the fan-in/fan-out device, and an external optical fiber is connected to one end of the single mode fiber and one core of a multicore fiber is connected to the other end of the same. 
     Patent Document 1 described below discloses a shape sensing technique for measuring a shape of the optical fiber sensor (a shape of an object where the optical fiber sensor is attached) by an Optical Frequency Domain Reflectometry (OFDR) method using an FBG type optical fiber sensor using a multicore fiber. In addition, Patent Documents 2 to 4 described below disclose examples of fan-in/fan-out devices that connect each core of a multicore fiber to an external optical fiber. 
     PATENT DOCUMENTS 
     
         
         [Patent Document 1] U.S. Pat. No. 7,781,724 
         [Patent Document 2] Japanese Patent No. 5782104 
         [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2015-1673 
         [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2015-152774 
       
    
     An optical device in which a fan-in/fan-out device is connected (fused) to a multicore fiber may have a small light loss. However, in such an optical device, the occurrence of light loss is inevitable due to the following causes. 
     (1) Manufacturing Error of Fan-In/Fan-Out Device 
     For example, the positional deviation of the hole formed in the glass preform, variation in inter-core distance in an elongating portion (due to the positional deviation in the hole of single-core fiber generated at the time of melting and elongating), and misalignment of the core in the elongated portion (caused by variation in a diameter reduction ratio) 
     (2) Coupling Loss Between Multicore Fiber and Fan-In/Fan-Out Device 
     For example, there is a loss due to a mismatch between the core diameter of the elongated portion and the core diameter of the multicore fiber fused to the elongated portion. Such a mismatch essentially occurs in the manufacture of a fan-in/fan-out device having an elongated portion, and occurs due to the variation of the diameter reduction ratio of the elongated portion when melting and elongating a glass preform in which a single-core fiber is inserted. 
     SUMMARY 
     One or more embodiments of the present invention provide a method for manufacturing an optical device capable of manufacturing an optical device having a small optical loss due to a manufacturing error. 
     One or more embodiments of the present invention provide a method for manufacturing an optical device including, a multicore fiber comprising a plurality of cores, and a fan-in/fan-out device including a plurality of single-core fibers arranged so as to be connectable to the cores respectively based on a plurality of combinations when the multicore fiber is rotated, the method comprising: a first step of determining an optical loss for each core while changing a combination of the single-core fibers of the fan-in/fan-out device to be connected to the cores of the multicore fiber (i.e., a connection combination); and a second step of selecting one combination of the single-core fibers of the fan-in/fan-out device to be connected to the cores of the multicore fiber according to the result of the first step, and connecting an end portion of the multicore fiber and an end portion of the fan-in/fan-out device so that the single-core fibers and the core of the multicore fibers of the selected combination are connected. 
     According to one or more embodiments of the present invention, in the first step, the optical loss is determined by measuring an intensity of an optical signal for each core input from either one of the multicore fiber or the fan-in/fan-out device and output from another one of the multicore fiber or the fan-in/fan-out device in a state where the end portion of the multicore fiber and the end portion of the fan-in/fan-out device are in proximity. 
     According to one or more embodiments of the present invention, in the first step, the optical loss is determined by determining a difference between a distance of the cores of the multicore fiber and a distance of the single core fibers of the fan-in/fan-out device from respective images obtained by capturing the end portion of the multicore fiber and the end portion of the fan-in/fan-out device. 
     According to one or more embodiments of the present invention, in the first step, the optical loss is determined by determining a difference between a core diameter of the multicore fiber and a core diameter of the single-core fibers of the fan-in/fan-out device from respective images obtained by capturing the end portion of the multicore fiber and the end portion of the fan-in/fan-out device. 
     According to one or more embodiments of the present invention, in the second step, the optical loss is determined by determining a difference between a mode field diameter of the multicore fiber and a mode field diameter of the single-core fibers of the fan-in/fan-out device obtained by a method of measuring optical characteristics at the end portion of the multicore fiber and the end portion of the fan-in/fan-out device. 
     According to one or more embodiments of the present invention, a distance between the single core fibers of the fan-in/fan-out device is different for each combination of the single-core fibers of the fan-in/fan-out device that are connected to the core of the multicore fiber. 
     According to one or more embodiments of the present invention, a core diameter of the single core fiber of the fan-in/fan-out device is different for each combination of the single-core fibers of the fan-in/fan-out device that are connected to the core of the multicore fiber. 
