Multi-core optical fiber, optical device, and method for manufacturing multi-core optical fiber

Abstract

Provided is a multi-core optical fiber in which the position of a mark can be easily measured. In the end surface (sigma1) of the multi-core optical fiber (MF), the plurality of cores (a1-an) are arranged in line symmetry with respect to an imaginary axis (L1) orthogonal to the inclination direction (v1) of the end surface (sigma1). The end surface (sigma1) is virtually divided into two regions, i.e., a first region and a second region by a virtual axis (L1), the core farthest from the virtual axis (L1) among the cores (a1, a4) provided in the first region is defined as a first core, the core farthest from the virtual axis (L1) among the cores (a2, a3) provided in the second region is defined as a second core, and in the end surface (sigma1), the end surface (sigma1) is defined as a second core. The center of the mark (c) is included in a region between a straight line that passes through the first core or the mode field of the first core and is parallel to the imaginary axis (L1) and a straight line that passes through the second core or the mode field of the second core and is parallel to the imaginary axis (L1).

Description

Technical Field

[0001] The present invention relates to a multi-core optical fiber or an optical device including the multi-core optical fiber. Background Art

[0002] In the field of optical communication, multi-core optical fibers having a plurality of cores are widely used. Patent Document 1 is an example of a document disclosing a multi-core optical fiber.

[0003] Patent Document 1: Japanese Patent Application Publication No. 2019-152866

[0004] In order to reduce reflection at the end face of an optical fiber, a process of tilting the end face in a manner not orthogonal to the extension direction of the core is widely performed. However, when the end face of a multi-core optical fiber in which a mark for identifying the core is formed in the cladding is tilted, for example, the following problems may occur.

[0005] That is, in order to evaluate the quality of a multi-core optical fiber or to align the rotation of a multi-core optical fiber, the positions of multiple cores and marks of the multi-core optical fiber are sometimes measured. This measurement is performed, for example, by observing the end face of the multi-core optical fiber using a microscope. At this time, if the end face is tilted, it may be difficult to focus on multiple cores and marks at the same time. This is because: the optical axis of the objective lens of the measuring optical system is sometimes configured to be consistent with the central axis of the multi-core optical fiber. In this case, the lens surface of the objective lens is not directly opposite to the end face of the multi-core optical fiber. As a result, it may be difficult to measure the position of the mark. Summary of the invention

[0006] One aspect of the present invention has been made in view of the above-mentioned problems, and an object of the present invention is to realize a multi-core optical fiber capable of easily measuring the position of a mark, an optical device including such a multi-core optical fiber, or a method for manufacturing such a multi-core optical fiber.

[0007] In a multi-core optical fiber involved in one embodiment of the present invention, the following structure is adopted, namely: it has: a cladding; a plurality of cores formed in the above-mentioned cladding; at least one mark formed in the above-mentioned cladding; and an end face, which is inclined in a manner not orthogonal to the extension direction of the above-mentioned plurality of cores, and in the above-mentioned end face, the above-mentioned plurality of cores are arranged in a line symmetrical state with respect to an imaginary axis orthogonal to the inclination direction of the above-mentioned end face. When the above-mentioned end face is virtually divided into two regions, a first region and a second region, the core farthest from the above-mentioned imaginary axis among the cores arranged in the above-mentioned first region among the above-mentioned plurality of cores is set as the first core, and the core farthest from the above-mentioned imaginary axis among the cores arranged in the above-mentioned second region among the above-mentioned plurality of cores is set as the second core, and in the above-mentioned end face, the center of the above-mentioned mark is included in the region between a straight line passing through the above-mentioned first core or the mode field of the above-mentioned first core and parallel to the above-mentioned imaginary axis and a straight line passing through the above-mentioned second core or the mode field of the above-mentioned second core and parallel to the above-mentioned imaginary axis.

[0008] According to one embodiment of the present invention, it is possible to realize a multi-core optical fiber capable of easily measuring the position of a mark, an optical device including such a multi-core optical fiber, or a method for manufacturing such a multi-core optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a diagram showing the structure of a multi-core optical fiber according to the first embodiment of the present invention. Figure 1 (a) is a side view of the multi-core optical fiber. Figure 1 (b) is a front view of an end face of one side of the multi-core optical fiber, Figure 1 (c) is a front view of the other end face of the multi-core optical fiber. Figure 1 (d) is a stereoscopic image of the multi-core optical fiber.

[0010] Figure 2 (a) to (c) represent Figure 1 FIG. 1 is a diagram of a first modified example of a multi-core optical fiber. Figure 2 (a) is a side view of the multi-core optical fiber. Figure 2 (b) is a front view of an end face of one side of the multi-core optical fiber, Figure 2 (c) is a front view of the other end face of the multi-core optical fiber. Figure 2 (d) to (f) represent Figure 1 FIG. 2 is a diagram of a second modified example of a multi-core optical fiber. Figure 2 (d) is a side view of the multi-core optical fiber. Figure 2 (e) is a front view of an end face of one side of the multi-core optical fiber, Figure 2 (f) is a front view of the other end face of the multi-core optical fiber.

[0011] Figure 3 It is a diagram showing the structure of a multi-core optical fiber according to a second embodiment of the present invention. Figure 3 (a) is a side view of the multi-core optical fiber. Figure 3 (b) is a front view of an end face of one side of the multi-core optical fiber, Figure 3 (c) is a front view of the other end face of the multi-core optical fiber. Figure 3 (d) is a stereoscopic image of the multi-core optical fiber.

[0012] Figure 4 Yes means Figure 3 FIG. 2 is a diagram showing a modified example of the multi-core optical fiber shown. Figure 4 (a) is a side view of the multi-core optical fiber. Figure 4 (b) is a front view of an end face of one side of the multi-core optical fiber, Figure 4 (c) is a front view of the other end face of the multi-core optical fiber. Figure 4(d) is a stereoscopic image of the multi-core optical fiber.

[0013] Figure 5 It is a diagram showing the structure of an optical device according to a third embodiment of the present invention. Figure 5 (a) is a side view of the optical device. Figure 5 (b) is a front view of an end face of one side of the optical device, Figure 5 (c) is a front view of the other end face of the optical device.

[0014] Figure 6 It is a diagram showing the structure of an optical device according to a fourth embodiment of the present invention. Figure 6 (a) is a side view of the optical device. Figure 6 (b) is a front view of an end face of one side of the optical device, Figure 6 (c) is a front view of the other end face of the optical device.

[0015] Figure 7 Yes means Figure 6 A diagram showing a modified example of the optical device shown. Figure 7 (a) is a front view of one end surface of the optical device involved in the first modification example, Figure 7 (b) is a front view of one end surface of the optical device involved in the second modification.

[0016] Figure 8 It is a diagram showing the structure of a multi-core optical fiber according to a fifth embodiment of the present invention. Figure 8 (a) is a side view of the multi-core optical fiber. Figure 8 (b) is a stereogram of the multi-core optical fiber. Figure 8 (c) is a side view showing a modified example of the multi-core optical fiber.

[0017] Fig. 9 It is a diagram showing the structure of a multi-core optical fiber according to a sixth embodiment of the present invention. Fig. 9 (a) is a side view of the multi-core optical fiber. Fig. 9 (b) is a stereogram of the multi-core optical fiber. Fig. 9 (c) is a side view showing a modified example of the multi-core optical fiber. DETAILED DESCRIPTION

[0018] In order to evaluate the quality of a multi-core optical fiber or to align the rotation of a multi-core optical fiber, the positions of multiple cores and marks of a multi-core optical fiber are sometimes measured. This measurement is performed, for example, by observing the end face of the multi-core optical fiber using a microscope. At this time, if the end face is tilted, it is difficult to focus on multiple cores and marks at the same time. This is because: the optical axis of the objective lens of the measuring optical system is usually configured to be consistent with the central axis of the multi-core optical fiber, so the lens surface of the objective lens is not directly opposite to the end face of the multi-core optical fiber, and as a result, it is difficult to accommodate multiple cores and marks within the depth of field at the same time. In addition, as long as the orientation of the measuring optical system or the multi-core optical fiber is adjusted in a manner that the lens surface of the objective lens is directly opposite to the end face of the multi-core optical fiber, such problems can be avoided. However, in this way, a mechanism for adjusting the orientation of the measuring optical system or the multi-core optical fiber is required, resulting in other problems of large-scale and complicated observation devices. In addition, as long as each of the multiple cores and marks is focused in sequence, such problems can be avoided. However, in this way, other problems such as the extension of observation time are caused.

[0019] The above-mentioned difficulty in observation is particularly significant for the marker. This is because the marker is smaller than the core, or has a different refractive index from the core, or is formed outside the core, and the defocus effect is greater than that of the core. The embodiment described below is proposed in view of such problems, and its purpose is to realize a multi-core optical fiber that is easy to observe the marker.

[0020] [First embodiment]

[0021] (Structure of multi-core optical fiber)

[0022] Reference Figure 1 The structure of the multi-core optical fiber MF according to the first embodiment of the present invention will be described. Figure 1 (a) is a side view of the multi-core optical fiber MF. Figure 1 (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core optical fiber MF as viewed from the line of sight E1. Figure 1 (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core optical fiber MF as viewed from the line of sight E2. Figure 1 (d) is a perspective view of the multi-core optical fiber MF in a state where the first end face σ1 and the second end face σ2 are butted against each other.

[0023] The multi-core optical fiber MF includes n cores a1 to an and a cladding b. Here, n is an arbitrary natural number greater than 2 (in Figure 1, the case of n=4 is illustrated). The cladding b is a cylindrical component. The cladding b is made of, for example, quartz glass. Each core ai (i is a natural number greater than 1 and less than n) is a cylindrical region arranged inside the cladding b, having a higher refractive index than the cladding b and extending in the same direction as the cladding b. Each core ai is made of, for example, quartz glass to which a rising dopant such as germanium is added. In addition, the cladding b only needs to be columnar, and its cross-sectional shape is arbitrary. The cross-sectional shape of the cladding b may also be, for example, a polygonal shape such as a quadrilateral or hexagon. In addition, the multi-core optical fiber MF may also have cores other than the n cores a1 to an of interest in this embodiment. For example, as a core other than the n cores a1 to an of interest, a core arranged in the center of the cladding b may also be provided. The n cores a1 to an of interest are, for example, cores used in communications, in which case, preferably cores that meet the standards determined by ITUT. Cores other than the n cores a1 to an of interest may be cores used for communication or cores not used for communication (dummy cores). In the latter case, they may be cores that do not satisfy the standards determined by ITUT.

