Preform for optical fibers, method for measuring refractive index profile of preform for optical fibers, and method for producing preform for optical fibers

Abstract

A preform (1P) for optical fibers according to the present invention has a refractive index profile that contains a fluctuation region in which a fluctuation (20), wherein the refractive index increases or decreases, is repeated; at least a part of the fluctuation region is contained in an outer region (OR) that is at a distance (L) of 7 mm or more from the center of the preform (1P) for optical fibers; and the width (20W) of the fluctuation 20 in the radial direction within the outer region (OR) is less than 2 µm.

Description

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

[0001] The present invention relates to an optical fiber preform, a method for measuring the refractive index profile of an optical fiber preform, and a method for manufacturing an optical fiber preform.

[0002] Known methods for producing optical fiber preforms used in the manufacture of optical fibers include depositing glass particles in multiple layers on a glass rod rotating around its axis using an OVD (Outside Vapor Deposition) method, a VAD (Vapor Phase Axial Deposition) method, or the like to form a porous glass body, and then sintering the porous glass body.

[0003] The refractive index profile of the optical fiber preform thus obtained may be measured using a preform analyzer, which measures the refractive index profile of the optical fiber preform by irradiating a laser beam perpendicular to the longitudinal direction of the optical fiber preform and scanning it radially, thereby measuring the refraction angle of the laser beam emitted from the optical fiber preform.

[0004] As described above, optical fiber preforms are formed by sintering a porous glass body in which glass particles are deposited in multiple layers. Therefore, glass layers corresponding to each layer of glass particles are formed in the optical fiber preform. The refractive index distribution of the optical fiber preform exhibits minute fluctuations in the refractive index corresponding to these glass layers, resulting in a distribution in which the refractive index repeatedly increases and decreases. This is thought to be caused by layer-by-layer variations in the bulk density of the glass particle layers and the concentration of dopants contained in the layers for adjusting the refractive index. Such refractive index variations are sometimes called striae. The laser light from a preform analyzer may be diffracted by these striae. In this case, the refraction angle of the laser light cannot be accurately measured, resulting in a disturbance in the measured refractive index distribution.

[0005] Patent Document 1 below discloses that glass particles are deposited while changing the rotation speed of a glass rod between two or more set values. In the manufacturing method of Patent Document 1 below, by doing so, the thickness of the glass particle layer in the porous glass body becomes random, and as a result, the width of each layer constituting the striae becomes random, which makes it possible to prevent diffraction of the laser light from the preform analyzer. Therefore, the manufacturing method of Patent Document 1 below is said to be able to measure the refractive index distribution of an optical fiber preform.

[0006] JP 2013-56794 A

[0007] However, it has been found that even if glass particles are deposited while changing the rotation speed of the glass rod between two or more set values, as in Patent Document 1, it may not be possible to measure the refractive index distribution of the optical fiber preform.

[0008] Therefore, an object of the present invention is to provide an optical fiber preform, a method for measuring the refractive index profile of an optical fiber preform, and a method for manufacturing an optical fiber preform, which can prevent the refractive index profile from being unable to be measured by a preform analyzer.

[0009] Aspect 1 of the present invention is an optical fiber preform having a refractive index profile including a fluctuation region in which the refractive index repeatedly increases and decreases, wherein at least a portion of the fluctuation region is included in an outer region that is at a distance of 7 mm or more from the center of the optical fiber preform, and the radial width of the fluctuation in the outer region is less than 2 μm.

[0010] The present inventors have investigated optical fiber preforms whose refractive index profile cannot be measured using a preform analyzer. As a result, they have found that the greater the distance from the center of the optical fiber preform, the more difficult it is to accurately measure the refraction angle of laser light, whereas the shorter the distance, the easier it is to accurately measure the refraction angle of laser light. They have also found that even if the distance is great, the refraction angle of laser light is easier to accurately measure if the range of fluctuation in the refractive index is narrow. The range of fluctuation in the refractive index can also be considered the width of each layer constituting the striae. Therefore, as a result of further intensive research, the present inventors have found that the refractive index profile can be measured using a preform analyzer when the range of fluctuation in the refractive index profile in an outer region at a distance of 7 mm or more from the center of the optical fiber preform is less than 2 μm. Therefore, according to aspect 1, it is possible to prevent the refractive index profile from becoming unmeasurable using a preform analyzer.

[0011] A second aspect of the present invention is characterized in that, in the optical fiber preform of the first aspect, the radial width of the fluctuation in the entire outer region is less than 2 μm.

[0012] A third aspect of the present invention is characterized in that, in the optical fiber preform of the first or second aspect, the width of the fluctuation increases toward the center.

[0013] In the refractive index distribution of an optical fiber preform obtained by sintering a porous glass body formed using the VAD method, the above-mentioned fluctuation range increases toward the center. Therefore, the second aspect of the present invention is suitable for an optical fiber preform manufactured in this manner.

