Preform to be processed into hollow-core optical fiber, method for manufacturing preform to be processed into hollow-core optical fiber, and method for manufacturing hollow-core optical fiber

Abstract

This preform to be processed into a hollow-core optical fiber includes: a jacket tube having an inner surface; and a plurality of tubular preforms disposed apart from each other within a space enclosed by the inner surface of the jacket tube. Regarding this preform, the softening point of at least one of the plurality of tubular preforms is higher than the softening point of the jacket tube.

Description

Preform to be processed into hollow-core optical fiber, method for manufacturing preform to be processed into hollow-core optical fiber, and method for manufacturing hollow-core optical fiber

[0001] This disclosure relates to a preform to be processed into a hollow-core optical fiber, a method for manufacturing a preform to be processed into a hollow-core optical fiber, and a method for manufacturing a hollow-core optical fiber. This application claims priority to Japanese Application No. 2024-111940, filed on July 11, 2024, and incorporates by reference all the contents of said Japanese application.

[0002] Patent Documents 1 and 2 disclose a preform to be processed into a hollow-core optical fiber, a method for manufacturing the preform, and a method for manufacturing a hollow-core optical fiber using the preform.

[0003] International Publication No. 2019 / 053412 International Publication No. 2021 / 009236

[0004] A preform to be processed into a hollow-core optical fiber according to one embodiment of the present disclosure includes a jacket tube having an inner surface, and a plurality of cylindrical preforms arranged apart from one another within a space enclosed by the inner surface of the jacket tube, wherein at least one of the plurality of cylindrical preforms has a softening point higher than the softening point of the jacket tube.

[0005]

[0013] Fig. 1 is a cross-sectional view showing an embodiment of a hollow-core optical fiber. Fig. 2 is a cross-sectional view showing a modified example of a hollow-core optical fiber. Fig. 3A is a view for explaining a method of manufacturing a preform to be processed into a hollow-core optical fiber. Fig. 3B is a view for explaining a method of manufacturing a preform to be processed into a hollow-core optical fiber. Fig. 3C is a cross-sectional view showing a preform to be processed into a hollow-core optical fiber. Fig. 4 is a schematic view showing a drawing apparatus for manufacturing a hollow-core optical fiber by drawing a preform. Fig. 5 is a view for explaining the arrangement of cladding tubes in a preform. Fig. 6 is a view for explaining that the ratio of the outer diameter of the cladding tube to the distance between the cladding tubes varies from upstream to downstream of the drawing. Fig. 7 is a view showing the relationship between the ratio of the outer diameter of the cladding to the distance between the cladding tubes and the drawing tension, and is a plot of the peak value of the ratio of the outer diameter of the cladding tube to the distance between the cladding tubes under conditions of constant internal pressure. Fig. 8 is a view showing the decrease in softening point when phosphorus is added to silica glass. Fig. 9 is a graph showing the decrease in melt viscosity for each softening point when fluorine is added to silica glass. Fig. 10A is a graph showing the relationship between the ratio of the cladding tube outer diameter to the distance between the cladding tubes and the difference in softening point for each drawing tension, for a 20 mm diameter preform. Fig. 10B is a graph showing the relationship between the ratio of the cladding tube outer diameter to the distance between the cladding tubes and the difference in softening point for each drawing tension, for a 50 mm diameter preform. Fig. 10C is a graph showing the relationship between the ratio of the cladding tube outer diameter to the distance between the cladding tubes and the difference in softening point for each drawing tension, for a 100 mm diameter preform. Fig. 10D is a graph showing the relationship between the ratio of the cladding tube outer diameter to the distance between the cladding tubes and the difference in softening point for each drawing tension, for a 200 mm diameter preform. Fig. 11 is a cross-sectional view showing a preform to be processed into a hollow-core optical fiber according to a first modified example. FIG. 12 is a cross-sectional view showing a preform to be processed into a hollow-core optical fiber according to a second modification.Fig. 13 is a cross-sectional view showing a preform to be processed into a hollow core optical fiber according to a third modified example. Fig. 14 is a cross-sectional view showing a preform to be processed into a hollow core optical fiber according to a fourth modified example. Fig. 15 is a cross-sectional view showing a preform to be processed into a hollow core optical fiber according to a fifth modified example. Fig. 16 is a cross-sectional view showing a preform to be processed into a hollow core optical fiber according to a sixth modified example. Fig. 17 is a cross-sectional view showing a preform to be processed into a hollow core optical fiber according to a seventh modified example.

[0006] In the preform disclosed in Patent Document 1, multiple cladding tubes are arranged within a jacket tube. The cladding tubes are arranged within the jacket tube so as to be spaced apart from each other. When such a preform is drawn to produce a hollow-core optical fiber, internal pressure is applied to each cladding tube to prevent the cladding tube from collapsing. This internal pressure is controlled to be smaller than the surface tension acting on the cladding tube downstream of drawing so that the outer diameter of a capillary (formed from the cladding tube) serving as the hollow-core optical fiber has a desired dimension (i.e., does not expand beyond the desired dimension). Note that the internal pressure is constant from upstream to downstream of drawing. Here, "downstream of drawing" refers to the downstream side during drawing, and refers to the area near where the preform is melted and drawn until its outer diameter approaches the fiber diameter. "Upstream of drawing" refers to the upstream side during drawing, and refers to the area near where the preform is melted.

[0007] On the other hand, the surface tension generated in the cladding tube is inversely proportional to the radius of curvature of the cladding tube and differs upstream and downstream of the drawing. That is, because the radius of curvature of the cladding tube decreases from upstream to downstream of the drawing, the surface tension acting on the cladding tube upstream is smaller than the surface tension acting downstream. Therefore, near the upstream of the drawing, the surface tension acting on the outside of the cladding tube may become smaller than the internal pressure of the cladding tube. In this case, the cladding tubes, which were previously arranged apart from each other, may expand, and the capillaries may come into contact with each other when they are formed into an optical fiber. If the capillaries come into contact with each other, the optical properties of the manufactured optical fiber may deteriorate.

[0008] An object of the present disclosure is to provide a preform to be processed into a hollow-core optical fiber, which can prevent cylindrical preforms such as cladding tubes from contacting each other in the production of the hollow-core optical fiber, and a method for manufacturing the preform to be processed into a hollow-core optical fiber. Another object of the present disclosure is to provide a method for manufacturing a hollow-core optical fiber, which can improve the optical properties.

[0009] According to the present disclosure, it is possible to prevent cylindrical preforms from coming into contact with each other when manufacturing a hollow-core optical fiber, and it is also possible to improve the optical properties of the hollow-core optical fiber.

[0010] First, the contents of the embodiments of the present disclosure will be listed and explained. [1] According to one embodiment, a preform to be processed into a hollow-core optical fiber includes a jacket tube having an inner surface, and a plurality of cylindrical preforms arranged apart from each other within a space enclosed by the inner surface of the jacket tube. In this preform, the softening point of at least one of the plurality of cylindrical preforms is higher than the softening point of the jacket tube.

[0011] In the preform of [1] above, the softening point of the tubular preform is higher than that of the jacket tube. In this case, when the preform is melted and drawn, the outer jacket tube can be sufficiently melted, while softening of the tubular preform at the melting temperature can be suppressed, thereby maintaining the strength of the tubular preform. As a result, when the preform is drawn to manufacture an optical fiber, even if the internal pressure applied to the tubular preform near the upstream of the drawing becomes greater than the surface tension acting on the tubular preform, it is possible to prevent a tubular preform from expanding and coming into contact with an adjacent tubular preform. Note that the softening point referred to here is the temperature at which the material forming the tubular preform and the jacket tube begins to soften and deform under its own weight. Typically, the softening point is the temperature at which the "glass viscosity is about 10" measured in accordance with JIS R 3103-1 (2001) ７．６ The viscosity of the material can be defined as "the temperature at which the viscosity becomes dPa·s."

[0012] [2] In the preform of [1] above, the softening point of the cylindrical preform may be higher than the softening point of the jacket tube by 60° C. or more. In this case, when the preform is drawn to produce an optical fiber, it is possible to more reliably prevent a certain cylindrical preform from expanding and coming into contact with an adjacent cylindrical preform.

[0013] [3] In the preform of [1] above, the softening point of the tubular preform may be higher than the softening point of the jacket tube by 190° C. or more. In this case, even when an optical fiber is produced by drawing a large-diameter preform, such as a preform with a diameter of 100 mm or more (for example, a diameter of 200 mm), it is possible to more reliably prevent a tubular preform from expanding and coming into contact with an adjacent tubular preform while properly melting the jacket tube.

