Method for producing multicore optical fiber

Abstract

The present invention comprises: a step for preparing a plurality of communication core rods serving as communication cores, a cladding body serving as a cladding, and a marker core rod serving as a marker core; a step for combining the plurality of communication core rods, the cladding body, and the marker core rod to obtain an optical fiber preform; and a step for drawing the optical fiber preform to thereby obtain a multicore optical fiber equipped with a plurality of communication cores, a cladding surrounding the communication cores, and at least one marker core provided to the cladding. The marker core has a propagation characteristic conforming to a first standard relating to the propagation characteristics of light when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of any of the communication cores by adjusting the outside diameter of the marker core.

Description

Multi-core optical fiber manufacturing method

[0001] The present invention relates to a method for manufacturing a multi-core optical fiber. This application claims priority to Japanese Patent Application No. 2024-090619, 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 becomes easy to determine the splicing position. The multi-core optical fiber can be manufactured by, for example, a drilling method.

[0003] Japanese Patent No. 5267481

[0004] To manufacture the multi-core optical fiber, it is necessary to prepare a dedicated marker rod as a marker in addition to a cladding body that serves as the cladding and a plurality of core rods that serve as the cores, which results in a problem that productivity is lower than that of a multi-core optical fiber that does not have a marker. Therefore, there is a demand for a method that does not require the preparation of a dedicated marker rod even for a multi-core optical fiber provided with a marker, and that can manufacture a multi-core optical fiber with productivity similar to that of a multi-core optical fiber that does not have a marker.

[0005] An object of one aspect of the present invention is to provide a method for manufacturing a multi-core optical fiber that does not require the preparation of a dedicated marker rod even for a multi-core optical fiber provided with a marker.

[0006] A manufacturing method of a multi-core optical fiber according to a first aspect of the present invention is a manufacturing method of 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 manufacturing method including the steps of: preparing a plurality of communication core rods to become the communication cores, a cladding body to become the cladding, and a marker core rod to become the marker cores; obtaining an optical fiber preform by combining the plurality of communication core rods, the cladding body, and the marker core rod; and obtaining the multi-core optical fiber by drawing the optical fiber preform, wherein the marker core has propagation characteristics that comply with a first standard specification regarding optical propagation characteristics when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of any of the communication cores by adjusting the outer diameter of the marker core.

[0007] According to this method, by adjusting the outer diameter of the marker core, a core rod with the same specifications as a communication core rod can be used as the marker core rod, which eliminates the need to prepare a core material (core rod) specifically for the marker core, thereby improving the productivity of multi-core optical fibers.

[0008] In a second aspect of the present invention, in the method for manufacturing a multi-core optical fiber of the first aspect, the plurality of communication cores may have optical propagation characteristics that comply with a second standard, and the marker core may have optical propagation characteristics that comply with the first standard, which is the same as the second standard of the plurality of communication cores, when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of any one of the communication cores by adjusting the outer diameter of the marker core.

[0009] In a third aspect of the present invention, in the method for manufacturing a multi-core optical fiber according to the first or second aspect, the marker core rod may be produced by drawing a core rod having the same specifications as the communication core rod.

[0010] In a fourth aspect of the present invention, in the method for manufacturing a multi-core optical fiber according to any one of the first to third aspects, a normalized propagation constant b of the marker core may be b≦0.2 in the communication wavelength band of the communication core, and b>0.2 at least at one wavelength in a region of wavelengths shorter than the communication wavelength band of the communication core.

[0011] One aspect of the present invention provides a method for manufacturing a multi-core optical fiber that does not require the preparation of a dedicated marker rod even for a multi-core optical fiber provided with a marker.

[0012] FIG. 1 is a cross-sectional view perpendicular to the longitudinal direction of a multi-core optical fiber obtained by the manufacturing method of a multi-core optical fiber according to a first embodiment. FIG. 2 is a schematic view showing the relationship between a normalized propagation constant and wavelength. FIG. 3 is a schematic view showing the relationship between a normalized propagation constant and wavelength. FIG. 4 is an explanatory view of a process of preparing a communication core rod, a cladding body, and a marker core rod. FIG. 5 is an explanatory view of a process of combining a communication core rod, a cladding body, and a marker core rod. FIG. 6 is a schematic structural view of an optical fiber preform. FIG. 7 is a front view of a glass material unit as seen from the central axis direction. FIG. 8 is a configuration diagram of a drawing apparatus for manufacturing a multi-core optical fiber from an optical fiber preform. FIG. 9 is a cross-sectional view of a structure used in the manufacturing method of a multi-core optical fiber according to a second embodiment.

