Method for producing optical fiber preform, and optical fiber preform

Abstract

A method for manufacturing an optical fiber preform includes: an addition step of adding one or more elements from an alkali metal group to an inner surface of a first glass tube; a collapsing step of solidifying the first glass tube after the adding step by heating to obtain a glass rod; and a sleeve collapsing step of inserting a rod including the glass rod into the second glass tube, integrating the rod and the second glass tube by heating, and performing a collapsing step and a sleeve collapsing step while traversing the external heat source in the first direction or the second direction, the difference between the number of traverse movements in the first direction and the number of traverse movements in the second direction is 1 or less.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing an optical fiber preform and an optical fiber preform. This application claims priority based on Japanese application No. 2022-135668 filed on August 29, 2022, and all the contents described in the Japanese application are incorporated herein by reference. Background Art

[0002] If the core composed of silica-based glass contains an alkali metal element or an alkaline earth metal element, the viscosity of the core is reduced when the optical fiber base material is drawn to produce an optical fiber, and the rearrangement of the glass is promoted. Therefore, the transmission loss caused by Rayleigh scattering of the optical fiber is reduced. As a result, the transmission loss can be reduced.

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

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: International Publication No. 2004 / 020357

[0007] Patent Document 2: International Publication No. 2005 / 021455

[0008] Patent Document 3: International Publication No. 2013 / 111470 Summary of the invention

[0009] One aspect of the present disclosure relates to a method for manufacturing an optical fiber mother material, including: an adding step of adding one or more elements of an alkali metal group consisting of alkali metal elements and alkaline earth metal elements to the inner surface of a first glass tube consisting of a silica-based glass; a collapsing step of solidifying the first glass tube after the adding step by heating to obtain a glass rod; and one or more sleeve collapsing steps of inserting a rod including the glass rod into a second glass tube, integrating the rod and the second glass tube by heating, and performing the collapsing step and the one or more sleeve collapsing steps while traversing an external heat source in a first direction from a first end of the glass rod toward a second end or in a second direction from the second end toward the first end. In the collapsing step and the one or more sleeve collapsing steps, the difference between the number of traverses in the first direction and the number of traverses in the second direction is less than 1. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber preform according to the first embodiment.

[0011] Figure 2This is a flowchart showing the method for manufacturing the optical fiber preform according to the first embodiment.

[0012] Figure 3 This is a diagram for explaining the adding process.

[0013] Figure 4 It is a cross-sectional view along the longitudinal direction of the optical fiber preform according to the first embodiment.

[0014] Figure 5 It is a cross-sectional view along the longitudinal direction of the optical fiber preform according to the first comparative example.

[0015] Figure 6 It is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber preform according to the second embodiment.

[0016] Figure 7 This is a flowchart showing a method for manufacturing an optical fiber preform according to the second embodiment. DETAILED DESCRIPTION

[0017] [Technical Problems to be Solved by the Present Disclosure]

[0018] In the above-mentioned method for manufacturing the optical fiber mother material, multiple collapses are sometimes performed, including when manufacturing a core rod to which an alkali metal element or an alkaline earth metal element is added. In the glass portion to which an alkali metal element or an alkaline earth metal element is added (hereinafter referred to as the alkali-added portion), the viscosity decreases. In the case of collapse, the alkali-added portion is sometimes crushed by the application of an external force due to its low viscosity. As a result, the diameter of the alkali-added portion sometimes increases or decreases depending on the position in the longitudinal direction. In particular, at the terminal portion during the lateral movement of the collapse, the increase in the diameter of the alkali-added portion sometimes becomes significant.

[0019] If the diameter of the alkali-doped portion increases or decreases, the concentration of the alkali metal element or alkaline earth metal element in the cross section increases or decreases when the fiber is formed. Rayleigh scattering loss depends on the concentration of the alkali metal element or alkaline earth metal element. Therefore, if the concentration of the alkali metal element or alkaline earth metal element cannot be controlled, the transmission loss cannot be controlled with high accuracy, and there is a possibility that the number of failures will increase.

[0020] 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 the concentration variation of an alkali metal group in the longitudinal direction.

[0021] [Effects of the present disclosure]

[0022] 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 the concentration variation of the alkali metal group in the longitudinal direction.

