Optical fiber grating device and methods of manufacturing the same

Abstract

To provide a polarization-independent optical fiber grating device using a simple configuration.SOLUTION: An optical fiber grating device (100) is provided, comprising an optical fiber (10) having a grating formed therein, as well as cores (11) formed at positions where refractive indices are polarization-independent.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical fiber grating device and a method for manufacturing an optical fiber grating device. [Background technology]

[0002] It has been known that in conventional optical fiber gratings in which periodic refractive index changes are formed in an optical fiber, differences in refractive index occur depending on the polarization direction of the transmitted light (i.e., birefringence occurs).Patent Document 1 discloses a polarization-maintaining optical fiber grating that outputs single-polarized laser light by forming orthogonal axes (birefringence axes) with different refractive indexes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-177469 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the optical fiber grating disclosed in Patent Document 1, it is necessary to adjust the orientation of the optical fiber grating to match the polarization direction of the incident light. Also, in order to realize orthogonal axes with different refractive indices, it is necessary to form a stress-applying part inside the optical fiber, which makes the manufacturing process complicated.

[0005] An object of one aspect of the present invention is to realize an optical fiber grating device that has no polarization dependency and has a simple configuration. [Means for solving the problem]

[0006] In order to solve the above problems, the optical fiber grating device according to aspect 1 of the present invention has an optical fiber in which a grating is formed and in which a core is formed at a position where the refractive index does not depend on polarization.

[0007] In the optical fiber grating device according to a second aspect of the present invention, in the first aspect, the core may be located at a position offset from the center of the optical fiber.

[0008] In an optical fiber grating device according to a third aspect of the present invention, in the first or second aspect, a grating may be formed in the optical fiber by periodically applying pressure to a side surface of the optical fiber.

[0009] In an optical fiber grating device according to a fourth aspect of the present invention, in the third aspect, a marker indicating a pressing position where the pressure is applied may be printed on the optical fiber.

[0010] An optical fiber grating device according to a fifth aspect of the present invention is, in the third or fourth aspect described above, further comprising a pressing member that applies periodic pressure to the side of the optical fiber from the outside, the pressing member comprising a first pressing portion having periodically arranged convex portions and a second pressing portion having a flat surface that is arranged on the opposite side of the first pressing portion with respect to the optical fiber, and in a cross section of the optical fiber, when a first line is a line passing through the center of the optical fiber and the pressing position where the pressure is applied, and a second line is a line passing through the center of the optical fiber and the center of the core, the core may be arranged at a position where the angle between the first line and the second line is greater than 0° and less than 90°.

[0011] In order to solve the above problems, a manufacturing method for an optical fiber grating device according to a sixth aspect of the present invention is a manufacturing method for an optical fiber grating device comprising an optical fiber, and includes a first identifying step of identifying a first region in a cross section of the optical fiber in which the refractive index does not have polarization dependence when a refractive index change is caused in the optical fiber, a second identifying step of identifying a second region in the cross section of the optical fiber in which the refractive index modulation degree is equal to or greater than a predetermined threshold when a refractive index change is caused in the optical fiber, and a forming step of forming a core in a region where the first region and the second region overlap.

[0012] In order to solve the above problems, a manufacturing method for an optical fiber grating device according to aspect 7 of the present invention is a manufacturing method for an optical fiber grating device comprising an optical fiber having a core, and includes a preparation step of preparing the optical fiber, a rotation step of rotating the optical fiber around its axis so that the core is positioned in an appropriate position, and an application step of periodically applying pressure to the side of the optical fiber to form a grating in the optical fiber when the core is positioned in the appropriate position.

[0013] In order to solve the above problems, a manufacturing method of an optical fiber grating device according to aspect 8 of the present invention is a manufacturing method of an optical fiber grating device comprising an optical fiber having a core, and includes a preparation step of preparing the optical fiber, a rotation step of rotating the optical fiber by a predetermined angle around its axis, an application step of periodically applying pressure to the side of the optical fiber to form a grating in the optical fiber, and a measurement step of measuring the polarization dependence of the refractive index of the core in a refractive index change region, and the rotation step, the application step, and the measurement step are repeated to position the core in a position where there is no polarization dependence. [Effects of the Invention]

