Fiber optic cable

Abstract

An optical fiber cable (1) comprises: an optical fiber tape (3) in which three or more optical fiber core wires (30) each propagating two or more modes are integrated; and a linear material (4) the outer periphery of which is covered with the optical fiber tape (3), the linear material (4) abutting on the optical fiber core wires (30) in the outer periphery. The width in a predetermined direction of the linear material (4) on a cross-section perpendicular to the longitudinal direction of the linear material changes with a predetermined period (P) in the longitudinal direction.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber cable.

Background Art

[0002] In recent years, in optical fiber communication systems, for the purpose of expanding the transmission capacity, a technique has been studied to transmit different information to each mode using a few-mode fiber (hereinafter, FMF) (Non-Patent Document 1).

[0003] In addition, in recent years, a small-diameter high-density structure with a non-slot cable structure has been developed and put into practical use (Patent Document 1). In this technology, since optical fiber cores are mounted in a high density in the cable core, the mounting density of optical fiber cores per cross-sectional area can be increased.

[0004] Since the propagation constants of each mode propagating in the FMF are different except for a combination of some modes, the delay times during transmission are different. Further, since the signals propagating in each mode of the FMF are mixed in the receiving section, it is necessary to independently restore the mixed signals using Multi Input Multi Output (hereinafter, MIMO) technology (Non-Patent Document 2). When the delay times during transmission of each mode are different, the mixed signal of the mode with the smallest delay time and the mode with the largest delay time is restored using a digital signal processor (hereinafter, DSP). Since the circuit scale of the DSP becomes larger as the difference in delay times between modes becomes larger, it is desirable that the difference in delay times is small.

[0005] As a method of reducing the difference in delay times, there is a long-period fiber grating (hereinafter, LPFG) (Non-Patent Document 3). The LPFG is to apply a periodic lateral pressure to the optical fiber, and by the lateral pressure, the refractive index of the optical fiber core can be periodically changed, and it is possible to promote the coupling between modes. In particular, when the LPFG is applied over the entire length of the optical fiber cable, there is a method of providing a mode coupling section for applying a periodic lateral pressure inside the cable (Patent Document 2).

Prior Art Documents

[0006]

Patent Document 1

Patent document 2

Non-licensed literature

[0007] [Non-licensed document 1] D. Soma et al., "10.16 Petabit / 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. [Non-licensed document 2] PJ Winzer, H. Chen, R. Ryf, K. Guan and S. Randel, “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. [Non-licensed document 3] H. Liu, H. Wen, R. Amezcua-Correa, P. Sillard and G. Li, “Reducing group delay spread in a 9-LPmode FMF using uniform long-periodgratings”, 2017 Optical Fiber Communications Conference and Exhibition (OFC), 2017, pp. 1-3. [Overview of the project] [Problems that the invention aims to solve]

[0008] Patent Document 2 describes a non-slotted cable structure in which a sheet with an uneven surface acting as an LPFG is embedded inside the cable so as to make even contact with all the mounted optical fibers. In the structure of Patent Document 2, especially when a large number of optical fibers are mounted inside the cable, the sheet must be wide in order for the LPFG to make contact with all the optical fibers, and the proportion of the sheet's cross-sectional area that occupies is large. Furthermore, since there is no mention of the arrangement of the LPFG and optical fibers on the sheet, it is not guaranteed that the LPFG will make even contact with all the optical fibers. Moreover, when connecting an optical fiber at the end of the cable to another optical fiber, the optical fibers inside the cable are covered by the sheet, which greatly impairs the ease of removing the optical fibers. Therefore, there has been a need for a thin-diameter optical fiber cable that applies an LPFG along its entire length and allows for easy removal of the optical fibers.

