Optical fiber processing device and optical fiber processing method

Abstract

An optical fiber processing device (10) is provided with: a roller (11) that has, on the outer periphery thereof, a recess (15) on which an optical fiber (1) is hung; and an irradiation part (12) that irradiates the inner surface (15a) of the recess (15) with a light pulse (16). The inner surface (15a) includes a plurality of reflection surfaces (20) that periodically change the reflectance of the light pulse (16) along the rotation direction of the roller (11), and reflect the light pulse (16) so as to condense the light pulse (16) into the core (2) of the optical fiber (1).

Description

Optical fiber processing device and optical fiber processing method

[0001] The present disclosure relates to an optical fiber processing device and an optical fiber processing method.

[0002] To increase transmission capacity, research is being conducted into technologies that use few-mode optical fibers (hereinafter referred to as FMF) to transmit different information in each mode. The delay time during transmission in FMF differs for each mode (see Non-Patent Document 1).

[0003] Each mode propagating through the FMF has a different propagation constant, with the exception of some mode combinations, resulting in different delay times during transmission. Signals propagating through each mode of the FMF are mixed at the receiving unit. Therefore, it is necessary to independently restore the mixed signals using multi-input multi-output (MIMO) technology. The larger the difference in delay times, the larger the decoding circuitry becomes (see Non-Patent Document 2).

[0004] Mode coupling is expected to reduce differential delay. Mode coupling can be promoted by using a long-period fiber grating (LPFG). The efficiency of mode coupling can be increased by setting the period of the LPFG to a value appropriate for the effective refractive index difference between each mode of the optical fiber (see Non-Patent Document 3).

[0005] Patent Document 1 discloses an optical fiber cable incorporating an LPFG for promoting mode coupling. This LPFG is formed in a sheet shape. Typically, the LPFG is in contact with the surface of the optical fiber coating. This results in little change in the refractive index of the core. Therefore, sufficient lateral pressure is required to generate mode coupling. Furthermore, applying a large lateral pressure causes optical loss due to mode coupling.

[0006] A method for identifying optical fiber cores by applying color at regular intervals along the length of the optical fiber core is already known (see Non-Patent Document 4). The timing of the discharge of paint or the like is controlled as the optical fiber is transported. This makes it possible to color the optical fiber cores at regular intervals. However, the coloring of the optical fiber cores is applied across the entire width of the core, and it is not possible to target specific parts of the optical fiber.

[0007] A method for changing the refractive index of a silica optical waveguide, such as an optical fiber or a planar optical circuit, by irradiating it with an optical pulse is known (Patent Document 2). For example, the configuration shown in Patent Document 2 allows the refractive index of a desired position in the optical fiber to be changed by irradiating the core of the optical fiber with an optical pulse. Furthermore, by appropriately sliding the stage, any processing can be performed in the longitudinal direction. Therefore, a periodic change in the refractive index can be generated along the longitudinal direction of the optical fiber.

[0008] JP 2018-036339 A JP 2005-257719 A

[0009] 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, 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.H. Liu, H. Wen, R. Amezcua-Correa, P. Sillard and G. Li, “Reducing group delay spread in a 9-LP mode FMF using uniform long-period gratings," 2017 Optical Fiber Communications Conference and Exhibition (OFC), 2017, pp. 1-3.M. Isaji et al., "Ultra-High density wrapping tube optical fiber cable with 12-fiber spider web ribbon," the proceedings of 62nd International Wire and Connectivity Symposium, pp. 605-609, 2013.

[0010] As described above, the technique of Patent Document 2 can cause a periodic change in refractive index. However, since the operating range of the stage is limited, it is not possible to process the entire area of ​​a long optical fiber.

[0011] The present disclosure has been made in consideration of the above circumstances, and aims to provide an optical fiber processing device and an optical fiber processing method that are capable of continuously imparting periodic refractive index changes to a long optical fiber.

[0012] An optical fiber processing device according to a first aspect of the present disclosure comprises a roller having a recess on its outer periphery in which an optical fiber is hung, and an irradiation unit that irradiates an inner surface of the recess with a light pulse, the inner surface including a plurality of reflective surfaces that periodically change the reflectivity of the light pulse along the rotation direction of the roller and reflect the light pulse so as to focus it within the core of the optical fiber.