[0014] According to one aspect of the present invention, an optical fiber grating device that has no polarization dependency can be realized with a simple configuration. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram for explaining an optical fiber grating. [Figure 2] 1 is a schematic diagram showing a schematic configuration of an optical fiber grating device according to a first embodiment. [Figure 3] 1 shows a view of the optical fiber of the optical fiber grating device as viewed from the Z direction. [Figure 4] FIG. 10 is a diagram showing the distribution of refractive index change Δnx for X-polarized light in the cross section of an optical fiber. [Figure 5] FIG. 10 is a diagram showing the distribution of refractive index change Δny for Y-polarized light in the cross section of an optical fiber. [Figure 6] FIG. 1 is a diagram showing the distribution of birefringence Δnxy in the cross section of an optical fiber. [Figure 7] 10 is a flowchart showing an example of a method for manufacturing an optical fiber grating device according to the second embodiment. [Figure 8] 10 is a flowchart showing another example of the method for manufacturing the optical fiber grating device according to the third embodiment. [Figure 9] 10 is a process chart showing another example of the method for manufacturing the optical fiber grating device according to the third embodiment. [Figure 10] 10 is a flowchart showing yet another example of the method for manufacturing an optical fiber grating device according to the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] [Embodiment 1] (Features of one aspect of the present invention) Fig. 1 is a schematic diagram illustrating an optical fiber grating device 100A. As shown in Fig. 1, the optical fiber grating device 100A generates periodic refractive index changes along the transmission direction in a core 11A of an optical fiber 10A. Fig. 1 particularly shows an LPFG (Long Period Fiber Grating) as the optical fiber grating device 100A. The optical fiber grating device 100A functions, for example, as a wavelength filter that selectively reflects light of a specific wavelength.

[0017] Here, in the refractive index change region 111A of the optical fiber grating device 100A, polarization dependency (for example, birefringence) may occur, in which the optical characteristics vary depending on the polarization direction of the incident light. Therefore, in order to obtain the desired optical characteristics, it is necessary to adjust the orientation of the optical fiber grating device 100A to match the polarization direction of the incident light. This has caused a problem that the manufacturing process of the optical fiber grating device 100A becomes complicated. Furthermore, for example, when birefringence occurs, there is a possibility that the polarized waves of light oscillating in different directions may interfere with each other, or the signal waveform may be dispersed.

[0018] The present inventors have found that there exists a position within the cross section of an optical fiber where the polarization dependency is small when a grating is formed. Hereinafter, such a position will be referred to as the "appropriate position." The present inventors have found that by forming a core at the appropriate position, an optical fiber grating with small polarization dependency can be provided. Specific examples of such optical fiber gratings will be described below.

[0019] (Schematic configuration of an optical fiber grating device) 2 is a schematic diagram showing the general configuration of an optical fiber grating device 100 according to this embodiment. The optical fiber grating device 100 includes an optical fiber 10 and a pressing member 20. The optical fiber grating device 100 is an MLPFG (mechanical LPFG) that periodically changes the refractive index due to stress generated in the optical fiber 10 by an external force. Hereinafter, the region in the optical fiber 10 where the refractive index changes due to the external force will be referred to as a refractive index change region 111.

[0020] The pressing member 20 is a member that periodically applies pressure to the side surface of the optical fiber 10. The pressing member 20 includes an uneven plate 21 (first pressing portion) and a support plate 22 (second pressing portion). The pressing member 20 periodically applies pressure to the side surface of the optical fiber 10, thereby forming a grating in the optical fiber 10.

[0021] The uneven plate 21 is disposed on one side of the optical fiber 10 and has a plurality of convex portions 21a periodically arranged along the transmission direction of the optical fiber 10. The support plate 22 is disposed on the other side of the optical fiber 10 and has a flat surface 22a. The flat surface 22a of the support plate 22 supports the side of the optical fiber 10, while the convex portions 21a of the uneven plate 21 are pressed against the side of the optical fiber 10, thereby periodically applying pressure to the side of the optical fiber 10.

[0022] Hereinafter, the direction in which the concave-convex plate 21 is located relative to the support plate 22 will be referred to as the upward direction, and the opposite direction will be referred to as the downward direction. The direction perpendicular to the transmission direction and the up-down direction of the optical fiber 10 will be referred to as the left-right direction. The left-right direction, up-down direction, and transmission direction will also be referred to as the X direction, Y direction, and Z direction, respectively.

