Optical fiber bundle and fan-in / fan-out device

Abstract

This optical fiber bundle comprises: a cylindrical capillary which is formed of glass and has a first opening and a second opening; and a plurality of optical fibers which each comprise a core and a cladding, and in which the cladding has, on the tip side, a small-diameter part that is thinner than the rear-end side, and the small-diameter part enters the capillary from the first opening. The cladding is formed of glass, and the capillary has a joint part where the cladding is fused in a fixed range from the second opening side. The distance M between the cores of the adjacent optical fibers in the joint part and the diameter D of the small-diameter part which is not joined to the capillary satisfy M / D≥0.92.

Description

Fiber optic bundles and fan-in / fan-out devices

[0001] The present invention relates to an optical fiber bundle. It also refers to a fan-in / fan-out device using the optical fiber bundle. This application claims priority to Japanese Patent Application No. 2024-014407, filed February 1, 2024, the contents of which are incorporated herein by reference.

[0002] In communication networks using optical fibers, the transmission capacity is increasing year by year. Currently used communication networks using single-core fiber (SCF), which has a single core in one optical fiber, are approaching their transmission capacity limit, and the application of multi-core fiber (MCF), which has multiple cores in one optical fiber, is expected to become a reality.

[0003] In order to apply MCF to an already operating communication network, a fan-in / fan-out (FIFO) device is required as a conversion device between MCF and SCF when connecting to existing communication equipment or amplifiers that use SCF, and research and development is currently underway.

[0004] One known method for manufacturing a FIFO device is a technique called melt drawing, in which a bundle of SCFs is inserted into a cladding tube (capillary) and heated while being pulled in the axial direction to fusion-bond the SCF cladding to the capillary. Patent Document 1 aims to suppress the diameter reduction rate of the core at the tip side of the capillary connected to the MCF by reducing the degree of drawing during melt drawing, thereby suppressing the occurrence of crosstalk.

[0005] International Publication No. 2022 / 130974

[0006] In addition to the crosstalk problem described in Patent Document 1, there are several other problems with fusion drawing. On the other hand, for example, optical cable systems installed on the ocean floor are expected to be used for long periods of time without failure due to the difficulty of repair, and in such applications, durability of the fusion-spliced ​​joint between the glass cladding and the capillary is essentially essential. The inventors conducted further studies in light of the above circumstances and completed the present invention.

[0007] An object of the present invention is to provide an optical fiber bundle that has a structure in which a capillary and a clad are fused together, and that is capable of suppressing degradation of communication characteristics.

[0008] A first aspect of the present invention is an optical fiber bundle including a cylindrical capillary made of glass and having a first opening and a second opening, and a plurality of optical fibers each having a core and a cladding, the cladding of which has a thin narrow-diameter portion at its front end that is thinner than its rear end, the thin narrow-diameter portion extending into the capillary from the first opening. The cladding is made of glass, and the capillary has a splice portion at which the cladding is fused to a certain area from the second opening. In this optical fiber bundle, the distance M between the cores of adjacent optical fibers at the splice portion and the diameter D of the thin narrow-diameter portion not spliced ​​to the capillary satisfy M / D≧0.92.

[0009] A second aspect of the present invention is a fan-in / fan-out device comprising the optical fiber bundle according to the first aspect and a multi-core fiber having the same number of cores as the number of optical fibers in the optical fiber bundle. In this fan-in / fan-out device, an end face of the second opening of the capillary and an end face of the multi-core fiber are joined by fusion splicing.

[0010] According to the present invention, it is possible to provide an optical fiber bundle that has a structure in which the capillary and the cladding are fused together, and in which degradation of communication characteristics is suppressed.

[0011] Fig. 1 is a schematic diagram showing an optical fiber bundle according to one embodiment of the present invention. Fig. 2 is a cross-sectional view taken along line II in Fig. 1. Fig. 3 is a cross-sectional view taken along line II-II in Fig. 1. Fig. 4 is a graph showing the relationship between the degree of cladding melting in a fusion splicing process and the circularity of the core. Fig. 5 is a schematic diagram showing a fan-in / fan-out device according to one embodiment. Fig. 6 is a schematic cross-sectional view showing a multi-core fiber associated with the fan-in / fan-out device.