[0014] A fourth aspect of the present invention is characterized in that, in the optical fiber preform according to any one of the first to third aspects, the inner region at a distance of less than 7 mm from the center includes a region in which the width of the fluctuation is 2 μm or more.

[0015] Aspect 5 of the present invention is characterized in that, in the optical fiber preform of any one of Aspects 1 to 4, the optical fiber preform comprises a rod-shaped core glass body and a cladding glass body having a refractive index different from that of the core glass body and surrounding the outer surface of the core glass body, and the core glass body is located only in an inner region at a distance of less than 7 mm from the center.

[0016] In the fifth aspect of the present invention, the range of the above-mentioned fluctuations in the core glass body is allowed to be large, and therefore, according to the fifth aspect of the present invention, it is possible to realize an optical fiber preform including a core glass body with a high degree of freedom in the range of the above-mentioned fluctuations.

[0017] A sixth aspect of the present invention is a method for measuring the refractive index distribution of an optical fiber preform, which involves relatively moving the optical fiber preform and a light emitting unit that emits the laser light in a direction toward or away from the center of the optical fiber preform so that the laser light is incident on the optical fiber preform from a direction perpendicular to the longitudinal direction of the optical fiber preform, thereby scanning the laser light, and measuring the refractive index distribution of the optical fiber preform based on the refraction angle of the laser light emitted from the optical fiber preform, wherein the optical fiber preform is any one of the optical fiber preforms of the first to fifth aspects, and the diameter of the laser light when incident on the optical fiber preform is 20 μm or more and 40 μm or less.

[0018] According to the sixth aspect of the present invention, the refractive index profile of the optical fiber preform according to any one of the first to fifth aspects can be measured.

[0019] A seventh aspect of the present invention is a method for manufacturing an optical fiber preform, comprising: a glass member-forming step of depositing glass particles in multiple layers on a glass rod rotating around its axis to form a porous glass body, and sintering the porous glass body to form a rod-shaped glass member having a refractive index distribution including a fluctuation region in which the refractive index repeatedly increases and decreases; and a stretching step of stretching the glass member, wherein the rotation speed of the glass rod in the glass member-forming step and the stretching rate of the glass member in the stretching step are set so that at least a part of the fluctuation region in the refractive index distribution of the glass member after the stretching step is included in an outer region that is 7 mm or more away from the center of the glass member, and the radial width of the fluctuation in the outer region is less than 2 μm.

[0020] As the rotation speed of the glass rod increases in the glass member-forming step, the thickness of each layer of glass particles in the porous glass body decreases, narrowing the range of variation in the refractive index distribution of the resulting glass member. Furthermore, as the elongation rate of the glass member increases in the elongation step, the range of variation in the refractive index distribution of the elongated glass member decreases. In Aspect 7, as described above, the rotation speed of the glass rod and the elongation rate of the glass member are set so that at least a portion of the variation region in the refractive index distribution of the elongated glass member is included in an outer region at a distance of 7 mm or more from the center of the glass member, and the radial width of the variation in the outer region is less than 2 μm. Therefore, Aspect 7 makes it possible to produce an optical fiber preform in which the variation region in the outer region at a distance of 7 mm or more from the center is less than 2 μm, thereby making it possible to produce an optical fiber preform that can prevent the refractive index distribution from becoming unmeasurable using a preform analyzer.

[0021] Aspect 8 of the present invention is characterized in that in the method for manufacturing an optical fiber preform according to aspect 7, the rotation speed of the glass rod is 20 rpm or more and 40 rpm or less.

[0022] 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 fluctuation in the outer region is less than 2 μm.

[0023] A ninth aspect of the present invention is characterized in that, in the manufacturing method of an optical fiber preform according to the seventh or eighth aspect, the elongation ratio is set so that the radial width of the fluctuation in the entire area of ​​the outer region is less than 2 μm.

[0024] As described above, the present invention provides an optical fiber preform that can prevent the refractive index profile from becoming unmeasurable with a preform analyzer, and a method for manufacturing an optical fiber preform.

[0025] 1 is a diagram schematically illustrating a cross section perpendicular to the longitudinal direction of an optical fiber preform according to an embodiment of the present invention. FIG. 2 is a diagram schematically illustrating a refractive index profile in a cross section perpendicular to the longitudinal direction of the optical fiber preform shown in FIG. 1. FIG. 3 is a conceptual diagram illustrating an enlarged view of a portion of the refractive index profile shown in FIG. 2. FIG. 4 is a flowchart illustrating steps in a method for manufacturing an optical fiber preform according to an embodiment. FIG. 5 is a diagram schematically illustrating a preform analyzer according to an embodiment. FIG. 6 is a graph illustrating the relationship between the distance from the center of the optical fiber preform and the range of fluctuation in the refractive index profile in Examples 1 to 3 and Comparative Examples 1 to 3.

