Optical Fiber Evaluation Apparatus and Optical Fiber Evaluation Method

Abstract

An object of the present invention is to provide an optical fiber evaluation apparatus and an optical fiber evaluation method for easily and highly accurately evaluating the deviation of the center of the cladding of an MCF and the centers of the respective cores of the MCF from the design values. The optical fiber evaluation apparatus of the present invention approximates the outer diameter of the cladding as a circle based on the cross-sectional image of the MCF, and takes the center of this circle as the cladding center. Further, the optical fiber evaluation apparatus of the present invention obtains the center coordinates of the respective cores with the center of this circle as the origin, rotates the cross-sectional image so that the difference between the center coordinates of the respective cores and the design coordinates becomes minimum, and derives this minimum value as the offset amount of the center of each core.

Description

Technical Field

[0001] The present invention relates to an optical fiber evaluation device and an optical fiber evaluation method for evaluating the geometric structure of a multi-core optical fiber. Background Art

[0002] In current single-mode optical fibers or multi-mode optical fibers, in order to ensure interconnectivity, optical characteristics and geometric structure parameters are standardized (Non-Patent Documents 1 and 2). These optical fibers are single-core optical fibers having a core at the center of the cladding, and thus test methods for evaluating the offset between the center of the cladding and the center of the core are shown (Non-Patent Documents 3 and 4).

[0003] Prior Art Documents

[0004] Non-Patent Document 1: ITU-T G.652, “Characteristics of a single-mode fib re andcable,” 2016.

[0005] Non-Patent Document 2: ITU-T G.651.1, “Characteristics of a 50 / 125μm mul timodegraded index optical fibre cable for the optical access networ k,” 2018.

[0006] Non-Patent Document 3: ITU-T G.650.1, “Definitions and test methods for l inear,deterministic attributes of single-mode fibre and cable,” 2018.

[0007] Non-Patent Document 4: JIS-C 6822, “Optical fiber structure parameter test method - dimensional characteristics,” 2009.

[0008] Non-Patent Document 5: T.Matsui et.al., “118.5Tbit / s Transmission over 316km-LongMulti-Core Fiber with Standard Cladding Diameter”,OECC2017,2-s2892,2017.

[0009] In a multi-core fiber (MCF: Multi Core Fiber, such as Non-Patent Document 5) having a plurality of cores within a cladding, in addition to the center of the cladding, it is also necessary to evaluate the offset of the center positions of the plurality of cores. If it is an MCF with a core disposed at the center of the cladding, the techniques of Non-Patent Documents 3 and 4 can be used to grasp the relative relationship between the center of the cladding and the center of the core, but there is a problem that it is difficult to measure the offset of other cores. In addition, in the MCF disclosed in Non-Patent Document 5, since there is no core at the center of the optical fiber, there is also a problem that it is difficult to grasp the relative relationship between the center of the cladding and the center of the core. Summary of the Invention

[0010] Therefore, in order to solve the above problems, an object of the present invention is to provide an optical fiber evaluation apparatus and an optical fiber evaluation method for simply and highly accurately evaluating the deviation of the center of the cladding of an MCF and the center of each core of the MCF from the design values.

[0011] In order to achieve the above object, the optical fiber evaluation apparatus of the present invention approximates the outer diameter of the cladding as a circle based on the cross-sectional image of the MCF, and takes the center of the circle as the center of the cladding. In addition, the optical fiber evaluation apparatus of the present invention obtains the center coordinates of each core with the center of the circle as the origin, rotates the cross-sectional image so that the difference between the center coordinates of each core and the design coordinates becomes the minimum, and derives the minimum value as the offset of each core center.

