Optical fiber base material, method for measuring refractive index distribution of optical fiber base material, and method for manufacturing optical fiber base material

Abstract

The optical fiber base material (1P) has a refractive index distribution including a variation region in which variations (20) in which the refractive index increases and decreases are repeated, at least a portion of the variation region being included in an outer region (OR) having a distance (L) of 7 mm or more from the center of the optical fiber base material (1P), and the width (20W) of the variations (20) in the radial direction in the outer region (OR) being less than 2 [mu] m.

Description

Technical Field

[0001] The present invention relates to an optical fiber base material, a method for measuring the refractive index distribution of the optical fiber base material, and a method for manufacturing the optical fiber base material. Background Art

[0002] As a method for manufacturing an optical fiber base material used in the manufacture of an optical fiber, the following method is known, in which glass particles are deposited in multiple layers on a glass rod rotating around an axis using an OVD method (Outside Vapor Deposition method), a VAD method (Vapor Phase Axial Deposition method), or the like to form a porous glass body, and the porous glass body is sintered.

[0003] The refractive index distribution of the optical fiber base material thus obtained is sometimes measured using a preform analyzer. The preform analyzer measures the refractive index distribution of the optical fiber base material by causing laser light to be incident from a direction perpendicular to the longitudinal direction of the optical fiber base material and scanning in the radial direction, thereby measuring the refraction angle of the laser light emitted from the optical fiber base material.

[0004] As described above, the optical fiber base material is formed by sintering a porous glass body on which multiple layers of glass particles are deposited. Therefore, glass layers corresponding to the layers of glass particles are formed in the optical fiber base material, and in the refractive index distribution of the optical fiber base material, slight fluctuations in the refractive index corresponding to these glass layers are generated, and the refractive index distribution becomes a distribution of repeated changes in the refractive index increase and decrease. It is believed that this is caused by the layer-by-layer changes in the packing density of the layer of glass particles that become the glass layer, the concentration of the dopant contained in the layer for adjusting the refractive index, etc. Such changes in the refractive index are sometimes called ribs. The laser of the preform analyzer sometimes diffracts due to the ribs, etc. In this case, the refraction angle of the laser cannot be accurately measured, and the measured refractive index distribution is disordered.

[0005] Patent document 1 below discloses that glass particles are deposited while the rotation speed of the glass rod is changed between two or more set values. In the manufacturing method of Patent document 1 below, by doing so, the thickness of the layer of glass particles in the porous glass body becomes random, and as a result, the width of each layer constituting the ribs becomes random, thereby preventing the diffraction of the laser of the preform analyzer. Therefore, according to the manufacturing method of Patent document 1 below, the refractive index distribution of the optical fiber base material can be measured.

[0006] Patent Document 1: Japanese Patent Application Publication No. 2013-56794 Summary of the invention

[0007] 1. Technical issues to be resolved

[0008] However, it is known that even if glass fine particles are deposited while changing the rotation speed of the glass rod between two or more set values ​​as in Patent Document 1, the refractive index distribution of the optical fiber base material may not be measured.

[0009] Therefore, an object of the present invention is to provide an optical fiber preform, a method for measuring the refractive index distribution of an optical fiber preform, and a method for manufacturing an optical fiber preform that can suppress the inability to measure the refractive index distribution using a preform analyzer.

[0010] (II) Technical solution

[0011] Mode 1 of the present invention is a base material for optical fiber, which has a refractive index distribution, wherein the refractive index distribution includes a variable region with repeated changes in refractive index increase and decrease, and is characterized in that at least a part of the variable region is included in an outer region at a distance of more than 7 mm from the center of the base material for optical fiber, and the radial width of the change in the outer region is less than 2 μm.

[0012] The inventors of this case have conducted research on optical fiber base materials whose refractive index distribution cannot be measured by a preform analyzer. It was found that the farther the distance from the center of the optical fiber base material is, the more difficult it is to accurately measure the refraction angle of the laser, and the closer the distance is, the easier it is to accurately measure the refraction angle of the laser. In addition, it was found that even if the distance is far, as long as the width of the change in the refractive index increase or decrease is narrow, it is easy to accurately measure the refraction angle of the laser. In addition, the width of the change in the refractive index increase or decrease can also be referred to as the width of each layer constituting the rib. Therefore, the inventors of this case have further repeatedly conducted in-depth research and found that when the width of the above-mentioned change in the outer area of ​​the refractive index distribution that is more than 7 mm away from the center of the optical fiber base material is less than 2μm, the refractive index distribution can be measured based on the preform analyzer. Therefore, according to method 1, it is possible to suppress the inability to measure the refractive index distribution based on the preform analyzer.

[0013] A second aspect of the present invention is the optical fiber preform of the first aspect, wherein the radial width of the variation in the entire region of the outer region is less than 2 μm.

[0014] A third aspect of the present invention is the optical fiber preform of the first aspect or the second aspect, wherein the width of the variation increases toward the center.

[0015] In the refractive index distribution of the optical fiber preform obtained by sintering the porous glass body formed by the VAD method, the above-mentioned variation width becomes larger toward the center. Therefore, the second aspect of the present invention is applicable to the optical fiber preform produced in this way.