[0024] The multi-core optical fiber MF also has a mark c for identifying the core numbers 1 to n of the cores a1 to an. The mark c is a columnar region arranged inside the cladding b, having a refractive index different from that of the cladding b and extending in the same direction as the cladding b. The cross-sectional shape of the mark c is arbitrary, for example, a circular shape, a triangular shape, a quadrilateral shape, etc. The mark c is composed of, for example, quartz glass to which a reducing dopant such as fluorine or boron is added. In this case, the refractive index of the mark c is lower than the refractive index of the cladding b. Alternatively, the mark c is composed of quartz glass to which a rising dopant such as germanium, aluminum, phosphorus, or chlorine is added. In this case, the refractive index of the mark c is higher than the refractive index of the cladding b. The mark c can be formed, for example, using a hole opening method, a stacking & stretching method, etc. The outer diameter of the mark c is usually smaller than the outer diameter of the core ai. In addition, the mark c can also be a hole. In this case, the refractive index of the mark c is lower than the refractive index of the cladding b. In addition, the multi-core optical fiber MF can also have marks other than the mark c that is concerned in this embodiment.

[0025] The core numbers 1 to n of the cores a1 to an can be identified based on the distance from the mark c. For example, when the cores a1 to an are arranged on a circle, the core numbers 1 to n of the cores a1 to an can be identified as follows. First, the core number of the core a1 closest to the mark c is set to "1". Next, the core number of the core a2 that is the second closest to the mark c is set to "2". Moreover, when the above-mentioned circle is drawn in a manner passing through the cores a1 and a2 in sequence, the core number of the third core a3 passing through is set to "3", the core number of the fourth core a4 passing through is set to "4",..., and the core number of the nth core an passing through is set to "n".

[0026] In the multi-core optical fiber MF, the first end face σ1 is tilted in a manner that is not orthogonal to the extension direction of the cores a1 to an. Hereinafter, when the plane orthogonal to the extension direction of the cores a1 to an is regarded as a horizontal plane, the direction in which the downward slope of the first end face σ1 is the largest is recorded as the tilt direction v1 of the first end face σ1, and the maximum value of the downward slope of the first end face σ1 is recorded as the tilt angle θ1 of the first end face σ1. The tilt angle θ1 of the first end face σ1 is preferably greater than 2° and less than 88°, more preferably greater than 4° and less than 12°, more preferably greater than 7° and less than 9°, and more preferably greater than 7.8° and less than 8.2°. The tilt angle θ1 of the first end face σ1 is, for example, 8° or 6°.

[0027] In the multi-core optical fiber MF involved in the present embodiment, in the first end face σ1, the cores a1~an are arranged in a linear symmetric state (basically linear symmetric) with respect to the imaginary axis L1 that is orthogonal to the tilt direction v1. Here, the cores a1~an are arranged in a linear symmetric state with respect to the imaginary axis L1 means that when the first end face σ1 is flipped relative to the imaginary axis L1, each of the cores a1~an at least partially overlaps with any of the cores a1~an. In addition, according to the description "in the first end face σ1" at the beginning of this paragraph, the imaginary axis L1 is a straight line within the first end face σ1. In addition, in the present embodiment, the imaginary axis L1 passes through the center of the first end face σ1 (the center of the cladding). However, the imaginary axis L1 only needs to be orthogonal to the tilt direction v1 and does not need to pass through the center of the first end face σ1.

[0028] Therefore, for two multi-core optical fibers MF, when the first end face σ1 of one multi-core optical fiber MF is connected to the first end face σ1 of the other multi-core optical fiber MF in a manner such that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the cores a1~an of the two multi-core optical fibers MF can be optically coupled.

[0029] In addition, in the first end face σ1, it is further preferred that the cores a1 to an are arranged in line symmetry (complete line symmetry) with respect to the imaginary axis L1 orthogonal to the tilt direction v1. Here, the cores a1 to an are arranged in line symmetry with respect to the imaginary axis L1 means that when the first end face σ1 is flipped with respect to the imaginary axis L1, each of the cores a1 to an overlaps with any of the cores a1 to an exactly.

[0030] Therefore, for two multi-core optical fibers MF, when the first end face σ1 of one multi-core optical fiber MF is connected with the first end face σ1 of the other multi-core optical fiber MF in a manner such that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the coupling efficiency of the cores a1~an of the two multi-core optical fibers MF can be further improved.

[0031] In the multi-core optical fiber MF according to the present embodiment, the above-mentioned virtual axis L1 does not intersect any of the cores a1 to an at the first end face σ1.

[0032] Therefore, for two multi-core optical fibers MF, when the first end face σ1 of one multi-core optical fiber MF is connected to the first end face σ1 of the other multi-core optical fiber MF in such a way that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the cores with different core numbers are connected to each other. Figure 1 In the example shown, the core a1 of one multi-core optical fiber MF is connected to the core a2 of the other multi-core optical fiber MF, and the core a3 of one multi-core optical fiber MF is connected to the core a4 of the other multi-core optical fiber MF.

[0033] The first point that should be particularly mentioned in the multi-core optical fiber MF involved in the present embodiment is that in the first end face σ1, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged as points sandwiching the imaginary axis L1. As a result, the mark c is arranged at a position close to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easy to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L1 or near the imaginary axis L1 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be suppressed to a small value.

[0034] Especially in Figure 1In the multi-core optical fiber MF shown, in the first end face σ1, the mark c is arranged in an area sandwiched by a straight line P passing through the center of the core a1 closest to the mark c and parallel to the imaginary axis L1, and a straight line Q passing through the center of the core a2 second closest to the mark c and parallel to the imaginary axis L1. Thus, the mark c is arranged at a position closer to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easier to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on or near the imaginary axis L1 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be further suppressed to a small value.

[0035] In addition, in the multi-core optical fiber MF, the second end face σ2 is tilted in a manner that is not orthogonal to the extension direction of the cores a1 to an. Hereinafter, when the plane orthogonal to the extension direction of the cores a1 to an is regarded as a horizontal plane, the direction in which the downward slope of the second end face σ2 is the largest is recorded as the tilt direction v2 of the second end face σ2, and the maximum value of the downward slope of the second end face σ2 is recorded as the tilt angle θ2 of the second end face σ2. The tilt angle θ2 of the second end face σ2 is preferably greater than 2° and less than 88°, more preferably greater than 4° and less than 12°, more preferably greater than 7° and less than 9°, and more preferably greater than 7.8° and less than 8.2°. The tilt angle θ2 of the second end face σ2 is, for example, 8° or 6°.

[0036] In the multi-core optical fiber MF involved in the present embodiment, in the second end face σ2, the cores a1~an are arranged in a linear symmetric state (basically linear symmetric) relative to the imaginary axis L2 that is orthogonal to the tilt direction v2. Here, the cores a1~an are arranged in a linear symmetric state relative to the imaginary axis L2 means that when the second end face σ2 is flipped relative to the imaginary axis L2, each of the cores a1~an overlaps at least partially with any of the cores a1~an. In addition, according to the description "in the second end face σ2" at the beginning of this paragraph, the imaginary axis L2 is a straight line within the second end face σ2. In addition, in the present embodiment, the imaginary axis L2 passes through the center of the second end face σ2 (the center of the cladding). However, the imaginary axis L2 only needs to be orthogonal to the tilt direction v2 and does not need to pass through the center of the second end face σ2.

[0037] Therefore, for two multi-core optical fibers MF, when the second end face σ2 of one multi-core optical fiber MF is connected with the second end face σ2 of the other multi-core optical fiber MF in a manner that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the cores a1~an of the two multi-core optical fibers MF can be optically coupled.

[0038] In addition, in the second end face σ2, it is further preferred that the cores a1 to an are arranged in line symmetry (complete line symmetry) with respect to the imaginary axis L2 orthogonal to the tilt direction v2. Here, the cores a1 to an are arranged in line symmetry with respect to the imaginary axis L2 means that when the second end face σ2 is flipped with respect to the imaginary axis L2, each of the cores a1 to an overlaps with any of the cores a1 to an exactly.

[0039] Therefore, for two multi-core optical fibers MF, when the second end face σ2 of one multi-core optical fiber MF is connected with the second end face σ2 of the other multi-core optical fiber MF in a manner that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the coupling efficiency of the cores a1~an of the two multi-core optical fibers MF can be further improved.

[0040] In the multi-core optical fiber MF according to the present embodiment, the above-mentioned virtual axis L2 does not intersect any of the cores a1 to an at the second end face σ2.

[0041] Therefore, for two multi-core optical fibers MF, when the second end face σ2 of one multi-core optical fiber MF is connected to the second end face σ2 of the other multi-core optical fiber MF in such a way that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the cores with different core numbers are connected to each other. Figure 1 In the example shown, the core a1 of one multi-core optical fiber MF is connected to the core a2 of the other multi-core optical fiber MF, and the core a3 of one multi-core optical fiber MF is connected to the core a4 of the other multi-core optical fiber MF.

[0042] The second point that should be particularly mentioned in the multi-core optical fiber MF involved in this embodiment is that in the second end face σ2, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged as points sandwiching the imaginary axis L2. As a result, the mark c is arranged at a position close to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easy to observe the mark c together with the cores a1~an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L2 or near the imaginary axis L2 in order to reduce the difference in the degree of defocusing of the cores a1~an in the tilt direction, the defocusing of the mark c can be suppressed to a small value.

[0043] Especially in Figure 1In the multi-core optical fiber MF shown, in the second end face σ2, the mark c is arranged in an area sandwiched by a straight line R passing through the center of the core a1 closest to the mark c and parallel to the imaginary axis L2, and a straight line S passing through the center of the core a2 second closest to the mark c and parallel to the imaginary axis L2. Thus, the mark c is arranged at a position closer to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is further easy to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L2 or near the imaginary axis L2 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be further suppressed to a small value.

[0044] In the multi-core optical fiber MF according to the present embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so as to satisfy the following condition 1.

[0045] Condition 1: When the first end face σ1 and the second end face σ2 are in surface contact in a manner that minimizes the angle formed by the extension direction of the cores a1~an in the first end face σ1 and the extension direction of the cores a1~an in the second end face σ2, each of the cores a1~an in the first end face σ1 at least partially overlaps with any one of the cores a1~an in the second end face σ2.

[0046] In addition, the form in which the two cores at least partially overlap includes a form in which only a portion of the core of one side overlaps with only a portion of the core of the other side, a form in which only a portion of the core of one side overlaps with the entire core of the other side, and a form in which the entire core of one side overlaps with the entire core of the other side (i.e., a form in which the two cores overlap with each other neither more nor less).