[0014] [4] In the preform of any one of [1] to [3] above, the jacket tube may be doped with at least one of phosphorus, fluorine, chlorine, and a hydroxyl group. In this case, the softening point of the cylindrical preform can be made higher than the softening point of the jacket tube by a simple method.

[0015] [5] In any of the preforms [1] to [4] above, aluminum may be added to the cylindrical preform, and in this case, the softening point of the cylindrical preform can be made higher than the softening point of the jacket tube by a simple method.

[0016] [6] In the preform of any of the above [1] to [5], the plurality of cylindrically formed preforms may include at least one of a cylindrical cladding tube and a cavity-forming wall. The cavity-forming wall may have a constant shape along the central axis of the space surrounded by the inner surface of the jacket tube. The cavity-forming wall may have a curved surface that protrudes toward the central axis on a cross section perpendicular to the central axis. The cavity-forming wall may form, together with other elements of the preform, a cylindrically enclosed cavity that is a closed region that does not include the central axis on a cross section perpendicular to the central axis. When a cavity-forming wall is used, a hollow-core optical fiber with equivalent optical properties can be realized with fewer cylindrically formed preforms than when a cladding tube is used.

[0017] [7] In the preform of any one of [1] to [6] above, the plurality of cylindrical preforms may be fixed so as to contact the inner surface of the jacket tube, in which case the core region defined by the plurality of cylindrical preforms can be stabilized.

[0018] [8] The preform of any one of [1] to [7] above may further include a support element preform disposed between the inner surface of the jacket tube and each of the plurality of tube-forming preforms. The support element preform may be fixed so as to contact at least one of the plurality of tube-forming preforms and the inner surface of the jacket tube. In this case, the tube-forming preforms can be positioned with high precision.

[0019] [9] In any of the preforms [1] to [8] above, the jacket tube may have an inner jacket tube and an outer jacket tube covering the inner jacket tube. The softening point of at least one of the plurality of cylindrical preforms may be higher than the softening point of the outer jacket tube. In this case, the outer jacket tube having a larger volume is prepared separately from the cylindrical preform and the inner jacket tube to prepare the preforms. Even when such preforms are drawn to produce optical fiber, it is possible to prevent a cylindrical preform from expanding and coming into contact with an adjacent cylindrical preform.

[0020]

[10] In the preform of [9] above, the softening point of the cylindrical preform may be higher than the softening point of the outer jacket tube by 60° C. or more. In this case, when the preform is drawn to produce an optical fiber, it is possible to more reliably prevent a cylindrical preform from expanding and coming into contact with an adjacent cylindrical preform.

[0021]

[11] In the preform of [9] or

[10] above, the outer jacket tube may be doped with at least one of phosphorus, fluorine, chlorine, and a hydroxyl group. In this case, the softening point of the cylindrical preform can be made higher than that of the outer jacket tube by a simple method.

[0022]

[12] According to one embodiment, a method for manufacturing a preform to be processed into a hollow-core optical fiber includes the steps of: arranging a plurality of cylindrical preforms in a space surrounded by an inner surface of an inner jacket tube so that the plurality of cylindrical preforms are spaced apart from one another; and fixing the plurality of cylindrical preforms in the space surrounded by the inner surface of the inner jacket tube. At least one of the plurality of cylindrical preforms has a softening point higher than the softening point of the inner jacket tube. In the fixing step, the cylindrical preform and the inner jacket tube are heated.

[0023] In the method

[12] , the softening point of the tubular preform is set higher than that of the inner jacket tube, and the tubular preform and the inner jacket tube are heated when the tubular preform is fixed. This prevents a tubular preform from expanding and coming into contact with an adjacent tubular preform when the internal pressure applied to the tubular preform near the upstream of the drawing may exceed the surface tension acting on the tubular preform when the preform is drawn to produce an optical fiber as described above.

[0024]

[13] The method for manufacturing a preform according to

[12] above may further include a step of shrinking the first preform formed in the fixing step in the radial direction while heating it, and providing an outer jacket tube to cover the shrunken first preform to form a second preform. In this case, by making the inner jacket tube thin, it is possible to heat it from the outside with a burner or the like, and fix the cylindrically formed preform by melting it. Furthermore, by adding a thick outer jacket tube, it is possible to protect the structurally weak core portion when it is processed into an optical fiber.

[0025]

[14] According to one embodiment, a method for manufacturing a preform to be processed into a hollow-core optical fiber includes the steps of: arranging a plurality of cylindrical preforms in a space surrounded by an inner surface of an inner jacket tube so that the plurality of cylindrical preforms are spaced apart from one another; fixing the plurality of cylindrical preforms in the space surrounded by the inner surface of the inner jacket tube; and forming a second preform by heating and radially shrinking a first preform formed in the fixing step, and further providing an outer jacket tube to cover the shrunken first preform. The softening point of at least one of the plurality of cylindrical preforms is higher than the softening point of the outer jacket tube. In the fixing step, the cylindrical preform and the inner jacket tube are heated.

[0026] In the method

[14] above, the softening point of the tubular preform is set higher than that of the outer jacket tube, and the tubular preform and the inner jacket tube are heated when the tubular preform is fixed. This prevents a tubular preform from expanding and coming into contact with an adjacent tubular preform when the internal pressure applied to the tubular preform near the upstream of the drawing may exceed the surface tension acting on the tubular preform when the preform is drawn to produce an optical fiber as described above.

[0027]

[15] In the preform manufacturing method of the above

[13] or

[14] , the softening point of the outer jacket tube may be lower than that of the inner jacket tube. In this case, since the outer jacket tube has a large cross-sectional area and therefore has a lower softening point, the difference between the softening point of the outer jacket tube and the softening point of the cylindrical preform can be further increased, thereby suppressing softening of the cylindrical preform and maintaining the strength of the cylindrical preform.

[0028]

[16] In the preform manufacturing method of any one of the above

[12] to

[15] , the softening point of the cylindrically formed preform may be higher by 60° C. or more than the softening point of at least one of the inner jacket tube and the outer jacket tube. In this case, it is possible to provide a preform in which the cylindrically formed preforms do not come into contact with each other when manufacturing a hollow-core optical fiber by drawing.

[0029]

[17] In the preform manufacturing method of any of the above

[12] to

[16] , at least one of phosphorus, fluorine, chlorine, and a hydroxyl group may be added to at least one of the inner jacket tube and the outer jacket tube. In this case, by using a simple method to make the softening point of the cylindrical preform higher than the softening points of the jacket tubes, it is possible to easily provide cylindrical preforms that do not come into contact with each other when manufacturing a hollow-core optical fiber.

[0030]

[18] In the preform manufacturing method of any one of

[12] to

[17] above, the plurality of cylindrically formed preforms may include at least one of a cylindrical cladding tube and a cavity-forming wall. The cavity-forming wall may have a constant shape along the central axis of the space surrounded by the inner surface of the inner jacket tube. The cavity-forming wall may have a curved surface that protrudes toward the central axis on a cross section perpendicular to the central axis. The cavity-forming wall may form, together with other elements of the preform, a cylindrically enclosed cavity that is a closed region that does not include the central axis on a cross section perpendicular to the central axis. When a cavity-forming wall is used, a hollow-core optical fiber with equivalent optical properties can be realized with fewer cylindrically formed preforms than when a cladding tube is used.

[0031]

[19] In the preform manufacturing method according to any one of

[12] to

[18] above, in the fixing step, the plurality of cylindrical preforms may be fixed so that the plurality of cylindrical preforms contact the inner surface of the jacket tube. In this case, the core region defined by the plurality of cylindrical preforms can be stabilized.

[0032]

[20] In the preform manufacturing method according to any one of

[12] to

[18] above, in the fixing step, support element preforms may be provided so as to contact each of the plurality of cylindrical preforms with the inner surface of the jacket tube. In this case, the cylindrical preforms can be positioned with high precision.

[0033]

[21] A method for manufacturing a hollow-core optical fiber according to one embodiment includes the steps of preparing a preform according to any one of [1] to

[11] above, and heating and drawing the preform.

[0034] In the hollow-core optical fiber manufacturing method of

[21] above, an optical fiber is manufactured by drawing a cylindrical preform whose softening point is higher than that of the jacket tube. In this case, when the preform is heated and melted and drawn, the outer jacket tube can be sufficiently melted, while softening of the cylindrical preform at the heating temperature can be suppressed, thereby maintaining the strength of the cylindrical preform. As a result, when manufacturing the optical fiber, even if the internal pressure applied to the cylindrical preform near the upstream of the drawing becomes greater than the surface tension acting on the cylindrical preform, it is possible to prevent one cylindrical preform from expanding and coming into contact with an adjacent cylindrical preform. Therefore, this manufacturing method can manufacture a hollow-core optical fiber with improved optical properties.

