Hollow-core optical fiber and method for manufacturing hollow-core optical fiber

Abstract

The hollow-core optical fiber is provided with a hollow region that extends in the longitudinal direction. The hollow region has: a glass structure that constitutes an optical transmission path in which a hollow section is provided; and an outer cladding layer surrounding the glass structure. If 2a is the diameter of a circle having the same area as the total area of a cross section orthogonal to the longitudinal direction of the glass structure, n1 is the average refractive index of the glass structure, n2 is the average refractive index of the outer cladding layer, and lambda is the wavelength of light propagating through the light transmission path, (2) is the average refractive index of a circle having the same area as the total area of the cross section orthogonal to the longitudinal direction of the glass structure. N1gt when the relative refractive index difference between the glass structure and the outer cladding layer is set to be delta represented by formula (1); # imgabs0 # n2, normalization frequency v represented by formula (2) of 2.56 or more

Description

Technical Field

[0001] The present disclosure relates to a hollow core optical fiber and a method for manufacturing a hollow core optical fiber. This application claims priority based on Japanese application No. 2022-181193 filed on November 11, 2022, and all the contents described in the Japanese application are incorporated herein by reference. Background Art

[0002] As hollow core fibers, for example, photonic crystal hollow core fibers and anti-resonant hollow core fibers are known. Photonic crystal hollow core fibers are disclosed in Patent Document 1. Anti-resonant hollow core fibers are disclosed in Patent Documents 2 and 3 and Non-Patent Documents 1 and 2.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. 2009 / 044100

[0006] Patent Document 2: Japanese Patent Application No. 2017-52084

[0007] Patent Document 3: International Publication No. 2020 / 217052

[0008] Non-patent literature

[0009] Non-patent document 1: W. Belardi et al., "Hollow antiresonant fibers with reduced attenuation", OPTICS LETTERS, Vol. 39, No. 7, April 1, 2014, p. 1853-1856

[0010] Non-Patent Literature 2: Jeff Hecht, “Is Nothing Better Than Something?”, OPTICS&PHOTONICS NEWS MARCH 2021, p.28-34 Summary of the invention

[0011] The hollow-core optical fiber disclosed in the present invention has a hollow region, which is a region extending in the length direction. The hollow region has: a glass structure constituting a light transmission path provided with a hollow portion; and an outer cladding surrounding the glass structure. When the diameter of a circle having an area equal to the total area of ​​the cross-section of the glass structure orthogonal to the length direction is set to 2a, the average refractive index of the glass structure is set to n1, the average refractive index of the outer cladding is set to n2, the wavelength of light propagating in the light transmission path is set to λ, and the relative refractive index difference between the glass structure and the outer cladding is set to Δ represented by formula (1), n1>n2, and the normalized frequency v represented by formula (2) is greater than 2.56,

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a cross-sectional view along the longitudinal direction of the hollow-core optical fiber according to the first embodiment.

[0014] Figure 2 yes Figure 1 A cross-sectional view of a hollow-core optical fiber perpendicular to the length direction.

[0015] Figure 3 It is a diagram for explaining the drawing step of the method for producing the hollow-core optical fiber according to the first embodiment.

[0016] Figure 4 It is a cross-sectional view along the longitudinal direction of the hollow-core optical fiber according to the second embodiment.

[0017] Figure 5 is a cross-sectional view of a solid area.

[0018] Figure 6 It is a diagram for explaining the wire drawing process including the solidification process.

[0019] Figure 7 This is a diagram for explaining a method of detecting an abnormal portion by OTDR measurement.

[0020] Figure 8 It is a cross-sectional view of a hollow region of a hollow-core optical fiber according to the third embodiment.

[0021] Fig. 9 is a cross-sectional view showing a connection portion between the hollow region and the general optical fiber.

[0022] Fig.10 is a cross-sectional view showing a connection portion between a solid region and a common optical fiber.

[0023] Fig.11 This is a graph showing the relationship between the v value and the loss involved in one connection portion.

