Multicore optical fiber and marker identification method for multicore optical fiber

Abstract

The purpose of the present disclosure is to provide a multicore optical fiber which enables suppression of crosstalk between a core and a marker and easy confirmation of the marker, and a marker identification method for the multicore optical fiber.　A multicore optical fiber (10) comprises: a plurality of communication cores (11); a cladding (12) that surrounds the communication cores (11); and at least one marker core (13) that is provided to the cladding (12). A normalized propagation constant (b) of the marker core (13) is b≤0.2 in a communication wavelength band of the communication cores (11), and is b>0.2 at at least one wavelength in a region having a wavelength shorter than that of the communication wavelength band of the communication cores (11).

Description

Multi-core optical fiber and method for identifying markers in multi-core optical fiber

[0001] This application claims priority to Japanese Patent Application No. 2024-090618, filed on Jun. 4, 2024, the contents of which are incorporated herein by reference.

[0002] In a multi-core optical fiber, a marker having a refractive index higher than that of the cladding in at least a part of the region is sometimes provided to identify the multiple cores (see, for example, Patent Document 1). In a multi-core optical fiber provided with such a marker, the cores can be easily identified. Therefore, when splicing the multi-core optical fiber with another optical product, it is easy to determine the splicing position.

[0003] Japanese Patent No. 5267481

[0004] In the multi-core optical fiber, light can be guided through the markers, which can cause crosstalk between the cores and the markers.

[0005] An aspect of the present disclosure aims to provide a multi-core optical fiber and a method for identifying a marker in a multi-core optical fiber that can suppress crosstalk between cores and markers and that can easily identify the markers.

[0006] A multi-core optical fiber according to a first aspect of the present disclosure comprises a plurality of communication cores, a cladding surrounding the communication cores, and at least one marker core provided in the cladding, wherein the normalized propagation constant b of the marker core satisfies b≦0.2 in the communication wavelength band of the communication cores, and b>0.2 at least at one wavelength in a region shorter than the communication wavelength band of the communication cores.

[0007] With this configuration, the normalized propagation constant b of the marker core is b≦0.2 in the communication wavelength band of the communication core. Therefore, the amount of light propagating through the marker core is small in the communication wavelength region. Therefore, crosstalk between the marker core and the communication core can be suppressed in the communication wavelength region.

[0008] The normalized propagation constant b of the marker core satisfies b>0.2 for at least one wavelength in a region shorter than the communication wavelength band of the communication core. Therefore, during inspection using light including the wavelength, a sufficient amount of light can be propagated in the marker core. Therefore, the marker core can be easily identified during inspection.

[0009] In a second aspect of the present disclosure, in the multi-core optical fiber of the first aspect, the wavelength at which b>0.2 is satisfied is a wavelength shorter than the wavelength at which light in an LP mode that is one order higher than the LP mode of light in a communication wavelength band that propagates through the communication core can propagate through the communication core.

[0010] A third aspect of the present disclosure is the multi-core optical fiber according to the first or second aspect, wherein the wavelength at which b>0.2 is satisfied is a wavelength in the visible light region.

[0011] A fourth aspect of the present disclosure is a multi-core optical fiber according to any one of the first to third aspects, wherein at least one of the optical propagation characteristics of the marker core complies with a standard specification for optical propagation characteristics when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of one of the communication cores by adjusting the diameter of the marker core.

[0012] A fifth aspect of the present disclosure is a multi-core optical fiber according to any one of the first to fourth aspects, wherein the relative refractive index difference of the marker core is within a range from the maximum value to the minimum value of the relative refractive index differences of the plurality of communication cores.

[0013] A sixth aspect of the present disclosure is a multi-core optical fiber according to any one of the first to fifth aspects, wherein a mode field diameter MFD of the marker core at the wavelength where b>0.2 and a distance OCT from the center of the marker core to the outer peripheral surface of the cladding satisfy MFD / 2<OCT.