[0026] The optical fiber preform and the method for manufacturing the optical fiber preform according to the present invention will be exemplified below with reference to the accompanying drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved without departing from the spirit and scope of the present invention. In the drawings referred to below, the dimensions of each component may be changed to facilitate understanding.

[0027] Fig. 1 is a schematic diagram showing a cross section perpendicular to the longitudinal direction of an optical fiber preform according to an embodiment of the present invention. As shown in Fig. 1, the optical fiber preform 1P of this embodiment mainly comprises a rod-shaped core glass body 10P and a clad glass body 11P surrounding the outer peripheral surface of the core glass body 10P. The core glass body 10P is a component that will become the core of an optical fiber obtained from the optical fiber preform 1P, and the clad glass body 11P is a component that will become the clad of an optical fiber obtained from the optical fiber preform 1P. In this embodiment, the outer shapes of the core glass body 10P and the clad glass body 11P in the cross section are approximately circular, and the core glass body 10P is disposed at the center of the clad glass body 11P. The outer diameter of the core glass body 10P is 15 mm, and the outer diameter of the clad glass body 11P is 50 mm, but these outer diameters are not limited.

[0028] 2 is a diagram schematically illustrating the refractive index profile in a cross section perpendicular to the longitudinal direction of the optical fiber preform 1P shown in FIG. 1 , in which the refractive index of the core glass body 10P is higher than that of the cladding glass body 11P. In this embodiment, the core glass body 10P is made of silica glass doped with a dopant that increases the refractive index, such as germanium (Ge), and the cladding glass body 11P is made of silica glass without any dopant. Alternatively, the core glass body 10P may be made of silica glass without any dopant, and the cladding glass body 11P may be made of silica glass doped with a dopant that decreases the refractive index, such as fluorine (F). Alternatively, the core glass body 10P may be made of silica glass doped with a dopant that increases the refractive index, and the cladding glass body 11P may be made of silica glass doped with a dopant that decreases the refractive index. Furthermore, the dopants that increase the refractive index and the dopants that decrease the refractive index are not limited.

[0029] FIG. 3 is a conceptual diagram showing an enlarged portion of the refractive index distribution shown in FIG. 2 , and is a conceptual diagram showing an enlarged portion of the refractive index distribution of the core glass body 10P. As shown in FIG. 3 , the refractive index distribution of the optical fiber preform 1P includes a variation region in which the refractive index repeatedly increases and decreases. 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 alternately arranged. The increase and decrease in the refractive index in this variation 20, when the refractive index distribution is expressed in terms of relative refractive index, are, for example, approximately 0.04% or less. Furthermore, the radial width 20W of this variation 20 is, for example, approximately 0.3 mm to 0.3 μm. The width 20W of the variation 20 is sometimes referred to as the width or period of the striae, and can also be referred to as the width of the repetition of the variation 20 or the width of each layer constituting the striae. As will be described in detail later, the optical fiber preform 1P is a sintered porous glass body in which glass particles are deposited in multiple layers. Therefore, the optical fiber preform 1P has glass layers corresponding to each layer of glass particles, and the refractive index distribution of the optical fiber preform 1P has minute refractive index fluctuations corresponding to these glass layers. As a result, the refractive index distribution includes a fluctuation region in which the above-described fluctuations 20 are repeated. In this embodiment, the entire refractive index distribution of the optical fiber preform 1P is a fluctuation region. Furthermore, in the outer region OR, where the distance L from the center of the optical fiber preform 1P is 7 mm or more, the radial width 20W of the fluctuations 20 is less than 2 μm, and the width 20W of the fluctuations 20 increases toward the center of the optical fiber preform 1P. Furthermore, in this embodiment, the radial width 20W of the fluctuations 20 throughout the entire outer region OR is less than 2 μm. Furthermore, in this embodiment, the inner region IR, where the distance L is less than 7 mm, includes a region where the width 20W of the fluctuations 20 is 2 μm or more, but the inner region IR does not necessarily need to include a region where the width 20W of the fluctuations 20 is 2 μm or more. In addition, in this embodiment, a portion of the core glass body 10P is located in the outer region OR, but the core glass body 10P does not have to be located in the outer region OR. In other words, the core glass body 10P may be located only in the inner region IR.

[0030] Next, a method for manufacturing an optical fiber preform according to this embodiment will be described.

[0031] 4 is a flowchart showing steps of a method for manufacturing an optical fiber preform 1P according to this embodiment. As shown in FIG. 4, the method for manufacturing an optical fiber preform 1P according to this embodiment includes a glass member forming step P1 and a drawing step P2.