[0024] Fig.12 is a graph showing the relationship between the v value and the loss involved in a solid area. DETAILED DESCRIPTION

[0025] [Technical Problems to be Solved by the Present Disclosure]

[0026] The inventors studied the above-mentioned prior art and found the following technical problems. That is, in the above-mentioned prior art, if the end of the hollow-core optical fiber is left open, the air and moisture will enter the hole from the open end. The intrusion of moisture will increase the loss in the 1.38μm band, and the carbon dioxide (CO 2 ) will increase the loss in the 1.55μm band. If dust enters with the atmosphere, metal components such as metal particles react with halogen components from the glass to form corrosive metal halides, which show erosion and corrosion. As a result, it may not be able to be used stably for a long time due to deterioration over time.

[0027] In order to prevent various infiltrations from the open end of the hollow-core optical fiber, a method of connecting a general optical fiber to both ends of the hollow-core optical fiber and sealing the ends is often used. However, Fresnel reflection occurs at the interface between the hollow part of the hollow-core optical fiber and the solid part of the general optical fiber. In addition, there is a large difference in mode field diameter between the hollow-core optical fiber and the general optical fiber. Although the connection loss caused by the difference in mode field diameter can be suppressed by TEC (Thermal Expansion Core) technology, the effect is limited. As can be seen from the above, connection loss will occur in the sealing using a general optical fiber, and the characteristics of the hollow-core optical fiber cannot be fully utilized.

[0028] Consider solidifying the end of a hollow core optical fiber. However, the refractive index of the solidified portion is generally uniform, so it does not have the function of confining light to the center portion, and thus light may leak from the end, making it impossible to propagate light.

[0029] An object of the present disclosure is to provide a hollow-core optical fiber and a method for manufacturing the hollow-core optical fiber that can propagate light while suppressing connection loss caused by sealing the end portion when connecting a general-purpose optical fiber.

[0030] [Effects of the present disclosure]

[0031] The hollow-core optical fiber of the present disclosure can propagate light while suppressing connection loss caused by sealing the end portion when connecting a general optical fiber.

[0032] [Description of the implementation aspects of the present disclosure]

[0033] First, implementation aspects of the present disclosure are listed for description.

[0034] (1) One aspect of the present disclosure relates to a hollow-core optical fiber having a hollow region, which is a region extending in the length direction. The hollow region has: a glass structure constituting an optical transmission path provided with a hollow portion; and an outer cladding surrounding the glass structure. When the diameter of a circle having the same area as the total area of ​​the cross-section of the glass structure orthogonal to the length direction is set to 2a, the average refractive index of the glass structure is set to n1, the average refractive index of the outer cladding is set to n2, the wavelength of light propagating in the optical transmission path is set to λ, and the relative refractive index difference between the glass structure and the outer cladding is set to Δ represented by formula (1), n1>n2, and the normalized frequency v represented by formula (2) is greater than 2.56,

[0035]

[0036] In the hollow-core optical fiber, even when a portion of the length direction of the hollow region is solidified, the solid region can appropriately relay light because the normalized frequency is within the above range. Therefore, light can be propagated while suppressing the influence of cutting.

[0037] (2) The hollow-core optical fiber of (1) above may be a hollow-core optical fiber further including a solid region, wherein the solid region is a region extending in the length direction, and the solid region includes: a core made of the same material as the glass structure; and a cladding made of the same material as the outer cladding and surrounding the core, wherein the area of ​​the cross section of the core perpendicular to the length direction is the same as the total area of ​​the glass structure, and the area of ​​the cross section of the cladding perpendicular to the length direction is the same as the area of ​​the cross section of the outer cladding perpendicular to the length direction. In this case, the solid region has a structure equivalent to that obtained by solidifying the hollow region, and thus light can be propagated while suppressing the influence of cutting.

[0038] (3) In the above (2), a pair of solid regions may be provided at both ends of the hollow region. In this case, it is possible to reliably suppress the intrusion of air, moisture, etc. into the hollow region.

[0039] (4) In the above (2) or (3), the pair of hollow regions may be provided at both ends of the solid region. In this case, even if air, moisture, etc. intrude into one of the pair of hollow regions, the influence on the other hollow region can be suppressed.

[0040] (5) In any of the above (2) to (4), the solid region may be provided at the end of the hollow core optical fiber and connected to an optical fiber having a core and a cladding. In this case, connectivity with a so-called universal optical fiber at the end can be ensured. Thus, connectivity with a connector or the like can also be ensured.

