Optical fiber base material, and method and device for measuring refractive index change position of optical fiber

Abstract

The method for measuring the refractive index change position of the optical fiber base material is a method for measuring the refractive index change position of the optical fiber base material by using a measuring device. The measuring device is provided with: a holding unit that holds an optical fiber base material so as to be rotatable about a central axis; a light source and a camera that are disposed on an observation axis orthogonal to the central axis; a polarizer; and a detector that has a transmission axis orthogonal to the transmission axis of the polarizer. The measurement method includes: rotating an optical fiber base material about a central axis so that a straight line that is orthogonal to the central axis and passes through two or more desired refractive index change positions to be measured is orthogonal to an observation axis; light emitted from a light source is irradiated to the side surface of the optical fiber base material through a polarizer; receiving the light transmitted through the optical fiber base material by a camera through a detector to acquire a light intensity image; detecting a refractive index change position of the optical fiber base material based on the light intensity image; and moving the optical fiber base material or the observation axis parallel to the central axis.

Description

Technical Field

[0001] The present disclosure relates to a method and a device for measuring the refractive index change position of an optical fiber base material and an optical fiber. This application claims priority based on International Application No. PCT / JP2022 / 018193 filed on April 19, 2022, and all the contents recorded in the international application are cited. Background Art

[0002] In recent years, with the development of technologies such as IoT (Internet of Things) and Metaverse, there is a trend of further increasing data traffic. This trend is expected to continue in the future. In order to achieve large-capacity optical communication networks, not only the existing general-purpose single-mode optical fiber (SMF) but also various optical fibers are being actively studied. As such optical fibers, for example, thin-diameter SMF, multi-mode optical fiber (MMF), and multi-core optical fiber (MCF) can be listed. According to the thin-diameter SMF, the number of optical fibers per optical cable can be increased. According to MMF and MCF, the spatial multiplexing density of the signal per optical fiber can be improved. In order to realize an optical communication system using such optical fibers, it is necessary not only to establish a design method for the optical fiber itself, but also to establish the manufacturing technology of the optical fiber mother material, and the connection technology such as fusion splicer / optical connector. For the establishment of these technologies, it is necessary to have a technology that can non-destructively measure the position of the refractive index change inside the optical fiber mother material and the optical fiber.

[0003] Patent document 1 describes the necessity of controlling the core spacing during the rod collapse process. Patent document 2 describes a method for measuring the core eccentricity of an optical fiber mother material using a polarization filter. By removing the reflected light component from the outer surface of the optical fiber mother material with a polarization filter, the boundary line caused by the difference in glass composition between the core and cladding can be clearly identified.

[0004] Patent document 3 describes a method for measuring the refractive index distribution of an optical fiber by generating a phase image based on two intensity images obtained by observing the optical fiber from the side from different imaging directions and performing an inverse Abel transform of the phase image. Non-patent document 1 describes a method for non-destructively measuring the refractive index distribution of an optical fiber by using an optical system based on a Mach-Zehnder interferometer to detect light irradiated from the side of the optical fiber with an imaging element and performing a fast Fourier transform on the interference fringe signal.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2014-159348

[0008] Patent Document 2: Japanese Patent Application Publication No. 2000-213912

[0009] Patent Document 3: Japanese Patent Application Publication No. 2021-156761

[0010] Non-patent literature

[0011] Non-patent document 1: ADYablon, "Multi-Wavelength Optical Fiber RefractiveIndex Profiling by Spatially Resolved Fourier Transform Spectroscopy," Journal of Lightwave Technology, vol.28, no.4, pp.360-364, Feb.15, 2010 Summary of the invention

[0012] A method for measuring a refractive index change position of an optical fiber base material according to one aspect of the present disclosure is a method for measuring a refractive index change position of an optical fiber base material using a measuring device, the measuring device comprising: a holding portion for holding the optical fiber base material so as to be rotatable about a central axis; a light source and a camera, which are arranged on an observation axis orthogonal to the central axis with the optical fiber base material interposed therebetween; a polarizer, which is arranged between the light source and the optical fiber base material; and a detector, which has a transmission axis orthogonal to the transmission axis of the polarizer and is arranged between the optical fiber base material and the camera. The method for measuring a refractive index change position of an optical fiber base material comprises the following steps: rotating the optical fiber base material about the central axis so that a straight line orthogonal to the central axis and passing through two or more desired refractive index change positions as measurement objects is orthogonal to the observation axis; irradiating light emitted from the light source to a side surface of the optical fiber base material via the polarizer; receiving light transmitted through the optical fiber base material via the detector by the camera to acquire a light intensity image; detecting the refractive index change positions at two or more points based on the light intensity image; and moving the optical fiber base material or the observation axis parallel to the central axis. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a diagram showing a cross section of an optical fiber preform and the refractive index distribution in the direction of the arrow in the cross section.

[0014] Figure 2 is a flowchart showing a method for manufacturing an optical fiber.

[0015] Figure 3 It is a diagram showing the structure of a device for measuring a refractive index change position according to an embodiment.

[0016] Figure 4 : is a flowchart showing a method for measuring a refractive index change position according to an embodiment.

[0017] Figure 5It is a cross-sectional view showing the relationship between the rotational motion of the optical fiber preform and the observation axis.

[0018] Figure 6 This is a schematic diagram for explaining the correction of image distortion caused by the lens effect on the side surface of the optical fiber base material.

[0019] Figure 7 It is a diagram showing the structure of a device for measuring a refractive index change position according to a modification example.

[0020] Figure 8 It is a perspective view showing the relationship between the container and the observation axis. DETAILED DESCRIPTION

[0021] [Technical Problems to be Solved by the Present Disclosure]

[0022] In the method for measuring the refractive index distribution of an optical fiber mother material using a preform analyzer, the laser is scanned point by point from the side of the optical fiber mother material along a straight line (scanning axis) that is orthogonal to the central axis of the optical fiber mother material and to the emission direction of the laser. For this reason, if the diameter of the optical fiber mother material is thickened, the measurement time increases due to the increase in the number of scanning points. In the method described in Patent Document 2, when the refractive index difference between dissimilar materials is small, it is difficult to detect the boundary line on the intensity image with high precision, and the measurement error is likely to increase. In the method described in Patent Document 3 and the method described in Non-Patent Document 1, the measurement time increases because the time required for numerical calculation is long.

[0023] An object of the present disclosure is to provide a method and a device for measuring a refractive index change position of an optical fiber preform and an optical fiber, which can reduce measurement time and improve measurement accuracy.

[0024] [Effects of the present disclosure]

[0025] According to the present disclosure, it is possible to provide a method and a device for measuring a refractive index change position of an optical fiber preform and an optical fiber, which can improve measurement accuracy while reducing measurement time.