     According to one or more embodiments of the present invention, it is possible to manufacture an optical device having a small light loss due to a manufacturing error. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view showing an optical device according to one or more embodiments of the present invention. 
         FIG.  2    is a cross-sectional view showing one example of a multicore fiber according to one or more embodiments of the present invention. 
         FIG.  3    is a perspective view showing a fan-in/fan-out device according to one or more embodiments of the present invention. 
         FIG.  4    is a cross-sectional view showing one example of a single-core fiber according to one or more embodiments of the present invention. 
         FIG.  5    is a cross-sectional view showing one example of an arrangement of a single-core fiber according to one or more embodiments of the present invention. 
         FIG.  6    is a flowchart showing one example of a manufacturing method of the optical device according to one or more embodiments of the present invention. 
         FIG.  7 A  is a cross-sectional view showing one example of a multicore fiber and a fan-in/fan-out device according to one or more embodiments of the present invention. 
         FIG.  7 B  is a cross-sectional view showing one example of a multicore fiber and a fan-in/fan-out device according to one or more embodiments of the present invention. 
         FIG.  8 A  is a cross-sectional view showing one example of a multicore fiber and a fan-in/fan-out device according to one or more embodiments of the present invention. 
         FIG.  8 B  is a cross-sectional view showing one example of a multicore fiber and a fan-in/fan-out device according to one or more embodiments of the present invention. 
         FIG.  9 A  is a cross-sectional view showing another example of a multicore fiber and a fan-in/fan-out device. 
         FIG.  9 B  is a cross-sectional view showing another example of a multicore fiber and a fan-in/fan-out device. 
         FIG.  10 A  is a cross-sectional view showing another example of a multicore fiber and a fan-in/fan-out device. 
         FIG.  10 B  is a cross-sectional view showing another example of a multicore fiber and a fan-in/fan-out device. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a method for manufacturing an optical device according to one or more embodiments of the present invention will be described in detail with reference to the drawings. In the drawings referred to in the following, in order to facilitate understanding, the scale of dimensions of each member is appropriately changed and illustrated as necessary. 
     (Configuration of Optical Device) 
       FIG.  1    is a schematic view showing an optical device according to one or more embodiments of the present invention. As shown in  FIG.  1   , the optical device  1  according to one or more embodiments includes a multicore fiber  10 , a fan-in/fan-out device  20 , and a plurality of signal transmission fibers  30 . Such an optical device  1  makes an optical signal propagated through the signal transmission fiber  30  enter the multicore fiber  10  via the fan-in/fan-out device  20  or makes an optical signal propagated through the multicore fiber  10  enter the signal transmission fiber  30  through the fan-in/fan-out device  20 . The optical device  1  shown in  FIG.  1    is applicable to an optical fiber sensor using, for example, a multicore fiber  10  as a sensor. 
       FIG.  2    is a cross-sectional view showing one example of the multicore fiber according to one or more embodiments of the present invention. As shown in  FIG.  2   , the multicore fiber  10  has a plurality of cores  11  and a common cladding  12  covering the periphery of the cores  11 . The multicore fiber  10  illustrated in  FIG.  2    includes a total of four cores  11  where one core  11  is disposed at the center of the multicore fiber  10  and three cores  11  are disposed concentrically around the center core  11 . Such a multicore fiber  10  can propagate individual optical signals to the respective cores  11 . 
       FIG.  3    is a perspective view showing a fan-in/fan-out device according to one or more embodiments of the present invention. As shown in  FIGS.  1  and  3   , the fan-in/fan-out device  20  includes a plurality of single-core fibers  21  and a capillary  22 , and relay-connects the multicore fiber  10  and a plurality of signal transmission fibers  30 . That is, the fan-in/fan-out device  20  causes optical signals propagated through the plurality of signal transmission fibers  30  to be incident on the plurality of cores  11  of the multicore fiber  10  and causes optical signals propagated through the plurality of cores  11  of the multicore fiber  10  to be incident on the plurality of signal transmission fibers  30 , respectively. 
       FIG.  4    is a cross-sectional view showing an example of a single-core fiber according to one or more embodiments of the present invention. As shown in  FIG.  4   , the single-core fiber  21  includes a core C and a cladding CL covering the periphery of the core C. The cladding CL has a lower refractive index than the core C. The number of the single-core fibers  21  is greater than the number of cores  11  of the multicore fibers  10 , and can be connected to the cores  11  based on a plurality of combinations when the multicore fibers  10  are rotated. With such an arrangement, it is possible to realize an optical device  1  with a small optical loss due to a manufacturing error. 