[0012] Specifically, the optical fiber evaluation apparatus of the present invention includes: an imaging unit that images a cross-sectional image of one end of a multi-core fiber; and an arithmetic circuit,

[0013] The arithmetic circuit performs:

[0014] Approximates the outer periphery of the cladding of the multi-core fiber as a circle based on the cross-sectional image;

[0015] Takes the center and diameter of the circle as the center and diameter of the multi-core fiber respectively, and defines an arbitrary coordinate system with the center of the multi-core fiber as the origin in the cross-sectional image;

[0016] Detects the measurement center that is the center of each core of the multi-core fiber from the cross-sectional image;

[0017] Overlaps the design center, which is the center of each core of the multi-core fiber in design, with the cross-sectional image with the center of the multi-core fiber in design as the origin;

[0018] Detects the core-to-core distance between the measurement center and the design center for each core of the multi-core fiber, and calculates the average value or the mean square value of the core-to-core distances of all the cores of the multi-core fiber;

[0019] Derive the rotation angle in the coordinate system where the average value or the mean square sum is minimized; and

[0020] Use the inter-core distances at the rotation angle as the offsets of the respective cores of the multi-core optical fiber.

[0021] Furthermore, the optical fiber evaluation method of the present invention

[0022] Take a cross-sectional image of one end of the multi-core optical fiber;

[0023] Approximate the outer circumference of the cladding of the multi-core optical fiber as a circle based on the cross-sectional image;

[0024] Use the center and diameter of the circle as the center and diameter of the multi-core optical fiber, and define an arbitrary coordinate system with the center of the multi-core optical fiber as the origin in the cross-sectional image;

[0025] Detect the measurement centers that are the centers of the respective cores of the multi-core optical fiber from the cross-sectional image;

[0026] With the designed center of the multi-core optical fiber as the origin, overlap the designed centers that are the centers of the respective cores of the multi-core optical fiber in design with the cross-sectional image;

[0027] Detect the inter-core distances between the measurement centers and the designed centers for each core of the multi-core optical fiber, and calculate the average value or the mean square sum of the inter-core distances of all the cores of the multi-core optical fiber;

[0028] Derive the rotation angle in the coordinate system where the average value or the mean square sum is minimized; and

[0029] Use the inter-core distances at the rotation angle as the offsets of the respective cores of the multi-core optical fiber.

[0030] In particular, the optical fiber evaluation apparatus and method of the present invention preferably obtain a cross-sectional image of the MCF in the following manner.

[0031] The photographing unit of the optical fiber evaluation apparatus of the present invention is characterized by including:

[0032] A holding mechanism that holds the multi-core optical fiber in a straight line;

[0033] A light source that emits light into the other end of the multi-core optical fiber;

[0034] An image photographing device that photographs the entire cross-section of one end of the multi-core optical fiber as an overall image;

[0035] A light image photographing device that obtains the intensity distribution of the light emitted from the respective cores at one end of the multi-core optical fiber; and

[0036] A switcher switches to the image shooter or the optical image shooter to shoot one end of the multi-core optical fiber.

[0037] The arithmetic circuit uses the peak position of the intensity distribution as the measurement center.

[0038] In addition, the optical fiber evaluation method of the present invention is characterized in that when shooting the cross-sectional image,

[0039] Light is incident on the other end of the multi-core optical fiber;

[0040] An overall image of the entire cross-section of one end of the multi-core optical fiber is obtained;

[0041] The intensity distribution of the light emitted from each core is obtained at one end of the multi-core optical fiber; and

[0042] The peak position of the intensity distribution is used as the measurement center.

[0043] The optical fiber evaluation device and method of the present invention synthesize an image of the entire cross-section of the MCF and an image of the intensity distribution of the light emitted from the core of the optical fiber. On the synthesized cross-sectional image, the outer diameter of the cladding is approximately circular, and the center of this circle is used as the cladding center. In addition, the optical fiber evaluation device and method of the present invention perform fitting of the cross-section on the synthesized image and the designed cross-section in such a way that the (square) average value of the distances between the observed positions of each core based on the cladding center and the designed positions is minimized, and calculate the offset of each core center.

[0044] Therefore, the present invention can provide an optical fiber evaluation device and an optical fiber evaluation method for simply and highly accurately evaluating the cladding center of the MCF and the deviation of the center of each core of the MCF from the design value.

[0045] The present invention can provide an optical fiber evaluation device and an optical fiber evaluation method for simply and highly accurately evaluating the cladding center of the MCF and the deviation of the center of each core of the MCF from the design value. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a cross-sectional view illustrating a multi-core optical fiber.

