Optical fiber preform manufacturing method and optical fiber preform

Abstract

A production method for an optical fiber base material made of silica glass, the method comprising: forming a core part; forming, around the core part, a cladding part having a refractive index lower than that of the core part, wherein the forming the core part includes adding alkali elements including an alkali metal element and an alkaline-earth metal element to an inner surface of a glass pipe made of silica glass, and, after the adding, integrating the glass pipe with a glass rod disposed inside the glass pipe to form an integrated rod.

Description

[Technical Field]

[0001] This application claims priority to Japanese Application No. 2020-148202, filed on September 3, 2020, and incorporates by reference all of the contents of said Japanese application.

[0002] The present disclosure relates to a method for manufacturing an optical fiber preform and an optical fiber preform. [Background technology]

[0003] If the core made of silica-based glass contains an alkali metal element or an alkaline earth metal element, the viscosity of the core is reduced and the rearrangement of the glass is promoted when the optical fiber is produced by drawing the optical fiber preform. Therefore, the transmission loss of the optical fiber caused by Rayleigh scattering is reduced. As a result, the transmission loss can be reduced.

[0004] Patent Documents 1, 2, and 3 describe methods of doping an alkali metal element or an alkaline earth metal element into the core portion of an optical fiber preform by a diffusion method. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2004 / 020357 [Patent Document 2] International Publication No. 2005 / 021455 [Patent Document 3] International Publication No. 2013 / 111470 Summary of the Invention

[0006] The method for manufacturing an optical fiber preform according to the present disclosure is a method for manufacturing an optical fiber preform made of silica-based glass, and includes forming a core portion and forming a cladding portion surrounding the core portion and having a refractive index lower than that of the core portion. The forming of the core portion includes adding an alkali element group consisting of an alkali metal element and an alkaline earth metal element to the inner surface of a glass pipe made of silica-based glass, and, after the addition, integrating the glass pipe with a glass rod disposed in the glass pipe to form an integrated rod.

[0007] The optical fiber preform of the present disclosure is an optical fiber preform made of silica-based glass, and includes a core containing an alkali element group consisting of alkali metal elements and alkaline earth metal elements, and a cladding surrounding the core and having a refractive index lower than that of the core. The core includes a region having a chlorine mass fraction lower than the chlorine mass fraction on the central axis of the core. The mass fraction of the alkali element group in the core has a maximum value outside the central axis. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a flowchart showing a method for manufacturing an optical fiber according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of an optical fiber preform according to the embodiment. [Figure 3] FIG. 3 is a graph showing the alkali element concentration distribution and the chlorine concentration distribution in the core portion. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Problem to be solved by this disclosure] In the methods described in Patent Documents 1, 2, and 3, an alkali metal element or an alkaline earth metal element is added to the inner surface of a glass pipe, and the glass pipe is then subjected to diameter reduction, etching, and other processes before being solidified to produce a glass body that will become the core of an optical fiber preform. However, because the inside of the glass pipe is hollow, the volume (amount of glass) of the glass pipe is smaller than that of a glass cylinder with the same outer diameter. This results in low productivity.

[0010] An object of the present disclosure is to provide a method for manufacturing an optical fiber preform that can improve productivity while suppressing transmission loss.

[0011] [Effects of this disclosure] According to the present disclosure, it is possible to provide an optical fiber preform manufacturing method and an optical fiber preform that can improve productivity while suppressing transmission loss. [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be described. A method for manufacturing an optical fiber preform according to one embodiment of the present disclosure is a method for manufacturing an optical fiber preform made of silica-based glass, and includes forming a core portion and forming a cladding portion surrounding the core portion and having a refractive index lower than that of the core portion. Forming the core portion includes adding an alkali element group consisting of alkali metal elements and alkaline earth metal elements to the inner surface of a glass pipe made of silica-based glass, and, after the addition, integrating the glass pipe with a glass rod disposed in the glass pipe to form an integrated rod.