[0014] A seventh aspect of the present disclosure is the multi-core optical fiber according to any one of the first to sixth aspects, wherein a diameter of the marker core is larger than an inspection wavelength.

[0015] An eighth aspect of the present disclosure is a multi-core optical fiber according to any one of the first to seventh aspects, wherein the plurality of communication cores are arranged so that their centers are linearly symmetric with respect to at least one axis of symmetry, and the center of the marker core is located off the axis of symmetry.

[0016] A marker identification method for a multi-core optical fiber according to a ninth aspect of the present disclosure is a method for identifying a marker core in a multi-core optical fiber including a plurality of communication cores, a cladding surrounding the communication cores, and at least one marker core provided in the cladding, the method including using the multi-core optical fiber in which a normalized propagation constant b of the marker core satisfies b≦0.2 in a communication wavelength band of the communication cores and b>0.2 for at least one wavelength in a region of wavelengths shorter than the communication wavelength band of the communication cores, and inputting light including the wavelength at which b satisfies b>0.2 into the multi-core optical fiber and observing output light.

[0017] One aspect of the present disclosure provides a multi-core optical fiber and a method for identifying a marker in a multi-core optical fiber, which can suppress crosstalk between cores and markers and can easily identify the markers.

[0018] The present invention relates to a multi-core optical fiber and a multi-core optical fiber according to an embodiment of the present invention.

[0019] Hereinafter, a multi-core optical fiber and a method for identifying a marker in a multi-core optical fiber according to an embodiment will be described with reference to the drawings.

[0020] [Multi-core optical fiber] Fig. 1 is a cross-sectional view perpendicular to the longitudinal direction of a multi-core optical fiber 10 according to an embodiment. The left-right direction in Fig. 1 is the X direction. The up-down direction in Fig. 1 is the Y direction. The X direction is perpendicular to the central axis C1 of the cladding 12. The Y direction is perpendicular to the central axis C1. The X direction and the Y direction are perpendicular to each other. The central axis C1 coincides with the central axis of the multi-core optical fiber 10.

[0021] The positions of the components of the multi-core optical fiber 10 will be provisionally defined according to Fig. 1 . In the following description, the upper side (+Y side) in Fig. 1 may be defined as the upper side. The communication core 11A is located above the communication core 11B. The communication core 11C is located above the communication core 11D. The positional relationships defined here do not limit the posture of the multi-core optical fiber 10 during use.

[0022] 1 , the multi-core optical fiber 10 includes a plurality of communication cores 11, a cladding 12 surrounding the communication cores 11, and marker cores 13 provided in the cladding 12. The multi-core optical fiber 10 may include one or more coating layers (not shown) that coat the cladding 12.

[0023] In this embodiment, the multi-core optical fiber 10 has four communication cores 11. The cross section of each communication core 11 is circular. The four communication cores 11 are referred to as communication cores 11A to 11D, respectively.

[0024] The communication cores 11A and 11B are arranged side by side at an interval in the Y direction. The communication cores 11C and 11D are arranged side by side at an interval in the Y direction. The communication cores 11A and 11C are arranged side by side at an interval in the X direction. The communication cores 11B and 11D are arranged side by side at an interval in the X direction. The outer diameters of the multiple communication cores 11 are, for example, the same as each other.

[0025] The refractive index of the communication core 11 includes a region higher than the refractive index of the cladding 12. The communication core 11 may be made of, for example, silica glass doped with a dopant (such as germanium) that increases the average refractive index. The cladding 12 may be made of, for example, pure silica glass.

[0026] The multiple communication cores 11 are arranged so that their centers 11a are line-symmetric with respect to the symmetry axis A1. Specifically, the centers 11a of the communication cores 11A and 11B and the centers 11a of the communication cores 11C and 11D are line-symmetric with respect to the symmetry axis A1. The symmetry axis A1 is a straight line that passes through the center of the cladding 12 and is parallel to the Y direction.