[0012] An embodiment of the present invention will be described with reference to Fig. 1 to Fig. 6. Fig. 1 is a schematic diagram showing an optical fiber bundle (hereinafter, sometimes simply referred to as a "bundle") 1 according to this embodiment. The bundle 1 according to this embodiment includes four optical fibers 10 and a capillary 20 through which the optical fibers 10 are passed. One end of each of the four optical fibers 10 is inserted into the capillary 20 and integrated with the capillary 20, thereby forming the bundle 1 in which the four optical fibers 10 are bundled.

[0013] Each optical fiber 10 has a known basic structure including a core and a glass cladding surrounding the core, and the cladding at the first end that enters the capillary 20 is thinner than the cladding behind it. As a result, the optical fiber 10 has a thin-diameter portion 11 at the tip, and the portion behind the thin-diameter portion 11 has a larger diameter than the thin-diameter portion 11. The thin-diameter portion 11 can be produced by thinning the cladding by processing the optical fiber, such as etching or cutting. The diameter of the thin-diameter portion 11 is, for example, less than 125 μm, and can also be approximately 40 to 50 μm.

[0014] The capillary 20 is a cylindrical member made of glass. The small diameter portion 11 of each optical fiber 10 enters the capillary 20 through a first opening 21 of the capillary 20. The position of the tip of the small diameter portion 11 is generally aligned with the end face of the second opening 22 in the capillary 20.

[0015] 2 is a cross-sectional view taken along line II in FIG. 1. The optical fiber 10 and the capillary 20 are spliced ​​by fusion within a certain range from the second opening 22. In the following description, the region of the bundle 1 where the optical fiber 10 and the capillary 20 are fused is referred to as the splice 30. As shown in FIG. 2, in the splice 30, in each optical fiber 10, a portion of the cladding 14 surrounding the core 13 and a portion of the inner surface of the capillary 20 are fused and formed integrally. Furthermore, adjacent claddings 14 of the optical fibers 10 are also fused and formed integrally. The length of the splice 30 can be set as appropriate. The length of the splice 30 can be set to, for example, approximately 2 mm.

[0016] Fig. 3 is a cross-sectional view taken along line II-II in Fig. 1. In the capillaries 20 that do not correspond to the splice portions 30, the optical fibers 10 and the capillaries 20 are arranged close to each other and may even be in contact with each other in some places, but are not spliced ​​together.

[0017] Considering the risk of core deformation and other problems inherent in the melt-drawing technique, the inventors first considered fusion splicing the optical fiber 10 and the capillary 20 without applying any pulling force in the axial direction when forming the splice 30. However, during this process, they found a number of prototypes that had poor polarization mode dispersion (PMD) despite not being pulled in the axial direction at all.

[0018] As the inventors continued their research, they discovered that the cause was that the degree of melting of the integrated cladding and capillary exceeded a certain level, causing minute deformations in the core, resulting in a decrease in circularity compared to when the optical fiber was manufactured.

[0019] When the circularity of the core decreases, polarization becomes more likely due to the occurrence of long and short diameter portions, and the PMD value generally increases. The fact that the circularity of the core decreases even when the capillary and cladding are fused together without stretching is extremely difficult to predict from the details of the fusion process, and was discovered for the first time by the inventors.

[0020] When the clads of four optical fibers whose outer surfaces are in contact are melted and integrated, the cores of the four optical fibers gradually approach each other as the clads are integrated. When two optical fibers of the same diameter are in contact with each other at their outer surfaces without being integrated, the distance between the cores of the two optical fibers (the distance between the central axes of each core) is usually the same as the diameter of the optical fiber 10. Therefore, by comparing the distance M between the cores of two adjacent optical fibers 10 at the splice 30 with the diameter D of the small-diameter portion 11 of the optical fiber 10 located within the capillary 20 and obtaining the change in distance associated with integration, the degree of melting of the clad during the fusion process can be determined.

