Method and device for measuring refractive index variation position of optical fiber preform and optical fiber

Abstract

This method for measuring the refractive index variation position of an optical fiber preform uses a measurement device provided with: a holding part that holds an optical fiber preform in a manner so as to be rotatable about a central axis; a light source and camera that are disposed on an observation axis orthogonal to the central axis; a polarizer; and a detector that has a transmission axis which is orthogonal to the transmission axis of the polarizer. The method comprises: a step for rotating the optical fiber preform about the central axis such that a straight line which is orthogonal to the central axis and which passes through two more refractive index variation positions of choice that are subject to measurement is orthogonal to the observation axis; a step for irradiating a side surface of the optical fiber preform with light emitted from the light source, via the polarizer; a step for receiving, with the camera and via the detector, the light that has been transmitted by the optical fiber preform, and acquiring a light intensity image; a step for detecting a refractive index variation position of the optical fiber preform on the basis of the light intensity image; and a step for moving the optical fiber preform or the observation axis parallel to the central axis.

Description

[Technical field]

[0001] The present disclosure relates to a method and an apparatus for measuring a refractive index change position of an optical fiber preform and an optical fiber. This application claims priority to International Application No. PCT / JP2022 / 018193 filed on April 19, 2022, and incorporates by reference all of the contents of said international application. [Background technology]

[0002] In recent years, with the development of technologies such as IoT (Internet of Things) and metaverse, data traffic is increasing. This trend is expected to continue in the future. In order to increase the capacity of optical communication networks, research on various optical fibers, in addition to the conventional general-purpose single-mode optical fiber (SMF), is being actively conducted. Examples of such optical fibers include thin SMF, multimode optical fiber (MMF), and multicore optical fiber (MCF). Thin SMF can increase the number of optical fibers per optical cable. MMF and MCF can improve the spatial multiplexing density of signals per optical fiber. In order to realize an optical communication system using such optical fibers, it is necessary to establish not only the design method of the optical fiber itself, but also the manufacturing technology of the optical fiber preform and the connection technology such as fusion splicer and optical connector. To establish these technologies, a technology that can measure the refractive index change position inside the optical fiber preform and the optical fiber nondestructively is required.

[0003] Patent Document 1 describes the necessity of controlling the core spacing during the rod-in collapse process. Patent Document 2 describes a method of measuring the eccentricity of the core of an optical fiber preform using a polarizing filter. The polarizing filter removes the reflected light components from the outer surface of the optical fiber preform, making it possible to clearly identify the boundary line caused by the difference in glass composition between the core and cladding.

[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 Abel inverse 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 image sensor and performing a fast Fourier transform on the interference fringe signal. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2014-159348 A [Patent Document 2] JP 2000-213912 A [Patent Document 3] JP 2021-156761 A [Non-patent literature]

[0006] [Non-Patent Document 1] AD Yablon, "Multi-Wavelength Optical Fiber Refractive Index 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

[0007] A method for measuring a refractive index change position of an optical fiber preform according to one embodiment of the present disclosure is a method for measuring a refractive index change position of an optical fiber preform using a measurement device including a holding part that holds the optical fiber preform rotatably around a central axis, a light source and a camera arranged to sandwich the optical fiber preform on an observation axis perpendicular to the central axis, a polarizer arranged between the light source and the optical fiber preform, and a detector having a transmission axis perpendicular to the transmission axis of the polarizer and arranged between the optical fiber preform and the camera, the method including the steps of: rotating the optical fiber preform around the central axis so that a straight line that is perpendicular to the central axis and passes through two or more desired refractive index change positions to be measured is perpendicular to the observation axis; irradiating light emitted from the light source onto a side surface of the optical fiber preform via the polarizer; receiving the light that has transmitted through the optical fiber preform via the detector with the camera to obtain a light intensity image; detecting the two or more refractive index change positions based on the light intensity image; and moving the optical fiber preform or the observation axis parallel to the central axis. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing a cross section of an optical fiber preform and a refractive index distribution in the cross section in the direction of an arrow. [Diagram 2] FIG. 2 is a flow chart showing a method for manufacturing an optical fiber. [Diagram 3] FIG. 3 is a configuration diagram of a measuring device for measuring a refractive index change position according to the embodiment. [Figure 4] FIG. 4 is a flowchart showing a method for measuring a refractive index change position according to the embodiment. [Diagram 5] FIG. 5 is a cross-sectional view showing the relationship between the rotational operation of the optical fiber preform and the observation axis. [Figure 6] FIG. 6 is a schematic diagram for explaining correction of image distortion caused by the lens effect of the side surface of the optical fiber preform. [Figure 7] FIG. 7 is a configuration diagram of a measurement device for measuring the refractive index change position according to a modified example. [Figure 8] FIG. 8 is a perspective view showing the relationship between the container and the observation axis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Problem that this disclosure aims to solve] In the method for measuring the refractive index distribution of an optical fiber preform using a preform analyzer, the laser light is scanned point by point from the side of the optical fiber preform along a straight line (scan axis) perpendicular to the central axis of the optical fiber preform and perpendicular to the emission direction of the laser light. Therefore, when the diameter of the optical fiber preform increases, the measurement time increases due to the increase in the number of scan points. In the method described in Patent Document 2, when the refractive index difference between different materials is small, it is difficult to detect the boundary line on the intensity image with high accuracy, and there is a risk of increasing measurement errors. In the method described in Patent Document 3 and the method described in Non-Patent Document 1, the measurement time increases because it takes a long time for numerical calculation.

[0010] An object of the present disclosure is to provide a method and apparatus for measuring the refractive index change position of an optical fiber preform and an optical fiber, which can improve measurement accuracy while reducing measurement time.

[0011] [Effects of this disclosure] According to the present disclosure, it is possible to provide a method and an apparatus for measuring the refractive index change position of an optical fiber preform and an optical fiber, which can improve measurement accuracy while reducing measurement time.