[0027] The multiple communication cores 11 are arranged so that their centers 11a are line-symmetric with respect to the symmetry axis A2. Specifically, the centers 11a of the communication cores 11A and 11C and the centers 11a of the communication cores 11B and 11D are line-symmetric with respect to the symmetry axis A2. The symmetry axis A2 is a straight line that passes through the center of the cladding 12 and is parallel to the X direction.

[0028] The multiple communication cores 11 are arranged so that their centers 11a are line-symmetric with respect to the symmetry axis A3. Specifically, the center 11a of communication core 11A and the center 11a of communication core 11D are located line-symmetric with respect to the symmetry axis A3. The symmetry axis A3 is a straight line passing through the center 11a of communication core 11B, the center of the cladding 12, and the center 11a of communication core 11C.

[0029] The multiple communication cores 11 are arranged so that their centers 11a are line-symmetric with respect to the symmetry axis A4. Specifically, the center 11a of communication core 11B and the center 11a of communication core 11C are located line-symmetric with respect to the symmetry axis A4. The symmetry axis A4 is a straight line passing through the center 11a of communication core 11A, the center of the cladding 12, and the center 11a of communication core 11D.

[0030] The centers 11a of the four communication cores 11 (11A to 11D) are equidistant from the central axis C1 and are located at positions that are rotationally symmetrical with respect to the central axis C1.

[0031] The marker core 13 is surrounded by, for example, the cladding 12. The refractive index of the marker core 13 is different from the refractive index of the surrounding area. For example, the marker core 13 includes a region having a higher refractive index than the cladding 12. If there is a low refractive index region around the marker core 13 that has a lower refractive index than the cladding 12, the refractive index of the marker core 13 may be the same as the refractive index of the cladding 12. If there is a low refractive index region around the marker core 13 that has a lower refractive index than the cladding 12, the refractive index of the marker core 13 may be lower than the refractive index of the cladding 12 and higher than the low refractive index region.

[0032] The refractive index of the marker core 13 is preferably smaller than that of the communication core 11. This configuration can reduce crosstalk between the two. The marker core 13 may be formed of, for example, silica glass doped with a dopant (such as germanium) that increases the average refractive index.

[0033] The cross-sectional shape of the marker core 13 is circular. The outer diameter of the marker core 13 is smaller than the outer diameter of the communication core 11, for example. The cross-sectional area of ​​the marker core 13 is smaller than the cross-sectional area of ​​the communication core 11, for example.

[0034] The marker core 13 is formed above the communication core 11A and spaced apart from the communication core 11A. The marker core 13 is formed outside the rectangular region having the four communication cores 11 as vertices. In other words, the marker core 13 is formed outside the rectangular region having the centers of gravity (area centers of gravity) of the four communication cores 11 as vertices.

[0035] The center 13a of the marker core 13 is located off the symmetry axes A1 to A4. Therefore, the marker core 13 is formed at a position that breaks the symmetry (axial symmetry) of the multiple communication cores 11. This makes it easier to identify the communication cores 11 in the multi-core optical fiber 10. Therefore, when splicing the multi-core optical fiber 10 with another optical product (for example, a multi-core optical fiber), it becomes easy to determine the splicing position.

[0036] The marker core 13 is formed at a position that disrupts the symmetry (rotational symmetry about the central axis C1) of the multiple communication cores 11. This makes it easier to identify the communication cores 11 in the multi-core optical fiber 10.

[0037] When the marker core 13 is disposed in a position where its center 13a is off the symmetry axes A1 to A4, and when at least one of the symmetry axes A1 and A2 passes through the marker core 13, crosstalk can be reduced and the communication core 11 can be easily identified for the following reason. The symmetry axes A1 and A2 are symmetry axes that do not pass through the communication core 11. Because the symmetry axes A1 and A2 are far from the communication core 11, when the marker core 13 is positioned so as to overlap with at least one of the symmetry axes A1 and A2, the marker core 13 can be disposed at a position away from the communication core 11. This makes it possible to reduce crosstalk. Because the center 13a of the marker core 13 is disposed in a position where it is off the symmetry axes A1 to A4, the communication cores 11 can be easily identified from each other for the reason described above.