       FIG.  5    is a cross-sectional view showing an example of the arrangement of single-core fibers according to one or more embodiments of the present invention. In the fan-in/fan-out device  20  illustrated in  FIG.  5   , one single-core fiber  21  ( 21   a ) is disposed at the center of the fan-in/fan-out device  20 , and six single-core fibers  21  ( 21   b ,  21   c ) are arranged concentrically around the single-core fiber  21   a . In the example shown in  FIG.  5   , the single-core fibers  21   b  and the single-core fibers  21   c  are alternately arranged along the circumferential direction. 
     In the example shown in  FIG.  5   , when the multicore fiber  10  is rotated, combinations capable of being connected to the plurality of cores  11  are i) a combination of a single-core fiber  21   a  and three single-core fibers  21   b , and ii) a combination of a single-core fiber  21   a  and three single-core fibers  21   c . In the combination i), the splice is possible when, for example, the core  11  of the multicore fiber  10  is in a state shown in  FIG.  2   , and in the combination ii), the splice is possible when, for example, the core  11  of the multicore fiber  10  is rotated by 180° from a state shown in  FIG.  2   . The combination i) and the combination ii) can be referred to as rotationally symmetric (or substantially rotationally symmetric in consideration of manufacturing errors and the like). 
     In addition, it is also possible to arrange the single-core fibers  21  so that connection with the core  11  becomes possible in a plurality of combinations when the multicore fiber  10  is moved in parallel in the radial direction. However, in the case of such an arrangement, the multicore fiber  10  and the fan-in/fan-out device  20  may be connected in a state of being eccentric in the radial direction (state of axial deviation), and the mechanical strength may be reduced. On the other hand, according to one or more embodiments, the multicore fiber  10  and the fan-in/fan-out device  20  are connected without causing the above-mentioned axial deviation by arranging in rotational symmetry (or substantially rotational symmetry) and the mechanical strength can be increased. For this reason, the arrangement of the single-core fibers  21  may be a rotationally symmetrical (or substantially rotationally symmetrical) arrangement. 
     As shown in  FIGS.  1  and  3   , the single-core fiber  21  has a large diameter portion P 1  which is one end side portion, and an elongated portion P 2  which is the other end side portion extending in the length direction from the large diameter portion P 1 . The large diameter portion P 1  is a portion which is not extended, and an outer diameter thereof is constant in the length direction. Among the single-core fibers  21 , the end portion E 1  of the large diameter portion P 1  of the single-core fiber  21  to be connected to the core  11  of the multicore fiber  10  is connected to the end portion E 10  of the signal transmission fiber  30  at the connection portion C 1 . The large diameter portion P 1  may be fusion-spliced to the signal transmission fiber  30 . In the example shown in  FIG.  1   , the end portion E 1  of the large diameter portion P 1  of each of the four single-core fibers  21  is respectively connected to the end portions E 10  of each of the four signal transmission fibers  30 . 
     The end portion E 1  of the large diameter portion P 1  of the remaining single-core fibers  21  (the single-core fibers  21  which are not connected to the cores  11  of the multicore fiber  10 ) is not connected to the end portions E 10  of the signal transmission fiber  30 . Such a large diameter portion P 1  which is not connected to the end portion E 10  of the signal transmission fiber  30  may be cut so as to shorten its length (or not to extend from the capillary  22 ) as necessary. 
     The elongated portion P 2  includes a diameter-reduced portion P 3  extending from the large diameter portion P 1  while reducing the diameter in the length direction, and a small diameter portion P 4  extending from the diameter-reduced portion P 3 . At the diameter-reduced portion P 3 , the core diameter of the single-core fiber  21  gradually decreases along the extending direction. The ratio (minimum outer diameter/maximum outer diameter) of the minimum outer diameter to the maximum outer diameter of the single-core fiber  21  at the diameter-reduced portion P 3  may fall into a range of, for example, 1/1.5 to 1/2.5. The maximum outer diameter of the diameter-reduced portion P 3  may be the same as the outer diameter of the large diameter portion P 1 . 
