Manufacturing method for optical fiber preforms

Abstract

To provide a method for manufacturing an optical fiber preform, capable of preventing the productivity of an optical fiber from decreasing.SOLUTION: A method for manufacturing an optical fiber preform 1P comprises: an intermediate preform formation step P1 of forming an intermediate preform 1PP consisting of a core glass body 10P formed as a body part and a first glass layer 21 being a part of a clad glass body 11P formed as an outer part; and a glass layer formation step P3 of depositing glass fine particles on the outer peripheral surface of the first glass layer 21 to form a coated porous glass body and sintering the coated porous glass body to form a second glass layer 22 being an other part of the clad glass body 11P. The glass layer formation step P3 comprises depositing the glass fine particles on the outer peripheral surface of the first glass layer 21 so that the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate preform 1PP is a value to be set on the basis of the refractive index distribution normally measured by a preform analyzer having a reference preform in which the profile of the refractive index distribution is similar to the profile of the refractive index distribution of the intermediate preform 1PP.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a method for producing an optical fiber preform.

Background Art

[0002] As a method for producing an optical fiber preform used in the production of an optical fiber, it is known to produce an optical fiber preform from an intermediate preform formed by sintering a porous glass body. In this case, for example, an optical fiber preform is produced by forming a coated porous glass body surrounding the outer peripheral surface of the intermediate preform, and sintering the coated porous glass body to form a glass layer surrounding the outer peripheral surface of the intermediate preform. Furthermore, the above porous glass body and coated porous glass body are formed by depositing glass fine particles in multiple layers using methods such as the OVD method (Outside Vapor Deposition method) or the VAD method (Vapor Phase Axial Deposition method).

[0003] In such a production method, for example, the intermediate preform is composed of a core glass body serving as a core, and a first glass layer that is formed of a glass body serving as a part of a clad and surrounds the outer peripheral surface of the core glass body, and the glass layer surrounding the outer peripheral surface of the intermediate preform may be a second glass layer formed of a glass body serving as another part of the clad. In this case, before forming the coated porous glass body, the ratio of the outer diameter of the core glass body to the outer diameter of the intermediate preform, etc. is obtained based on the refractive index distribution of the intermediate preform, and the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate preform is set from such ratio. Then, the coated porous glass body is formed such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate preform becomes a set value. As a method for measuring the refractive index distribution of an intermediate preform, measurement using a preform analyzer is known. A preform analyzer measures the refractive index distribution of an intermediate preform by making a laser beam incident from a direction perpendicular to the longitudinal direction of the intermediate preform, scanning the laser beam in the radial direction, and measuring the refraction angle of the laser beam emitted from the intermediate preform.

[0004] As described above, the intermediate matrix is ​​a porous glass body in which glass nanoparticles are deposited in multiple layers and then sintered. Therefore, the refractive index distribution of the intermediate matrix has minute fluctuations in the refractive index corresponding to each layer of glass nanoparticles in the porous glass body, and these fluctuations are sometimes called striations. The laser beam of the preform analyzer may diffract due to these striations, in which case the refraction angle of the laser beam cannot be accurately measured, and the measured refractive index distribution becomes disordered. When such measurement anomalies occur, it is not possible to set the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate matrix, and it becomes impossible to manufacture a matrix for optical fibers with the desired optical properties.

[0005] Patent Document 1 below discloses a method for suppressing measurement abnormalities caused by striations by stretching a preform for optical fibers that is formed by sintering a porous glass body, thereby shortening the period of striations. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-56786 [Overview of the project] [Problems that the invention aims to solve]

[0007] If the measurement results of the refractive index distribution of the intermediate matrix are disturbed by striations, it is conceivable to stretch the intermediate matrix to prevent disturbances in the refractive index distribution measurement results and then set the above ratio for the stretched intermediate matrix, as described in Patent Document 1 above. However, stretching the intermediate matrix reduces the diameter of the final optical fiber matrix. As a result, the length of the optical fiber that can be manufactured from the optical fiber matrix becomes shorter, and the productivity of optical fibers decreases.

[0008] Therefore, the present invention aims to provide a method for manufacturing optical fiber preforms that can suppress a decrease in the productivity of optical fibers. [Means for solving the problem]

[0009] One aspect of the present invention is a method for manufacturing a preform for optical fibers, comprising a rod-shaped main body and an outer part having a different refractive index from the main body and surrounding the outer surface of the main body, comprising: an intermediate preform formation step of sintering a porous glass body to form an intermediate preform consisting of the main body and a first glass layer which is part of the outer part; and a glass layer formation step of depositing glass fine particles on the outer surface of the first glass layer to form a coated porous glass body, and sintering the coated porous glass body to form a second glass layer which is another part of the outer part, wherein in the glass layer formation step, the glass fine particles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate preform is a value set based on the refractive index distribution measured correctly with a preform analyzer of a reference preform whose refractive index distribution profile is similar to that of the intermediate preform.

