Method for producing optical fiber preform, and optical fiber preform

Abstract

This method for producing an optical fiber preform comprises: an addition step in which one or more elements in the alkali metal group are added to the inner surface of a first glass pipe; a collapse step in which the first glass pipe after the addition step is made solid by means of heating, thereby obtaining a glass rod; and a rod-in collapse step in which a rod comprising the glass rod is inserted into a second glass pipe, and the rod and the second glass pipe are integrated with each other by means of heating. The collapse step and the rod-in collapse step are carried out, while traversing an external heat source in a first direction or in a second direction; and in the collapse step and the rod-in collapse step, the difference between the number of times that the heat source is traversed in the first direction and the number of times that the heat source is traversed in the second direction is 1 or less.

Description

Optical fiber preform manufacturing method and optical fiber preform

[0001] The present disclosure relates to a method for manufacturing an optical fiber preform and an optical fiber preform. This application claims priority to Japanese Patent Application No. 2022-135668, filed on August 29, 2022, and incorporates the entire contents of said Japanese application by reference.

[0002] 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 caused by Rayleigh scattering in the optical fiber is reduced. As a result, the transmission loss can be reduced.

[0003] Patent Documents 1, 2, and 3 describe methods of doping an alkali metal element or an alkaline earth metal element into the core of an optical fiber preform by a diffusion method.

[0004] International Publication No. 2004 / 020357 International Publication No. 2005 / 021455 International Publication No. 2013 / 111470

[0005] A method for manufacturing an optical fiber preform according to one aspect of the present disclosure includes: an adding step of adding one or more elements from an alkali metal group consisting of alkali metal elements and alkaline earth metal elements to the inner surface of a first glass pipe made of silica-based glass; a collapsing step of heating the first glass pipe after the adding step to solidify it into a glass rod; and one or more rod-in-collapse steps of inserting a rod containing a glass rod into a second glass pipe and integrating the rod and the second glass pipe by heating, wherein the collapse step and the one or more rod-in-collapse steps are performed while an external heat source traverses in a first direction from a first end of the glass rod to a second end, or in a second direction from the second end to the first end, and the difference between the number of traverses in the first direction and the number of traverses in the second direction in the collapse step and the one or more rod-in-collapse steps is 1 or less.

[0006] Fig. 1 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber preform according to the first embodiment. Fig. 2 is a flowchart showing a method for manufacturing the optical fiber preform according to the first embodiment. Fig. 3 is a diagram illustrating an addition step. Fig. 4 is a cross-sectional view along the longitudinal direction of the optical fiber preform according to the first embodiment. Fig. 5 is a cross-sectional view along the longitudinal direction of the optical fiber preform according to a first comparative example. Fig. 6 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber preform according to the second embodiment. Fig. 7 is a flowchart showing a method for manufacturing the optical fiber preform according to the second embodiment.

[0007] [Problem to be Solved by the Present Disclosure] In the above-described method for manufacturing an optical fiber preform, collapse may be performed multiple times, including when manufacturing a core rod doped with an alkali metal element or an alkaline earth metal element. The viscosity of the glass portion doped with an alkali metal element or an alkaline earth metal element (hereinafter referred to as the alkali-doped portion) decreases. When collapse is performed, the alkali-doped portion may be crushed by the application of an external force due to its low viscosity. This may cause the diameter of the alkali-doped portion to increase or decrease depending on the location in the longitudinal direction. In particular, the increase in the diameter of the alkali-doped portion may be significant at the end portion of the collapse traverse.

[0008] When the diameter of the alkali-doped portion increases or decreases, the concentration of alkali metal elements or alkaline earth metal elements in the cross section of the fiber increases or decreases. Rayleigh scattering loss depends on the concentration of alkali metal elements or alkaline earth metal elements. Therefore, if the concentration of alkali metal elements or alkaline earth metal elements cannot be controlled, the transmission loss cannot be controlled with high precision, which may increase the risk of defects.

[0009] An object of the present disclosure is to provide a method for manufacturing an optical fiber preform and an optical fiber preform capable of suppressing fluctuations in the concentration of an alkali metal group in the longitudinal direction.

[0010] Effect of the Present Disclosure According to the present disclosure, it is possible to provide a method for manufacturing an optical fiber preform and an optical fiber preform capable of suppressing fluctuations in the concentration of an alkali metal group in the longitudinal direction.

