Fiber optic cable

Abstract

The purpose of the present disclosure is to make it possible to apply LPG over the entire length of an optical fiber without hampering the work for removing the optical fiber.　The present disclosure is an optical fiber cable (90) in which are gathered one or more optical fibers (91) through which at least two modes propagate, the optical fiber cable (90) being characterized by comprising a linear material (94) that is in contact with at least one of the optical fibers (91), wherein the thickness of the linear material (94) changes periodically in the longitudinal direction of the linear material (94).

Description

[Technical Field]

[0001] This disclosure relates to optical fiber cables. [Background technology]

[0002] In recent years, in order to increase transmission capacity in optical fiber communication systems, research has been conducted on technologies that enable the transmission of different information in each mode using optical fibers that can propagate multiple modes in a single core (hereinafter referred to as FMF (Few Mode Fiber)) (Non-Patent Literature 1).

[0003] Each mode propagating FMF has a different propagation constant, except for some mode combinations, resulting in different transmission delay times. Furthermore, since the signals propagated by each mode of FMF are mixed at the receiver, it is necessary to independently reconstruct the mixed signal using Multi Input Multi Output (MIMO) technology (Non-Patent Literature 2). When the transmission delay times of each mode differ, a mixed signal of the mode with the smallest delay time and the mode with the largest delay time is reconstructed using a digital signal processor (DSP). The circuit size of the digital signal processor increases as the difference in delay times between modes increases, so it is desirable that the difference in delay times is small.

[0004] One method for reducing the difference in delay time is long-period optical fiber grating (LPG) (Non-Patent Document 3). LPG involves applying periodic lateral pressure to the optical fiber, which periodically changes the refractive index of the optical fiber core and promotes coupling between modes. In particular, when applying LPG along the entire length of an optical fiber cable, there is a method of equipping the cable with a mode coupling section for applying periodic lateral pressure (Patent Document 2). Embodiment 5 of this document describes an embodiment example in a non-slot structure. In that embodiment example, a sheet with irregularities is embedded inside the cable so as to make even contact with all the mounted optical fibers. [Prior art documents] [Chartered documents]

[0005]

Patent Document 1

Patent document 2

Non-licensed literature

[0006]

Non-licensed literature 1

Non-licensed Document 2

[0007] However, in the structure described in Patent Document 2, the sheet must be wide to ensure contact with all optical fibers, especially when a large number of optical fiber cables are mounted. Furthermore, because the sheet is used to accommodate all optical fibers, when connecting optical fibers within the cable, the optical fibers are covered by the sheet, significantly impairing the ease of extracting the optical fibers. Therefore, there was a need for an optical fiber cable that could be coated with LPG along its entire length and that allowed for easy extraction of the optical fibers.

[0008] This disclosure aims to enable the application of LPG along the entire length of an optical fiber without interfering with the optical fiber extraction process. [Means for solving the problem]

[0009] The optical fiber cable disclosed herein is An optical fiber cable comprising at least one optical fiber that propagates in at least two modes, It comprises a linear material in contact with at least one optical fiber, The thickness of the linear material changes periodically with respect to the longitudinal direction of the linear material.

[0010] The linear material may be formed by combining at least two linear materials into one body. In this case, the thickness of the at least two linear materials may be constant in the longitudinal direction. Further, a tension may be applied to at least one of the at least two linear materials, and the tension applied to one linear material may be greater than that of the other linear materials.

[0011] The position of the optical fiber may vary randomly in the longitudinal direction of the optical fiber cable. Further, the linear material may function as a bundle tape that bundles at least one optical fiber. Further, the linear material may be configured using yarn. For example, the linear material may have water absorption.

[0012] Note that the above disclosures can be combined as much as possible.

Advantages of the Invention

[0013] According to the present disclosure, it is possible to realize an optical fiber cable that can maintain a small-diameter structure while applying LPG over the entire length. Therefore, the optical fiber cable of the present disclosure can apply LPG over the entire length of the optical fiber without hindering the work of taking out the optical fiber.

Brief Description of the Drawings

[0014] [Figure 1] It is an example of the configuration of an optical fiber cable according to Embodiment Example 1, where (a) shows a side view and (b) shows a cross-sectional view. [Figure 2] It is an example of the shape of a linear material, where (a) shows a side view and (b) shows a cross-sectional view. [Figure 3] An example of the linear material of the present disclosure using a bundle tape is shown. [Figure 4] It is an example of the configuration of an optical fiber cable according to Embodiment Example 2, where (a) shows a side view and (b) shows a cross-sectional view. [Figure 5]This is an example of the configuration of an optical fiber cable according to Embodiment Example 3, where (a) is a side view and (b) is a cross-sectional view. [Modes for carrying out the invention]

[0015] Embodiments of this disclosure will be described in detail below with reference to the drawings. However, this disclosure is not limited to the embodiments shown below. These examples are illustrative, and this disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In this specification and in the drawings, components with the same reference numerals refer to the same components.