[0032] <Glass Member Forming Process P1> This process involves depositing glass particles in multiple layers on a glass rod rotating around its axis to form a porous glass body, and then sintering the porous glass body to form a rod-shaped glass member. Although not illustrated, in this embodiment, the porous glass body is formed using the VAD method, in which glass particles are deposited in multiple layers from one end of a glass rod rotating around its axis in the axial direction of the glass rod. The porous glass body is sintered to form a rod-shaped glass member. The glass member thus formed has a configuration in which the optical fiber preform 1P shown in FIG. 1 is expanded in the radial direction and contracted in the longitudinal direction. Therefore, the glass member 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 member is approximately the same as that in the optical fiber preform 1P. Furthermore, the refractive index distribution of the glass member is a radially elongated distribution of the refractive index distribution of the optical fiber preform 1P shown in FIG. 2. That is, to form such a glass member, glass particles are deposited in multiple layers to form a porous glass body, and the porous glass body is then sintered. In this embodiment, the rotation speed of the glass rod is constant, but the rotation speed of the glass rod may be changed while depositing the glass particles. Furthermore, the method for depositing the glass particles is not limited, and may be, for example, an OVD method in which glass particles are deposited in multiple layers on the outer peripheral surface of a glass rod rotating around its axis.

[0033] <Drawing step P2> This step is a step of heating the glass member formed in the glass member forming step P1 and drawing it in the longitudinal direction. As described above, the glass member formed in the glass member forming step P1 has a configuration in which the optical fiber preform 1P shown in Figure 1 has been expanded in the radial direction and reduced in the longitudinal direction. Therefore, by drawing the glass member in the longitudinal direction, the glass member becomes the optical fiber preform 1P.

[0034] Incidentally, as the rotation speed of the glass rod in the glass member-forming process P1 increases, the thickness of each layer of glass particles in the porous glass body decreases, narrowing the width 20W of fluctuations in the refractive index distribution of the resulting glass member. Furthermore, as the elongation ratio of the glass member in the elongation process P2 increases, the width 20W of fluctuations in the refractive index distribution of the elongated glass member decreases. The elongation ratio is the ratio of the length of the elongated glass member to the length of the glass member before elongation. In this embodiment, the rotation speed of the glass rod in the glass member-forming process P1 and the elongation ratio of the glass member in the elongation process P2 are set so that at least a portion of the fluctuation region in the refractive index distribution of the elongated glass member is included in an outer region OR that is 7 mm or more away from the center of the glass member, and the width 20W of fluctuations in the outer region OR is less than 2 μm. Therefore, the refractive index distribution of the resulting optical fiber preform 1P includes a fluctuation region in which the refractive index repeatedly increases and decreases. At least a part of the fluctuation region is included in an outer region OR having 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.

[0035] 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. This configuration makes it easier to manufacture an optical fiber preform 1P in which the width of the fluctuation 20 in the outer region OR is less than 2 μm.

[0036] Next, a preform analyzer for measuring the refractive index distribution of the optical fiber preform according to this embodiment will be described.

[0037] Fig. 5 is a diagram schematically illustrating a preform analyzer according to this embodiment. As shown in Fig. 5, in this embodiment, a preform analyzer 30 mainly comprises a light-emitting unit 31 that emits light, a light-receiving unit 32 that receives light, a light-transmitting housing unit 33 having a housing space, and a measurement unit 34.

[0038] The light emitting section 31 of this embodiment emits laser light with a power peak wavelength of 632 nm, but the power peak wavelength of the laser light is not limited.

[0039] The light receiving unit 32 is an optical element that converts light received by a light receiving surface 32s into an electrical signal and outputs the electrical signal. In this embodiment, the light receiving surface 32s is composed of light receiving surfaces of multiple light receiving elements that receive light, and the light receiving unit 32 outputs information related to the position of light irradiated onto the light receiving surface 32s to the measurement unit 34. An example of the information related to the position of light is a two-dimensional image. The light emitting unit 31 and the light receiving unit 32 are arranged so that the light emitting portion of the light emitting unit 31 faces the light receiving surface 32s of the light receiving unit 32 with a gap between them.

[0040] The accommodation section 33 is disposed between the light-emitting section 31 and the light-receiving section 32. The optical fiber preform 1P is accommodated in the accommodation space of the accommodation section 33 so that the longitudinal direction of the optical fiber preform 1P is perpendicular to the direction in which the light-emitting section 31 and the light-receiving section 32 face each other, and the accommodation space is filled with matching oil 35 having the same refractive index as the clad glass body 11P. The accommodation section 33 is movable in a predetermined direction perpendicular to the longitudinal direction of the optical fiber preform 1P and perpendicular to the direction in which the light-emitting section 31 and the light-receiving section 32 face each other.