[0011] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be described. (1) A method for producing an optical fiber preform according to one aspect of the present disclosure includes: a doping step of doping one or more elements from an alkali metal group consisting of alkali metal elements and alkaline earth metal elements onto the inner surface of a first glass pipe made of silica-based glass; a collapse step of heating the first glass pipe after the doping step to solidify it to obtain a glass rod; and one or more rod-in-collapse steps of inserting a rod containing a glass rod into a second glass pipe and integrating the rod and the second glass pipe by heating, wherein the collapse step and the one or more rod-in-collapse steps are performed while an external heat source traverses the glass rod in a first direction from the first end toward the second end or in a second direction from the second end toward the first end, and the difference between the number of traverses in the first direction and the number of traverses in the second direction in the collapse step and the one or more rod-in-collapse steps is 1 or less. In this method for manufacturing an optical fiber preform, it is possible to suppress fluctuations in the diameter of the alkali-added portion in the longitudinal direction, and therefore to suppress fluctuations in the concentration of the alkali metal group in the longitudinal direction.

[0012] (2) In the above (1), in the collapse step and the one or more rod-in collapse steps, the traverse in the first direction and the traverse in the second direction may be alternately performed, which can reliably suppress fluctuations in the diameter of the alkali-added portion in the longitudinal direction.

[0013] (3) In the method for manufacturing a multi-core optical fiber preform having a plurality of core portions in the above (1) or (2), in the collapse step and the one or more rod-in collapse steps, the difference between the number of traverses in the first direction and the number of traverses in the second direction for each of the plurality of core portions may be 1 or less. In this case, it is possible to suppress a variation in the diameter of the alkali-added portion in the longitudinal direction for each core portion. Therefore, when the preform is made into a fiber, it is possible to suppress a variation in transmission loss in the longitudinal direction for each core.

[0014] (4) The method for manufacturing an optical fiber preform according to any one of (1) to (3) above may further include a step of applying a glass layer to the outside of the rod including the glass rod by a method other than the rod-in-collapse method. In this case, since the rod-in-collapse method is not used, the diameter of the alkali-added portion is less likely to vary in the longitudinal direction.

[0015] (5) The method for manufacturing an optical fiber preform according to any one of (1) to (3) above may further include a step of applying a glass layer to the outside of the rod including the glass rod by the OVD method or the VAD method. In this case, unlike the rod-in-collapse method, the diameter of the alkali-added portion is less likely to vary in the longitudinal direction.

[0016] (6) In any of the above (1) to (5), the doping step may include doping at least one element selected from the group consisting of sodium, potassium, rubidium, and cesium as an alkali metal. In this case, the transmission loss of the optical fiber caused by Rayleigh scattering is reliably reduced.

[0017] (7) An optical fiber preform according to an aspect of the present disclosure is an optical fiber preform doped with one or more elements from an alkali metal group consisting of alkali metal elements and alkaline earth metal elements, the optical fiber preform having longitudinal ends and a central portion, and a difference in the concentration of the element at the ends and the central portion being less than 15% by mass fraction. In this optical fiber preform, it is possible to suppress fluctuations in the concentration of the alkali metal group in the longitudinal direction.

[0018] (8) In the above (7), the concentration of the alkali metal group at the end portion may be higher than that at the center portion. In this case, the transmission loss at the end portion is equal to or lower than that at the center portion. Therefore, there is little risk of a decrease in manufacturing yield due to an increase in transmission loss during mass production.

[0019] [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.

[0020] First Embodiment Fig. 1 is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber preform according to a first embodiment. As shown in Fig. 1, an optical fiber preform 10 according to the first embodiment includes a core 11, a first cladding 12, and a second cladding 13. The first cladding 12 and the second cladding 13 form a cladding 14.

[0021] The core region 11 is made of silica-based glass. The core region 11 contains one or more elements from an alkali metal group consisting of alkali metal elements and alkaline earth metal elements, chlorine, and fluorine. The alkali metal group includes, for example, sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and calcium (Ca). The core region 11 contains, as the alkali metal group, at least one element from the alkali metal group, for example, sodium, potassium, rubidium, and cesium. The concentrations of other dopants and impurities contained in the core region 11 are 10 ppm by mass fraction or less.

[0022] The proportion of silica glass, which is the main component of silica-based glass, may be 50% or more, 90% or more, 95% or more, 98% or more, or 99% or more by mass ratio, where mass ratio means mass fraction.

[0023] The first cladding portion 12 is provided outside the core portion 11 and surrounds the core portion 11. The first cladding portion 12 is made of silica-based glass. The first cladding portion 12 contains fluorine. The refractive index of the first cladding portion 12 is lower than the refractive index of the core portion 11.

[0024] The second cladding portion 13 is provided outside the first cladding portion 12 and surrounds the first cladding portion 12. The second cladding portion 13 is made of silica-based glass. The second cladding portion 13 contains fluorine. The refractive index of the second cladding portion 13 is lower than the refractive index of the core portion 11 and higher than the refractive index of the first cladding portion 12.