[0023] [Description of the implementation aspects of the present disclosure]

[0024] First, the implementation aspects of the present disclosure are listed for explanation. (1) A method for manufacturing an optical fiber base material according to one aspect of the present disclosure includes: an adding step of adding one or more elements of an alkali metal group consisting of alkali metal elements and alkaline earth metal elements to the inner surface of a first glass tube consisting of a silica-based glass; a collapsing step of solidifying the first glass tube after the adding step by heating to obtain a glass rod; and one or more sleeve collapsing steps of inserting a rod including the glass rod into a second glass tube, integrating the rod and the second glass tube by heating, and performing the collapsing step and the one or more sleeve collapsing steps while traversing an external heat source in a first direction from a first end toward a second end of the glass rod or in a second direction from the second end toward the first end, wherein the difference between the number of traversing in the first direction and the number of traversing in the second direction in the collapsing step and the one or more sleeve collapsing steps is 1 or less. In the method for manufacturing an optical fiber base material, the diameter variation of the alkali-added portion in the longitudinal direction can be suppressed. Therefore, the concentration variation of the alkali metal group in the longitudinal direction can be suppressed.

[0025] (2) In the above (1), in the collapsing step and in one or more sleeve collapsing steps, the traverse in the first direction and the traverse in the second direction may be performed alternately. In this case, the diameter variation of the alkali added portion in the longitudinal direction can be reliably suppressed.

[0026] (3) In the above (1) or (2), in the method for manufacturing a multi-core optical fiber preform having a plurality of cores, in the collapsing step and the one or more sleeve collapsing steps, the difference between the number of traverses performed in the first direction and the number of traverses performed in the second direction for each of the plurality of cores may be 1 or less. In this case, the diameter variation of the alkali-doped portion in the longitudinal direction can be suppressed for each core. Therefore, when the fiber is formed into an optical fiber, the variation of the transmission loss in the longitudinal direction can be suppressed for each core.

[0027] (4) The method for manufacturing an optical fiber preform according to any one of (1) to (3) above may further include a step of imparting a glass layer to the outer side of the rod including the glass rod by a method other than the tube collapse method. In this case, since the tube collapse method is not used, the diameter of the alkali-added portion is less likely to vary in the longitudinal direction.

[0028] (5) The method for manufacturing an optical fiber preform according to any one of (1) to (3) above may further include a step of imparting a glass layer to the outer side of the rod including the glass rod by an OVD method or a VAD method. In this case, unlike the sleeve collapse method, the diameter of the alkali-doped portion is less likely to vary in the longitudinal direction.

[0029] (6) In any of the above (1) to (5), at least one element selected from sodium, potassium, rubidium, and cesium may be added as an alkali metal group in the adding step. In this case, the transmission loss caused by Rayleigh scattering of the optical fiber can be reliably reduced.

[0030] (7) An optical fiber base material according to one aspect of the present disclosure is doped with one or more elements of an alkali metal group consisting of alkali metal elements and alkaline earth metal elements, the optical fiber base material having end portions and a central portion in a longitudinal direction, the difference between the concentration of the element at the end portions and the concentration of the element at the central portion being less than 15% by mass fraction. In the optical fiber base material, the concentration variation of the alkali metal group in the longitudinal direction can be suppressed.

[0031] (8) In the above (7), the concentration of the alkali metal group at the end portion may be higher than the concentration of the alkali metal group at the central portion. In this case, the transmission loss at the end portion is equal to or lower than the transmission loss at the central portion. Therefore, the risk of a decrease in manufacturing yield due to an increase in transmission loss during mass production is small.

[0032] [Details of the embodiments of the present disclosure]

[0033] Hereinafter, the manufacturing method of the optical fiber mother material and the specific examples of the optical fiber mother material disclosed in the present invention are described with reference to the accompanying drawings. It should be noted that the present invention is not limited to these examples, but is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are marked with the same figure numbers, and repeated descriptions are omitted.

[0034] (First Embodiment)

[0035] Figure 1 FIG. 2 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material according to the first embodiment. Figure 1 As shown in FIG. 1 , the optical fiber preform 10 according to the first embodiment includes a core portion 11 , a first cladding portion 12 , and a second cladding portion 13 . The first cladding portion 12 and the second cladding portion 13 constitute a cladding portion 14 .

[0036] The core 11 is made of silica glass. The core 11 contains one or more elements of the 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 11 contains, for example, at least one of sodium, potassium, rubidium and cesium as the alkali metal group. The concentration of other dopants and impurities contained in the core 11 is less than 10 ppm by mass.

[0037] The proportion of silica glass as 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 in terms of mass ratio. Here, mass ratio means mass fraction.

[0038] The first cladding 12 is provided outside the core 11 and surrounds the core 11. The first cladding 12 is made of silica-based glass. The first cladding 12 contains fluorine. The refractive index of the first cladding 12 is lower than the refractive index of the core 11.