[0047] Figure 2 It is a flowchart illustrating the optical fiber evaluation method of the present invention.

[0048] Figure 3 It is a schematic diagram illustrating the optical fiber evaluation method of the present invention. The solid circle represents the measured core position, and the dashed circle represents the designed core position.

[0049] Figure 4 It is a schematic diagram illustrating the optical fiber evaluation device of the present invention.

[0050] Figure 5 This is a diagram showing the holding mechanism of the optical fiber evaluation device of the present invention. Detailed implementation mode

[0051] The implementation modes of the present invention will be described with reference to the accompanying drawings. The implementation modes described below are examples of the present invention, and the present invention is not limited to the following implementation modes. In addition, in this specification and the drawings, components with the same reference numerals represent the same components.

[0052] Figure 1 This is a diagram showing the cross-sectional image of the optical fiber to be measured. In this implementation mode, a multi-core optical fiber 10 in which four cores 11 are arranged in a square lattice pattern with a spacing Λ within a single circular cladding 12 is used as the optical fiber to be measured. The spacing Λ refers to the center-to-center distance of the cores 11. In addition, the optical fiber evaluation method of the present invention is not limited to Figure 1 the evaluation of the multi-core optical fiber 10 shown. The optical fiber evaluation method of the present invention can also be the evaluation of the following multi-core optical fibers.

[0053] (1) Multi-core optical fibers having a number of cores other than four

[0054] (2) Multi-core optical fibers having a core at the center 13 of the cladding

[0055] (3) Multi-core optical fibers in which a plurality of cores are arranged in concentric circles at different distances from the center 13 of the cladding (multi-core optical fibers in which the cores are arranged in multiple layers)

[0056] (4) Multi-core optical fibers in which a plurality of cores are arranged linearly (non-rotationally symmetric) within the cladding

[0057] Figure 2 This is a flowchart showing the optical fiber evaluation method of this implementation mode. This optical fiber evaluation method performs:

[0058] Taking a cross-sectional image of one end of the multi-core optical fiber (steps S01, S02);

[0059] Approximating the outer periphery of the cladding of the multi-core optical fiber as a circle based on the cross-sectional image (step S03);

[0060] Taking the center and diameter of the circle as the center and diameter of the multi-core optical fiber respectively, and defining an arbitrary coordinate system with the center of the multi-core optical fiber as the origin in the cross-sectional image (steps S03, S04);

[0061] Detecting the measurement centers that are the centers of the respective cores of the multi-core optical fiber based on the cross-sectional image (step S04);

[0062] Taking the design center of the multi-core optical fiber as the origin, and overlapping the design centers that are the centers of the respective cores of the multi-core optical fiber in design with the cross-sectional image (step S05);

[0063] Detect the core-to-core distance between the measurement center and the design center for each core of the multi-core optical fiber, and calculate the average value or the mean square value of the sum of the core-to-core distances of all the cores of the multi-core optical fiber (step S05);

[0064] Derive the rotation angle in the coordinate system where the average value or the mean square value is the smallest (step S06); and

[0065] Use the core-to-core distances at the rotation angle as the offset amounts of the respective cores of the multi-core optical fiber (step S06).

[0066] Figure 3 FIG. is a diagram showing an evaluation image for explaining this optical fiber evaluation method. Figure 3 (a) of FIG. is an implementation image of steps S03 and S04. Figure 3 (b) of FIG. is Figure 2 an implementation image of step S05 of FIG.

[0067] Use Figure 2 and Figure 3 to explain this optical fiber evaluation method in more detail.

[0068] In step S01, cut the optical fiber to be measured. An existing optical fiber cutter can be used for cutting the optical fiber. In addition, it is preferable that the cut end face is cut perpendicularly to the long side direction of the optical fiber, and it is also effective to grind the cut end face to improve flatness.

[0069] In step S02, obtain an overall image of the cladding of the optical fiber to be measured.

[0070] In step S03, as Figure 3 (a) of FIG. shows, approximate the outer periphery of the cladding as a circle, and while deriving its center coordinates, calculate the cladding diameter as the diameter of the approximate circle.