[0026] [Description of the implementation aspects of the present disclosure]

[0027] First, the implementation aspects of the present disclosure are listed for description. (1) A method for measuring a refractive index change position of an optical fiber base material according to one aspect of the present disclosure is a method for measuring a refractive index change position of an optical fiber base material using a measuring device, the measuring device comprising: a holding portion that holds the optical fiber base material so as to be rotatable about a central axis; a light source and a camera that are arranged on an observation axis orthogonal to the central axis with the optical fiber base material interposed therebetween; a polarizer that is arranged between the light source and the optical fiber base material; and a detector that has a transmission axis orthogonal to the transmission axis of the polarizer and is arranged between the optical fiber base material and the camera, the method for measuring a refractive index change position of an optical fiber base material comprising the following steps: rotating the optical fiber base material about the central axis so that a straight line that is orthogonal to the central axis and passes through two or more desired refractive index change positions as measurement objects is orthogonal to the observation axis; irradiating light emitted from the light source to a side surface of the optical fiber base material via the polarizer; receiving light that has passed through the optical fiber base material via the detector by the camera to acquire a light intensity image; detecting the refractive index change positions at two or more points based on the light intensity image; and moving the optical fiber base material or the observation axis parallel to the central axis.

[0028] In this measurement method, a measurement device having a polarizer and a detector having mutually orthogonal transmission axes is used. When the polarized wave after passing through the polarizer passes through the strain existing at the boundary position between different materials, that is, the position where the refractive index changes, the polarization state changes due to the photoelastic effect. For this reason, the light after passing through the strain can pass through the detector. Therefore, in the light intensity image obtained by the camera, a difference in brightness is generated depending on the presence or absence of strain. According to this measurement method, it is possible to improve the measurement accuracy while suppressing the measurement time.

[0029] (2) The method for measuring the refractive index change position of the optical fiber base material of (1) above may further include a step of immersing at least a portion of the optical fiber base material in a central axis direction in a medium having a refractive index greater than that of air and less than that of glass, wherein in the method for measuring the refractive index change position of the optical fiber base material, light emitted from a light source is irradiated to the side surface of the optical fiber base material via a polarizer and a medium, and light transmitted through the optical fiber base material is received by a camera via the medium and a detector to acquire a light intensity image. In this case, the influence of the lens effect originating from the shape of the side surface of the optical fiber base material is suppressed.

[0030] (3) In the above (1) or (2), the optical fiber base material may be a polarization-maintaining optical fiber base material. In this case, the eccentricity and size of not only the core portion but also the stress-applying portion can be measured.

[0031] (4) In the above (1) or (2), the optical fiber preform may be a multi-core optical fiber preform. In this case, the eccentricity and size of the core portion can be measured.

[0032] (5) In any of the above (1) to (4), the optical fiber base material may include a cladding portion and a pair of core portions arranged across a central axis and having a maximum center-to-center distance, the two or more refractive index change positions are positions of two boundaries close to the outer peripheral surface of the cladding portion among boundaries between the cladding portion and the pair of core portions existing on a straight line, and in the method for measuring the refractive index change position of the optical fiber base material, the center-to-center distance of the pair of core portions is measured based on the distance between the two boundaries detected in the detection step. In this case, the refractive index change position can be measured without being affected by other core portions.

[0033] (6) Alternatively, the method for measuring the refractive index change position of the optical fiber base material according to any one of (1) to (5) above may further include a step of correcting the light intensity image for image distortion caused by the lens effect of the side surface of the optical fiber base material using the following formula:

[0034]

[0035] Here, a is the apparent distance from the refractive index change position to the central axis, x is the actual distance from the refractive index change position to the central axis, n0 is the refractive index around the cladding part, n2 is the refractive index of the cladding part, and r is the radius of the cladding part. In this case, image distortion caused by the lens effect of the side of the optical fiber mother material can be easily corrected.

[0036] (7) An apparatus for measuring a refractive index change position of an optical fiber base material according to one aspect of the present disclosure measures a refractive index change position of the optical fiber base material, the apparatus comprising: a holding portion holding the optical fiber base material so as to be rotatable about a central axis; a light source and a camera arranged on an observation axis orthogonal to the central axis with the optical fiber base material interposed therebetween; a polarizer arranged between the light source and the optical fiber base material; and a detector having a transmission axis orthogonal to the transmission axis of the polarizer and arranged between the optical fiber base material and the camera, wherein the holding portion holds the optical fiber base material in such a manner that a straight line orthogonal to the central axis and passing through two or more desired refractive index change positions as measurement targets is orthogonal to the observation axis, the light source irradiates light toward a side surface of the optical fiber base material via the polarizer, and the camera receives light transmitted through the optical fiber base material via the detector to obtain a light intensity image having a light-dark difference at the two or more refractive index change positions, and the holding portion holds the optical fiber base material so as to be movable in a direction parallel to the central axis, or the light source, the camera, the polarizer, and the detector are configured to be movable in a direction parallel to the central axis.

[0037] In this measuring device, a polarizer and a detector having mutually orthogonal transmission axes are used. When the polarized wave after passing through the polarizer passes through the strain existing at the boundary position between different materials, that is, the position where the refractive index changes, the polarization state changes due to the photoelastic effect. For this reason, the light after passing through the strain can pass through the detector. Therefore, in the light intensity image acquired by the camera, a difference in brightness is generated depending on the presence or absence of strain. According to this measuring device, it is possible to improve the measurement accuracy while suppressing the measurement time.

[0038] (8) In the above (7), a container for accommodating at least a portion of the central axis direction of the optical fiber mother material may be further provided, and the container may be filled with a medium having a refractive index greater than that of air and less than that of glass. In this case, the influence of the lens effect originating from the shape of the side surface of the optical fiber mother material is suppressed. Therefore, the light emitted from the light source can be irradiated to the side surface of the optical fiber mother material via the polarizer and the medium, and the light after passing through the optical fiber mother material can be received by the camera via the medium and the detector.

[0039] (9) One aspect of the present disclosure relates to a method for measuring a refractive index change position of an optical fiber, which is a method for measuring a refractive index change position of an optical fiber using a measuring device, the measuring device comprising: a holding portion for holding the optical fiber so as to be rotatable about a central axis; a light source and a camera, which are arranged on an observation axis orthogonal to the central axis via the optical fiber; a polarizer, which is arranged between the light source and the optical fiber; and a detector, which has a transmission axis orthogonal to the transmission axis of the polarizer and is arranged between the optical fiber and the camera. The method for measuring a refractive index change position of an optical fiber comprises the following steps: rotating the optical fiber about the central axis so that a straight line orthogonal to the central axis and passing through two or more desired refractive index change positions as measurement objects is orthogonal to the observation axis; irradiating light emitted from the light source to the side of the optical fiber via the polarizer; causing light after passing through the optical fiber to be received by the camera via the detector to obtain a light intensity image; and detecting the refractive index change positions at two or more points based on the light intensity image.