[0047] Thus, for two multi-core optical fibers MF, when the first end face σ1 of the multi-core optical fiber MF on one side is connected with the second end face σ2 of the multi-core optical fiber MF on the other side in a manner that the two multi-core optical fibers MF are arranged in a straight line as much as possible, the cores a1 to an of the two multi-core optical fibers MF can be optically coupled. Here, connecting the first end face σ1 with the second end face σ2 "in a manner that the two multi-core optical fibers MF are arranged in a straight line as much as possible" means connecting the first end face σ1 with the second end face σ2 "in a manner that the angle formed by the extension direction of the cores a1 to an of the multi-core optical fiber MF on one side and the extension direction of the cores a1 to an of the other multi-core optical fiber MF is minimized."

[0048] In addition, the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 may be consistent or inconsistent. However, the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 are preferably equal (substantially consistent), and more preferably consistent (completely consistent). Here, the inclination angle θ1 of the first end face σ1 and the inclination angle θ2 of the second end face σ2 are equal (substantially consistent) to mean that, for example, the difference |θ1-θ2| is less than 2° or less than 0.4°.

[0049] When the inclination angle θ1 of the first end face σ1 is consistent with the inclination angle θ2 of the second end face σ2, the minimum value of the angle formed by the extension direction of the cores a1 to an in the first end face σ1 and the extension direction of the cores a1 to an in the second end face σ2 is 0°. That is, minimizing the angle formed by the extension direction of the cores a1 to an in the first end face σ1 and the extension direction of the cores a1 to an in the second end face σ2 is equivalent to aligning the extension direction of the cores a1 to an in the first end face σ1 with the extension direction of the cores a1 to an in the second end face σ2. Therefore, the above-mentioned condition 1 is equivalent to the following condition 1'.

[0050] Condition 1': When the first end face σ1 and the second end face σ2 are brought into surface contact in such a way that the extension direction of the cores a1~an in the first end face σ1 is consistent with the extension direction of the cores a1~an in the second end face σ2, each of the cores a1~an in the first end face σ1 at least partially overlaps with any one of the cores a1~an in the second end face σ2.

[0051] Thus, for two multi-core optical fibers MF, when the first end face σ1 of one multi-core optical fiber MF is connected to the second end face σ2 of the other multi-core optical fiber MF in a manner that the two multi-core optical fibers MF are arranged on a straight line, the cores a1 to an of the two multi-core optical fibers MF can be optically coupled. Here, connecting the first end face σ1 to the second end face σ2 "in a manner that the two multi-core optical fibers MF are arranged on a straight line" means connecting the first end face σ1 to the second end face σ2 "in a manner that the extension direction of the cores a1 to an of one multi-core optical fiber MF is consistent with the extension direction of the cores a1 to an of the other multi-core optical fiber MF." In addition, even if the inclination angle θ1 of the first end face σ1 is not strictly consistent with the inclination angle θ2 of the second end face σ2, as long as the inclination angle θ1 of the first end face σ1 is the same as the inclination angle θ2 of the second end face σ2 (as long as they are basically consistent), the same effect can be obtained.

[0052] In the multi-core optical fiber MF involved in this embodiment, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 are determined so that the projections onto the plane orthogonal to the optical axis L0 are in opposite directions to each other. Therefore, in addition to satisfying the above-mentioned condition 1, the multi-core optical fiber MF involved in this embodiment also satisfies the following condition 2.

[0053] Condition 2: When the first end face σ1 and the second end face σ2 are in surface contact in a manner that minimizes the angle between the extending directions of the cores a1 to an in the first end face σ1 and the extending directions of the cores a1 to an in the second end face σ2, the core numbers of the cores that at least partially overlap are identical.

[0054] exist Figure 1 In the example shown, the pairs of cores that at least partially overlap are (1) a pair of core a1 in the first end face σ1 and a pair of core a1 in the second end face σ2, (2) a pair of core a2 in the first end face σ1 and a pair of core a2 in the second end face σ2, (3) a pair of core a3 in the first end face σ1 and a pair of core a3 in the second end face σ2, and (4) a pair of core a4 in the first end face σ1 and a pair of core a4 in the second end face σ2. For any of the above four pairs, the core numbers of the two cores constituting the pair are the same.

[0055] Therefore, for two multi-core optical fibers MF, when the first end face σ1 of one multi-core optical fiber MF and the second end face σ2 of the other multi-core optical fiber MF are connected in a manner such that the two multi-core optical fibers MF are arranged on a straight line as much as possible, the cores with the same core numbers can be optically coupled to each other.

[0056] In addition, in the multi-core optical fiber MF, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 may not satisfy the above relationship. For example, the inclination direction v1 of the first end face σ1 and the inclination direction v2 of the second end face σ2 may be determined so that the projections onto the plane orthogonal to the optical axis L0 are in opposite directions to each other, or may be determined so that the projections onto the plane orthogonal to the optical axis L0 are orthogonal to each other. In this case, the multi-core optical fiber MF involved in this embodiment satisfies the following condition 3 instead of the above condition 2.

[0057] Condition 3: When the first end face σ1 and the second end face σ2 are in surface contact in a manner that minimizes the angle between the extending directions of the cores a1 to an in the first end face σ1 and the extending directions of the cores a1 to an in the second end face σ2, the core numbers of the cores that at least partially overlap are different.

[0058] In addition, in the multi-core optical fiber MF, only one of the first end face σ1 and the second end face σ2 may be tilted. For example, when the first end face σ1 is tilted as described above, the second end face σ2 may be tilted or not. Alternatively, when the second end face σ2 is tilted as described above, the first end face σ1 may be tilted or not.

[0059] (Variation of multi-core optical fiber)

[0060] Reference Figure 2 Two modified examples of the multi-core optical fiber MF will be described. Figure 2 (a) is a side view of the multi-core optical fiber MF involved in the first modification. Figure 2 (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core optical fiber MF involved in the first modification example as viewed from the line of sight E1. Figure 2 (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core optical fiber MF involved in the first modification example as viewed from the direction of the line of sight E2. Figure 2 (d) is a side view of the multi-core optical fiber MF involved in the second modification. Figure 2 (e) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core optical fiber MF involved in the second modification example as viewed from the direction of the line of sight E1. Figure 2 (f) is a front view of the other end face (hereinafter referred to as “second end face”) σ2 of the multi-core optical fiber MF according to the second modification example as viewed from the direction of the line of sight E2.

[0061] exist Figure 1 In the multi-core optical fiber MF shown in the figure, (1) in the first end face σ1, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axis L1, and (2) in the second end face σ2, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axis L2. Figure 2 In the multi-core optical fiber MF involved in the first variant example shown in (a) to (c), (1) in the first end face σ1, the core a1 closest to the mark c is arranged on the imaginary axis L1, and (2) in the second end face σ2, the core a1 closest to the mark c is arranged on the imaginary axis L2.

[0062] Thus, in the first end face σ1, the mark c is arranged at a position close to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, the mark c can be easily observed together with the cores a1 to an. In addition, in the second end face σ2, the mark c is arranged at a position close to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, the mark c can be easily observed together with the cores a1 to an.

[0063] exist Figure 1 In the multi-core optical fiber MF shown, (1) at the first end face σ1, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axis L1, and (2) at the second end face σ2, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axis L2. Figure 2 In the multi-core optical fiber MF involved in the second variant example shown in (d) to (f), (1) in the first end face σ1, the core a2 that is second closest to the mark c is arranged on the imaginary axis L1, and (2) in the second end face σ2, the core a2 that is second closest to the mark c is arranged on the imaginary axis L2.

[0064] Thus, in the first end face σ1, the mark c is arranged at a position close to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, the mark c can be easily observed together with the cores a1 to an. In addition, in the second end face σ2, the mark c is arranged at a position close to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, the mark c can be easily observed together with the cores a1 to an.

[0065] (Common Features)

[0066] As Figure 1 and Figure 2 The common feature of the multi-core optical fiber MF shown in each of them is that "at each of the first end face σ1 and the second end face σ2, the center of the mark c is included in the area between the imaginary axis L1 and a straight line passing through the center of the core farthest from the imaginary axis L1 among the cores a1 to an and parallel to the imaginary axis L1." Therefore, compared with the case where the mark c is not included in the above-mentioned area, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easy to observe the mark c together with the cores a1 to an. In addition, regarding the later-described Figure 3 and Figure 4 The multi-core optical fiber MF shown in each example also has this feature. This point will be described in a little more detail below.

[0067] First, about Figure 1 The first end face σ1 of the multi-core optical fiber MF shown in FIG. 1 can be described as follows. Consider dividing the first end face σ1 into a first region (in Figure 1 the area above the imaginary axis L1) and the second area (in Figure 1 The cores a1 and a4 disposed in the first region are the cores farthest from the imaginary axis L1, and the cores a2 and a3 disposed in the second region are the cores farthest from the imaginary axis L1. At this time, the mark c is disposed on the straight line (in the middle) passing through the cores a1 and a4 and parallel to the imaginary axis L1. Figure 1 and the straight line P in the figure) and the straight line passing through the cores a2 and a3 and parallel to the imaginary axis L1 (in the figure Figure 1 , which is consistent with the above-mentioned straight line Q). Therefore, it can be said that the mark c is formed in the area sandwiched between the straight line passing through the cores a1 and a4 farthest from the imaginary axis L1 among the cores a1 and a4 arranged in the first area and parallel to the imaginary axis L1, and the straight line passing through the cores a2 and a3 farthest from the imaginary axis L1 among the cores a2 and a3 arranged in the second area and parallel to the imaginary axis L1. In addition, for the straight line passing through the cores a1 and a4 and parallel to the imaginary axis L1, (1) it can be a straight line passing through the point closest to the imaginary axis L1 among the points included in the cores a1 and a4, (2) it can also be a straight line passing through the centers of the cores a1 and a4, and (3) it can also be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the cores a1 and a4. Similarly, for a straight line passing through the cores a2 and a3 and parallel to the imaginary axis L1, (1) it may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the cores a2 and a3, (2) it may be a straight line passing through the center of the cores a2 and a3, or (3) it may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the cores a2 and a3. Figure 1 The same can be said of the second end face σ2 of the multi-core optical fiber MF shown.