[0038] The normalized propagation constant b of the marker core 13 satisfies the following conditions (A) and (B): (A) b≦0.2 in the communication wavelength band of the communication core 11. (B) b>0.2 at least at one wavelength in a region shorter than the communication wavelength band of the communication core 11.

[0039] To satisfy conditions (A) and (B), a core design is required in which the normalized propagation constant b is small in the communication wavelength band and large in the region of wavelengths shorter than the communication wavelength band.

[0040] Using the step approximation method, we calculated the relationship between the normalized propagation constant b of the communication core and the wavelength of light propagating through the communication core. The results are shown in Figure 2. Figure 2 is a schematic diagram showing the relationship between the normalized propagation constant b and wavelength for the first to third communication cores, which have different diameters. As shown in Figure 2, the normalized propagation constant b tends to decrease as the wavelength increases.

[0041] Fig. 3 is a schematic diagram showing the relationship between the normalized propagation constant and wavelength. The horizontal axis of Fig. 3 represents the wavelength of light incident on the communication core 11 or the marker core 13. The vertical axis of Fig. 3 represents the normalized propagation constant b.

[0042] In Figure 3, in Example 1, b > 0.2 in a portion of the inspection wavelength range, which is shorter than the communication wavelength band, and b ≤ 0.2 in the entire range of communication wavelengths. Therefore, Example 1 satisfies conditions (A) and (B). In Example 2, b > 0.2 in the entire range of inspection wavelengths, and b ≤ 0.2 in the entire range of communication wavelengths. Therefore, Example 2 satisfies conditions (A) and (B).

[0043] In contrast, in Example 3, b is less than 0.2 over the entire range of test wavelengths, so condition (B) is not satisfied. In Example 4, b is greater than 0.2 over the entire range of communication wavelengths, so condition (A) is not satisfied.

[0044] The following test was conducted to verify the relationship between the normalized propagation constant b and wavelength. A multi-core optical fiber was fabricated having a reference communication core and first to third communication cores. The diameter of the first communication core was 0.2 times that of the reference communication core. The diameter of the second communication core was 0.3 times that of the reference communication core. The diameter of the third communication core was 0.5 times that of the reference communication core. In other words, the core diameter magnifications of the first to third communication cores when the reference communication core is used as a reference were 0.2 times, 0.3 times, and 0.5 times, respectively.

[0045] For each communication core, the normalized propagation constant b was calculated when light with a wavelength of 650 nm, which is in a wavelength range shorter than the communication wavelength band, was used as the test light. In this example, the wavelength of 650 nm is the test wavelength. For the first communication core (core diameter magnification 0.2 times), the normalized propagation constant b was 0.047. For the second communication core (core diameter magnification 0.3 times), the normalized propagation constant b was 0.247.

[0046] Light with a wavelength of 650 nm was input into this multi-core optical fiber, and the amount of output light was determined as the "light propagation amount." The first communication core output a very small amount of light, but the second communication core output a sufficient amount of light. The results are shown in Table 1. These results show that when light with a wavelength of 650 nm was used, a sufficient amount of light propagation could be ensured in communication cores with a normalized propagation constant b greater than 0.2.

[0047] Next, for the third communication core (core diameter magnification 0.5 times), the normalized propagation constant b was calculated when light of wavelengths 1310 nm and 1383 nm, which are in the communication wavelength band (for example, 1310 nm to 1550 nm), was used. The normalized propagation constant b when light of wavelength 1310 nm was used was 0.205. The normalized propagation constant b when light of wavelength 1383 nm was used was 0.173.

[0048] When light with a wavelength of 1310 nm was incident on the third communication core, a sufficient amount of light was emitted. On the other hand, when light with a wavelength of 1383 nm was incident on the third communication core, it was found that the amount of light emitted was very small. The results are shown in Table 1. These results show that when light with wavelengths of 1310 nm and 1383 nm, which are in the communication wavelength band, is used, the amount of light propagated is small in communication cores with a normalized propagation constant b of 0.2 or less.