[0040] In this measurement method, a measurement device having a polarizer and a detector having mutually orthogonal transmission axes is used. When the polarized wave after passing through the polarizer passes through the strain existing at the boundary position between different materials, that is, the position where the refractive index changes, the polarization state changes due to the photoelastic effect. For this reason, the light after passing through the strain can pass through the detector. Therefore, in the light intensity image obtained by the camera, a difference in brightness is generated depending on the presence or absence of strain. According to this measurement method, it is possible to improve the measurement accuracy while suppressing the measurement time.

[0041] (10) The method for measuring the refractive index change position of an optical fiber of (9) above may further include a step of moving the optical fiber or the observation axis parallel to the central axis. In this case, the internal structure of the optical fiber can be easily measured at any position in the longitudinal direction.

[0042] (11) Alternatively, the method for measuring the refractive index change position of an optical fiber of (9) or (10) further includes a step of immersing at least a portion of the optical fiber in a medium having a refractive index greater than that of air and less than that of glass, wherein in the method for measuring the refractive index change position of an optical fiber, light emitted from a light source is irradiated onto the side surface of the optical fiber via a polarizer and a medium, and light transmitted through the optical fiber is received by a camera via the medium and a detector, thereby acquiring a light intensity image. In this case, the influence of the lens effect originating from the shape of the side surface of the optical fiber is suppressed.

[0043] (12) In any of the above (9) to (11), the optical fiber may be a polarization-maintaining optical fiber. In this case, not only the core but also the eccentricity and size of the stress-applying portion can be measured.

[0044] (13) In any of the above (9) to (11), the optical fiber may be a multi-core optical fiber. In this case, the eccentricity and size of the core can be measured.

[0045] (14) In the above (13), the optical fiber may include a cladding and a pair of cores arranged across a central axis and having a maximum center-to-center distance, the two or more refractive index change positions are positions of two boundaries close to the outer peripheral surface of the cladding among the boundaries between the cladding and the pair of cores existing on a straight line, and in the method for measuring the refractive index change position of the optical fiber, the center-to-center distance of the pair of cores is measured based on the distance between the two boundaries detected in the detection step. In this case, the refractive index change position can be measured without being affected by other core parts.

[0046] (15) The method for measuring the refractive index change position of an optical fiber according to any one of (9) to (14) above may also include a step of correcting the image distortion caused by the lens effect on the side of the optical fiber using the following formula.

[0047]

[0048] Here, a may be the apparent distance from the refractive index change position to the central axis, x may be the actual distance from the refractive index change position to the central axis, n0 may be the refractive index around the cladding, n2 may be the refractive index of the cladding, and r may be the radius of the cladding. In this case, image distortion caused by the lens effect on the side of the optical fiber can be easily corrected.

[0049] (16) An optical fiber refractive index change position measuring device according to one aspect of the present disclosure measures the refractive index change position of the optical fiber, and the optical fiber refractive index change position measuring device comprises: a holding portion, which holds the optical fiber so as to be rotatable around a central axis; a light source and a camera, which are arranged on an observation axis orthogonal to the central axis via the optical fiber; a polarizer, which is arranged between the light source and the optical fiber; and a detector, which has a transmission axis orthogonal to the transmission axis of the polarizer and is arranged between the optical fiber and the camera, wherein the holding portion holds the optical fiber in such a manner that a straight line which is orthogonal to the central axis and passes through two or more desired refractive index change positions as measurement objects is orthogonal to the observation axis, the light source irradiates light toward the side of the optical fiber via the polarizer, and the camera receives light transmitted through the optical fiber via the detector to obtain a light intensity image having a light-dark difference at the two or more refractive index change positions.

[0050] In this measuring device, a polarizer and a detector having mutually orthogonal transmission axes are used. When the polarized wave after passing through the polarizer passes through the strain existing at the boundary position between different materials, that is, the position where the refractive index changes, the polarization state changes due to the photoelastic effect. For this reason, the light after passing through the strain can pass through the detector. Therefore, in the light intensity image acquired by the camera, a difference in brightness is generated depending on the presence or absence of strain. According to this measuring device, it is possible to improve the measurement accuracy while suppressing the measurement time.

[0051] (17) In the above (16), the holding portion may hold the optical fiber so as to be movable in a direction parallel to the central axis, or the light source, camera, polarizer, and detector may be configured so as to be movable in a direction parallel to the central axis. In this case, the internal structure of the optical fiber can be easily measured at any position in the longitudinal direction.

[0052] [Details of the embodiments of the present disclosure]

[0053] Hereinafter, specific examples of the method and device for measuring the refractive index change position of the optical fiber mother material and the optical fiber disclosed in the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is not limited to these examples, but is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are marked with the same reference numerals, and repeated descriptions are omitted.

[0054] (Optical fiber base material and optical fiber)

[0055] First, an optical fiber preform and an optical fiber that are symmetrical in measurement of a method and an apparatus for measuring a refractive index change position according to an embodiment will be described. Figure 1 : is a diagram showing the cross section of the optical fiber mother material and the refractive index distribution in the direction of the arrow in the cross section. Figure 1As shown, the optical fiber base material 1 has a plurality of cores 2, a common cladding portion 3 and a marking portion 4. The optical fiber base material 1 of this embodiment is a multi-core optical fiber (MCF) base material. The optical fiber base material 1 can also be a single-mode optical fiber (SMF) base material, a multi-mode optical fiber (MMF) base material, a polarization-maintaining optical fiber (PMF) base material, etc.

[0056] The plurality of cores 2 extend along the central axis C of the optical fiber base material 1. The plurality of cores 2 are arranged at positions that are rotationally symmetrical with respect to the center of the optical fiber base material 1 in a cross section orthogonal to the central axis C. The cross-sectional shapes of the plurality of cores 2 are circular shapes that are identical to each other. Here, the number of cores 2 is 4, and the optical fiber base material 1 is, for example, a square four-core MCF optical fiber, but is not limited thereto.

[0057] The core 2 includes a central core 5 and a recessed portion 6. The recessed portion 6 surrounds the central core 5 and is disposed in contact with the outer peripheral surface of the central core 5. The recessed portion 6 is a separate cladding portion or an optical cladding portion. The cladding portion 3 surrounds a plurality of cores 2 and a marking portion 4. The cladding portion 3 is disposed in contact with the outer peripheral surfaces of each of the plurality of cores 2 and the marking portion 4. The core 2 may not include the recessed portion 6.

[0058] The refractive index n1 of the central core 5 is higher than the refractive index n2 of the cladding 3. The refractive index nd of the recessed portion 6 is lower than the refractive index n2 of the cladding 3. The central core 5 is a high refractive index region having a refractive index n1 higher than the refractive index n2 of the cladding 3. The central core 5, the recessed portion 6 and the cladding 3 are respectively composed of silica-based glass. The central core 5, the recessed portion 6 and the cladding 3 are respectively composed of silica glass as a main component and contain a dopant for adjusting the refractive index. Germanium is widely known as an additive element for increasing the refractive index. Fluorine and boron are widely known as additive elements for reducing the refractive index.