[0013] An optical fiber processing method according to a second aspect of the present disclosure includes hanging an optical fiber in a recess formed on the outer periphery of a roller and irradiating a parallel light pulse toward the inner surface of the recess, wherein the inner surface includes a plurality of reflective surfaces that periodically change the reflectivity of the light pulse along the rotation direction of the roller and reflect the light pulse so as to focus it within the core of the optical fiber.

[0014] According to the present disclosure, it is possible to provide an optical fiber processing device and an optical fiber processing method that are capable of continuously imparting a periodic refractive index change to a long optical fiber.

[0015] Fig. 1 is a configuration diagram of an optical fiber processing device according to this embodiment. Fig. 2 is a partial cross-sectional view of a roller according to this embodiment. Fig. 3A is a cross-sectional view of a reflecting surface. Fig. 3B is a cross-sectional view of a reflecting surface. Fig. 4 is a partial cross-sectional view of a roller according to a modified example of this embodiment. Fig. 5 is a diagram showing an example of a focal point of a light pulse.

[0016] Hereinafter, an optical fiber processing apparatus and an optical fiber processing method according to an embodiment of the present disclosure will be described with reference to the drawings. In the description of the drawings, identical parts are designated by the same reference numerals and description thereof will be omitted. For convenience of explanation, mutually orthogonal X, Y, and Z directions are defined. The X direction is the width direction of the roller 11 and the extension direction of the rotation axis 13 of the roller 11. The Y direction coincides with the radial direction of the roller 11.

[0017] The optical fiber processing device 10 according to this embodiment irradiates an optical fiber 1 with a light pulse 16, periodically changing the refractive index of the core 2 along the longitudinal direction of the optical fiber 1. The optical fiber 1 applied to this embodiment is not limited in length. That is, the refractive index of the core 2 can be periodically changed for a long optical fiber 1. The optical fiber 1 irradiated with the light pulse 16 is, for example, a few-mode optical fiber. By imparting a periodic refractive index change to the core of a few-mode optical fiber, an optical fiber with improved mode coupling efficiency can be obtained.

[0018] 1 is a configuration diagram of an optical fiber processing device 10 according to this embodiment. For ease of explanation, the optical fiber processing device 10 will be referred to as the processing device 10 hereinafter. As shown in FIG. 1 , the processing device 10 includes a roller 11 that can rotate around a rotation axis 13, and an irradiation unit 12 that irradiates the roller 11 with a light pulse 16.

[0019] The roller 11 is a disk rotatable about a rotation axis 13, and is rotated by a drive unit 14 such as a motor. The roller 11 has a recess 15 on its outer periphery. The recess 15 is recessed toward the rotation axis 13 of the roller 11. The optical fiber 1 is hung in the recess 15.

[0020] The inner surface 15a of the recess 15 includes a plurality of reflective surfaces (first surfaces) 20 that periodically change the reflectivity of the light pulse 16 along the rotation direction T of the roller 11 and reflect the light pulse 16 so as to focus it within the core 2 of the optical fiber 1. As shown in FIG. 1 , the plurality of reflective surfaces 20 are arranged at intervals in the rotation direction T. Furthermore, a non-reflective surface (second surface) 21 is provided between two reflective surfaces 20 adjacent to each other in the rotation direction T. That is, the inner surface 15a includes reflective surfaces 20 and non-reflective surfaces 21 that are alternately arranged along the rotation direction T. These reflective surfaces 20 and non-reflective surfaces 21 will be described later.

[0021] The irradiation unit 12 is a so-called femtosecond laser device, which repeatedly irradiates the recess 15, in which the optical fiber 1 is hung, with light pulses 16 having a pulse width of about femtoseconds. The period of the light pulses 16 is sufficiently shorter than the time it takes for one reflecting surface 20 to pass through the irradiation area of ​​the light pulses 16. In other words, the period of the light pulses 16 is set to a value that causes the refractive index change in the core 2, formed by one reflecting surface 20, to be continuous in the longitudinal direction of the optical fiber 1.

[0022] The light pulse 16 emitted from the irradiation unit 12 is converted into a parallel or nearly parallel light by a lens (not shown). The diameter of this light is larger than the diameter of the optical fiber 1. Furthermore, the optical axis 16a of the light pulse 16 directed toward the recess 15 is located on a cross section perpendicular to the rotation direction T of the roller 11. The optical axis 16a is the central axis of the beam bundle formed by the light pulse 16. As will be described later, the focal point 17 of the light pulse 16 is located somewhere within the core 2 (see FIG. 2 ) of the optical fiber 1 due to reflection by the reflecting surface 20. The light pulse 16 has an intensity that changes the refractive index of the core 2 at and near the focal point 17.