[0049]

[0050] The wavelength (marker core identification wavelength) of the marker core 13 at which b>0.2 is preferably shorter than the cutoff wavelength of the communication core 11 (the wavelength at which LP mode light, which is one order higher than the LP mode of light of wavelengths in the communication wavelength band propagating through the communication core 11, can propagate through the communication core 11). Specifically, when the communication core 11 complies with ITU-T, the wavelength (inspection wavelength) at which b>0.2 is preferably 1530 nm or less (see, for example, G.654) or 1260 nm or less (see, for example, G.652). ITU-T is the International Telecommunication Union Telecommunication Standardization Sector.

[0051] It is preferable that the wavelength at which b>0.2 of the marker core 13 (marker core identification wavelength) is a wavelength shorter than the cutoff wavelength of the communication core 11, because the LP mode, which is one order higher, is not used for communication, so there is no problem even if crosstalk occurs.

[0052] The propagation loss at the wavelength where b>0.2 of the marker core 13 (the marker core identification wavelength) is preferably 10 dB / km or less. If the transmission loss can be kept within this range, it becomes easy to identify the marker core 13.

[0053] The light of the wavelength (marker core identification wavelength) of the marker core 13 where b>0.2 may be visible light (360 nm or more and less than 830 nm). With this configuration, the light emitted from the marker core 13 can be visually confirmed, making it easy to identify the marker core 13.

[0054] The wavelength of light (marker core identification wavelength) of the marker core 13 where b>0.2 may be 800 nm or more and 950 nm or less. The wavelength of light used in multimode fiber communication is sometimes 800 nm or more and 950 nm or less. Therefore, optical devices using this wavelength band are widely used. In the multi-core optical fiber 10, the marker core 13 can be identified using this optical device.

[0055] It is desirable that the refractive index profile of the marker core 13 and the refractive index profile of the communication core 11 have the same shape by adjusting the diameter (core diameter) of the marker core 13. For example, assume that the relative refractive index difference Δ of the marker core 13 and the relative refractive index difference Δ of the communication core 11 are the same, and the diameter of the marker core 13 is smaller than the diameter of the communication core 11. When the diameter of the marker core 13 is increased to be the same as the diameter of the communication core 11, it is desirable that the refractive index profile of the marker core 13 have the same shape as the refractive index profile of the communication core 11.

[0056] According to this configuration, the marker cores 13 can be fabricated using a preform containing a core material having the same refractive index profile as the communication cores 11. Since there is no need to prepare a core material specifically for the marker cores, the productivity of the multi-core optical fiber 10 can be improved.

[0057] Marker core 13 has propagation characteristics that comply with the standard specifications for optical propagation characteristics when the cutoff wavelength is made the same as the cutoff wavelength of one of communication cores 11 by adjusting the diameter of marker core 13. The standard specifications are preferably any one of G. 651, G. 652, G. 653, G. 654, G. 655, G. 656, and G. 657 defined by ITU-T.

[0058] It is preferable that at least one of the light propagation characteristics of the marker core 13 conforms to the standard specifications for light propagation characteristics when the cutoff wavelength is made the same as the cutoff wavelength of one of the communication cores 11 by adjusting the diameter of the marker core 13.

[0059] The refractive index profile of the marker core 13 preferably complies with the standard specifications for the propagation characteristics of light when the cutoff wavelength is set to the same as the cutoff wavelength of one of the communication cores 11 by adjusting the diameter of the marker core 13. The refractive index profile of the marker core 13 preferably has propagation characteristics that comply with the same standard specifications as those of the communication core 11 described above.

[0060] The optical propagation characteristics of the marker core 13 preferably satisfy the range of the standard specification for at least one of the mode field diameter (MFD), macrobend loss, zero-dispersion wavelength, and dispersion slope. It is more preferable that the optical propagation characteristics of the marker core 13 satisfy the range of the standard specification for two or more of the MFD, macrobend loss, zero-dispersion wavelength, and dispersion slope. It is even more preferable that the optical propagation characteristics of the marker core 13 satisfy the range of the standard specification for all of the MFD, macrobend loss, zero-dispersion wavelength, and dispersion slope.