[0041] The laser light emitted from the light-emitting unit 31 passes through the optical fiber preform 1P and is irradiated onto the light-receiving surface 32s of the light-receiving unit 32. In this embodiment, the diameter of the laser light incident on the optical fiber preform 1P is, for example, 20 μm to 40 μm, and in this embodiment, it is 30 μm. In this embodiment, the laser light is scanned by moving the storage unit 33 in a predetermined direction. The predetermined direction is, for example, the direction of incidence of the laser light on the optical fiber preform 1P and a direction perpendicular to the longitudinal direction of the optical fiber preform 1P. In this case, the laser light is incident on the optical fiber preform 1P stored in the storage unit 33 from a direction perpendicular to the longitudinal direction of the optical fiber preform 1P. Furthermore, the optical fiber preform 1P is moved relative to the light-emitting unit 31 in a direction D1 toward the center 1Pc of the optical fiber preform 1P or a direction D2 away from the center 1Pc of the optical fiber preform 1P, thereby scanning the laser light. The laser beam can be scanned by relatively moving the optical fiber preform 1P and the light-emitting unit 31 in a direction D1 toward the center 1Pc of the optical fiber preform 1P or a direction D2 away from the center 1Pc of the optical fiber preform 1P so that the laser beam is incident on the optical fiber preform 1P in a direction perpendicular to the longitudinal direction of the optical fiber preform 1P. For example, the light-emitting unit 31 may move in the predetermined direction, the optical fiber preform 1P may move in the predetermined direction, or the optical fiber preform 1P and the light-emitting unit 31 may move in the predetermined direction. The term "perpendicular" as used herein does not necessarily mean perfectly perpendicular, but also includes cases where the optical fiber preform 1P deviates from perfectly perpendicular due to curvature of the optical fiber preform 1P caused by manufacturing errors, for example.

[0042] The light receiving unit 32 outputs information relating to the position of the laser light irradiated onto the light receiving surface 32s to the measurement unit 34. The measurement unit 34 is configured to measure the refraction angle θ of the laser light in the optical fiber preform 1P based on this information, measure the refractive index at the position in the optical fiber preform 1P through which the laser light passes based on the refraction angle θ, and measure the refractive index distribution from the refractive index. The measurement unit 34 is formed, for example, by an integrated circuit such as a microcontroller, an integrated circuit (IC), a large-scale integrated circuit (LSI), or an application specific integrated circuit (ASIC), or an NC (numerical control) device.

[0043] In this embodiment, the refractive index distribution of the optical fiber preform 1P is measured using such a preform analyzer 30. First, the optical fiber preform 1P is accommodated in the accommodation space of the accommodation unit 33 as described above. Next, laser light is emitted from the light-emitting unit 31. Next, the optical fiber preform 1P and the light-emitting unit 31 are moved relative to each other in a direction toward or away from the center 1Pc of the optical fiber preform 1P so that the laser light is incident on the optical fiber preform 1P from a direction perpendicular to the longitudinal direction of the optical fiber preform 1P, thereby scanning the laser light. In this embodiment, the accommodation unit 33 is moved to move the optical fiber preform 1P relative to the light-emitting unit 31. Then, the laser light emitted from the optical fiber preform 1P is received by the light-receiving unit 32. The light receiving unit 32 outputs information related to the position of the light irradiated onto the light receiving surface 32s, and the measurement unit 34 measures the refraction angle θ of the laser light at the optical fiber preform 1P based on the two-dimensional image, and measures the refractive index distribution of the optical fiber preform 1P based on the refraction angle θ. In other words, the preform analyzer 30 scans the laser light by relatively moving the optical fiber preform 1P and the light emitting unit 31 in a direction toward or away from the center 1Pc of the optical fiber preform 1P so that the laser light is incident on the optical fiber preform 1P from a direction perpendicular to the longitudinal direction of the optical fiber preform 1P, and measures the refractive index distribution of the optical fiber preform 1P based on the refraction angle of the laser light emitted from the optical fiber preform 1P.

[0044] As described above, the optical fiber preform 1P of this embodiment has a refractive index profile including a fluctuation region in which the refractive index repeatedly increases and decreases, i.e., a fluctuation 20. At least a part of this fluctuation region is included in the outer region OR, the distance L from the center of the optical fiber preform 1P being 7 mm or more, and the radial width of the fluctuation 20 in the outer region OR is less than 2 μm.

[0045] The present inventors have investigated optical fiber preforms whose refractive index profile cannot be measured using a preform analyzer. As a result, they have found that the greater the distance from the center of the optical fiber preform, the more difficult it is to accurately measure the refraction angle of laser light, and the closer the distance, the easier it is to accurately measure the refraction angle of laser light. They have also found that even if the distance is great, the refraction angle of laser light is easier to accurately measure if the range of fluctuation in the refractive index is narrow. Therefore, as a result of further intensive research, the present inventors have found that the refractive index profile can be measured using a preform analyzer when the radial width of the fluctuation in the refractive index profile in an outer region at a distance of 7 mm or more from the center of the optical fiber preform is less than 2 μm. Therefore, the optical fiber preform 1P of this embodiment can prevent the refractive index profile from being unable to be measured using a preform analyzer.