[0010] In intermediate matrix materials with similar refractive index distribution profiles, the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate matrix material is approximately the same. Therefore, the refractive index distribution profiles of each intermediate matrix material on which the second glass layer is formed are similar to each other. Consequently, the refractive index distribution profiles of the resulting optical fiber matrix materials are similar to each other, and the optical properties of the manufactured optical fibers are also similar. In Embodiment 1, as described above, glass nanoparticles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate matrix material is set to a value based on the refractive index distribution of a reference matrix material measured correctly with a preform analyzer. The refractive index distribution profile of this reference matrix material is similar to that of the intermediate matrix material. Therefore, according to Embodiment 1, even if an intermediate matrix material exhibits disturbance due to striations in the measurement results of the refractive index distribution measured by a preform analyzer, the above ratio can be set without stretching the intermediate matrix material to eliminate this disturbance. Accordingly, according to Embodiment 1, it is possible to suppress the reduction in the diameter of the manufactured optical fiber matrix material and suppress the decrease in optical fiber productivity. Furthermore, examples of profiles similar to the refractive index distribution profile of the intermediate matrix include, for example, a profile that is roughly the same as the refractive index distribution profile of the intermediate matrix obtained by reducing it radially. In addition, examples of reference matrix materials having such a profile include, for example, a glass member obtained by stretching another intermediate matrix formed on the same day under the same conditions and with the same equipment as the intermediate matrix formation process, a glass member formed on a different day, a glass member obtained by stretching an intermediate matrix formed under the same conditions as the intermediate matrix formation process but with different equipment, and a glass member obtained by stretching a part of the intermediate matrix that has been separated from the intermediate matrix. Since these glass members have a smaller outer diameter than the intermediate matrix, the striation period is shorter than that of the intermediate matrix, and disturbances due to striations may not occur.

[0011] Aspect 2 of the present invention is a method for manufacturing a preform for optical fibers according to aspect 1, characterized in that the reference preform is a glass member obtained by stretching another intermediate preform formed under the same conditions and with the same apparatus as the intermediate preform formation step.

[0012] The refractive index distribution profile of a glass member stretched from another intermediate base material formed under the same conditions and with the same equipment as the intermediate base material is approximately the same as the refractive index distribution profile of the intermediate base material, but scaled radially according to the stretching ratio. In other words, the refractive index distribution of the glass member, where the horizontal axis is the radial position normalized by dividing the radial distance from the center by the outer diameter of the glass member, is approximately the same as the refractive index distribution of the intermediate base material, where the horizontal axis is the radial position normalized by dividing the radial distance from the center by the outer diameter of the intermediate base material. In Embodiment 2, such a glass member is the reference base material. Therefore, according to Embodiment 2, even if the intermediate base material causes disturbance due to striations in the measurement results of the refractive index distribution, the above ratio can be set without stretching the intermediate base material in a way that eliminates this disturbance. Furthermore, according to Embodiment 2, the reference base material can be easily prepared.

[0013] A third aspect of the present invention is a method for manufacturing a preform for optical fibers according to the first aspect, characterized in that the reference preform is a glass member obtained by stretching a portion of the intermediate preform that has been separated from the intermediate preform.

[0014] The refractive index distribution profile of a glass member obtained by stretching a portion of the intermediate matrix is, as in Embodiment 2, generally the same as the profile obtained by radially scaling down the refractive index distribution profile of the intermediate matrix. In Embodiment 3, such a glass member is the reference matrix. Therefore, according to Embodiment 3, even if the intermediate matrix is ​​such that striations cause disturbances in the measurement results of the refractive index distribution, the above ratio can be set without stretching the entire intermediate matrix to eliminate these disturbances. Furthermore, the refractive index distribution profile of a glass member obtained by stretching a portion of the intermediate matrix tends to closely resemble the profile obtained by radially scaling down the refractive index distribution profile of the intermediate matrix according to the stretching ratio, more so than the refractive index distribution profile of a glass member obtained by stretching another intermediate matrix formed under the same conditions and with the same equipment as the intermediate matrix. This is because, for example, even if another intermediate matrix is ​​formed under the same conditions and with the same equipment as the intermediate matrix, slight differences occur in the way glass nanoparticles are deposited, etc. Therefore, according to embodiment 3, the above ratio can be set to a more appropriate value compared to the case where the reference base material is a glass member stretched from another intermediate base material formed under the same conditions and with the same apparatus as the intermediate base material.

[0015] Aspect 4 of the present invention is a method for manufacturing a preform for optical fibers according to aspect 1, wherein in the glass layer formation step, glass fine particles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate preform is a value set based on a correlation obtained from the refractive index distributions of a plurality of reference preforms, the refractive index distributions of the intermediate preform and each of the reference preforms include a feature portion having a predetermined characteristic based on a change in refractive index, the feature portion of the intermediate preform is located within a range that can be normally measured by the preform analyzer, and the correlation is a correlation between the ratio of the distance from the center of each reference preform to the feature portion in the reference preform with respect to the radius of each reference preform and the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate preform, which is set based on the refractive index distribution of each reference preform.