[0025] 2 is a flowchart showing a method for manufacturing an optical fiber preform according to the first embodiment. As shown in FIG. 2, the method for manufacturing an optical fiber preform 10 according to the first embodiment includes a preparation step S1, an addition step S2, a diameter reduction step S3, an etching step S4, a collapse step S5, a first stretch grinding step S6, a rod-in collapse step S7, a second stretch grinding step S8, and an OVD (Outside Vapor Deposition) step S9. The optical fiber preform 10 is manufactured through these steps S1 to S9. Furthermore, an optical fiber is manufactured by performing a drawing step (not shown).

[0026] In the preparation step S1, a glass pipe 1 (see FIG. 2) made of silica-based glass into which a dopant such as an alkali metal group is to be diffused is prepared. The glass pipe 1 contains a certain concentration of chlorine and fluorine, and the mass fraction of other dopants and impurities is 10 ppm or less (hereinafter, mass fraction is referred to as "concentration"). The outer diameter of the glass pipe 1 is 30 mm to 50 mm, and the inner diameter is 10 mm to 30 mm.

[0027] The glass pipe 1 contains chlorine at an average concentration of 0 ppm to 1500 ppm and fluorine at an average concentration of 500 ppm to 5000 ppm. Here, the average concentration, for example, the average chlorine concentration, is the concentration expressed by the following formula: Cl(r) represents the local chlorine concentration at a position of radius r. i represents the inner diameter of the glass pipe 1, and d represents the outer diameter of the glass pipe 1. Fluorine and other dopants are calculated in a similar manner. In the case of a glass rod, calculations are made assuming i to be 0 and d to be the outer diameter of the glass rod.

[0028] The local concentration was measured as follows: The chlorine concentration was measured at each position along a line passing through the center of the end face of the glass pipe 1 and the glass rod using an electron probe micro analyzer (EPMA). The EPMA measurement conditions were, for example, an acceleration voltage of 20 kV, a probe beam diameter of 1 μm or less, and a measurement interval of 100 nm or less.

[0029] In the doping step S2, one or more elements from an alkali metal group are doped as dopants onto the inner surface of the glass pipe 1 (first glass pipe) made of silica-based glass. In the doping step S2, at least one of sodium, potassium, rubidium, and cesium is doped as the alkali metal. Here, the doping of potassium (K) is described. For example, 6 g to 20 g of potassium bromide (KBr) is used as a raw material. Depending on the type of alkali metal to be doped, one or more of KBr, KI, RbBr, RbI, etc. may be used as the raw material.

[0030] Fig. 3 is a diagram illustrating the doping step. As shown in Fig. 3, a handling glass pipe 5 disposed in an electric furnace 2 is connected to one end of a glass pipe 1. A part of the handling glass pipe 5 is used as a raw material reservoir, and raw materials 3 are placed therein. A part of the glass pipe 1 may also be used as the raw material reservoir. An oxyhydrogen burner 4 is disposed outside the glass pipe 1. The electric furnace 2 is an external heat source for heating the raw materials 3. The oxyhydrogen burner 4 is an external heat source for heating the glass pipe 1. An induction furnace, a resistance furnace, or the like may be used instead of the oxyhydrogen burner 4.

[0031] The raw material 3 is heated to a temperature of 700°C to 850°C in an electric furnace 2 to generate raw material vapor. The generated raw material vapor is introduced into the glass pipe 1 together with a carrier gas consisting of oxygen, while the glass pipe 1 is heated from the outside by an oxyhydrogen burner 4. The flow rate of the carrier gas is 1 SLM to 3 SLM (1 liter / min converted to standard conditions (25°C, 100 kPa)). The glass pipe 1 is heated by traversing the oxyhydrogen burner 4 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 1 reaches 1400°C to 2000°C. This allows potassium to be diffused and added to the inner surface of the glass pipe 1.

[0032] In the diameter-reducing step S3, the potassium-doped glass pipe 1 is reduced in diameter. At this time, oxygen is flowed through the glass pipe 1 at a rate of 0.5 SLM to 1.0 SLM, and the glass pipe 1 is heated by an external heat source so that the outer surface of the glass pipe 1 reaches a temperature of 2000° C. to 2300° C. The external heat source is traversed for a total of 6 to 10 turns, and the glass pipe 1 is reduced in diameter until the inner diameter is 3 mm to 5 mm.