[0059] The marking portion 4 is provided for identifying each core portion 2. The marking portion 4 is provided at a position away from the central axis C of the optical fiber base material 1. The marking portion 4 is provided at a position away from each core portion 2. The diameter of the marking portion 4 is, for example, smaller than the diameter of the core portion 2, and is 2 μm or more and 10 μm or less. The marking portion 4 is distinguished from the core portion 2, for example, by coloring. The marking portion 4 has, for example, a refractive index different from the refractive index n2 of the cladding portion 3. The marking portion 4 is composed of silica-based glass. The marking portion 4 has silica glass as a main component and contains, for example, a dopant for coloring or refractive index adjustment.

[0060] Although not shown in the figure, the optical fiber obtained from the optical fiber mother material 1 has a structure corresponding to the optical fiber mother material 1. The cross-sectional structure of the optical fiber is similar to the cross-sectional structure of the optical fiber mother material 1. The optical fiber has the same refractive index distribution as the optical fiber mother material 1. The optical fiber has a plurality of cores composed of a plurality of core portions 2, a common cladding composed of a cladding portion 3, and a marker composed of a marker portion 4. The core includes a central core composed of a central core portion 5, and a depression composed of a depression portion 6. The core may not include a depression. The depression is a single cladding or an optical cladding. The diameter of the central core portion 5 is, for example, greater than 6 μm and less than 12 μm. The diameter of the cladding portion 3 (cladding diameter) is, for example, greater than 124 μm and less than 126 μm. The optical fiber is not limited to MCF, and may also be SMF, MMF, PMF, etc.

[0061] (Optical fiber base material and method for manufacturing optical fiber)

[0062] Next, the optical fiber preform 1 and the method for manufacturing the optical fiber will be described. One method for manufacturing an MCF preform is a hole-forming method. Here, the case where the optical fiber preform 1 is manufactured by the hole-forming method will be described. Figure 2 FIG. 1 is a flow chart showing a method for manufacturing an optical fiber. Figure 2 As shown, the method for manufacturing an optical fiber includes a cladding material hole opening step S1, a core material insertion step S2, a heating integration step S3, a peripheral grinding step S4, and a drawing step S5. The optical fiber is manufactured, for example, by sequentially performing the cladding material hole opening step S1, the core material insertion step S2, the heating integration step S3, the peripheral grinding step S4, and the drawing step S5. The optical fiber mother material 1 is manufactured by sequentially performing the cladding material hole opening step S1, the core material insertion step S2, the heating integration step S3, and the peripheral grinding step S4.

[0063] The cladding material hole opening step S1 is a step of providing a plurality of holes in a cylindrical glass material (cladding material) that penetrates the cladding portion 3 that will become the optical fiber base material 1 in the length direction. The core material insertion step S2 is a step of inserting the core material that will become the core portion 2 of the optical fiber base material 1 into each hole of the cladding material. The core material includes a region having a refractive index higher than that of the cladding material, that is, a region that will become the central core portion 5. Here, the marker material that will become the marker portion 4 is also inserted into the hole of the cladding material. The heating integration step S3 is a step of heating the cladding material, the core material, and the marker material to integrate them. In the heating integration step S3, the cladding material, the core material, and the marker material are heated axially while rotating them around the central axis of the cladding material. In this way, the cladding material, the core material, and the marker material are integrated to manufacture the optical fiber base material 1. The peripheral grinding step S4 is a step of grinding the periphery of the cladding portion 3. The drawing step S5 is a step of drawing the optical fiber base material 1. In this way, an optical fiber is manufactured.

[0064] As described in Patent Document 1, in the heating integration step S3, since the cladding material is rotated around the central axis while being heated axially symmetrically, the deformation of the glass near the core material is asymmetrical with respect to the core material located in the non-central area of ​​the cladding material. For this reason, the center of the hole portion and the center of the integrated core material are inconsistent with each other. Therefore, it is difficult to arrange the core 2 at the designed position with high precision. The cross-sectional structure of the optical fiber obtained by drawing is similar to the cross-sectional structure of the optical fiber mother material 1. Therefore, the inconsistency between the center position of the core material after heating integration and the designed position will cause the connection loss of the optical fiber. In order to suppress the connection loss, it is necessary to appropriately adjust the center position of the core 2 relative to the center position of the cladding portion 3. For this reason, a method for measuring the eccentricity of the core 2 is required.

[0065] The above-mentioned technical problem of core eccentricity when manufacturing optical fiber mother materials may also occur in manufacturing methods other than the hole-opening method. As manufacturing methods other than the hole-opening method, for example, MCVD (Modified Chemical Vapor Deposition) method, OVD (Outside Vapor Deposition) method, VAD (Vapor phase Axial Deposition) method can be cited. It is not limited to MCF mother materials. When manufacturing optical fiber mother materials such as SMF mother materials, MMF mother materials, and PMF mother materials, the technical problem of core eccentricity may also occur. The ratio of the core diameter of the optical fiber to the cladding diameter is an important parameter that affects the mode field diameter, the effective core cross-sectional area, the connection loss, and the nonlinear optical effect. Therefore, it is necessary to properly maintain it from the stage of the optical fiber mother material. Since the true roundness of the core greatly affects the polarization mode dispersion, it is also hoped that the non-circularity of the core is maintained uniformly in the length direction. In the case of PMF, it is hoped that the eccentricity and size of not only the core but also the stress-applying part can be measured. For example, in the stress-applying portion to which boron is added, the refractive index is lower than the refractive index of the cladding portion.

[0066] Thus, it is desirable to be able to measure the internal structure of an optical fiber mother material at arbitrary intervals in the length direction. For this purpose, a method is required to non-destructively measure the refractive index change position that becomes the boundary between different materials in the optical fiber mother material from the side of the optical fiber mother material. Based on the measurement results, it is possible to identify defective products before drawing and adjust the peripheral grinding amount of the cladding part.

[0067] For optical fibers, a method for non-destructively measuring the position of refractive index change from the side of the optical fiber is also required. For example, when optical fibers such as MCF and PMF are connected to each other through a fusion splicer, rotational alignment of the optical fiber is required. There is a method for end face alignment as follows: a reflector element with a 45° reflection surface is set between a pair of optical fibers arranged opposite to each other, and rotational alignment is performed while observing the end face images of the pair of optical fibers. In this method, the optical system inside the fusion splicer is complex, and the fusion splicer may be large-scale. On the other hand, in the side alignment method that performs rotational alignment based on the intensity distribution image obtained by observing the side of the optical fiber, a reflector element is not required, and the size of the fusion splicer can be suppressed. The side alignment method can also be applied to uses other than fusion splicers, such as when manufacturing optical fiber connectors and optical fiber arrays. By measuring the refractive index change position inside the optical fiber with high precision by observing from the side, higher-precision rotational alignment can be performed.