[0046] In the optical fiber preform 1P of this embodiment, the width 20W of the fluctuations 20 increases toward the center. In the refractive index distribution of an optical fiber preform obtained by sintering a porous glass body formed using the VAD method, the width 20W of the fluctuations 20 increases toward the center. Therefore, the optical fiber preform 1P of this embodiment is suitable for optical fiber preforms manufactured in this manner. The width 20W of the fluctuations 20 may be random or approximately constant in the radial direction.

[0047] The optical fiber preform 1P of this embodiment is a preform that is drawn to obtain an optical fiber. Therefore, with the optical fiber preform 1P of this embodiment, the drawing conditions can be appropriately set based on, for example, the refractive index profile measured by a preform analyzer. The optical fiber preform 1P can also be used as a so-called intermediate preform for obtaining a preform with a larger diameter than the optical fiber preform 1P. By providing a glass layer on the outer peripheral surface of the optical fiber preform 1P as an intermediate preform, a preform with a larger diameter than the optical fiber preform 1P can be obtained. For example, if this glass layer is made of the same glass body as the cladding glass body 11P, a preform with a larger outer diameter than the cladding glass body 11P shown in FIG. 1 can be obtained. In this case, for example, the refractive index profile 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 profile. Therefore, a preform with a desired ratio of the outer diameter of the core glass body 10P to the outer diameter of the cladding glass body 11P can be obtained.

[0048] In the method for measuring the refractive index profile of an optical fiber preform according to the present embodiment, the optical fiber preform and the light emitter 31 are moved relative to each other toward or away from the center 1Pc of the optical fiber preform 1P so that the laser beam is incident on the optical fiber preform 1P from a direction perpendicular to the longitudinal direction of the optical fiber preform 1P, scanning the laser beam. The refractive index profile of the optical fiber preform 1P is measured based on the refraction angle of the laser beam emitted from the optical fiber preform 1P. The optical fiber preform 1P has a refractive index profile including a fluctuation region in which the refractive index repeatedly increases and decreases. At least a portion of this fluctuation region is included in an 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 region 20 in the outer region OR is less than 2 μm. The diameter of the laser beam incident on the optical fiber preform 1P is 20 μm or more and 40 μm or less. This configuration allows the refractive index profile of the optical fiber preform 1P to be measured.

[0049] Although the present invention has been described above using the above-mentioned embodiments as examples, the present invention is not limited to these.

[0050] For example, in the above embodiment, an optical fiber preform 1P including a core glass body 10P and a cladding glass body 11P has been described as an example. However, the optical fiber preform 1P has a refractive index profile including a fluctuation region in which the refractive index repeatedly increases and decreases, and at least a portion of the fluctuation region is included in an outer region OR whose distance L from the center of the optical fiber preform 1P is 7 mm or more, and the width 20W of the fluctuation 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 surrounding the outer peripheral surface of the core glass body 10P and having a refractive index different from that of the core glass body 10P, and an outer glass layer surrounding the outer peripheral surface of the inner glass layer and having a refractive index different from that of the inner glass layer. Furthermore, when the optical fiber preform 1P is the aforementioned intermediate preform, the optical fiber preform 1P may be, for example, the core glass body 10P.

[0051] In the above embodiment, the optical fiber preform 1P has been described as an example in which the refractive index variation 20 is repeated throughout the entire radial direction of the optical fiber preform 1P, and the entire refractive index distribution is a variation region. However, it is sufficient that at least a part of the variation region is included in the outer region OR, which is located at a distance L from the center of the optical fiber preform 1P of less than 7 mm. For example, the variation 20 may not occur on the central side of the inner region IR, which is located at a distance L of less than 7 mm. Such an optical fiber preform 1P can be obtained, for example, by forming a porous glass body by the OVD method using a glass rod in which the refractive index variation 20 does not occur, and then sintering the porous glass body to form a glass member in the glass member forming step P1.

[0052] In the above embodiment, the optical fiber preform 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. A portion 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 located at a distance of less than 7 mm from the center of the optical fiber preform 1P. This configuration allows the width of fluctuation 20 in the core glass body 10P to be large. Therefore, this configuration can realize an optical fiber preform 1P including a core glass body 10P with a high degree of freedom in the width 20W of fluctuation 20.

[0053] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these.

[0054] Example 1 In this example, the optical fiber preform 1P shown in FIG. 1 was manufactured using the method for manufacturing an optical fiber preform shown in the above embodiment. Specifically, in the glass member forming step P1, a porous glass body was formed by the VAD method with the rotation speed of the glass rod set to 20 rpm, and the porous glass body was sintered to form a glass member. The outer diameter of this glass member was 100 mm. Furthermore, in the elongation step P2, this glass member was elongated to manufacture the optical fiber preform 1P. The elongation ratio of the glass member in the elongation step P2 was approximately 2.0, the outer diameter of the optical fiber preform 1P was 50 mm, and the radius of the core glass body 10P was 7.5 mm.