[0009] The purpose of this disclosure is to provide an optical fiber cable to which LPFG can be applied along the entire length of the optical fiber core without impairing the ease of extracting the optical fiber or the small diameter structure of the optical fiber cable. [Means for solving the problem]

[0010] The optical fiber cable according to this disclosure comprises an optical fiber tape in which three or more optical fiber cores, each propagating two or more modes, are integrated, and a linear material that covers the outer circumference of the optical fiber tape and contacts the optical fiber cores on the outer circumference, wherein the width of the linear material in a predetermined direction in a cross section perpendicular to the longitudinal direction of the linear material changes at a predetermined period in the longitudinal direction. [Effects of the Invention]

[0011] The optical fiber cable according to this disclosure allows for the application of LPFG along the entire length of the optical fiber core without impairing the ease of extracting the optical fiber or the small diameter structure of the optical fiber cable. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1(a) is a schematic diagram showing a side view of an optical fiber cable according to an embodiment, and Figure 1(b) is a schematic diagram showing a cross-section of the optical fiber cable. [Figure 2] Figure 2(a) is a side view of the linear material according to the embodiment, Figure 2(b) is a cross-sectional view of the large-diameter portion of the linear material according to the embodiment, and Figure 2(c) is a cross-sectional view of the small-diameter portion of the linear material according to the embodiment. [Figure 3] Figure 3(a) is a cross-sectional view of an optical fiber unit comprising an optical fiber tape consisting of four optical fiber cores and a linear material, and Figure 3(b) is a cross-sectional view of an optical fiber unit comprising an optical fiber tape consisting of five optical fiber cores and a linear material. [Figure 4] Figure 4(a) is a cross-sectional view showing the state before the coating of the optical fiber core undergoes elastic deformation due to a load applied to the optical fiber core by the large-diameter portion of the linear material, and Figure 4(b) is a cross-sectional view showing the state after the coating of the optical fiber core has undergone elastic deformation due to a load applied to the optical fiber core by the large-diameter portion of the linear material. [Figure 5] Figure 5(a) is a side view of the linear material according to the first modified example, Figure 5(b) is a cross-sectional view of the large-diameter portion of the linear material according to the first modified example, and Figure 5(c) is a cross-sectional view of the small-diameter portion of the linear material according to the first modified example. [Figure 6] FIG. 6(a) is a side view of the linear material according to the second modification, FIG. 6(b) is a cross-sectional view of the large-diameter portion of the linear material according to the second modification, and FIG. 6(c) is a cross-sectional view of the small-diameter portion of the linear material according to the second modification. [Figure 7] FIG. 7 is a perspective view of the optical fiber unit according to the third modification.

Embodiments for Carrying Out the Invention

[0013] The embodiments will be described with reference to the drawings. In the description of the drawings, the same reference numerals are assigned to the same parts and the description thereof is omitted.

[0014] (Embodiment) FIG. 1(a) is a schematic view showing a side surface of the optical fiber cable 1 according to the embodiment, and FIG. 1(b) is a schematic view showing a cross section of the optical fiber cable 1. The optical fiber cable 1 has an outer jacket 12 made of resin or the like covering the outer periphery of the cable core 11.

[0015] The cable core 11 includes one or more optical fiber units 2. The optical fiber unit 2 includes an optical fiber tape 3 in which at least three (four in FIG. 1) optical fiber cores 30 are integrated, and a linear material 4 covering the outer periphery so as to abut on each optical fiber core of the optical fiber tape 3. Each optical fiber core 30 of the optical fiber tape 3 is integrally formed by being intermittently adhered to an adjacent optical fiber core 30 with an adhesive or the like. By using the intermittently adhered type optical fiber tape 3, the optical fiber unit 2 can be stably produced, and the workability of taking out the optical fiber core 30 and the core mounting density of the optical fiber cable 1 can be improved.

[0016] The number of optical fiber cores 30 used for the optical fiber tape 3 may be three or more, and the number can be arbitrarily determined. Each optical fiber core 30 has a coating 32 made of resin or the like covering the outer periphery of an optical fiber 31 made of an optical material such as quartz. The optical fiber 31 uses an FMF (Few Mode Fiber) capable of propagating two or more modes.