[0068] (Method and device for measuring refractive index change position)

[0069] The method and device for measuring the refractive index change position involved in the present embodiment are described. For example, the observation object is set to the optical fiber mother material 1, and the method and device for measuring the refractive index change position involved in the present embodiment are used between the heating integration step S3 and the peripheral grinding step S4. In this way, the eccentricity of the core 2 of the optical fiber mother material 1 can be measured. Based on the measurement result, the peripheral grinding of the cladding part 3 is performed in the peripheral grinding step S4, so that the center position of the core 2 can be adjusted. The observation object can also be an optical fiber that is subjected to rotational centering. In this case, high-precision rotational centering of the optical fiber can be performed. The following describes the case where the observation object is the optical fiber mother material 1.

[0070] Figure 3 FIG. 2 is a diagram showing the structure of a device for measuring a refractive index change position according to an embodiment of the present invention. Figure 3 As shown, the measuring device 20 includes a light source 21 , a camera 22 , a polarizer 23 , a detector 24 , and a holding unit 25 .

[0071] The light source 21 irradiates the optical fiber base material 1 with light L. The light L may be collimated light or non-collimated light. In the case of collimated light, the effective diameter of the lens needs to be greater than the outer diameter of the optical fiber base material 1. In the case of non-collimated light, the lens 22b can be miniaturized. The camera 22 acquires a side observation image of the optical fiber base material 1. The camera 22 includes an imaging element 22a and a lens 22b. The lens 22b may be a telecentric lens or a non-telecentric lens of a variable magnification type. In the case of a telecentric lens, the effective diameter of the lens needs to be greater than the outer diameter of the optical fiber base material 1. Therefore, as the diameter of the optical fiber base material 1 increases, the size and introduction cost of the measuring device 20 increase. In the case of a non-telecentric lens of a variable magnification type, the camera field of view can be enlarged or reduced by simply adjusting the magnification of the lens and the position of the camera. Therefore, even when the diameter of the optical fiber base material 1 increases, it is possible to perform measurements with a cheap and compact device configuration compared to a telecentric lens.

[0072] The light source 21 and the camera 22 are arranged on the observation axis AX orthogonal to the central axis C of the optical fiber base material 1 via the optical fiber base material 1. The observation axis AX is located on a straight line orthogonal to the central axis C, that is, the observation axis AX is located on a straight line perpendicular to the central axis C and passing through the central axis C. The lens 22b is arranged between the light source 21 and the imaging element 22a. It can be said that the light source 21 and the camera 22 are arranged so that the observation axis AX connecting the light source 21 and the camera 22 is orthogonal to the central axis C of the optical fiber base material 1.

[0073] The polarizer 23 is arranged between the light source 21 and the optical fiber base material 1 on the observation axis AX. The polarizer 23 transmits only the light vibrating in the transmission axis direction of the polarizer 23 in the light L. The detector 24 is arranged between the optical fiber base material 1 and the camera 22 on the observation axis AX. The detector 24 transmits only the light vibrating in the transmission axis direction of the detector 24 in the light after passing through the optical fiber base material 1. The polarizer 23 and the detector 24 have transmission axes that are orthogonal to each other.

[0074] The holding part 25 holds the optical fiber preform 1 so that the optical fiber preform 1 can rotate around the central axis C. The holding part 25 is, for example, a glass lathe. The rotation of the optical fiber preform 1 may be performed manually or automatically using a rotary encoder or the like.

[0075] The measuring device 20 is configured so that at least one of the optical fiber base material 1 and the observation axis AX can be moved parallel to the central axis C. Thus, the optical fiber base material 1 can be observed from the side at any position in the longitudinal direction of the optical fiber base material 1. In the present embodiment, the light source 21, the camera 22, the polarizer 23, and the detector 24 are configured so as to be movable in the direction B parallel to the central axis C. Thus, the observation axis AX can be moved parallel to the central axis C from one end 1a to the other end 1b of the optical fiber base material 1. Instead of or in addition to the observation axis AX, the holding portion 25 may hold the optical fiber base material 1 so as to be movable in the direction B parallel to the central axis C.

[0076] The measuring principle of the measuring device 20 is described. In the optical fiber base material 1, for example, materials with different refractive indices such as the core 2 and the cladding part 3 have different compositions. At the boundary position between such dissimilar materials, strain is generated after heating and integration. The optical fiber obtained from the optical fiber base material 1 also has strain. When the polarized wave after passing through the polarizer 23 passes through the boundary position between the dissimilar materials with strain, the polarization state changes due to the photoelastic effect. For this reason, the light after passing through the strain can pass through the detector 24 whose transmission axis is orthogonal to the transmission axis of the polarizer 23 and is received by the camera 22. On the other hand, the polarized wave that has not passed through the strain cannot pass through the detector 24 because the polarization state has not changed, and is not received by the camera 22. Therefore, in the light intensity image obtained by the camera 22, a clear light and dark difference is generated according to the presence or absence of strain. Therefore, according to the measuring device 20, compared with the tiny refractive index difference of the refractive index change position of the observation object, the boundary position between the dissimilar materials, that is, the refractive index change position can be clearly detected.

[0077] Figure 4 2 is a flowchart showing a method for measuring a refractive index change position according to an embodiment. The method for measuring a refractive index change position according to an embodiment is performed using a measuring device 20. The method for measuring a refractive index change position includes a preparation step S11, a rotation step S12, an irradiation step S13, an image acquisition step S14, a detection step S15, a correction step S16, and a movement step S17.

[0078] The preparation step S11 includes: a step of holding the optical fiber base material 1 by the holding portion 25 so as to be rotatable about the central axis C; and a step of arranging the light source 21, the camera 22, the polarizer 23, and the detector 24 at predetermined positions on the observation axis AX. The rotation step S12 is to rotate the optical fiber base material 1 held by the holding portion 25 about the central axis C so that a straight line m (refer to FIG. 1 ) which is orthogonal to the central axis C and passes through the desired refractive index change positions of two or more points to be measured is rotated. Figure 5) is orthogonal to the observation axis AX, that is, the straight line m is perpendicular to the observation axis AX and passes through the observation axis AX. After performing this rotation action, the focus of the camera 22 is aligned with the central axis C, so that the ratio of the outer diameter of the optical fiber mother material 1 to the outermost core pitch can be accurately measured. Here, the outermost core pitch is the distance between the centers of a pair of core portions 2 arranged across the central axis C. The pair of core portions 2 that specifies the outermost core pitch is located on a straight line orthogonal to the central axis C. In the case where there are multiple groups of core portions 2 arranged across the central axis C on a straight line orthogonal to the central axis C, the group of core portions 2 with the largest center-to-center distance is the pair of core portions 2 that specifies the outermost core pitch.

[0079] The outer diameter of the optical fiber base material 1 is measured, for example, by a vernier caliper or a dimension measuring instrument. The outermost core pitch can also be easily calculated based on the outer diameter of the optical fiber base material 1 and the above-mentioned ratio. Furthermore, by focusing on the geometric properties of the cross-sectional structure, the adjacent core pitch is also calculated as 1 / √2 times the outermost core pitch. Here, the adjacent core pitch is the distance between the centers of a pair of core portions 2 adjacent to each other around the central axis C. It should be noted that in the case where the optical fiber base material 1 is a dual-core MCF base material in which two core portions 2 are arranged at positions symmetrical with respect to the central axis C, the outermost core pitch is equal to the adjacent core pitch.