[0055] Example 2 An optical fiber preform 1P was manufactured in the same manner as in Example 1, except that the elongation ratio of the glass member in the elongation step P2 was greater than that in Example 1. The elongation ratio of the glass member in the elongation step P2 in this example was approximately 2.3, the outer diameter of the optical fiber preform 1P in this example was 43 mm, and the radius of the core glass body 10P was 6.5 mm. In other words, the core glass body 10P in Example 2 was located only in the inner region IR.

[0056] Example 3 An optical fiber preform 1P was manufactured in the same manner as in Example 1, except that the rotational speed of the glass rod in the glass member forming step P1 was set to 40 rpm, and the elongation ratio of the glass member in the elongation step P2 was set to be smaller than that in Example 1. The elongation ratio of the glass member in the elongation step P2 in this example was approximately 1.9, the outer diameter of the optical fiber preform 1P in this example was 53 mm, and the radius of the core glass body 10P was 8 mm.

[0057] (Comparative Examples 1 to 3) Optical fiber preforms 1P were manufactured in the same manner as in Example 1, except that the elongation ratio 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 ratio of the glass member in the elongation step P2 was about 1.6, the outer diameter of the optical fiber preform 1P was 63 mm, and the radius of the core glass body 10P was 9.5 mm. In Comparative Example 2, the elongation ratio of the glass member in the elongation step P2 was about 1.8, the outer diameter of the optical fiber preform 1P was 57 mm, and the radius of the core glass body 10P was 8.5 mm. In Comparative Example 3, the elongation ratio of the glass member in the elongation step P2 was about 1.9, the outer diameter of the optical fiber preform 1P was 53 mm, and the radius of the core glass body 10P was 8 mm.

[0058] (Measurement of refractive index distribution) The refractive index distribution of the optical fiber preforms 1P obtained in Examples 1 to 3 and Comparative Examples 1 to 3 was measured using a preform analyzer 30 shown in Figure 5. The peak wavelength of the laser light power in the preform analyzer 30 was 632 nm, and the diameter of the laser light when incident on the optical fiber preform 1P was approximately 30 µm. Then, for Examples 1 to 3 and Comparative Examples 1 to 3, the relationship between the distance L from the center of the optical fiber preform 1P and the width 20W of the fluctuation 20 in the refractive index distribution was investigated based on detailed analysis and calculation. This relationship was obtained by changing the distance from the center of the optical fiber preform 1P in increments of 0.5 mm, and some of the results are shown in Figure 6. Table 1 also 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 preform 1P, the radius of the core glass body 10P, the elongation rate in the elongation process P2, the width 20W of the fluctuation 20 at the position where the distance L is 7 mm, whether the width 20W of the fluctuation 20 in the region where the distance L is 7 mm or more is less than 2 μm, and whether there were cases where the refraction angle of the laser light emitted from the preform analyzer 30 could not be measured.

[0059] As shown in Fig. 6, in Examples 1 to 3, the width 20W of the fluctuation 20 in the region where the distance L was 7 mm or more and 9.5 mm or less was less than 2 µm. Although not shown in Fig. 6, in Examples 1 to 3, the width 20W in the region where the distance L exceeded 9.5 mm was also less than 2 µm. In addition, in Examples 1 to 3, the refractive index distribution of the optical fiber preform 1P could be measured. In Fig. 6, the range where the distance L was 7 mm or more and the width 20W of the fluctuation 20 was 2 µm or more is hatched with multiple dots.

[0060] In Comparative Example 1, the refraction angle of the laser beam emitted from the preform analyzer 30 could not be measured in the region where the distance L was 7.0 mm or more and 9.0 mm or less. In Comparative Example 2, the refraction angle of the laser beam emitted from the preform analyzer 30 could not be measured in the region where the distance L was 7.0 mm or more and 8.0 mm or less. In Comparative Example 3, the refraction angle of the laser beam emitted from the preform analyzer 30 could not be measured in the region where the distance L was 7.0 mm or more and 7.5 mm or less. Furthermore, in Comparative Examples 1 to 3, the refractive index distribution could not be accurately measured. Although not shown in FIG. 6 , in Comparative Examples 1 to 3, the width 20W of the fluctuation 20 in the region where the distance L exceeded 9.5 mm was less than 2 μm. As described above, in the refractive index distribution of an optical fiber preform obtained by sintering a porous glass body formed using the VAD method, the width 20W of the fluctuation 20 increases toward the center. In Comparative Examples 1 to 3, the width 20W in the region where the distance L was less than 7.0 mm exceeded 2 μm. 6 also shows that in Examples 1 to 3, the width 20W decreases at a generally constant rate from the center toward the outside, and the rate at which the width 20W decreases becomes smaller in regions outside the distance L where the width 20W is approximately 1 μm. For this reason, it is considered that the width 20W also decreases from the center toward the outside in Comparative Examples 1 to 3, as in Examples 1 to 3, and that the width 20W in regions where the refraction angle of the laser light could not be measured in Comparative Examples 1 to 3 was 2 μm or more.