[0023] The control unit 18 is a so-called computer, and controls the drive unit 14 and the irradiation unit 12. The control unit 18 controls the rotation speed of the roller 11 caused by the drive unit 14, in other words, the transport amount of the optical fiber 1 caused by the rotation of the roller 11. The control unit 18 also controls the generation of the light pulse 16 by the irradiation unit 12. Note that the timing of generation of the light pulse 16 does not have to be synchronized with the operation of the roller 11.

[0024] The driving unit 14 may include a moving mechanism (not shown) such as a movable stage that controls the position of the roller 11. In this case, the control unit 18 controls the driving unit 14 to adjust the position of the roller 11.

[0025] Fig. 2 is a partial cross-sectional view of the roller 11, showing the recess 15. The cross-section shown in Fig. 2 includes the central axis of the rotation shaft 13. In other words, this cross-section is perpendicular to the rotation direction T of the roller 11. As shown in Fig. 2, the inner surface 15a of the recess 15 is recessed toward the rotation shaft 13.

[0026] A deepest part 19 is formed at or near the center of the recess 15 in the width direction (X direction). The deepest part 19 is the part of the recess 15 that is deepest in the radial direction (Y direction). The optical fiber 1 hung on the recess 15 is located at a position closest to the deepest part 19 due to the tension of the optical fiber 1. For example, in the example shown in Fig. 2, the optical fiber 1 is located in the recess 15 with its outer circumferential surface 1a in contact with the deepest part 19.

[0027] The inner surface 15a of the recess 15 has a plurality of reflective surfaces 20 shown in FIG. 2 . The reflective surfaces 20 are periodically arranged along the rotation direction T. The length and period of each reflective surface 20 along the rotation direction T correspond to the length and period of the refractive index change formed in the core 2. Each reflective surface 20 is a curved surface that reflects the light pulse 16 incident on the recess 15 and focuses the light within the core 2 of the optical fiber 1. For example, the inner surface 15a may form a parabola 30 in a cross section perpendicular to the rotation direction T of the roller 11. In this case, the focus 31 of the parabola 30 is located within the core 2 of the optical fiber 1 hung on the recess 15. Furthermore, when the light pulse 16 is a parallel light, the position of the focusing point 17 of the light pulse 16 can be set with high precision, enabling focusing with the most energy efficiency.

[0028] For ease of explanation, the following description will be given taking as an example a case where reflective surface 20 forms parabola 30. When reflective surface 20 forms parabola 30, optical axis 16a of light pulse 16 directed toward recess 15 is parallel to axis 32 of parabola 30 in a cross section perpendicular to rotation direction T of roller 11. Therefore, light pulse 16 is focused at focus 31 of parabola 30, forming focused point 17.

[0029] Here, assuming that the deepest part 19 is the origin of the parabola 30, the X axis is in the width direction (X direction), and the Y axis is in the radial direction (Y direction), the parabola 30 can be expressed by the following equation (1): Y=X ２ / 4p (1) When the variable p is equal to the radius R of the optical fiber 1, the focus 31 of the parabola 30 is located at the coordinates (0, R) and coincides with the center of the optical fiber 1 (core 2).

[0030] The value of the variable p in equation (1) is not limited to the radius R of the optical fiber 1. For example, by setting the variable p to a value smaller than the radius R of the optical fiber 1, the position of the focal point 31 can be set between the center of the optical fiber 1 (core 2) and the innermost part 19. Conversely, by setting the variable p in equation (1) to a value larger than the radius R of the optical fiber 1, the position of the focal point 31 can also be set to a position farther away from the innermost part 19 than the center of the optical fiber 1 (core 2). In other words, by adjusting the variable p, the focal point 17 of the light pulse 16 can be set to any position on the axis 32. In other words, the focal point 17 can be shifted in the Y direction.

[0031] 3A and 3B are cross-sectional views of the reflective surface 20. As shown in Fig. 3A, the reflective surface 20 may be formed directly on the inner surface 15a by surface treatment such as vapor deposition of a reflective material. Alternatively, as shown in Fig. 3B, a film (sheet) 22 having a reflective layer may be attached to the inner surface 15a.