     The small diameter portion P 4  is a portion of a constant diameter which is formed to extend in the same direction from the end portion E 2  in the extension direction of the diameter-reduced portion P 3 . The end portion E 3  in the extension direction of the small diameter portion P 4  is connected to the core  11  (see  FIG.  2   ) of the multicore fiber  10  at the connection point C 2 . The small diameter portion P 4  may be fusion-spliced to the multicore fiber  10 . According to one or more embodiments, four single-core fibers  21  in the small diameter portion P 4  of the seven single-core fibers  21  are connected to the cores  11  of the multicore fiber  10 , respectively. The outer diameter of the small diameter portion P 4  may be the same as the minimum outer diameter of the diameter-reduced portion P 3  and may be the same as the outer diameter of the multicore fiber  10 . 
     The single-core fiber  21  of the fan-in/fan-out device according to one or more embodiments includes a structure including a large diameter portion P 1  and an elongated portion P 2 , and the elongated portion P 2  includes a diameter-reduced portion P 3  and a small diameter portion P 4 . However, the structure can be, one or both of the large diameter portion P 1  and the small diameter portion P 4  may be omitted. When there is no small diameter portion P 4  the elongated portion P 2  is formed of only the diameter-reduced portion P 3 . 
     As shown in  FIG.  3   , the capillary  22  holds a plurality of single-core fibers  21  in a substantially bundled state, and is formed of glass or the like. The capillary  22  is provided in a portion of or all of the single-core fiber  21  in the length direction. The capillary  22  includes a base  22   a  and an extension portion  22   b  extending from the base  22   a . The extension portion  22   b  includes a tapered portion  22   c  extending from the base portion  22   a  while reducing the diameter in the length direction, and a tip portion  22   d  extending from the tapered portion  22   c . The base  22   a  can be formed on a portion of the large diameter portion P 1  of the single-core fiber  21 . The tapered portion  22   c  and the tip portion  22   d  are formed at positions corresponding to the diameter-reduced portion P 3  and the diameter-reduced portion P 4  of the single-core fiber  21 , respectively. 
     The signal transmission fibers  30  are optical fibers respectively connected to the plurality of cores  11  of the multicore fiber  10  via the fan-in/fan-out device  20 . The signal transmission fiber  30  transmits an optical signal to be incident on the plurality of cores  11  of the multicore fiber  10  or transmits an optical signal propagated through the plurality of cores  11  of the multicore fiber  10 . 
     (Method for Manufacturing Optical Device) 
       FIG.  6    is a flowchart showing an example of a method for manufacturing an optical device according to one or more embodiments of the present invention. As shown in  FIG.  6   , when the manufacture of the optical device  1  is started, the process of manufacturing the fan-in/fan-out device  20  having the plurality of single-core fibers  21  described with reference to  FIGS.  1  and  3    (Step S 11 ). 
     In Step S 11 , the process that the single-core fiber  21  is inserted in each of a plurality of holes of the cylindrical capillary  22  having the plurality of holes in which the single-core fiber  21  is inserted, and the process of melt-drawing the capillary  22  while partially heating the capillary  22  where the plurality of single-core fibers  21  is inserted therein to form the elongated portion P 2  (extension portion  22   b ) are sequentially performed. The holes of the capillary  22  are formed such that the single-core fibers  21  to be inserted are arranged as shown in  FIG.  5   . Thus, the fan-in/fan-out device  20  shown in  FIG.  3    is manufactured. 
     Next, while changing the combination of the single-core fibers  21  to be connected to each of the plurality of cores  11  of the multicore fiber  10 , a step of obtaining the coupling loss for each core  11  is performed (Step S 12 : first step). In Step S 12 , first, an end portion of the multicore fiber  10  and an end portion (end portion E 3 ) of the elongated portion P 2  of the fan-in/fan-out device  20  are respectively captured, and one combination of single-core fibers  21  to be connected to the plurality of cores  11  of the multicore fiber  10  is set based on the obtained image. For example, a combination of a single-core fiber  21   a  and three single-core fibers  21   b  shown in  FIG.  5    is set. 
     When the setting described above is completed, the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the extension portion  22   b  of the fan-in/fan-out device  20  is brought into a state of being in proximity so that the core of the single-core fiber  21  in the set combination faces the core  11  of the multicore fiber  10 . Then, when an optical signal is individually incident from the other end portion of the single-core fiber  21  (single-core fiber  21  disposed so that one end portion faces the core  11  of the multicore fiber  10 ), the light intensity of the light signal emitted from the opposed core  11  is separately measured. 