[0035]

[22] In the method for manufacturing a hollow-core optical fiber according to

[21] above, in the drawing step, the optical fiber may be drawn while applying internal pressure to the internal spaces of the plurality of tubular preforms. In the optical fiber being drawn, the internal pressure applied to each of the plurality of tubular preforms may be maintained, from upstream to downstream of the drawing, to be smaller than the surface tension of each of the molten plurality of tubular preforms. This more reliably prevents the tubular preforms from expanding and coming into contact with each other when manufacturing the optical fiber. Therefore, this manufacturing method makes it possible to manufacture a hollow-core optical fiber with improved optical properties.

[0036]

[23] In the method for manufacturing a hollow-core optical fiber according to

[21] or

[22] above, the jacket tube may have an inner jacket tube and an outer jacket tube covering the inner jacket tube. The softening point of at least one of the plurality of tubular preforms may be higher than the softening point of the outer jacket tube. In the drawing process, drawing may be performed while applying internal pressure to the internal space of the plurality of tubular preforms from upstream to downstream of drawing so that the molten tubular preforms do not come into contact with each other. In the drawing process, drawing may optionally be performed while reducing the pressure in the space between the inner jacket tube and the outer jacket tube. In this case, a large-volume outer jacket tube is prepared separately from the tubular preform and the inner jacket tube to prepare the preforms. Even when such preforms are drawn to manufacture an optical fiber, it is possible to prevent a tubular preform from expanding and coming into contact with an adjacent tubular preform.

[0037]

[24] In the method for manufacturing a hollow-core optical fiber according to any one of

[21] to

[23] above, in the drawing step, the drawing may be performed while applying internal pressure to the space inside the plurality of cylindrically formed preforms. In the drawing step, the molten plurality of cylindrically formed preforms may be maintained in a state where they do not come into contact with each other from upstream to downstream of the drawing. In this case, a hollow-core optical fiber having desired optical properties can be manufactured.

[0038]

[25] In the method for manufacturing a hollow-core optical fiber according to any one of the above

[21] to

[24] , when the outer diameter of the jacket tube is φ [mm], the tensile tension applied to the hollow-core optical fiber to be drawn is τ [N], and the temperature difference between the softening point of the cylindrical preform and the softening point of the jacket tube is ΔT [°C], the optical fiber may be manufactured so as to satisfy the following formula (1): In this case, it is possible to more reliably prevent the tube-forming preforms from coming into contact with each other during drawing.

[0039]

[26] In the method for manufacturing a hollow-core optical fiber according to any one of

[21] to

[24] above, the jacket tube may have an inner jacket tube and an outer jacket tube covering the inner jacket tube. The softening point of at least one of the plurality of cylindrical preforms may be higher than the softening point of the outer jacket tube (for example, higher by 60°C or more). When the outer diameter of the jacket tube is φ [mm], the tensile tension applied to the hollow-core optical fiber to be drawn is τ [N], and the temperature difference between the softening point of the cylindrical preform and the softening point of the jacket tube is ΔT [°C], the optical fiber may be manufactured so as to satisfy the following formula (2): In this case, it is possible to more reliably prevent the tube-forming preforms from coming into contact with each other during drawing.

[0040]

[27] In the method for manufacturing a hollow-core optical fiber according to any one of

[21] to

[24] above, the jacket tube may have an inner jacket tube and an outer jacket tube covering the inner jacket tube. The softening point of at least one of the plurality of cylindrical preforms may be higher than the softening point of the outer jacket tube (for example, higher by 60°C or more). When the outer diameter of the jacket tube is φ [mm], the tensile tension applied to the hollow-core optical fiber to be drawn is τ [N], and the temperature difference between the softening point of the cylindrical preform and the softening point of the jacket tube is ΔT [°C], the optical fiber may be manufactured so as to satisfy the following formula (3): In this case, it is possible to more reliably prevent the tube-forming preforms from coming into contact with each other during drawing.

[0041] Specific examples of a preform to be processed into a hollow-core optical fiber, a method for manufacturing a preform to be processed into a hollow-core optical fiber, and a method for manufacturing a hollow-core optical fiber according to embodiments of the present disclosure will be described below with reference to the drawings. In the following description, the same elements or elements having the same functions will be designated by the same reference numerals, and duplicated explanations will be omitted. The present invention 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.

[0042] [Configuration of hollow core optical fiber] One embodiment of a hollow core optical fiber will be described with reference to Figure 1. Figure 1 is a cross-sectional view showing one embodiment of a hollow core optical fiber. As shown in Figure 1, a hollow core optical fiber 10 has a jacket tube 11 and a plurality of capillaries 12. In the example shown in Figure 1, the hollow core optical fiber 10 has seven capillaries 12, but the number of capillaries 12 is not limited as long as it is plural. A hollow core optical fiber 10 having such a configuration is, for example, an anti-resonance hollow core optical fiber, and extends in a direction perpendicular to Figure 1.

[0043] The jacket tube 11 is made of, for example, silica glass (SiO ２ The jacket tube 11 is a hollow tube portion formed from a material such as silica glass (SiO ２ ) and has a smaller diameter than the jacket tube 11. In one example, the diameters of the capillaries 12 are the same. The thickness of the capillaries 12 is much thinner than the thickness of the jacket tube 11, for example, 0.25 μm or more and 0.50 μm or less. Such capillaries 12 are housed in the jacket tube 11 so as to be spaced apart from each other and fixed in contact with the inner surface 11 a of the jacket tube 11 at contact points 13. The gap G, which is the distance between the capillaries 12, is the same in one example.

[0044] With this arrangement, the hollow-core optical fiber 10 has a hollow core portion 14 formed in its central portion. The capillaries 12 and jacket tube 11 surrounding the hollow core portion 14 form a fiber cladding. With the capillaries 12 arranged to be spaced apart from each other, the hollow-core optical fiber 10 can propagate introduced light along the hollow core portion 14 by, for example, the anti-resonance effect. If the capillaries 12 come into contact with each other, an optical mode with high resonance is induced, causing transmission loss; however, by arranging the capillaries 12 with a gap G provided between them, it is possible to prevent the formation of such a mode that causes transmission loss.

[0045] 2 is a cross-sectional view showing a modified hollow-core optical fiber. As shown in FIG. 2, a hollow-core optical fiber 10A according to the modified example has a jacket tube 11, a plurality of capillaries 12, and a plurality of sub-capillaries 15. The sub-capillaries 15 are made of, for example, silica glass (SiO ２ ). In the hollow core optical fiber 10A, a corresponding sub-capillary 15 is provided inside each capillary 12. The sub-capillary 15 is fixed in contact with the inner surface of the capillary 12 at the same position as the contact point 13 where the capillary 12 is fixed in contact with the jacket tube 11. The sub-capillary 15 has an extremely thin thickness like the capillary 12, and has a smaller diameter than the capillary 12. The hollow core optical fiber 10A can further reduce optical transmission loss than the hollow core optical fiber 10. In a hollow core optical fiber, by using double or even triple capillaries in this way, the optical characteristics of the optical fiber can be improved.

[0046] [Method of Manufacturing Preform and Configuration of Preform] Next, a glass preform for manufacturing a hollow-core optical fiber having such a cross-sectional structure and a method of manufacturing the same will be described with reference to Figures 3A, 3B, and 3C. Figures 3A, 3B, and 3C are diagrams for explaining a method of manufacturing a preform to be processed into a hollow-core optical fiber. As shown in Figure 3A, an inner jacket tube 21 corresponding to the jacket tube 11 and multiple cladding tubes 22 (multiple cylindrical preforms) corresponding to the capillary 12 are prepared. Below, a case will be described in which cladding tubes are used as the multiple cylindrical preforms provided in the preform, but the same applies when a cavity-forming wall is used as the cylindrical preform. Note that the cavity-forming wall here has a constant shape along the central axis of the space surrounded by the inner surface of the jacket tube, has a curved surface that protrudes toward the central axis on a cross section perpendicular to the central axis, and together with other elements of the preform, forms a closed region that does not include the central axis, i.e., a cylindrically enclosed cavity, on a cross section perpendicular to the central axis.

[0047] As shown in FIG. 3A, the inner jacket tube 21 is made of, for example, silica glass (SiO ２ The inner jacket tube 21 may be doped with phosphorus, fluorine, chlorine, hydroxyl groups, etc., as described below. Such doping lowers the softening point of the inner jacket tube 21. Each of the multiple cladding tubes 22 is made of, for example, silica glass (SiO ２ ) and has a smaller diameter than the inner jacket tube 21. In one example, the diameters of the cladding tubes 22 are the same. The thickness of the cladding tube 22 is much thinner than the thickness of the inner jacket tube 21.