[0032] On the other hand, the non-reflective surface 21 (see FIG. 1 ) is formed so that the optical pulse 16 reflected by the non-reflective surface 21 does not cause a change in the refractive index of the core 2. For example, the non-reflective surface 21 may have the same cross-sectional shape as the reflective surface 20. However, in this case, the reflectance of the non-reflective surface 21 is lower than that of the reflective surface 20 and has a value that does not cause a change in the refractive index of the core 2. For example, the non-reflective surface 21 may have a minute uneven surface that causes diffuse reflection of the optical pulse 16. Alternatively, the reflectance of the non-reflective surface 21 may be equal to that of the reflective surface 20. In this case, the non-reflective surface 21 is formed so that the focus of its parabola is located outside the core 2, and no change in the refractive index of the core 2 occurs. In this way, the non-reflective surface 21 may have a cross-sectional shape different from that of the reflective surface 20, as long as the optical fiber 1 hung on the non-reflective surface 21 is stably positioned.

[0033] Similar to the reflective surface 20, the non-reflective surface 21 may be formed by the inner surface 15a itself, or a film (sheet) having a reflectance or shape that does not cause a change in the refractive index of the core 2 may be attached to the inner surface 15a. In the latter case, the reflective surface 20 and the non-reflective surface 21 may be provided alternately on a single film (sheet) (e.g., film 22) extending along the optical fiber 1.

[0034] As described above, the optical fiber 1 is located at a position closest to the deepest part 19. Therefore, as shown in Fig. 2, the center of the core 2 and the deepest part 19 are both located on the optical axis 16a. In this case, by irradiating the inner surface 15a of the recess 15 with the light pulse 16, the light pulse 16 is focused at a focal point 31. In the example of Fig. 2, the focal point 31 is located at the center of the core 2. Therefore, a distribution of refractive index changes centered on the center of the core 2 is obtained.

[0035] Furthermore, by continuing to repeatedly irradiate the optical fiber 1 with the light pulses 16 while the optical fiber 1 is being transported by the rotation of the rollers 11, a state in which the light pulses 16 are focused on the core 2 due to irradiation of the reflective surface 20 and a state in which the light pulses 16 are not focused on the core 2 due to irradiation of the non-reflective surface 21 occur alternately within the optical fiber 1. Therefore, portions in which the refractive index is changed within the core 2 can be periodically imparted (formed) along the longitudinal direction of the optical fiber 1.

[0036] 4 is a partial cross-sectional view of roller 11 according to a modified example of this embodiment. As shown in FIG. 4, in a cross section perpendicular to rotation direction T, the axis of symmetry of reflecting surface 20 may be inclined with respect to an imaginary plane (i.e., the Y-Z plane) that includes rotation direction T. For example, when reflecting surface 20 forms a parabola 30, axis 32, which is the axis of symmetry, may be inclined with respect to the imaginary plane. In this case, optical axis 16a of light pulse 16 is also inclined with respect to the imaginary plane, like axis 32.

[0037] 4, when the axis 32 of the parabola 30 is inclined with respect to the Y-Z plane, the deepest part 19 is located at a position shifted in the X direction along the parabola 30 from the intersection 33 of the parabola 30 and the axis 32. Accordingly, the position where the optical fiber 1 contacts the reflecting surface 20 is also located at a position shifted in the X direction along the parabola 30. The outer peripheral surface 1a of the optical fiber 1 contacts the reflecting surface 20 at the deepest part 19.

[0038] On the other hand, as shown in FIG. 2 , even if the axis 32 is tilted, the relative position of the focal point 31 with respect to the parabola 30 does not change. Therefore, tilting the axis 32 shifts the position of the core 2 with respect to the focal point 31. In other words, tilting the axis 32 shifts the position of the focal point 31 with respect to the center of the core 2. The relative positional shift between the center of the core 2 and the focal point 31 occurs in a direction approximately parallel to the X direction in the cross section shown in FIG. 2 . In other words, by tilting the parabola 30 of the reflecting surface 20 with respect to the Y-Z plane, the focal point 17 can be shifted in the X direction from its position on the axis 32. Furthermore, as described above, the focal point 17 can also be shifted in the Y direction by adjusting the variable p shown in equation (1). Therefore, in this embodiment, the focal point 17 can be shifted two-dimensionally within the core 2.