[0016] In a preform analyzer, a laser beam is scanned radially across the intermediate base material. As a result, the refractive index distribution measurement results show a normal refractive index distribution outside the area where striation-induced disturbances occur, and an abnormal refractive index distribution towards the center of that area. Therefore, the refractive index distribution profile in the normally measured range is similar to the refractive index distribution profile in the corresponding range of the reference base material. The inventors diligently studied the relationship between these similar profiles and the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material, which is set based on the refractive index distribution of the reference base material. As a result, they found that when the refractive index distributions of the intermediate base material and the reference base material include characteristic areas with predetermined features based on changes in refractive index, the above correlation is approximately proportional. Therefore, when the characteristic area of ​​the intermediate base material is located within the range that can be normally measured by the preform analyzer, the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material can be set based on this correlation. Accordingly, according to embodiment 4, even if the intermediate base material is such that the measurement result of the refractive index distribution is disturbed due to striations, the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material can be set based on the correlation described above without stretching the intermediate base material to eliminate the disturbance. Features include, for example, a region in the radial direction where the refractive index first reaches a predetermined value as you move from the outside towards the center. [Effects of the Invention]

[0017] As described above, the present invention provides a method for manufacturing a preform for optical fibers that can suppress a decrease in the productivity of optical fibers. [Brief explanation of the drawing]

[0018] [Figure 1] This figure schematically shows a cross-section perpendicular to the longitudinal direction of the optical fiber preform according to the first embodiment of the present invention. [Figure 2] This figure schematically shows a cross-section perpendicular to the longitudinal direction of the intermediate base material in the first embodiment. [Figure 3] This is a flowchart showing the steps for manufacturing a preform for optical fibers according to the first embodiment. [Figure 4]It is a flowchart of the setting step in the first embodiment. [Figure 5] It is a diagram showing the measured refractive index profile of the intermediate preform and the refractive index profile of the reference preform. [Figure 6] It is a diagram showing the relationship between the ratio of the distance from the center of each reference preform to the characteristic portion in the reference preform with respect to the radius of the reference preform, and the ratio of the outer diameter of the second glass layer with respect to the outer diameter of the intermediate preform set based on the refractive index profile of each reference preform. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, a method for manufacturing an optical fiber preform according to the present invention will be illustrated with reference to the accompanying drawings. The embodiments illustrated below are intended to facilitate understanding of the present invention, and are not intended to limit or interpret the present invention. The present invention can be modified and improved without departing from the gist thereof. In the drawings referenced below, the dimensions of each member may be shown modified for easy understanding.

[0020] (First Embodiment) FIG. 1 is a diagram schematically showing a cross section perpendicular to the longitudinal direction of an optical fiber preform according to the present embodiment. As shown in FIG. 1, the optical fiber preform 1P of the present embodiment includes a rod-shaped core glass body 10P and a clad glass body 11P. The clad glass body 11P surrounds the outer peripheral surface of the core glass body 10P. The outer shape of the clad glass body 11P in the cross section is substantially circular, and the core glass body 10P is arranged at the center of the clad glass body 11P. The outer diameter of the optical fiber preform 1P is not limited, and is, for example, 150 mm.

[0021] The refractive index of the core glass body 10P and the refractive index of the clad glass body 11P are different. In this embodiment, the core glass body 10P is made of silica glass to which a dopant that increases the refractive index, such as germanium (Ge), has been added, and the clad glass body 11P is made of silica glass with no additives. Alternatively, the core glass body 10P may be made of silica glass with no additives, and the clad glass body 11P may be made of silica glass to which a dopant that decreases the refractive index, such as fluorine (F), has been added. Alternatively, the core glass body 10P may be made of silica glass to which a dopant that increases the refractive index has been added, and the clad glass body 11P may be made of silica glass to which a dopant that decreases the refractive index has been added. Furthermore, the dopant that increases the refractive index and the dopant that decreases the refractive index are not limited.

[0022] In such an optical fiber preform 1P, if the core glass body 10P is considered the main body, then the clad glass body 11P can be understood as an outer part that surrounds the outer surface of the main body and has a different refractive index from the main body. Furthermore, as will be described in detail later, the optical fiber preform 1P is manufactured by forming a second glass layer on the outer surface of a rod-shaped intermediate preform. Next, the intermediate preform will be described.

[0023] Figure 2 is a schematic diagram showing a cross-section perpendicular to the longitudinal direction of the intermediate base material in this embodiment. As shown in Figure 2, the intermediate base material 1PP is composed of a core glass body 10P and a first glass layer 21 which is part of the clad glass body 11P. In Figure 1, the outer surface of the first glass layer 21 is shown by a dashed line, and the area inside this dashed line is the intermediate base material 1PP. Of the clad glass body 11P, the area inside this dashed line is the first glass layer 21, and the area outside is the aforementioned second glass layer 22, and the second glass layer 22 is the part of the clad glass body 11P other than the first glass layer 21. In addition, the outer diameter of the intermediate base material 1PP in this embodiment is 60 mm, but it is not limited to this.

[0024] Next, a method for manufacturing the optical fiber preform according to this embodiment will be described.

[0025] Figure 3 is a flowchart showing the steps for manufacturing the optical fiber base material 1P according to this embodiment. As shown in Figure 3, the manufacturing method for the optical fiber base material 1P according to this embodiment includes an intermediate base material formation step P1, a setting step P2, and a glass layer formation step P3.

[0026] <Intermediate base material formation process P1> This process involves depositing glass nanoparticles in multiple layers to form a porous glass body, and then sintering the porous glass body to form an intermediate base material 1PP, which is part of the optical fiber base material 1P. In this embodiment, glass nanoparticles are deposited in multiple layers to form a porous glass body, and the porous glass body is sintered, so that the intermediate base material 1PP shown in Figure 2 is formed. Examples of methods for depositing glass nanoparticles include the OVD method and the VAD method, and in this embodiment, glass nanoparticles are deposited by the VAD method to form a porous glass body.