[0016] A fourth aspect of the present invention is the optical fiber preform of any one of aspects 1 to 3, wherein the region where the width of the variation is 2 μm or more is included in an inner region less than 7 mm from the center.

[0017] Mode 5 of the present invention is in any one of modes 1 to 4 of the optical fiber base material, characterized in that the optical fiber base material comprises a rod-shaped core glass body and a cladding glass body, the cladding glass body having a refractive index different from that of the core glass body and surrounding the outer peripheral surface of the core glass body, and the core glass body is located only in an inner area less than 7 mm from the center.

[0018] In the fifth aspect of the present invention, the width of the above-mentioned variation in the core glass body is allowed to be larger. Therefore, according to the fifth aspect of the present invention, it is possible to realize an optical fiber preform including a core glass body having a high degree of freedom in the above-mentioned variation width.

[0019] In addition, mode 6 of the present invention is a method for measuring the refractive index distribution of an optical fiber base material, characterized in that a laser is incident on the optical fiber base material from a direction perpendicular to the length direction of the optical fiber base material, the optical fiber base material and a light-emitting portion that emits the laser are relatively moved in a direction toward the center of the optical fiber base material or in a direction away from the center of the optical fiber base material and the laser is scanned, and the refractive index distribution of the optical fiber base material is measured based on the refraction angle of the laser emitted from the optical fiber base material. The measurement method is characterized in that the optical fiber base material is any one of modes 1 to 5, and the diameter of the laser when it is incident on the optical fiber base material is not less than 20 μm and not more than 40 μm.

[0020] According to the sixth aspect of the present invention, the refractive index distribution of the optical fiber preform of any one of the first aspect to the fifth aspect can be measured.

[0021] In addition, mode 7 of the present invention is a method for manufacturing a base material for an optical fiber, characterized in that it comprises: a glass component forming step, in which glass particles are deposited in multiple layers on a glass rod rotating around an axis to form a porous glass body, and the porous glass body is sintered to form a rod-shaped glass component having a refractive index distribution, wherein the refractive index distribution includes a variable region in which the refractive index repeatedly increases and decreases; and a stretching step, in which the glass component is stretched, the rotation speed of the glass rod in the glass component forming step and the stretching ratio of the glass component in the stretching step being set so that at least a part of the variable region in the refractive index distribution of the glass component after the stretching step is included in an outer region that is at least 7 mm away from the center of the glass component, and the radial width of the variation in the outer region is less than 2 μm.

[0022] If the rotation speed of the glass rod in the glass component forming process becomes faster, the thickness of each layer of glass particles in the porous glass body becomes thinner, and the width of the above-mentioned variation in the refractive index distribution of the obtained glass component becomes narrower. In addition, if the elongation rate of the glass component in the stretching process becomes larger, the width of the above-mentioned variation in the refractive index distribution of the stretched glass component becomes narrower. In mode 7, as described above, the rotation speed of the glass rod and the elongation rate of the glass component are set so that at least a part of the variation region in the refractive index distribution of the stretched glass component is included in the outer region at a distance of 7 mm or more from the center of the glass component, and the radial width of the variation in the outer region is less than 2 μm. Therefore, according to mode 7, it is possible to manufacture an optical fiber base material in which the width of the above-mentioned variation in the outer region at a distance of 7 mm or more from the center is less than 2 μm, and it is possible to manufacture an optical fiber base material that can suppress the inability to measure the refractive index distribution based on the preform analyzer.

[0023] Aspect 8 of the present invention is the method for producing an optical fiber preform according to aspect 7, wherein the rotation speed of the glass rod is 20 rpm or more and 40 rpm or less.

[0024] According to the eighth aspect of the present invention, it is possible to easily manufacture an optical fiber preform in which the width of the above-mentioned variation in the above-mentioned outer region is less than 2 μm.

[0025] A ninth aspect of the present invention is the method for manufacturing an optical fiber base material according to the seventh or eighth aspect, wherein the elongation ratio is set so that the radial width of the variation in the entire outer region is less than 2 μm.

[0026] (III) Beneficial effects

[0027] As described above, according to the present invention, there are provided an optical fiber preform and a method for producing the optical fiber preform that can suppress the inability to measure the refractive index distribution using a preform analyzer. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a diagram schematically showing a state of a cross section perpendicular to the longitudinal direction of an optical fiber preform according to an embodiment of the present invention.

[0029] Figure 2 It is a rough representation Figure 1 The diagram shows the refractive index distribution in a cross section perpendicular to the longitudinal direction of the optical fiber base material.

[0030] Figure 3 is to magnify and represent Figure 2 Conceptual diagram of a portion of the refractive index profile shown.

[0031] Figure 4It is a flowchart showing the steps of the method for producing an optical fiber preform according to the embodiment.

[0032] Figure 5 It is a diagram schematically showing a preform analyzer according to the embodiment.

[0033] Figure 6 This is a graph showing the relationship between the distance from the center of the optical fiber preform and the width of the variation in the refractive index distribution in Examples 1 to 3 and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0034] The following is attached to the instruction manual Figure 1 The following examples illustrate the optical fiber base material and the method for manufacturing the optical fiber base material of the present invention. The following exemplary embodiments are used to make the present invention easy to understand and are not intended to limit the interpretation of the present invention. The present invention can be changed and improved without departing from its main purpose. In addition, in the following referenced drawings of the specification, there are cases where the dimensions of each component are changed for easy understanding.