[0061] When the refractive index profile of the marker core 13 conforms to the standard specification by adjusting the diameter of the marker core 13, the marker core 13 can be fabricated using a preform containing a core material having the same refractive index profile as the communication core 11. Since there is no need to prepare a core material dedicated to the marker core, the productivity of the multi-core optical fiber 10 can be improved.

[0062] The relative refractive index difference Δ of the marker core 13 does not have to be completely the same as the relative refractive index difference Δ of the communication core 11. For example, the relative refractive index difference Δ of the marker core 13 may correspond to any one of the following (i) to (iv).

[0063] (i) The relative refractive index difference Δ of the marker core 13 is included in the range from the minimum value to the maximum value of the relative refractive index differences Δ of the multiple communication cores 11 (11A to 11D).

[0064] (ii) The relative refractive index difference Δ of the marker core 13 is within the range from minus 10% to plus 10% of the minimum value of the relative refractive index differences Δ of the multiple communication cores 11 (11A to 11D). That is, the relative refractive index difference Δ of the marker core 13 is within the range from 0.9 × the minimum relative refractive index difference Δmin to 1.1 × the maximum relative refractive index difference Δmax.

[0065] (iii) The relative refractive index difference Δ of the marker core 13 is within a range of ±15% of the average value of the relative refractive index differences Δ of the multiple communication cores 11 (11A to 11D). That is, the relative refractive index difference Δ of the marker core 13 is within a range from 0.85 × the average relative refractive index difference Δavg to 1.15 × the average relative refractive index difference Δavg.

[0066] (iv) When the standard deviation of the relative refractive index differences Δ of the multiple communication cores 11 (11A to 11D) is σ, the relative refractive index difference Δ of the marker core 13 is within the range from the minimum value minus 1σ to the maximum value plus 1σ of the relative refractive index differences Δ of the multiple communication cores 11. In other words, the relative refractive index difference Δ of the marker core 13 is within the range from the minimum relative refractive index difference Δmin-1σ to the maximum relative refractive index difference Δmax+1σ.

[0067] When the relative refractive index difference Δ of the marker core 13 falls under any of the above-mentioned (i) to (iv), it may be possible to manufacture the marker core 13 using a preform containing a core material having a refractive index profile similar to that of the communication core 11. Therefore, the productivity of the multi-core optical fiber 10 can be improved.

[0068] It is desirable that the mode field diameter MFD of the marker core 13 at the wavelength where b>0.2 (marker core identification wavelength) of the marker core 13 and the OCT satisfy "MFD / 2<OCT." OCT is the distance from the center 13a of the marker core 13 to the outer peripheral surface of the cladding 12. In other words, OCT refers to the shortest distance from the center 13a of the marker core 13 to the outer peripheral surface of the cladding 12. With this configuration, light leakage in the marker core 13 is suppressed. Therefore, the identification ability of the marker core 13 can be improved.

[0069] In the multi-core optical fiber 10, the diameter of the marker core 13 may be smaller than the diameter of the communication core 11. The core pitch (center-to-center distance) between the marker core 13 and the communication core 11 closest to the marker core 13 is defined as P1. Comparing the MFD / 2 at the wavelength (marker core identification wavelength) where b>0.2 of the marker core 13 with the core pitch P1, it is preferable that MFD / 2>core pitch P1. With this configuration, even if the diameter of the marker core 13 is small, the communication core 11 can be easily visually recognized due to crosstalk between the marker core 13 and the communication core 11 during inspection. This makes it easy to identify the marker core 13.

[0070] The diameter of the marker core 13 is preferably larger than the marker core identification wavelength (inspection wavelength). With this configuration, the inspection light can be easily guided through the marker core 13.