[0012] [Description of the embodiments of the present disclosure] First, the embodiments of the present disclosure will be described. (1) A method for measuring a refractive index change position of an optical fiber preform according to one embodiment of the present disclosure is a method for measuring a refractive index change position of an optical fiber preform using a measuring device including a holder for rotatably holding the optical fiber preform around a central axis, a light source and a camera arranged on an observation axis perpendicular to the central axis with the optical fiber preform sandwiched therebetween, a polarizer arranged between the light source and the optical fiber preform, and a detector having a transmission axis perpendicular to the transmission axis of the polarizer and arranged between the optical fiber preform and the camera, the method including the steps of: rotating the optical fiber preform around the central axis so that a straight line perpendicular to the central axis and passing through two or more desired refractive index change positions to be measured is perpendicular to the observation axis; irradiating the side surface of the optical fiber preform with light emitted from the light source through the polarizer; receiving the light transmitted through the optical fiber preform with the camera through the detector to obtain a light intensity image; detecting two or more refractive index change positions based on the light intensity image; and moving the optical fiber preform or the observation axis parallel to the central axis. This measurement method uses a measurement device equipped with a polarizer and a detector having mutually orthogonal transmission axes. When the polarized wave passes through the polarizer and passes through the distortion that exists at the boundary position between different materials, i.e., the refractive index change position, the polarization state changes due to the photoelastic effect. Therefore, the light that has passed through the distortion can pass through the detector. Therefore, a light intensity image acquired by the camera has a difference in brightness depending on the presence or absence of distortion. This measurement method can improve measurement accuracy while reducing measurement time.

[0013] (2) The method for measuring the refractive index change position of the optical fiber preform described in (1) above may further include a step of immersing at least a part of the optical fiber preform in the central axis direction in a medium having a refractive index greater than that of air and equal to or less than that of glass, irradiating the side surface of the optical fiber preform with light emitted from a light source through a polarizer and the medium, receiving the light transmitted through the optical fiber preform with a camera through the medium and a detector, and acquiring a light intensity image. In this case, the influence of the lens effect derived from the shape of the side surface of the optical fiber preform is suppressed.

[0014] (3) In the above (1) or (2), the optical fiber preform may be a polarization-maintaining fiber preform. In this case, the eccentricity and dimensions of not only the core but also the stress-applying portion can be measured.

[0015] (4) In the above (1) or (2), the optical fiber preform may be a multi-core optical fiber preform, in which case the eccentricity and dimensions of the cores can be measured.

[0016] (5) In any of the above (1) to (4), the optical fiber preform includes a cladding portion and a pair of core portions arranged on either side of a central axis and having a maximum center-to-center distance, and the two or more refractive index change positions are positions of two boundaries between the cladding portion and the pair of core portions that exist on a straight line and are close to the outer peripheral surface of the cladding portion, and the center-to-center distance of the pair of core portions may be measured from the distance between the two boundaries detected in the detecting step. In this case, the refractive index change positions can be measured without being affected by the other core portions.

[0017] (6) The method for measuring a refractive index change position of an optical fiber preform according to any one of (1) to (5) above further comprises a step of correcting an image distortion caused by a lens effect of a side surface of the optical fiber preform for the light intensity image by the following formula:

number

[0018] (7) A measuring device for measuring refractive index change positions of an optical fiber preform according to one embodiment of the present disclosure is a device for measuring refractive index change positions of an optical fiber preform, and includes a holding unit that holds the optical fiber preform rotatably around a central axis, a light source and a camera arranged to sandwich the optical fiber preform on an observation axis perpendicular to the central axis, a polarizer arranged between the light source and the optical fiber preform, and a detector having a transmission axis perpendicular to the transmission axis of the polarizer and arranged between the optical fiber preform and the camera, wherein the holding unit holds the optical fiber preform so that a straight line that is perpendicular to the central axis and that passes through two or more desired refractive index change positions to be measured is perpendicular to the observation axis, the light source irradiates light onto a side surface of the optical fiber preform via the polarizer, the camera receives the light that has transmitted through the optical fiber preform via the detector, and obtains a light intensity image in which a light-dark difference occurs at the two or more refractive index change positions, and the holding unit holds the optical fiber preform movably 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. This measuring device uses a polarizer and a detector having mutually orthogonal transmission axes. When the polarized wave passes through the polarizer and passes through the distortion that exists at the boundary position between different materials, i.e., the position where the refractive index changes, the polarization state changes due to the photoelastic effect. Therefore, the light that has passed through the distortion can pass through the detector. Therefore, a difference in brightness occurs in the light intensity image acquired by the camera depending on the presence or absence of distortion. This measuring device can improve the measurement accuracy while reducing the measurement time.

[0019] (8) In the above (7), a container may be further provided that contains at least a portion of the optical fiber preform in the central axis direction, and is filled with a medium having a refractive index greater than that of air and equal to or less than that of glass. In this case, the influence of the lens effect resulting from the shape of the side surface of the optical fiber preform is suppressed. Thus, the light emitted from the light source can be irradiated onto the side surface of the optical fiber preform via the polarizer and the medium, and the light transmitted through the optical fiber preform can be received by the camera via the medium and the detector.

[0020] (9) A method for measuring a refractive index change position of an optical fiber according to one embodiment of the present disclosure is a method for measuring a refractive index change position of an optical fiber using a measurement device including a holding unit that holds the optical fiber rotatably around a central axis, a light source and a camera arranged to sandwich the optical fiber on an observation axis perpendicular to the central axis, a polarizer arranged between the light source and the optical fiber, and a detector having a transmission axis perpendicular to the transmission axis of the polarizer and arranged between the optical fiber and the camera, the method including the steps of: rotating the optical fiber around the central axis so that a straight line that is perpendicular to the central axis and passes through two or more desired refractive index change positions to be measured is perpendicular to the observation axis; irradiating light emitted from the light source onto a side surface of the optical fiber via the polarizer; receiving the light that has transmitted through the optical fiber via the detector with the camera to obtain a light intensity image; and detecting the two or more refractive index change positions based on the light intensity image. This measurement method uses a measurement device equipped with a polarizer and a detector having mutually orthogonal transmission axes. When the polarized wave passes through the polarizer and passes through the distortion that exists at the boundary position between different materials, i.e., the refractive index change position, the polarization state changes due to the photoelastic effect. Therefore, the light that has passed through the distortion can pass through the detector. Therefore, a light intensity image acquired by the camera has a difference in brightness depending on the presence or absence of distortion. This measurement method can improve measurement accuracy while reducing measurement time.