[0035] Figure 1 1 is a diagram schematically showing a cross section perpendicular to the longitudinal direction of an optical fiber base material according to an embodiment of the present invention. Figure 1 As shown in FIG. 1 , the optical fiber base material 1P of the present embodiment includes, as main structures, a rod-shaped core glass body 10P and a cladding glass body 11P surrounding the outer peripheral surface of the core glass body 10P. The core glass body 10P is a member that becomes the core of the optical fiber obtained from the optical fiber base material 1P, and the cladding glass body 11P is a member that becomes the cladding of the optical fiber obtained from the optical fiber base material 1P. In the present embodiment, the outer shape of the core glass body 10P and the outer shape of the cladding glass body 11P in the cross section are substantially circular, and the core glass body 10P is arranged at the center of the cladding glass body 11P. In addition, the outer diameter of the core glass body 10P is 15 mm, and the outer diameter of the cladding glass body 11P is 50 mm, but these outer diameters are not limited.

[0036] Figure 2 It is a rough representation Figure 1In the diagram of the refractive index distribution on the cross section perpendicular to the longitudinal direction of the optical fiber base material 1P shown in FIG. 1 , the refractive index of the core glass body 10P is higher than the refractive index of the cladding glass body 11P. In the present embodiment, the core glass body 10P is composed of silica glass to which a dopant such as germanium (Ge) is added to increase the refractive index, and the cladding glass body 11P is composed of silica glass without any additives. Alternatively, the core glass body 10P may be composed of silica glass to which a dopant such as fluorine (F) is added to decrease the refractive index, and the cladding glass body 11P may be composed of silica glass to which a dopant that increases the refractive index is added, and the cladding glass body 11P may be composed of silica glass to which a dopant that decreases the refractive index is added. There is no limitation on the dopant that increases the refractive index and the dopant that decreases the refractive index.

[0037] Figure 3 It is a magnified representation Figure 2 The conceptual diagram of a portion of the refractive index distribution shown in FIG. 1 is a conceptual diagram showing an enlarged portion of the refractive index distribution of the core glass body 10P. Figure 3As shown, the refractive index distribution of the optical fiber base material 1P is a distribution of a variation region including a variation 20 in which the refractive index increases and decreases repeatedly. That is, in the variation region, an increase region in which the refractive index increases and a decrease region in which the refractive index decreases are arranged alternately. For example, when the refractive index distribution is represented by a relative refractive index (Japanese: relative refractive index), the increase and decrease in the refractive index in the variation 20 are approximately less than 0.04%. In addition, the radial width 20W of the variation 20 is, for example, about 0.3 mm to 0.3 μm. In addition, the width 20W of the variation 20 is sometimes referred to as the width of the ribs, the period of the ribs, and may also be referred to as the repeated width of the variation 20, or the width of each layer constituting the ribs. The details will be described later. The optical fiber base material 1P is formed by sintering a porous glass body on which multiple layers of glass particles are deposited. Therefore, glass layers corresponding to the respective layers of glass particles are formed in the optical fiber base material 1P, and minute refractive index fluctuations corresponding to these glass layers are generated in the refractive index distribution of the optical fiber base material 1P. As a result, the refractive index distribution includes a variation region in which the above-mentioned variation 20 is repeated. In the present embodiment, the entire refractive index distribution of the optical fiber base material 1P is the variation region. In addition, the radial width 20W of the variation 20 in the outer region OR where the distance L from the center of the optical fiber base material 1P is 7 mm or more is less than 2 μm, and the width 20W of the variation 20 increases as it approaches the center of the optical fiber base material 1P. In addition, in the present embodiment, the radial width 20W of the variation 20 in the entire region of the outer region OR is less than 2 μm. In addition, in the present embodiment, the inner region IR where the distance L is less than 7 mm includes a region where the width 20W of the variation 20 is 2 μm or more, but the inner region IR may not include a region where the width 20W of the variation 20 is 2 μm or more. In the present embodiment, a part of the core glass body 10P is located in the outer region OR, but the core glass body 10P may not be located in the outer region OR. In other words, the core glass body 10P may be located only in the inner region IR.

[0038] Next, a method for producing an optical fiber preform according to the present embodiment will be described.

[0039] Figure 4 FIG. 1 is a flow chart showing the steps of a method for manufacturing an optical fiber base material 1P according to the present embodiment. Figure 4 As shown, the method for producing the optical fiber preform 1P according to the present embodiment includes a glass member forming step P1 and a stretching step P2.