[0068] In addition, about Figure 2 The first end face σ1 of the multi-core optical fiber MF shown in (a) to (c) can be described as follows. Consider dividing the first end face σ1 into a first region (in Figure 2 the area above the imaginary axis L1) and the second area (in Figure 2The two regions are as follows (region below the imaginary axis L1 in the first region). The core farthest from the imaginary axis L1 among the cores a1, a2, and a3 arranged in the first region is core a2, and the core farthest from the imaginary axis L1 among the cores a3, a4, and a1 arranged in the second region is core a4. At this time, the mark c is arranged in the region sandwiched between the straight line passing through the core a2 and parallel to the imaginary axis L1 and the straight line passing through the core a4 and parallel to the imaginary axis L1. Therefore, it can be said that the mark c is formed in the region sandwiched between the straight line passing through the core a2 farthest from the imaginary axis L1 among the cores a1, a2, and a3 arranged in the first region and parallel to the imaginary axis L1 and the straight line passing through the core a4 farthest from the imaginary axis L1 among the cores a3, a4, and a1 arranged in the second region and parallel to the imaginary axis L1. In addition, for the straight line passing through the core a2 and parallel to the imaginary axis L1, (1) may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the core a2, (2) may be a straight line passing through the center of the core a2, and (3) may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the core a2. Similarly, for the straight line passing through the core a4 and parallel to the imaginary axis L1, (1) may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the core a4, (2) may be a straight line passing through the center of the core a4, and (3) may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the core a4. About Figure 2 The same can be said about the second end face σ2 of the multi-core optical fiber MF shown in (a) to (c).

[0069] In addition, about Figure 2 The first end face σ1 of the multi-core optical fiber MF shown in (d) to (f) can be described as follows. Consider dividing the first end face σ1 into the first region (in Figure 2 the area above the imaginary axis L1) and the second area (in Figure 2The two regions are as follows (region below the imaginary axis L1 in the fiber cores a4, a1, and a2 arranged in the first region). The core farthest from the imaginary axis L1 among the cores a4, a1, and a2 arranged in the first region is core a1, and the core farthest from the imaginary axis L1 among the cores a2, a3, and a4 arranged in the second region is core a3. At this time, the mark c is arranged in the region sandwiched between the straight line passing through the core a1 and parallel to the imaginary axis L1 and the straight line passing through the core a3 and parallel to the imaginary axis L1. Therefore, it can be said that the mark c is formed in the region sandwiched between the straight line passing through the core a1 farthest from the imaginary axis L1 among the cores a4, a1, and a2 arranged in the first region and parallel to the imaginary axis L1 and the straight line passing through the core a3 farthest from the imaginary axis L1 among the cores a2, a3, and a4 arranged in the second region and parallel to the imaginary axis L1. In addition, for the straight line passing through the core a1 and parallel to the imaginary axis L1, (1) may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the core a1, (2) may be a straight line passing through the center of the core a1, and (3) may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the core a1. Similarly, for the straight line passing through the core a3 and parallel to the imaginary axis L1, (1) may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the core a3, (2) may be a straight line passing through the center of the core a3, and (3) may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the core a3. About Figure 2 The same can be said about the second end face σ2 of the multi-core optical fiber MF shown in (d) to (f).

[0070] As described above, it can be said that the multi-fiber MF according to the present embodiment has the following features 1 and 2.

[0071] Feature 1: When the first end face σ1 is divided into two regions, a first region and a second region, by an imaginary axis L1, a mark c is formed in a region sandwiched between a straight line that passes through a core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and is parallel to the imaginary axis L1, and a straight line that passes through a core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and is parallel to the imaginary axis L1.

[0072] Feature 2: When the second end face σ2 is divided into two regions, a first region and a second region, by the imaginary axis L1, a mark c is formed in a region sandwiched between a straight line that passes through a core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and is parallel to the imaginary axis L1, and a straight line that passes through a core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and is parallel to the imaginary axis L1.

[0073] Furthermore, even if the above-mentioned features 1 and 2 are replaced by the following features 1' and 2' to slightly expand the range in which the mark c can be formed, the same effect as the case with the above-mentioned features 1 and 2 can be expected. In the following features 1' and 2', the mode field of the core refers to the area where 86.5% of the optical power is concentrated with respect to the intensity distribution of the light of the fundamental mode propagating in the core at the operating wavelength.

[0074] Feature 1': When the first end face σ1 is divided into two regions, a first region and a second region, by an imaginary axis L1, a mark c is formed in a region sandwiched between a straight line that passes through a mode field of a core that is farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and is parallel to the imaginary axis L1, and a straight line that passes through a mode field of a core that is farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and is parallel to the imaginary axis L1.

[0075] Feature 2: When the second end face σ2 is divided into two regions, a first region and a second region, by the imaginary axis L1, a mark c is formed in a region sandwiched between a straight line passing through the mode field of the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and parallel to the imaginary axis L1, and a straight line passing through the mode field of the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and parallel to the imaginary axis L1.

[0076] [Second embodiment]

[0077] (Structure of multi-core optical fiber)

[0078] Reference Figure 3 The structure of a multi-core optical fiber MF according to a second embodiment of the present invention will be described. Figure 3 (a) is a side view of the multi-core optical fiber MF. Figure 3 (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core optical fiber MF as viewed from the line of sight E1. Figure 3 (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core optical fiber MF as viewed from the line of sight E2. Figure 3 (d) is a perspective view of the multi-core optical fiber MF in a state where the first end face σ1 and the second end face σ2 are butted against each other.

[0079] The multi-core optical fiber MF according to the first embodiment includes four cores a1 to a4 , whereas the multi-core optical fiber MF according to the second embodiment includes eight cores a1 to a8 .

[0080] The first point that should be particularly mentioned in the multi-core optical fiber MF involved in the second embodiment is that, similar to the multi-core optical fiber MF involved in the first embodiment, in the first end face σ1, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged as points sandwiching the imaginary axis L1. Thus, the mark c is arranged at a position close to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easy to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L1 or near the imaginary axis L2 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be suppressed to a small value.

[0081] Especially in Figure 3 In the multi-core optical fiber MF shown, in the first end face σ1, the mark c is arranged in an area sandwiched by a straight line P passing through the center of the core a1 closest to the mark c and parallel to the imaginary axis L1, and a straight line Q passing through the center of the core a2 second closest to the mark c and parallel to the imaginary axis L1. Thus, the mark c is arranged at a position closer to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easier to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L1 or near the imaginary axis L2 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be further suppressed to a small value.

[0082] The second point that should be particularly mentioned in the multi-core optical fiber MF involved in the second embodiment is that, similar to the multi-core optical fiber MF involved in the first embodiment, in the second end face σ2, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged as points sandwiching the imaginary axis L2. Thus, the mark c is arranged at a position close to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is easy to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L2 or near the imaginary axis L2 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be suppressed to a small value.

[0083] Especially in Figure 3In the multi-core optical fiber MF shown, in the second end face σ2, the mark c is arranged in an area sandwiched by a straight line R passing through the center of the core a1 closest to the mark c and parallel to the imaginary axis L2, and a straight line S passing through the center of the core a2 second closest to the mark c and parallel to the imaginary axis L2. Thus, the mark c is arranged at a position closer to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, it is further easy to observe the mark c together with the cores a1 to an. This is because: in particular, when the focus of the objective lens is set on the imaginary axis L2 or near the imaginary axis L2 in order to reduce the difference in the degree of defocusing of the cores a1 to an in the tilt direction, the defocusing of the mark c can be further suppressed to a small value.

[0084] (Variation of multi-core optical fiber)

[0085] Reference Figure 4 Modifications of the multi-core optical fiber MF will be described. Figure 4 (a) is a side view of the multi-core optical fiber MF. Figure 4 (b) is a front view of one end face (hereinafter referred to as "first end face") σ1 of the multi-core optical fiber MF as viewed from the line of sight E1. Figure 4 (c) is a front view of the other end face (hereinafter referred to as "second end face") σ2 of the multi-core optical fiber MF as viewed from the line of sight E2. Figure 4 (d) is a perspective view of the multi-core optical fiber MF in a state where the first end face σ1 and the second end face σ2 are butted against each other.

[0086] exist Figure 3 In the multi-core optical fiber MF shown, (1) in the first end face σ1, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axis L1, and (2) in the second end face σ2, the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axis L2. In contrast, in the multi-core optical fiber MF involved in this modification, (1) in the first end face σ1, the core a1 closest to the mark c is arranged on the imaginary axis L1, and (2) in the second end face σ2, the core a1 closest to the mark c is arranged on the imaginary axis L2.

[0087] Thus, in the first end face σ1, the mark c is arranged at a position close to the imaginary axis L1. As a result, when the first end face σ1 of the multi-core optical fiber MF is observed from the front using a microscope or the like, the mark c can be easily observed together with the cores a1 to an. In addition, in the second end face σ2, the mark c is arranged at a position close to the imaginary axis L2. As a result, when the second end face σ2 of the multi-core optical fiber MF is observed from the front using a microscope or the like, the mark c can be easily observed together with the cores a1 to an.

[0088] (Common Features)

[0089] First, about Figure 3 The first end face σ1 of the multi-core optical fiber MF shown in (b) can be described as follows. Consider dividing the first end face σ1 into a first region (in Figure 3 the area above the imaginary axis L1) and the second area (in Figure 3 The two regions are the region below the imaginary axis L1 in the first region. The cores farthest from the imaginary axis L1 among the cores a6, a7, a8, and a1 arranged in the first region are cores a7 and a8, and the cores farthest from the imaginary axis L1 among the cores a2, a3, a4, and a5 arranged in the second region are cores a3 and a4. At this time, the center of the mark c is arranged in the region sandwiched between the straight line passing through the cores a7 and a8 and parallel to the imaginary axis L1 and the straight line passing through the cores a3 and a4 and parallel to the imaginary axis L1. Therefore, it can be said that the center of the mark c is formed in the region sandwiched between the straight line passing through the cores a7 and a8 farthest from the imaginary axis L1 among the cores a6, a7, a8, and a1 arranged in the first region and parallel to the imaginary axis L1 and the straight line passing through the cores a3 and a4 farthest from the imaginary axis L1 among the cores a2, a3, a4, and a5 arranged in the second region and parallel to the imaginary axis L1. In addition, for the straight line passing through the cores a7 and a8 and parallel to the imaginary axis L1, (1) can be a straight line passing through the point closest to the imaginary axis L1 among the points included in the cores a7 and a8, (2) can be a straight line passing through the centers of the cores a7 and a8, and (3) can be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the cores a7 and a8. Similarly, for the straight line passing through the cores a3 and a4 and parallel to the imaginary axis L1, (1) can be a straight line passing through the point closest to the imaginary axis L1 among the points included in the cores a3 and a4, (2) can be a straight line passing through the centers of the cores a3 and a4, and (3) can be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the cores a3 and a4. About Figure 3 The same can be said about the second end face σ2 of the multi-core optical fiber MF shown in (c).