[0027] <Setting process P2> This step is to set the outer diameter of the second glass layer 22 to be formed in the glass layer formation step P3 described later. Figure 4 is a flowchart of this step in this embodiment. As shown in Figure 4, this step includes steps SP21 to SP25.

[0028] (Step SP21) This step involves measuring the refractive index distribution of the intermediate base material 1PP formed by the intermediate base material formation step P1 using a preform analyzer. The preform analyzer measures the refractive index distribution of the intermediate base material 1PP by irradiating it with laser light from a direction perpendicular to the longitudinal direction of the intermediate base material 1PP and scanning it radially, and measuring the angle at which the laser light emitted from the intermediate base material 1PP is refracted. In this embodiment, the peak wavelength of the laser light power in the preform analyzer is 632 nm, and the diameter of the laser light incident on the intermediate base material 1PP is approximately 30 μm, but the preform analyzer is not limited. Furthermore, the refractive index distribution in this embodiment is a graph with the radial position normalized by dividing the radial distance from the center of the intermediate base material 1PP by the outer diameter of the intermediate base material 1PP on the horizontal axis, and the specific refractive index Δ(%) normalized by dividing the measured refractive index by the refractive index at the outer edge of the intermediate base material 1PP on the vertical axis. The refractive index distribution is not limited; for example, the horizontal axis may represent the radial distance from the center of the intermediate base material 1PP. The vertical axis may represent the relative refractive index Δ(%) normalized by dividing the measured refractive index by the refractive index of pure quartz glass, or it may represent the measured refractive index.

[0029] (Step SP22) This step is performed after step SP21 and determines whether the refractive index distribution of the intermediate base material 1PP obtained in step SP21 is normal. As mentioned above, the intermediate base material 1PP is a porous glass body in which glass nanoparticles are deposited in multiple layers and then sintered. For this reason, striations occur in the refractive index distribution of the intermediate base material 1PP, and the laser beam of the preform analyzer is diffracted by these striations, causing disturbances in the measured refractive index distribution, which may prevent the refractive index distribution from being measured correctly. In this step, it is determined whether the refractive index distribution of the intermediate base material 1PP is normal and free from disturbances caused by striations. This determination may be made by an operator or by a device such as a microcontroller.

[0030] (Step SP23) This step is performed when it is determined in step SP22 that the refractive index distribution of the intermediate base material 1PP is normal. In this step, based on the refractive index distribution of the intermediate base material 1PP obtained in step SP21, the ratio of the outer diameter of the core glass body 10P to the outer diameter of the intermediate base material 1PP is determined, and the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP is set. In this embodiment, the above ratio is set so that the refractive index distribution of the intermediate base material 1PP on which the second glass layer 22 is formed is the same as the refractive index distribution of the optical fiber base material 1P shown in Figure 1.

[0031] (Step SP24) This step is performed if it is determined in step SP22 that the refractive index distribution of the intermediate base material 1PP is not normal. In this step, a reference base material is prepared whose refractive index distribution profile is similar to that of the intermediate base material 1PP, and whose refractive index distribution can be measured normally with a preform analyzer. The refractive index distribution of this reference base material is then measured using a preform analyzer. As an example of a profile similar to the refractive index distribution profile of the intermediate base material 1PP, one example would be a profile that is roughly the same as the refractive index distribution profile of the intermediate base material 1PP, but scaled down in the radial direction.

[0032] In this embodiment, the reference base material is a glass member obtained by stretching another intermediate base material, formed under the same conditions and with the same apparatus as the intermediate base material formation process P1 described above, in the longitudinal direction. This reference base material consists of a core glass body 10P and a first glass layer 21, similar to the intermediate base material 1PP. Although the outer diameter of the reference base material is smaller than the outer diameter of the intermediate base material 1PP, the refractive index distribution profile of the reference base material is approximately the same as the refractive index distribution profile of the intermediate base material 1PP, which is reduced radially according to the stretching ratio. In other words, the refractive index distribution of the reference base material, where the horizontal axis is the radial position normalized by dividing the radial distance from the center by the outer diameter of the reference base material, is approximately the same as the refractive index distribution of the intermediate base material 1PP, where the horizontal axis is the radial position normalized by dividing the radial distance from the center by the outer diameter of the intermediate base material 1PP. Furthermore, the period of striations in the refractive index distribution of the reference base material is shorter than the period of striations in the refractive index distribution of the intermediate base material 1PP, and is short enough that there is no disturbance due to striations in the measurement results of the refractive index distribution of the reference base material. In other words, another intermediate base material is stretched so that the measurement results of the refractive index distribution are free from disturbances caused by striations. In this embodiment, the reference base material is a glass member obtained by stretching another intermediate base material with an outer diameter of 60 mm to an outer diameter of 30 mm, but the degree of stretching of the other intermediate base material is not limited. For example, the reference base material can be formed by stretching another intermediate base material so that its outer diameter is 0.5 times or less.