[0041] (6) In any of the above (2) to (5), a plurality of hollow regions and a plurality of solid regions may be alternately provided. In this case, light can be propagated over a long distance while suppressing the influence of cutoff.

[0042] (7) In any of the above (2) to (6), the two closest solid regions may be provided with a hollow region having a length of 3 km or more and 20 km or less in between. In this case, since the length of the hollow region is 3 km or more, an increase in additional loss can be suppressed. In addition, since the length of the hollow region is 20 km or less, the effect of limiting the characteristic degradation region during fracture, which is caused by solidification, can be suppressed from being weakened.

[0043] (8) In any of the above (2) to (7), the length of the solid region may be 0.5 mm or more. In this case, the solid region can further appropriately relay light.

[0044] (9) In any of the above (2) to (8), the hollow region and the solid region may be coaxially arranged.

[0045] (10) In any of the above (2) to (9), the normalized frequency v may be equal to or less than 11.0. In this case, the loss per solid point after the hollow-core optical fiber is solidified can be equal to or less than 0.3 dB.

[0046] (11) In any of the above (2) to (9), the normalized frequency v may be 8.0 or less. In this case, the loss per solid point after the hollow-core optical fiber is solidified can be made 0.25 dB or less.

[0047] (12) In any of the above (2) to (9), the normalized frequency v may be 6.0 or less. In this case, the loss per solid point after the hollow-core optical fiber is solidified can be 0.20 dB or less.

[0048] (13) In any of the above (2) to (12), the outer diameter of the solid region may be smaller than the outer diameter of the hollow region.

[0049] (14) In any of the above (1) to (13), the glass structure may include a plurality of inner cladding units.

[0050] (15) One aspect of the present disclosure provides a method for manufacturing a hollow-core optical fiber, including the steps of: solidifying a glass optical fiber by melting a portion of the optical fiber in the longitudinal direction by heating from the side, wherein the glass optical fiber has a glass structure and an outer cladding, wherein the glass structure constitutes an optical transmission path having a hollow portion, and the outer cladding surrounds the glass structure.

[0051] In the above-mentioned method for producing a hollow-core optical fiber, a hollow-core optical fiber including a solid region can be easily produced.

[0052] [Details of the embodiments of the present disclosure]

[0053] As required, specific examples of hollow-core optical fibers involved in this embodiment are described with reference to the accompanying drawings. It should be noted that the present disclosure 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.

[0054] (First Embodiment)

[0055] Figure 1 It is a cross-sectional view along the longitudinal direction of the hollow-core optical fiber according to the first embodiment. Figure 2 yes Figure 1 The cross-sectional view of the hollow core optical fiber perpendicular to the length direction. Figure 1 and Figure 2 As shown, the hollow core fiber 1 involved in the first embodiment has the structure of an antiresonant hollow core fiber. The length direction of the hollow core fiber 1 is the direction along the central axis AX of the hollow core fiber 1. The hollow core fiber 1 has a hollow region Rh, which is a region extending in the length direction. The hollow core fiber 1 has a hollow region Rh across the entire region in the length direction. The hollow region Rh has a glass structure 11, an outer cladding 12, a sheath layer 13, and a resin coating 14. It should be noted that in Figure 1 In the figure, the structure of the inner region 12b is omitted.

[0056] The glass structure 11 surrounds a space that becomes a core region 15. The core region 15 functions as a hollow optical waveguide region. The core region 15 extends along the central axis AX. The glass structure 11 functions as an antiresonance layer that confines light at the center. The glass structure 11 is a transmission path constituent part that constitutes an optical transmission path provided with a hollow portion.

[0057] The glass structure 11 includes a plurality of first inner cladding units 16. In the present embodiment, the glass structure 11 includes six first inner cladding units 16. The plurality of first inner cladding units 16 are arranged so as to surround a space to be the core region 15 in a state where they are welded to the inner peripheral surface 12a of the outer cladding 12 at one point. The plurality of first inner cladding units 16 are arranged spaced apart from each other. The plurality of first inner cladding units 16 are arranged spaced apart at equal intervals. The first inner cladding unit 16 is a thin-walled tube having a tube shape extending along the central axis AX. The central axis of the first inner cladding unit 16 is arranged so as to deviate from the central axis AX.