[0090] In addition, about Figure 4 The first end face σ1 of the multi-core optical fiber MF shown in (c) can be said as follows. Consider dividing the first end face σ1 into a first region (in Figure 4 the area above the imaginary axis L1) and the second area (in Figure 4The core farthest from the imaginary axis L1 among the cores a1, a2, a3, a4, and a5 arranged in the first region is core a3, and the core farthest from the imaginary axis L1 among the cores a5, a6, a7, a8, and a1 arranged in the second region is core a7. At this time, the center of the mark c is set in the region sandwiched between the straight line passing through the core a3 and parallel to the imaginary axis L1 and the straight line passing through the core a7 and parallel to the imaginary axis L1. Therefore, it can be said that the center of the mark c is formed in the region sandwiched between the straight line passing through the core a3 farthest from the imaginary axis L1 among the cores a1, a2, a3, a4, and a5 arranged in the first region and parallel to the imaginary axis L1 and the straight line passing through the core a7 farthest from the imaginary axis L1 among the cores a5, a6, a7, a8, and a1 arranged in the second region and parallel to the imaginary axis L1. In addition, for the straight line passing through the core a3 and parallel to the imaginary axis L1, (1) may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the core a3, (2) may be a straight line passing through the center of the core a3, and (3) may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the core a3. Similarly, for the straight line passing through the core a7 and parallel to the imaginary axis L1, (1) may be a straight line passing through the point closest to the imaginary axis L1 among the points included in the core a7, (2) may be a straight line passing through the center of the core a7, and (3) may be a straight line passing through the point farthest from the imaginary axis L1 among the points included in the core a7. About Figure 4 The same can be said about the second end face σ2 of the multi-core optical fiber MF shown in (c).

[0091] As described above, it can be said that the multi-fiber MF according to the present embodiment has the following features 1 and 2, similarly to the multi-core optical fiber MF according to the first embodiment.

[0092] Feature 1: When the first end face σ1 is divided into two regions, a first region and a second region, by the imaginary axis L1, the center of the mark c is formed in a region sandwiched between a straight line that passes through the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and is parallel to the imaginary axis L1, and a straight line that passes through the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and is parallel to the imaginary axis L1.

[0093] Feature 2: When the second end face σ2 is divided into two regions, a first region and a second region, by the imaginary axis L1, the center of the mark c is formed in a region sandwiched between a straight line that passes through the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and is parallel to the imaginary axis L1, and a straight line that passes through the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and is parallel to the imaginary axis L1.

[0094] Furthermore, even if the above-mentioned features 1 and 2 are replaced by the following features 1' and 2' to slightly expand the range in which the mark c can be formed, the same effect as the case with the above-mentioned features 1 and 2 can be expected. In the following features 1' and 2', the mode field of the core refers to the area where 86.5% of the optical power is concentrated with respect to the intensity distribution of the light of the fundamental mode propagating in the core at the operating wavelength.

[0095] Feature 1': When the first end face σ1 is divided into two regions, a first region and a second region, by the imaginary axis L1, the center of the mark c is formed in a region sandwiched between a straight line that passes through the mode field of the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and is parallel to the imaginary axis L1, and a straight line that passes through the mode field of the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and is parallel to the imaginary axis L1.

[0096] Feature 2': When the second end face σ2 is divided into two regions, a first region and a second region, by the imaginary axis L1, the center of the mark c is formed in a region sandwiched between a straight line passing through the mode field of the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the first region and parallel to the imaginary axis L1, and a straight line passing through the mode field of the core farthest from the imaginary axis L1 among the cores a1 to an arranged in the second region and parallel to the imaginary axis L1.

[0097] [Third embodiment]

[0098] (Structure of optical device)

[0099] Reference Figure 5 The structure of the optical device OD1 according to the third embodiment of the present invention will be described. Figure 5 (a) is a side view of the optical device OD1. In addition, Figure 5 (b) is a front view of one end surface of the optical device OD1 observed from the direction of the line of sight E1. Figure 5 (c) is a front view of the other end surface of the optical device OD1 as viewed from the direction of the line of sight E2.

[0100] The optical device OD1 includes a multi-core optical fiber MF and single-core connectors C1 and C2 provided at both ends of the multi-core optical fiber MF.

[0101] Multi-core optical fiber MF can be Figure 1 to Figure 4 Any one of the multi-core optical fibers MF shown. Figure 5 In the figure, as a multi-core optical fiber MF, Figure 1 The multi-core optical fiber MF is shown.

[0102] The first single-core connector C1 is provided at one end of the multi-core optical fiber MF. The end face of the first single-core connector C1 is inclined in a manner coplanar with the first end face σ1 of the multi-core optical fiber MF. In addition, a key K1 is provided on the side face located at the front end of the inclination direction v1 of the first end face σ1 among the four side faces of the first single-core connector C1. The key K1 is, for example, a rectangular convex portion protruding from the side face of the first single-core connector C1.

[0103] The second single-core connector C2 is provided at the other end of the multi-core optical fiber MF. The end face of the second single-core connector C2 is inclined in a manner coplanar with the second end face σ2 of the multi-core optical fiber MF. In addition, a key K2 is provided on the side face located at the front end of the inclination direction v2 of the second end face σ2 among the four side faces of the second single-core connector C2. The key K2 is, for example, a rectangular convex portion protruding from the side face of the second single-core connector C2.

[0104] Therefore, for two optical devices OD1, when the first single-core connector C1 of one optical device OD1 and the second single-core connector C2 of the other optical device OD1 are connected in a manner in which the positions of key K1 and key K2 are reversed, the cores a1~an of the multi-core optical fiber MF included in the above two optical devices OD1 can be optically coupled.

[0105] In addition, here, the structure of providing single-core connectors at both ends of the multi-core optical fiber MF is described, but the present invention is not limited to this. That is, the structure of providing a single-core connector at one end of the multi-core optical fiber MF is also included in the scope of the present invention. Figure 5 The structure in which either the first single-core connector C1 or the second single-core connector C2 is omitted in the optical device OD1 shown is also included in the scope of the present invention. In this case, the end face of the multi-core optical fiber MF without the single-core connector may be inclined or not inclined.

[0106] [Fourth embodiment]

[0107] (Structure of optical device)

[0108] Reference Figure 6 The structure of the optical device OD2 according to the fourth embodiment of the present invention will be described. Figure 6(a) is a side view of the optical device OD2. In addition, Figure 6 (b) is a front view of one end face of the optical device OD2 observed from the direction of the line of sight E1. Figure 6 (c) is a front view of the other end surface of the optical device OD2 as viewed from the direction of the line of sight E2.

[0109] The optical device OD2 includes: a multi-core optical fiber bundle MFB composed of a plurality of multi-core optical fibers MF; and multi-core connectors C3 and C4 provided at both ends of the multi-core optical fiber bundle MFB.

[0110] The multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB may be Figure 1 to Figure 4 Any one of the multi-core optical fibers MF shown. Figure 6 In the figure, as a multi-core optical fiber MF constituting a multi-core optical fiber MFB, Figure 1 The multi-core optical fiber MF is shown.

[0111] The first multi-core connector C3 is provided at one end of the multi-core optical fiber bundle MFB. Each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB is fixed to the first multi-core connector C3 in such a manner that the inclination direction (in the example shown in the figure, the inclination direction v1) of the end face on the side of the first multi-core connector C3 (in the example shown in the figure, the first end face σ1) is consistent with a specific direction and these end faces are coplanar. In other words, each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB is fixed to the first multi-core connector C3 in such a manner that the above-mentioned imaginary axis L1 is arranged on the same straight line.

[0112] In addition, a key K3 is provided on the side surface of the first multi-core connector C3 at the front end of the inclination direction (in the example shown in the figure, the inclination direction v1) of the end surface (in the example shown in the figure, the first end surface σ1) located on the first multi-core connector C3 side of each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB. The key K3 is, for example, a rectangular parallelepiped convex portion protruding from the side surface of the first multi-core connector C3.

[0113] The second multi-core connector C4 is provided at the other end of the multi-core optical fiber bundle MFB. Each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB is fixed to the second multi-core connector C4 in such a manner that the inclination direction (in the illustrated example, the inclination direction v2) of the end face (in the illustrated example, the second end face σ2) on the side of the second multi-core connector C4 is consistent with a specific direction and these end faces are coplanar. In other words, each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB is fixed to the second multi-core connector C4 in such a manner that the above-mentioned imaginary axis L2 is arranged on the same straight line.

[0114] In addition, a key K4 is provided on the side surface of the end surface (in the example shown in the figure, the first end surface σ1) located on the second multi-core connector C4 side of each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB, in the four side surfaces of the second multi-core connector C4, at the front end in the inclination direction (in the example shown in the figure, the inclination direction v2). The key K4 is, for example, a rectangular parallelepiped convex portion protruding from the side surface of the second multi-core connector C4.

[0115] Therefore, for two optical devices OD2, when the first multi-core connector C3 of the optical device OD2 on one side is connected with the second multi-core connector C4 of the optical device OD2 on the other side in a manner in which the positions of key K3 and key K4 are reversed, the cores a1~an of each multi-core optical fiber MF included in the above two optical devices OD2 can be optically coupled.

[0116] In addition, a structure in which a single-core connector is provided at the end of one side of each multi-core optical fiber MF and these single-core connectors are integrated can be adopted to replace the first multi-core connector C3. In this case, these single-core connectors are integrated in a manner that the imaginary axis L1 related to each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB is arranged on the same straight line. Similarly, a structure in which a single-core connector is provided at the end of the other side of each multi-core optical fiber MF and these single-core connectors are integrated can be adopted to replace the second multi-core connector C4. In this case, these single-core connectors are integrated in a manner that the imaginary axis L2 related to each multi-core optical fiber MF constituting the multi-core optical fiber bundle MFB is arranged on the same straight line.

[0117] In addition, here, the structure of providing a multi-core connector at both ends of the multi-core optical fiber bundle MFB is described, but the present invention is not limited to this. That is, the structure of providing a multi-core connector at one end of the multi-core optical fiber bundle MFB is also included in the scope of the present invention. Figure 6 The structure in which either the first multicore connector C3 or the second multicore connector C4 is omitted in the optical device OD2 shown is also included in the scope of the present invention. In this case, the end face of each multicore optical fiber MF without the multicore connector may be inclined or not inclined.