[0033] Figure 5 shows the refractive index distribution of the measured intermediate base material 1PP and the refractive index distribution of the reference base material. In Figure 5, the refractive index distribution of the intermediate base material 1PP is shown by a solid line, and the refractive index distribution of the reference base material is shown by a dotted line. The horizontal axis represents the radial position normalized by dividing the radial distance from the center of the intermediate base material 1PP by the outer diameter of the intermediate base material 1PP, and the vertical axis represents the specific refractive index Δ(%) normalized by dividing the measured refractive index by the refractive index at the outer edge of the intermediate base material 1PP. In the refractive index distribution of the intermediate base material 1PP shown in Figure 5, there is a range EA in which the specific refractive index Δ decreases sharply to a negative value, indicating an abnormal refractive index distribution. This is thought to be due to disturbances caused by striations in the measurement results of the refractive index distribution of the intermediate base material 1PP.

[0034] (Step SP25) This step is performed after step SP24. In this step, based on the refractive index distribution of the reference base material measured in step SP24, the ratio of the outer diameter of the core glass body 10P in the reference base material to the outer diameter of the reference base material is determined, and the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the reference base material is set. In this embodiment, the above ratio is set so that the refractive index distribution profile of the reference base material on which the second glass layer 22 is formed is the same as the refractive index distribution profile of the optical fiber base material 1P shown in Figure 1, which is reduced radially according to the stretching ratio during the manufacturing of the reference base material, and this ratio is taken as the ratio in the intermediate base material 1PP.

[0035] Thus, in this embodiment, if the refractive index distribution of the intermediate base material 1PP can be measured correctly, the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP is set based on the refractive index distribution of the intermediate base material 1PP. If the refractive index distribution of the intermediate base material 1PP cannot be measured correctly, the above ratio is set based on the refractive index distribution measured correctly using a preform analyzer of a reference base material whose refractive index distribution profile is similar to that of the intermediate base material 1PP.

[0036] <Glass layer formation process P3> This process involves depositing glass nanoparticles on the outer surface of the first glass layer 21 of the intermediate base material 1PP to form a coated porous glass body surrounding the outer surface of the first glass layer 21, and then sintering the coated porous glass body to form a second glass layer 22 which will become another part of the clad glass body 11P. When forming the coated porous glass body, glass nanoparticles are deposited on the outer surface of the first glass layer 21 by the OVD method so that the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP is the value set in setting step P2, thereby forming the coated porous glass body.

[0037] As described above, in setting step P2, if the measured refractive index distribution of the intermediate base material 1PP is normal, the above ratio is set based on the refractive index distribution of the intermediate base material 1PP such that the refractive index distribution of the intermediate base material 1PP on which the second glass layer 22 is formed becomes the same as the refractive index distribution of the optical fiber base material 1P shown in Figure 1. As a result, with the formation of the second glass layer 22, the portion consisting of the first glass layer 21 and the second glass layer 22 becomes the clad glass body 11P of the optical fiber base material 1P, and the optical fiber base material 1P is obtained.

[0038] Furthermore, in setting step P2, if the measured refractive index distribution of the intermediate base material 1PP is not normal, the above ratio is set so that the refractive index distribution profile of the reference base material on which the second glass layer 22 is formed becomes the same as the refractive index distribution profile of the optical fiber base material 1P shown in Figure 1, which is reduced radially according to the stretching ratio during the manufacturing of the reference base material. As mentioned above, the refractive index distribution profile of the reference base material is approximately the same as the refractive index distribution profile of the intermediate base material 1PP which is reduced radially. For this reason, the ratio of the outer diameter of the core glass body 10P in the reference base material to the outer diameter of the reference base material is approximately the same as the ratio of the outer diameter of the core glass body 10P in the intermediate base material 1PP to the outer diameter of the intermediate base material 1PP. For this reason, the above ratio set in setting step P2 when the measured refractive index distribution of the intermediate base material 1PP is not normal is approximately the same as the ratio set when the measured refractive index distribution of the intermediate base material 1PP is normal. Therefore, even in this case, the formation of the second glass layer 22 results in the portion consisting of the first glass layer 21 and the second glass layer 22 becoming the clad glass body 11P of the optical fiber base material 1P, thus obtaining the optical fiber base material 1P.

[0039] As described above, the manufacturing method for the optical fiber base material of this embodiment comprises an intermediate base material formation step P1 and a glass layer formation step P3. The optical fiber base material 1P includes a rod-shaped core glass body 10P and a clad glass body 11P surrounding the outer surface of the core glass body 10P. In the intermediate base material formation step, the porous glass body is sintered to form an intermediate base material 1PP consisting of the core glass body 10P and a first glass layer 21 which is part of the clad glass body 11P. In the glass layer formation step P3, glass fine particles are deposited on the outer surface of the first glass layer 21 to form a coated porous glass body, and the coated porous glass body is sintered to form a second glass layer 22 which becomes another part of the clad glass body 11P. If the refractive index distribution of the intermediate base material 1PP cannot be measured correctly with the preform analyzer, in the glass layer formation process P3, glass microparticles are deposited on the outer surface of the first glass layer 21 so that the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP is set to a value based on the refractive index distribution of the reference base material measured correctly with the preform analyzer. The refractive index distribution profile of this reference base material is similar to that of the intermediate base material 1PP, and the reference base material is a glass member obtained by stretching another intermediate base material formed under the same conditions and with the same equipment as in the intermediate base material formation process P1.