[0071] [Method for identifying markers in multi-core optical fibers] A method for connecting multi-core optical fibers including a method for identifying markers in multi-core optical fibers according to an embodiment will be described. The method for identifying markers in multi-core optical fibers according to the present embodiment is a method for identifying marker cores 13 of a multi-core optical fiber 10 when connecting the multi-core optical fiber 10 to another multi-core optical fiber.

[0072] <End Face Opposing Step> The multi-core optical fiber 10 (see FIG. 1) and another multi-core optical fiber (optical product) are placed on respective mounting tables, and the end face of the multi-core optical fiber 10 is made to face the end face of the other multi-core optical fiber.

[0073] In this case, inspection light including a wavelength (marker core identification wavelength, inspection wavelength) that satisfies the normalized propagation constant b>0.2 is incident on the multi-core optical fiber 10, and the output light is observed. The marker core identification wavelength may be, for example, a wavelength in the visible light region (360 nm or more and less than 830 nm) or 800 nm or more and 950 nm or less.

[0074] The marker cores 13 allow the inspection light to propagate through them, and therefore a sufficient amount of emitted light can be obtained. Therefore, the marker cores 13 can be easily confirmed by capturing an image of the end face of the multi-core optical fiber 10 with a camera. Therefore, when splicing the multi-core optical fiber 10 with another multi-core optical fiber, it is easy to determine the splicing position.

[0075] <Core Alignment Step> The optical axes of the communication cores in the multi-core optical fiber 10 and other multi-core optical fibers are adjusted.

[0076] <Fusion Step> The end face of the multi-core optical fiber 10 is fused to the end face of another multi-core optical fiber, thereby enabling the multi-core optical fiber 10 to be connected to the other multi-core optical fiber.

[0077] [Effects of the multi-core optical fiber and marker identification method according to the embodiment] In the multi-core optical fiber 10 according to the embodiment, the normalized propagation constant b of the marker core 13 is b≦0.2 in the communication wavelength band of the communication core 11. Therefore, the amount of light propagating through the marker core 13 is small in the communication wavelength region. Therefore, crosstalk between the marker core 13 and the communication core 11 can be suppressed in the communication wavelength region.

[0078] The normalized propagation constant b of the marker core 13 satisfies b>0.2 at least at one wavelength in a region shorter than the communication wavelength band of the communication core 11, so that a sufficient amount of light can be propagated in the marker core 13 during inspection. This makes it easy to check the marker core 13. This makes it easy to determine the splicing position when splicing the multi-core optical fiber 10 to another multi-core optical fiber.

[0079] The technical scope of the present disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present disclosure. The multi-core optical fiber 10 shown in Fig. 1 has four communication cores 11, but the number of communication cores 11 is not particularly limited. The number of communication cores 11 may be plural (any number equal to or greater than two). The number of communication cores 11 may be, for example, six, eight, twelve, etc. In the multi-core optical fiber 10 shown in Fig. 1 , the outer diameters of the multiple communication cores 11 are the same, but the outer diameters of the multiple communication cores 11 do not have to be the same. For example, two or more of the multiple communication cores 11 may have different outer diameters.

[0080] 1 has one marker core 13, but the number of marker cores 13 is not particularly limited. There may be a plurality (any number equal to or greater than two) of marker cores 13. In the multi-core optical fiber 10 shown in Fig. 1 , the outer diameter of the marker core 13 is smaller than the outer diameter of the communication core 11, but the outer diameter of the marker core 13 may be the same as or larger than the outer diameter of the communication core 11.

[0081] In the multi-core optical fiber 10 shown in Figure 1, the centers 11a of the multiple communication cores 11 are arranged so as to be line-symmetric with respect to the symmetry axes A1 to A4, but the multiple communication cores 11 may be arranged so as to be line-symmetric with respect to at least one symmetry axis.

[0082] In the multi-core optical fiber 10 shown in Figure 1, the center 13a of the marker core 13 is located off the symmetry axes A1 to A4, but the center 13a of the marker core 13 may also be located off at least one of the symmetry axes.