[0033] In the etching step S4, the inner surface of the glass pipe 1 is etched. ６ Vapor-phase etching is performed by heating the glass pipe 1 with an external heat source while introducing a mixed gas of chlorine (0.2 SLM to 1.0 SLM) and chlorine (0.5 SLM to 1.0 SLM) into the glass pipe 1. This allows the inner surface of the glass pipe 1, which contains a high concentration of impurities added together with the target dopant, to be polished and the impurities removed. The steps from the preparation step S1 to the etching step S4 constitute a diffusion doping step for diffusing the dopant into the glass pipe 1.

[0034] In the collapse step S5, a glass rod is obtained from the glass pipe 1 after the addition step S2 by a collapse method. That is, the glass pipe 1 after the addition step S2 is heated to be collapsed to obtain a glass rod. For example, 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 1, and the absolute pressure inside the glass pipe 1 is reduced to 97 kPa or less while the surface temperature is increased to 2000°C to 2300°C, thereby sealing and collapsing the glass pipe 1. This results in a glass rod with an outer diameter of 20 mm to 40 mm. In the collapse step S5, the glass pipe 1 is collapsed by heating while a heat source is moved. The movement of the heat source during the collapse is specifically referred to as a traverse in this specification.

[0035] In the elongation grinding step S6, the glass rod obtained in the collapse step S5 is elongated to a diameter of 20 mm to 25 mm, and the outer periphery of the glass rod is further ground to a diameter of 15 mm to 25 mm. This results in the core 11 of the optical fiber preform 10. In other words, the steps from the preparation step S1 to the elongation grinding step S6 constitute a core manufacturing step for manufacturing the core 11.

[0036] In the rod-in-collapse process S7, the first cladding 12 is provided outside the core 11 by the rod-in-collapse method. Specifically, the core 11 is inserted into a glass pipe (second glass pipe) that will become the first cladding 12, and the core 11 and the glass pipe are integrated by heating. Here, a fluorine-doped silica-based glass pipe is used. The core 11 is used as a rod including the glass rod obtained in the collapse process S5. The relative refractive index difference between the core 11 and the first cladding 12 is approximately 0.34% at most. In this embodiment, the relationship between the relative refractive index difference in the state of the optical fiber preform 10 and the state of the optical fiber is the same. Adding the first cladding 12 by the rod-in-collapse method makes it possible to sufficiently reduce the moisture content of the core 11 and the first cladding 12 in the vicinity thereof. In the rod-in-collapse process S7, the glass pipe and the glass rod are integrated as described above by collapsing the glass pipe while moving a heat source. The movement of the heat source during the collapse is specifically referred to as a traverse in this specification.

[0037] In the OVD process S8, the rod formed by integrating the core 11 and the first cladding 12 is stretched to a predetermined diameter, and then the second cladding 13, which is a glass layer containing fluorine, is synthesized on the outside of the rod by the OVD method. The OVD process S8 can be said to be a process of applying a glass layer to the outside of the glass rod obtained in the collapse process S5 by the OVD method, which is a method other than the collapse method. In this way, the optical fiber preform 10 is manufactured. A VAD (Vapor-phase Axial Deposition) method may be used instead of the OVD method.

[0038] An optical fiber can be manufactured by a drawing process of drawing the optical fiber preform 10. The drawing speed is, for example, 800 m / min or more and 2300 m / min or less. The drawing tension is, for example, 0.5 N.

[0039] Steps S5 and S7 are performed while an external heat source is traversed in a first direction from the first end to the second end of the glass rod obtained in step S5, or in a second direction from the second end to the first end of the glass rod. When the external heat source is traversed in the first direction, the first end of the glass rod becomes the starting end of the collapse, and the second end of the glass rod becomes the terminal end of the collapse. In steps S5 and S7, the starting end of the collapse differs from each other, and the terminal end of the collapse differs from each other.

[0040] In the two collapses consisting of steps S5 and S7, the difference between the number of traverses in the first direction and the number of traverses in the second direction is 1 or less. For example, in steps S5 and S7, traverses in the first direction and traverses in the second direction are alternately performed. That is, if a traverse in the first direction is performed in step S5, a traverse in the second direction is performed in step S7. If a traverse in the second direction is performed in step S5, a traverse in the first direction is performed in step S7.

[0041] The external heat source is used to heat the glass pipe 1 to become the core 11 and the glass pipe to become the first cladding 12 from the outside. The external heat source may be the same as the oxyhydrogen burner 4 used in the doping step S2. The same external heat source may be used in steps S5 and S7.