[0021] (10) The method for measuring the refractive index change position of an optical fiber according to (9) above may further include a step of moving the optical fiber or the observation axis parallel to the central axis, in which case the internal structure of the optical fiber can be easily measured at any position in the longitudinal direction.

[0022] (11) The method for measuring the refractive index change position of an optical fiber according to (9) or (10) above may further include a step of immersing at least a part of the optical fiber in a central axial direction in a medium having a refractive index greater than that of air and equal to or less than that of glass, irradiating light emitted from a light source onto a side surface of the optical fiber via a polarizer and the medium, and receiving the light transmitted through the optical fiber by a camera via the medium and a detector to obtain a light intensity image. In this case, the influence of the lens effect resulting from the shape of the side surface of the optical fiber is suppressed.

[0023] (12) In any one of the above (9) to (11), the optical fiber may be a polarization maintaining fiber. In this case, the eccentricity and dimensions of not only the core but also the stress applying portion can be measured.

[0024] (13) In any one of the above (9) to (11), the optical fiber may be a multi-core optical fiber. In this case, the eccentricity and dimensions of the cores can be measured.

[0025] (14) In the above (13), the optical fiber includes a cladding and a pair of cores arranged on either side of a central axis and having a maximum center-to-center distance, and the two or more refractive index change positions are positions of two boundaries between the cladding and the pair of cores that exist on a straight line and are close to the outer peripheral surface of the cladding, and the center-to-center distance of the pair of cores may be measured from the distance between the two boundaries detected in the detection step. In this case, the other cores may be A The position of the refractive index change can be measured without being affected.

[0026] (15) The method for measuring the refractive index change position of an optical fiber according to any one of (9) to (14) above may further include a step of correcting image distortion of the light intensity image caused by the lens effect of the side of the optical fiber by the following equation:

number

[0027] (16) A device for measuring refractive index change positions of an optical fiber according to one embodiment of the present disclosure is a device for measuring refractive index change positions of an optical fiber, comprising: a holding unit that holds the optical fiber rotatably around a central axis; a light source and a camera arranged to sandwich the optical fiber on an observation axis perpendicular to the central axis; a polarizer arranged between the light source and the optical fiber; and a detector having a transmission axis perpendicular to the transmission axis of the polarizer and arranged between the optical fiber and the camera, wherein the holding unit holds the optical fiber so that a straight line that is perpendicular to the central axis and passing through two or more desired refractive index change positions to be measured is perpendicular to the observation axis, the light source irradiates light onto a side of the optical fiber via the polarizer, and the camera receives the light that has transmitted through the optical fiber via the detector to obtain a light intensity image in which light and dark differences occur at the two or more refractive index change positions. This measuring device uses a polarizer and a detector having mutually orthogonal transmission axes. When the polarized wave passes through the polarizer and passes through the distortion that exists at the boundary position between different materials, i.e., the position where the refractive index changes, the polarization state changes due to the photoelastic effect. Therefore, the light that has passed through the distortion can pass through the detector. Therefore, a difference in brightness occurs in the light intensity image acquired by the camera depending on the presence or absence of distortion. This measuring device can improve the measurement accuracy while reducing the measurement time.

[0028] (17) In the above (16), the holder may hold 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 may be configured 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.

[0029] [Details of the embodiment of the present disclosure] Specific examples of the method and device for measuring the refractive index change position of an optical fiber preform and an optical fiber according to the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicated descriptions will be omitted.

[0030] (Optical fiber preform and optical fiber) First, a measurement target of the refractive index change position measurement method and measurement device according to the embodiment elephant An optical fiber preform and an optical fiber that are the above-mentioned will be described. FIG. 1 is a diagram showing a cross section of an optical fiber preform and a refractive index distribution in the direction of an arrow in the cross section. As shown in FIG. 1, an optical fiber preform 1 includes a plurality of core portions 2, a common cladding portion 3, and a marker portion 4. The optical fiber preform 1 of this embodiment is a multi-core optical fiber (MCF) preform. The optical fiber preform 1 may be a single mode fiber (SMF) preform, a multimode fiber (MMF) preform, a polarization maintaining fiber (PMF) preform, or the like.

[0031] Each of the core portions 2 extends along the central axis C of the optical fiber preform 1. The core portions 2 are arranged at positions rotationally symmetrical with respect to the center of the optical fiber preform 1 in a cross section perpendicular to the central axis C. The cross-sectional shapes of the core portions 2 are the same circular shapes. Here, the number of core portions 2 is four, and the optical fiber preform 1 is, for example, a square 4-core MCF fiber, but is not limited thereto.

[0032] The core portion 2 includes a central core portion 5 and a depressed portion 6. The depressed portion 6 surrounds the central core portion 5 and is provided in contact with the outer peripheral surface of the central core portion 5. The depressed portion 6 is an individual cladding portion or an optical cladding portion. The cladding portion 3 surrounds the multiple core portions 2 and the marker portion 4. The cladding portion 3 is provided in contact with the outer peripheral surfaces of each of the multiple core portions 2 and the marker portion 4. The core portion 2 does not have to include the depressed portion 6.

[0033] The refractive index n1 of the central core portion 5 is higher than the refractive index n2 of the cladding portion 3. The refractive index nd of the depressed portion 6 is lower than the refractive index n2 of the cladding portion 3. The central core portion 5 is a high refractive index region having a refractive index n1 higher than the refractive index n2 of the cladding portion 3. The central core portion 5, the depressed portion 6, and the cladding portion 3 are each made of silica-based glass. The central core portion 5, the depressed portion 6, and the cladding portion 3 each contain silica glass as the main component and a dopant for adjusting the refractive index. Germanium is well known as an additive element that increases the refractive index. Fluorine and boron are well known as additive elements that decrease the refractive index.