[0040] <Glass component forming process P1>

[0041] This process is a process of forming a porous glass body by depositing glass particles in multiple layers on a glass rod rotating around an axis, and sintering the porous glass body to form a rod-shaped glass component. The description based on the diagram is omitted. In this embodiment, the porous glass body is formed by the VAD method of depositing glass particles in multiple layers from one end of a glass rod rotating around an axis in the axial direction of the glass rod. The porous glass body is sintered to form a rod-shaped glass component. The glass component thus formed is Figure 1 The optical fiber base material 1P shown in the figure is a structure that is enlarged in the radial direction and reduced in the length direction. Therefore, the glass component is composed of a core glass body 10P and a cladding glass body 11P, and the ratio of the outer diameter of the core glass body 10P to the outer diameter of the cladding glass body 11P in the glass component is approximately the same as that in the optical fiber base material 1P. In addition, the refractive index distribution of the glass component is Figure 2 The refractive index distribution of the optical fiber mother material 1P shown is stretched in the radial direction. That is to say, in the manner of forming such a glass component, glass particles are piled up in multiple layers to form a porous glass body, and the porous glass body is sintered. In addition, in the present embodiment, the rotation speed of the glass rod is constant, but the rotation speed of the glass rod can also be changed when the glass particles are piled up. In addition, there is no restriction on the method for piling up the glass particles, for example, it can also be an OVD method in which glass particles are piled up in multiple layers on the peripheral surface of the glass rod rotating around the axis.

[0042] <Stretching process P2>

[0043] This step is a step of heating the glass member formed in the glass member forming step P1 and stretching it in the longitudinal direction. Figure 1 The optical fiber preform 1P shown is a structure that is expanded in the radial direction and contracted in the longitudinal direction. Therefore, by stretching the glass member in the longitudinal direction, the glass member becomes the optical fiber preform 1P.

[0044] However, if the rotation speed of the glass rod in the glass component forming step P1 becomes faster, the thickness of each layer of the glass particles in the porous glass body becomes thinner, and the width 20W of the variation 20 in the refractive index distribution of the obtained glass component becomes narrower. In addition, if the elongation rate of the glass component in the stretching step P2 becomes larger, the width 20W of the variation 20 in the refractive index distribution of the stretched glass component becomes narrower. In addition, the elongation rate is the ratio of the length of the stretched glass component to the length of the glass component before stretching. In this embodiment, the rotation speed of the glass rod in the glass component forming step P1 and the elongation rate of the glass component in the stretching step P2 are set so that at least a part of the variation region in the refractive index distribution of the stretched glass component is included in the outer region OR with a distance of 7 mm or more from the center of the glass component, and the width 20W of the variation 20 in the outer region OR is less than 2μm. Therefore, the refractive index distribution of the obtained optical fiber base material 1P includes a variation region of the variation 20 in which the refractive index increases and decreases repeatedly. Furthermore, at least a part of the fluctuation region is included in the outer region OR at a distance L of 7 mm or more from the center of the optical fiber preform 1P, and the radial width of the fluctuation 20 in the outer region OR is less than 2 μm.

[0045] In addition, the rotation speed of the glass rod in the glass member forming step P1 is preferably 20 rpm or more and 40 rpm or less. With such a configuration, the optical fiber preform 1P having a width of the fluctuation 20 in the outer region OR of less than 2 μm can be easily manufactured.

[0046] Next, a preform analyzer for measuring the refractive distribution of the optical fiber preform according to the present embodiment will be described.

[0047] Figure 5 FIG. 2 is a diagram schematically showing a preform analyzer according to the present embodiment. Figure 5 As shown, in this embodiment, the preform analyzer 30 includes, as main structures, a light emitting unit 31 for emitting light, a light receiving unit 32 for receiving light, a light-transmitting housing unit 33 having a housing space, and a measuring unit 34 .

[0048] The light emitting unit 31 of the present embodiment emits laser light having a peak wavelength of power of 632 nm, but the peak wavelength of the laser light power is not limited.

[0049] The light receiving unit 32 is an optical element that converts the light received by the light receiving surface 32s into an electrical signal and outputs it. In the present embodiment, the light receiving surface 32s is composed of the light receiving surfaces of a plurality of light receiving elements that receive light, and the light receiving unit 32 outputs information related to the position of the light irradiated to the light receiving surface 32s to the measuring unit 34. As information related to the position of the light, for example, a two-dimensional image can be cited. The light emitting unit 31 and the light receiving unit 32 are arranged in such a manner that the portion of the light emitting unit 31 that emits light is spaced apart from the light receiving surface 32s of the light receiving unit 32.

[0050] The housing portion 33 is disposed between the light emitting portion 31 and the light receiving portion 32. The optical fiber base material 1P is housed in the housing space of the housing portion 33 so that the length direction of the optical fiber base material 1P is perpendicular to the direction in which the light emitting portion 31 and the light receiving portion 32 are opposed to each other, and the housing space is filled with a matching oil 35 having the same refractive index as the cladding glass body 11P. The housing portion 33 is movable in a predetermined direction perpendicular to the length direction of the optical fiber base material 1P and perpendicular to the direction in which the light emitting portion 31 and the light receiving portion 32 are opposed to each other.