[0042] 4 is a longitudinal cross-sectional view of the optical fiber preform according to the first embodiment. As shown in FIG. 4, the optical fiber preform 10 has a first end portion 10a, a second end portion 10b, and a central portion 10c in the longitudinal direction. The central portion 10c is located between the first end portion 10a and the second end portion 10b. The first end portion 10a includes the first end of the glass rod obtained in step S5. The second end portion 10b includes the second end of the glass rod obtained in step S5.

[0043] The optical fiber preform 10 includes an alkali-added portion 20 to which one or more elements from the alkali metal group are added. Fig. 4 conceptually illustrates the alkali-added portion 20. The alkali-added portion 20 is disposed at the center of the cross section of the optical fiber preform 10 perpendicular to the longitudinal direction, and extends along the longitudinal direction.

[0044] The diameter of the alkali addition portion 20 at the first end 10a is equal to the diameter of the alkali addition portion 20 at the second end 10b. The diameters of the alkali addition portion 20 at the first end 10a and the second end 10b are equal to or longer than the diameter of the alkali addition portion 20 at the central portion 10c.

[0045] The diameter of the alkali-added portion 20 correlates with the concentration of the alkali metal group. Therefore, the concentration of the alkali metal group at the first end 10a is equivalent to the concentration of the alkali metal group at the second end 10b. The concentrations of the alkali metal group at the first end 10a and the second end 10b are equivalent to or higher than the concentration of the alkali metal group at the central portion 10c. The difference between the concentration of the alkali metal group at the first end 10a and the second end 10b and the concentration of the alkali metal group at the central portion 10c is less than 15% by mass fraction, and more preferably 5% or less.

[0046] (First Comparative Example) Fig. 5 is a longitudinal cross-sectional view of an optical fiber preform according to a first comparative example. As shown in Fig. 5, the optical fiber preform 110 according to the first comparative example differs from the optical fiber preform 10 in that it has an alkali-added portion 120 whose diameter increases from the second end portion 110b toward the first end portion 110a. The diameter of the alkali-added portion 120 at the first end portion 110a is larger than the diameter of the alkali-added portion 120 at the second end portion 110b. In other words, the concentration of the alkali metal group at the first end portion 110a is higher than the concentration of the alkali metal group at the second end portion 110b.

[0047] The manufacturing method of the optical fiber preform 110 according to the first comparative example differs from the manufacturing method according to the first embodiment in that the traversal direction of the external heat source is not reversed in steps S5 and S7. That is, in both steps S5 and S7, the traversal direction of the external heat source is the same, and the external heat source traverses in either the first direction or the second direction. In steps S5 and S7, the starting points of the collapses coincide with each other, and the ending points of the collapses coincide with each other. In this example, traversal in the first direction is performed in both steps S5 and S7.

[0048] Second Embodiment Fig. 6 is a cross-sectional view perpendicular to the longitudinal direction of an optical fiber preform according to a second embodiment. As shown in Fig. 6, an optical fiber preform 10A according to the second embodiment includes a first core region 15, a second core region 16, a first cladding region 12, and a second cladding region 13. The first core region 15 and the second core region 16 form a core region 17. The first cladding region 12 and the second cladding region 13 form a cladding region 14.

[0049] The first core region 15 has a configuration equivalent to that of the core region 11 of the optical fiber preform 10. The second core region 16 is provided outside the first core region 15 and surrounds the first core region 15. The second core region 16 is made of silica-based glass. The second core region 16 contains chlorine and fluorine, but does not contain alkali metals. The first cladding region 12 differs from the first cladding region 12 of the optical fiber preform 10 in that it is provided outside the second core region 16 and surrounds the second core region 16, but otherwise has a configuration equivalent to that of the first cladding region 12 of the optical fiber preform 10. The second cladding region 13 has a configuration equivalent to that of the second cladding region 13 of the optical fiber preform 10.

[0050] 7 is a flowchart showing a method for manufacturing an optical fiber preform according to the second embodiment. As shown in FIG. 7 , the method for manufacturing an optical fiber preform 10A according to the second embodiment includes a preparation step S11, an addition step S12, a diameter reduction step S13, an etching step S14, a collapse step S15, a first stretch-grinding step S16, a first rod-in-collapse step S17, a second stretch-grinding step S18, a second rod-in-collapse step S19, a third stretch-grinding step S20, and a third rod-in-collapse step S21. The optical fiber preform 10A is manufactured through these steps S11 to S21. Furthermore, an optical fiber is manufactured by performing a drawing step (not shown).

[0051] Steps S11 to S16 are substantially the same as steps S1 to S6 in the first embodiment, respectively, and therefore will not be described further. The first core portion 15 is obtained through steps S11 to S16.