[0034] The marker portion 4 is provided to identify each core portion 2. The marker portion 4 is provided at a position away from the central axis C of the optical fiber preform 1. The marker portion 4 is provided at a position away from each core portion 2. The diameter of the marker 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 marker portion 4 is distinguished from the core portion 2 by, for example, being colored. The marker portion 4 has a refractive index different from the refractive index n2 of the cladding portion 3, for example. The marker portion 4 is made of silica-based glass. The marker portion 4 is mainly composed of silica glass and contains, for example, a dopant for coloring or refractive index adjustment.

[0035] Although not shown in the figure, an optical fiber obtained from the optical fiber preform 1 has a configuration corresponding to the optical fiber preform 1. The cross-sectional structure of the optical fiber is similar to that of the optical fiber preform 1. The optical fiber has the same refractive index distribution as the optical fiber preform 1. The optical fiber includes a plurality of cores consisting of a plurality of core portions 2, a common cladding consisting of a cladding portion 3, and a marker consisting of a marker portion 4. The core includes a central core consisting of a central core portion 5 and a depressed portion consisting of a depressed portion 6. The core does not have to include a depressed portion. The depressed portion is an individual cladding or an optical cladding. The diameter of the central core portion 5 is, for example, 6 μm or more and 12 μm or less. The diameter of the cladding portion 3 (cladding diameter) is, for example, 124 μm or more and 126 μm or less. The optical fiber is not limited to an MCF, and may be an SMF, an MMF, a PMF, or the like.

[0036] (Optical fiber preform and method of manufacturing optical fiber) Next, a method for manufacturing the optical fiber preform 1 and the optical fiber will be described. One method for manufacturing the MCF preform is a hole drilling method. Here, a case where the optical fiber preform 1 is manufactured by the hole drilling method will be described. FIG. 2 is a flow chart showing a method for manufacturing the optical fiber. As shown in FIG. 2, the method for manufacturing the optical fiber includes a cladding hole drilling step S1, a core material insertion step S2, a heating and integrating step S3, a periphery grinding step S4, and a drawing step S5. The optical fiber is manufactured, for example, by carrying out the cladding hole drilling step S1, the core material insertion step S2, the heating and integrating step S3, the periphery grinding step S4, and the drawing step S5 in this order. The optical fiber preform 1 is manufactured by carrying out the cladding hole drilling step S1, the core material insertion step S2, the heating and integrating step S3, and the periphery grinding step S4 in this order.

[0037] The cladding material hole drilling step S1 is a step of providing a plurality of holes that penetrate the cylindrical glass material (cladding material) that will become the cladding portion 3 of the optical fiber preform 1 in the longitudinal direction. The core material insertion step S2 is a step of inserting a core material that will become the core portion 2 of the optical fiber preform 1 into each hole of the cladding material. The core material includes a region with a higher refractive index than the refractive index 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 and integrating step S3 is a step of heating and integrating the cladding material, the core material, and the marker material. In the heating and integrating step S3, the cladding material, the core material, and the marker material are heated axially symmetrically while rotating them around the central axis of the cladding material. As a result, the cladding material, the core material, and the marker material are integrated, and the optical fiber preform 1 is manufactured. The outer periphery grinding step S4 is a step of grinding the outer periphery of the cladding portion 3. The drawing step S5 is a step of drawing the optical fiber preform 1. In this way, an optical fiber is manufactured.

[0038] As described in Patent Document 1, in the heating and unifying step S3, the cladding material is heated axially symmetrically while rotating around the central axis, so that the deformation of the glass near the core material is not symmetric in the core material located in the non-central region of the cladding material. Therefore, the center of the hole and the center of the core material after unification do not match each other. Therefore, it is difficult to accurately position the core portion 2 at the designed position. The cross-sectional structure of the optical fiber obtained by drawing is similar to the cross-sectional structure of the optical fiber preform 1. Therefore, the mismatch between the center position of the core material after heating and unification and the designed position causes a 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 portion 2 relative to the center position of the cladding portion 3. For this reason, a method for measuring the eccentricity of the core portion 2 is required.

[0039] The above-mentioned problem of core eccentricity during the manufacture of optical fiber preforms can occur in manufacturing methods other than the hole-drilling method. Examples of manufacturing methods other than the hole-drilling method include the MCVD (Modified Chemical Vapor Deposition) method, the OVD (Outside Vapor Deposition) method, and the VAD (Vapor phase Axial Deposition) method. The problem of core eccentricity can occur not only during the manufacture of MCF preforms, but also during the manufacture of optical fiber preforms such as SMF preforms, MMF preforms, and PMF preforms. The ratio of the core diameter to the cladding diameter of an optical fiber is an important parameter that affects the mode field diameter, effective core cross-sectional area, connection loss, and nonlinear optical effect, so it is necessary to maintain it appropriately from the stage of the optical fiber preform. Since the circularity of the core has a large effect on the polarization mode dispersion, it is desirable to keep the noncircularity of the core uniform in the longitudinal direction. In the case of PMF, it is desirable to be able to measure the eccentricity and dimensions of not only the core but also the stress-applying part. For example, in a stress-applying part to which boron is added, the refractive index is lower than that of the cladding part.

[0040] It is desirable to be able to measure the internal structure of the optical fiber preform at any interval in the longitudinal direction. To achieve this, a method is required to measure the refractive index change position, which is the boundary between different materials in the optical fiber preform, from the side of the optical fiber preform nondestructively. Based on the measurement results, it is possible to determine whether the optical fiber preform is defective before drawing and to adjust the amount of peripheral grinding of the cladding.

[0041] A method for measuring the refractive index change position from the side of an optical fiber nondestructively is also required for optical fibers. For example, when optical fibers such as MCF and PMF are connected to each other using a fusion splicer, it is necessary to perform rotational alignment of the optical fiber. There is an end face alignment method in which a mirror element with a 45° reflecting surface is placed between a pair of optical fibers arranged opposite to each other, and rotational alignment is performed while observing the end face image of the pair of optical fibers. In this method, the optical system inside the fusion splicer becomes complicated, and the fusion splicer may become large. On the other hand, in the side alignment method, in which rotational alignment is performed based on an intensity distribution image obtained by side observation of the optical fiber, a mirror element is not required, and it is possible to suppress the size of the fusion splicer. The side alignment method can be applied to applications other than fusion splicers, such as, for example, during the manufacture of optical fiber connectors and optical fiber arrays. By measuring the refractive index change position inside the optical fiber with high accuracy from side observation, more accurate rotational alignment can be performed.