[0013] Hereinafter, a method for manufacturing a multi-core optical fiber according to an embodiment will be described with reference to the drawings.

[0014] [Multi-core optical fiber] Fig. 1 is a cross-sectional view perpendicular to the longitudinal direction of a multi-core optical fiber 10 obtained by the manufacturing method of a multi-core optical fiber according to the first 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The refractive index of the marker core 13 is preferably smaller than the refractive index of the communication core 11. This configuration can reduce crosstalk. 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.

[0027] 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.

[0028] The marker core 13 is formed above the communication core 11 A and spaced apart from the communication core 11 A. The marker core 13 is formed outside the rectangular area having the four communication cores 11 as vertices.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] It is preferable that the normalized propagation constant b of the marker core 13 satisfy 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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. b ≤ 0.2 in the entire range of communication wavelengths. Therefore, conditions (A) and (B) are satisfied. In Example 2, b > 0.2 in the entire range of inspection wavelengths. b ≤ 0.2 in the entire range of communication wavelengths. Therefore, conditions (A) and (B) are satisfied.

[0037] 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.

[0038] 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 were 0.2 times, 0.3 times, and 0.5 times, respectively.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043]

[0044] 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 that 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). 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.

[0045] 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 than the LP mode, is not used for communication, so there is no problem even if crosstalk occurs.

[0046] 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.

[0047] 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.

[0048] 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 set to 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.

[0049] 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.

[0050] According to this configuration, 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. For example, in the optical fiber preform 210, a core rod having the same specifications as the communication core rod 1 can be drawn to have a smaller diameter and used as the marker core rod 3 (see FIG. 6 ). Since there is no need to prepare a core material specifically for the marker core, the productivity of the multi-core optical fiber 10 can be improved.

[0051] Marker core 13 has propagation characteristics that comply with a standard (first standard) related to optical propagation characteristics when the cutoff wavelength is made the same as the cutoff wavelength of one of communication cores 11 by adjusting the outer diameter of marker core 13. The standard is preferably one of G. 651, G. 652, G. 653, G. 654, G. 655, G. 656, and G. 657 defined by ITU-T.

[0052] The plurality of communication cores 11 preferably have optical propagation characteristics that conform to the standard (second standard). The marker core 13 may have propagation characteristics that conform to the same standard as the plurality of communication cores 11 when the cutoff wavelength of the marker core 13 is made the same as the cutoff wavelength of one of the communication cores 11 by adjusting the outer diameter of the marker core 13.

[0053] 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 outer diameter of the marker core 13.

[0054] 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 made the same as the cutoff wavelength of one of the communication cores 11 by adjusting the outer 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.

[0055] 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.

[0056] If the optical propagation characteristics of the marker core 13 satisfy the range of at least one standard specification among the mode field diameter (MFD), macrobend loss, zero-dispersion wavelength, and dispersion slope, it may be possible to reuse unused communication core rods (e.g., defective communication core rods) as marker core rods by adjusting the outer diameter of the marker core rod. In this case, there is no need to prepare a dedicated core material (marker core rod). When a defective communication core rod is used, the rate of defective products can be reduced. As a result, the productivity of multi-core optical fibers can be increased.

[0057] If the optical propagation characteristics of the marker core 13 satisfy the ranges of the standard specifications for all of the mode field diameter (MFD), macrobend loss, zero-dispersion wavelength, and dispersion slope, even if there are no surplus communication core rods (e.g., defective communication core rods), the outer diameter of a core material manufactured as a communication core rod can be adjusted and used as a marker core rod. Therefore, there is no need to prepare a dedicated core material (marker core rod). This makes it possible to increase the productivity of multi-core optical fibers.

[0058] If the light propagation characteristics of the marker core 13 comply with the same standard as that of the communication core 11, there is no need to prepare a dedicated core material (marker core rod) even when only one type of communication core 11 is manufactured, which can increase the productivity of the multi-core optical fiber.

[0059] When the refractive index profile of the marker core 13 conforms to the standard 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. For example, in the optical fiber preform 210, a core rod having the same specifications as the communication core rod 1 can be drawn to have a thinner diameter and used as the marker core rod 3 (see FIG. 6 ). Since there is no need to prepare a core material specifically for the marker core, the productivity of the multi-core optical fiber 10 can be improved.

[0060] 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).

[0061] (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).

[0062] (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.

[0063] (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.

[0064] (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.