[0051] The laser light emitted from the light emitting section 31 passes through the optical fiber base material 1P and irradiates the light receiving surface 32s of the light receiving section 32. In the present embodiment, the diameter of the laser light when incident on the optical fiber base material 1P is, for example, not less than 20 μm and not more than 40 μm, and is 30 μm in the present embodiment. In the present embodiment, the laser light is scanned by moving the housing 33 in a prescribed direction. The prescribed direction is, for example, a direction perpendicular to the incident direction of the laser light on the optical fiber base material 1P and the longitudinal direction of the optical fiber base material 1P. In this case, the laser light is incident on the optical fiber base material 1P accommodated in the housing 33 from a direction perpendicular to the longitudinal direction of the optical fiber base material 1P. In addition, the optical fiber base material 1P moves in a direction D1 toward the center 1Pc of the optical fiber base material 1P or in a direction D2 away from the center 1Pc of the optical fiber base material 1P relative to the light emitting section 31, and is scanned by the laser light. In addition, the optical fiber base material 1P and the light emitting unit 31 are relatively moved in a direction D1 toward the center 1Pc of the optical fiber base material 1P or in a direction D2 away from the center 1Pc of the optical fiber base material 1P so that the laser beam is incident on the optical fiber base material 1P from a direction perpendicular to the longitudinal direction of the optical fiber base material 1P, and the laser beam is scanned. For example, the light emitting unit 31 may be moved in the above-mentioned prescribed direction, or the optical fiber base material 1P may be moved in the above-mentioned prescribed direction, or the optical fiber base material 1P and the light emitting unit 31 may be moved in the above-mentioned prescribed direction. In addition, the above-mentioned perpendicularity includes, in addition to the completely perpendicularity, the case where, for example, the optical fiber base material 1P is bent due to a manufacturing error and thus deviates from the completely perpendicularity.

[0052] The light receiving unit 32 outputs information about the position of the laser beam irradiated to the light receiving surface 32s to the measuring unit 34. The measuring unit 34 is configured to measure the refraction angle θ of the laser beam in the optical fiber base material 1P based on the information, measure the refractive index of the position where the laser beam is transmitted in the optical fiber base material 1P based on the refraction angle θ, and measure the refractive index distribution based on the refractive index. The measuring unit 34 is composed of, for example, a microcontroller, an integrated circuit such as an IC (Integrated Circuit), an LSI (Large-scale Integrated Circuit), an ASIC (Application Specific Integrated Circuit), or an NC (Numerical Control) device.

[0053] In the present embodiment, the refractive distribution of the optical fiber base material 1P is measured using such a preform analyzer 30. First, as described above, the optical fiber base material 1P is accommodated in the accommodation space of the accommodation portion 33. Next, laser light is emitted from the light emitting portion 31. Next, the optical fiber base material 1P and the light emitting portion 31 are relatively moved toward the center 1Pc of the optical fiber base material 1P or away from the center 1Pc so that the laser light enters the optical fiber base material 1P from a direction perpendicular to the longitudinal direction of the optical fiber base material 1P, and the laser light is scanned. In the present embodiment, the optical fiber base material 1P is moved relative to the light emitting portion 31 by moving the accommodation portion 33. Then, the light receiving portion 32 receives the laser light emitted from the optical fiber base material 1P. The light receiving portion 32 outputs information related to the position of the light irradiated to the light receiving surface 32s, and the measuring portion 34 measures the refraction angle θ of the laser light in the optical fiber base material 1P based on the two-dimensional image, and measures the refractive index distribution of the optical fiber base material 1P based on the refraction angle θ. That is, the preform analyzer 30 causes the optical fiber base material 1P and the light emitting portion 31 to move relative to each other in a direction toward or away from the center 1Pc of the optical fiber base material 1P and scans the laser beam so as to cause the laser beam to be incident on the optical fiber base material 1P from a direction perpendicular to the length direction of the optical fiber base material 1P, thereby measuring the refractive index distribution of the optical fiber base material 1P based on the refraction angle of the laser beam emitted from the optical fiber base material 1P.

[0054] As described above, the optical fiber base material 1P of the present embodiment has a refractive index distribution including a variation region in which the refractive index increases and decreases repeatedly 20. At least a portion of the variation region is included in the outer region OR at a distance L of 7 mm or more from the center of the optical fiber base material 1P, and the radial width of the variation 20 in the outer region OR is less than 2 μm.

[0055] The inventors of this case have conducted research on optical fiber base materials whose refractive index distribution cannot be measured by a preform analyzer. It was found that the farther the distance from the center of the optical fiber base material is, the more difficult it is to accurately measure the refraction angle of the laser, and the closer the distance is, the easier it is to accurately measure the refraction angle of the laser. In addition, it was found that even if the distance is far, as long as the width of the change in the refractive index increase or decrease is narrow, it is easy to accurately measure the refraction angle of the laser. Therefore, the inventors of this case have further repeatedly conducted in-depth research and found that when the radial width of the above-mentioned change in the outer area of ​​the refractive index distribution that is more than 7 mm away from the center of the optical fiber base material is less than 2μm, the refractive index distribution can be measured based on the preform analyzer. Therefore, according to the optical fiber base material 1P of this embodiment, it is possible to suppress the inability to measure the refractive index distribution based on the preform analyzer.

[0056] The closer to the center side the optical fiber base material 1P of this embodiment is, the larger the width 20W of the change 20 is. In the refractive index distribution of the optical fiber base material obtained by sintering the porous glass body formed by the VAD method, the closer to the center side the above-mentioned width 20W of the change 20 is. Therefore, the optical fiber base material 1P of this embodiment is suitable for the optical fiber base material manufactured in this way. In addition, the width 20W of the change 20 can be random in the radial direction or can be approximately constant.

