Optical fiber cable

Abstract

An optical fiber cable (10) comprises at least one optical fiber unit (11) and at least one linear material (12) that are twisted together. The optical fiber unit (11) includes a plurality of optical fibers (14) through which two or more modes of light propagate, and the spiral formed by the optical fiber unit (11) has a radius determined by the bending stiffness and the outer diameter of each of the optical fiber unit (11) and the linear material (12). The curvature of the optical fiber unit (11) has a value determined by the radius of the spiral and the twisting pitch of the optical fiber unit (11) and the linear material (12).

Description

Fiber optic cable

[0001] This disclosure relates to optical fiber cables.

[0002] Non-patent document 1 discloses a technology that enables the transmission of different information in each mode using multimode optical fiber (hereinafter referred to as FMF) in order to increase transmission capacity. Patent document 1 discloses a non-slot cable structure that increases the mounting density of optical fibers.

[0003] Each mode propagating FMF has a different transmission delay time (group delay), with the exception of some mode combinations. Since the signals propagated by each mode of FMF are mixed at the receiver, it is necessary to independently reconstruct the mixed signals using Multi Input Multi Output (MIMO) technology. The larger the differential modal delay (DMD) between modes, the larger the decoding circuit becomes. Non-patent document 2 describes the mode-dependent loss, mode-dependent gain, and noise figure in a Multi Input Multi Output (MIMO) system employing spatial division multiplexing (SDM).

[0004] The DMD of FMF changes by applying curvature to the optical fiber. Depending on the structure of the optical fiber, the absolute value of the DMD can be reduced. On the other hand, changing the curvature increases optical loss (Patent Document 3). In this regard, Non-Patent Document 1 focuses on the curvature of the optical fiber in high-density cables and proposes a method for controlling the DMD in 2LP-mode-graded index (GI) optical fibers.

[0005] Patent No. 4774337

[0006] D. Soma et al., “10.16 Peta-bit / s Dense SDM / WDM transmission over Low-DMD 6-Mode 19-Core Fiber Across C+L Band,” 2017 European Conference on Optical Communication (ECOC), 2017, pp. 1-3, doi: 10.1109 / ECOC.2017.8346082.PJ Winzer et al. al., “Mode-dependent loss, gain, and noise in MIMO-SDM systems,” 2014 The European Conference on Optical Communication (ECOC), 2014, pp. 1-3, doi: 10.1109 / ECOC.2014.6963888.M. Kikuchi et al., “A method for differential modal delay reduction by using curvature of few-mode optical fiber in high-density cable,” Optical Fiber Communication (OFC) Conference 2023, M4B.1, 2023, doi: 10.1364 / OFC.2023.M4B.1

[0007] To give a desired curvature to an optical fiber unit containing multiple FMFs, one can consider wrapping a linear material around the optical fiber unit and applying tension to the linear material, as shown in Non-Patent Document 3. However, in order to maintain the tension applied to the linear material, it is necessary to ensure sufficient friction between the constituent materials of the optical cable, such as optical fibers and linear materials. In this case, limitations tend to arise in the design of the friction coefficient of the constituent materials and the inner diameter of the optical cable.

[0008] This disclosure aims to provide an optical fiber cable that can impart a desired curvature to an optical fiber without maintaining the tension applied to the linear material.

[0009] An optical fiber cable according to one aspect of the present disclosure comprises at least one twisted optical fiber unit and a linear material, wherein the optical fiber unit includes a plurality of optical fibers from which two or more modes of light propagate, and the helix formed by the optical fiber unit has a radius determined by the bending stiffness and outer diameter of the optical fiber unit and the linear material, respectively, and the curvature of the optical fiber unit has a value determined by the radius of the helix and the twisting pitch of the optical fiber unit and the linear material.

[0010] According to this disclosure, it is possible to provide an optical fiber cable that can be given a desired curvature to an optical fiber without maintaining the tension applied to the linear material.