[0048] Additionally, spacers 23, 24, and 25 are prepared for positioning the multiple cladding tubes 22 at predetermined positions within the inner jacket tube 21. The spacers 23, 24, and 25 are, for example, solid or hollow quartz rods. The spacer 23 is a member disposed in the center of the inner jacket tube 21. The spacer 24 is a member disposed between the cladding tubes 22. The spacer 25 is a member disposed between the spacer 23 and the cladding tube 22.

[0049] After the preparation of the inner jacket tube 21, the multiple cladding tubes 22, and the spacers 23, 24, and 25 is complete, the multiple cladding tubes 22 are placed inside the inner jacket tube 21. At this time, each cladding tube 22 is placed so that it contacts the inner surface 21a of the inner jacket tube 21 at contact points 26 and the cladding tubes 22 are spaced apart from one another. The cladding tubes 22 are spaced apart, for example, by equal distances. To maintain this arrangement, a spacer 23 is placed in the center of the inner jacket tube 21, and spacers 24 and 25 are placed between the cladding tubes 22 and between the cladding tubes 22 and the spacer 23. As a result, the cladding tubes 22 are temporarily fixed in place inside the inner jacket tube 21, as shown in FIG. 3A .

[0050] Next, while maintaining the temporarily fixed state shown in FIG. 3A , the cladding tube 22 is fixed to the inner surface 21 a of the inner jacket tube 21 at the contact point 26 by heating. For example, a flame (e.g., an oxyhydrogen flame) is applied from the outside of the inner jacket tube 21 toward the cladding tube 22 at the contact point 26 in an appropriately controlled manner, or radiant heat is applied from a heating furnace, or a laser is irradiated to melt and bond the two together. At this time, the spacers 23, 24, and 25 are prevented from welding to other elements. To prevent fusion between the cladding tube 22 and the inner jacket tube 21, the cladding tube 22 may be temporarily fixed using the spacers 23 and 25 instead of the spacer 24. Once the cladding tube 22 is fixed to the inner jacket tube 21, the spacers 23, 24, and 25 are removed, and the first preform 20 shown in FIG. 3B is formed. The center of the first preform 20 becomes a hollow core portion 27 corresponding to the hollow core portion 14.

[0051] Subsequently, once the first preform 20 is formed, the first preform 20 may be heated and melted while being shrunk in the radial direction to obtain a first preform 20 having an appropriate size. This shrinking process can be performed, for example, by elongation heating. When the outer diameter of the second preform 20A is set to 20 mm, the outer diameter of the first preform 20 may be set to 2 mm or more and 10 mm or less. When the outer diameter of the second preform 20A is set to 200 mm, the outer diameter of the first preform 20 may be set to 20 mm or more and 100 mm or less. In the shrunken first preform 20, the inner jacket tube 21 and the cladding tube 22 are also shrunk in the radial direction while maintaining their arrangement. Note that a first preform 20 having an appropriate size (e.g., the size described above) may be produced from the beginning without elongation. Once the first preform 20 having an appropriate size is prepared, an outer jacket tube 28 is disposed so as to surround the outside of the first preform 20 having the appropriate size. The outer jacket tube 28 may be made of, for example, silica glass (SiO ) similar to the inner jacket tube 21. ２) As will be described later, phosphorus, fluorine, chlorine, hydroxyl groups, etc. may be added to the outer jacket tube 28. Such addition lowers the softening point of the outer jacket tube 28, similar to that of the inner jacket tube 21. The softening point of the outer jacket tube 28 may be equal to or lower than that of the inner jacket tube 21. The thickness of the outer jacket tube 28 may be thicker or thinner than that of the inner jacket tube 21. Alternatively, when the cross-sectional area of ​​the outer jacket tube 28 is sufficiently larger than that of the inner jacket tube 21 and the outer jacket tube 28 is the main element of the jacket tube, the softening point of the inner jacket tube 21 may be not lower than that of the cladding tube 22 (tubular preform), and the softening point of the outer jacket tube 28 may be lower than that of the cladding tube 22, so that the softening point of the jacket tube 29 (the midpoint as an average of the entire jacket tube including the inner jacket tube 21 and the outer jacket tube 28) may be lower than that of the cladding tube 22. For example, the cross-sectional area of ​​the outer jacket tube 28 may be 5 times or more, 10 times or more, 50 times or more, or 100 times or more than the cross-sectional area of ​​the inner jacket tube 21. In this case, the softening point of the jacket tube 29 may be considered to be a weighted average of the softening points of the inner jacket tube 21 and the outer jacket tube 28, calculated based on the ratio of their cross-sectional areas. Thereafter, the first preform 20 and the outer jacket tube 28 are integrated by melting or the like to form a preform PF (second preform 20A). Note that the preform PF may be formed while maintaining a gap between the first preform 20 and the outer jacket tube 28.

[0052] This forms the second preform 20A, which is the preform PF to be used for drawing when manufacturing the hollow-core optical fiber 10. In the second preform 20A, the inner jacket tube 21 and the outer jacket tube 28 are integrated to form the jacket tube 29. The outer diameter of the first preform 20 may be reduced to match the outer diameter of the second preform 20A. Note that the first preform 20 may be used as the preform PF for drawing without providing the outer jacket tube 28. In this case, the inner jacket tube 21 becomes the jacket tube 29. Alternatively, rod-in drawing may be performed in which the outer jacket tube 28 is not integrated at the preform stage, but is integrated with the inner jacket tube 21 during drawing.

[0053] [Method of manufacturing optical fiber using preform] Next, a method of manufacturing a hollow-core optical fiber HOF from the above-mentioned preform PF using a drawing apparatus 30 shown in Fig. 4 will be described. Fig. 4 is a schematic diagram showing an apparatus for manufacturing a hollow-core optical fiber by drawing a preform. As shown in Fig. 4, the drawing apparatus 30 includes a holding mechanism 31, a heating device 32, a cooling device 33, a resin coating device 34, an outer diameter measuring instrument 35, a direct below roller 36, and a take-up roller 37. In the drawing process, the upper part of the preform PF is held by the holding mechanism 31, and the lower part (tip portion) of the preform PF is heated by the heating device 32 to be in a molten state. The tip portion of the molten preform PF is drawn downward, and is finally elongated to, for example, a glass diameter of 80 µm to 400 µm and a core diameter (diameter of the hollow core portion 14) of 10 µm to 50 µm. The glass fiber GF drawn from the preform PF is cooled by a cooling device 33, and the outer periphery of the cooled glass fiber GF is coated with a resin layer by a resin coating device 34. Thereafter, the outer diameter of the resin-coated glass fiber GF is measured by an outer diameter measuring device 35, and the glass fiber GF is taken up by a take-up roller 37 via a roller 36 directly below. In this way, a hollow-core optical fiber HOF is produced from the preform PF.

[0054] Here, the application of pressure when melting and heating the preform PF to produce the hollow-core optical fiber HOF will be described. Since the hollow-core optical fiber HOF has a configuration in which holes are provided inside, the preform PF is provided with a cladding tube 22, and a hollow core portion 27 is provided at the center of the preform PF surrounded by the cladding tube 22. When melting and drawing such a preform PF, an internal pressure P1 is applied to the hollow core portion 27 from above the preform PF, and an internal pressure P2 is applied to each cladding tube 22. By controlling this internal pressure difference |P1-P2|, the arrangement configuration in the cross section of the preform PF (e.g., the cross section shown in FIG. 3C ) during drawing is maintained even when the hollow-core optical fiber HOF is produced. Incidentally, during drawing, drawing may be performed while reducing the pressure in the space between the inner jacket tube 21 and the outer jacket tube 28.

[0055] However, while internal pressure P2 is applied to prevent the cladding tube 22 from collapsing, the radius of the cladding tube 22 is not the same for the glass fiber GF upstream of the drawing device 30 and the glass fiber GF downstream of the drawing device 30. The radius Rc of the cladding tube 22 decreases from upstream to downstream of the drawing. Because the surface tension of the cladding tube 22 is inversely proportional to the radius Rc, the surface tension of the cladding tube 22 upstream of the drawing is smaller than the surface tension of the cladding tube 22 downstream of the drawing. Meanwhile, the internal pressure P2 applied to the cladding tube 22 is constant from upstream to downstream. From the above, the following relationships are established: Upstream of the drawing, the radius Rc is large and the surface tension is small, so the internal pressure P2 is larger than the surface tension. Downstream of the drawing, the radius Rc is small and the surface tension is large, so the internal pressure P2 is smaller than the surface tension.