[0039] 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 that of the core portion 11, but higher than that of the first cladding portion 12.

[0040] Figure 2 FIG. 1 is a flow chart showing a method for manufacturing an optical fiber preform according to the first embodiment. Figure 2 As shown, the method for manufacturing the optical fiber base material 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 extension grinding step S6, a sleeve collapse step S7, a second extension grinding step S8, and an OVD (Outside Vapor Deposition) step S9. The optical fiber base material 10 is manufactured through these steps S1 to S9. Furthermore, the optical fiber is manufactured by performing a drawing step (not shown).

[0041] In the preparation step S1, a glass tube 1 of silica-based glass in which a dopant such as an alkali metal group is to be diffused is prepared (see Figure 2 The glass tube 1 contains chlorine and fluorine at a certain concentration, and the mass fraction of other dopants and impurities is less than 10 ppm (hereinafter, the mass fraction is referred to as "concentration"). The outer diameter of the glass tube 1 is greater than 30 mm and less than 50 mm, and the inner diameter is greater than 10 mm and less than 30 mm.

[0042] The glass tube 1 contains chlorine at an average concentration of 0 ppm or more and 1500 ppm or less and fluorine at an average concentration of 500 ppm or more and 5000 ppm or less. Here, the so-called average concentration, for example, the average chlorine concentration, refers to the concentration expressed by the following formula. Cl(r) represents the local chlorine concentration at the radius r position. i represents the inner diameter of the glass tube 1, and d represents the outer diameter of the glass tube 1. The same consideration method is used to calculate fluorine or other dopants. In the case of a glass rod, i is set to 0 and d is set to the outer diameter of the glass rod for calculation.

[0043] [Mathematical formula 1]

[0044]

[0045] The method for measuring the local concentration is as follows. The chlorine concentration is measured at each position along a straight line passing through the center position on a certain end surface of the glass tube 1 and the glass rod using an electron beam microanalyzer (EPMA). The conditions for measuring using EPMA are, for example, an accelerating voltage of 20 kV, a probe beam diameter of 1 μm or less, and a measurement interval of 100 nm or less.

[0046] In the adding step S2, one or more elements of the alkali metal group are added as dopants to the inner surface of the glass tube 1 (first glass tube) composed of silica-based glass. In the adding step S2, for example, at least one or more of sodium, potassium, rubidium and cesium is added as the alkali metal group. Here, the case of adding potassium (K) element is described. For example, potassium bromide (KBr) of more than 6 g and less than 20 g is used as a raw material. Depending on the type of alkali metal group to be added, one or more of KBr, KI, RbBr, RbI, etc. can also be used as a raw material.

[0047] Figure 3 This is a diagram for explaining the addition process. Figure 3 As shown, an operating glass tube 5 disposed in an electric furnace 2 is connected to one end of the glass tube 1. A portion of the operating glass tube 5 is used as a raw material storage portion, in which the raw material 3 is placed. A portion of the glass tube 1 may also be used as a raw material storage portion. An oxyhydrogen burner 4 is disposed outside the glass tube 1. The electric furnace 2 is an external heat source for heating the raw material 3. The oxyhydrogen burner 4 is an external heat source for heating the glass tube 1. An induction furnace or a resistance furnace may also be used instead of the oxyhydrogen burner 4.

[0048] The raw material 3 is heated to a temperature of 700°C to 850°C by an electric furnace 2 to generate raw material vapor. The generated raw material vapor is introduced into the interior of the glass tube 1 together with a carrier gas composed of oxygen, while the glass tube 1 is heated from the outside by an oxyhydrogen burner 4. The flow rate of the carrier gas is set to be 1 SLM or more (converted to 1 liter / min in standard state (25°C, 100 kPa)) and 3 SLM or less. The oxyhydrogen burner 4 is moved horizontally at a speed of 30 mm / min to 60 mm / min for a total of 8 or more and 15 or less to heat the glass tube 1, so that the temperature of the outer surface of the glass tube 1 reaches 1400°C to 2000°C. Thus, the potassium element is diffused and added to the inner surface of the glass tube 1.

[0049] In the diameter reduction step S3, the glass tube 1 to which the potassium element is added is reduced in diameter. At this time, while oxygen is allowed to flow through the interior of the glass tube 1 at a rate of 0.5 SLM or more and 1.0 SLM or less, the glass tube 1 is heated by an external heat source so that the outer surface of the glass tube 1 reaches 2000° C. or more and 2300° C. or less. The external heat source is traversed for a total of 6 or more and 10 or less turns to heat the glass tube 1, and the diameter of the glass tube 1 is reduced until the inner diameter becomes 3 mm or more and 5 mm or less.