[0017] As the cable core 11 of the optical fiber cable 1, a small-diameter high-density structure with a non-slot structure without a slot rod is used, enabling reduction in diameter and weight (see, for example, Patent Document 1). Further, the cable core 11 of the optical fiber cable 1 is not limited to the small-diameter high-density structure, and may be a slot structure, a loose tube structure, or the like.

[0018] FIG. 2(a) is a side view of the linear material 4. FIG. 2(b) is a cross-sectional view taken along line 2B-2B' of FIG. 2(a), and is a cross-sectional view of the large-diameter portion 41 of the linear material 4. FIG. 2(c) is a cross-sectional view taken along line 2C-2C' of FIG. 2(a), and is a cross-sectional view of the small-diameter portion 42 of the linear material 4. As shown in FIGS. 2(a) to 2(c), the cross-sectional shape of the linear material 4 orthogonal to the longitudinal direction is circular, and its diameter varies in a sinusoidal wave shape with a predetermined period P along the longitudinal direction of the linear material 4. In the linear material 4, a large-diameter portion 41 with a maximum width t1 and a small-diameter portion 42 with a minimum width t2 are formed at a predetermined period P over the entire length. However, it is not limited thereto, and in the linear material 4, the large-diameter portion 41 and the small-diameter portion 42 may be formed partially or intermittently at a predetermined period P.

[0019] Further, the diameter of the linear material 4 may vary in an arbitrary periodic shape such as a rectangular wave shape, a triangular wave shape, or a sawtooth wave shape with a predetermined period P along the longitudinal direction of the linear material 4. The cross-sectional shape orthogonal to the longitudinal direction of the linear material 4 is not limited to a circle, and may be non-circular such as an ellipse, an oval, a square, a rectangle, or a regular polyhedron. For example, the cross-sectional shape of the linear material 4 may be a regular polygon corresponding to the number of optical fiber cores 30 constituting the optical fiber tape 3. The cross-sectional shapes of the large-diameter portion 41 and the small-diameter portion 42 may be different.

[0020] For the linear material 4, for example, a hard resin, a flexible metal material, or the like can be used. The linear material 4 may be a material that can apply side pressure to the optical fiber core 30 and can bend together when the optical fiber cable 1 is bent.

[0021] Figure 3(a) is a cross-sectional view of an optical fiber unit 2 comprising an optical fiber tape 3 consisting of four optical fiber cores 30 and a linear material 4, and Figure 3(b) is a cross-sectional view of an optical fiber unit 2 comprising an optical fiber tape 3 consisting of five optical fiber cores 30 and a linear material 4. As shown in Figures 3(a) and 3(b), the optical fiber unit 2 is formed by wrapping the optical fiber tape 3 around the outer circumference of the linear material 4 and bringing them into contact. At this time, the linear material 4 comes into contact with each optical fiber core 30 of the optical fiber tape 3. Then, due to the effect of the large-diameter portion 41 and small-diameter portion 42 provided at a predetermined period P in the longitudinal direction of the linear material 4, lateral pressure is applied to each optical fiber core 30 at a predetermined period P in the longitudinal direction.

[0022] Here, it is desirable that the predetermined period P in the longitudinal direction of the linear material 4 be a value that efficiently generates coupling between modes of the optical fiber 31, which is an FMF. If the propagation constants of the two propagation modes that we want to couple in the optical fiber 31 are βL and βM (βL > βM), respectively, then the longitudinal period P that yields the strongest mode coupling can be expressed by the following equation (1). P = 2π / (βL - βM) (1)

[0023] Therefore, by applying lateral pressure to the optical fiber core 30 using a linear material 4 in which the large-diameter portion 41 and the small-diameter portion 42 are varied with a period P set to a value according to formula (1), coupling between modes of the optical fiber 31 can be efficiently generated. In other words, LPFG can be applied over the entire length of the optical fiber 31 of each optical fiber core 30.