[0012] In this method for manufacturing an optical fiber preform, the alkali element group is added to the inner surface of the glass pipe, thereby reducing transmission loss, and the glass pipe and the glass rod disposed therein are integrated, thereby improving productivity compared to when the glass pipe is solidified to form a glass body that serves as the core.

[0013] The formation of the core portion may further include reducing the diameter of the glass pipe between the doping and the integration. If the diameter of the glass rod used is significantly different from the hole diameter of the glass pipe, the core portion after integration is likely to become non-circular. In this case, by reducing the diameter, the hole diameter of the glass pipe can be made closer to the diameter of the glass rod, thereby suppressing the occurrence of non-circularity in the core portion.

[0014] The forming of the core portion may further include etching the inner surface of the glass pipe between the doping and the integrating, in which case impurities that have been doped to the inner surface of the glass pipe together with the alkali elements can be removed.

[0015] The average mass fraction of chlorine in the glass rod may be 20 ppm or more and 2000 ppm or less.

[0016] The average mass fraction of chlorine in the glass pipe may be 20 ppm or more and 2000 ppm or less.

[0017] The average mass fraction of fluorine in the glass rod may be 200 ppm or more and 5000 ppm or less. In this case, the number of abnormalities in the optical fiber preform can be reduced. Here, the abnormalities refer to, for example, foreign matter or glass crystals formed due to compounds of alkali elements with chlorine or fluorine, which will become defective parts when the optical fiber is subsequently made.

[0018] The average mass fraction of fluorine in the glass pipe may be 200 ppm or more and 5000 ppm or less, in which case the number of defective portions in the optical fiber preform can be reduced.

[0019] The glass rod may have an outer peripheral portion having a thickness of 0.5 mm, including an outer peripheral surface of the glass rod, and the average mass fraction of chlorine in the outer peripheral portion may be lower than the average mass fraction of chlorine in the entire glass rod, thereby suppressing the number of defects in the optical fiber preform.

[0020] The average mass fraction of chlorine in the outer circumferential portion may be 20 ppm or more and 2000 ppm or less, in which case the number of defective portions in the optical fiber preform can be reduced.

[0021] Forming the core may further include providing a glass layer around the monolithic rod having a higher refractive index than the cladding, which allows for more flexibility in designing optical properties such as the effective area (Aeff) or cutoff wavelength.

[0022] Glass layer The average mass fraction of chlorine in the glass may be 100 ppm or more and 2000 ppm or less. In this case, by making the mass fraction 100 ppm or more, an increase in loss due to glass defects can be suppressed, and transmission loss can be suppressed. If the mass fraction is higher than 2000 ppm, the frequency of base material abnormalities increases, resulting in a decrease in yield.

[0023] The average mass fraction of the alkali elements contained in the integrated rod may be 0.2 ppm or more and 300 ppm or less, in which case transmission loss can be suppressed.

[0024] The mass fraction of the alkali element group in the integrated rod may have a maximum value at a location other than the central axis of the integrated rod. This is because the alkali element group is disposed on the outer periphery of the glass rod immediately after integration. By having a maximum value at a location other than the central axis, the maximum value can be kept lower than when the same total amount is added so that the mass fraction is maximum at the central axis, and defects such as crystallization can be suppressed.

[0025] The core may contain any one of sodium, potassium, rubidium, cesium, and calcium as an alkali element, which can reduce transmission loss.

[0026] An optical fiber preform according to an embodiment of the present disclosure is an optical fiber preform made of silica-based glass, and includes: a core containing an alkali element group consisting of alkali metal elements and alkaline earth metal elements; and a cladding surrounding the core and having a refractive index lower than that of the core. The core includes a region having a chlorine mass fraction lower than the chlorine mass fraction on the central axis of the core. The mass fraction of the alkali element group in the core has a maximum value other than at the central axis. The mass fraction of the alkali element group in the core may have a maximum value other than at the central axis in a region within 50% of the radius of the core. The mass fraction of the alkali element group in the core may have a maximum value other than at the central axis in a region within 30% of the radius of the core.

[0027] This optical fiber preform can achieve both a reduction in the number of abnormal portions and a reduction in glass defect loss by increasing the average chlorine concentration in the core portion.