[0042] (Method and device for measuring refractive index change position) A method and device for measuring the refractive index change position according to the present embodiment will be described. The method and device for measuring the refractive index change position according to the present embodiment are used, for example, between the heating and integrating step S3 and the outer periphery grinding step S4, with the optical fiber preform 1 as the observation target. This makes it possible to measure the amount of eccentricity of the core portion 2 of the optical fiber preform 1. Based on the measurement result, the cladding portion 3 is outer periphery ground in the outer periphery grinding step S4, thereby adjusting the center position of the core portion 2. The observation target may be an optical fiber to be subjected to rotational alignment. In this case, the optical fiber can be rotated and aligned with high accuracy. Below, a case where the observation target is the optical fiber preform 1 will be described.

[0043] 3 is a configuration diagram of a measuring device for measuring a refractive index change position according to an embodiment. As shown in FIG. 3, the measuring device 20 includes a light source 21, a camera 22, a polarizer 23, a detector 24, and a holder 25.

[0044] The light source 21 irradiates the optical fiber preform 1 with light L. The light L may be collimated light or non-collimated light. In the case of collimated light, the lens effective diameter needs to be equal to or larger than the outer diameter of the optical fiber preform 1. In the case of non-collimated light, the lens 22b can be made smaller. The camera 22 acquires a side observation image of the optical fiber preform 1. The camera 22 includes an image sensor 22a and a lens 22b. The lens 22b may be a telecentric lens or a non-telecentric lens of variable magnification. In the case of a telecentric lens, the lens effective diameter needs to be equal to or larger than the outer diameter of the optical fiber preform 1. Therefore, as the diameter of the optical fiber preform 1 increases, the size and introduction cost of the measuring device 20 increase. In the case of a non-telecentric lens of variable magnification, the camera field of view can be enlarged and reduced simply by adjusting the magnification of the lens and the camera position. Therefore, even if the diameter of the optical fiber preform 1 increases, the measurement can be performed with a device configuration that is less expensive and smaller than that of a telecentric lens.

[0045] The light source 21 and the camera 22 are disposed on an observation axis AX perpendicular to the central axis C of the optical fiber preform 1, sandwiching the optical fiber preform 1 therebetween. The observation axis AX is on a straight line perpendicular to the central axis C, i.e., the observation axis AX is perpendicular to the central axis C and on a straight line passing through the central axis C. The lens 22b is disposed between the light source 21 and the imaging element 22a. It can be said that the light source 21 and the camera 22 are disposed such that the observation axis AX connecting the light source 21 and the camera 22 is perpendicular to the central axis C of the optical fiber preform 1.

[0046] The polarizer 23 is disposed between the light source 21 and the optical fiber preform 1 on the observation axis AX. The polarizer 23 transmits only light vibrating in the transmission axis direction of the polarizer 23, out of the light L. The detector 24 is disposed between the optical fiber preform 1 and the camera 22 on the observation axis AX. The detector 24 transmits only light vibrating in the transmission axis direction of the detector 24, out of the light transmitted through the optical fiber preform 1. The polarizer 23 and the detector 24 have transmission axes perpendicular to each other.

[0047] The holding unit 25 holds the optical fiber preform 1 rotatably around the central axis C. The holding unit 25 is, for example, a glass lathe. The optical fiber preform 1 may be rotated manually or automatically by a rotary encoder or the like.

[0048] The measuring device 20 is configured so that at least one of the optical fiber preform 1 and the observation axis AX can move parallel to the central axis C. This allows side observation of the optical fiber preform 1 at any position in the longitudinal direction of the optical fiber preform 1. In this embodiment, the light source 21, the camera 22, the polarizer 23, and the detector 24 are configured so as to be movable in a direction B parallel to the central axis C. This allows the observation axis AX to move parallel to the central axis C from one end 1a to the other end 1b of the optical fiber preform 1. Instead of or in addition to the observation axis AX, the holder 25 may hold the optical fiber preform 1 so as to be movable in the direction B parallel to the central axis C.

[0049] The measurement principle of the measuring device 20 will be described. In the optical fiber preform 1, for example, materials with different refractive indices, such as the core portion 2 and the cladding portion 3, have different compositions. At the boundary position between such different materials, distortion occurs after heating and integration. The optical fiber obtained from the optical fiber preform 1 also has distortion. When the polarized wave after passing through the polarizer 23 passes through the boundary position between the different materials where distortion exists, the polarization state changes due to the photoelastic effect. For this reason, the light that passes through the distortion 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 does not pass through the distortion does not change its polarization state, so it cannot pass through the detector 24 and is not received by the camera 22. Therefore, a clear difference in brightness depending on the presence or absence of distortion occurs in the light intensity image obtained by the camera 22. Therefore, according to the measuring device 20, the boundary position between the different materials, that is, the refractive index change position, can be clearly detected rather than focusing on the minute refractive index difference at the refractive index change position of the observation target.

[0050] 4 is a flowchart showing a method for measuring a refractive index change position according to the embodiment. The method for measuring a refractive index change position according to the embodiment is performed using a measurement 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.

[0051] The preparation step S11 includes a step of holding the optical fiber preform 1 by the holding part 25 so as to be rotatable around 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 a step of rotating the optical fiber preform 1 held by the holding part 25 around the central axis C so that a straight line m (see FIG. 5) that is perpendicular to the central axis C and passes through two or more desired refractive index change positions to be measured is perpendicular 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 operation, the focus of the camera 22 is adjusted to the central axis C, thereby making it possible to accurately measure the ratio between the outer diameter of the optical fiber preform 1 and the outermost core pitch. Here, the outermost core pitch is the center-to-center distance between a pair of core parts 2 arranged on either side of the central axis C. The pair of core parts 2 that define the outermost core pitch are located on a straight line perpendicular to the central axis C. When there are multiple sets of core parts 2 arranged on either side of the central axis C on a straight line perpendicular to the central axis C, the set of core parts 2 with the longest center-to-center distance becomes the pair of core parts 2 that defines the outermost core pitch.