[0040] Therefore, as mentioned above, the above ratio is approximately the same as the ratio set when the refractive index distribution of the measured intermediate base material 1PP is normal. Accordingly, according to the manufacturing method of optical fiber base material of this embodiment, even if the intermediate base material 1PP has disturbances due to striations in the measurement result of the refractive index distribution measured by the preform analyzer, the above ratio can be set without stretching the intermediate base material 1PP in a way that eliminates these disturbances. Accordingly, according to the manufacturing method of optical fiber base material of this embodiment, it is possible to suppress the reduction in the diameter of the manufactured optical fiber base material and suppress the decrease in optical fiber productivity. Furthermore, according to the manufacturing method of optical fiber base material of this embodiment, a reference base material can be easily prepared.

[0041] (Second Embodiment) Next, a second embodiment of the present invention will be described in detail. Note that components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and redundant descriptions are omitted. In the manufacturing method for optical fiber preforms of this embodiment, step SP24 of the setting process P2 differs from step SP24 in the first embodiment. Therefore, step SP24 will be described below, and descriptions of other steps will be omitted.

[0042] In step SP24 of this embodiment, a reference base material different from the reference base material in the first embodiment is prepared. In step SP24 of this embodiment, one end in the longitudinal direction of the intermediate base material 1PP is cut, and the other long end becomes the intermediate base material 1PP. Therefore, the other short end is a part of the intermediate base material 1PP that has been separated from it, and a glass member stretched from this part is used as the reference base material. The refractive index distribution of this reference base material is then measured using a preform analyzer. The reference base material of this embodiment consists of a core glass body 10P and a first glass layer 21, similar to the intermediate base material 1PP. Furthermore, the outer diameter of the reference base material is smaller than the outer diameter of the intermediate base material 1PP, and the refractive index distribution profile of the reference base material is approximately the same as the refractive index distribution profile of the intermediate base material 1PP, but scaled down in the radial direction. Furthermore, the period of striations in the refractive index distribution of the reference base material is shorter than the period of striations in the refractive index distribution of the intermediate base material 1PP, and is short enough that the measurement results of the refractive index distribution of the glass component are free from disturbances caused by striations. In other words, a portion of the intermediate base material 1PP is stretched so that the refractive index distribution can be measured normally by a preform analyzer. For example, a portion of the intermediate base material 1PP is stretched so that the outer diameter is 0.5 times or less to form a reference base material. Then, the refractive index distribution of this reference base material is measured using a preform analyzer.

[0043] According to the manufacturing method for optical fiber preforms of this embodiment, similar to the first embodiment, even if the intermediate preform 1PP exhibits disturbances due to striations in the measurement results of the refractive index distribution, the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate preform 1PP can be set without stretching the entire intermediate preform 1PP to eliminate the disturbances. Furthermore, the refractive index distribution profile of a glass member obtained by stretching a portion of the intermediate preform 1PP tends to closely resemble the profile of the refractive index distribution of the intermediate preform 1PP, which is radially reduced according to the stretching ratio during the production of the reference preform, more so than the refractive index distribution profile of a glass member obtained by stretching another intermediate preform formed under the same conditions and with the same equipment as the intermediate preform 1PP. This is because, for example, even if another intermediate preform is formed under the same conditions and with the same equipment as the intermediate preform 1PP, slight differences occur in the way glass nanoparticles are deposited, etc. Therefore, according to the manufacturing method for optical fiber preforms of this embodiment, the above ratio can be set to a more appropriate value compared to the case where the reference preform is a glass member obtained by stretching another intermediate preform formed under the same conditions and with the same equipment as the intermediate preform 1PP.

[0044] (Third embodiment) Next, a third embodiment of the present invention will be described in detail. Note that components identical or equivalent to those in the first embodiment are denoted by the same reference numerals unless otherwise specified, and redundant descriptions are omitted. In the manufacturing method of the optical fiber preform of this embodiment, the setting step P2 differs from the setting step P2 of the first embodiment.

[0045] Although not illustrated, the setting step P2 of this embodiment does not include step SP24. Furthermore, in this embodiment, step SP25 is performed if it is determined in step SP22 that there is an abnormality in the refractive index profile of the intermediate base material 1PP, and this step SP25 differs from step SP25 of the first embodiment. For this reason, only step SP25 will be described below, and the descriptions of the other steps will be omitted.

[0046] In this embodiment, the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material 1PP is set based on the correlation obtained from the refractive index profiles of a plurality of reference base materials that are similar to the refractive index profile of the intermediate base material 1PP. The plurality of reference base materials in this embodiment are a plurality of glass members obtained by stretching each of a plurality of other intermediate base materials, which were formed under the same conditions and with the same apparatus as in the intermediate base material formation process P1, in the longitudinal direction so that there is no disturbance due to striations in the measurement result of the refractive index distribution. For this reason, these reference base materials consist of a core glass body 10P and a first glass layer 21, similar to the intermediate base material 1PP, and the refractive index distribution profile of these reference base materials is similar to the refractive index distribution profile of the intermediate base material 1PP. Furthermore, the refractive index distribution of these reference base materials can be measured normally with a preform analyzer. In this embodiment, these reference base materials are prepared in advance, and the refractive index distribution of each reference base material is measured using a preform analyzer.