[0057] The optical fiber base material 1P of the present embodiment is a base material that is drawn in order to obtain an optical fiber. Therefore, according to the optical fiber base material 1P of the present embodiment, for example, the conditions in the drawing can be appropriately set based on the refractive index distribution measured by the preform analyzer. In addition, the optical fiber base material 1P can also be used as a so-called intermediate base material for obtaining a base material having a diameter thicker than that of the optical fiber base material 1P. By providing a glass layer on the outer peripheral surface of the optical fiber base material 1P as an intermediate base material, a base material having a diameter thicker than that of the optical fiber base material 1P can be obtained. For example, in the case where the glass layer is composed of the same glass body as the cladding glass body 11P, it can be obtained. Figure 1 In this case, for example, the refractive index distribution of the optical fiber preform 1P can be measured by a preform analyzer, and the outer diameter of the glass layer can be set based on the refractive index distribution. Therefore, a preform can be obtained in which the ratio of the outer diameter of the core glass body 10P to the outer diameter of the cladding glass body 11P is a desired ratio.

[0058] In addition, in the method for measuring the refractive index distribution of the optical fiber base material of the present embodiment, the optical fiber base material 1P and the light emitting unit 31 emitting the laser light are relatively moved in a direction toward the center 1Pc of the optical fiber base material 1P or in a direction away from the center 1Pc of the optical fiber base material 1P so that the laser light is incident on the optical fiber base material 1P from a direction perpendicular to the longitudinal direction of the optical fiber base material 1P, and the laser light is scanned, and the refractive index distribution of the optical fiber base material 1P is measured based on the refraction angle of the laser light emitted from the optical fiber base material 1P. The optical fiber base material 1P has a refractive index distribution including a variation region in which the variation 20 of the refractive index increase and decrease is repeated. At least a part of the variation region is included in an outer region OR at a distance L of 7 mm or more from the center of the optical fiber base material 1P, and the radial width of the variation 20 in the outer region OR is less than 2 μm. Moreover, the diameter of the laser light when incident on the optical fiber base material 1P is 20 μm or more and 40 μm or less. According to such a structure, the refractive index distribution of the optical fiber base material 1P described above can be measured.

[0059] As mentioned above, although the present invention has been described by taking the above-mentioned embodiment as an example, the present invention is not limited thereto.

[0060] For example, in the above-mentioned embodiment, an optical fiber base material 1P having a core glass body 10P and a cladding glass body 11P is described as an example. However, as long as the optical fiber base material 1P has a refractive index distribution including a change region of change 20 that repeats refractive index increase and decrease, at least a part of the change region is included in the outer region OR at a distance L of 7 mm or more from the center of the optical fiber base material 1P, and the width 20W of the change 20 in the outer region OR is less than 2 μm. For example, the cladding glass body 11P may be composed of an inner glass layer that surrounds the outer peripheral surface of the core glass body 10P and has a refractive index different from that of the core glass body 10P, and an outer glass layer that surrounds the outer peripheral surface of the inner glass layer and has a refractive index different from that of the inner glass layer. In addition, in the case where the optical fiber base material 1P is the above-mentioned intermediate base material, for example, the optical fiber base material 1P may also be the core glass body 10P.

[0061] In addition, in the above-mentioned embodiment, the optical fiber base material 1P in which the change 20 of the refractive index is repeated throughout the radial direction of the optical fiber base material 1P and the entire refractive index distribution is a change region is described as an example. However, it is sufficient as long as at least a part of the change region is included in the outer region OR where the distance L from the center of the optical fiber base material 1P is less than 7 mm. For example, it is also possible that in the inner region IR where the distance L is less than 7 mm, no change 20 is generated on the center side. Such an optical fiber base material 1P is obtained, for example, in the following manner: in the glass component forming step P1, a porous glass body is formed by the OVD method using a glass rod in which no change 20 is generated in the refractive index distribution, and the porous glass body is sintered to form a glass component.

[0062] In addition, in the above-mentioned embodiment, the optical fiber base material 1P includes: a rod-shaped core glass body 10P; and a cladding glass body 11P having a refractive index different from that of the core glass body 10P and surrounding the outer peripheral surface of the core glass body 10P. With respect to the core glass body 10P, a part of the core glass body 10P is located in the outer region OR. However, as described above, the core glass body 10P may be located only in the inner region IR which is less than 7 mm from the center of the optical fiber base material 1P. According to such a structure, the width of the variation 20 in the core glass body 10P is allowed to be larger. Therefore, according to such a structure, the optical fiber base material 1P having the core glass body 10P with a high degree of freedom of the width 20W of the variation 20 can be realized.

[0063] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

[0064] (Example 1)

[0065] In this embodiment, the optical fiber base material manufacturing method shown in the above embodiment is manufactured. Figure 1 The optical fiber base material 1P shown in the figure. Specifically, in the glass component forming step P1, a porous glass body is formed by the VAD method with the rotation speed of the glass rod, in other words, the rotation speed is set to 20 rpm, and the porous glass body is sintered to form a glass component. The outer diameter of the glass component is 100 mm. In addition, in the stretching step P2, the optical fiber base material 1P is manufactured by stretching the glass component. The stretching rate of the glass component in the stretching step P2 is about 2.0, the outer diameter of the optical fiber base material 1P is 50 mm, and the radius of the core glass body 10P is 7.5 mm.