     Subsequently, another combination of the single-core fibers  21  to be connected to the plurality of cores  11  of the multicore fiber  10  is set. For example, a combination of a single-core fiber  21   a  and three single-core fibers  21   c  shown in  FIG.  5    is set. When the setting is completed, for example, the multicore fiber  10 , which is brought into a state of being in proximity to the end portion (end portion E 3 ) of the elongated portion P 2  of the fan-in/fan-out device  20 , is rotated such that the core of the fiber  21  of the set combination faces the core  11  of the multicore fiber  10 . Then, when the optical signal is again separately incident from the other end of the single-core fiber  21  (single-core fiber  21  disposed so that one end faces the core  11  of the multicore fiber  10 ), the light intensity of the optical signal emitted from the facing core  11  is separately measured. 
     Based on the above, the light intensity emitted from each core  11  of the multicore fiber  10  is measured when the combination of the single-core fiber  21  to be connected to the plurality of cores  11  of the multicore fiber  10  is changed. The measured light intensity reflects the coupling loss between the core  11  of the multicore fiber  10  and the single-core fiber  21 . For this reason, the coupling loss for each core  11  is determined when the combination of the single-core fibers  21  to be connected to the plurality of cores  11  of the multicore fiber  10  is changed. 
     Next, a combination of single-core fibers  21  to be connected to each core  11  of the multicore fiber  10  is selected according to the determined coupling loss (Step S 13 : second step). For example, the measurement results for each combination of single-core fibers  21  are compared, and a combination of single-core fibers  21  with high intensity of the optical signal emitted from each core  11  and small variation is selected. Here, it is assumed that a combination of the single-core fiber  21   a  and the three single-core fibers  21   b  shown in  FIG.  5    is selected. 
     Next, the process of connecting the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the elongated portion P 2  of the fan-in/fan-out device  20  is performed so that the single-core fiber  21  and the core  11  of the multicore fiber  10  of the selected combination are connected (Step S 14 : second step). For example, the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the elongated portion P 2  of the fan-in/fan-out device  20  are fusion-spliced such that the combination of the single-core fiber  21   a  and the three single-core fibers  21   b  shown in  FIG.  5    and the core  11  of the multicore fiber  10  are connected. 
     When the process described above is completed, the process is performed such that the end portion E 10  of the signal transmission fiber  30  is connected to the end portion E 1  of the single-core fiber  21  connected to the core  11  of the multicore fiber  10  (Step S 15 ). The end portion E 10  of the signal transmission fiber  30  may be connected to the end portion E 1  of all the single-core fibers  21  after the fan-in/fan-out device  20  is manufactured in Step S 11 . In such a case, Step S 15  is omitted. 
     According to one or more embodiments described above, in Step S 12 , optical signals are individually incident from the other end of the single-core fiber  21  (the single-core fiber  21  disposed so that one end faces the core  11  of the multicore fiber  10 ). Occasionally, the light intensity of the light signal emitted from the opposing core  11  is measured individually. However, conversely to this, optical signals may be made incident from a side of the multicore fiber  10 , and optical signals emitted from the other end of the single-core fiber  21  may be individually measured. 
     As described above, according to one or more embodiments, a fan-in/fan-out device  20  is manufactured that has a plurality of single-core fibers  21  arranged so as to enable splicing with the core  11  in a plurality of combinations when the multicore fiber  10  is rotated, and the coupling loss for each core  11  is determined while changing the combination of the single-core fibers  21  to be connected to the core  11  of the multicore fiber  10 . Then, according to the result, one combination of the single-core fibers  21  is selected, and the end portion of the multicore fiber  10  and the end portion E 3  of the elongated portion P 2  of the fan-in/fan-out device  20  is connected so that the single-core fiber  21  and the core  11  of the multicore fiber  10  of the selected combination are connected. 
     Thus, according to one or more embodiments, among the combinations of the plurality of single-core fibers  21  that can be connected to the core  11  of the multicore fiber  10 , the combination of the single-core fibers  21  with small coupling loss is selected to connect to the core  11  of the multicore fiber  10 . Therefore, the optical device  1  can be manufactured with a small optical loss due to the manufacturing error. 
     According to one or more embodiments described above, the example of selecting the combination of the single-core fibers  21  in which the coupling loss decreases is described. However, the combination of the single-core fibers  21  in which the variation of the coupling loss decreases may be selected. By selecting such a combination, variations in intensity among the cores can be reduced and optical signals with uniform intensity can be obtained which is advantageous from the point of view of a Signal to Noise (S/N) ratio. 