[0071] In step S04, as Figure 3 (a) of FIG. shows, determine a coordinate system with the center coordinates as the origin, and derive the center coordinates of each core 11. In this example, the coordinate system is described as an orthogonal coordinate system. For example, when obtaining an image in step S02, visible light can be incident on each core of the optical fiber to be measured, and the center coordinates of the core can be determined based on the brightness distribution of the obtained image. This method can obtain core coordinates more simply and with high precision.

[0072] In addition, when the outer circle of the cladding and the core can be sufficiently recognized based on the image obtained in step S02 and the cladding center and the core center can be sufficiently measured, it is also possible to omit the step of incident visible light on each core of the optical fiber to be measured and obtain the coordinates of the core center based on its intensity distribution.

[0073] The coordinates of the core 11 obtained from the image in the above-described manner are recorded as the "measurement center coordinates".

[0074] In step S05, using the core pitch Λ in the design of the optical fiber to be measured, the center coordinates of each core 11a in the design are plotted on the orthogonal coordinates. The center coordinates of the core 11a in the design are recorded as the "design center coordinates". In addition, as shown in (b) of Figure 3 , the distance δi (where i represents the core number) between the design center coordinates and the measurement center coordinates of each core is calculated, and the average or the mean square sum of the distances δi is calculated.

[0075] In step S06, the orthogonal coordinates are rotated by θ with the center coordinates as the origin, and the evaluation in step S05 is performed. θ is rotated by 360 degrees, and the θ at which the average value or the mean square sum average of the distances δi is the minimum is determined. The distance δi at this θ is determined as the offset of the center of each core of the optical fiber to be measured.

[0076] Figure 4 It is a diagram for explaining an optical fiber evaluation apparatus 301 that implements the optical fiber evaluation method described in Figure 2 . The optical fiber evaluation apparatus 301 includes an imaging unit 20 and an arithmetic circuit 30.

[0077] The imaging unit 20 captures a cross-sectional image of one end of the multi-core optical fiber 10.

[0078] The arithmetic circuit 30 performs the following:

[0079] Approximate the outer periphery of the cladding 12 of the multi-core optical fiber 10 as a circle based on the cross-sectional image;

[0080] Use the center and diameter of the circle as the center and diameter of the multi-core optical fiber 10 respectively, and define an arbitrary coordinate system with the center of the multi-core optical fiber 10 as the origin in the cross-sectional image;

[0081] Detect the measurement center that is the center of each core 11 of the multi-core optical fiber 10 from the cross-sectional image;

[0082] With the center in the design of the multi-core optical fiber 10 as the origin, overlap the design center that is the center of each core 11a in the design of the multi-core optical fiber 10 with the cross-sectional image;

[0083] For each core of the multi-core optical fiber 10, detect the core-to-core distance δi between the measurement center and the design center, and calculate the average or the mean square sum average of the core-to-core distances δi of all the cores of the multi-core optical fiber 10;

[0084] Derive the rotation angle θ in the coordinate system where the average value or the mean square sum average is the minimum; and

[0085] The core-to-core distance at the rotation angle θ is taken as the offset of each core 11 of the multi-core optical fiber 10, respectively.

[0086] The photographing unit 20 includes:

[0087] A holding mechanism 21 that holds the multi-core optical fiber 10 in a straight line;

[0088] A light source 22 that injects light into the other end of the multi-core optical fiber 10;

[0089] An image camera 23 that photographs the entire cross-section of one end of the multi-core optical fiber 10 as an overall image;

[0090] An optical image camera 24 that obtains the intensity distribution of light emitted from each core at one end of the multi-core optical fiber 10; and

[0091] A switch 25 that switches to photograph one end of the multi-core optical fiber 10 using the image camera or the optical image camera.

[0092] The optical fiber evaluation device 301 includes: a light source 22, a holding mechanism 21, an objective lens 26, an image camera 23 for photographing the cross-section of the multi-core optical fiber 10, an optical image camera 24 for determining the centers of the cores of the multi-core optical fiber 10, and a photographing switching unit 25 that switches between these two cameras.