[0028] [Details of the embodiments of the present disclosure] Specific examples of the manufacturing method of the optical fiber preform and the optical fiber preform of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0029] 1 is a flowchart illustrating a method for manufacturing an optical fiber according to this embodiment. The following description also includes an example of specific conditions. The optical fiber according to this embodiment is manufactured through a preparation step S1, an addition step S2, a diameter reduction step S3, an etching step S4, an integration step S5, a stretch grinding step S6, a collapse step S7, an OVD (Outside Vapor Deposition) step S8, and a fiber drawing step S9, in that order.

[0030] The optical fiber preform 1 (see FIG. 2) according to this embodiment is manufactured by sequentially performing a core-forming step S10 for forming a core 10 (see FIG. 2) made of silica (quartz) glass, and a cladding-forming step S20 for forming a cladding 20 surrounding the core 10. That is, the manufacturing method of the optical fiber preform 1 includes the core-forming step S10 and the cladding-forming step S20. The core-forming step S10 includes a preparing step S1, an adding step S2, a diameter-reducing step S3, an etching step S4, an integrating step S5, and a stretch-grinding step S6. The cladding-forming step S20 includes a collapsing step S7 and an OVD step S8.

[0031] The preparation step S1 is a step of preparing a glass pipe and a glass rod for forming the core 10. In this embodiment, the preparation step S1 is performed before the addition step S2, but the glass rod may be prepared by the integration step S5. That is, the preparation step S1 may be composed of a glass pipe preparation step performed before the addition step S2 and a glass rod preparation step performed before the integration step S5.

[0032] The glass pipe is made of silica-based glass. The glass pipe is a glass pipe into which an alkali element group is to be diffused as a dopant. Here, the alkali element group is a general term for alkali metal elements and alkaline earth metal elements. In other words, the alkali element group consists of alkali metal elements and alkaline earth metal elements. The outer diameter (2d) of the glass pipe is 30 mm or more and 50 mm or less. The inner diameter (2i) of the glass pipe is 10 mm or more and 30 mm or less.

[0033] The glass rod is made of silica-based glass. The glass rod is synthesized, for example, by the VAD (Vapor Phase Axial Deposition) method. The glass rod may be processed by elongation, grinding, or other processes to adjust the diameter of the glass rod to a desired value. The glass rod is integrated with a glass pipe in the integration step S5 to form an integrated rod. The diameter of the glass rod is 3 mm or more and 15 mm or less.

[0034] Each of the glass pipe and the glass rod contains a certain mass fraction of chlorine and fluorine. The mass fractions of other dopants and impurities contained in each of the glass pipe and the glass rod are 10 ppm or less. "Mass fraction" is the ratio of the mass of a target element to the total mass of the object, and is expressed as (mass of the target element) / (total mass). Hereinafter, mass fraction is also referred to as "concentration."

[0035] The average chlorine concentration of the glass pipe is 20 ppm or more and 2000 ppm or less. This allows the refractive index of the core region 10 to be higher than the refractive index of the cladding region 20. As a result, transmission loss can be suppressed. The average fluorine concentration of the glass pipe is 200 ppm or more and 5000 ppm or less. This allows the number of defective portions of the optical fiber preform 1 to be suppressed.

[0036] The average chlorine concentration of the glass rod is 20 ppm or more and 2000 ppm or less. This allows the refractive index of the core region 10 to be higher than the refractive index of the cladding region 20. As a result, transmission loss can be suppressed. The average fluorine concentration of the glass rod is 200 ppm or more and 5000 ppm or less. This allows the number of defective portions in the optical fiber preform 1 to be suppressed.

[0037] The glass rod has an outer peripheral portion having a thickness of 0.5 mm, including the outer peripheral surface of the glass rod. The outer peripheral portion is, for example, a portion that is 70% to 100% or 90% to 100% of the radius of the glass rod. The average chlorine concentration of the outer peripheral portion is lower than the average chlorine concentration of the entire glass rod. The average chlorine concentration of the outer peripheral portion is 20 ppm to 2000 ppm.