[0023] FIG. 3 shows an optical fiber 10 as viewed from the Z direction. As shown in FIG. 3, the optical fiber 10 includes at least one core 11 and a cladding 12 surrounding the core 11. The core 11 is provided at a position (appropriate position) where a grating can be formed and where the refractive index in the cross section of the optical fiber 10 does not have polarization dependency. In the example shown in FIG. 3, two appropriate positions (regions R1 and R2) are formed in the cross section of the optical fiber 10 pressed by the pressing member 20. Two cores 11 are provided corresponding to these two appropriate positions. Note that the core 11 may be provided in only one of the two appropriate positions.

[0024] In this embodiment, "no polarization dependence of the refractive index" refers to a state in which the birefringence Δnxy in the refractive index changing region 111 is 0.0005 or less, or 0.0001 or less. Here, the birefringence Δnxy is defined as the difference nx-ny between the refractive index nx of X-polarized light (light polarized in the X direction) and the refractive index ny of Y-polarized light (light polarized in the Y direction). Alternatively, "no polarization dependence of the refractive index" refers to a state in which the polarization mode dispersion (PMD) occurring in the refractive index changing region 111 is 0.25 ps or less, or 0.05 ps or less. Here, the polarization mode dispersion is defined as the time difference required for X-polarized light and Y-polarized light to propagate through the optical fiber grating device 100.

[0025] The core 11 is located at a position off-center of the optical fiber 10. Specifically, the core 11 is provided at a position where the angle θ formed by the first line L1 and the second line L2 is greater than 0° and less than 90°. The first line L1 is a line passing through the center of the optical fiber 10 and the pressing position where pressure is applied from the pressing member 20. The second line L2 is a line passing through the center of the optical fiber 10 and the center of the core 11. In other words, the core 11 is provided on the pressing position side, diagonally downward to the left and diagonally downward to the right of the pressing position. In this embodiment, such positions are typically appropriate positions. The angle θ may be an angle other than 30°, 45°, or 60°.

[0026] Furthermore, a marker 30 is marked on the end face of the optical fiber 10 to indicate the pressing position where pressure from the pressing member 20 is applied (to serve as a guide for the pressing position). The marker is preferably marked at a position 0 degrees, 90 degrees, or 180 degrees from the pressing position, and more preferably at the 0-degree position. When the marker is marked at a position 0 degrees from the pressing position, the worker connects the optical fiber 10 to the optical fiber of another optical element so that the marker is positioned at the pressing position where pressure from the pressing member 20 is applied. This allows the core 11 to be positioned appropriately. Therefore, when connecting the optical fiber 10 to the optical fiber of another optical element, the worker can easily determine the appropriate orientation of the optical fiber 10. The marker 30 may be made of any material as long as it is visible, and may be made of a material having a refractive index different from that of the cladding 12, for example. The material of the marker 30 may be the same as that of the core 11, for example.

[0027] (Simulation results) A simulation was performed to determine the distribution of refractive index changes for X-polarized light and Y-polarized light within the optical fiber 10 when stress was applied to the optical fiber 10. In this simulation, a force of 1 N was applied to the optical fiber 10. The diameter of the optical fiber 10 was set to 0.125 m. The material of the optical fiber 10 was set to quartz glass. Note that as long as a stress is applied that does not break the optical fiber 10, the stress is proportional to the magnitude of the external force. Therefore, the distribution of refractive index changes does not change depending on the magnitude of the external force.

[0028] Fig. 4 is a diagram showing the distribution of refractive index change Δnx for X-polarized light in the cross section of optical fiber 10. Fig. 5 is a diagram showing the distribution of refractive index change Δny for Y-polarized light in the cross section of optical fiber 10. Fig. 6 is a diagram showing the distribution of birefringence Δnxy in the cross section of optical fiber 10. Note that Figs. 4 and 5 show the refractive index change caused by pressing by the pressing member 20 shown in Fig. 2. Furthermore, because the refractive index distribution in the optical fiber before being pressed by the pressing member 20 is approximately uniform, it can also be said that the birefringence Δnxy is the difference between the refractive index change Δnx for X-polarized light and the refractive index change Δny for Y-polarized light.

[0029] As shown in FIGS. 4 to 6, the distribution of the refractive index change Δnx for X-polarized light is different from the distribution of the refractive index change Δny for Y-polarized light. This indicates that there are regions in the refractive index change region 111A where polarization dependence occurs, in which the optical properties vary depending on the polarization direction of the incident light. On the other hand, as shown in FIG. 6, the refractive index change region 111A also has regions (first regions) where there is no polarization dependence. For example, regions R1, R2, and R3 in FIG. 6 are regions where there is no polarization dependence.