[0065] 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. For example, in the optical fiber preform 210, a core rod having the same specifications as the communication core rod 1 is drawn to have a thinner diameter, and this core rod can be used as the marker core rod 3 (see FIG. 6 ). Therefore, the productivity of the multi-core optical fiber 10 can be improved.

[0066] 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. With this configuration, light leakage in the marker core 13 is suppressed. Therefore, the identification ability of the marker core 13 can be improved.

[0067] 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. Therefore, it is easy to identify the marker core 13.

[0068] 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.

[0069] [Manufacturing method of multi-core optical fiber] (First embodiment) Fig. 4 is an explanatory diagram of a process of preparing a communication core rod 1, a cladding body 2, and a marker core rod 3. Fig. 5 is an explanatory diagram of a process of combining the communication core rod 1, the cladding body 2, and the marker core rod 3. Fig. 6 is a schematic structural diagram of an optical fiber preform 210. Fig. 7 is a front view of a glass material unit U1 as viewed from the central axis direction. Fig. 8 is a configuration diagram of a drawing device for manufacturing a multi-core optical fiber 10 from the optical fiber preform 210.

[0070] The manufacturing method of the multi-core optical fiber according to the first embodiment will be described with reference to Fig. 4 to Fig. 8. The manufacturing method of the multi-core optical fiber according to this embodiment includes a step of manufacturing an optical fiber preform 210, and a step of manufacturing the multi-core optical fiber 10 by drawing the optical fiber preform 210 (drawing step).

[0071] <Optical fiber preform> The following describes the configuration of the optical fiber preform 210. As shown in Fig. 6, the optical fiber preform 210 includes a plurality of communication core rods 1, a cladding body 2, a marker core rod 3, a tip connecting portion 4, and a dummy tube 5. C2 is the central axis of the cladding body 2.

[0072] The clad body 2 is formed in a cylindrical (rod-like) shape. The clad body 2 is formed of, for example, quartz glass. The clad body 2 has a plurality of main inner holes 2a and marker inner holes 2b formed therein. The main inner holes 2a and the marker inner holes 2b are formed parallel to the central axis C2 of the clad body 2. The first end 2c is one end of the clad body 2. The second end 2d is the other end of the clad body 2.

[0073] As shown in Figure 7, the main bores 2a are formed so that the centers 2e of the main bores 2a are line-symmetric with respect to the symmetry axes B1 to B4. The centers 2f of the marker bores 2b are located outside the symmetry axes B1 to B4. Therefore, the marker bores 2b are formed at positions that disrupt the symmetry (line symmetry) of the main bores 2a.

[0074] As shown in Fig. 6, the communication core rod 1 is formed in a cylindrical (rod-like) shape. The communication core rod 1 includes a region having a higher refractive index than the cladding body 2. The communication core rod 1 may be made of, for example, silica glass doped with a dopant (such as germanium) that increases the average refractive index. The multiple communication core rods 1 are each inserted into the main bore 2a.

[0075] The marker core rod 3 is formed in a cylindrical (rod-like) shape. The refractive index of the marker core rod 3 includes, for example, a region having a higher refractive index than the cladding body 2. The refractive index of the marker core rod 3 is determined so that the refractive index of the marker core 13 is different from the refractive index of the surrounding area in the multi-core optical fiber 10 (see FIG. 1 ). When the cladding body 2 has a low-refractive-index region surrounding the marker core rod 3 and having a lower refractive index than the cladding body 2, the refractive index of the marker core rod 3 may be the same as the refractive index of the cladding body 2. The refractive index of the marker core rod 3 may be lower than the refractive index of the cladding body 2 and higher than the low-refractive-index region.

[0076] The refractive index of the marker core rod 3 is desirably smaller than the refractive index of the communication core rod 1. This configuration can reduce crosstalk in the multi-core optical fiber 10 (see FIG. 1 ). The marker core rod 3 may be made of, for example, silica glass doped with a dopant (such as germanium) that increases the average refractive index.

[0077] The outer diameter of the marker core rod 3 is, for example, smaller than the outer diameter of the communication core rod 1. The marker core rod 3 is inserted into the marker inner hole 2b.

[0078] The plurality of communication core rods 1, the cladding body 2, and the marker core rod 3 constitute a "glass material unit U1." The cladding body 2 becomes the cladding 12 (see FIG. 1) in the multi-core optical fiber 10. The communication core rod 1 becomes the communication core 11 (see FIG. 1). The marker core rod 3 becomes the marker core 13 (see FIG. 1).