[0083] In the above-described method for connecting multi-core optical fibers, an example has been given in which the multi-core optical fiber 10 is connected to another multi-core optical fiber, but the connection destination of the multi-core optical fiber 10 is not limited to a multi-core optical fiber, and it may also be an optical product such as a transceiver device.

[0084] The marker identification method according to the embodiment can be applied to a manufacturing method of an optical connector (a method of assembling an optical connector). When connecting the optical connector to the tip end of a multi-core optical fiber for manufacturing (assembling) the optical connector, it is necessary to align the relative position of a key (e.g., a flange portion) of the optical connector with the multi-core optical fiber. At this time, by observing the emitted light from the marker core according to the above-described marker identification method, the relative position of the optical connector and the multi-core optical fiber can be grasped. Therefore, the optical connector and the multi-core optical fiber can be connected with high accuracy.

[0085] The marker identification method according to the embodiment can also be applied to an optical connector inspection method. To inspect an optical connector, the optical connector is connected to the tip end of a multi-core optical fiber. At this time, by observing the light emitted from the marker core according to the above-described marker identification method, the relative positions of the optical connector and the multi-core optical fiber can be grasped. Therefore, the optical connector and the multi-core optical fiber can be connected with high accuracy.

[0086] In addition, within the scope of the present disclosure, it is possible to replace the components in the above-described embodiments with well-known components as appropriate, and the above-described embodiments and variations may be combined as appropriate.

[0087] 10... multi-core optical fiber, 11... communication core, 12... cladding, 13... marker core

Claims

1. A multi-core optical fiber comprising a plurality of communication cores, a cladding surrounding the communication cores, and at least one marker core provided in the cladding, wherein the normalized propagation constant b of the marker core satisfies b≦0.2 in the communication wavelength band of the communication cores, and b>0.2 at least at one wavelength in a region shorter than the communication wavelength band of the communication cores.

2. The multi-core optical fiber according to claim 1, wherein the wavelength at which b>0.2 is satisfied is a wavelength shorter than the wavelength at which light in an LP mode that is one order higher than the LP mode of light in the communication wavelength band that propagates through the communication core can propagate through the communication core.

3. The multi-core optical fiber according to claim 1 or 2, wherein the wavelengths at which b>0.2 is satisfied are wavelengths in the visible light region.

4. The multi-core optical fiber according to any one of claims 1 to 3, wherein at least one of the optical propagation characteristics of the marker core complies with a standard specification for optical propagation characteristics when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of one of the communication cores by adjusting the diameter of the marker core.

5. A multi-core optical fiber according to any one of claims 1 to 4, wherein the relative refractive index difference of the marker core is included in the range from the maximum value to the minimum value of the relative refractive index differences of the plurality of communication cores.

6. The multi-core optical fiber according to any one of claims 1 to 5, wherein a mode field diameter MFD of the marker core at the wavelength where b > 0.2 and a distance OCT from the center of the marker core to the outer peripheral surface of the cladding satisfy MFD / 2 < OCT.

7. The multi-core optical fiber according to any one of claims 1 to 6, wherein the diameter of the marker core is larger than the inspection wavelength.

8. A multi-core optical fiber according to any one of claims 1 to 7, wherein the plurality of communication cores are arranged so that their centers are linearly symmetric with respect to at least one axis of symmetry, and the center of the marker core is located off the axis of symmetry.

9. A marker identification method for a multi-core optical fiber, the method comprising: a multi-core optical fiber having a plurality of communication cores, a cladding surrounding the communication cores, and at least one marker core provided in the cladding, the method comprising: using the multi-core optical fiber in which the normalized propagation constant b of the marker core satisfies b≦0.2 in the communication wavelength band of the communication cores and b>0.2 for at least one wavelength in a region shorter than the communication wavelength band of the communication cores; and inputting light including the wavelength at which b satisfies b>0.2 into the multi-core optical fiber, and observing the output light.

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