[0052] In the first rod-in-collapse step S17, a second core portion 16 is provided outside the first core portion 15 by a rod-in-collapse method. That is, the first core portion 15 is inserted into a glass pipe (first and second glass pipes) that will become the second core portion 16, and the first core portion 15 and the glass pipe are integrated by heating. Here, a glass pipe made of silica-based glass doped with chlorine and fluorine is used. The first core portion 15 is used as a rod including the glass rod obtained in the collapse step S15.

[0053] In the second elongation grinding step S18, the glass rod obtained in the first rod-incollapse step S17 is elongated to a diameter of 20 mm to 25 mm, and the outer periphery of the glass rod is further ground to a diameter of 15 mm to 25 mm. This results in the core 17 of the optical fiber preform 10A. That is, each step from the preparation step S1 to the second elongation grinding step S18 constitutes a core manufacturing step for manufacturing the core 17.

[0054] In the second rod-in-collapse step S19, the first cladding portion 12 is provided outside the core portion 17 by the rod-in-collapse method. That is, the core portion 17 is inserted into a glass pipe (a second glass pipe) that will become the first cladding portion 12, and the core portion 17 and the glass pipe are integrated by heating. In this step, a glass pipe made of silica-based glass doped with fluorine is used. The core portion 17 is used as a rod, including the glass rod obtained in the collapse step S15.

[0055] In the third elongation grinding step S20, the glass rod obtained in the second rod incollapse step S19 is elongated to a diameter of 20 mm to 35 mm, and the outer periphery of the glass rod is further ground to a diameter of 15 mm to 25 mm.

[0056] In the third rod-in collapse step S21, a second cladding portion 13 is provided on the outside of a glass rod consisting of a core portion 17 and a first cladding portion 12 by a rod-in collapse method. That is, the glass rod consisting of the core portion 17 and the first cladding portion 12 is inserted into a glass pipe (a second glass pipe) that will become the second cladding portion 13, and the glass rod consisting of the core portion 17 and the first cladding portion 12 and the glass pipe are integrated by heating. Here, a glass pipe made of silica-based glass doped with fluorine is used. The glass rod consisting of the core portion 17 and the first cladding portion 12 is used as a rod, including the glass rod obtained in the collapse step S15. In this way, an optical fiber preform 10A is manufactured.

[0057] The optical fiber preform 10A is drawn at a drawing speed of 800 m / min to 2300 m / min, for example, and the drawing tension is 0.5 N, for example.

[0058] Steps S15, S17, S19, and S21 are performed while an external heat source is traversed in a first direction from the first end to the second end of the glass rod obtained in step S15, or in a second direction from the second end to the first end of the glass rod.

[0059] In the four collapses consisting of steps S15, S17, S19, and S21, the difference between the number of times traverses are performed in the first direction and the number of times traverses are performed in the second direction is equal to or less than 1. For example, in steps S15, S17, S19, and S21, traverses in the first direction and traverses in the second direction are performed alternately.

[0060] In the four collapses consisting of steps S15, S17, S19, and S21, the direction in which the external heat source traverses is reversed. For example, a first direction traverse is performed in step S15, a second direction traverse is performed in step S17, a first direction traverse is performed in step S19, and a second direction traverse is performed in step S21.

[0061] In the optical fiber preform 10A according to the second embodiment, an alkali-added portion 20 (see FIG. 4) with little fluctuation in outer diameter is formed, similar to the optical fiber preform 10. Therefore, in the optical fiber preform 10A, similar to the optical fiber preform 10, the concentrations of the alkali metal group in the first end portion 10a (see FIG. 4) and the second end portion 10b (see FIG. 4) are equal to or higher than the concentrations of the alkali metal group in the central portion 10c (see FIG. 4). The difference in mass fraction between the alkali metal group concentrations in the first end portion 10a and the second end portion 10b and the alkali metal group concentration in the central portion 10c is less than 15%, and more preferably 5% or less.

[0062] (Second Comparative Example) The manufacturing method according to the second comparative example differs from the manufacturing method according to the second embodiment in that the direction in which the external heat source traverses is not reversed in steps S15, S17, S19, and S21. That is, traversal is performed in the same direction in all of steps S15, S17, S19, and S21. In this example, traversal is performed in the first direction in all of steps S15, S17, S19, and S21.

[0063] In the optical fiber preform according to the second comparative example, similar to the optical fiber preform 110 according to the first comparative example, an alkali-added portion 120 (see FIG. 5) is formed, the diameter of which increases from the second end 110b (see FIG. 5) toward the first end 110a (see FIG. 5). Thus, in the optical fiber preform according to the second comparative example, similar to the optical fiber preform 110, the diameter of the alkali-added portion 120 at the first end 110a is larger than the diameter of the alkali-added portion 120 at the second end 110b. In other words, the concentration of the alkali metal group at the first end 110a is higher than the concentration of the alkali metal group at the second end 110b.