[0038] Here, the average concentration is, for example, an average chlorine concentration, which is expressed by the following formula:

[0039]

number

[0040] In the above formula, Cl(r) represents the local chlorine concentration at the position of radius r. i represents the inner radius of the glass pipe. d represents the outer radius of the glass pipe. Fluorine is calculated in a similar manner. In the case of a glass rod, the average chlorine and fluorine concentrations are calculated using the above formula by setting i to 0 and d to the radius of the glass rod. The local concentrations are measured at each position along a line passing through the center position on the end face of the glass pipe and the glass rod using an electron probe micro analyzer (EPMA). The measurement conditions using EPMA are, for example, an acceleration voltage of 20 kV, a probe beam diameter of 0.5 μm to 1 μm, and a measurement interval of 100 nm or less.

[0041] The doping step S2 is a step of doping an alkali element group onto the inner surface of the glass pipe made of silica-based glass. When potassium (K) is doped as a dopant of the alkali element group, for example, 6 g to 20 g of potassium bromide (KBr) is used as a raw material. Depending on the type of alkali element group to be doped, one or more of KBr, potassium iodide (KI), rubidium bromide (RbBr), rubidium iodide (RbI), etc. may be used as the raw material.

[0042] In the addition step S2, the raw material is heated to a temperature of 700°C or more and 850°C or less by a first external heat source to generate raw material vapor. The first external heat source is, for example, an electric furnace, and is provided for heating the raw material. The generated raw material vapor is introduced into the glass pipe together with a carrier gas consisting of oxygen, while the glass pipe is heated from the outside by a second external heat source. The second external heat source is, for example, an oxyhydrogen burner, an induction furnace, or a resistance furnace, and is provided for heating the glass pipe. The flow rate of the carrier gas is 1 SLM (standard condition (0°C, 1.01 x 10 5 Pa) and is expressed as 1 liter / min.

[0043] In the doping step S2, the glass pipe is heated by moving a second external heat source along the longitudinal direction of the glass pipe. The glass pipe is heated by traversing the second external heat source at a speed of 30 mm / min to 60 mm / min for a total of 8 to 15 turns so that the temperature of the outer surface of the glass pipe is 1400°C to 2000°C. This allows alkali elements such as K to be diffused and doped into the inner surface of the glass pipe.

[0044] The diameter-reducing step S3 is a step of reducing the diameter of the glass pipe to which the alkali element group has been added in the adding step S2. The diameter-reducing step S3 is performed between the adding step S2 and the integrating step S5. At this time, the glass pipe is heated from the outside by a second external heat source while oxygen is flowed through the glass pipe at a rate of 0.5 SLM to 1.0 SLM. In the diameter-reducing step S3, the glass pipe is heated by moving the second external heat source along the longitudinal direction of the glass pipe. The glass pipe is heated by traversing the second external heat source for a total of 6 to 10 turns so that the outer surface of the glass pipe is heated to a temperature of 1300°C to 2000°C. The glass pipe is reduced in diameter until its inner diameter is approximately 1 mm to 3 mm larger than the diameter of the glass rod to be integrated in the integrating step S5.

[0045] The etching step S4 is a step of etching the inner surface of the glass pipe after the diameter-reducing step S3. The etching step S4 is performed between the doping step S2 and the integration step S5. In the etching step S4, a mixed gas of SF6 (0.2 SLM to 0.4 SLM) and chlorine (0.5 SLM to 1.0 SLM) is introduced into the glass pipe, while the glass pipe is heated from the outside by a second external heat source to perform vapor-phase etching. This allows the inner surface of the glass pipe, which contains a high concentration of impurities added together with the target dopant, to be scraped and these impurities to be removed. In the etching step S4, the glass pipe is heated by moving the second external heat source along the glass pipe's longitudinal direction. The glass pipe is heated by traversing the second external heat source for a total of one to five turns so that the outer surface of the glass pipe is heated to a temperature of 1300°C to 2000°C.