[0050] In the etching step S4, the inner surface of the glass tube 1 is etched. At this time, a mixed gas of SF6 (0.2 SLM or more and 1.0 SLM or less) and chlorine (0.5 SLM or more and 1.0 SLM or less) is introduced into the interior of the glass tube 1, and the glass tube 1 is heated by an external heat source to perform vapor phase etching. By doing so, the inner surface of the glass tube 1 containing impurities added together with the target dopant at a high concentration can be scraped, and the impurities can be removed. Each step from the preparation step S1 to the etching step S4 constitutes a diffusion addition step for diffusing and adding the dopant to the glass tube 1.

[0051] In the collapsing step S5, a glass rod is obtained from the glass tube 1 after the adding step S2 by a collapsing method. That is, the glass tube 1 after the adding step S2 is solidified by heating to obtain a glass rod. For example, a mixed gas of oxygen (0.1 SLM or more and 0.5 SLM or less) and He (0.5 SLM or more and 1.0 SLM or less) is introduced into the interior of the glass tube 1, and the absolute pressure in the glass tube 1 is reduced to less than 97 kPa while the surface temperature is made greater than 2000°C and less than 2300°C, and the glass tube 1 is sealed and solidified. Thus, a glass rod with an outer diameter of greater than 20 mm and less than 40 mm is obtained. In the collapsing step S5, the glass tube 1 is solidified by heating while the heat source is moved. In this specification, the movement of the heat source during the solidification is particularly referred to as traverse.

[0052] In the elongation and grinding step S6, the glass rod obtained in the collapse step S5 is elongated to a diameter of 20 mm or more and 25 mm or less, and the outer periphery of the glass rod is further ground to a diameter of 15 mm or more and 25 mm or less. Thus, the core 11 of the optical fiber mother material 10 is obtained. That is, each step from the preparation step S1 to the elongation and grinding step S6 constitutes a core manufacturing step for manufacturing the core 11.

[0053] In the sleeve collapse step S7, the first cladding portion 12 is provided on the outside of the core 11 by the sleeve collapse method. That is, the core 11 is inserted into the interior of the glass tube (second glass tube) that becomes the first cladding portion 12, and the core 11 and the glass tube are integrated by heating. Here, a glass tube of silica-based glass to which fluorine is added is used. The core 11 is used as a rod including the glass rod obtained in the collapse step S5. The maximum relative refractive index difference between the core 11 and the first cladding portion 12 is about 0.34%. In this embodiment, the relationship of the relative refractive index difference between the state of the optical fiber base material 10 and the state of the optical fiber is the same. As a result of adding the first cladding portion 12 by the sleeve collapse method, the moisture content of the core 11 and the first cladding portion 12 in the vicinity thereof can be suppressed to a sufficiently low level. In the sleeve collapse step S7, the glass tube and the glass rod are collapsed as described above while the heat source is moved. In this specification, the movement of the heat source during the collapse is particularly referred to as traverse.

[0054] In the OVD step S8, after the rod formed by integrating the core 11 and the first cladding 12 is extended to a predetermined diameter, the second cladding 13 is synthesized on the outside of the rod by the OVD method. The second cladding 13 is a glass layer containing fluorine. The OVD step S8 can be said to be a step of providing a glass layer on the outside of the glass rod obtained in the collapse step S5 by the OVD method, which is a method other than the collapse method. In this way, the optical fiber base material 10 is manufactured. Instead of the OVD method, a VAD (Vapor-phase Axial Deposition) method may be used.

[0055] The optical fiber can be manufactured by subjecting the optical fiber preform 10 to a drawing process. 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.

[0056] Steps S5 and S7 are performed while the external heat source is moved in a first direction from the first end of the glass rod obtained in step S5 to the second end or in a second direction from the second end of the glass rod to the first end. When the external heat source is moved in the first direction, the first end side of the glass rod becomes the start end of collapse, and the second end side of the glass rod becomes the end end of collapse. In steps S5 and S7, the start ends of collapse are different from each other, and the end ends of collapse are different from each other.

[0057] In the double collapse consisting of step S5 and step S7, the difference between the number of traverses in the first direction and the number of traverses in the second direction is less than 1. For example, in step S5 and step S7, the traverse in the first direction and the traverse in the second direction are performed alternately. That is, when the traverse in the first direction is performed in step S5, the traverse in the second direction is performed in step S7. When the traverse in the second direction is performed in step S5, the traverse in the first direction is performed in step S7.