[0064] (Experimental Example) An experimental example will be described below.

[0065] In the first experimental example, an optical fiber preform was manufactured by the manufacturing method according to the first comparative example, and then a drawing process was performed to manufacture an optical fiber. That is, in the first experimental example, a total of two collapse processes, consisting of steps S5 and S7, were performed. In both steps S5 and S7, a traverse in the first direction was performed.

[0066] In the second experimental example, an optical fiber preform was manufactured by the manufacturing method according to the first embodiment, and then a drawing process was performed to manufacture an optical fiber. That is, in the second experimental example, a total of two collapses, consisting of steps S5 and S7, were performed. In step S5, a traverse in the first direction was performed, and in step S7, a traverse in the second direction was performed.

[0067] In the third experimental example, an optical fiber preform was manufactured by the manufacturing method according to the second comparative example, and then a drawing process was carried out to manufacture an optical fiber. That is, in the third experimental example, a total of four collapse processes, consisting of steps S15, S17, S19, and S21, were carried out. In each of steps S15, S17, S19, and S21, a traverse in the first direction was carried out.

[0068] In the fourth experimental example, an optical fiber preform was manufactured by the manufacturing method according to the second embodiment, and then a drawing process was performed to manufacture an optical fiber. That is, in the fourth experimental example, a total of four collapses were performed, consisting of steps S15, S17, S19, and S21. In steps S15 and S19, a traverse in the first direction was performed. In steps S17 and S21, a traverse in the second direction was performed.

[0069] The optical fiber according to each experimental example has an effective area (Aeff) of 105 μm ２ 115 μm or more ２ Here, the fibers were manufactured so that the cutoff wavelength λc was 1400 nm or more and 1520 nm or less, and the relative refractive index difference (relative relative refractive index difference) between the core and the cladding was 0.34%±0.01%. Here, the "relative refractive index difference between the core and the cladding" refers to the relative refractive index difference between the core and the first cladding in the first and second experimental examples, and refers to the relative refractive index difference between the second core and the first cladding in the third and fourth experimental examples.

[0070] The potassium concentration at the first end, the center, and the second end of the optical fiber preforms according to each experimental example was measured using the EPMA described above. The transmission loss at a wavelength of 1550 nm was measured for the optical fibers drawn from the first end, the center, and the second end of the optical fiber preforms according to each experimental example.

[0071] Table 1 is a table summarizing the specifications and conditions of the optical fiber preforms and optical fibers in each experimental example.

[0072] In the first and third experimental examples, the K concentration decreases from the first end to the second end. As a result, the transmission loss increases from the first end to the second end. In the second and fourth experimental examples, the traverse direction, i.e., the collapse direction, is reversed, so the K concentration is approximately constant throughout the entire longitudinal direction. As a result, the transmission loss is stable throughout the entire longitudinal direction.

[0073] As described above, in the manufacturing method according to the embodiment, a collapse step and one or more rod-in collapse steps are performed, and in the collapse step and one or more rod-in collapse steps, the difference between the number of traverses in the first direction and the number of traverses in the second direction is 1 or less. This suppresses the diameter of the alkali-added section 120 from increasing or decreasing in the longitudinal direction. Therefore, fluctuations in the concentration of the alkali metal group in the longitudinal direction are suppressed. If the concentration of the alkali metal group is increased or the diameter of the alkali-added section 120 is increased, the concentration of the alkali metal group is likely to fluctuate in the longitudinal direction. While lowering the collapse temperature is also conceivable, this may result in insufficient melting of the interface, which may result in manufacturing defects. Therefore, the manufacturing method according to the embodiment, in which the collapse direction is reversed, is effective.

[0074] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments and modifications, and various modifications are possible without departing from the spirit of the present disclosure.

[0075] The optical fiber preform 10 may be a multi-core optical fiber preform having a plurality of cores. In this case, in steps S5 and S7, the difference between the number of traverses in the first direction and the number of traverses in the second direction for each of the plurality of cores is 1 or less. This makes it possible to suppress fluctuations in the concentration of the alkali metal group in the longitudinal direction for each core.

[0076] The optical fiber preform 10A may be a multi-core optical fiber preform having a plurality of cores. In this case, in steps S15, S17, S19, and S21, the difference between the number of traverses in the first direction and the number of traverses in the second direction for each of the plurality of cores is 1 or less. This makes it possible to suppress the variation in diameter of the alkali-added portion 120 in the longitudinal direction for each core. Therefore, when the fiber is fabricated, it is possible to suppress the variation in transmission loss in the longitudinal direction for each core.