[0046] The integration step S5 is a step of integrating the glass pipe and the glass rod placed inside the glass pipe after the etching step S4. In the integration step S5, first, the glass rod is inserted into the glass pipe and fixed at the center of the glass pipe. Next, a mixed gas of oxygen (0.1 SLM to 0.5 SLM) and He (0.5 SLM to 1.0 SLM) is introduced into the glass pipe, and the surface temperature is increased to 2000°C while the absolute pressure inside the glass pipe is reduced to 97 kPa or less. ℃ The glass pipe and the glass rod are integrated at a temperature of 2300°C or higher. As a result, an integrated rod is formed by integrating the glass pipe and the glass rod. The diameter of the integrated rod is 20 mm or higher and 40 mm or lower. The average mass fraction of the alkali element group contained in the integrated rod is 0.2 ppm or higher and 300 ppm or lower. This makes it possible to suppress transmission loss.

[0047] In the drawing grinding process S6, the integrated rod is drawn to a diameter of 20 mm to 25 mm, and the outer periphery of the integrated rod is further ground to a diameter of 15 mm to 20 mm, thereby obtaining a core rod that constitutes the core region 10 (see FIG. 2) of the optical fiber preform 1.

[0048] As a modified example, the core portion forming step S10 may further include a glass layer providing step of providing a glass layer around the integrated rod after the elongation grinding step S6. In this case, the glass layer and the integrated rod are used together as the core rod that constitutes the core portion 10 (see FIG. 2) of the optical fiber preform 1. The glass layer is provided by a known method such as the OVD method or the collapse method.

[0049] The glass layer has a refractive index higher than that of the cladding portion 20 (first cladding portion 21 and second cladding portion 22; see FIG. 2). The glass layer does not contain an alkali element group. The glass layer contains chlorine. The average chlorine concentration of the glass layer is 100 ppm or more and 2000 ppm or less.

[0050] In the rod-in-collapse process S7, the first cladding portion 21 (see FIG. 2) is provided on the outside of the core portion 10. The rod-in-collapse method is used here, in which the core portion 10 is inserted into a glass pipe made of fluorine-doped silica-based glass, and the two are heated and integrated by an external heat source. The difference in refractive index between the core portion 10 and the first cladding portion 21, normalized to the refractive index of pure silica glass, is a maximum of approximately 0.34%. Adding the first cladding portion 21 using this rod-in-collapse method makes it possible to sufficiently reduce the moisture content of the core portion 10 and the first cladding portion 21 in its vicinity.

[0051] In the OVD process S8, a rod formed by integrating the core portion 10 and the first cladding portion 21 is stretched to a predetermined diameter, and then a second cladding portion 22 (see Figure 2) containing fluorine is synthesized on the outside of the rod by the OVD method to produce an optical fiber preform 1.

[0052] In the drawing step S9, an optical fiber can be obtained by drawing the optical fiber preform 1. The drawing speed is 800 m / min or more and 2300 m / min or less, and the drawing tension is, for example, 0.5 N.

[0053] FIG. 2 is a cross-sectional view of an optical fiber preform according to this embodiment. As shown in FIG. 2, the optical fiber preform 1 includes a core 10 including a central axis C and a cladding 20. The core 10 contains alkali elements, chlorine, and fluorine. This reduces the viscosity of the core during drawing and promotes glass rearrangement. This reduces the transmission loss caused by Rayleigh scattering in the optical fiber, thereby enabling a reduction in transmission loss. The core 10 contains any one of sodium, potassium, rubidium, cesium, and calcium as the alkali elements. The average concentration of the alkali elements in the core 10 is 3 ppm or more and 200 ppm or less. The average concentration of chlorine in the core 10 is 30 ppm or more and 2000 ppm or less. The average concentration of fluorine in the core 10 is 500 ppm or more and 5000 ppm or less.

[0054] The cladding portion 20 is provided outside the core portion 10 and surrounds the core portion 10. 10 The cladding portion 20 has a refractive index lower than that of the core portion 10. The cladding portion 20 has a first cladding portion 21 and a second cladding portion 22. The first cladding portion 21 is provided outside the core portion 10 and surrounds the core portion 10. The first cladding portion 21 is made of silica-based glass. The first cladding portion 21 contains fluorine. The difference in refractive index between the core portion 10 and the first cladding portion 21 normalized to the refractive index of pure silica glass is at most about 0.34%.