[0052] The outer diameter of the optical fiber preform 1 is measured, for example, by a vernier caliper or a dimension measuring device. The outermost core pitch can also be easily calculated from the outer diameter of the optical fiber preform 1 and the above ratio. Furthermore, by focusing on the geometric properties of the cross-sectional structure, the adjacent core pitch can also be calculated as 1 / √2 times the outermost core pitch. Here, the adjacent core pitch is the center-to-center distance between a pair of core portions 2 adjacent around the central axis C. Note that when the optical fiber preform 1 is a two-core MCF preform in which two core portions 2 are arranged symmetrically with respect to the central axis C, the outermost core pitch is equal to the adjacent core pitch.

[0053] FIG. 5 is a cross-sectional view showing the relationship between the rotation operation of the optical fiber preform and the observation axis. As shown in FIG. 5, when the optical fiber preform 1 is a 4-core MCF preform, a plurality of orthogonal relationships that satisfy the conditions are generated as shown in (b) and (c) of FIG. 5. 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 core parts 2 arranged on either side of the central axis C. In FIG. 5, one straight line m is shown by a solid line, and the other straight line m is shown by a dashed line. In the initial state (a), neither of the two straight lines m is orthogonal to the observation axis AX. The optical fiber preform 1 may 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 operation b), or the optical fiber preform 1 may be rotated around the central axis C so that the straight line m shown by the dashed line is orthogonal to the observation axis AX (rotation operation c). (b) the outermost core pitch measured in a state after the rotational motion b and (c) the outermost core pitch measured in a state after the rotational motion c may be averaged, and the average value may be used as the outermost core pitch of the optical fiber preform 1. By determining the outer periphery grinding amount based on the average value, the pitches of multiple adjacent cores can be made more uniform.

[0054] On each straight line m, there are four boundaries between a pair of cores 2 and cladding 3. Of these, two boundaries close to the central axis C may overlap with other cores 2 and may not be clearly detected on the light intensity image. In this case, the distance between two boundaries close to the outer peripheral surface of the cladding 3 that can be clearly detected is measured. In other words, the desired two or more refractive index change positions to be measured are the positions of two boundaries close to the outer peripheral surface of the cladding 3 among the boundaries between a pair of cores 2 and cladding 3 that exist on the straight line m. The outer diameter of the core 2 is equal to the outer diameter of the core material. Therefore, the outermost core pitch can be obtained by subtracting the outer diameter of the core material measured in advance from the distance between the two boundaries detected by the detection step S15.

[0055] The rotation step S12 is performed, for example, 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 gradually changing the angle by the holding unit 25, and the longest measured value may be taken as the outermost core pitch.

[0056] The irradiation step S13 is a step of irradiating the side surface of the optical fiber preform 1 with the light L emitted from the light source 21 via the polarizer 23. Of the light L, only the light whose vibration direction coincides with the direction of the transmission axis of the polarizer 23 passes through the polarizer 23 and is irradiated onto the side surface of the optical fiber preform 1. The image acquisition step S14 is a step of receiving the light transmitted through the optical fiber preform 1 by the camera 22 via the detector 24 and acquiring a light intensity image. Of the light transmitted through the optical fiber preform 1, only the light whose vibration direction coincides with the direction of the transmission axis of the detector 24 passes through the detector 24 and is received by the camera 22.

[0057] The detection step S15 is a step of detecting two or more desired refractive index change positions to be measured based on the acquired light intensity image. The boundary positions of a pair of core portions 2 and cladding portions 3 arranged on either side of the central axis C are detected based on the brightness difference of the light intensity image. This makes it possible to obtain the outermost core pitch and the adjacent core pitch.

[0058] 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. FIG. 6 is a schematic diagram for explaining the correction of image distortion caused by the lens effect of the side surface of the optical fiber preform. The circumference shown in FIG. 6 represents the outer surface of the cladding 3. The center point of the circle represents the central axis C. The radius of the circle (i.e., the radius of the cladding 3) is r. The refractive index of the periphery of the cladding 3 is n0, and the refractive index of the cladding 3 is n2 (n0≦n2). n0 is, for example, the refractive index of air. The angle of incidence when the light ray is traced back is θ0, and the angle of refraction is θ2. In this case, ∠CP D =θ0, ∠CPX=θ2. If we focus on a ray L1 parallel to the optical axis, the light passing through point P on the circumference will appear as an apparent point X on the image due to the lens effect, not as a true passing point X. D It is observed as if it corresponds to The true distance x is smaller than the apparent distance da. (x< d a).

[0059] This image distortion causes the apparent distance dWe will derive an equation for correcting a and the true distance x. First, equation (1) holds true from Snell's law.

number

number

[0060] The moving step S17 is a step of moving the optical fiber preform 1 or the observation axis AX parallel to the central axis C with the observation axis AX perpendicular to the central axis C. While moving the optical fiber preform 1 or the observation axis AX by the moving step S17, the rotation step S12, the irradiation step S13, the image acquisition step S14, the detection step S15, and the correction step S16 are repeatedly performed to measure the internal structure at any interval in the longitudinal direction of the optical fiber preform 1.

[0061] As described above, in this embodiment, the polarizer 23 and the detector 24 having mutually orthogonal transmission axes are used. When the polarized wave transmitted through the polarizer 23 passes through the boundary position between different materials, i.e., the distortion present at the refractive index change position, the polarization state changes due to the photoelastic effect. Therefore, the light transmitted through the distortion can pass through the detector 24. Therefore, the light intensity image acquired by the camera 22 has a difference in brightness depending on the presence or absence of distortion. According to this embodiment, it is possible to improve the measurement accuracy while suppressing the measurement time.