[0118] (Modification of Optical Device)

[0119] Reference Figure 7 A modification example of the optical device OD2 will be described. Figure 7 (a) is a front view of the optical device OD2 involved in the first modification example, Figure 7 (b) is a front view of the optical device OD2 involved in the second modification.

[0120] In the optical device OD2 involved in the first variant, as Figure 7As shown in (a), the multi-core optical fiber bundle MFB includes at least two multi-core optical fibers MF whose cores with the same core number at least partially overlap each other when the end surface on the first multi-core connector C3 side is translated and flipped. Figure 7 (a) shows the situation where the leftmost multi-core optical fiber MF and the second multi-core optical fiber MF from the left satisfy this relationship. Therefore, the at least two multi-core optical fibers MF can satisfy the condition that the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axes L1 and L2, the condition that the core a1 closest to the mark c is arranged on the imaginary axis L1, or the condition that the core a2 second closest to the mark c is arranged on the imaginary axis. Figure 7 In the example shown in (a), the leftmost multi-core optical fiber MF and the second multi-core optical fiber MF from the left can satisfy the condition that the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axes L1 and L2. In addition, when the first multi-core connectors C3 are connected to each other, the same core number can be replaced in the above-mentioned at least two multi-core optical fibers MF. Figure 7 In the example shown in (a), in the leftmost multi-core optical fiber MF and the second multi-core optical fiber MF from the left, core a1 is connected to core a2 (producing a replacement of core number 1 and core number 2), and core a3 is connected to core a4 (producing a replacement of core number 3 and core number 4).

[0121] In addition, as the above-mentioned flipping, flipping relative to an axis parallel to the inclination direction of each multi-core optical fiber MF and flipping relative to an axis perpendicular to the inclination direction of each multi-core optical fiber MF are considered. In either case, the above-mentioned effect is achieved. In addition, as the above-mentioned parallel movement, parallel movement in a direction parallel to the inclination direction of the end face of the first multi-core connector C3 and parallel movement in a direction perpendicular to the inclination direction of the end face of the first multi-core connector C3 are considered. In the former case, the above-mentioned at least two multi-core optical fibers MF are arranged parallel to the inclination direction of the end face of the first multi-core connector C3. On the other hand, in the latter case, the above-mentioned at least two multi-core optical fibers MF are arranged perpendicular to the inclination direction of the end face of the first multi-core connector C3. In either case, the above-mentioned effect is achieved. In addition, in the latter case, in particular, the distance from the imaginary axis perpendicular to the inclination direction of the end face of the first multi-core connector C3 to the center of the mark c of the above-mentioned at least two multi-core optical fibers MF is equal. As a result, when observing the end face of the multi-core connector C3 from the front using a microscope or the like, focusing on the virtual axis makes it easy to simultaneously observe the marks c of the at least two multi-core optical fibers MF.

[0122] In addition, it is preferred that all the multi-core optical fibers MF constituting the multi-core optical fiber bundle MFB satisfy the above-mentioned translation and flipping relationship. Thus, in all the multi-core optical fibers MF constituting the multi-core optical fiber bundle MFB, replacement of the same core number can be produced. In particular, when the above-mentioned parallel movement is a parallel movement in a direction perpendicular to the inclination direction of the end face of the first multi-core connector C3, it is easy to simultaneously observe the marks c of all the multi-core optical fibers MF constituting the multi-core optical fiber bundle MFB.

[0123] In the optical device OD2 according to the second variant, Figure 7 As shown in (b), the multi-core optical fiber bundle MFB includes at least two multi-core optical fibers MF whose cores with the same core number at least partially overlap each other when the end surface on the first multi-core connector C3 side is translated and rotated. Figure 7 In (b), the leftmost multi-core optical fiber MF and the second multi-core optical fiber MF from the left satisfy this relationship. Thus, when the first multi-core connectors C3 are connected to each other, different core numbers can be replaced in the at least two multi-core optical fibers MF. Figure 7 In the example shown in (b), in the leftmost multi-core optical fiber MF, core a1 is connected to core a2 (generating a replacement of core number 1 and core number 2), and core a3 is connected to core a4 (generating a replacement of core number 3 and core number 4). On the other hand, in the second multi-core optical fiber MF from the left, core a1 is connected to core a4 (generating a replacement of core number 1 and core number 4), and core a2 is connected to core a3 (generating a replacement of core number 2 and core number 3). Thus, the wiring freedom when using the optical device OD2 to construct a network can be improved.

[0124] In addition, as the above-mentioned rotation, when the configuration of the cores a1 to an of the multi-core optical fiber MF has n-fold symmetry, a rotation of m×360° / n (m is a natural number greater than 1 and less than n-1) is considered. In the present embodiment, a rotation of 180° is adopted. Thus, the multi-core optical fibers MF on both sides can satisfy the condition that the core a1 closest to the mark c and the core a2 second closest to the mark c are arranged to sandwich the imaginary axes L1 and L2, or the condition that the core a1 closest to the mark c is arranged on the imaginary axis L1.

[0125] [Fifth embodiment]

[0126] Reference Figure 8 A multi-core optical fiber MF according to a fifth embodiment of the present invention will be described. Figure 8 (a) is a side view of the multi-core optical fiber MF. Figure 8 (b) is a stereoscopic diagram of the multi-core optical fiber MF.

[0127] The multi-core optical fiber MF includes a first multi-core optical fiber MF1 and a second multi-core optical fiber MF2. The first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are connected (for example, connected by a connector or fusion spliced).

[0128] The first multi-core optical fiber MF1 is configured as Figure 1 However, in the first multi-core optical fiber MF1, although the first end face σ1 must be inclined, the second end face σ2 does not have to be inclined. Figure 1 The end face corresponding to the first end face σ1 of the multi-core optical fiber MF shown is referred to as a first end face Σ1. The first end face Σ1 of the first multi-core optical fiber MF1 is connected to a second end face Σ2 of a second multi-core optical fiber MF2 described later.

[0129] The second multi-core optical fiber MF2 is configured as Figure 1 However, in the second multi-core optical fiber MF2, although the second end face σ2 must be tilted, the first end face σ1 does not have to be tilted. Figure 1 The end face corresponding to the second end face σ2 of the multi-core optical fiber MF shown is referred to as the second end face Σ2. The second end face Σ2 of the second multi-core optical fiber MF2 is connected to the first end face Σ1 of the first multi-core optical fiber MF1 described above.

[0130] The inclination direction v1 of the first end face Σ1 and the inclination direction v2 of the second end face Σ2 are determined so as to satisfy the following condition 1.

[0131] Condition 1: When the first end face Σ1 and the second end face Σ2 are in surface contact in a manner that minimizes the angle formed by the extension direction of the fiber cores a1~an in the first end face Σ1 and the extension direction of the fiber cores a1~an in the second end face Σ2, each of the fiber cores a1~an in the first end face Σ1 at least partially overlaps with any one of the fiber cores a1~an in the second end face Σ2.

[0132] Thus, when the first end face Σ1 of the first multi-core optical fiber MF1 and the second end face Σ2 of the second multi-core optical fiber MF2 are connected in a manner that the first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are arranged as much as possible in a straight line, the cores a1~an of the first multi-core optical fiber MF1 and the cores a1~an of the second multi-core optical fiber MF2 can be optically coupled. Here, connecting the first end face Σ1 and the second end face Σ2 "in a manner that the first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are arranged as much as possible in a straight line" means connecting the first end face Σ1 and the second end face Σ2 "in a manner that the angle formed by the extension direction of the cores a1~an of the first multi-core optical fiber MF1 and the extension direction of the cores a1~an of the second multi-core optical fiber MF2 is minimized."

[0133] In addition, the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 may be consistent or inconsistent. However, the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are preferably equal (basically consistent), and more preferably consistent (completely consistent). Here, the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are equal (basically consistent) to mean that, for example, the difference |θ1-θ2| is, for example, less than 2° or less than 0.4°.

[0134] When the inclination angle θ1 of the first end face Σ1 is consistent with the inclination angle θ2 of the second end face Σ2, the minimum value of the angle formed by the extension direction of the cores a1 to an in the first end face Σ1 and the extension direction of the cores a1 to an in the second end face Σ2 is 0°. That is, minimizing the angle formed by the extension direction of the cores a1 to an in the first end face Σ1 and the extension direction of the cores a1 to an in the second end face Σ2 is equivalent to aligning the extension direction of the cores a1 to an in the first end face Σ1 with the extension direction of the cores a1 to an in the second end face Σ2. Therefore, the above-mentioned condition 1 is equivalent to the following condition 1'.

[0135] Condition 1': When the first end face Σ1 and the second end face Σ2 are brought into surface contact in such a way that the extension direction of the cores a1~an in the first end face Σ1 is consistent with the extension direction of the cores a1~an in the second end face Σ2, each of the cores a1~an in the first end face Σ1 at least partially overlaps with any one of the cores a1~an in the second end face Σ2.

[0136] Thus, when the first end face Σ1 of the first multi-core optical fiber MF1 and the second end face Σ2 of the second multi-core optical fiber MF2 are connected in a manner that the first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are arranged on a straight line, the cores a1 to an of the first multi-core optical fiber MF1 and the cores a1 to an of the second multi-core optical fiber MF2 can be optically coupled. Here, connecting the first end face Σ1 and the second end face Σ2 "in a manner that the first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are arranged on a straight line" means connecting the first end face Σ1 and the second end face Σ2 "in a manner that the extension direction of the cores a1 to an of the first multi-core optical fiber MF1 and the extension direction of the cores a1 to an of the second multi-core optical fiber MF2 are consistent." In addition, even if the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are not strictly consistent, as long as the inclination angle θ1 of the first end face Σ1 and the inclination angle θ2 of the second end face Σ2 are the same (as long as they are basically consistent), the same effect can be obtained.

[0137] In the multi-core optical fiber MF according to the present embodiment, the tilt direction v1 of the first end face Σ1 of the first multi-core optical fiber MF1 and the tilt direction v2 of the second end face Σ2 of the second multi-core optical fiber MF2 are determined to satisfy the following condition 2 in addition to the above condition 1.

[0138] Condition 2: When the first end face Σ1 and the second end face Σ2 are in contact with each other in such a way as to minimize the angle between the extension direction of the cores a1 to an in the first end face Σ1 and the extension direction of the cores a1 to an in the second end face Σ2, the core numbers of the cores that overlap at least partially are consistent.