[0055] The second cladding portion 22 is provided outside the first cladding portion 21 and surrounds the first cladding portion 21. The second cladding portion 22 is made of silica-based glass. The second cladding portion 22 contains fluorine. The difference in refractive index between the first cladding portion 21 and the second cladding portion 22, normalized to the refractive index of pure silica glass, is approximately 0.05% to 0.2%.

[0056] FIG. 3 is a graph showing an example of the alkali element concentration distribution and chlorine concentration distribution in the core portion. The horizontal axis indicates the distance (radial position) from the central axis C of the core portion 10. The vertical axis indicates the alkali element concentration or chlorine concentration. The lowest part of the chlorine concentration distribution is the boundary between the glass rod and the glass pipe. On the pipe side of the boundary, there is a region where alkali elements are added. Furthermore, in order to simultaneously suppress crystallization and glass defects, the chlorine concentration is high in the center of the rod where alkali elements are not added, and the chlorine concentration in other parts, including the pipe portion, is lower than that of the rod center.

[0057] The concentration of the alkali element group in the core portion 10 has a maximum value at a position other than the central axis C. The alkali element group in the core portion 10 is added to the inner surface of the glass pipe in the adding step S2. Therefore, the position of the maximum value is a position corresponding to the inner peripheral portion of the glass pipe used in the integrating step S5. The concentration of the alkali element group in the core portion 10 has a maximum value at a position other than the central axis C in a region within 50% of the radius of the core portion 10. The concentration of the alkali element group in the core portion 10 may also have a maximum value at a position other than the central axis C in a region within 30% of the radius of the core portion 10.

[0058] Table 1 summarizes the average chlorine concentration in the outer peripheral portion of the glass rod used in the integration step S5, with the post-integration state (number of defects) for prototypes 1 to 8 of integrated rods (integrated rods immediately after the integration step S5 and before the elongation grinding step S6) manufactured by the above-mentioned manufacturing method. In the integration step S5, the glass rod comes into contact with the alkali elements added to the inner peripheral surface of the glass pipe. Therefore, a high chlorine concentration in the outer peripheral portion of the glass rod increases the incidence of defects (defects). When the chlorine mass fraction reaches 2500 ppm, the increase in the incidence of defects becomes significant, and the number of defects increases.

[0059] [Table 1]

[0060] In the samples 1 to 8, potassium (K) was used as a dopant of the alkali elements in the doping step S2, and the local K concentration added to the inner surface of the glass pipe was standardized to a range of 100 ppm to 200 ppm. Therefore, the increase in the number of abnormalities is considered to be due to the chlorine concentration on the outer periphery of the glass rod, rather than the influence of the K concentration.

[0061] Table 2 summarizes the properties of prototypes 9 to 13 of the integrated rods manufactured using the above-mentioned manufacturing method. In prototypes 9 to 13, the size and composition of the integrated rods were all the same. It can be seen that increasing the average chlorine concentration of the integrated rod reduces transmission loss. This is thought to be because the chlorine contained in the integrated rod repairs structural defects in the glass that occur during drawing, thereby reducing transmission loss caused by defects. Furthermore, in prototypes 9 to 13, the average chlorine concentration in the outer periphery of the 0.5 mm thick glass rod was standardized to approximately 1000 ppm, which reduced the number of defects after integration. [Table 2]

[0062] Table 3 summarizes the properties of the integrated rods manufactured by the above-mentioned manufacturing method, Samples 14 to 19. It can be seen that increasing the average fluorine concentration of the glass rod increases the number of base material defects (number of defects). Therefore, the average fluorine concentration of the glass rod is preferably 7000 ppm or less, and more preferably 5500 ppm or less.