[0024] If there are multiple combinations of modes to be coupled in the optical fiber 31, a large-diameter section 41 and a small-diameter section 42 should be provided in the linear material 4 to correspond to the multiple periods P determined by equation (1).

[0025] Furthermore, the greater the contact force between the linear material 4 and the optical fiber core 30, the stronger the mode coupling of the optical fiber 31. The contact force between the linear material 4 and the optical fiber core 30 can be increased by methods such as increasing the number of optical fiber cores 30 per unit cross-sectional area of ​​the optical fiber cable 1 or applying pressure to the sides of the optical fiber cable 1.

[0026] Figure 4(a) is a cross-sectional view showing the state before the coating 32 of the optical fiber core 30 is elastically deformed by applying a load F to the optical fiber core 30 by the large-diameter portion 41 of the linear material 4. Figure 4(b) is a cross-sectional view showing the state after the coating 32 of the optical fiber core 30 has been elastically deformed by applying a load F to the optical fiber core 30 by the large-diameter portion 41 of the linear material 4.

[0027] The optical fiber core 30 is composed of a coating 32 with a low Young's modulus and an optical fiber 31 with a high Young's modulus. Therefore, even if a load F from the linear material 4 is applied to the optical fiber core 30, the difference in Young's modulus is sufficiently large that only the coating 32 of the optical fiber core 30 undergoes elastic deformation, as shown in Figure 4(b).

[0028] As shown in Figures 3(a) and 3(b), the maximum width t1, which is the diameter of the large-diameter portion 41 of the linear material 4, should be larger than the inscribed circle of the optical fiber core 30 when the axial centers of the optical fiber core 30 are arranged on the outer circumference of the linear material 4 to form the vertices of a regular polygon. Conversely, the minimum width t2, which is the diameter of the small-diameter portion 42 of the linear material 4, should be smaller than the inscribed circle of the optical fiber core 30. However, if the diameter t1 of the large-diameter portion 41 of the linear material 4 is too large, the optical fiber tape 3 cannot be wrapped around the outer circumference of the linear material 4. Therefore, the upper limit of the diameter t1 of the large-diameter portion 41 is determined by the range in which the coating 32 of the optical fiber core 30 undergoes elastic deformation, as shown in Figure 4(b). By arranging the axial centers of the optical fiber cores 30 of the optical fiber tape 3 on the outer circumference of the linear material 4 to form the vertices of a regular polygon corresponding to the number of optical fiber cores 30, the linear material 4 and each optical fiber core 30 come into contact simply by covering the outer circumference of the linear material 4 with the optical fiber tape 3.

[0029] Therefore, in the optical fiber cable 1, the coupling efficiency of the optical fibers 31 of each optical fiber core 30 can be improved, and LPFG can be applied along the entire length of the optical fiber core 30.

[0030] In the optical fiber cable 1, the linear material 4 in the optical fiber unit 2 does not cover the optical fiber tape 3, but is arranged along the longitudinal direction, thus ensuring ease of extraction of the optical fiber core 30.

[0031] In the optical fiber cable 1, in the optical fiber unit 2, multiple optical fiber cores 30 constituting the optical fiber tape 3 are in contact with a single linear material 4 along the longitudinal direction. Therefore, the cross-sectional area of ​​the linear material can be reduced compared to the optical fiber cable disclosed in Patent Document 2. Furthermore, the linear material 4 can be reliably brought into contact with all optical fiber cores 30 within the optical fiber cable 1 without compromising the small diameter structure of the optical fiber cable 1.