[0021] Figure 4 is a graph showing the relationship between the degree of cladding melting during the fusion process and the core circularity. The core noncircularity, shown on the vertical axis of Figure 4, was calculated using the following method. The core's outer periphery is obtained from a radial cross-sectional image of the optical fiber, such as that shown in Figures 2 and 3. The obtained outer periphery is then approximated by an ellipse to obtain the core's approximate cross-sectional shape, and the difference df between the major axis and minor axis of the approximate cross-sectional shape is obtained. In parallel, the core's cross-sectional area is obtained from the radial cross-sectional image, and the diameter Id of a perfect circle having the same area is calculated. Based on the obtained df and Id, the ratio of the two, df / Id, is calculated and used as the noncircularity. That is, if the radial cross-sectional shape of the core is a perfect circle, the noncircularity is 0%, and the noncircularity value increases as the circularity decreases.

[0022] The horizontal axis in Figure 4 shows the ratio M / D, which is the relationship between the core distance M between two adjacent optical fibers 10 and the diameter D of the thin-diameter portion 11 of the optical fiber 10 inserted into the capillary 20. In other words, M / D = 1 corresponds to a state in which the outer surfaces of two adjacent optical fibers are in contact without their claddings being integrated, i.e., the state before fusion splicing. Therefore, the practical upper limit of M / D is 1. In actual manufacturing, although a core with a perfectly circular cross-sectional shape is manufactured, it is difficult to achieve a perfectly circular cross-sectional shape. For this reason, in Figure 4, the non-circularity value is just over 1% even when M / D = 1.

[0023] The inventors conducted prototype fabrication and measurements while varying the degree of cladding melting during the fusion process, and found that, as shown in Figure 4, the core noncircularity hardly changed until the M / D ratio fell below 0.97. When the M / D ratio fell below 0.97, the noncircularity value began to increase, and when the M / D ratio fell below 0.95, the noncircularity exceeded 3%. However, by performing fusion while maintaining the M / D ratio at 0.92 or greater, it was found that the core noncircularity at the splice 30 could be kept within 5%. The inventors' investigations revealed that the PMD of an optical fiber with a core noncircularity of 5% was approximately 0.005 ps, which is approximately one-tenth of the general PMD requirement of 0.05 ps. Therefore, it was believed that if the noncircularity was 5% or less, bundles and FIFO devices that fully satisfied the required PMD level could be reliably manufactured, even if various loads acted on the optical fiber in subsequent processes related to the fabrication of bundles and FIFO devices.

[0024] Based on the above findings, the inventors formed the splice 30 so that the M / D value is 0.92 or more, and completed the bundle 1 according to this embodiment. The M / D value at the splice 30 can be set to a desired value by appropriately setting the heating temperature, heating time, etc. in the fusion process. As an example, the inventors performed a fusion process using an oxyhydrogen torch to heat a capillary with an outer diameter of 200 μm and four optical fibers with a narrow diameter portion with an outer diameter of 45 μm, and were able to obtain a bundle satisfying the above M / D value under heating conditions of 240 sccm of hydrogen and 40 sccm of oxygen for 500 seconds.

[0025] As described above, the bundle 1 of this embodiment has excellent performance because the cladding and capillaries are fusion-bonded, making it robust enough to withstand long-term installation in harsh environments such as the seabed, while also maintaining good communication characteristics such as polarization characteristics.

[0026] 5 shows a FIFO device 100 according to this embodiment. The FIFO device 100 can be formed by bringing the end face of the multicore fiber 50 into contact with the end face of the capillary 20 of the bundle 1 on the second opening 22 side, and aligning and joining the four cores 13 of the bundle 1 with the cores of the multicore fiber 50 by fusion splicing. Studies by the inventors have confirmed that when the noncircularity of the cores in the bundle 1 is within 3%, the occurrence of connection loss can be suitably suppressed in connection with the multicore fiber 50. From this perspective, it can be said that the M / D value of the bundle 1 is preferably 0.95 or more.

[0027] 6 shows an end face of a multi-core fiber 50 according to an example. The multi-core fiber 50 has a configuration in which a plurality of cores 51 are arranged in a single cladding 52. When forming a FIFO device using the bundle 1, the optical fiber 10 may be selected in consideration of the number and diameter of the cores of the multi-core fiber to be connected, and the bundle 1 may be fabricated in consideration of the positional relationship of the cores in the multi-core fiber.