[0016] (Example of Embodiment 1) Figure 1 shows a first embodiment of the present disclosure. It is an optical fiber cable 90 comprising an optical fiber unit 92 which is an assembly of at least one optical fiber 91, each comprising an optical fiber 91 which propagates at least two or more modes, and an outer sheath 93 which covers the optical fiber unit 92. Thus, the optical fiber cable 90 has a slotless structure that eliminates slot rods, which allows for a smaller diameter and lighter weight (see, for example, Patent Document 1).

[0017] The cable core 96 of the optical fiber cable 90 comprises linear material 94 assembled inside the outer sheath 93 by being longitudinally attached to or twisted with the optical fiber unit 92, and the thickness of the cross-section of the linear material 94 is denoted as t. The thickness t is determined by the length of a line segment that crosses the cross-section of the linear material 94 and changes periodically with respect to the longitudinal direction of the linear material 94. This line segment length may be the maximum length in the cross-section or the average length.

[0018] Figure 2 shows an example of the shape of the linear material 94. The linear material 94 has a thickness of t in the longitudinal direction. A Region L A And the thickness is t B Region L B and are arranged alternately. 94a has a thickness of t A The 2A-2A' cross-sectional shape is shown, and 94b has a thickness of tB The cross-sectional shape of 2B-2B’ is shown. Thus, in this embodiment, the thickness of the linear material 94 is periodically changed with the period P.

[0019] In this embodiment, the cross-sectional shapes 94a and 94b are rectangular, and an example in which the lengths of two opposite sides are periodically changed is shown, but the present disclosure is not limited thereto. For example, the cross-sectional shapes 94a and 94b are rectangular, and all sides may be periodically changed. Further, the cross-sectional shape of the linear material 94 is not limited to a rectangle, and any shape capable of applying a lateral pressure to the optical fiber 91 such as a circle can be adopted.

[0020] Here, the period of the thickness t is desirably set to a value at which efficient coupling occurs between the modes of the FMF. Let the propagation constants of the propagation modes of the FMF be β L , β M . Then, the period P of the thickness t at which the strongest mode coupling can be obtained can be expressed by Equation (1). P = 2π / (β L - β M ) (1) Therefore, by applying a lateral pressure to the optical fiber 91 using the change in the cross-sectional shapes 94a and 94b with the period P corresponding to the propagation constants β L and β M of the optical fiber 91, efficient coupling between the modes of the FMF can be generated.

[0021] Since the linear material 94 is assembled in the optical fiber unit 92, it abuts against at least one optical fiber 91 included in the optical fiber unit 92. By periodically changing the thickness of the linear material 94 with the period P, a lateral pressure corresponding to the periodically changing thickness t can be applied to the optical fiber 91. Moreover, since the linear material 94 does not cover the optical fiber unit 92, it is possible to maintain good workability for taking out the optical fiber 91 when connecting the optical fiber cable 90.

[0022] When multiple optical fibers 91 are implemented, it is desirable to apply periodic lateral pressure to all optical fibers 91. The following methods can be used to apply a periodic lateral pressure to all optical fibers 91 along their entire length. (i) One method is to attach one linear material lengthwise to each optical fiber. (ii) Alternatively, in a fiber optic cable with multiple optical fibers to be implemented, a fiber optic tape is formed by integrating multiple optical fibers, and the fiber optic tape is deformed to cover the periphery of the linear material and attached vertically.

[0023] The position of the optical fibers 91, which are arranged on the cross-section of the thin-diameter, high-density optical fiber cable 90, changes randomly in the longitudinal direction. Therefore, the optical fibers 91 that come into contact with the linear material 94 are replaced depending on their position in the longitudinal direction, and as a result, a periodically changing lateral pressure can be applied to all the optical fibers in the manner of (ii).

[0024] Furthermore, when the optical fiber cable 90 is damaged, water that seeps in from the damaged area can travel through the inside of the optical fiber cable, resulting in water damage over a long distance. As a countermeasure, a yarn that prevents water ingress may be applied. Here, yarn is a string-like member made by braiding fiber threads, and in the case of an optical fiber cable, it is used as an intermediary so that the cross-sectional shape of the cable core 96, which includes the optical fiber unit 92 and the linear material 94, approaches a circular shape, and is also used to enhance the water-sealing performance of the optical fiber cable 90 by sprinkling water-absorbing powder inside. In this embodiment, the yarn may also serve as the linear material 94. In this case, the linear material 94 may have any function that the yarn has, such as water absorption.