[0011] Figure 1 is a side view of an optical fiber cable according to one embodiment of the present disclosure. Figure 2 is a cross-sectional view of an example of an optical fiber cable according to one embodiment of the present disclosure. Figure 3A is a cross-sectional view taken along line A-A, showing the position of one optical fiber relative to a linear material when the optical fiber unit itself is twisted. Figure 3B is a cross-sectional view taken along line B-B, showing the position of one optical fiber relative to a linear material when the optical fiber unit itself is twisted. Figure 4A is a cross-sectional view of a first modified example of the linear material. Figure 4B is a cross-sectional view of a second modified example of the linear material. Figure 5 is a cross-sectional view of another example of an optical fiber cable.

[0012] The optical fiber cable 10 according to the embodiment of this disclosure will be described below. In each figure, common parts are denoted by the same reference numerals, and redundant explanations will be omitted. For the convenience of explanation, the Z direction will be defined below. The Z direction is the extension direction of the optical fiber cable 10, and is also the extension direction of each helix formed by the optical fiber unit 11 and the linear material 12.

[0013] Figure 1 is a side view of an example of an optical fiber cable 10. Figure 2 is a cross-sectional view of an example of an optical fiber cable 10. As shown in Figure 1, the optical fiber cable 10 comprises at least one optical fiber unit 11, a linear material 12, and an outer sheath 13. The optical fiber unit 11 and the linear material 12 are twisted together.

[0014] The optical fiber unit 11 is a so-called fiber bundle containing multiple optical fibers 14. The multiple optical fibers 14 may be bundled together only with linear material 12, or they may be partially bonded to each other.

[0015] The optical fiber 14 is a so-called fuse-mode optical fiber (FMF) or multimode optical fiber (MMF) through which two or more modes of light propagate. The outer surface of the cladding of the optical fiber 14 may be covered with at least one layer of coating (not shown). The optical fiber 14 may be a graded-index optical fiber or a step-index optical fiber. However, the reduction of DMD (intermode group delay difference) by applying bending is more effectively obtained with the former than with the latter. The optical fiber 14 may also be a multicore optical fiber having multiple cores.

[0016] The linear material 12 is a member that extends in the Z direction and is made of the same material as the optical fiber 14, or a material (for example, resin) having a coefficient of thermal expansion close to that of the optical fiber 14. Depending on its bending stiffness, the linear material 12 may be spirally stretched together with the optical fiber unit 11, or it may be linearly stretched relative to the spiral optical fiber unit 11. The cross-sectional shape of the linear material 12 perpendicular to the Z direction is, for example, circular as shown in Figure 2. However, this cross-sectional shape is not limited to a circle and may be other shapes.

[0017] When the optical fiber unit 11 and the linear material 12 are formed from different materials, a difference in the coefficient of linear expansion generally occurs between them. When this difference in coefficient of linear expansion exists, the twisted state of the optical fiber unit 11 and the linear material 12 changes depending on the temperature, and the curvature ρ imparted to the optical fiber unit 11 is prone to change. Therefore, it is desirable to use materials with a small difference in the coefficient of linear expansion for the optical fiber unit 11 and the linear material 12.

[0018] For example, the linear material 12 may be composed of multiple optical fibers other than the optical fibers constituting the optical fiber unit 11, or multiple dummy fibers (not shown) made of the same material as the optical fiber 14. In this case, the coefficient of thermal expansion of the linear material 12 will be equal to that of the optical fiber unit 11.

[0019] The outer sheath 13 is provided on the outer circumference of the twisted optical fiber unit 11 and the linear material 12, and protects them. The outer sheath 13 may be formed to cover the outer circumference of the optical fiber unit 11 and the linear material 12 without any gaps, or it may be formed in the shape of a hollow cylinder extending in the Z direction.

[0020] In other words, the optical fiber cable 10 according to this embodiment may have a small-diameter, high-density structure in which optical fibers 14 are densely bundled, or it may have a loose tube structure in which a gap is created between either the optical fiber unit 11 or the linear material 12 and the inner surface of the outer sheath 13. The optical fiber cable 10 may also have a slot structure. In this case, the optical fiber unit 11 and the linear material 12 are twisted together and housed in slots (not shown) of a slot rod housed in the outer sheath 13.