[0061] Therefore, it was found that when the width 20W in the entire region where the distance L is 7 mm or more is less than 2 μm, it is possible to measure the refractive index distribution using the preform analyzer 30. It was also found that the width 20W at the position where the distance L is 7 mm may be 1.8 μm or more and less than 2.0 μm.

[0062] The refractive index distribution was measured using a laser beam from the preform analyzer 30 with a power peak wavelength of 405 nm. The relationship between the distance L and the width 20W was similar to the results shown in FIG. 6 and Table 1. Therefore, it is believed that the wavelength of the laser beam from the preform analyzer 30 does not significantly affect whether or not the refractive index distribution can be measured. The refractive index distribution was also measured by changing the laser beam from the preform analyzer 30 to white light emitted from an LED (Light Emitting Diode). In this case, the degree of diffusion of the light emitted from the optical fiber preform was greater than when the light was laser beam. However, the refraction angle of the light could be measured, and the relationship between the distance L and the width 20W was similar to the results shown in FIG. 6 and Table 1. Therefore, it is believed that the diameter of the light incident on the optical fiber preform does not significantly affect whether or not the refractive index distribution can be measured. Furthermore, when the refractive index distribution was measured with the diameter of the laser light incident on the optical fiber preform 1P set to 20 μm, and when the refractive index distribution was measured with this diameter set to 40 μm, the relationship between the distance L and the width 20W was similar to the results shown in Fig. 6 and Table 1. Therefore, if the diameter is 20 μm or more and 40 μm or less, it is considered possible to measure the refractive index distribution of the optical fiber preform 1P in which the width 20W is less than 2 μm in the region where the distance L is 7 mm or more, as in the case of 30 μm.

[0063] As described above, according to the present invention, an optical fiber preform, a method for measuring the refractive index profile of an optical fiber preform, and a method for manufacturing an optical fiber preform are provided, which are capable of preventing the refractive index profile from being measured by a preform analyzer, and are expected to be used in fields such as optical fiber communications.

Claims

1. A preform for an optical fiber having a refractive index distribution including a variable region that repeats fluctuations in refractive index increase and decrease, wherein at least a part of the variable region is included in an outer region where the distance from the center of the preform for the optical fiber is 7 mm or more, and the radial width of the fluctuation in the outer region is less than 2 μm A preform for an optical fiber, characterized by this.

2. The radial width of the fluctuation in the entire region of the outer region is less than 2 μm The preform for an optical fiber according to claim 1, characterized by this.

3. The width of the fluctuation becomes larger toward the center side The preform for an optical fiber according to claim 1 or 2, characterized by this.

4. In the inner region where the distance from the center is less than 7 mm, a region where the width of the fluctuation is 2 μm or more is included The preform for an optical fiber according to claim 1 or 2, characterized by this.

5. A rod-shaped core glass body and a clad 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, wherein the core glass body is located only in the inner region where the distance from the center is less than 7 mm The preform for an optical fiber according to claim 1 or 2, characterized by this.

6. A method for measuring the refractive index distribution of a preform for an optical fiber, wherein a laser beam is scanned by relatively moving the preform for the optical fiber and a light emitting part that emits the laser beam in a direction toward the center of the preform for the optical fiber or in a direction away from the center of the preform for the optical fiber so that the laser beam is incident on the preform for the optical fiber from a direction perpendicular to the longitudinal direction of the preform for the optical fiber, and the refractive index distribution of the preform for the optical fiber is measured based on the refraction angle of the laser beam emitted from the preform for the optical fiber, wherein the preform for the optical fiber is the preform for the optical fiber according to claim 1 or 2, and the diameter of the laser beam when the laser beam is incident on the preform for the optical fiber is 20 μm or more and 40 μm or less A method for measuring the refractive index distribution of a preform for an optical fiber, characterized by this.

7. A glass member forming step of forming a porous glass body by depositing glass fine particles in multiple layers on a glass rod rotating around an axis, and sintering the porous glass body to form a rod-shaped glass member having a refractive index distribution including a variable region that repeats fluctuations in refractive index increase and decrease, and a drawing step of drawing the glass member Comprising The rotation speed of the glass rod in the glass member forming step and the drawing ratio of the glass member in the drawing step are set such that at least a part of the variation region in the refractive index distribution of the glass member after the drawing step is included in the outer region where the distance from the center of the glass member is 7 mm or more, and the radial width of the variation in the outer region is less than 2 μm. A method for manufacturing a preform for an optical fiber, characterized by the above. **Claim 8** The rotation speed of the glass rod is 20 rpm or more and 40 rpm or less. A method for manufacturing a preform for an optical fiber according to claim 7, characterized by the above. **Claim 9** The drawing ratio is set such that the radial width of the variation in the entire outer region is less than 2 μm. A method for manufacturing a preform for an optical fiber according to claim 7 or 8, characterized by the above.