[0056] The above-mentioned internal pressure P2 is adjusted to an appropriate value in relation to the internal pressure P1 and surface tension in the final stage of fabrication of the hollow-core optical fiber HOF (i.e., downstream of drawing). Therefore, upstream of drawing, the internal pressure P2 may become greater than the surface tension. This causes the cladding tube 22 to expand. In this case, as shown in FIG. 5 , in the preform PF (before drawing), the cladding tubes 22 are arranged so as to be spaced apart from each other, and the center-to-center distance, as shown by "2*Rc_contact," is twice the radius Rc, i.e., greater than the outer diameter (2*Rc) of the cladding tube. However, if the cladding tubes 22 expand, they may come into contact with adjacent cladding tubes 22. If the cladding tubes 22 come into contact, the optical properties of the resulting hollow-core optical fiber HOF will deteriorate, as described above.

[0057] FIG. 6 illustrates how the ratio of the cladding tube outer diameter 2*Rc to the distance between the cladding tubes (2*Rc_contact) varies from upstream to downstream of the drawing. As shown in FIG. 6 , if this ratio exceeds 1, the cladding tubes 22 may come into contact with each other. Note that z [mm] in FIG. 6 refers to the distance from a point upstream of the drawing (e.g., the entrance of the drawing furnace). As shown in FIG. 6 , near the upstream (50 mm), the internal pressure P2 becomes greater than the surface tension, causing the ratio to exceed 1.

[0058] 7 shows the relationship between the peak value of the ratio of the cladding tube outer diameter 2*Rc to the distance between the cladding tubes (2*Rc_contact) and the drawing tension [gf]. As shown in FIG. 7, by increasing the drawing tension higher than normal, it is possible to stretch the cladding tube 22 before it deforms. However, for example, a preform PF with a diameter of 20 mm requires a drawing tension of 800 gf or more, and a large-diameter preform PF with a diameter of 200 mm requires an even higher drawing tension of 8000 gf or more. Drawing at such a high drawing tension may increase the frequency of optical fiber breakage.

[0059] Therefore, in the preform PF according to this embodiment, the softening point of the cladding tube 22 is adjusted to be higher than the softening point of the jacket tube 29 (the inner jacket tube 21 and the outer jacket tube 28). During the heat melting process during fiber drawing, the heating temperature is adjusted so that the jacket tube 29 melts appropriately. Therefore, by setting the softening point of the cladding tube 22 higher than the softening point of the jacket tube 29, the molten state of the cladding tube 22 can be made harder than the jacket tube during fiber drawing. This ensures that the internal pressure P2 acting on the cladding tube 22 remains lower than the strength of the molten cladding tube 22, even if the internal pressure P2 is greater than the surface tension of the cladding tube 22 upstream of fiber drawing. This prevents the cladding tube 22 from expanding due to the internal pressure P2. The softening point here refers to the temperature at which the glass forming the capillary and jacket tube begins to soften and deform under its own weight. Typically, the softening point is measured in accordance with JIS R 3103-1 (2001) when the glass viscosity is about 10 ７．６ The viscosity of the material can be defined as "the temperature at which the viscosity becomes dPa·s."

[0060] The softening point of the glass member can be adjusted in this way by adding, for example, phosphorus (P), fluorine (F), chlorine (Cl), or hydroxyl (OH) or a combination thereof to the jacket tube 29.２ ) and is a table shown in figure 1 of "Hammond, CR (1978) Fusion temperatures of SiO2-P2O5 binary glasses. Physics and Chemistry of Glasses, 19(3), 41-42." Figure 9 shows the decrease in softening point when phosphorus is added to silica glass (SiO ２ ) shows the decrease in softening point when fluorine is added to the preform PF. For example, by adding any of phosphorus, fluorine, chlorine, and hydroxyl groups to the jacket tube 29, the softening point of the jacket tube 29 can be made lower than that of the cladding tube 22. In this case, when drawing the hollow-core optical fiber HOF, the heating temperature of the preform PF by the heating device 32 can be reduced by the difference in softening points, so that the preform PF can be melted and drawn at, for example, 2000°C or less. In other words, it can be melted at a lower temperature than usual. Conversely, the softening point of the cladding tube 22 itself may be increased by adding a material that increases the softening point (e.g., aluminum (Al)) to the cladding tube 22. By using the various methods described above, the softening point of the cladding tube 22 in the preform PF can be made higher than that of the jacket tube 29. Note that methods other than those described above may also be used, as they are obvious to those skilled in the art, and therefore will not be described here.

[0061] 10A to 10D, the relationship between the peak value of the ratio of the cladding tube outer diameter 2*Rc to the distance between the cladding tubes (2*Rc_contact) and the difference [K] between the softening points of the cladding tube 22 and the jacket tube 29 will be described when the diameter of the preform PF is changed. Figures 10A to 10D show the results of simulations of these relationships.

[0062] Figure 10A is a graph showing the relationship between the ratio of the cladding tube outer diameter to the distance between the cladding tubes and the softening point difference for each drawing tension for a 20 mm diameter preform. Figure 10B is a graph showing the relationship between the ratio of the cladding tube outer diameter to the distance between the cladding tubes and the softening point difference for each drawing tension for a 50 mm diameter preform. Figure 10C is a graph showing the relationship between the cladding tube outer diameter to the distance between the cladding tubes and the softening point difference for each drawing tension for a 100 mm diameter preform. Figure 10D is a graph showing the relationship between the cladding tube outer diameter to the distance between the cladding tubes and the softening point difference for each drawing tension for a 200 mm diameter preform. 10A to 10D show the relationship for drawing tensions of 100 gf (0.980665 N), 200 gf (1.96133 N), 500 gf (4.903325 N), and 1000 gf (9.80665 N).

[0063] 10A , if the difference in softening points is 60° C. or more, even when a preform PF with a diameter of 20 mm is drawn with a normal drawing tension of 200 gf, it is possible to prevent the cladding tubes 22 from contacting each other due to expansion of the cladding tubes 22. Furthermore, if the difference in softening points is 110° C. or more, it is possible to prevent the cladding tubes 22 from contacting each other due to expansion of the cladding tubes 22, even when the preform PF is drawn with an even lower tension of 100 gf.

[0064] As shown in Figure 10B, if the difference in softening point is 60°C or more, even if a preform PF with a diameter of 50 mm is drawn with a drawing tension of 500 gf, contact between the cladding tubes 22 due to expansion of the cladding tubes 22 can be prevented.

[0065] As shown in Figure 10C, if the difference in softening point is 60°C or more, even if a preform PF with a diameter of 100 mm is drawn with a drawing tension of 1000 gf, contact between the cladding tubes 22 due to expansion of the cladding tubes 22 can be prevented.

[0066] 10D , if the softening point difference is 190° C. or more, even when a preform PF with a diameter of 200 mm is drawn at a normal drawing tension of 100 gf or 200 gf, it is possible to prevent the cladding tubes 22 from coming into contact with each other due to expansion of the cladding tubes 22. More specifically, in the case of a preform with a diameter of 200 mm, the softening point difference is 90° C. or more when the drawing tension is 1000 gf, the softening point difference is 120° C. or more when the drawing tension is 500 gf, the softening point difference is 160° C. or more when the drawing tension is 200 gf, and the softening point difference is 190° C. or more when the drawing tension is 100 gf, allowing for drawing to be performed without any problems.

[0067] Here, if the outer diameter of the jacket tube 29 is φ [mm], the drawing tension (tensile tension) applied to the hollow core optical fiber being drawn is τ [N], and the temperature difference between the softening point of the cladding tube 22 and the softening point of the jacket tube 29 is ΔT [°C], then by manufacturing the optical fiber so that the following formula (1) is satisfied, drawing can be performed without any problems. The above formula 1 is calculated as an empirical formula based on the conditions under which Rc / Rc_contact is less than 1 from FIGS. 10A to 10D.

[0068] Furthermore, the prevention of contact between the cladding tubes 22 due to the temperature difference in softening point described above can be further defined by the following formula. In this case, the jacket tube 29 has an inner jacket tube 21 and an outer jacket tube 28 that covers the inner jacket tube 21. The softening points of the multiple cladding tubes 22 are higher by 60°C or more than the softening point of the outer jacket tube 28. In this case, when the outer diameter of the jacket tube 29 is φ [mm], the drawing tension (tensile tension) applied to the hollow-core optical fiber being drawn is τ [N], and the temperature difference between the softening points of the cladding tube 22 and the jacket tube 29 is ΔT [°C], the optical fiber may be manufactured so as to further satisfy the following formula (2). The above formula 2 was calculated empirically from FIGS. 10A to 10D to determine the condition under which the value of Rc / Rc_contact increases by less than 10% compared to the value when the softening point difference is 300K.