[0080] Figure 5 is a cross-sectional view showing the relationship between the rotation of the optical fiber base material and the observation axis. Figure 5 As shown, in the case where the optical fiber mother material 1 is a four-core MCF mother material, as shown in Figure 5 As shown in (b) and (c) of FIG. 1 , multiple orthogonal relationships satisfying the conditions are generated. This is because there are two straight lines m that are orthogonal to the central axis C and pass through the central axes of a pair of cores 2 arranged across the central axis C. Figure 5 In the figure, one straight line m is represented by a solid line and the other straight line m is represented by a dotted line. In the initial state (a), neither of the two straight lines m is orthogonal to the observation axis AX. The optical fiber base material 1 can be rotated around the central axis C so that the straight line m shown by the solid line is orthogonal to the observation axis AX (rotation action b), or the optical fiber base material 1 can be rotated around the central axis C so that the straight line m shown by the dotted line is orthogonal to the observation axis AX (rotation action c). It is also possible to average the outermost core pitch measured in the state after (b) rotation action b and the outermost core pitch measured in the state after (c) rotation action c, and set the average value as the outermost core pitch of the optical fiber base material 1. By determining the peripheral grinding amount based on the average value, multiple adjacent core pitches can be made more uniform.

[0081] There are four pairs of boundaries between the core 2 and the cladding 3 on each straight line m. Among them, the two boundaries close to the central axis C sometimes overlap with other cores 2 and cannot be clearly detected on the light intensity image. In this case, the distance between the two boundaries close to the outer peripheral surface of the cladding 3 that can be clearly detected is measured. That is, the refractive index change positions of the desired two points or more as the measurement object are the positions of the two boundaries close to the outer peripheral surface of the cladding 3 among the boundaries of a pair of cores 2 and the cladding 3 on the straight line m. The outer diameter of the core 2 is the same as the outer diameter of the core material. Therefore, by subtracting the outer diameter of the core material measured in advance from the distance between the two boundaries detected by the detection process S15, the outermost core pitch can be calculated.

[0082] For example, the rotating step S12 is performed while visually observing the end face of the optical fiber preform 1. After roughly adjusting the angle visually, the outermost core pitch may be measured multiple times while the angle is changed little by little by the holding portion 25, and the longest measured value is set as the outermost core pitch.

[0083] The irradiation step S13 is a step of irradiating the light L emitted from the light source 21 to the side of the optical fiber preform 1 via the polarizer 23. Only the light L whose vibration direction coincides with the direction of the transmission axis of the polarizer 23 passes through the polarizer 23 and is irradiated to the side of the optical fiber preform 1. The image acquisition step S14 is a step of acquiring a light intensity image by causing the light transmitted through the optical fiber preform 1 to be received by the camera 22 via the detector 24. Only the light whose vibration direction coincides with the direction of the transmission axis of the detector 24, among the light transmitted through the optical fiber preform 1, passes through the detector 24 and is received by the camera 22.

[0084] The detection step S15 is a step of detecting the refractive index change positions of two or more desired points as the measurement object based on the acquired light intensity image. The boundary position of a pair of core portions 2 and cladding portions 3 arranged across the central axis C is detected based on the light and dark difference of the light intensity image. In this way, the outermost core pitch and the adjacent core pitch can be obtained.

[0085] The correction step S16 is a step of correcting image distortion caused by the lens effect of the side surface of the optical fiber preform 1 . Figure 6 This is a schematic diagram for explaining the correction of image distortion caused by the lens effect on the side surface of the optical fiber base material. Figure 6The circle shown represents the outer surface of the cladding portion 3. The center point of the circle represents the central axis C. Let the radius of the circle (i.e., the radius of the cladding portion 3) be r. Let the refractive index around the cladding portion 3 be n0, and let the refractive index of the cladding portion 3 be n2 (n0≤n2). n0 is, for example, the refractive index of air. Let the angle of incidence when tracing the light in the reverse direction be θ0, and the angle of refraction be θ2. At this time, ∠CPA=θ0, ∠CPX=θ2. If we focus on the light ray L1 parallel to the optical axis, then due to the lens effect, the light passing through point P on the circumference appears to correspond to the apparent point A on the image, rather than actually passing through point X (x <a)。

[0086] An equation for correcting the apparent distance a and the true distance x caused by the image distortion is derived. First, according to Snell's law, equation (1) holds.

[0087] n0sin00=n2sin02(1)

[0088] Here, according to formula (1) and Figure 6 The ratio of line segment CX to line segment CA shown in FIG. 1 is used to derive equation (2).

[0089]

[0090] According to equation (2), the apparent distance a caused by image distortion can be corrected and the true distance x can be obtained.

[0091] The moving step S17 is a step of moving the optical fiber base material 1 or the observation axis AX parallel to the central axis C in a state where the observation axis AX is orthogonal to the central axis C. While the optical fiber base material 1 or the observation axis AX is moved by the moving step S17, the rotating step S12, the irradiation step S13, the image acquisition step S14, the detection step S15, and the calibration step S16 are repeatedly performed to measure the internal structure of the optical fiber base material 1 at arbitrary intervals in the longitudinal direction.

[0092] As described above, in this embodiment, the polarizer 23 and the detector 24 having mutually orthogonal transmission axes are used. When the polarized wave after passing through the polarizer 23 passes through the strain existing at the boundary position between different materials, that is, the position where the refractive index changes, the polarization state changes due to the photoelastic effect. For this reason, the light after passing through the strain can pass through the detector 24. Therefore, in the light intensity image acquired by the camera 22, a difference in brightness is generated depending on the presence or absence of the strain. According to this embodiment, the measurement accuracy can be improved while the measurement time is suppressed.

[0093] In the measurement method using a preform analyzer, the refractive index distribution is measured by scanning light point by point along the scanning axis from the side of the optical fiber mother material, and performing an inverse Abel transform on the measurement result of the polarization angle. As disadvantages of this method, there are the increase in the number of laser scanning points and the increase in measurement time as the mother material becomes thicker, the increase in introduction cost due to the large size of the optical system, and the calculation time required for numerical calculation processing. In addition, if the purpose is only to measure the position of the refractive index change, the polarization angle alone is sufficient, and the refractive index distribution is not required, so the measurement method using a preform analyzer becomes redundant. Non-patent document 1 and patent document 3 disclose methods for measuring the refractive index distribution inside an optical fiber, but either method requires numerical calculation processing, which increases the calculation time and the measurement time. In addition, like the preform analyzer, if the purpose is limited to only measuring the position of the refractive index change, it becomes redundant.