[0079] The communication core rod 1 and the marker core rod 3 may have a region at the outermost periphery that will become part of the cladding 12 (see FIG. 1).

[0080] The tip connecting portion 4 is made of, for example, quartz glass. The tip connecting portion 4 is connected to the first end 2c of the clad body 2 by welding. The tip connecting portion 4 is coaxial with the clad body 2. The tip connecting portion 4 closes the openings of all of the main bores 2a and marker bores 2b of the clad body 2. The outer diameter of the tip connecting portion 4 is preferably the same as the outer diameter of the clad body 2. The tip connecting portion 4 is, for example, a solid glass rod.

[0081] The dummy tube 5 is connected to the second end 2d of the glass material unit U1. The optical fiber preform 210 may be configured without the dummy tube 5.

[0082] <Process for manufacturing an optical fiber preform> The process for manufacturing an optical fiber preform 210 will be described with reference to Fig. 4 to Fig. 7. Note that the scale of the drawings used in the following description may be changed so that the components are recognizable.

[0083] 4, a plurality of communication core rods 1, a cladding body 2, and a marker core rod 3 are prepared. The marker core rod 3 may be produced by drawing a core rod having the same specifications as the communication core rod 1 to reduce its diameter.

[0084] (Process for Producing Optical Fiber Preform) As shown in Figures 5 and 7, a communication core rod 1, a cladding body 2, and a marker core rod 3 are combined. Specifically, the communication core rods 1 are inserted into the main inner holes 2a of the cladding body 2, respectively. The marker core rods 3 are inserted into the marker inner holes 2b. In this way, a glass material unit U1 is obtained.

[0085] As shown in Figure 6, a dummy tube 5 is provided at the second end 2d of the glass material unit U1. A glass rod is fused to the first end 2c of the glass material unit U1, and then the glass rod is fused. The glass rod remains at the first end 2c of the glass material unit U1 as the tip-connected portion 4. In this manner, an optical fiber preform 210 is obtained, which includes a communication core rod 1, a cladding body 2, and a marker core rod 3.

[0086] <Drawing Process> As shown in Fig. 8, the drawing apparatus 50 includes a preform lifting device 51 and a heating furnace 52. The preform lifting device 51 supports the optical fiber preform 210 so that it can be raised and lowered. The preform lifting device 51 has a lifting frame 51a and a lifting device main body 51b that raises and lowers the lifting frame 51a. The heating furnace 52 heats the lower end (tip) of the optical fiber preform 210. The heating furnace 52 is configured in a ring shape.

[0087] The dummy tube 5 of the optical fiber preform 210 is attached to the lifting frame 51a. The optical fiber preform 210 is supported by the lifting frame 51a with the tip connecting portion 4 facing downward. The lower end of the optical fiber preform 210 is introduced into the inner space 52a of the heating furnace 52.

[0088] The lower end of the optical fiber preform 210 is heated and softened by the heating furnace 52. The multi-core optical fiber 10 is drawn while the optical fiber preform 210 is lowered by the lifting frame 51a. At the lower end of the optical fiber preform 210, the cladding body 2 shrinks due to heating, so that the communication core rod 1 and the marker core rod 3 are integrated with the cladding body 2. As a result, the multi-core optical fiber 10 (see FIG. 1 ) in which the communication cores 11 and the marker cores 13 are integrated with the cladding 12 is obtained.

[0089] In the drawing device 50, a coating layer may be formed on the outer circumferential surface of the multi-core optical fiber 10 (see FIG. 1) by a coating unit (not shown).

[0090] [Method for connecting multi-core optical fibers] A method for connecting multi-core optical fibers obtained by the manufacturing method of a multi-core optical fiber according to the embodiment will be described. This method for connecting multi-core optical fibers is a method for connecting the multi-core optical fiber 10 to another multi-core optical fiber.

[0091] <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.

[0092] 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.

[0093] 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.

[0094] <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.

[0095] <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.

[0096] [Effects of the manufacturing method of the multi-core optical fiber according to the embodiment] In the manufacturing method of the multi-core optical fiber according to the embodiment, the marker core 13 has propagation characteristics that comply with the standard specifications regarding the propagation characteristics of light when the cutoff wavelength of the marker core 13 is made the same as the cutoff wavelength of any of the communication cores 11 by adjusting the outer diameter of the marker core 13. Therefore, by adjusting the outer diameter of the marker core 13, a core rod having the same specifications as the communication core rod 1 can be used as the marker core rod 3 (see FIG. 6 ). Therefore, there is no need to prepare a dedicated marker rod when manufacturing the multi-core optical fiber 10 provided with a marker. This makes it possible to improve the productivity of the multi-core optical fiber 10.