[0058] The outer cladding 12 surrounds the glass structure 11. The outer cladding 12 functions as an optical cladding. The outer cladding 12 is a thin-walled tube having a tube shape extending along the central axis AX. The central axis of the outer cladding 12 coincides with the central axis AX. The inner circumferential surface 12a of the outer cladding 12 defines an internal region 12b for accommodating the glass structure 11. The remaining portion of the internal region 12b, except for the portion occupied by the plurality of first inner cladding units 16, is a hollow portion of the light transmission path. The internal region 12b is equivalent to a light transmission path provided with a hollow portion.

[0059] The sheath layer 13 surrounds the outer cladding layer 12. The sheath layer 13 is in contact with the outer circumference of the outer cladding layer 12. The sheath layer 13 covers the outer circumference of the outer cladding layer 12. The sheath layer 13 functions as a physical cladding. The resin coating 14 surrounds the sheath layer 13. The resin coating 14 is in contact with the outer circumference of the sheath layer 13. The resin coating 14 covers the outer circumference of the sheath layer 13.

[0060] When the diameter of a circle having the same area as the total area of ​​the cross section perpendicular to the longitudinal direction of the glass structure 11 is 2a, the average refractive index of the glass structure 11 is n1, and the average refractive index of the outer cladding 12 is n2, n1>n2. Furthermore, when the wavelength of light propagating in the optical transmission path is λ, and the relative refractive index difference between the glass structure 11 and the outer cladding 12 is Δ represented by equation (1), the normalized frequency v represented by equation (2) is greater than or equal to 2.56 and less than or equal to 11.0. Here, the total area of ​​the cross section of the glass structure 11 is the area of ​​only the glass portion excluding the area of ​​the space region between the plurality of first inner cladding units 16 and the area of ​​the space region inside each first inner cladding unit 16.

[0061]

[0062] By making the normalized frequency v within this range, a structure is formed that can propagate signal light in the central part of the hollow-core optical fiber 1 even when the hollow-core optical fiber 1 is solidified by heating or the like. As a result, the loss of each solid point after the hollow-core optical fiber 1 is solidified can be made less than 0.3 dB. The normalized frequency v can also be greater than 2.56 and less than 8.0. In this case, the loss of each solid point can be made less than 0.25 dB. The normalized frequency v can also be greater than 2.56 and less than 6.0. In this case, the loss of each solid point can be made less than 0.20 dB. It should be noted that the wavelength refers to the communication wavelength assumed to use the hollow-core optical fiber 1, that is, the wavelength of light propagating through the hollow-core optical fiber 1, for example, 1.55 μm.

[0063] Next, a description will be given of a method for producing the hollow-core optical fiber 1. The method for producing the hollow-core optical fiber 1 includes a drawing step. Figure 3It is a diagram for explaining the drawing step of the method for producing the hollow-core optical fiber 1 according to the first embodiment.

[0064] Figure 3 The drawing device 10 shown in the figure includes a pressurizing device 300, a heater 400, a resin coating device 500, a winding device 600, and a roller 610. The pressurizing device 300 presses the inside of the optical fiber base material 100 to be drawn. The heater 400 heats one end of the optical fiber base material 100. The resin coating device 500 coats the surface of the drawn glass optical fiber 2 with resin to form a hollow core optical fiber 1. The winding device 600 winds the hollow core optical fiber 1. The roller 610 adjusts the traveling direction of the hollow core optical fiber 1.

[0065] The optical fiber preform 100 is composed of an outer cladding portion 120 having a tubular shape and becoming an outer cladding 12 after drawing, a plurality of first inner cladding unit portions 160 having a tubular shape and becoming first inner cladding units 16 after drawing, and a sheath portion 130 becoming a sheath layer 13 after drawing. In an inner region 120b surrounded by an inner peripheral surface 120a of the outer cladding portion 120, the plurality of first inner cladding unit portions 160 are respectively arranged to surround the center of the outer cladding portion 120 in a state of contact with the inner peripheral surface 120a. In addition, the sheath portion 130 is provided on the outer periphery of the outer cladding portion 120.