       FIGS.  7 A and  7 B  are cross-sectional views showing an example of a multicore fiber and a fan-in/fan-out device according to one or more embodiments of the present invention.  FIG.  7 A  is a cross-sectional view of the multicore fiber  10  at the connection point C 2 , and  FIG.  7 B  is a cross-sectional view of the fan-in/fan-out device  20  at the connection point C 2 . 
     The optical device according to one or more embodiments is manufactured basically through the steps shown in the flow chart of  FIG.  6    as in the optical device of the above-described embodiments. However, according to one or more embodiments discussed below, the configuration of the fan-in/fan-out device  20  manufactured in Step S 11  in  FIG.  6    and the method of obtaining the coupling loss in Step S 12  in  FIG.  6    are different from those according to the above-described embodiments. In particular, in the fan-in/fan-out device  20  manufactured according to one or more embodiments, the distance between the single-core fibers  21  is different for each combination of single-core fibers  21  to be connected to the core  11  of the multicore fiber  10 . Furthermore, according to one or more embodiments, without making an optical signal incident on the multicore fiber  10  and the fan-in/fan-out device  20 , the coupling loss is estimated from an image obtained by capturing the end portion of the multicore fiber  10  and the fan-in/fan-out device  20 . 
     In the fan-in/fan-out device  20  according to one or more embodiments, the combination that can be connected to the plurality of cores  11  when the multicore fiber  10  is rotated is the same as that of the above-described embodiments. That is, i) a combination of a single-core fiber  21   a  and three single-core fibers  21   b , and ii) a combination of a single-core fiber  21   a  and three single-core fibers  21   c.    
     As shown in  FIG.  7 B , the inter-core distance (inter-core distance between the single-core fiber  21   a  and the three single-core fibers  21   b ) of the combination i) is set to r1, and the inter-core distance (inter-core distance between the core fiber  21   a  and the three single-core fibers  21   c ) of the combination ii) is set to r2. As shown in  FIG.  7 A , the inter-core distance (distance between the cores  11 ) of the multicore fiber  10  is set to r3. 
     When obtaining the coupling loss of each core  11  of the multicore fiber  10  shown in  FIG.  7 A  and the fan-in/fan-out device  20  shown in  FIG.  7 B , first, the end portion of the multicore fiber  10  and the end portion E 2  of the fan-in/fan-out device  20  are captured. Next, the difference between the distance between the cores  11  of the multicore fiber  10  and the distance between the single-core fibers  21  of the fan-in/fan-out device  20  is determined from the respective images obtained by capturing. Then, the coupling loss for each core  11  is estimated based on the difference. In the case of step index type fiber, for example, the following equation (1) can be used to estimate the coupling loss. 
     
       
         
           
             
               
                 
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                   ) 
                 
               
             
           
         
       
     
     In the equation (1) described above, D is the core diameter, and c is the positional displacement amount between the cores to be connected. 
     When the coupling loss for each core  11  is determined, for example, steps S 13  to S 15  in  FIG.  6    are sequentially performed. For example, when the relationship |r3−r1|≤|r3−r2| is satisfied, the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the fan-in/fan-out device  20  are fusion-spliced so that the combination of the single-core fiber  21   a  and the three single-core fibers  21   b  shown in  FIG.  7 B  and the core  11  of the multicore fiber  10  are connected. On the other hand, when the relationship of |r3−r1|&gt;|r3−r2| is satisfied, the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the fan-in/fan-out device  20  are fusion-spliced so that the combination of the single-core fiber  21   a  and the three single-core fibers  21   c  shown in  FIG.  7 B  and the core  11  of the multicore fiber  10  are connected. 
     According to one or more embodiments, based on an image obtained by capturing the end portion of the multicore fiber  10  and the fan-in/fan-out device  20  without making the optical signal incident on the multicore fiber  10  and the fan-in/fan-out device  20 , the coupling loss is determined. However, as in the above-described embodiments, an optical signal may be made incident on the multicore fiber  10  and the fan-in/fan-out device  20  to determine the coupling loss. 
     Further, in the embodiments described above, the inter-core distance indicates a distance between the core (core  11 , single-core fiber  21   a ) disposed at the center of the multicore fiber  10  or the fan-in/fan-out device  20  and the other cores (core  11 , single-core  21   b  or single-core fiber  21   c ). If there is no centrally placed core, the inter-core distance may be a distance from the center of the multicore fiber  10  or the fan-in/fan-out device  20 , or a distance between two adjacent cores. 