[0139] exist Figure 8 In the example shown, the pairs of cores that overlap at least partially are (1) the pair of core a1 in the first end face Σ1 and the pair of core a1 in the second end face Σ2, (2) the pair of core a2 in the first end face Σ1 and the pair of core a2 in the second end face Σ2, (3) the pair of core a3 in the first end face Σ1 and the pair of core a3 in the second end face Σ2, and (4) the pair of core a4 in the first end face Σ1 and the pair of core a4 in the second end face Σ2. For any of the above four pairs, the core numbers of the two cores constituting the pair are the same. That is, Figure 8 The connection form shown can be said to be a connection form in which (1) the core a1 closest to the mark c in the first multi-core optical fiber MF1 and the core a1 closest to the mark c in the second multi-core optical fiber MF2 at least partially overlap, and (2) the core a2 second closest to the mark c in the first multi-core optical fiber MF1 and the core a2 second closest to the mark c in the second multi-core optical fiber MF2 at least partially overlap.

[0140] Therefore, when the first end face Σ1 of the first multi-core optical fiber MF1 and the second end face Σ2 of the second multi-core optical fiber MF2 are connected so that the first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are arranged on a straight line as much as possible, the cores with the same core number can be optically coupled to each other.

[0141] In addition, if Figure 8 As shown in (c), a first single-core connector C1 may be provided at the end of the first end face Σ1 of the first multi-core optical fiber MF1. Figure 8 As shown in (c), a second single-core connector C2 may be provided at the end of the second end face Σ2 of the second multi-core optical fiber MF2. Figure 5 The description is the same as what has been explained, so it will not be repeated here.

[0142] In addition, in the present embodiment, as the first multi-core optical fiber MF1, a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a4 and the core a1 is used, but the first multi-core optical fiber MF1 is not limited to this. (1) A multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a1 and the core a2, (2) A multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a2 and the core a3, (3) A multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a3 and the core a4 can also be used as the first multi-core optical fiber MF1. In addition, (1) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a1, (2) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a2, (3) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a3, and (4) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a4 can also be used as the first multi-core optical fiber MF1. The same can be said about the second multi-core optical fiber MF2.

[0143] [Sixth embodiment]

[0144] Reference Fig. 9 A multi-core optical fiber MF according to a sixth embodiment of the present invention will be described. Fig. 9 (a) is a side view of the multi-core optical fiber MF. Fig. 9 (b) is a stereoscopic diagram of the multi-core optical fiber MF.

[0145] The multi-core optical fiber MF includes a first multi-core optical fiber MF1 and a second multi-core optical fiber MF2. The first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are connected (for example, fusion-connected).

[0146] The first multi-core optical fiber MF1 is configured as Figure 1 However, in the first multi-core optical fiber MF1, although the first end face σ1 must be inclined, the second end face σ2 does not need to be inclined. Figure 1 An end face corresponding to the first end face σ1 of the multi-core optical fiber MF shown is referred to as a first end face Σ1.

[0147] The second multi-core optical fiber MF2 is configured as Figure 1 However, in the second multi-core optical fiber MF2, although the first end face σ1 must be tilted, the second end face σ2 does not need to be tilted. Figure 1 An end face corresponding to the first end face σ1 of the multi-core optical fiber MF shown is referred to as a second end face Σ2.

[0148] In the multi-core optical fiber MF involved in the fifth embodiment, the inclination direction v1 of the first end face Σ1 and the inclination direction v2 of the second end face Σ2 are determined to satisfy the above-mentioned condition 2 in addition to the above-mentioned condition 1. In contrast, in the multi-core optical fiber MF involved in the sixth embodiment, the inclination direction v1 of the first end face Σ1 and the inclination direction v2 of the second end face Σ2 are determined to satisfy the following condition 3 in addition to the above-mentioned condition 1.

[0149] Condition 3: When the first end face Σ1 and the second end face Σ2 are in surface contact in a manner that minimizes the angle between the extension direction of the cores a1 to an in the first end face Σ1 and the extension direction of the cores a1 to an in the second end face Σ2, the core numbers of the cores that at least partially overlap are different.

[0150] exist Fig. 9 In the example shown, the pairs of cores that overlap at least partially are (1) the pair of core a1 in the first end face Σ1 and the pair of core a2 in the second end face Σ2, (2) the pair of core a2 in the first end face Σ1 and the pair of core a1 in the second end face Σ2, (3) the pair of core a3 in the first end face Σ1 and the pair of core a4 in the second end face Σ2, and (4) the pair of core a4 in the first end face Σ1 and the pair of core a3 in the second end face Σ2. For any of the above four pairs, the core numbers of the two cores constituting the pair are different. That is, Fig. 9 The connection form shown can be said to be a connection form in which (1) the fiber core a1 closest to the mark c in the first multi-core optical fiber MF1 and the fiber core a2 second closest to the mark c in the second multi-core optical fiber MF2 at least partially overlap, and (2) the fiber core a2 second closest to the mark c in the first multi-core optical fiber MF1 and the fiber core a1 closest to the mark c in the second multi-core optical fiber MF2 at least partially overlap.

[0151] Thus, when the first end face Σ1 of the first multi-core optical fiber MF1 and the second end face Σ2 of the second multi-core optical fiber MF2 are connected so that the first multi-core optical fiber MF1 and the second multi-core optical fiber MF2 are arranged on a straight line as much as possible, cores with different core numbers can be optically coupled to each other.

[0152] In addition, if Fig. 9 As shown in (c), a first single-core connector C1 may be provided at the end of the first end face Σ1 of the first multi-core optical fiber MF1. Fig. 9 As shown in (c), a second single-core connector C2 may be provided at the end of the second end face Σ2 of the second multi-core optical fiber MF2. Figure 5 The description is the same as what has been explained, so it will not be repeated here.

[0153] In addition, in the present embodiment, as the first multi-core optical fiber MF1, a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a4 and the core a1 is used, but the first multi-core optical fiber MF1 is not limited to this. (1) A multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a1 and the core a2, (2) A multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a2 and the core a3, (3) A multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the midpoint between the core a3 and the core a4 can also be used as the first multi-core optical fiber MF1. In addition, (1) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a1, (2) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a2, (3) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a3, and (4) a multi-core optical fiber with an inclined direction v1 from the center of the first end face Σ1 toward the core a4 can also be used as the first multi-core optical fiber MF1. The same can be said about the second multi-core optical fiber MF2.

[0154] 〔Summarize〕

[0155] In the multi-core optical fiber involved in mode 1 of the present invention, the following structure is adopted, namely, it has: a cladding; a plurality of cores formed in the above-mentioned cladding; at least one mark formed in the above-mentioned cladding; and an end face, which is inclined in a manner not orthogonal to the extension direction of the above-mentioned plurality of cores, and in the above-mentioned end face, the above-mentioned plurality of cores are arranged in a linear symmetric state with respect to an imaginary axis which is orthogonal to the inclination direction of the above-mentioned end face and passes through the center of the above-mentioned cladding, and the center of the above-mentioned mark is included in the area between a straight line passing through a core farthest from the above-mentioned imaginary axis or a region farthest from the above-mentioned imaginary axis in a mode field of light propagating through the core and parallel to the above-mentioned imaginary axis and a straight line passing through a core farthest from the above-mentioned imaginary axis or a region farthest from the above-mentioned imaginary axis in a mode field of light propagating through the core on the side opposite to the side where the core farthest from the above-mentioned imaginary axis is provided and parallel to the above-mentioned imaginary axis.

[0156] In the multi-core optical fiber involved in mode 2 of the present invention, based on the structure of mode 1, the following structure is adopted, that is, in the above-mentioned end face, (1) the core closest to the above-mentioned mark among the above-mentioned multiple cores and the core second closest to the above-mentioned mark are arranged to sandwich the above-mentioned imaginary axis, or (2) the core closest to the above-mentioned mark among the above-mentioned multiple cores is arranged on the above-mentioned imaginary axis, or (3) the core second closest to the above-mentioned mark among the above-mentioned multiple cores is arranged on the above-mentioned imaginary axis.

[0157] In the multi-core optical fiber involved in mode 3 of the present invention, based on the structure of mode 2, the following structure is adopted, that is, in the above-mentioned end face, the center of the above-mentioned mark is arranged in an area sandwiched by a straight line passing through the center of the fiber core closest to the above-mentioned mark and parallel to the above-mentioned axis and a straight line passing through the center of the fiber core second closest to the above-mentioned mark and parallel to the above-mentioned imaginary axis.

[0158] In the multi-core optical fiber according to the fourth aspect of the present invention, in addition to the structure of any one of the first to third aspects, a structure is adopted in which the virtual axis does not intersect any of the plurality of cores.

[0159] In the multi-core optical fiber involved in mode 5 of the present invention, on the basis of the structure of any one of modes 1 to 4, the following structure is adopted, that is, it has another end face inclined in a manner not orthogonal to the extension direction of the above-mentioned multiple cores, in the above-mentioned other end face, the above-mentioned multiple cores are arranged in a linear symmetrical state with respect to an imaginary axis orthogonal to the inclination direction of the above-mentioned end face, (1) the core closest to the above-mentioned mark and the core second closest to the above-mentioned mark among the above-mentioned multiple cores are arranged to sandwich the above-mentioned imaginary axis, or (2) the core closest to the above-mentioned mark among the above-mentioned multiple cores is arranged on the above-mentioned imaginary axis.

[0160] In the multi-core optical fiber involved in method 6 of the present invention, the following structure is adopted, namely, it comprises: a first multi-core optical fiber, which is the multi-core optical fiber involved in any one of methods 1 to 5; and a second multi-core optical fiber, which is the multi-core optical fiber involved in any one of methods 1 to 5, and the above-mentioned end face of the above-mentioned first multi-core optical fiber and the above-mentioned end face of the second multi-core optical fiber are connected in a manner that each of the above-mentioned multiple cores of the above-mentioned first multi-core optical fiber at least partially overlaps with any one of the above-mentioned multiple cores of the above-mentioned second multi-core optical fiber.

[0161] In the multi-core optical fiber involved in method 7 of the present invention, on the basis of the multi-core optical fiber involved in method 6, the following structure is adopted, that is, the core among the above-mentioned multiple cores of the above-mentioned first multi-core optical fiber that is closest to the above-mentioned mark of the above-mentioned first multi-core optical fiber in the above-mentioned end face of the above-mentioned first multi-core optical fiber at least partially overlaps with the core among the above-mentioned multiple cores of the above-mentioned second multi-core optical fiber that is closest to the above-mentioned mark of the above-mentioned second multi-core optical fiber in the above-mentioned end face of the above-mentioned second multi-core optical fiber, and the core among the above-mentioned multiple cores of the above-mentioned first multi-core optical fiber that is second closest to the above-mentioned mark of the above-mentioned first multi-core optical fiber in the above-mentioned end face of the above-mentioned second multi-core optical fiber at least partially overlaps with the core among the above-mentioned multiple cores of the above-mentioned second multi-core optical fiber that is second closest to the above-mentioned mark of the above-mentioned second multi-core optical fiber in the above-mentioned end face of the above-mentioned second multi-core optical fiber.