[0094] In contrast, in the method for measuring the refractive index change position using the measuring device 20, it is not necessary to scan the light point by point along the scanning axis from the side of the optical fiber base material 1, and the measuring time is not likely to increase even if the diameter of the optical fiber base material 1 is increased. Since the light L irradiated by the light source 21 can also be non-collimated light, the size of the optical system can be reduced, and the increase in the introduction cost of the measuring device 20 can be suppressed. In addition, since complex numerical calculation processing is not required, the measuring time can be suppressed.

[0095] As mentioned above, although embodiment was described, this disclosure is not necessarily limited to the said embodiment and modification, Various changes can be made within the range which does not deviate from the summary.

[0096] Figure 7 It is a diagram showing the structure of a device for measuring a refractive index change position according to a modification example. Figure 8 is a perspective view showing the relationship between the container and the observation axis. Figure 7 and Figure 8 As shown, the measuring device 20A for the refractive index change position involved in the modified example is different from the measuring device 20 in that it has a transparent container 26 and a refractive index matching material 27 filled in the interior of the container 26. The container 26 is, for example, a transparent glass tank. The container 26 only needs to be transparent, and may be made of acrylic. The container 26 is configured to accommodate at least a portion of the optical fiber base material 1 in the longitudinal direction. In the case where the observation object is the optical fiber base material 1, the observation object may be accommodated in the container 26 as a whole. In this case, the shape of the container 26 can be simplified. In the present embodiment, a portion of the observation object in the longitudinal direction is accommodated in the container 26. Thus, the amount of the refractive index matching material 27 can be saved.

[0097] The container 26 has, for example, a rectangular parallelepiped shape. The container 26 has a pair of side surfaces 26a and 26b that are orthogonal to the observation axis AX. The pair of side surfaces 26a and 26b are opposed to each other with the optical fiber base material 1 sandwiched therebetween. The side surface 26a is disposed between the polarizer 23 and the optical fiber base material 1, and allows the light irradiated to the optical fiber base material 1 to pass through. The side surface 26b is disposed between the optical fiber base material 1 and the detector 24, and allows the light emitted from the optical fiber base material 1 to pass through. The container 26 has a pair of side surfaces 26c and 26d that intersect (orthogonal in this case) the central axis C. A circular through hole that allows the optical fiber base material 1 to pass through is provided on the pair of side surfaces 26c and 26d.

[0098] The refractive index matching material 27 is filled in the interior of the container 26, that is, the gap between the container 26 and the optical fiber base material 1. The refractive index matching material 27 is a medium having a refractive index greater than that of air and less than that of glass. The refractive index matching material 27 may be water, for example. By immersing the optical fiber base material 1 in the refractive index matching material 27, the influence of the circular lens effect originating from the outer surface of the optical fiber base material 1 can be suppressed. By using the refractive index matching material 27 having a refractive index equivalent to that of glass, a side image without distortion can be obtained. Therefore, the work of correcting image distortion can be omitted. Even when the refractive index of the refractive index matching material 27 is lower than that of glass, the lens effect of the outer surface of the optical fiber base material 1 can be alleviated compared with the case of air atmosphere. For this reason, without enlarging the camera 22, the entire refractive index change position inside the optical fiber base material 1 can be included in the camera field of view for observation.

[0099] In the measurement method according to the modified example using the measurement device 20A, the preparation step S11 further includes a step of immersing at least a portion of the optical fiber preform 1 in the longitudinal direction in the refractive index matching material 27 (medium). In the irradiation step S13, the light L emitted from the light source 21 is irradiated to the side surface of the optical fiber preform 1 via the polarizer 23 and the refractive index matching material 27. In the image acquisition step S14, the light transmitted through the optical fiber preform 1 is received by the camera 22 via the refractive index matching material 27 and the detector 24, and a light intensity image is acquired.

[0100] In the measurement method according to the modified example using the measurement device 20A, when the refractive index matching material 27 having a refractive index equivalent to that of glass is used, the calibration step S16 can be omitted.

[0101] In the above content, the observation axis AX is actually orthogonal to the central axis C and the straight line m. However, in the case where the observation axis AX can be regarded as being substantially orthogonal to the central axis C and the straight line m through correction, the observation axis AX may not be completely orthogonal to the central axis C and the straight line m. That is, the observation axis AX may deviate from the original observation axis AX orthogonal to the central axis C and the straight line m. For example, the observation axis AX may be located on a straight line after the original observation axis AX is translated, or on a straight line after the original observation axis AX is rotated around the central axis C, or on a straight line after the original observation axis AX is translated and rotated around the central axis C, in a plane orthogonal to the central axis C. In these cases, the correction of the influence of the translation distance and the rotation angle may be included in the correction step S16.

[0102] Description of Reference Numerals

[0103] 1: optical fiber base material; 1a: one end; 1b: the other end; 2: core; 3: cladding; 4: marking; 5: central core; 6: recessed portion; 20, 20A: measuring device; 21: light source; 22: camera; 22a: imaging element; 22b: lens; 23: polarizer; 24: detector; 25: holding portion; 26: container; 26a, 26b, 26c, 26d: side; 27: refractive index matching material; A: apparent position; AX: observation axis; B: direction; C: central axis; b, c: rotational movement; n0: refractive index around the cladding; n1: refractive index of the central core; n2: refractive index of the cladding; nd: refractive index of the recessed portion; L: light; L1: ray; P: point; X: passing point; A: apparent point; x: actual passing point; a: apparent distance; θ0: angle of incidence; θ2: angle of refraction; r: radius.

Claims

1. A method for measuring a refractive index change position of an optical fiber base material, the method using a measuring device to measure the refractive index change position of the optical fiber base material, the measuring device comprising: a holding portion for holding the optical fiber base material so as to be rotatable about a central axis; a light source and a camera, arranged on an observation axis orthogonal to the central axis via the optical fiber base material; a polarizer, arranged between the light source and the optical fiber base material; and a detector, having a transmission axis orthogonal to the transmission axis of the polarizer, and arranged between the optical fiber base material and the camera. The method for measuring the refractive index change position of the optical fiber mother material comprises the following steps: Rotating the optical fiber base material around the central axis so that a straight line that is orthogonal to the central axis and passes through two or more desired refractive index change positions to be measured is orthogonal to the observation axis; Allowing the light emitted from the light source to irradiate the side surface of the optical fiber mother material via the polarizer; The light transmitted through the optical fiber mother material is received by the camera via the detector to obtain a light intensity image; Based on the light intensity image, detecting the refractive index change positions at the two or more points; as well as The optical fiber preform or the observation axis is moved parallel to the central axis.

2. The method for measuring the refractive index change position of an optical fiber base material according to claim 1, wherein: The method for measuring the refractive index change position of the optical fiber base material further includes the step of immersing at least a portion of the optical fiber base material in the central axis direction in a medium having a refractive index greater than that of air and less than that of glass. In the method for measuring the refractive index change position of the optical fiber mother material, The light emitted from the light source is irradiated to the side of the optical fiber mother material via the polarizer and the medium, and The light transmitted through the optical fiber preform is received by the camera via the medium and the detector to acquire a light intensity image.