[0063] [Table 3] [Explanation of symbols]

[0064] 1...Optical fiber preform 10...Core part 20...Clad section 21...First cladding section 22...Second clad section C…Central axis

Claims

1. A method for manufacturing an optical fiber preform made of silica-based glass, comprising: forming a core portion; forming a clad portion having a refractive index lower than that of the core portion and surrounding the core portion; The forming of the core portion includes: adding an alkali element group consisting of alkali metal elements and alkaline earth metal elements to the inner surface of a glass pipe made of silica-based glass; After the adding step, integrating the glass pipe with a glass rod disposed in the glass pipe to form an integrated rod; the mass fraction of chlorine in the core portion is highest at the center of the glass rod and lowest at the boundary between the glass rod and the glass pipe; a region doped with the alkali elements exists on the glass pipe side of the boundary of the core portion; A method for manufacturing an optical fiber preform.

2. forming the core portion further includes reducing the diameter of the glass pipe between the adding and the integrating. The method for manufacturing an optical fiber preform according to claim 1 .

3. The forming of the core portion further includes etching the inner surface of the glass pipe between the adding and the integrating.

3. The method for manufacturing an optical fiber preform according to claim 1.

4. the average mass fraction of chlorine in the glass rod is 20 ppm or more and 2000 ppm or less; The method for manufacturing the optical fiber preform according to any one of claims 1 to 3.

5. the average mass fraction of chlorine in the glass pipe is 20 ppm or more and 2000 ppm or less; The method for manufacturing the optical fiber preform according to any one of claims 1 to 4.

6. the average mass fraction of fluorine in the glass rod is 200 ppm or more and 5000 ppm or less; The method for manufacturing the optical fiber preform according to any one of claims 1 to 5.

7. the average mass fraction of fluorine in the glass pipe is 200 ppm or more and 5000 ppm or less; The method for manufacturing the optical fiber preform according to any one of claims 1 to 6.

8. the glass rod has an outer periphery having a thickness of 0.5 mm, including the outer periphery of the glass rod; the average mass fraction of chlorine in the outer circumferential portion is lower than the average mass fraction of chlorine in the entire glass rod; The method for manufacturing the optical fiber preform according to any one of claims 1 to 7.

9. the average mass fraction of chlorine in the outer periphery is 20 ppm or more and 2000 ppm or less; The method for manufacturing an optical fiber preform according to claim 8.

10. forming the core portion further includes applying a glass layer around the integrated rod having a refractive index higher than that of the cladding portion; The method for manufacturing an optical fiber preform according to any one of claims 1 to 9.

11. the average mass fraction of chlorine in the glass layer is 100 ppm or more and 2000 ppm or less; The method for manufacturing an optical fiber preform according to claim 10.

12. the average mass fraction of the alkali element group contained in the integrated rod is 0.2 ppm or more and 300 ppm or less; The method for manufacturing an optical fiber preform according to any one of claims 1 to 11.

13. the mass fraction of the alkali element group in the integrated rod has a maximum value other than at the central axis of the integrated rod; The method for manufacturing an optical fiber preform according to any one of claims 1 to 12.

14. the core portion contains any one of sodium, potassium, rubidium, cesium, and calcium as the alkali element group; The method for manufacturing an optical fiber preform according to any one of claims 1 to 13.

15. An optical fiber preform made of silica-based glass, a core portion including an alkali element group consisting of alkali metal elements and alkaline earth metal elements; a clad portion having a refractive index lower than that of the core portion and surrounding the core portion, the core portion includes a region having a mass fraction of chlorine that is lower than the mass fraction of chlorine on a central axis of the core portion, the mass fraction of the alkali element group in the core portion has a maximum value other than at the central axis, the region of the core portion containing the alkali element group is a portion of the core portion having the lowest mass fraction of chlorine; Optical fiber base material.

16. the mass fraction of the alkali element group in the core portion has a maximum value in a region within 50% of the radius of the core portion other than the central axis; 16. The optical fiber preform according to claim 15.

17. the mass fraction of the alkali element group in the core portion has a maximum value in a region within 30% of the radius of the core portion other than the central axis; 16. The optical fiber preform according to claim 15.

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