[0069] Furthermore, the prevention of contact between the cladding tubes 22 due to the temperature difference in softening point described above can be further defined by the following formula. In this case, the jacket tube 29 has an inner jacket tube 21 and an outer jacket tube 28 that covers the inner jacket tube 21. The softening points of the multiple cladding tubes 22 are higher by 60°C or more than the softening point of the outer jacket tube 28. In this case, when the outer diameter of the jacket tube 29 is φ [mm], the drawing tension (tensile tension) applied to the hollow-core optical fiber being drawn is τ [N], and the temperature difference between the softening points of the cladding tube 22 and the jacket tube 29 is ΔT [°C], the optical fiber may be manufactured so as to further satisfy the following formula (3). The above formula was empirically calculated from FIGS. 10A to 10D under the condition that the value of Rc / Rc_contact increases by less than 5% compared to the value when the softening point difference is 300K.

[0070] As described above, in the preform PF according to this embodiment, the softening point of the cladding tube 22 is higher than the softening point of the jacket tube 29. As a result, when the preform PF is melted and drawn, the jacket tube 29 (the inner jacket tube 21 and the outer jacket tube 28) is sufficiently melted, while softening of the cladding tube 22 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 22. As a result, when the preform PF is drawn to produce a hollow-core optical fiber HOF, even if the internal pressure P2 applied to the cladding tube 22 becomes larger than the surface tension acting on the cladding tube 22 near the upstream of drawing, it is possible to prevent the cladding tube 22 from expanding and coming into contact with adjacent cladding tubes 22.

[0071] Furthermore, when manufacturing a hollow-core optical fiber HOF, the optical fiber is manufactured by drawing a preform PF in which the softening point of the cladding tube 22 is higher than the softening point of the jacket tube 29. In this case, when the preform PF is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cladding tube 22 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 22. As a result, when manufacturing a hollow-core optical fiber HOF, even if the internal pressure P2 applied to the cladding tube 22 near the upstream of drawing becomes greater than the surface tension acting on the cladding tube 22, it is possible to prevent the cladding tube 22 from expanding and coming into contact with adjacent cladding tubes 22. Therefore, according to this manufacturing method, it is possible to manufacture a hollow-core optical fiber HOF with improved optical characteristics.

[0072] The preform to be processed into a hollow-core optical fiber, the method for manufacturing the preform to be processed into a hollow-core optical fiber, and the method for manufacturing the hollow-core optical fiber according to the embodiments of the present disclosure have been described in detail above, but the present invention is not limited to the above embodiments and can be applied to various embodiments and modifications. For example, the above-mentioned embodiments illustrate a technique for lowering the softening point by adding phosphorus, fluorine, chlorine, or a hydroxyl group to the jacket tube 29, but the present invention is not limited to this. For example, the softening point may be lowered by adding chlorine or nitrogen to the jacket tube 29.

[0073] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF1 according to a first modified example shown in Fig. 11. The preform PF1 includes a jacket tube 29, a plurality of cavity-forming walls 122 (cylindrical preforms), and sub-tubes 123 arranged within each cavity-forming wall 122. Each of the cavity-forming walls 122 has a constant shape along the central axis C of the space surrounded by the inner surface 29a of the jacket tube 29. Each of the cavity-forming walls 122 has a curved surface that protrudes toward the central axis C on a cross section perpendicular to the central axis C. As an example, the cavity-forming wall 122 has a sector-shaped cross section. On a cross section perpendicular to the central axis C, the cavity-forming wall 122, together with the jacket tube 29, which is another element of the preform PF1, forms a closed region 124 that does not include the central axis C, i.e., a cylindrically enclosed cavity. For example, a pair of sub-tubes 123 is arranged in the closed region 124. The pair of sub-tubes 123 are arranged in parallel and fixed by melting or the like to the inner surface 29a of the jacket tube 29. Both the cavity-forming wall 122 and the sub-tubes 123 have the same thickness as the cladding tube 22 described above.

[0074] In the preform PF1 according to the first modification, the softening points of the cavity-forming walls 122 and the sub-tubes 123 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF1 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cavity-forming walls 122 and the sub-tubes 123 at the melting temperature is suppressed, thereby maintaining the strength of the cavity-forming walls 122 and the sub-tubes 123. As a result, when the preform PF1 is drawn to manufacture a hollow-core optical fiber, even if the internal pressure P2 applied to the cavity-forming walls 122 and the sub-tubes 123 near the upstream of the drawing becomes larger than the surface tension acting on the cavity-forming walls 122 and the sub-tubes 123, it is possible to prevent the cavity-forming walls 122 and the sub-tubes 123 from expanding and coming into contact with adjacent cavity-forming walls 122 and sub-tubes 123.

[0075] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF2 according to a second modified example shown in Fig. 12. The preform PF2 includes a jacket tube 29, a plurality of cavity-forming walls 122A (cylindrical preforms), and sub-tubes 125, 126 arranged within the cavity-forming walls 122A. Each of the cavity-forming walls 122A has a constant shape along the central axis C of the space surrounded by the inner surface 29a of the jacket tube 29. Each of the cavity-forming walls 122A has a curved surface that protrudes toward the central axis C on a cross section perpendicular to the central axis C. The cavity-forming walls 122A have, as an example, a sector-shaped cross section. On a cross section perpendicular to the central axis C, the cavity-forming walls 122A, together with the jacket tube 29, which is another element of the preform PF2, form a closed region 124A that does not include the central axis C, i.e., a cylindrically enclosed cavity. The sub-tubes 125 and 126 are disposed within the closed region 124. The sub-tube 126 is disposed within the sub-tube 125. The sub-tubes 125 and 126 are fixed to the inner surface 29a of the jacket tube 29. The cavity-forming wall 122A and the sub-tubes 125 and 126 all have the same thickness as the cladding tube 22 described above.

[0076] In the preform PF2 according to the second modification, the softening points of the cavity-forming walls 122A and the sub-tubes 125, 126 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF2 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cavity-forming walls 122A and the sub-tubes 125, 126 at the melting temperature is suppressed, thereby maintaining the strength of the cavity-forming walls 122A and the sub-tubes 125, 126. As a result, when the preform PF2 is drawn to manufacture a hollow-core optical fiber, even if the internal pressure P2 applied to the cavity-forming walls 122A and the sub-tubes 125, 126 near the upstream of the drawing becomes larger than the surface tension acting on the cavity-forming walls 122A and the sub-tubes 125, 126, it is possible to prevent the cavity-forming walls 122A and the sub-tubes 125, 126 from expanding and coming into contact with the adjacent cavity-forming walls 122A, etc.

[0077] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF3 according to a third modified example shown in Fig. 13. The preform PF3 comprises a jacket tube 29, a plurality of cladding tubes 122B (tubular preforms), and a sub-tube 127 arranged within the cladding tube 122B. In this modified example, a support element preform 128 that contacts the inner surface of the cladding tube 122B is provided in a region inside the cladding tube 122B closer to the jacket tube 29. The sub-tube 127 is arranged within the cladding tube 122B and is fixed in contact with the support element preform 128.

[0078] In PF3 according to the third modification, the softening points of the cladding tube 122B and the sub-tubes 127 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF3 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cladding tube 122B and the sub-tubes 127 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 122B and the sub-tubes 127. As a result, when the preform PF3 is drawn to produce a hollow-core optical fiber, even if the internal pressure P2 applied to the cladding tube 122B and the sub-tubes 127 near the upstream of drawing becomes greater than the surface tension acting on the cladding tube 122B and the sub-tubes 127, it is possible to prevent the cladding tube 122B and the sub-tubes 127 from expanding and coming into contact with the adjacent cladding tubes 122B, etc.

[0079] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF4 according to a fourth modified example shown in Fig. 14. The preform PF4 includes a jacket tube 29 and a plurality of cladding tubes 122B (tubular preforms). In this modified example, support element preforms 128 that contact the inner surface of the cladding tube 122B are provided in a region inside the cladding tube 122B closer to the jacket tube 29. The sub-tube 127 is disposed within the cladding tube 122B and is fixed in contact with the support element preforms 128. Furthermore, in the preform PF4, support element preforms 129 are further provided between the inner surface 29a of the jacket tube 29 and each of the plurality of support element preforms 128. Each of the support element preforms 129 is fixed so as to contact each of the plurality of support element preforms 128 and the inner surface 29a of the jacket tube 29. Support element preform 129 supports cladding tube 122B together with support element preform 128. Support element preform 129 may be made of the same material as cladding tube 122B (e.g., a glass material), or may be made of another material.