3. The method for measuring the refractive index change position of an optical fiber base material according to claim 1 or 2, wherein: The optical fiber mother material is a polarization-maintaining optical fiber mother material.

4. The method for measuring the refractive index change position of an optical fiber base material according to claim 1 or 2, wherein: The optical fiber mother material is a multi-core optical fiber mother material.

5. The method for measuring the refractive index change position of an optical fiber base material according to claim 4, wherein: The optical fiber base material includes a cladding portion and a pair of core portions arranged across the central axis and having a maximum center-to-center distance. The two or more refractive index change positions are positions of two boundaries close to the outer peripheral surface of the cladding portion among the boundaries between the cladding portion and the pair of core portions existing on the straight line, In the method for measuring the refractive index change position of the optical fiber preform, the center-to-center distance of the pair of core portions is measured based on the distance between the two boundaries detected in the step of performing the detection.

6. The method for measuring a refractive index change position of an optical fiber base material according to any one of claims 1 to 5, wherein: The method for measuring the refractive index change position of the optical fiber mother material further includes the step of correcting the image distortion caused by the lens effect of the side surface of the optical fiber mother material on the light intensity image using the following formula: Here, a is the apparent distance from the refractive index change position to the central axis, x is the actual distance from the refractive index change position to the central axis, n0 is the refractive index around the cladding part, n2 is the refractive index of the cladding part, and r is the radius of the cladding part.

7. A device for measuring a refractive index change position of an optical fiber mother material, the device for measuring a refractive index change position of an optical fiber mother material comprising: A holding portion, which holds the optical fiber mother material so as to be rotatable around a central axis; A light source and a camera are arranged on an observation axis orthogonal to the central axis through the optical fiber base material; A polarizer, disposed between the light source and the optical fiber mother material; as well as a detector having a transmission axis orthogonal to the transmission axis of the polarizer and arranged between the optical fiber base material and the camera, The holding unit holds the optical fiber preform in such a manner that a straight line that is orthogonal to the central axis and passes through two or more desired refractive index change positions to be measured is orthogonal to the observation axis. The light source irradiates light toward the side of the optical fiber mother material via the polarizer. The camera receives the light transmitted through the optical fiber mother material via the detector to obtain a light intensity image with a light-dark difference at the refractive index change position at more than two points. The holding portion holds the optical fiber preform so as to be movable in a direction parallel to the central axis, or the light source, the camera, the polarizer, and the detector are configured so as to be movable in a direction parallel to the central axis.

8. The device for measuring the refractive index change position of an optical fiber base material according to claim 7, wherein: The device for measuring the refractive index change position of the optical fiber preform further includes a container for accommodating at least a portion of the optical fiber preform in the central axis direction, wherein the container is filled with a medium having a refractive index greater than that of air and less than that of glass.

9. A method for measuring a refractive index change position of an optical fiber, the method using a measuring device to measure the refractive index change position of the optical fiber, the measuring device comprising: a holding portion for holding the optical fiber so as to be rotatable about a central axis; a light source and a camera, arranged on an observation axis orthogonal to the central axis via the optical fiber; a polarizer, arranged between the light source and the optical fiber; and a detector, having a transmission axis orthogonal to the transmission axis of the polarizer and arranged between the optical fiber and the camera. The method for determining the refractive index change position of the optical fiber comprises the following steps: Rotating the optical fiber around the central axis so that a straight line that is orthogonal to the central axis and passes through two or more desired refractive index change positions to be measured is orthogonal to the observation axis; Allowing the light emitted from the light source to irradiate the side surface of the optical fiber via the polarizer; The light transmitted through the optical fiber is received by the camera via the detector to obtain a light intensity image; as well as Based on the light intensity image, the refractive index change positions at the two or more points are detected.

10. The method for measuring the refractive index change position of an optical fiber according to claim 9, wherein: The method for measuring a refractive index change position of an optical fiber further includes a step of moving the optical fiber or the observation axis parallel to the central axis.

11. The method for measuring the refractive index change position of an optical fiber according to claim 9 or 10, wherein: The method for measuring the refractive index change position of the optical fiber further includes the step of immersing at least a portion of the optical fiber in the central axis direction in a medium having a refractive index greater than that of air and less than that of glass. In the method for measuring the refractive index change position of the optical fiber, The light emitted from the light source is irradiated to the side of the optical fiber via the polarizer and the medium, and The light transmitted through the optical fiber is received by the camera via the medium and the detector to acquire a light intensity image.

12. The method for measuring a refractive index change position of an optical fiber according to any one of claims 9 to 11, wherein: The optical fiber is a polarization-maintaining optical fiber.

13. The method for measuring a refractive index change position of an optical fiber according to any one of claims 9 to 11, wherein: The optical fiber is a multi-core optical fiber.

14. The method for measuring the refractive index change position of an optical fiber according to claim 13, wherein: The optical fiber comprises a cladding and a pair of cores arranged across the central axis and having a maximum center-to-center distance. The two or more refractive index change positions are positions of two boundaries close to the outer peripheral surface of the cladding among the boundaries between the cladding and the pair of cores existing on the straight line, In the method for measuring the refractive index change position of the optical fiber, the distance between the centers of the pair of cores is measured based on the distance between the two boundaries detected in the step of performing the detection.

15. The method for measuring a refractive index change position of an optical fiber according to any one of claims 9 to 14, wherein: The method for measuring the refractive index change position of the optical fiber further includes the step of correcting the image distortion caused by the lens effect of the side surface of the optical fiber on the light intensity image using the following formula: Here, a is the apparent distance from the refractive index change position to the central axis, x is the actual distance from the refractive index change position to the central axis, n0 is the refractive index around the cladding, n2 is the refractive index of the cladding, and r is the radius of the cladding.

16. A device for measuring a refractive index change position of an optical fiber, the device for measuring a refractive index change position of an optical fiber comprising: A holding portion, which holds the optical fiber so as to be rotatable around a central axis; A light source and a camera are arranged on an observation axis orthogonal to the central axis via the optical fiber; a polarizer, disposed between the light source and the optical fiber; as well as a detector having a transmission axis orthogonal to the transmission axis of the polarizer and disposed between the optical fiber and the camera, The holding unit holds the optical fiber in such a manner that a straight line that is orthogonal to the central axis and passes through two or more desired refractive index change positions to be measured is orthogonal to the observation axis. The light source irradiates light toward the side of the optical fiber via the polarizer, The camera receives the light transmitted through the optical fiber via the detector to obtain a light intensity image having a brightness difference at the refractive index change positions at or above the two points.

17. The device for measuring the refractive index change position of an optical fiber according to claim 16, wherein: The holding portion holds the optical fiber so as to be movable in a direction parallel to the central axis, or the light source, the camera, the polarizer, and the detector are configured so as to be movable in a direction parallel to the central axis.

Images