[0030] Furthermore, to form a grating in the optical fiber 10, the core 11 needs to be formed in a region (second region) where the refractive index modulation degree is equal to or greater than a predetermined threshold (for example, 10 to the power of minus four, which is the refractive index modulation degree of a typical LPFG). The refractive index modulation degree is a dimensionless quantity that indicates the difference in refractive index. The refractive index modulation degree can be expressed, for example, as (ΔH-ΔL) / (ΔH+ΔL). Here, ΔH is the maximum value of the relative refractive index difference Δ of the optical fiber 10, and ΔL is the minimum value of the relative refractive index difference Δ of the optical fiber 10. In this simulation, the refractive index modulation degree in regions R1 and R2 was equal to or greater than the predetermined threshold, while the refractive index modulation degree in region R3 was less than the predetermined threshold. Therefore, regions R1 and R2 are appropriate positions. On the other hand, region R3 is not an appropriate position because a grating cannot be formed in region R3.

[0031] [Embodiment 2] Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0032] In this embodiment, an example of a method for manufacturing an optical fiber grating device 100 will be described. Fig. 7 is a flowchart showing an example of a method for manufacturing an optical fiber grating device 100. A method for forming a core at an appropriate position in the cross section of an optical fiber 10 will be described with reference to Fig. 7.

[0033] First, a first region in the cross section of the optical fiber 10 is identified (first identifying step S1), in which the refractive index does not have polarization dependency when a refractive index change is caused in the optical fiber 10. Specifically, the first region is identified by the numerical simulation described above with reference to FIG.

[0034] Next, a second region is identified in the cross section of the optical fiber 10, which region exhibits a refractive index modulation degree equal to or greater than a predetermined threshold when a refractive index change is caused in the optical fiber 10 (second identification step S2). Specifically, the second region is identified by the numerical simulation described above with reference to FIGS.

[0035] Next, a core 11 is formed in the region where the first region and the second region overlap (forming step S3). The region where the first region and the second region overlap is the appropriate position. This makes it possible to provide an optical fiber grating device 100 that is not polarization dependent.

[0036] [Embodiment 3] Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0037] In this embodiment, another example of the method for manufacturing the optical fiber grating device 100 will be described. Fig. 8 is a flowchart showing another example of the method for manufacturing the optical fiber grating device 100. Fig. 9 is a process chart showing another example of the method for manufacturing the optical fiber grating device 100. A method for positioning the core 11 of the optical fiber 10 at an appropriate position will be described with reference to Figs. 8 and 9.

[0038] First, an optical fiber 10 having a core 11 is prepared as indicated by reference numeral 9001 in Fig. 9 (preparation step S11). In the example shown in Fig. 9, four cores 11a, 11b, 11c, and 11d are provided around the axis of the optical fiber 10. In this embodiment, a case will be described in which only the core 11a of these four cores is used.

[0039] Next, as shown by reference numeral 9002 in Fig. 9, the optical fiber 10 is rotated around its axis so that the core 11a is positioned in the appropriate position (region R2) (rotation step S12). Here, the appropriate position is a position determined with respect to the pressing position where the concave-convex plate 21 of the pressing member 20 comes into contact.

[0040] 9, when the core 11a is positioned at an appropriate position, pressure is periodically applied to the side surface of the optical fiber 10 by the pressing member 20. As a result, a grating is formed in the optical fiber 10.

[0041] According to the above configuration, an optical fiber grating device that is not polarization dependent can be realized by a simple manufacturing process in which the optical fiber 10 is rotated around its axis.

[0042] [Embodiment 4] Other embodiments of the present invention will be described below. For ease of explanation, the same reference numerals will be used to designate components having the same functions as those described in the above embodiment, and the description thereof will not be repeated.

[0043] In this embodiment, a description will be given of yet another example of the method for manufacturing the optical fiber grating device 100. Fig. 10 is a flowchart showing yet another example of the method for manufacturing the optical fiber grating device 100. With reference to Fig. 10, a description will be given of a method for identifying an appropriate position on the cross section of the optical fiber 10 and positioning the core 11 at the appropriate position.

[0044] First, an optical fiber 10 having a core 11 is prepared (preparation step S21). Next, pressure is periodically applied to the side surface of the optical fiber 10 by the pressing member 20 (application step S22). This forms a grating in the optical fiber 10. Here, the core 11 is positioned at any position relative to the pressing position.