[0039] FIG. 5 is a diagram showing an example of the focal point 17 of the optical pulse 16. In this embodiment, the relative position of the core 2 with respect to the deepest part 19 is stable. Meanwhile, by adjusting the inclination and shape of the parabola 30 formed by the reflecting surface 20, the part where the refractive index changes can be set to any position within the core 2. For example, as shown in FIG. 5, the focal point 17 of the optical pulse 16 may be located at a position where a maximum value of the overlap component between the propagation modes occurs in the optical fiber 1 hung on the recess 15. The position where a maximum value of the overlap component occurs refers to the position where the overlap between the electric field intensities E of the different propagation modes is greatest in the electric field intensity distributions of the different propagation modes. In other words, it can be said to be the position where the intensity ratio of the electric field intensities E of the different propagation modes is smallest.

[0040] FIG. 5 shows the LP ０１ Mode and LP １１ This shows the electric field intensity distribution of the modes. Mode coupling occurs when the electric field energy transfers at the point where the propagation constants match. １１ Higher-order modes such as the 1000-2000 mode do not necessarily have a maximum electric field distribution at the center of the core 2. Therefore, the focusing point 17 of the optical pulse 16 is positioned at a position where the overlap component between the propagation modes has a maximum value, and the refractive index in that area is locally changed. This makes it possible to improve the coupling efficiency between different modes.

[0041] The position where the maximum value is generated varies depending on the combination of modes to be combined. Therefore, the processing device 10 may include a plurality of irradiation units 12, each having a focal point 17 set at a different position.

[0042] When a refractive index change is imparted to the optical fiber 1 using the processing device 10, a light pulse 16 is repeatedly irradiated onto the inner surface 15a of the recess 15 in which the optical fiber 1 is hung. During this irradiation, the optical fiber 1 is transported by the rotation of the roller 11. Meanwhile, reflective surfaces 20 and non-reflective surfaces 21 are alternately formed on the outer periphery of the roller 11 along the rotation direction T. Therefore, within the optical fiber 1, a state in which the light pulse 16 is focused on the core 2 due to irradiation of the reflective surface 20 and a state in which the light pulse 16 is not focused on the core 2 due to irradiation of the non-reflective surface 21 alternately occur. As a result, a periodic refractive index change that results in mode coupling can be imparted to the optical fiber 1. Furthermore, the period of the refractive index change is determined by the arrangement of the reflective surface 20 and the non-reflective surface 21. In other words, there is no need to control the irradiation timing of the light pulse 16.

[0043] In this embodiment, the optical fiber 1 to be irradiated with the light pulse 16 is transported by rollers 11, and the optical fiber 1 is irradiated with the light pulse 16 on the rollers 11. Therefore, the length of the optical fiber 1 is not limited. That is, a periodic refractive index change can be continuously imparted to a long optical fiber 1. Furthermore, the optical fiber 1 is stably positioned at a position closest to the deepest part 19 of the recess 15 due to its own tension. Therefore, a refractive index change can be stably imparted to a desired position in the width direction of the optical fiber 1, for example.

[0044] REFERENCE SIGNS LIST 1 Optical fiber 2 Core 10 Optical fiber processing device (processing device) 11 Roller 12 Irradiation unit 15 Recess 16 Light pulse 16a Optical axis 17 Focus point 19 Deepest part 20 Reflecting surface 21 Non-reflecting surface 30 Parabola 31 Focus 32 Axis

Claims

1. An optical fiber processing device comprising: a roller having a recess on its outer periphery in which an optical fiber is hung; and an irradiation unit that irradiates the inner surface of the recess with a light pulse, wherein the inner surface includes a plurality of reflective surfaces that periodically change the reflectivity of the light pulse along the direction of rotation of the roller and reflect the light pulse so as to be focused within the core of the optical fiber.

2. The optical fiber processing device according to claim 1, wherein each of the reflecting surfaces forms a parabola in a cross section perpendicular to the direction of rotation, and the focus of the parabola is located within the core of the optical fiber hung in the recess.

3. The optical fiber processing device according to claim 2, wherein the focus of the parabola is located at a position where a maximum value of overlapping components between propagation modes occurs in the optical fiber when it is hung on the recess.

4. An optical fiber processing method comprising: hanging an optical fiber in a recess formed on the outer periphery of a roller; and irradiating the inner surface of the recess with a light pulse, wherein the inner surface includes a plurality of reflective surfaces that periodically change the refractive index of the light pulse along the rotation direction of the roller and reflect the light pulse so as to be focused within the core of the optical fiber.

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