[0093] The light source 22 is a white light source such as a halogen lamp that generates white light including visible light and near-infrared light. As long as the entire cladding of the multi-core optical fiber 10 can be irradiated, the light can also be condensed using a lens or the like.

[0094] Figure 5 It is a diagram illustrating the holding mechanism 21. The holding mechanism 21 includes a substrate 21a formed with a V-groove or the like for arranging the multi-core optical fiber 10, and a clamping portion 21b that restrains the multi-core optical fiber 10 from moving.

[0095] The objective lens 26 has a magnification that can photograph the entire cladding in the cross-section of the multi-core optical fiber 10. In the case where the objective lens 26 has a magnification that can only photograph a part of the cladding in the cross-section of the multi-core optical fiber 10, the objective lens 26 and the photographing switching unit 25 (including the image camera and the optical image camera) can also be moved simultaneously to photograph the entire cross-section of the multi-core optical fiber 10 while scanning the cross-section of the multi-core optical fiber 10.

[0096] The light source 22, the multi-core optical fiber 10, and the objective lens 26 are arranged so that the optical axis L coincides. In addition, "aligning the multi-core optical fiber 10 with the optical axes of the light source 22 and the objective lens 26" means that the central axis of the multi-core optical fiber 10 substantially coincides with the optical axis so that the light from the light source 22 can be incident on the multi-core optical fiber. Conversely, if the light from the light source 22 can be incident on the multi-core optical fiber, it is not necessary to make the central axis of the multi-core optical fiber 10 exactly coincide with the optical axis.

[0097] The image pick-up device 23 and the optical image pick-up device 24 have a photographing range capable of sufficiently photographing the field of view of the objective lens 26. The image pick-up device 23 can photograph the entire cross-section of the multi-core optical fiber 10. The optical image pick-up device 24 can photograph the intensity distribution of the near-infrared light emitted from the cores of the multi-core optical fiber 10. In addition, when the outer circle of the cladding and the cores can be sufficiently identified by the image pick-up device 23 and the center of the cladding and the center of the cores can be sufficiently measured, the measurement of the core center coordinates using the optical image pick-up device 24 can be omitted.

[0098] The photographing switching unit 25 is configured to be able to photograph two types of images of the cross-section of the multi-core optical fiber 10. For example, the photographing switching unit 25 arranges the image pick-up device 23 and the optical image pick-up device 25 on the same track 25a. The photographing switching unit 25 may arrange the image pick-up device 23 and the optical image pick-up device 25 not on a track but on a rotator or the like. When photographing the entire cross-section of the multi-core optical fiber 10 by the image pick-up device 23, the photographing switching unit 25 moves the image pick-up device 23 to the position of the optical axis L on the track 25a. In addition, when photographing the intensity distribution of the near-infrared light emitted from the cores of the multi-core optical fiber 10 by the optical image pick-up device 24, the photographing switching unit 25 moves the optical image pick-up device 24 to the position of the optical axis L on the track 25a.

[0099] The arithmetic circuit 30 synthesizes the image of the entire cladding photographed by the photographing unit 20 and the image of the intensity distribution of each core. And, the arithmetic circuit 30 can measure the cladding diameter of the multi-core optical fiber 10 and the offset amount of the center coordinates of each core by performing steps S03 to S06 described in Figure 2 In this way.

[0100] As described above, the optical fiber evaluation device 301 can easily and highly accurately evaluate the geometric structure (cladding center, cladding diameter, offset amount of each core center) of the multi-core optical fiber 10 in which a plurality of cores are arranged in an arbitrary state.

[0101] [Invention Highlights]

[0102] By using the cladding center and the design center as a reference, it is possible to evaluate the geometric structure of any optical fiber that does not have a core at the center of the optical fiber.

[0103] In order to highly accurately evaluate the center of each core of the multi-core structure, in addition to the need to grasp the offset amount of each core center, it is also necessary to grasp the offset direction (rotation angle offset) of each core. However, in the present invention, since the cladding center and the design center are used as a reference, it is not necessary to separately evaluate the rotation angle offset.