[0032] (First variation) Figure 5(a) is a side view of the linear material 4A according to the first modified example of the embodiment. Figure 5(b) is a cross-sectional view of Figure 5(a) taken between 5B and 5B', and is a cross-sectional view of the large-diameter portion 41A of the linear material 4A. Figure 5(c) is a cross-sectional view of Figure 5(a) taken between 5C and 5C', and is a cross-sectional view of the small-diameter portion 42A of the linear material 4A.

[0033] In the first modified embodiment, a linear material 4A having the shape shown in Figures 5(a) to 5(c) is used instead of the linear material 4 shown in Figure 2. The other components of the first modified embodiment are configured the same as those of the embodiment described above, so their description is omitted.

[0034] The linear material 4A according to the first modified example is configured such that, as shown in Figures 5(a) to 5(c), the rectangular cross-sectional shape, with the long side having a maximum width t1 and the short side having a minimum width t2, is twisted along its entire length with a predetermined period P around the longitudinal axis. The cross-sectional shape of the linear material 4A is not limited to a rectangle; it may also be a non-circular shape such as an ellipse or oval. The cross-sectional shape of the linear material 4A is simply a non-circular shape where the direction with the maximum width t1 is approximately perpendicular to the direction with the minimum width t2.

[0035] By twisting a linear material 4A having such a cross-section with a predetermined period P, the width of a cross-section perpendicular to the longitudinal direction of the linear material 4A in a predetermined direction changes between a maximum width t1 and a minimum width t2 with a predetermined period P.

[0036] In the first modified example, the period P can be arbitrarily set according to the number of twists of the linear material 4A, so that an appropriate period P can be set for the linear material 4A to accommodate various FMFs.

[0037] Even if the linear material 4 according to the embodiment is replaced with the linear material 4A according to the first modified example, the same effects and advantages as those of the embodiment described above can be obtained.

[0038] (Second variation) Figure 6(a) is a side view of the linear material 4B according to a second modified example of the embodiment. Figure 6(b) is a cross-sectional view of Figure 6(a) taken between 6B and 6B', showing the large-diameter portion of the linear material 4B. Figure 6(c) is a cross-sectional view of Figure 6(a) taken between 6C and 6C', showing the small-diameter portion of the linear material 4B.

[0039] In the second modified embodiment, a linear material 4B having the shape shown in Figures 6(a) to 6(c) is used instead of the linear material 4 shown in Figure 2. The other components of the second modified embodiment are configured the same as those of the embodiment described above, so their description is omitted.

[0040] The linear material 4B according to the second modified example is constructed by twisting together multiple (two in Figures 6(a) to 6(c)) linear members 43 and 44 around a longitudinal axis at a predetermined period P, as shown in Figures 6(a) to 6(c). In the linear material 4B, the arrangement of the linear members 43 and 44 differs depending on their position in the longitudinal direction, so the width of the line segment in a predetermined direction of the cross-section perpendicular to the longitudinal direction of the linear material 4B changes between a maximum width t1 and a minimum width t2 at a predetermined period P. In Figures 6(a) to 6(c), the linear members 43 and 44 are shown as having two identical circular cross-sections, but the invention is not limited to this, and the cross-sections may be non-circular, and the cross-sectional shapes of linear member 43 and linear member 44 may be different. The linear material 4B may consist of three or more linear members.

[0041] In the second modified example, the period P can be arbitrarily set according to the number of linear members 43 and 44 constituting the linear material 4B, their cross-sectional shapes, and the number of twisting rotations. Therefore, an appropriate period P can be set for the linear material 4B to accommodate various FMFs.

[0042] Even if the linear material 4 according to the embodiment is replaced with the linear material 4B according to the second modified example, the same effects and advantages as those of the embodiment described above can be obtained.

[0043] (Third variation) Figure 7 is a perspective view of an optical fiber unit 2A according to a third modified example of the embodiment. The optical fiber unit 2A according to the third modified example differs from the optical fiber unit 2 according to the embodiment in that a bundle material 5 is wrapped around an optical fiber tape 3 that is rolled up to cover the outer circumference of a linear material 4. The other components of the third modified example are configured the same as those of the embodiment described above, so their description is omitted.