[0021] The helix formed by the optical fiber unit 11 has a radius determined by the bending stiffness and outer diameter of the optical fiber unit 11 and the linear material 12, respectively. Furthermore, the curvature ρ of the optical fiber unit 11 has a value determined by the radius R of the helix and the twisting pitch P of the optical fiber unit 11 and the linear material 12.

[0022] For example, the bending rigidity and outer diameter of the optical fiber unit 11 are EI f , 2R f This is how it is written. On the other hand, the bending rigidity and outer diameter of the linear material 12 are expressed as EI, respectively. b , 2R b This is how it is expressed. Furthermore, the twisting pitch of the optical fiber unit 11 and the linear material 12 is denoted as P (see Figure 1). The twisting pitch P is the distance in the Z direction that the spiral of the optical fiber unit 11 (linear material 12) travels when it makes one rotation.

[0023] The radius R of the spiral of the optical fiber unit 11 can be expressed by the following equation (1) using the bending stiffness and outer diameter described above. Furthermore, the curvature ρ assigned to the optical fiber unit 11, based on the spiral and twisting pitch P of the optical fiber unit 11, can be expressed by the following equation (2). In other words, by twisting the optical fiber unit 11 and the linear material 12, each having the above-mentioned bending rigidity and outer diameter, together at a twisting pitch P, the optical fiber unit 11 can be given a curvature ρ represented by equation (2), and each optical fiber 14 can be given a curvature based on the curvature ρ. That is, by controlling the curvature of each optical fiber 14, the DMD of each optical fiber 14 can be controlled.

[0024] According to this embodiment, by performing the above-described twisting without applying tension to the linear material 12, the optical fiber unit 11 can be deformed into a spiral shape, and curvature can be imparted to each optical fiber 14. Therefore, design to ensure frictional force within the optical fiber cable 10 becomes unnecessary. Generally, the twisting pitch P is sufficiently larger than the radius R of the spiral. Therefore, by setting the radius R to a large value, the curvature ρ can be increased.

[0025] Figure 3A is a cross-sectional view taken along line A-A, showing the position of a single optical fiber 14 relative to the linear material 12 when the optical fiber unit 11 itself is twisted. Figure 3B is a cross-sectional view taken along line B-B, showing the position of a single optical fiber 14 relative to the linear material 12 when the optical fiber unit 11 itself is twisted.

[0026] As shown in Figures 3A and 3B, the optical fiber unit 11 may be twisted itself. That is, the optical fiber unit 11 may be twisted around its own central axis. If the optical fiber unit 11 itself is not twisted, the curvature of each optical fiber 14 will vary slightly depending on its position within the optical fiber unit 11.

[0027] For example, let's focus on the nth optical fiber 14n that makes up the optical fiber unit 11. Also, let D be the distance from the center of the linear material 12 to the optical fiber 14n. f,nis expressed as. When there is no twist in the optical fiber unit 11 itself, the distance D f,n is constant. That is, the optical fiber 14n shown in FIG. 3A is located at the position indicated by the dotted line in FIG. 3B. Therefore, the curvature of the optical fiber 14n also takes a constant value according to its position. That is, depending on the degree of twisting of the optical fiber unit 11 and the linear material 12, the distance D f,n there may be a case where there coexist optical fibers 14 with a constantly small distance D f,n and optical fibers 14 with a constantly large distance D. As a result, the curvatures of the respective optical fibers 14 in the optical fiber unit 11 will have different values according to their positions.

[0028] On the other hand, when the optical fiber unit 11 itself is twisted, the position of the optical fiber 14n on the cross-section of the optical fiber unit 11 changes according to the position of the cross-section in the Z direction. That is, as shown in FIGS. 3A and 3B, the distance D f,n changes according to each position in the Z direction. Therefore, the spiral shapes of the respective optical fibers 14 become substantially the same, and the variation in the curvature generated in each optical fiber 14 can be suppressed or eliminated. Thus, the control of the decrease in DMD generated in each optical fiber 14 becomes easy.