[0104] Explanation of Reference Numerals

[0105] 10: Multi-core optical fiber

[0106] 11: Core

[0107] 11a: Core position in design

[0108] 12: Cladding

[0109] 13: Cladding center

[0110] 20: Imaging unit

[0111] 21: Holding mechanism

[0112] 21a: Substrate

[0113] 21b: Clamping part

[0114] 22: Light source

[0115] 23: Image imager

[0116] 24: Optical image imager

[0117] 25: Imaging switching part

[0118] 25a: Track

[0119] 26: Objective lens

[0120] 30: Arithmetic circuit

[0121] 301: Optical fiber evaluation device

Claims

1. An optical fiber evaluation device, characterized in that: It includes: A photographing unit that photographs a cross-sectional image of one end of a multi-core optical fiber; and an arithmetic circuit, The arithmetic circuit performs: Approximating the outer periphery of the cladding of the multi-core optical fiber as a circle according to the cross-sectional image; Taking the center and diameter of the circle as the center and diameter of the multi-core optical fiber respectively, and defining an arbitrary coordinate system with the center of the multi-core optical fiber as the origin in the cross-sectional image; Detecting the measurement centers that are the centers of the respective cores of the multi-core optical fiber from the cross-sectional image; Taking the designed center of the multi-core optical fiber as the origin, and overlapping the designed centers that are the centers of the respective cores of the multi-core optical fiber in design with the cross-sectional image; Detecting the inter-core distances between the measurement centers and the designed centers for each core of the multi-core optical fiber, calculating the average value of the inter-core distances of all the cores of the multi-core optical fiber or calculating the average value of the sum of squares of the inter-core distances of all the cores of the multi-core optical fiber; Deriving the rotation angle in the coordinate system where the average value or the average value of the sum of squares is the smallest; And Taking the inter-core distances at the rotation angle as the offset amounts of the respective cores of the multi-core optical fiber.

2. The optical fiber evaluation device according to claim 1, characterized in that: The photographing unit includes: A holding mechanism that holds the multi-core optical fiber in a straight line; A light source that emits light into the other end of the multi-core optical fiber; An image camera that photographs the entire cross-section of one end of the multi-core optical fiber as an overall image; A light image camera that obtains the intensity distribution of the light emitted from the respective cores at one end of the multi-core optical fiber; and A switch that switches to the image camera or the light image camera to photograph one end of the multi-core optical fiber, The arithmetic circuit takes the peak position of the intensity distribution as the measurement center.

3. An optical fiber evaluation method, characterized in that: Photographing a cross-sectional image of one end of a multi-core optical fiber; Approximating the outer periphery of the cladding of the multi-core optical fiber as a circle according to the cross-sectional image; Taking the center and diameter of the circle as the center and diameter of the multi-core optical fiber respectively, and defining an arbitrary coordinate system with the center of the multi-core optical fiber as the origin in the cross-sectional image; Detecting the measurement centers that are the centers of the respective cores of the multi-core optical fiber from the cross-sectional image; Taking the designed center of the multi-core optical fiber as the origin, and overlapping the designed centers that are the centers of the respective cores of the multi-core optical fiber in design with the cross-sectional image; Detecting the inter-core distances between the measurement centers and the designed centers for each core of the multi-core optical fiber, calculating the average value of the inter-core distances of all the cores of the multi-core optical fiber or calculating the average value of the sum of squares of the inter-core distances of all the cores of the multi-core optical fiber; Deriving the rotation angle in the coordinate system where the average value or the average value of the sum of squares is the smallest; And Taking the inter-core distances at the rotation angle as the offset amounts of the respective cores of the multi-core optical fiber.

4. The optical fiber evaluation method according to claim 3, characterized in that: When photographing the cross-sectional image, Emitting light into the other end of the multi-core optical fiber; Obtain an overall image of the entire cross-section of one end of the multi-core optical fiber; Obtain the intensity distribution of the light emitted from each of the cores at one end of the multi-core optical fiber; and Use the peak position of the intensity distribution as the measurement center.

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