[0097] The marker core 13 preferably has propagation characteristics that comply with the same standard as those of the communication core 11 when the cutoff wavelength is made the same as that of any of the communication cores 11 by adjusting the outer diameter of the marker core 13. According to this configuration, by adjusting the outer diameter of the marker core 13, a core rod having the same specifications as the communication core rod 1 can be used as the marker core rod 3. Therefore, the productivity of the multi-core optical fiber 10 can be improved.

[0098] In the preparation step, the marker core rod 3 can be fabricated by drawing and thinning a core rod having the same specifications as the communication core rod 1. Therefore, the productivity of the multi-core optical fiber 10 can be further improved.

[0099] In the manufacturing method of the multi-core optical fiber according to this embodiment, the multi-core optical fiber 10 is obtained through the above-mentioned steps. The normalized propagation constant b of the marker cores 13 in the multi-core optical fiber 10 is b≦0.2 in the communication wavelength band of the communication cores 11. Therefore, the amount of light propagating through the marker cores 13 is small in the communication wavelength region. Therefore, crosstalk between the marker cores 13 and the communication cores 11 can be suppressed.

[0100] 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.

[0101] [Manufacturing method of multi-core optical fiber] (Second embodiment) Fig. 9 is a cross-sectional view of a structure used in a manufacturing method of a multi-core optical fiber according to a second embodiment. In this manufacturing method, a stack-and-draw method is adopted. The manufacturing method of a multi-core optical fiber according to this embodiment will be described with reference to Fig. 9.

[0102] 9, there are prepared a clad glass tube 81 (clad body), a plurality of core-coated rods 82 (communication core rods), one or a plurality of glass rods 83 (clad bodies), and a marker core rod 3. The core-coated rod 82 is a glass rod that will become the communication core 11 (see FIG. 1) and is coated with a clad glass layer.

[0103] (Process for Producing Optical Fiber Preform) The core coating rod 82, the glass rod 83, and the marker core rod 3 are inserted into the cladding glass tube 81. The glass rod 83 is inserted, for example, into the gap between the cladding glass tube 81 and the core coating rod 82. The cladding glass tube 81, the core coating rod 82, the glass rod 83, and the marker core rod 3 are heated and melted together to form an integrated optical fiber preform. This produces an optical fiber preform.

[0104] By using this optical fiber preform and drawing it in the same manner as in the manufacturing method of the first embodiment (see FIG. 8), a multi-core optical fiber can be manufactured.

[0105] The method for manufacturing a multi-core optical fiber according to this embodiment has the same effects as the method for manufacturing a multi-core optical fiber according to the first embodiment.

[0106] The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] In addition, 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, without departing from the spirit of the present invention.

[0112] The present disclosure provides a method for manufacturing a multi-core optical fiber that does not require the preparation of a dedicated marker rod, even for a multi-core optical fiber provided with a marker.

[0113] 1... communication core rod, 2... cladding body, 3... marker core rod, 10... multi-core optical fiber, 11... communication core, 12... cladding, 13... marker core, 210... optical fiber preform

Claims

1. A method for manufacturing 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, comprising the steps of: preparing a plurality of communication core rods to become the communication cores, a cladding body to become the cladding, and a marker core rod to become the marker cores; combining the plurality of communication core rods, the cladding body, and the marker core rod to obtain an optical fiber preform; and drawing the optical fiber preform to obtain the multi-core optical fiber, wherein the marker core has propagation characteristics that comply with a first standard specification for optical propagation characteristics when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of any of the communication cores by adjusting the outer diameter of the marker core.

2. The method for manufacturing a multi-core optical fiber according to claim 1, wherein the plurality of communication cores have optical propagation characteristics that comply with a second standard, and the marker core has optical propagation characteristics that comply with the first standard, which is the same as the second standard of the plurality of communication cores, when the cutoff wavelength of the marker core is made the same as the cutoff wavelength of any one of the communication cores by adjusting the outer diameter of the marker core.

3. The method for manufacturing a multi-core optical fiber according to claim 1 or 2, wherein the marker core rod is manufactured by drawing a core rod having the same specifications as the communication core rod.

4. The method for manufacturing a multi-core optical fiber according to any one of claims 1 to 3, wherein the normalized propagation constant b of the marker core satisfies b≦0.2 in the communication wavelength band of the communication core, and b>0.2 at least at one wavelength in a region shorter than the communication wavelength band of the communication core.

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