[0077] The optical fiber preform 10, 10A may not include the second cladding portion 13. That is, the manufacturing method of the optical fiber preform 10 may not include the OVD step S9. The manufacturing method of the optical fiber preform 10A may not include the third rod-in collapse step S21. The optical fiber preform 10, 10A may further include one or more glass layers provided on the outside of the second cladding portion 13. That is, the manufacturing method of the optical fiber preform 10, 10A may further include a step of providing a glass layer on the outside of the second cladding portion 13 by a known method such as a rod-in collapse method, an OVD method, or a VAD method. Even in these cases, the manufacturing method of the optical fiber preform 10, 10A includes a collapse step and one or more rod-in collapse steps, and the difference between the number of traverses in the first direction and the number of traverses in the second direction in the collapse step and the one or more rod-in collapse steps may be one or less.

[0078] The optical fiber preform 10 may further include one or more glass layers provided outside the core region 11 and inside the first cladding region 12. That is, the manufacturing method of the optical fiber preform 10 may further include a step of providing a glass layer outside the core region 11 and inside the first cladding region 12 by a known method such as a rod-in-collapse method, an OVD method, or a VAD method. The optical fiber preform 10A may further include one or more glass layers provided outside the second core region 16 and inside the first cladding region 12. That is, the manufacturing method of the optical fiber preform 10A may further include a step of providing a glass layer outside the second core region 16 and inside the first cladding region 12 by a known method such as a rod-in-collapse method, an OVD method, or a VAD method. Even in these cases, in the manufacturing method of the optical fiber preform 10, 10A, a collapse process and one or more rod-in collapse processes are performed, and in the collapse process and one or more rod-in collapse processes, the difference between the number of traverses performed in the first direction and the number of traverses performed in the second direction may be 1 or less.

[0079] REFERENCE SIGNS LIST 1...glass pipe 2...electric furnace 3...raw material 4...oxyhydrogen burner 5...handling glass pipe 10, 10A...optical fiber preform 10a...first end 10b...second end 10c...center 11...core 12...first cladding 13...second cladding 14...cladding 15...first core 16...second core 17...core 110...optical fiber preform 110a...first end 110b...second end

Claims

1. an addition step of adding one or more elements selected from an alkali metal group consisting of alkali metal elements and alkaline earth metal elements to an inner surface of a first glass pipe consisting of silica-based glass; a collapsing step of heating the first glass pipe after the adding step to obtain a glass rod; one or more rod-in-collapse steps of inserting a rod including the glass rod into a second glass pipe and integrating the rod and the second glass pipe by heating; The collapse step and one or more of the rod-in collapse steps are performed while traversing an external heat source in a first direction from a first end of the glass rod to a second end thereof or in a second direction from the second end to the first end thereof; In the collapse step and the one or more rod-in collapse steps, a difference between the number of times traverses are performed in the first direction and the number of times traverses are performed in the second direction is 1 or less. A method for manufacturing an optical fiber preform.

2. In the collapse step and one or more of the rod-in collapse steps, the traverse in the first direction and the traverse in the second direction are alternately performed. The method for producing the optical fiber preform according to claim 1 .

3. A method for manufacturing a multi-core optical fiber preform having a plurality of cores, comprising the steps of: In the collapse step and one or more of the rod-in collapse steps, a difference between the number of times that traverses are performed in the first direction and the number of times that traverses are performed in the second direction for each of the plurality of core portions is 1 or less.

3. The method for producing an optical fiber preform according to claim 1 or 2.

4. The method further includes a step of providing a glass layer on the outside of the rod including the glass rod by a method other than the rod-in-collapse method.

3. The method for producing an optical fiber preform according to claim 1 or 2.

5. The method further comprises the step of applying a glass layer to the outside of the rod including the glass rod by an OVD method or a VAD method; 3. The method for producing an optical fiber preform according to claim 1 or 2.

6. In the addition step, at least one element selected from the group consisting of sodium, potassium, rubidium, and cesium is added as an alkali metal.

3. The method for producing an optical fiber preform according to claim 1 or 2.

7. An optical fiber preform doped with one or more elements selected from an alkali metal group consisting of alkali metal elements and alkaline earth metal elements, having longitudinal ends and a central portion, The difference between the concentration of the element at the end portion and the concentration of the element at the center portion is less than 15% by mass. Optical fiber base material.

8. the concentration of the alkali metal group at the end portion is higher than the concentration of the alkali metal group at the central portion; 8. The optical fiber preform according to claim 7.

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