[0025] In addition, a colored bundle tape 95 may be wrapped around the optical fiber unit 92 to identify it. The bundle tape 95 may also serve as the linear material 94.

[0026] Figure 3 shows an example of bundle tape 95. In the figure, 95a has a thickness of t. A The 3A-3A' cross-sectional shape is shown, and 95b has a thickness of t B This shows the 3B-3B' cross-sectional shape. For example, the bundle tape 95 has a thickness of t, similar to the linear material 94 shown in Figure 2. A Region L A And the thickness is t B Region L B The and are arranged alternately. This allows the thickness of the bundle tape 95 to be periodically changed with a period P.

[0027] (Example of Embodiment 2) Figure 4 is a diagram showing the configuration of an optical fiber cable illustrating a second embodiment of the present disclosure. The linear material 94 of Embodiment 1 is formed by twisting together two linear materials 94-1 and 94-2. 94a has a thickness of t A The 4A-4A' cross-sectional shape is shown, and 94b has a thickness of t B The 4B-4B' cross-sectional shape is shown. The linear material 94 of this disclosure may have its thickness periodically changed by combining linear materials 94-1 and 94-2 having a constant cross-sectional shape.

[0028] With the above configuration, a helical shape is formed on the surface of the twisted linear materials 94-1 and 94-2. Since the optical fiber 91 is in contact with the side of the helix in one direction, a periodic change in lateral pressure occurs in the optical fiber 91. Therefore, the same effect as in Embodiment Example 1 can be achieved.

[0029] Furthermore, by changing the thickness of the twisted linear materials 94-1 and 94-2 and the number of twists, the helical shape allows for arbitrary setting of the period P during which the lateral pressure changes.

[0030] Furthermore, the cross-sectional shapes of the linear materials 94-1 and 94-2 are identical and may be constant in the longitudinal direction. For this reason, in this embodiment, the optical fiber cable described in Embodiment Example 1, in which the period of the thickness t is set to a desired value, can be realized using only one type of linear material.

[0031] (Example of Embodiment 3) Figure 5 is a structural diagram of an optical fiber cable showing a third embodiment of the present disclosure. In the configurations of Embodiments 1 and 2, it was necessary to increase the mounting density (the ratio of the cross-sectional area occupied by the optical fiber units 92 and linear material 94 mounted on the cable core to the cross-sectional area of ​​the cable core 96) in order to achieve a sufficient magnitude of periodic lateral pressure. However, increasing the mounting density presented the challenge of increasing the cabling loss of the optical fiber 91.

[0032] In this embodiment, in the configuration of Embodiment Example 2, the tension T1 of one of the twisted linear materials 94-1 and 94-2 is set to be greater than the tension T2 of the other twisted linear material 94-2.

[0033] With the above configuration, the linear material 94-1 with high tension takes on a straight shape, and the other linear materials 94-2 spirally surround it. As a result, the linear material 94-2 contacts the optical fiber 91 in a wavy manner, and a larger lateral pressure can be obtained compared to the configuration of Embodiment Example 2, even if the number of linear materials 94-1 and 94-2 is the same. In other words, the desired periodic lateral pressure can be achieved while keeping the proportion of the linear material 94 that occupies the cross-sectional area inside the optical fiber cable 90 small. [Explanation of Symbols]

[0034] 91: Fiber Optic 92: Fiber Optic Unit 93: Outer cover 94, 94-1, 94-2: Linear materials 95: Bundle Tape 96: Cable core

Claims

1. An optical fiber cable comprising at least one optical fiber that propagates in at least two modes, A linear material in contact with at least one of the optical fibers, An outer covering that surrounds the optical fiber and the linear material, It is equipped with, The aforementioned linear material is formed by twisting together multiple linear materials to form a single unit. By twisting together the multiple linear materials, mode coupling is periodically generated in the longitudinal direction of the linear materials at a period corresponding to the propagation constant of the propagation mode of the optical fiber. A fiber optic cable characterized by the following features.

2. Tension is applied to at least one of the plurality of linear materials, and the tension applied to the one linear material is greater than that applied to the other linear materials. The optical fiber cable according to feature 1.

3. The thickness of the aforementioned plurality of linear materials is constant in the longitudinal direction. The optical fiber cable according to feature 2.

4. The plurality of linear materials are twisted together at a period such that the difference in delay time between modes propagating in the optical fiber becomes small. The optical fiber cable according to feature 1.

5. The linear material functions as a bundle tape that bundles at least one or more optical fibers together. The optical fiber cable according to feature 1.

6. The linear material has water absorption properties The optical fiber cable according to feature 1.

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