[0044] Even if the optical fiber tape 3 is rolled up to cover the linear material 4, depending on the rigidity of the adhesive joints that intermittently bond adjacent optical fiber cores 30, there is a risk that the adjacent optical fiber cores 30 may separate. To prevent the adjacent optical fiber cores 30 from separating easily, it is necessary to increase the number of optical fiber cores 30 per unit cross-sectional area in the cable core 11 of the optical fiber cable 1 (core mounting density). However, increasing the core mounting density may lead to an increase in the loss of optical fiber 31 in each optical fiber core 30. Therefore, in the third modified example, by wrapping the bundle material 5 around the optical fiber tape 3 rolled up to cover the linear material 4, the separation of adjacent optical fiber cores 30 can be suppressed without increasing the core mounting density. The bundle material 5 can be, for example, a linear or tape-shaped resin or flexible metal material.

[0045] Furthermore, the linear material 4A according to the first modification and the linear material 4B according to the second modification can also be applied to the optical fiber unit 2A according to the third modification.

[0046] Even if the optical fiber unit 2 according to the embodiment is replaced with the optical fiber unit 2A according to the third modified example, the same effects and advantages as those of the embodiment described above can be obtained.

[0047] As described above, several embodiments have been presented, but the statements and drawings that constitute part of this disclosure should not be understood as limiting the invention. From this disclosure, various alternative embodiments, examples, and operational techniques will become apparent to those skilled in the art. [Explanation of symbols]

[0048] 1. Fiber optic cable 2.2A Fiber Optic Unit 3 Fiber optic tape 4, 4A, 4B linear material 5 Bundling materials 11 Cable cores 12 Outer cover 30 Optical fiber core 31 Optical Fiber 32 Covering 41 Large diameter section 42 Small diameter section 43,44 Linear members

Claims

1. An optical fiber tape in which three or more optical fiber cores, each propagating in two or more modes, are integrated, The optical fiber tape comprises a linear material that covers the outer circumference and contacts the optical fiber core at the outer circumference, An optical fiber cable in which the width of the linear material in a predetermined direction in a cross section perpendicular to the longitudinal direction of the linear material changes at a predetermined period in the longitudinal direction, thereby applying lateral pressure to the optical fiber core at the predetermined period and generating coupling between the two or more modes.

2. The optical fiber cable according to claim 1, wherein the optical fiber cores are intermittently bonded.

3. The optical fiber cable according to claim 1, wherein when the optical fiber cores constituting the optical fiber tape are arranged such that the axial centers of the optical fiber cores form the vertices of a regular polygon in a cross section perpendicular to the longitudinal direction, the maximum value of the width in the predetermined direction is greater than the inscribed circle of the optical fiber cores arranged in the regular polygon.

4. The optical fiber cable according to any one of claims 1 to 3, wherein the cross-section of the linear material perpendicular to the longitudinal direction is circular, and the diameter of the circle changes in the longitudinal direction with a predetermined period.

5. The optical fiber cable according to any one of claims 1 to 3, wherein the cross-section of the linear material perpendicular to the longitudinal direction is non-circular, and the shape of the non-circular cross-section changes in the longitudinal direction at a predetermined period.

6. The optical fiber cable according to any one of claims 1 to 3, wherein the cross-section of the linear material perpendicular to the longitudinal direction is non-circular, and the linear material is twisted around the longitudinal axis at a predetermined period.

7. The optical fiber cable according to any one of claims 1 to 3, wherein the linear material comprises at least two linear members twisted together around the longitudinal axis at a predetermined period.

8. An optical fiber unit comprising the optical fiber tape and the linear material, The optical fiber cable according to any one of claims 1 to 3, further comprising a bundle material wrapped around the optical fiber unit.

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