[0162] In the multi-core optical fiber involved in mode 8 of the present invention, on the basis of the structure of mode 6, the following structure is adopted, that is, the core among the above-mentioned multiple cores of the above-mentioned first multi-core optical fiber that is closest to the above-mentioned mark of the above-mentioned first multi-core optical fiber in the above-mentioned end face of the above-mentioned first multi-core optical fiber and the core among the above-mentioned multiple cores of the above-mentioned second multi-core optical fiber that is second closest to the above-mentioned mark of the above-mentioned second multi-core optical fiber in the above-mentioned end face of the above-mentioned second multi-core optical fiber at least partially overlap, and the core among the above-mentioned multiple cores of the above-mentioned first multi-core optical fiber that is second closest to the above-mentioned mark of the above-mentioned first multi-core optical fiber in the above-mentioned end face of the above-mentioned second multi-core optical fiber and the core among the above-mentioned multiple cores of the above-mentioned second multi-core optical fiber that is closest to the above-mentioned mark of the second multi-core optical fiber in the above-mentioned end face of the above-mentioned second multi-core optical fiber at least partially overlap.

[0163] An optical device according to aspect 9 of the present invention has a structure comprising: a multi-core optical fiber according to any one of aspects 1 to 8; and a single-core connector provided at one or both ends of the multi-core optical fiber.

[0164] In the optical device involved in mode 10 of the present invention, the following structure is adopted, namely, it comprises: a multi-core optical fiber bundle composed of the multi-core optical fibers involved in any one of modes 1 to 8; and a multi-core connector or an integrated single-core connector group arranged at one or both ends of the above-mentioned multi-core optical fiber bundle.

[0165] In the optical device involved in mode 11 of the present invention, based on the structure of mode 10, the following structure is adopted, that is, the above-mentioned multi-core optical fiber bundle includes a plurality of multi-core optical fibers arranged in the above-mentioned multi-core connector in a manner such that the above-mentioned imaginary axis is on the same straight line, or the above-mentioned single-core connector group includes a plurality of single-core connectors arranged in a manner such that the above-mentioned imaginary axis is on the same straight line.

[0166] In the manufacturing method of a multi-core optical fiber involved in method 12 of the present invention, the multi-core optical fiber involved in any one of manufacturing methods 6 to 8 adopts the following structure, namely, the end face of the first multi-core optical fiber is connected with the end face of the second multi-core optical fiber in a manner that each of the above-mentioned multiple cores of the above-mentioned first multi-core optical fiber at least partially overlaps with any one of the above-mentioned multiple cores of the above-mentioned second multi-core optical fiber.

[0167] 〔Remarks〕

[0168] The present invention is not limited to the above-mentioned embodiments, and various changes can be made within the scope shown in the technical solution. The embodiments obtained by appropriately combining the various technical means disclosed in the above-mentioned embodiments are also included in the technical scope of the present invention. For example, the multi-core optical fiber can also be twisted. Regardless of whether it is twisted or not, the multi-core optical fiber that meets the conditions described in the technical solution is included in the technical scope of the present invention. In addition, the shape of the connector is arbitrary. For example, an optical device with a connector of the type that inserts and fixes the multi-core optical fiber in the optical fiber hole, or an optical device with a connector of the type that accommodates and fixes the multi-core optical fiber in the V-groove is also included in the technical scope of the present invention. In addition, the end face of the multi-core optical fiber can be a flat surface, or it can be a curved surface that can be approximated by a flat surface (for example, a convex spherical surface, or a concave spherical surface).

[0169] In addition, the multi-core optical fiber described above has the following structure, that is, in the end face, the plurality of cores are arranged in a linear symmetric shape with respect to an imaginary axis orthogonal to the tilt direction of the end face, and the center of the mark is included in the region between the imaginary axis and a straight line passing through the center of the core farthest from the imaginary axis among the plurality of cores and parallel to the imaginary axis. However, the multi-core optical fiber described above may also be configured such that in the end face, the plurality of cores are arranged in a linear symmetric shape with respect to the imaginary axis orthogonal to the tilt direction of the end face, and the center of the mark is included in the region between a straight line passing through the core farthest from the imaginary axis among the plurality of cores or a region farthest from the imaginary axis in the mode field of light propagating through the core and parallel to the imaginary axis and a straight line passing through the core farthest from the imaginary axis on the side opposite to the side where the core farthest from the imaginary axis is provided or a region farthest from the imaginary axis in the mode field of light propagating through the core and parallel to the imaginary axis. Here, the above-mentioned mode field refers to a region defined by a mode field diameter when light of any wavelength of 850 nm to 1700 nm propagates through the core in a region inside the outer side of the cladding.

[0170] Description of Reference Numerals

[0171] MF…multi-core optical fiber; a1~an…core; b…cladding; c…mark; σ1…first end face; σ2…second end face; v1, v2…tilt direction; θ1, θ2…tilt angle; OD1, OD2…optical device; C1, C2…single-core connector; C3, C4…multi-core connector.

Claims

1. A multi-core optical fiber, characterized in that: A fiber core having: a cladding; a plurality of cores formed in the cladding; at least one mark formed in the cladding; and an end face inclined in a manner not orthogonal to the extending direction of the plurality of cores, In the end face, the plurality of cores are arranged in a line-symmetrical manner with respect to an imaginary axis orthogonal to the inclination direction of the end face, When the end face is imaginarily divided into two regions, a first region and a second region, by the imaginary axis, a core farthest from the imaginary axis among the cores arranged in the first region among the multiple cores is set as the first core, and a core farthest from the imaginary axis among the cores arranged in the second region among the multiple cores is set as the second core, and in the end face, the center of the mark is included in the region between a straight line passing through the first core or the mode field of the first core and parallel to the imaginary axis and a straight line passing through the second core or the mode field of the second core and parallel to the imaginary axis.

2. The multi-core optical fiber according to claim 1, characterized in that: In the end face, (1) the core closest to the mark among the multiple cores and the core second closest to the mark are arranged to sandwich the imaginary axis, or (2) the core closest to the mark among the multiple cores is arranged on the imaginary axis, or (3) the core second closest to the mark among the multiple cores is arranged on the imaginary axis.

3. The multi-core optical fiber according to claim 2, characterized in that: In the end face, the center of the mark is arranged in a region sandwiched by a straight line passing through the center of the core closest to the mark and parallel to the imaginary axis and a straight line passing through the center of the core second closest to the mark and parallel to the imaginary axis.

4. The multi-core optical fiber according to any one of claims 1 to 3, characterized in that: The imaginary axis does not intersect any of the plurality of cores.

5. The multi-core optical fiber according to any one of claims 1 to 4, characterized in that: having another end face inclined in a manner not orthogonal to the extending direction of the plurality of cores, In the other end face, the multiple cores are arranged in a line symmetrical shape with respect to an imaginary axis orthogonal to the inclination direction of the end face, (1) the core closest to the mark among the multiple cores and the core second closest to the mark are arranged to sandwich the imaginary axis, or (2) the core closest to the mark among the multiple cores is arranged on the imaginary axis, or (3) the core second closest to the mark among the multiple cores is arranged on the imaginary axis.

6. A multi-core optical fiber, characterized in that: The method comprises: a first multi-core optical fiber which is the multi-core optical fiber according to any one of claims 1 to 5; and a second multi-core optical fiber which is the multi-core optical fiber according to any one of claims 1 to 5, The end face of the first multi-core optical fiber and the end face of the second multi-core optical fiber are connected in such a manner that each of the plurality of cores of the first multi-core optical fiber at least partially overlaps with any of the plurality of cores of the second multi-core optical fiber.

7. The multi-core optical fiber according to claim 6, characterized in that: The core of the plurality of cores of the first multi-core optical fiber that is closest to the mark of the first multi-core optical fiber in the end face of the first multi-core optical fiber at least partially overlaps with the core of the plurality of cores of the second multi-core optical fiber that is closest to the mark of the second multi-core optical fiber in the end face of the second multi-core optical fiber, A core among the multiple cores of the first multi-core optical fiber that is second closest to the mark of the first multi-core optical fiber in the end face of the first multi-core optical fiber at least partially overlaps with a core among the multiple cores of the second multi-core optical fiber that is second closest to the mark of the second multi-core optical fiber in the end face of the second multi-core optical fiber.

8. The multi-core optical fiber according to claim 6, characterized in that: The core of the plurality of cores of the first multi-core optical fiber that is closest to the mark of the first multi-core optical fiber in the end face of the first multi-core optical fiber at least partially overlaps with the core of the plurality of cores of the second multi-core optical fiber that is second closest to the mark of the second multi-core optical fiber in the end face of the second multi-core optical fiber, A core among the multiple cores of the first multi-core optical fiber that is second closest to the mark of the first multi-core optical fiber in the end face of the first multi-core optical fiber at least partially overlaps with a core among the multiple cores of the second multi-core optical fiber that is closest to the mark of the second multi-core optical fiber in the end face of the second multi-core optical fiber.

9. An optical device, characterized in that: have: The multi-core optical fiber according to any one of claims 1 to 8; and The single-core connector is arranged at one or both ends of the multi-core optical fiber.

10. An optical device, characterized in that: have: A multi-core optical fiber bundle consisting of the multi-core optical fiber according to any one of claims 1 to 8; and A multi-core connector or an integrated single-core connector group is provided at one or both ends of the multi-core optical fiber bundle.

11. The optical device according to claim 10, characterized in that: The multi-core optical fiber bundle includes a plurality of multi-core optical fibers arranged in the multi-core connector such that the imaginary axes are on the same straight line, or the single-core connector group includes a plurality of single-core connectors arranged such that the imaginary axes are on the same straight line.

12. A method for manufacturing a multi-core optical fiber, comprising manufacturing the multi-core optical fiber according to any one of claims 6 to 8, characterized in that: The end face of the first multi-core optical fiber is connected to the end face of the second multi-core optical fiber in such a manner that each of the plurality of cores of the first multi-core optical fiber at least partially overlaps with any of the plurality of cores of the second multi-core optical fiber.

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