[0045] Next, the polarization dependence of the optical fiber 10 is measured (measurement step S23). Specifically, birefringence or polarization mode dispersion is measured when X-polarized light and Y-polarized light are propagated through the stressed optical fiber 10. If the optical fiber 10 does not have polarization dependence (YES in S24), the process ends. On the other hand, if the optical fiber 10 has polarization dependence (NO in S24), the optical fiber 10 is rotated around its axis by a predetermined angle (rotation step S25), and the process returns to S22. That is, the rotation step S25, the application step S22, and the measurement step S23 are repeated until the core 11 is positioned appropriately.

[0046] According to the above configuration, it is possible to identify an appropriate position on the cross section of the optical fiber 10 and position the core 11 at the appropriate position.

[0047] (Action and effect) This configuration will facilitate the development of communications infrastructure for Beyond 5G services, which will contribute to the achievement of Goal 9 of the United Nations' Sustainable Development Goals (SDGs), which states, "Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation."

[0048] (Additional notes) In this embodiment, a method of applying stress to an optical fiber has been exemplified as a method of forming a grating in an optical fiber. However, the method of forming a grating in an optical fiber is not limited to this. For example, a method of irradiating the side surface of the optical fiber with ultraviolet light to cause a periodic refractive index change based on the amount of ultraviolet light absorbed can also be used. Another method of irradiating the side surface of the optical fiber with a CO laser to cause a refractive index change can also be used. These methods also allow for the formation of a grating, and there is an appropriate position in the cross section of the optical fiber where the refractive index is not polarization dependent. Therefore, by forming a core in such an appropriate position, an optical fiber grating device that is not polarization dependent can be realized.

[0049] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0050] 100 Optical Fiber Grating Device 10 Optical Fiber 11 cores 20 Pressing member 21 Uneven plate (first pressing part) 21a Convex part 22 Support plate (second pressing portion) 22a flat surface 30 markers 111 Refractive index change region

Claims

1. An optical fiber grating device comprising an optical fiber in which a grating is formed, the core being formed at a position where the refractive index does not have polarization dependence.

2. 2. The optical fiber grating device of claim 1, wherein the core is located off-center of the optical fiber.

3. 2. The optical fiber grating device according to claim 1, wherein a grating is formed in the optical fiber by periodically applying pressure to the side of the optical fiber.

4. 4. The optical fiber grating device according to claim 3, wherein a marker indicating the pressing position where the pressure is applied is printed on the optical fiber.

5. a pressing member that periodically applies pressure to the side surface of the optical fiber from the outside, the pressing member includes a first pressing portion having periodically arranged convex portions, and a second pressing portion having a flat surface and provided on the opposite side of the first pressing portion with respect to the optical fiber; 4. The optical fiber grating device according to claim 3, wherein, in a cross section of the optical fiber, a line passing through a center of the optical fiber and a pressing position where the pressure is applied is defined as a first line, and a line passing through the center of the optical fiber and the center of the core is defined as a second line, and the core is provided at a position where an angle formed between the first line and the second line is greater than 0° and less than 90°.

6. A method for manufacturing an optical fiber grating device including an optical fiber, comprising: a first identifying step of identifying a first region in a cross section of the optical fiber, the first region having a refractive index that does not exhibit polarization dependence when a refractive index change is caused in the optical fiber; a second identifying step of identifying a second region in a cross section of the optical fiber that exhibits a refractive index modulation degree equal to or greater than a predetermined threshold when a refractive index change is caused in the optical fiber; forming a core in a region where the first region and the second region overlap.

7. A method for manufacturing an optical fiber grating device comprising an optical fiber having a core, comprising: a preparation step of providing the optical fiber; a rotating step of rotating the optical fiber around its axis so that the core is positioned at a proper position; and an applying step of periodically applying pressure to the side of the optical fiber when the core is positioned in a proper position to form a grating in the optical fiber.

8. A method for manufacturing an optical fiber grating device comprising an optical fiber having a core, comprising: a preparation step of providing the optical fiber; a rotating step of rotating the optical fiber about an axis by a predetermined angle; applying pressure periodically to the side of the optical fiber to form a grating in the optical fiber; a measuring step of measuring the polarization dependence of the refractive index of the core in a refractive index changing region, The method for manufacturing an optical fiber grating device includes repeating the rotating step, the applying step, and the measuring step to position the core at a position where there is no polarization dependency.

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