[0080] In the preform PF4 according to the fourth modification, the softening points of the cladding tube 122B and the sub-tubes 127 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF4 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cladding tube 122B and the sub-tubes 127 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 122B and the sub-tubes 127. As a result, when the preform PF4 is drawn to produce a hollow-core optical fiber, even if the internal pressure P2 applied to the cladding tube 122B and the sub-tubes 127 near the upstream of drawing becomes greater than the surface tension acting on the cladding tube 122B and the sub-tubes 127, it is possible to prevent the cladding tube 122B and the sub-tubes 127 from expanding and coming into contact with the adjacent cladding tubes 122B, etc.

[0081] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF5 according to a fourth modified example shown in Fig. 15. The preform PF5 includes a jacket tube 29, a plurality of cladding tubes 132 (tubular preforms), and a plurality of sub-tubes 133. Each of the sub-tubes 133 is disposed within the corresponding cladding tube 132. The sub-tubes 133 are fixed in contact with a portion of the inner surface of the cladding tube 132 that is closer to the jacket tube 29. In this modified example, a pair of support element preforms 134 is provided between each cladding tube 132 and the inner surface 29a of the jacket tube 29. The pair of support element preforms 134 support and fix the cladding tube 132. The pair of support element preforms 134 may be formed from the same material (e.g., glass material) as the cladding tube 132, or may be formed from another material.

[0082] In the preform PF5 according to the fifth modification, the softening points of the cladding tube 132 and the sub-tubes 133 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF5 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cladding tube 132 and the sub-tubes 133 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 132 and the sub-tubes 133. As a result, when the preform PF5 is drawn to produce a hollow-core optical fiber, even if the internal pressure P2 applied to the cladding tube 132 and the sub-tubes 133 near the upstream of drawing becomes greater than the surface tension acting on the cladding tube 132 and the sub-tubes 133, it is possible to prevent the cladding tube 132 and the sub-tubes 133 from expanding and coming into contact with the adjacent cladding tube 132, etc.

[0083] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF6 according to a sixth modified example shown in Fig. 16. The preform PF6 includes a jacket tube 29, a plurality of cladding tubes 132 (tubular preforms), and a plurality of sub-tubes 133. Each of the sub-tubes 133 is disposed within the corresponding cladding tube 132. In this modified example, a pair of support element preforms 135 is provided between the inner surface of each cladding tube 132 and the outer surface of the sub-tube 133. The pair of support element preforms 135 support the sub-tube 133 and fix the sub-tube 133 to the cladding tube 132. The pair of support element preforms 135 may be formed from the same material (e.g., glass material) as the cladding tube 132, or may be formed from another material.

[0084] In the preform PF6 according to the sixth modification, the softening points of the cladding tube 132 and the sub-tubes 133 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF6 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cladding tube 132 and the sub-tubes 133 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 132 and the sub-tubes 133. As a result, when the preform PF6 is drawn to produce a hollow-core optical fiber, even if the internal pressure P2 applied to the cladding tube 132 and the sub-tubes 133 near the upstream of drawing becomes greater than the surface tension acting on the cladding tube 132 and the sub-tubes 133, it is possible to prevent the cladding tube 132 and the sub-tubes 133 from expanding and coming into contact with the adjacent cladding tube 132, etc.

[0085] Furthermore, the preform processed into the hollow-core optical fiber according to this embodiment may be, for example, a preform PF7 according to a seventh modified example shown in Fig. 17. The preform PF7 includes a jacket tube 29, a plurality of cladding tubes 132 (tubular-forming preforms), and a plurality of sub-tubes 133. Each of the sub-tubes 133 is disposed within the corresponding cladding tube 132. The sub-tubes 133 are disposed within the cladding tube 132 closer to the jacket tube 29. In this modified example, a pair of support element preforms 134 is provided between each cladding tube 132 and the inner surface 29a of the jacket tube 29. The pair of support element preforms 134 supports the cladding tube 132. In this modified example, a pair of support element preforms 135 is further provided between the inner surface of each cladding tube 132 and the outer surface of the sub-tube 133. The pair of support element preforms 135 supports and fixes the sub-tubes 133. The pair of support element preforms 134, 135 may be made of the same material as the cladding tube 132 (for example, a glass material), or may be made of another material.

[0086] In the preform PF7 according to the seventh modification, the softening points of the cladding tube 132 and the sub-tubes 133 are also higher than the softening point of the jacket tube 29. As a result, when the preform PF7 is melted and drawn, the jacket tube 29 is sufficiently melted, while softening of the cladding tube 132 and the sub-tubes 133 at the melting temperature is suppressed, thereby maintaining the strength of the cladding tube 132 and the sub-tubes 133. As a result, when the preform PF7 is drawn to produce a hollow-core optical fiber, even if the internal pressure P2 applied to the cladding tube 132 and the sub-tubes 133 near the upstream of drawing becomes greater than the surface tension acting on the cladding tube 132 and the sub-tubes 133, it is possible to prevent the cladding tube 132 and the sub-tubes 133 from expanding and coming into contact with the adjacent cladding tube 132, etc.

[0087] It should be understood that at least one configuration or feature described in each embodiment, example, or modification can be combined with other embodiments, examples, or modifications, or can be modified in various ways.

[0088] Here, the following are further appended as aspects included in the present disclosure. [Appendix 1] A preform to be processed into a hollow-core optical fiber, comprising: a jacket tube having an inner surface; and a plurality of capillaries fixed so as to be in contact with the inner surface of the jacket tube while being spaced apart from each other, wherein the softening point of the capillaries is higher than that of the jacket tube. [Appendix 2] The preform of Appendices 1, wherein the softening point of the capillaries is higher than that of the jacket tube by 60°C or more. [Appendix 3] The preform of Appendices 1, wherein the softening point of the capillaries is higher than that of the jacket tube by 190°C or more. [Appendix 4] The preform of any of Appendices 1 to 3, wherein the jacket tube is doped with at least one of phosphorus, fluorine, and a hydroxyl group. [Appendix 5] The preform of any of Appendices 1 to 4, wherein the capillaries are doped with aluminum. [Appendix 6] A method for manufacturing a preform to be processed into a hollow-core optical fiber, comprising the steps of: arranging a plurality of capillaries inside an inner jacket tube so that the capillaries are spaced apart and in contact with the inner surface of the inner jacket tube; and fixing the capillaries to the inner surface of the inner jacket tube, wherein in the fixing step, the softening points of the capillaries are set higher than the softening point of the inner jacket tube, and the capillaries and the inner jacket tube are heated. [Appendix 7] A method for manufacturing a preform according to Appendix 7, further comprising the steps of: shrinking a first preform formed in the fixing step in a radial direction while heating it, and further providing an outer jacket tube so as to cover the shrunken first preform, thereby forming a second preform. [Appendix 8] A method for manufacturing a preform according to Appendix 7, wherein the softening point of the outer jacket tube is lower than the softening point of the inner jacket tube. [Supplementary Note 9] The method for producing a preform according to Supplementary Note 7 or Supplementary Note 8, wherein the softening point of the capillary is higher by 60° C. or more than the softening point of at least one of the inner jacket tube and the outer jacket tube.[Supplementary Note 10] A method for manufacturing a preform according to any one of Supplementary Notes 7 to 9, wherein at least one of the inner jacket tube and the outer jacket tube is doped with at least one of phosphorus, fluorine, and a hydroxyl group. [Supplementary Note 11] A method for manufacturing a hollow-core optical fiber, comprising the steps of preparing a preform according to any one of Supplementary Notes 1 to 5, and heating and drawing the preform. [Supplementary Note 12] A method for manufacturing a hollow-core optical fiber according to Supplementary Note 11, wherein in the drawing step, drawing is performed while applying internal pressure to a plurality of the capillaries, and the optical fiber being drawn is maintained in a state where the internal pressure applied to the capillaries is smaller than the strength of the molten capillaries from upstream to downstream of drawing.

[0089] DESCRIPTION OF SYMBOLS 10, 10A... Hollow-core optical fiber 11... Jacket tube 11a... Inner surface 12... Capillary 13... Contact point 14... Hollow core portion 15... Sub-capillary 20... First preform 20A... Second preform 21... Inner jacket tube 21a... Inner surface 22, 122B, 132... Cladding tube 23, 24, 25... Spacer 26... Contact point 27... Hollow core portion 28... Outer jacket tube 29... Jacket tube 29a... Inner surface 30... Fiber drawing device 31... Holding mechanism 32... Heating device 33... Cooling device 34... Resin coating device 35... Outer diameter measuring device 36... Directly below roller 37... Winding roller 122, 122A... Cavity forming wall 123, 125, 126, 127, 133... Sub-tube 124, 124A...Closed area 128...Support element preform 129, 134, 135...Support element preform 2*Rc...Outer diameter of cladding tube 2*Rc_contact...Distance between cladding tubes C...Central axis G...Gap GF...Glass fiber PF, PF1, PF2, PF3, PF4, PF5, PF6, PF7...Preform P1, P2...Internal pressure HOF...Hollow-core optical fiber

Claims

1. A preform to be processed into a hollow-core optical fiber, comprising: a jacket tube having an inner surface; and a plurality of cylindrical preforms arranged apart from one another within a space enclosed by the inner surface of the jacket tube, wherein the softening point of at least one of the plurality of cylindrical preforms is higher than the softening point of the jacket tube.