[0047] In the preform analyzer, a laser beam is scanned radially across the intermediate base material 1PP. As shown in Figure 5, the refractive index distribution measurement results show a normal refractive index distribution outside the area where disturbance due to striations occurs, and an abnormal refractive index distribution towards the center of that area. Therefore, the refractive index distribution profile in the normally measured range is similar to the refractive index distribution profile in the corresponding range of the reference base material. The inventors diligently studied the relationship between these similar profiles and the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material, which is set based on the refractive index distribution of the reference base material. As a result, they found that when the refractive index distributions of the intermediate base material and the reference base material include a characteristic area having a predetermined characteristic based on the change in refractive index, the correlation between the ratio of the distance from the center of the reference base material to the characteristic area in the reference base material to the radius of the respective reference base material and the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP, which is set based on the refractive index distribution of the respective reference base material, is approximately proportional. Therefore, if the above-mentioned characteristic portion is within the normally measured range of the intermediate base material, the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP can be set based on the above correlation.

[0048] In this embodiment, as shown in Figure 5, within the range of the refractive index distribution of the intermediate base material 1PP that is measured normally, the specific refractive index Δ increases from the outside towards the center, and the rate of change of the specific refractive index Δ becomes large from a certain point toward the center. In consideration of these characteristics, in this embodiment, the characteristic part is defined as part PT1, where the specific refractive index Δ first reaches a predetermined value when moving radially from the outside towards the center. The predetermined value is set to a value where part PT1 is closer to the center than part PT2, where the rate of change of the specific refractive index Δ becomes large, and a dashed line indicating the predetermined value is shown in Figure 5. In this embodiment, the characteristic part is defined as part PT1, and the correlation relationship described above is determined.

[0049] Figure 6 is a graph showing the relationship between the ratio of the distance from the center of each reference base material to the characteristic portion of that base material to the radius of each reference base material, and the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP, which is set based on the refractive index distribution of each reference base material. As shown in Figure 6, there is a correlation between these two ratios that is roughly proportional, and a straight line showing this proportional relationship is shown as a dashed line in Figure 6. In this embodiment, the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP is set based on this correlation. Thus, according to the manufacturing method of optical fiber base materials of this embodiment, even if the intermediate base material 1PP has disturbances due to striations in the measurement results of the refractive index distribution, the ratio of the outer diameter of the second glass layer 22 to the outer diameter of the intermediate base material 1PP can be set without stretching the intermediate base material 1PP in a way that eliminates the disturbances.

[0050] Furthermore, the characteristic portion having predetermined features based on the change in refractive index is not limited. This characteristic portion is a part that is uniquely determined based on the change in refractive index, and it is sufficient that the characteristic portion of the intermediate base material 1PP is located within a measurable range.

[0051] Although the present invention has been described above with reference to the above embodiments, the present invention is not limited to these.

[0052] For example, in the above embodiment, a preform 1P for optical fibers was described as including a core glass body 10P as the main body and a clad glass body 11P as the outer part. However, the preform 1P for optical fibers only needs to include a rod-shaped main body and an outer part that has a different refractive index from the main body and surrounds the outer surface of the main body.

[0053] For example, the main body may consist of a rod-shaped inner core glass body and an outer core glass body having a lower refractive index than the inner core glass body and surrounding the outer surface of the inner core glass body. In this case, an optical fiber can be manufactured in which the refractive index distribution of the core increases in steps toward the center. Alternatively, in this case, the main body may consist of an inner core glass body, and the outer part may consist of an outer core glass body and a clad glass body 11P. In this case, for example, the first glass layer 21 in the intermediate base material 1PP may be a part of the outer core glass body, and the second glass layer 22 formed in the glass layer formation process P3 may be the remaining part of the outer core glass body and the clad glass body 11P. Alternatively, the first glass layer 21 may be the entirety of the outer core glass body and a part of the clad glass body 11P, and the second glass layer 22 may be the remaining part of the clad glass body 11P.

[0054] Furthermore, the outer portion may consist, for example, an inner clad glass body surrounding the outer circumferential surface of the main body and an outer clad glass body with a different refractive index from the inner clad glass body surrounding the outer circumferential surface of the inner clad glass body. In this case, for example, the first glass layer 21 in the intermediate base material 1PP may be a part of the inner clad glass body, and the second glass layer 22 formed in the glass layer formation process P3 may be the remaining part of the inner clad glass body and the outer clad glass body. Alternatively, the first glass layer 21 may be the entire inner clad glass body and a part of the outer clad glass body, and the second glass layer 22 may be the remaining part of the outer clad glass body.

[0055] Furthermore, in the above embodiment, the setting step P2 was described as an example in which the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material is set to a ratio such that the formation of the second glass layer 22 yields the optical fiber base material 1P. However, the above ratio may be smaller than the ratio that yields the optical fiber base material 1P. In this case, for example, the optical fiber base material 1P can be obtained by performing the setting step P2 and the glass layer formation step P3 at least once on the intermediate base material 1PP on which the second glass layer 22 has been formed.