[0028] 5 and 6 , in the FIFO device, the diameter D1 of the capillary 20 (i.e., the diameter of the end face on the side of the second opening 22) is larger than the diameter of the multicore fiber 50. In this way, the capillary 20 becomes thicker at the connection portion between the bundle 1 and the multicore fiber 50, and the heat capacity becomes larger, which makes it easier to control the degree of melting of the cladding in the fusion process between the bundle 1 and the multicore fiber 50, and the occurrence of splice loss can be suppressed. For example, when the multicore fiber to be spliced ​​has a diameter of 125 μm and four cores, the above-mentioned effect can be fully expected when the diameter of the capillary 20 is 180 μm or more. In this case, when the diameter of the capillary 20 exceeds 220 μm, the amount of heat required for fusion increases excessively, which may reduce the effect. Therefore, it is preferable that the diameter of the capillary 20 is 220 μm or less. The above-described effect is achieved by the capillaries having a thick second opening, and therefore, the same effect can be obtained even if a bundle is formed using, for example, truncated cone-shaped capillaries whose wall thickness gradually increases from the first opening side to the second opening side, or capillaries whose only diameter is large at the joint 30. Moreover, the same effect can be obtained even if a bundle is formed using truncated cone-shaped capillaries whose wall thickness gradually decreases from the first opening side to the second opening side, or capillaries whose only diameter is small at the joint 30.

[0029] Although each embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and configuration changes and combinations within the scope of the gist of the present invention are also included. Some examples of changes are shown below, but these are not all inclusive, and other changes are also possible. These changes can be applied to any embodiment, and two or more of them may be combined as appropriate.

[0030] The bundle according to the present invention is not limited to the four optical fibers described above, but may be composed of three or fewer optical fibers, or five or more optical fibers.

[0031] In the bundle according to the present invention, if the capillaries are short, the claddings of the optical fibers may be fused to the inner surface of the capillaries along the entire length of the capillaries. In this case, the bundle has a splice portion along the entire length of the capillaries, and there are no unspliced ​​thin-diameter portions inside the capillaries. Therefore, the M / D can be calculated using the thin-diameter portions located outside the capillaries.

[0032] In the bundle according to the present invention, if the capillary is long, the inner diameter of the capillary on the second opening side may be increased according to the outer diameter of the optical fiber 10, and may cover the non-thin diameter portion.

[0033] According to the present invention, it is possible to provide an optical fiber bundle that has a structure in which the capillary and the cladding are fused together, and in which degradation of communication characteristics is suppressed.

[0034] 1...optical fiber bundle, 10...optical fiber, 11...thin diameter portion, 13...core, 14...clad, 20...capillary, 21...first opening, 22...second opening, 30...junction portion, 50...multicore fiber, 100...fan-in / fan-out device

Claims

1. An optical fiber bundle comprising: a cylindrical capillary made of glass and having a first opening and a second opening; and a plurality of optical fibers each having a core and a cladding, the cladding having a thin portion at the front end that is thinner than the rear end, the thin portion entering the capillary from the first opening; wherein the cladding is made of glass, the capillary has a splice where the cladding is fused within a certain range from the second opening, and the distance M between the cores of adjacent optical fibers at the splice and the diameter D of the thin portion not spliced with the capillary satisfy M / D≧0.

92.

2. The optical fiber bundle according to claim 1, wherein the M / D is 0.95 or more.

3. The optical fiber bundle according to claim 1, wherein the M / D is 0.97 or more.

4. The optical fiber bundle according to any one of claims 1 to 3, wherein the diameter of the small diameter portion is 125 μm or less.

5. A fan-in / fan-out device comprising: an optical fiber bundle according to any one of claims 1 to 4; and a multi-core fiber having the same number of cores as the number of optical fibers in said optical fiber bundle, wherein an end face of said second opening of said capillary and an end face of said multi-core fiber are joined by fusion splicing.

6. The fan-in / fan-out device according to claim 5, wherein the diameter of the end face of the second opening is larger than the diameter of the multi-core fiber.

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