[0062] In the measurement method using a preform analyzer, the optical fiber preform is scanned point by point along the scan axis from the side, and the refractive index distribution is measured by performing an Abel inverse transform on the measurement result of the polarization angle. The disadvantages of this method include the increase in the number of scanning points of the laser light and the increase in measurement time as the preform diameter increases, the increase in the size of the optical system increases the introduction cost, and the need for calculation time due to numerical calculation processing. In addition, if the purpose is to measure only the refractive index change position, the polarization angle alone is sufficient and the refractive index distribution is not necessary, so the measurement method using the preform analyzer is over-specified. Non-Patent Document 1 and Patent Document 3 disclose methods for measuring the refractive index distribution inside the optical fiber, but both methods require numerical calculation processing, which increases the calculation time and therefore the measurement time. In addition, like the preform analyzer, if the purpose is limited to measuring only the refractive index change position, it will be over-specified.

[0063] In contrast, in the method of measuring the refractive index change position using the measuring device 20, it is not necessary to scan the optical fiber preform 1 from the side along the scan axis one point at a time, and the measurement time is unlikely to increase even if the diameter of the optical fiber preform 1 increases. Since the light L irradiated by the light source 21 may 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 measurement time can be suppressed.

[0064] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the gist of the present disclosure.

[0065] FIG. 7 is a configuration diagram of a measurement device for a refractive index change position according to a modified example. FIG. 8 is a perspective view showing the relationship between a container and an observation axis. As shown in FIG. 7 and FIG. 8, a measurement device 20A for a refractive index change position according to a modified example is different from the measurement device 20 in that it includes a transparent container 26 and a refractive index matching material 27 filled inside the container 26. The container 26 is, for example, a transparent glass cell. The container 26 may be made of acrylic as long as it is transparent. The container 26 is configured to be able to accommodate at least a part of the optical fiber preform 1 in the longitudinal direction. When the observation target is the optical fiber preform 1, the entire observation target may be accommodated in the container 26. In this case, the shape of the container 26 can be simplified. In this embodiment, a part of the observation target in the longitudinal direction is accommodated in the container 26. This allows the amount of the refractive index matching material 27 to be saved.

[0066] The container 26 has, for example, a rectangular parallelepiped shape. The container 26 has a pair of side surfaces 26a, 26b perpendicular to the observation axis AX. The pair of side surfaces 26a, 26b face each other with the optical fiber preform 1 interposed therebetween. The side surface 26a is disposed between the polarizer 23 and the optical fiber preform 1, and the light irradiated to the optical fiber preform 1 passes through the side surface 26a. The side surface 26b is disposed between the optical fiber preform 1 and the detector 24, and the light emitted from the optical fiber preform 1 passes through the side surface 26a. The container 26 has a pair of side surfaces 26c, 26d intersecting (here, perpendicular to) the central axis C. The pair of side surfaces 26c, 26d are provided with a circular through hole through which the optical fiber preform 1 passes.

[0067] The refractive index matching material 27 is filled in the inside of the container 26, that is, in the space between the container 26 and the optical fiber preform 1. . TiredThe refractive index matching material 27 is a medium having a refractive index greater than that of air and equal to or less than that of glass. The refractive index matching material 27 may be, for example, water. By immersing the optical fiber preform 1 in the refractive index matching material 27, the influence of the lens effect resulting from the circular shape of the outer surface of the optical fiber preform 1 can be suppressed. By using the refractive index matching material 27 having a refractive index equivalent to that of glass, a lateral image without distortion can be obtained. Therefore, the effort of correcting image distortion can be omitted. Even if the refractive index matching material 27 has a lower refractive index than that of glass, the lens effect of the outer surface of the optical fiber preform 1 can be mitigated compared to the case of air atmosphere. Therefore, the entire refractive index change position inside the optical fiber preform 1 can be observed within the field of view of the camera without increasing the size of the camera 22.

[0068] 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 part of the longitudinal direction of the optical fiber preform 1 in a refractive index matching material 27 (medium). In the irradiation step S13, the light L emitted from the light source 21 is irradiated onto 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 to acquire a light intensity image.

[0069] 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 correction step S16 can be omitted.

[0070] In the above, an example was described in which the observation axis AX is actually perpendicular to the central axis C and the straight line m. However, when the observation axis AX is approximately perpendicular and can be regarded as being perpendicular to the central axis C and the straight line m by correction, the observation axis AX does not have to be completely perpendicular to the central axis C and the straight line m. That is, the observation axis AX may be shifted from the original observation axis AX perpendicular to the central axis C and the straight line m. For example, the observation axis AX may be on a straight line obtained by translating the original observation axis AX in a plane perpendicular to the central axis C, may be on a straight line obtained by rotating the original observation axis AX around the central axis C, or may be on a straight line obtained by rotating the straight line obtained by translating the original observation axis AX around the central axis C. In these cases, the correction of the influence of the distance of translation and the angle of rotation may be included in the correction step S16. [Explanation of symbols]

[0071] 1...Optical fiber base material 1a…one end 1b...other end 2…Core section 3…Clad section 4…Marker section 5…Central core 6…Depressed section 20, 20A…Measuring device 21…Light source 22...Camera 22a...imaging element 22b…Lens 23...Polarizer 24…Detector 25...Holding part 26…Container 26a,26b,26c,26d…side 27...Refractive index matching Material A X: 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 depressed part L…Light L1…ray of light P…point X… True passing point D …apparent point x…True distance d a...apparent distance θ0…Incidence angle θ2…Refraction angle r…radius

Claims

1. A method for measuring a refractive index change position of an optical fiber preform using a measuring device including: a holder that holds the optical fiber preform rotatably around a central axis; a light source and a camera that are arranged to sandwich the optical fiber preform on an observation axis perpendicular to the central axis; a polarizer that is arranged between the light source and the optical fiber preform; and a detector that has a transmission axis perpendicular to a transmission axis of the polarizer and is arranged between the optical fiber preform and the camera, the method comprising: rotating the optical fiber preform around the central axis so that a straight line that is perpendicular to the central axis and passes through two or more desired refractive index change positions to be measured is perpendicular to the observation axis; irradiating a side surface of the optical fiber preform with light emitted from the light source via the polarizer; receiving the light transmitted through the optical fiber preform with the camera via the detector to obtain a light intensity image; detecting the two or more refractive index change positions based on the light intensity image; and moving the optical fiber preform or the observation shaft parallel to the central axis. A method for measuring the location of refractive index change in optical fiber preform.