[0058] The external heat source is used to heat the glass tube 1 that becomes the core 11 and the glass tube that becomes the first cladding portion 12 from the outside. The external heat source may be the same as the oxyhydrogen burner 4 used in the adding step S2. The same external heat source may also be used in steps S5 and S7.

[0059] Figure 4 FIG. 1 is a cross-sectional view along the length direction of the optical fiber base material involved in the first embodiment. Figure 4 As shown, the optical fiber mother material 10 has a first end 10a, a second end 10b and a central portion 10c in the length direction. The central portion 10c is located between the first end 10a and the second end 10b. The first end 10a is configured to include the first end of the glass rod obtained in step S5. The second end 10b is configured to include the second end of the glass rod obtained in step S5.

[0060] The optical fiber preform 10 includes an alkali-doped portion 20 to which one or more elements of the alkali metal group are added. Figure 4 , the alkali-doped portion 20 is conceptually shown. The alkali-doped portion 20 is disposed at the center portion of the cross section of the optical fiber preform 10 perpendicular to the longitudinal direction, and extends along the longitudinal direction.

[0061] The diameter of the alkali added part 20 at the first end 10a is equal to the diameter of the alkali added part 20 at the second end 10b. The diameters of the alkali added part 20 at the first end 10a and the second end 10b are equal to or longer than the diameter of the alkali added part 20 at the central part 10c.

[0062] The diameter of the alkali addition part 20 is correlated with the concentration of the alkali metal group. Therefore, the concentration of the alkali metal group at the first end 10a is equal to the concentration of the alkali metal group at the second end 10b. The concentration of the alkali metal group at the first end 10a and the second end 10b is equal to or higher than the concentration of the alkali metal group at the central part 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 part 10c is less than 15% by mass, and more preferably less than 5%.

[0063] (First Comparative Example)

[0064] Figure 5 is a cross-sectional view along the length direction of the optical fiber mother material involved in the first comparative example. Figure 5 As shown in FIG. 1 , the optical fiber preform 110 according to the first comparative example is different from the optical fiber preform 10 in that it has an alkali-doped portion 120 whose diameter increases from the second end portion 110b toward the first end portion 110a. The diameter of the alkali-doped portion 120 at the first end portion 110a is larger than the diameter of the alkali-doped 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.

[0065] The manufacturing method of the optical fiber preform 110 involved in the first comparative example is different from the manufacturing method involved in the first embodiment in that the direction of traversing the external heat source is not reversed in step S5 and step S7. That is, the direction of traversing the external heat source is the same in both step S5 and step S7, and the external heat source is traversed in the first direction or the second direction. In step S5 and step S7, the starting ends of the collapse are consistent with each other, and the terminal ends of the collapse are consistent with each other. In this example, traversing in the first direction is performed in both step S5 and step S7.

[0066] (Second Embodiment)

[0067] Figure 6 FIG. 2 is a cross-sectional view perpendicular to the longitudinal direction of the optical fiber base material according to the second embodiment. Figure 6 As shown, the optical fiber preform 10A according to the second embodiment includes a first core portion 15, a second core portion 16, a first cladding portion 12, and a second cladding portion 13. The first core portion 15 and the second core portion 16 constitute a core portion 17. The first cladding portion 12 and the second cladding portion 13 constitute a cladding portion 14.

[0068] The first core 15 has the same structure as the core 11 of the optical fiber base material 10. The second core 16 is arranged outside the first core 15 and surrounds the first core 15. The second core 16 is composed of silica glass. The second core 16 contains chlorine and fluorine, but does not contain an alkali metal group. The first cladding 12 is different from the first cladding 12 of the optical fiber base material 10 in that the first cladding 12 is arranged outside the second core 16 and surrounds the second core 16. The first cladding 12 has the same structure as the first cladding 12 of the optical fiber base material 10 in other points. The second cladding 13 has the same structure as the second cladding 13 of the optical fiber base material 10.

[0069] Figure 7 FIG. 1 is a flow chart showing a method for manufacturing an optical fiber preform according to the second embodiment. Figure 7As shown, the method for manufacturing the optical fiber base material 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 extension grinding step S16, a first sleeve collapse step S17, a second extension grinding step S18, a second sleeve collapse step S19, a third extension grinding step S20, and a third sleeve collapse step S21. The optical fiber base material 10A is manufactured through these steps S11 to S21. Furthermore, an optical fiber is manufactured by performing a wire drawing step (not shown).

[0070] Steps S11 to S16 are substantially the same as steps S1 to S6 of the first embodiment, and thus their description is omitted. Through steps S11 to S16, the first core portion 15 is obtained.