2. The preform according to claim 1, wherein the softening point of the cylindrical preform is higher than the softening point of the jacket tube by 60°C or more.

3. The preform according to claim 1, wherein the softening point of the cylindrical preform is higher than the softening point of the jacket tube by 190°C or more.

4. A preform according to any one of claims 1 to 3, wherein at least one of phosphorus, fluorine, chlorine, and a hydroxyl group is added to the jacket tube.

5. A preform according to any one of claims 1 to 4, wherein aluminum is added to the cylindrical preform.

6. A preform according to any one of claims 1 to 5, wherein the plurality of cylindrically formed preforms include at least one of a cylindrical cladding tube and a cavity-forming wall, the cavity-forming wall having a constant shape along the central axis of the space surrounded by the inner surface of the jacket tube, the cavity-forming wall having a curved surface that protrudes towards the central axis on a cross section perpendicular to the central axis, and the cavity-forming wall, together with other elements of the preform, forming a cylindrically enclosed cavity that is a closed region that does not include the central axis on a cross section perpendicular to the central axis.

7. A preform according to any one of claims 1 to 6, wherein the plurality of cylindrical preforms are fixed so as to contact the inner surface of the jacket tube.

8. A preform according to any one of claims 1 to 7, further comprising a support element preform disposed between the inner surface of the jacket tube and each of the plurality of tube-forming preforms, each of the support element preforms being fixed so as to contact at least one of the plurality of tube-forming preforms and the inner surface of the jacket tube.

9. A preform according to any one of claims 1 to 8, wherein the jacket tube comprises an inner jacket tube and an outer jacket tube covering the inner jacket tube, and the softening point of at least one of the plurality of cylindrical preforms is higher than the softening point of the outer jacket tube.

10. A preform according to claim 9, wherein the softening point of the cylindrical preform is at least 60°C higher than the softening point of the outer jacket tube.

11. A preform according to claim 9 or claim 10, wherein the outer jacket tube is doped with at least one of phosphorus, fluorine, chlorine, and a hydroxyl group.

12. A method for manufacturing a preform to be processed into a hollow-core optical fiber, comprising: a step of arranging a plurality of tubular preforms in a space surrounded by the inner surface of an inner jacket tube, wherein the plurality of tubular preforms are arranged so as to be spaced apart from one another; and a step of fixing the plurality of tubular preforms in the space surrounded by the inner surface of the inner jacket tube, wherein the softening point of at least one of the plurality of tubular preforms is higher than the softening point of the inner jacket tube, and wherein the fixing step includes heating the tubular preform and the inner jacket tube.

13. The method for manufacturing a preform according to claim 12, further comprising the step of shrinking the first preform formed in the fixing step in the radial direction while heating it, and further providing an outer jacket tube so as to cover the shrunken first preform, thereby forming a second preform.

14. A method for manufacturing a preform to be processed into a hollow-core optical fiber, comprising: a step of arranging a plurality of tubular preforms in a space surrounded by the inner surface of an inner jacket tube, wherein the plurality of tubular preforms are arranged so as to be spaced apart from one another; a step of fixing the plurality of tubular preforms in the space surrounded by the inner surface of the inner jacket tube; and a step of heating a first preform formed in the fixing step while shrinking it in the radial direction, and further providing an outer jacket tube to cover the shrunken first preform, to form a second preform, wherein the softening point of at least one of the plurality of tubular preforms is higher than the softening point of the outer jacket tube, and wherein the fixing step involves heating the tubular preform and the inner jacket tube.

15. A method for producing a preform according to claim 13 or 14, wherein the softening point of the outer jacket tube is lower than the softening point of the inner jacket tube.

16. A method for manufacturing a preform according to any one of claims 12 to 15, wherein the softening point of the cylindrically formed preform is 60°C or more higher than the softening point of at least one of the inner jacket tube and the outer jacket tube.

17. A method for producing a preform according to any one of claims 12 to 16, wherein at least one of the inner jacket tube and the outer jacket tube is doped with at least one of phosphorus, fluorine, chlorine, and a hydroxyl group.

18. A method for manufacturing a preform as claimed in any one of claims 12 to 17, wherein the plurality of cylindrically formed preforms include at least one of a cylindrical cladding tube and a cavity-forming wall, the cavity-forming wall having a constant shape along the central axis of a space surrounded by the inner surface of the inner jacket tube, the cavity-forming wall having a curved surface that protrudes towards the central axis on a cross section perpendicular to the central axis, and the cavity-forming wall, together with other elements of the preform, forms a cylindrically enclosed cavity that is a closed region that does not include the central axis on a cross section perpendicular to the central axis.

19. A method for manufacturing a preform according to any one of claims 12 to 18, wherein in the fixing step, the plurality of cylindrical preforms are fixed so that they contact the inner surface of the jacket tube.

20. A method for manufacturing a preform according to any one of claims 12 to 18, wherein in the fixing step, a support element preform is provided so as to contact each of the plurality of cylindrical preforms and the inner surface of the jacket tube.

21. A method for producing a hollow-core optical fiber, comprising the steps of: preparing a preform according to any one of claims 1 to 11; and heating and drawing the preform.

22. A method for manufacturing a hollow-core optical fiber as set forth in claim 21, wherein in the drawing step, drawing is performed while applying internal pressure to the space inside the plurality of tubular preforms, and in the optical fiber being drawn, the internal pressure applied to each of the plurality of tubular preforms from upstream to downstream of drawing is maintained in a state smaller than the surface tension of each of the plurality of molten tubular preforms.

23. A method for manufacturing a hollow-core optical fiber as set forth in claim 21 or claim 22, wherein the jacket tube has an inner jacket tube and an outer jacket tube covering the inner jacket tube, the softening point of at least one of the plurality of tubular preforms is higher than the softening point of the outer jacket tube, and in the drawing step, drawing is performed while applying internal pressure to the internal space of the plurality of tubular preforms from upstream to downstream of drawing so that the molten plurality of tubular preforms do not come into contact with each other, and in the drawing step, drawing is optionally performed while reducing the pressure in the space between the inner jacket tube and the outer jacket tube.

24. A method for manufacturing a hollow-core optical fiber as set forth in any one of claims 21 to 23, wherein in the drawing step, drawing is performed while applying internal pressure to the space inside the plurality of tubular preforms, and in the drawing step, a state in which the molten plurality of tubular preforms are kept out of contact with each other from upstream to downstream of drawing.

25. When the outer diameter of the jacket tube is φ [mm], the tensile tension applied to the hollow-core optical fiber being drawn is τ [N], and the temperature difference between the softening point of the cylindrical preform and the softening point of the jacket tube is ΔT [°C], an optical fiber is produced so as to satisfy the following formula (1):

25. A method for manufacturing a hollow-core optical fiber according to any one of claims 21 to 24.

26. The jacket tube has an inner jacket tube and an outer jacket tube covering the inner jacket tube, and the softening point of at least one of the plurality of cylindrical preforms is higher than the softening point of the outer jacket tube, and the outer diameter of the jacket tube is φ [mm], the tensile force applied to the hollow-core optical fiber being drawn is τ [N], and the temperature difference between the softening point of the cylindrical preform and the softening point of the jacket tube is ΔT [°C], the optical fiber is manufactured so as to satisfy the following formula (2):

25. A method for manufacturing a hollow-core optical fiber according to any one of claims 21 to 24.

27. The jacket tube has an inner jacket tube and an outer jacket tube covering the inner jacket tube, and the softening point of at least one of the plurality of cylindrical preforms is higher than the softening point of the outer jacket tube, and the outer diameter of the jacket tube is φ [mm], the tensile force applied to the hollow-core optical fiber being drawn is τ [N], and the temperature difference between the softening point of the cylindrical preform and the softening point of the jacket tube is ΔT [°C], the optical fiber is manufactured so as to satisfy the following formula (3):

25. A method for manufacturing a hollow-core optical fiber according to any one of claims 21 to 24.

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