[0056] Furthermore, the reference base material in the first and third embodiments was a glass member stretched from another intermediate base material formed under the same conditions and with the same equipment as the intermediate base material formation process P1. However, in the first and third embodiments, the reference base material is not limited to any glass member that can have its refractive index distribution measured normally with a preform analyzer and whose refractive index distribution profile is similar to that of the intermediate base material 1PP. Examples of reference base materials include a glass member stretched from another intermediate base material formed on the same day under the same conditions and with the same equipment as the intermediate base material formation process P1, a glass member formed on a different day, and a glass member stretched from an intermediate base material formed under the same conditions as the intermediate base material formation process P1 but with a different equipment. In addition, the radial measurement interval when measuring the refractive index distribution of such a reference base material with a preform analyzer may be wider than in the case of the intermediate base material 1PP. By doing so, the measurement result of the refractive index distribution of the obtained reference base material becomes coarser, and it is possible to make it less likely for disturbances due to striations to occur in the measurement result.

[0057] Furthermore, in the first and second embodiments, the setting process P2 including steps SP21 to SP25 was described as an example. However, in the first and second embodiments, steps SP21, SP22, and SP23 may be omitted. In other words, the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material may be set based on the refractive index distribution of the reference base material, without measuring the refractive index distribution of the intermediate base material 1PP.

[0058] Furthermore, the method for manufacturing the optical fiber preform may include a step of stretching the intermediate preform 1PP before the setting step P2. [Industrial applicability]

[0059] As described above, the present invention provides a method for manufacturing optical fiber preforms that can suppress the decrease in productivity of optical fibers, and is expected to be used in fields such as optical fiber communications. [Explanation of Symbols]

[0060] 1P... Pre-fabricated material for optical fibers 1PP...Intermediate base material 10P...Core glass body (main body) 11P...Claded glass body (outer part) 21. First glass layer 22. Second glass layer P1...Intermediate base material formation process P2...Setting process P3...Glass layer formation process

Claims

1. A method for manufacturing a preform for optical fibers, comprising a rod-shaped main body and an outer portion having a different refractive index from the main body and surrounding the outer surface of the main body, An intermediate base material formation step involves sintering a porous glass body to form an intermediate base material consisting of the main body portion and a first glass layer which is part of the outer portion, A glass layer forming step involves depositing glass nanoparticles on the outer surface of the first glass layer to form a coated porous glass body, and sintering the coated porous glass body to form a second glass layer which will become another part of the outer portion. Equipped with, In the glass layer formation step, the glass nanoparticles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material is set to a value based on the refractive index distribution measured correctly by a preform analyzer of a reference base material whose refractive index distribution profile is similar to that of the intermediate base material. The reference base material is a glass member obtained by stretching another intermediate base material formed under the same conditions and with the same apparatus as the intermediate base material forming process. A method for manufacturing a preform for optical fibers, characterized by the following features.

2. A method for manufacturing a preform for optical fibers, comprising a rod-shaped main body and an outer portion having a different refractive index from the main body and surrounding the outer surface of the main body, An intermediate base material formation step involves sintering a porous glass body to form an intermediate base material consisting of the main body portion and a first glass layer which is part of the outer portion, A glass layer forming step involves depositing glass nanoparticles on the outer surface of the first glass layer to form a coated porous glass body, and sintering the coated porous glass body to form a second glass layer which will become another part of the outer portion. Equipped with, In the glass layer formation step, the glass nanoparticles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material is set to a value based on the refractive index distribution measured correctly by a preform analyzer of a reference base material whose refractive index distribution profile is similar to that of the intermediate base material. The reference base material is a glass member obtained by stretching a portion of the intermediate base material that has been separated from the intermediate base material. A method for manufacturing a preform for optical fibers, characterized by the following features.

3. A method for manufacturing a preform for optical fibers, comprising a rod-shaped main body and an outer portion having a different refractive index from the main body and surrounding the outer surface of the main body, An intermediate base material formation step involves sintering a porous glass body to form an intermediate base material consisting of the main body portion and a first glass layer which is part of the outer portion, A glass layer forming step involves depositing glass nanoparticles on the outer surface of the first glass layer to form a coated porous glass body, and sintering the coated porous glass body to form a second glass layer which will become another part of the outer portion. Equipped with, In the glass layer formation step, the glass nanoparticles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material is set to a value based on the refractive index distributions of a plurality of reference base materials whose refractive index distribution profiles are similar to those of the intermediate base material, as measured by a preform analyzer. The multiple reference base materials are multiple glass members obtained by stretching each of several other intermediate base materials, each formed under the same conditions and with the same apparatus as the intermediate base material formation process, in the longitudinal direction so that the measurement results of the refractive index distribution are free from disturbances due to striations. A method for manufacturing a preform for optical fibers, characterized by the following features.

4. In the glass layer formation step, the glass fine particles are deposited on the outer surface of the first glass layer such that the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material is set to a value based on the correlation obtained from the refractive index distributions of the plurality of reference base materials. The refractive index distribution of the intermediate base material and each of the reference base materials includes a feature portion having a predetermined characteristic based on the change in refractive index. The characteristic portion of the intermediate base material is located within a range that can be normally measured by the preform analyzer. The correlation is the correlation between the ratio of the distance from the center of each reference base material to the characteristic portion of the reference base material with respect to the radius of each reference base material, and the ratio of the outer diameter of the second glass layer to the outer diameter of the intermediate base material, which is set based on the refractive index distribution of each reference base material. A method for manufacturing a preform for optical fibers according to any one of claims 1 to 3.

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