2. The method further includes a step of immersing at least a portion of the optical fiber preform in a medium having a refractive index greater than that of air and equal to or less than that of glass, irradiating a side surface of the optical fiber preform with light emitted from the light source via the polarizer and the medium; The light transmitted through the optical fiber preform is received by the camera via the medium and the detector to obtain a light intensity image.

2. A method for measuring a refractive index change position of an optical fiber preform according to claim 1.

3. The optical fiber preform is a polarization-maintaining fiber preform.

3. A method for measuring a refractive index change position of an optical fiber preform according to claim 1 or 2.

4. The optical fiber preform is a multi-core optical fiber preform.

3. A method for measuring a refractive index change position of an optical fiber preform according to claim 1 or 2.

5. the optical fiber preform includes a cladding portion and a pair of core portions disposed on either side of the central axis and having a maximum center-to-center distance; the two or more refractive index change positions are positions of two boundaries between the cladding portion and the pair of core portions that are on the straight line and that are close to an outer circumferential surface of the cladding portion, measuring a center-to-center distance of the pair of core portions from a distance between the two boundaries detected by the detecting step; 5. The method for measuring a refractive index change position of an optical fiber preform according to claim 4.

6. The method further includes a step of correcting an image distortion caused by a lens effect of the side surface of the optical fiber preform for the light intensity image by the following formula: [0010] where da is the apparent distance of the refractive index change location from the central axis, x is the true distance of the refractive index change location from 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.

3. A method for measuring a refractive index change position of an optical fiber preform according to claim 1 or 2.

7. An apparatus for measuring a refractive index change position of an optical fiber preform, comprising: a holding part that holds the optical fiber preform so as to be rotatable around a central axis; a light source and a camera disposed on an observation axis perpendicular to the central axis with the optical fiber preform interposed therebetween; a polarizer disposed between the light source and the optical fiber preform; a detector having a transmission axis perpendicular to the transmission axis of the polarizer and disposed between the optical fiber preform and the camera; the holding unit holds the optical fiber preform so that a straight line passing through two or more desired refractive index change positions to be measured is perpendicular to the central axis and perpendicular to the observation axis; the light source irradiates the side surface of the optical fiber preform with light via the polarizer; the camera receives the light transmitted through the optical fiber preform via the detector, and obtains a light intensity image in which a brightness difference occurs at the two or more refractive index change positions; the holding unit 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 to be movable in a direction parallel to the central axis. A device for measuring the refractive index change position of optical fiber preform.

8. a container for accommodating at least a portion of the optical fiber preform in the central axis direction, the container being filled with a medium having a refractive index greater than that of air and equal to or less than that of glass; 8. The apparatus for measuring a refractive index change position of an optical fiber preform according to claim 7.

9. A method for measuring a refractive index change position of an optical fiber using a measuring device including: a holder that holds an optical fiber rotatably around a central axis; a light source and a camera that are arranged to sandwich the optical fiber on an observation axis perpendicular to the central axis; a polarizer that is arranged between the light source and the optical fiber; and a detector that has a transmission axis perpendicular to the transmission axis of the polarizer and is arranged between the optical fiber and the camera, the method comprising: rotating the optical fiber around the central axis so that a straight line that is perpendicular to the central axis and passes through two or more desired refractive index change positions to be measured is perpendicular to the observation axis; irradiating the light emitted from the light source onto a side surface of the optical fiber via the polarizer; receiving the light transmitted through the optical fiber with the camera via the detector and acquiring a light intensity image; detecting the two or more refractive index change positions based on the light intensity image; A method for measuring the location of refractive index changes in optical fiber.

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

11. The method 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 equal to or less than that of glass, irradiating a side surface of the optical fiber with light emitted from the light source via the polarizer and the medium; The light transmitted through the optical fiber is received by the camera via the medium and the detector to obtain a light intensity image.

11. A method for measuring a refractive index change position of an optical fiber according to claim 9 or 10.

12. The optical fiber is a polarization-maintaining fiber.

11. A method for measuring a refractive index change position of an optical fiber according to claim 9 or 10.

13. The optical fiber is a multi-core optical fiber.

11. A method for measuring a refractive index change position of an optical fiber according to claim 9 or 10.

14. The optical fiber includes a clad and a pair of cores arranged on either side of the central axis and having a maximum center-to-center distance; the two or more refractive index change positions are positions of two boundaries between the cladding and the pair of cores that exist on the straight line and that are close to an outer peripheral surface of the cladding, measuring a center-to-center distance of the pair of cores from a distance between the two boundaries detected by the detecting step; The method for measuring a refractive index change position of an optical fiber according to claim 13.

15. The method further includes a step of correcting image distortion caused by a lens effect of the side surface of the optical fiber for the light intensity image by the following formula: [0025] where da is the apparent distance of the refractive index change location from the central axis, x is the true distance of the refractive index change location from the central axis, n0 is the refractive index of the cladding periphery, n2 is the refractive index of the cladding, and r is the radius of the cladding.

11. A method for measuring a refractive index change position of an optical fiber according to claim 9 or 10.

16. An apparatus for measuring a refractive index change position of an optical fiber, comprising: a holder that holds the optical fiber rotatably around a central axis; a light source and a camera disposed on an observation axis perpendicular to the central axis and sandwiching the optical fiber; a polarizer disposed between the light source and the optical fiber; a detector having a transmission axis perpendicular to the transmission axis of the polarizer and disposed between the optical fiber and the camera; the holding portion holds the optical fiber so that a straight line passing through two or more desired refractive index change positions to be measured is perpendicular to the central axis and perpendicular to the observation axis; the light source irradiates light onto a side surface of the optical fiber through the polarizer; the camera receives the light transmitted through the optical fiber via the detector, and obtains a light intensity image in which a brightness difference occurs at the two or more refractive index change positions. A device for measuring the position of refractive index change in optical fiber.

17. The holding unit 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 to be movable in a direction parallel to the central axis. The device for measuring a refractive index change position of an optical fiber according to claim 16.

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