[0066] (Example 2)

[0067] The optical fiber base material 1P was manufactured in the same manner as in Example 1 except that the elongation rate of the glass member in the elongation step P2 was greater than that in Example 1. The elongation rate of the glass member in the elongation step P2 of this example was about 2.3, the outer diameter of the optical fiber base material 1P of this example was 43 mm, and the radius of the core glass body 10P was 6.5 mm. That is, the core glass body 10P of Example 2 was located only in the inner region IR.

[0068] (Example 3)

[0069] The optical fiber base material 1P was manufactured in the same manner as in Example 1, except that the rotation speed of the glass rod in the glass member forming step P1, in other words, the rotation speed was set to 40 rpm, and the elongation rate of the glass member in the elongation step P2 was set to be lower than that in Example 1. The elongation rate of the glass member in the elongation step P2 of this example was about 1.9, the outer diameter of the optical fiber base material 1P of this example was 53 mm, and the radius of the core glass body 10P was 8 mm.

[0070] (Comparative Examples 1 to 3)

[0071] The optical fiber base material 1P was manufactured in the same manner as in Example 1 except that the elongation rate of the glass member in the elongation step P2 was set to a value different from that in Example 1. In Comparative Example 1, the elongation rate of the glass member in the elongation step P2 was about 1.6, the outer diameter of the optical fiber base material 1P was 63 mm, and the radius of the core glass body 10P was 9.5 mm. In Comparative Example 2, the elongation rate of the glass member in the elongation step P2 was about 1.8, the outer diameter of the optical fiber base material 1P was 57 mm, and the radius of the core glass body 10P was 8.5 mm. In Comparative Example 3, the elongation rate of the glass member in the elongation step P2 was about 1.9, the outer diameter of the optical fiber base material 1P was 53 mm, and the radius of the core glass body 10P was 8 mm.

[0072] (Measurement of refractive index distribution)

[0073] For the optical fiber preform 1P obtained in each of Examples 1 to 3 and Comparative Examples 1 to 3, Figure 5 The preform analyzer 30 shown in the figure measures the refractive index distribution. The peak wavelength of the power of the laser in the preform analyzer 30 is 632nm, and the diameter of the laser when incident on the optical fiber base material 1P is approximately 30μm. Then, for these Examples 1 to 3 and Comparative Examples 1 to 3, the relationship between the distance L from the center of the optical fiber base material 1P and the width 20W of the change 20 in the refractive index distribution was investigated based on detailed analysis and calculation. This relationship is obtained by changing the distance from the center of the optical fiber base material 1P by 0.5mm each time, and a part of the result is shown in Figure 6 In addition, Table 1 shows the rotation speed of the glass rod in Examples 1 to 3 and Comparative Examples 1 to 3, the outer diameter of the optical fiber base material 1P, the radius of the core glass body 10P, the elongation rate in the elongation step P2, the width 20W of the variation 20 at a position where the distance L is 7 mm, whether the width 20W of the variation 20 in the region where the distance L is greater than 7 mm is less than 2 μm, and whether there is a situation where the refraction angle of the laser light emitted from the preform analyzer 30 cannot be measured.

[0074] Table 1

[0075]

[0076] like Figure 6 As shown in FIG. 1 , in Examples 1 to 3, the width 20W of the variation 20 in the region where the distance L is 7 mm or more and 9.5 mm or less is less than 2 μm. Figure 6 Although not shown in the figure, in Examples 1 to 3, the width 20W in the region where the distance L exceeds 9.5 mm is also less than 2 μm. Moreover, in Examples 1 to 3, the refractive index distribution of the optical fiber base material 1P can be measured. Figure 6 In the range where the distance L is 7 mm or more and the width 20W of the variation 20 is 2 μm or more, a shadow composed of a plurality of dots is applied.

[0077] In Comparative Example 1, the refraction angle of the laser light emitted from the preform analyzer 30 cannot be measured in the region where the distance L is 7.0 mm to 9.0 mm. In addition, in Comparative Example 2, the refraction angle of the laser light emitted from the preform analyzer 30 cannot be measured in the region where the distance L is 7.0 mm to 8.0 mm. In addition, in Comparative Example 3, the refraction angle of the laser light emitted from the preform analyzer 30 cannot be measured in the region where the distance L is 7.0 mm to 7.5 mm. Moreover, in Comparative Examples 1 to 3, the refractive index distribution cannot be accurately measured. In addition, although Figure 6 Although not shown in the figure, in Comparative Examples 1 to 3, the width 20W of the variation 20 in the region where the distance L exceeds 9.5 mm is less than 2 μm. As described above, in the refractive index distribution of the optical fiber base material obtained by sintering the porous glass body formed using the VAD method, the width 20W of the variation 20 increases as it approaches the center. In Comparative Examples 1 to 3, the width 20W in the region where the distance L is less than 7.0 mm exceeds 2 μm. In addition, according to Figure 6 As a result, it can be seen that in Examples 1 to 3, the width 20W decreases at a substantially constant rate from the center side toward the outside, and the rate of decrease of the width 20W becomes smaller in the region outside the distance L where the width 20W is approximately 1 μm. Therefore, it is considered that in Comparative Examples 1 to 3, the width 20W decreases from the center side toward the outside as in Examples 1 to 3, and it is considered that the width 20W in the region where the refraction angle of the laser light cannot be measured in Comparative Examples 1 to 3 is 2 μm or more.