     As described above, in one or more embodiments, among a plurality of combinations of the single-core fibers  21  that can be connected to the core  11  of the multicore fiber  10 , a combination of the single-core fibers  21  where the coupling loss or the variation of the coupling loss is small can be selected and connected to the core  11  of the multicore fiber  10 . For this reason, it is possible to manufacture an optical device with a small optical loss due to a manufacturing error. 
       FIGS.  8 A and  8 B  are cross-sectional views showing an example of a multicore fiber and a fan-in/fan-out device according to one or more embodiments of the present invention.  FIG.  8 A  is a cross-sectional view of the multicore fiber  10  at the connection point C 2 , and  FIG.  8 B  is a cross-sectional view of the fan-in/fan-out device  20  at the connection point C 2 . 
     According to one or more embodiments discussed below, the configuration of the fan-in/fan-out device  20  manufactured in Step S 11  in  FIG.  6    and the method of obtaining the coupling loss in Step S 12  in  FIG.  6    are different from those in the embodiments described above. In particular, in the fan-in/fan-out device  20  manufactured according to one or more embodiments, the diameter of the single-core fiber  21  is different for each combination of the single-core fibers  21  to be connected to the core  11  of the multicore fiber  10 . Furthermore, according to one or more embodiments, without making an optical signal incident on the multicore fiber  10  and the fan-in/fan-out device  20 , the coupling loss is estimated from an image obtained by capturing the end portions of the multicore fiber  10  and the fan-in/fan-out device  20 . The loss due to the difference in the core diameter can be estimated, for example, by using the proportion to the square of the core diameter ratio when light is transmitted from the larger core diameter to the smaller core diameter. 
     Also in the fan-in/fan-out device  20  according to one or more embodiments, the combinations that can be connected to the plurality of cores  11  when the multicore fiber  10  is rotated are the same as that of the above-described embodiments. That is, the combinations are: i) a combination of single-core fiber  21   a  and three single-core fibers  21   b , and ii) a combination of single-core fiber  21   a  and three single-core fibers  21   c.    
     As shown in  FIG.  8 B , the core diameter (core diameter of the three single-core fibers  21   b ) in the combination i) is set to a1, and the core diameter (core diameter of the three single-core fibers  21   c ) in the combination ii) is set to a2. Here, the diameter of the single-core fiber  21   a  common to the combinations i) and ii) can be arbitrarily set, but is set to, for example, (a1+a2)/2. As shown in  FIG.  8 A , the core diameter a3 of the core  11  of the multicore fiber  10  is set. 
     When obtaining the coupling loss for each core  11  of the multicore fiber  10  shown in  FIG.  8 A  and the fan-in/fan-out device  20  shown in  FIG.  8 B , according to one or more embodiments, first, the end portion of the multicore fiber  10  and the end portion E 2  of the fan-in/fan-out device  20  are captured. Next, the difference between the core diameter of the core  11  of the multicore fiber  10  and the core diameter of the single-core fiber  21  of the fan-in/fan-out device  20  is determined from each image obtained by capturing. Then, the coupling loss for each core  11  is estimated based on the difference. 
     When the coupling loss for each core  11  is determined, for example, steps S 13  to S 15  in  FIG.  6    are sequentially performed. For example, when the relationship |a3−a1|≤|a3−a2| is satisfied, the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the fan-in/fan-out device  20  are fusion-spliced so that the combination of the single-core fiber  21   a  and the three single-core fibers  21   b  shown in  FIG.  8 B  and the core  11  of the multicore fiber  10  are connected. On the other hand, when the relationship of |a3−a1|&gt;|a3−a2| is satisfied, the end portion of the multicore fiber  10  and the end portion (end portion E 3 ) of the fan-in/fan-out device  20  are fusion-spliced so that the combination of the single-core fiber  21   a  and the three single-core fibers  21   c  shown in  FIG.  8 B  and the core  11  of the multicore fiber  10  are connected. 
     According to one or more embodiments described above, the loss is estimated using the core diameter; however, a mode field diameter may be used instead. The mode field diameter can be measured by an optical characteristic measurement method (for example, a far-field scanning method, a near-field scanning method, or the like) at the end portion E 2  of the fan-in/fan-out device  20 . At this time, the coupling loss L can be expressed by the following equation (2) in consideration of the positional displacement amount by the image observation. 
                   (     Equation   ⁢         2     )                                     L   =       -   10     ⁢   log   ⁢   T       ⁢   
     T   =         (       2   ⁢     w   1     ⁢     w   2           w   1   2     +     w   2   2         )     2     ·     exp   ⁡   (     -       2   ⁢     σ   2           w   1   2     +     w   2   2           )                 (   2   )               
In the equation (2), w1 and w2 are the respective mode field diameters (corresponding to the above-mentioned core diameters a1 and a2), and σ is the positional displacement amount between the cores.