[0071] In the first sleeve collapse step S17, the second core 16 is arranged outside the first core 15 by the sleeve collapse method. That is, the first core 15 is inserted into the inside of the glass tube (first and second glass tubes) that becomes the second core 16, and the first core 15 and the glass tube are integrated by heating. Here, a glass tube of silica-based glass to which chlorine and fluorine are added is used. The first core 15 is used as a rod including the glass rod obtained in the collapse step S15.

[0072] In the second elongation and grinding step S18, the glass rod obtained in the first sleeve collapse step S17 is elongated to a diameter of 20 mm or more and 25 mm or less, and the outer periphery of the glass rod is further ground to a diameter of 15 mm or more and 25 mm or less. Thus, the core 17 of the optical fiber preform 10A is obtained. That is, each step from the preparation step S1 to the second elongation and grinding step S18 constitutes a core manufacturing step for manufacturing the core 17.

[0073] In the second sleeve collapse step S19, the first cladding portion 12 is provided outside the core 17 by the sleeve collapse method. That is, the core 17 is inserted into the glass tube (second second glass tube) that becomes the first cladding portion 12, and the core 17 and the glass tube are integrated by heating. Here, a glass tube of silica-based glass to which fluorine is added is used. The core 17 is used as a rod including the glass rod obtained in the collapse step S15.

[0074] In the third elongation and grinding step S20 , the glass rod obtained in the second sleeve collapsing 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.

[0075] In the third sleeve collapse step S21, the second cladding portion 13 is provided on the outside of the glass rod composed of the core 17 and the first cladding portion 12 by the sleeve collapse method. That is, the glass rod composed of the core 17 and the first cladding portion 12 is inserted into the interior of the glass tube (second second glass tube) which becomes the second cladding portion 13, and the glass rod composed of the core 17 and the first cladding portion 12 and the glass tube are integrated by heating. Here, a glass tube of silica-based glass to which fluorine is added is used. The glass rod composed of the core 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, the optical fiber base material 10A is manufactured.

[0076] The optical fiber can be manufactured by subjecting the optical fiber preform 10A to a drawing process. 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.

[0077] Steps S15, S17, S19, and S21 are performed while the external heat source is traversed in a first direction from the first end toward the second end of the glass rod obtained in step S15 or in a second direction from the second end toward the first end of the glass rod.

[0078] In the four-step collapse consisting of step S15, step S17, step S19, and step S21, the difference between the number of traverses in the first direction and the number of traverses in the second direction is less than 1. For example, in step S15, step S17, step S19, and step S21, the traverse in the first direction and the traverse in the second direction are performed alternately.

[0079] In the four-step collapse consisting of step S15, step S17, step S19, and step S21, the direction of traversing the external heat source is reversed. For example, traversing in the first direction is performed in step S15, traversing in the second direction is performed in step S17, traversing in the first direction is performed in step S19, and traversing in the second direction is performed in step S21.

[0080] In the optical fiber preform 10A according to the second embodiment, the alkali-added portion 20 (see FIG. 2 ) having a small outer diameter variation is formed similarly to the optical fiber preform 10. Figure 4 ). Therefore, in the optical fiber base material 10A, similarly to the optical fiber base material 10, the first end portion 10a (see Figure 4 ) and the second end portion 10b (refer to Figure 4 ) and the concentration of the alkali metal group in the central part 10c (refer to Figure 4 The difference between the concentration of the alkali metal group at the first end portion 10a and the second end portion 10b and the concentration of the alkali metal group at the central portion 10c is less than 15% by mass, and more preferably less than 5%.

[0081] (Second Comparative Example)

[0082] The manufacturing method according to the second comparative example is different from the manufacturing method according to the second embodiment in that the direction of traversing the external heat source is not reversed in steps S15, S17, S19, and S21. That is, traversing is performed in the same direction in any of steps S15, S17, S19, and S21. In this example, traversing is performed in the first direction in any of steps S15, S17, S19, and S21.

[0083] In the optical fiber preform according to the second comparative example, similarly to the optical fiber preform 110 according to the first comparative example, a plurality of optical fibers are formed along the second end portion 110b (see Figure 5 ) toward the first end portion 110a (refer to Figure 5 ) and the alkali adding part 120 with an enlarged diameter (refer to Figure 5 ). Therefore, in the optical fiber base material involved in the second comparative example, the diameter of the alkali-doped portion 120 at the first end portion 110a is larger than the diameter of the alkali-doped portion 120 at the second end portion 110b, similarly to the optical fiber base material 110. That is, 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.