[0029] FIG. 4A is a cross-sectional view of a first modified example of the linear material 12. FIG. 4B is a cross-sectional view of a second modified example of the linear material 12. As shown in FIGS. 4A and 4B, the linear material 12 may have a recess (first recess) 15 that contacts the optical fiber unit 11. The recess 15 may be provided on the side surface 12a of the linear material 12 as shown in FIG. 4A, or may be provided on the inner surface of a depressed portion (second recess) 16 formed on the side surface 12a as shown in FIG. 4B. In either case, the recess 15 is recessed toward the center of the linear material 12 with, for example, the same curvature as the outer periphery of the optical fiber unit 11.

[0030] The formation of the recess 15 increases the contact area between the linear material 12 and the optical fiber unit. This suppresses the lateral pressure applied to the optical fiber 14, thereby reducing optical loss. The recess 16 may also have a cross-sectional shape that accommodates the entire optical fiber unit 11. In this case, the entire optical fiber unit 11 is housed in the space formed by the recess 16. Therefore, even if the outer sheath 13 or other linear materials (not shown) approach the optical fiber unit 11, the possibility of contact between them and the optical fiber unit 11 is reduced, thereby suppressing optical loss due to lateral pressure caused by such contact.

[0031] Figure 5 is a cross-sectional view of another example of an optical fiber cable 10. As shown in Figure 5, at least one optical fiber unit 11 may include multiple optical fiber units 11. For example, as shown in Figure 5, two optical fiber units 11A and 11B are arranged around one linear material 12. Furthermore, the optical fiber units 11A and 11B and the linear material 12 are twisted together. That is, multiple optical fiber units 11 are arranged around one linear material 12, and these are twisted together. Multiple optical fiber units 11 share the linear material 12. Therefore, it is possible to suppress an increase in the number of linear materials 12 used.

[0032] Furthermore, the multiple optical fiber units 11 may be twisted together symmetrically around a single linear material 12. In other words, in a cross-section perpendicular to the Z direction, the multiple optical fiber units 11 may be arranged point-symmetrically around the linear material 12. Also, as long as this condition is met, the number of optical fiber units 11 may be three or more.

[0033] According to this example, the bending stiffness EI of the optical fiber unit 11 f The mutual effects can be canceled out or reduced. Therefore, the bending stiffness EI of the linear material 12 b The design conditions can be relaxed. f To minimize the impact, the symmetrically positioned optical fiber units 11 have the same bending rigidity EI f It is desirable to stop.

[0034] Each optical fiber unit 11 has the same bending rigidity EI f Consider the case where each optical fiber unit 11 has the same bending rigidity EI and is symmetrically twisted around the linear material 12. In this case, the effects of the bending rigidities of the optical fiber units 11 cancel each other out. Therefore, the linear material 12 extends linearly along the Z direction. At this time, the radius R of the helix of the optical fiber unit 11 can be expressed by the following equation (3) using the radius R of the optical fiber unit 11 f and the radius R of the linear material 12 b and the radius R of the linear material 12 Note that this equation (3) corresponds to the case where the bending rigidity EI in equation (1) is infinite. Therefore, it becomes possible to relax the design for the linear material 12 and give curvature to the optical fiber b and give curvature to the optical fiber

[0035] 10 Optical fiber cable 11 Optical fiber unit 12 Linear material 13 Outer covering 14 Optical fiber 15 Concave portion 16 Depression

Claims

1. An optical fiber cable comprising at least one optical fiber unit and a linear material twisted together, wherein the optical fiber unit includes a plurality of optical fibers through which two or more modes of light propagate, the helix formed by the optical fiber unit has a radius determined by the bending stiffness and outer diameter of the optical fiber unit and the linear material, and the curvature of the optical fiber unit has a value determined by the radius of the helix and the twisting pitch of the optical fiber unit and the linear material.

2. The optical fiber cable according to claim 1, wherein the optical fiber unit is itself twisted.

3. The optical fiber cable according to claim 1 or 2, wherein the linear material has a recess that contacts the optical fiber unit.

4. The optical fiber cable according to claim 1, wherein at least one optical fiber unit includes a plurality of optical fiber units arranged around the linear material, and the plurality of optical fiber units and the linear material are twisted together.

Images