 
     According to one or more embodiments, based on an image obtained by capturing the end portion of the multicore fiber  10  and the fan-in/fan-out device  20  without making the optical signal incident on the multicore fiber  10  and the fan-in/fan-out device  20 , the coupling loss is determined. However, according to one or more embodiments, an optical signal may be made incident on the multicore fiber  10  and the fan-in/fan-out device  20  to determine the coupling loss. 
     As described above, in one or more embodiments, among a plurality of combinations of the single-core fibers  21  that can be connected to the core  11  of the multicore fiber  10 , a combination of the single-core fibers  21  where the coupling loss or the variation of the coupling loss is small can be selected and connected to the core  11  of the multicore fiber  10 . For this reason, it is possible to manufacture an optical device with a small optical loss due to a manufacturing error. 
     As described above, although embodiments of the present invention are described, the present invention can be changed freely within the scope of the present invention, without being limited to the above-mentioned embodiments. 
     For example, the embodiments described above may be combined appropriately. For example, the method of determining the coupling loss described above may be applied to any embodiments. In addition, for each combination of single-core fibers  21  connected to the core  11  of the multicore fiber  10 , the distance between the single-core fibers  21  may be different, and also the core diameter and the mode field diameter may be different. 
     According to the embodiments described above, the multicore fiber  10  having four cores  11  has been described as an example; however, the multicore fiber  10  is not limited to having four cores  11 , and a multicore fiber  10  having any number of cores  11  can be used.  FIGS.  9 A to  10 B  are cross-sectional views showing another examples of a multicore fiber and a fan-in/fan-out device.  FIGS.  9 A,  9 B, and  10 A  are cross-sectional views at the connection point C 2  of the multicore fiber  10 , and  FIG.  10 B  is a cross-sectional view at the connection point C 2  of the fan-in/fan-out device  20 . 
     The multicore fiber  10  shown in  FIG.  9 A  has two cores  11  arranged concentrically around the center of the multicore fiber  10 . For such a multicore fiber  10 , for example, a fan-in/fan-out device  20  shown in  FIG.  9 B  is used. In the fan-in/fan-out device  20 , four single-core fibers  21  ( 21   b ,  21   c ) are arranged concentrically around the center of the fan-in/fan-out device  20 . In the example shown in  FIG.  9 B , the single-core fibers  21   b  and the single-core fibers  21   c  are alternately arranged along the circumferential direction. 
     In the example shown in  FIG.  9 A  and  FIG.  9 B , when the multicore fiber  10  is rotated, combinations that can be connected to the plurality of cores  11  are: iii) a combination of two single-core fibers  21   b , and iv) a combination of two single-core fibers  21   c . The combination iii) is possible, for example, in a case of the core  11  of the multicore fiber  10  shown in  FIG.  9 A , and the combination iv) is possible, for example, when the core  11  of the multicore fiber  10  shown in  FIG.  9 A  is rotated by 90°. 
     The multicore fiber  10  shown in  FIG.  10 A  has three cores  11  arranged concentrically around the center of the multicore fiber  10 . For such a multicore fiber  10 , for example, a fan-in/fan-out device  20  shown in  FIG.  10 B  is used. In the fan-in/fan-out device  20 , six single-core fibers  21  ( 21   b ,  21   c ) are arranged concentrically around the center of the fan-in/fan-out device  20 . In the example shown in  FIG.  10 B , the single-core fibers  21   b  and the single-core fibers  21   c  are alternately arranged along the circumferential direction. 
     In the example shown in  FIGS.  10 A and  10 B , the combinations that can be connected to the plurality of cores  11  when the multicore fiber  10  is rotated are: v) a combination of three single-core fibers  21   b , and vi) a combination of three single-core fibers  21   c . The combination v) is possible, for example, in a case of the core  11  of the multicore fiber  10  shown in  FIG.  10 A , and the combination vi) is possible, for example, when the core  11  of the multicore fiber  10  shown in  FIG.  10 A  is rotated by 180°. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 
     REFERENCE SIGNS LIST 
     
         
           1 : Optical device 
           10 : Multicore fiber 
           11 : Core 
           20 : Fan-in/fan-out device 
           21 : Single-core fiber 
         E 3 : End portion