[0084] (Experimental example)

[0085] Hereinafter, experimental examples will be described.

[0086] In the first experimental example, the optical fiber base material is manufactured by the manufacturing method involved in the first comparative example, and then the optical fiber is manufactured by performing a drawing process. That is, in the first experimental example, a total of two collapses consisting of steps S5 and S7 are performed. In both steps S5 and S7, traverse in the first direction is performed.

[0087] In the second experimental example, the optical fiber base material is manufactured by the manufacturing method involved in the first embodiment, and then the optical fiber is manufactured by performing a drawing process. That is, in the second experimental example, a total of two collapses consisting of steps S5 and S7 are performed. In step S5, a traverse in the first direction is performed, and in step S7, a traverse in the second direction is performed.

[0088] In the third experimental example, an optical fiber base material is manufactured by the manufacturing method involved in the second comparative example, and an optical fiber is manufactured by performing a drawing process. That is, in the third experimental example, a total of four collapses consisting of steps S15, S17, S19, and S21 are performed. In any of the steps S15, S17, S19, and S21, the traverse in the first direction is performed.

[0089] In the fourth experimental example, an optical fiber base material is manufactured by the manufacturing method involved in the second embodiment, and then an optical fiber is manufactured by performing a drawing process. That is, in the fourth experimental example, a total of four collapses consisting of steps S15, S17, S19, and S21 are performed. In steps S15 and S19, a traverse in the first direction is performed. In steps S17 and S21, a traverse in the second direction is performed.

[0090] The optical fiber involved in each experimental example was manufactured to have an effective cross-sectional area (Aeff) of 105 μm2 or more and 115 μm2 or less, a cutoff wavelength λc of 1400 nm or more and 1520 nm or less, and a relative refractive index difference between the core and the cladding of 0.34% ± 0.01%. Here, the "relative refractive index difference between the core and the cladding" is the relative refractive index difference between the core and the first cladding in the case of the first and second experimental examples, and the relative refractive index difference between the second core and the first cladding in the case of the third and fourth experimental examples.

[0091] For the optical fiber mother material involved in each experimental example, the potassium concentration at the first end, the central part and the second end was measured using the above-mentioned EPMA. For the optical fiber drawn from the first end, the central part and the second end of the optical fiber mother material involved in each experimental example, the transmission loss at a wavelength of 1550nm was measured.

[0092] Table 1 is a table summarizing the details of optical fiber preforms and optical fibers and conditions involved in each experimental example.

[0093] [Table 1]

[0094]

[0095] In the first and third experimental examples, the K concentration decreases from the first end toward the second end. Thus, the transmission loss increases from the first end toward the second end. In the second and fourth experimental examples, the K concentration is substantially constant in the entire length direction by reversing the traverse direction, that is, the collapse direction. Thus, the transmission loss is stable in the entire length direction.

[0096] As described above, in the manufacturing method involved in the above-mentioned embodiment, a collapse process and one or more sleeve collapse processes are performed, and in the collapse process and one or more sleeve collapse processes, the difference between the number of lateral movements in the first direction and the number of lateral movements in the second direction is less than 1. As a result, the increase or decrease in the diameter of the alkali addition part 120 in the length direction is suppressed. Therefore, the concentration of the alkali metal group is suppressed from changing in the length direction. In the case where the concentration of the alkali metal group is increased and the diameter of the alkali addition part 120 is increased, the concentration of the alkali metal group is likely to change in the length direction. Although lowering the collapse temperature is also considered, the melting of the interface becomes insufficient, and sometimes manufacturing defects occur. Therefore, the manufacturing method involved in the above-mentioned embodiment in which the collapse direction is reversed is effective.

[0097] As mentioned above, although embodiment was described, this disclosure is not necessarily limited to the said embodiment and modification, Various changes can be made within the range which does not deviate from the summary.

[0098] The optical fiber base material 10 may also be a multi-core optical fiber base material having a plurality of cores. In this case, in steps S5 and S7, for each of the plurality of cores, the difference between the number of traverses in the first direction and the number of traverses in the second direction is 1 or less. Thus, for each core, the concentration variation of the alkali metal group in the length direction can be suppressed.

[0099] The optical fiber base material 10A may be a multi-core optical fiber base material 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. Thus, the diameter variation of the alkali-doped portion 120 in the longitudinal direction can be suppressed for each core. Therefore, when the optical fiber is formed, the variation of the transmission loss in the longitudinal direction can be suppressed for each core.