[0078] Therefore, it is known that when the width 20W in the entire region where the distance L is 7 mm or more is less than 2 μm, the refractive index distribution can be measured by the preform analyzer 30. In addition, it is known that the width 20W at the position where the distance L is 7 mm can be 1.8 μm or more and less than 2.0 μm.

[0079] In addition, the laser of the preform analyzer 30 is set to a laser with a peak wavelength of 405 nm and the refractive index distribution is measured, but the relationship between the distance L and the width 20W is the same as Figure 6 The results are the same as those shown in Table 1. Therefore, it can be considered that the wavelength of the laser of the preform analyzer 30 has little effect on whether the refractive index distribution can be measured. In addition, the laser of the preform analyzer 30 is changed to white light emitted from an LED (Light Emitting Diode) and the refractive index distribution is measured. In this case, compared with the case where the light is laser, the diffusion degree of the light emitted from the optical fiber base material becomes larger, but the refraction angle of the light can be measured, and the relationship between the distance L and the width 20W is the same as Figure 6 The results are the same as those shown in Table 1. Therefore, it can be considered that the diameter of the light incident on the optical fiber base material has little effect on whether the refractive index distribution can be measured. In addition, whether the refractive index distribution is measured when the diameter of the laser light incident on the optical fiber base material 1P is 20μm or when the refractive index distribution is measured when the diameter is 40μm, the relationship between the distance L and the width 20W is the same as Figure 6 The results are the same as those shown in Table 1. Therefore, it is considered that if the diameter is 20 μm or more and 40 μm or less, the refractive index distribution of the optical fiber base material 1P having a width 20W less than 2 μm in the region where the distance L is 7 mm or more can be measured in the same manner as when the diameter is 30 μm.

[0080] As described above, according to the present invention, an optical fiber base material, a method for measuring the refractive index distribution of an optical fiber base material, and a method for manufacturing an optical fiber base material are provided, which can suppress the inability to measure the refractive index distribution based on a preform analyzer, and are expected to be used in the field of optical fiber communication.

Claims

1. An optical fiber base material having a refractive index distribution, wherein the refractive index distribution includes a variation region in which the refractive index repeatedly increases and decreases, characterized in that: At least a part of the variable region is included in an outer region which is 7 mm or more away from the center of the optical fiber base material. A radial width of the variation in the outer region is less than 2 μm.

2. The optical fiber base material according to claim 1, characterized in that A radial width of the variation in the entire outer region is less than 2 μm.

3. The optical fiber base material according to claim 1 or 2, characterized in that: The width of the variation increases as the distance toward the center increases.

4. The optical fiber base material according to any one of claims 1 to 3, characterized in that: An inner region that is less than 7 mm from the center includes a region where the width of the variation is greater than or equal to 2 μm.

5. The optical fiber base material according to any one of claims 1 to 4, characterized in that: The fiber comprises: a rod-shaped core glass body and a cladding glass body, wherein the cladding glass body has a refractive index different from that of the core glass body and surrounds the outer peripheral surface of the core glass body. The core glass body is only located in the inner region less than 7 mm from the center.

6. A method for measuring the refractive index distribution of a base material for optical fiber, The method comprises: causing the optical fiber base material and a light emitting portion emitting the laser light to be relatively moved toward or away from the center of the optical fiber base material in such a manner that the laser light is incident on the optical fiber base material from a direction perpendicular to the length direction of the optical fiber base material, and scanning the laser light, and measuring the refractive index distribution of the optical fiber base material based on the refraction angle of the laser light emitted from the optical fiber base material. The method is characterized in that: The optical fiber base material is the optical fiber base material according to any one of claims 1 to 5, The diameter of the laser light when the laser light is incident on the optical fiber preform is 20 μm or more and 40 μm or less.

7. A method for manufacturing a base material for an optical fiber, characterized in that: have: A glass component forming step of depositing glass particles in multiple layers on a glass rod rotating about an axis to form a porous glass body, and sintering the porous glass body to form a rod-shaped glass component having a refractive index distribution including a variation region in which the refractive index repeatedly increases and decreases; and a stretching step of stretching the glass component, The rotation speed of the glass rod in the glass component forming step and the stretching rate of the glass component in the stretching step are set so that at least a part of the variation region in the refractive index distribution of the glass component after the stretching step is included in an outer region at a distance of more than 7 mm from the center of the glass component, and the radial width of the variation in the outer region is less than 2 μm.

8. The method for manufacturing an optical fiber base material according to claim 7, wherein: The rotation speed of the glass rod is 20 rpm or more and 40 rpm or less.

9. The method for manufacturing an optical fiber base material according to claim 7 or 8, characterized in that: The stretching ratio is set so that the radial width of the variation in the entire outer region is less than 2 μm.

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