[0100] The optical fiber base material 10, 10A may not include the second cladding portion 13. That is, the method for manufacturing the optical fiber base material 10 may not include the OVD step S9. The method for manufacturing the optical fiber base material 10A may not include the third sleeve collapse step S21. The optical fiber base material 10, 10A may further include one or more glass layers provided on the outside of the second cladding portion 13. That is, the method for manufacturing the optical fiber base material 10, 10A may further include the step of providing a glass layer on the outside of the second cladding portion 13 by a known method such as a sleeve collapse method, an OVD method, or a VAD method. Even in these cases, in the method for manufacturing the optical fiber base material 10, 10A, a collapse step and one or more sleeve collapse steps are performed, and in the collapse step and one or more sleeve collapse steps, the difference between the number of traverses in the first direction and the number of traverses in the second direction may be 1 or less.

[0101] The optical fiber base material 10 may further include one or more glass layers disposed outside the core 11 and inside the first cladding portion 12. That is, the method for manufacturing the optical fiber base material 10 may further include a step of disposing a glass layer outside the core 11 and inside the first cladding portion 12 by a known method such as a tube collapse method, an OVD method, or a VAD method. The optical fiber base material 10A may further include one or more glass layers disposed outside the second core 16 and inside the first cladding portion 12. That is, the method for manufacturing the optical fiber base material 10A may further include a step of disposing a glass layer outside the second core 16 and inside the first cladding portion 12 by a known method such as a tube collapse method, an OVD method, or a VAD method. Even in these cases, in the method for manufacturing the optical fiber base materials 10 and 10A, the collapse step and one or more tube collapse steps are performed, and in the collapse step and one or more tube collapse steps, the difference between the number of traverses in the first direction and the number of traverses in the second direction may be 1 or less.

[0102] Description of Reference Numerals

[0103] 1 Glass tube

[0104] 2 Electric stove

[0105] 3 Raw materials

[0106] 4 Hydrogen Oxygen Burner

[0107] 5. Operating the glass tube

[0108] 10, 10A optical fiber mother material

[0109] 10a First end

[0110] 10b Second end

[0111] 10c Central

[0112] 11 Core

[0113] 12 First cladding part

[0114] 13 Second cladding section

[0115] 14 Cladding

[0116] 15 First core

[0117] 16 Second core

[0118] 17 Core

[0119] 110 Optical fiber mother material

[0120] 110a First end

[0121] 110b second end portion.

Claims

1. A method for manufacturing an optical fiber mother material, comprising: an adding step of adding one or more elements of an alkali metal group consisting of alkali metal elements and alkaline earth metal elements to the inner surface of the first glass tube consisting of silica-based glass; A collapsing step of solidifying the first glass tube after the adding step by heating to obtain a glass rod; and One or more sleeve collapse steps, inserting the rod including the glass rod into the interior of the second glass tube, integrating the rod and the second glass tube by heating, The collapsing step and one or more of the sleeve collapsing steps are performed while the external heat source is moved transversely in a first direction from the first end toward the second end of the glass rod or in a second direction from the second end toward the first end. In the collapsing step and one or more of the sleeve collapsing steps, a difference between the number of traverses performed in the first direction and the number of traverses performed in the second direction is 1 or less.

2. The method for manufacturing an optical fiber preform according to claim 1, wherein: During the collapsing process and one or more of the sleeve collapsing processes, the traverse movement in the first direction and the traverse movement in the second direction are performed alternately.

3. The method for manufacturing an optical fiber preform according to claim 1 or 2, wherein: The method for manufacturing an optical fiber base material is a method for manufacturing a multi-core optical fiber base material having multiple cores. In the collapsing step and one or more of the sleeve collapsing steps, for each of the plurality of cores, a difference between the number of traverses performed in the first direction and the number of traverses performed in the second direction is 1 or less.

4. The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, wherein: The method for producing an optical fiber preform further includes the step of providing a glass layer on the outer side of a rod including the glass rod by a method other than the tube collapse method.

5. The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, wherein: The method for manufacturing the optical fiber preform further includes the step of providing a glass layer on the outer side of the rod including the glass rod by an OVD method or a VAD method.

6. The method for manufacturing an optical fiber preform according to any one of claims 1 to 5, wherein: In the adding step, at least one element selected from sodium, potassium, rubidium, and cesium is added as an alkali metal group.

7. An optical fiber base material, to which one or more elements of an alkali metal group consisting of alkali metal elements and alkaline earth metal elements are added, The optical fiber mother material has an end portion and a central portion in the length direction. The difference between the concentration of the element at the end portion and the concentration of the element at the central portion is less than 15% by mass.

8. The optical fiber mother material according to claim 7, wherein: 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.

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