[0066] One end of the optical fiber base material 100 is heated and softened by the heater 400. The reel of the winding device 600 rotates in the direction indicated by the arrow S, so that the hollow glass optical fiber 2 is pulled out from one end of the optical fiber base material 100. At this time, the pressure control gas and air are supplied to the internal space of the outer cladding part 120 and the internal space of each of the plurality of first inner cladding unit parts 160 by the pressurizing device 300, and these internal spaces are pressurized so that the tube shape does not deform. The glass optical fiber 2 has a glass structure corresponding to the glass structure 11 surrounding the space to be the core region 15, and an outer cladding corresponding to the outer cladding 12 surrounding the glass structure.

[0067] The resin coating device 500 coats the surface of the glass optical fiber 2 drawn from the optical fiber preform 100 with resin, thereby obtaining the hollow-core optical fiber 1. The obtained hollow-core optical fiber 1 is finally wound around a reel of the winding device 600 via a roller 610.

[0068] (Second Embodiment)

[0069] Figure 4 is a cross-sectional view along the length direction of the hollow core optical fiber involved in the second embodiment. Figure 4As shown, the hollow-core optical fiber 1A involved in the second embodiment is different from the hollow-core optical fiber 1 in that, in addition to the hollow region Rh as a region extending in the length direction, it also has a solid region Rs as a region extending in the length direction. The hollow region Rh has the same structure as the hollow region Rh of the hollow-core optical fiber 1. The hollow region Rh and the solid region Rs are coaxially arranged. It should be noted that Figure 4 The inner region 12b of the hollow region Rh is omitted (see Figure 2 ) structure is shown.

[0070] Figure 5 is the cross-sectional view of the solid area Rs. Figure 5 As shown, the solid region Rs includes a core 21, a cladding 22, a sheath layer 23, and a resin coating 24. The central axis of the core 21 coincides with the central axis AX. The core 21 is made of the same material as the glass structure 11. The core 21 is a solid body without a hollow portion. The core 21 is a glass structure equivalent to the glass structure 11 that is heated, melted, and solidified. The area of ​​the cross section of the core 21 that is perpendicular to the longitudinal direction is the same as the total area of ​​the cross section of the glass structure 11 that is perpendicular to the longitudinal direction. Here, "same" also includes the case where it is different within an error range of about ±5%.

[0071] The cross-sectional area of ​​the core 21 is obtained, for example, from the refractive index distribution. That is, after the boundary between the core 21 and the cladding 22 is obtained as the position where the gradient of the refractive index is the largest, the diameter of the core 21 is obtained based on the boundary, and the cross-sectional area of ​​the core 21 can be obtained. The total cross-sectional area of ​​the glass structure 11 can be obtained, for example, by image analysis of the cross section of the hollow region Rh, or after solidifying the hollow region Rh by heating, it can be obtained from the refractive index distribution.

[0072] The cladding 22 surrounds the core 21. The cladding 22 is in contact with the outer peripheral surface of the core 21. The cladding 22 covers the outer peripheral surface of the core 21. The cladding 22 functions as an optical cladding. The cladding 22 is made of the same material as the material of the outer cladding 12. The area of ​​the cross section of the cladding 22 perpendicular to the longitudinal direction is the same as the area of ​​the cross section of the outer cladding 12 perpendicular to the longitudinal direction. Here, "same" also includes the case where it is different within an error range of about ±5%.

[0073] The sheath layer 23 surrounds the cladding 22. The sheath layer 23 is in contact with the outer peripheral surface of the cladding 22. The sheath layer 23 covers the outer peripheral surface of the cladding 22. The sheath layer 23 functions as a physical cladding. The sheath layer 23 is made of the same material as the sheath layer 13. The resin coating 24 surrounds the sheath layer 23. The resin coating 24 is in contact with the outer peripheral surface of the sheath layer 23. The resin coating 24 covers the outer peripheral surface of the sheath layer 23. The resin coating 24 is made of the same material as the resin coating 14.

[0074] Since the hollow region Rh includes a hollow portion, the outer diameter of the solid region Rs is smaller than the outer diameter of the hollow region Rh. Figure 4 In FIG. 1 , the outer diameter of the hollow region Rh and the outer diameter of the solid region Rs are simply and equally shown.

[0075] By the structure of the solid region Rs, the deviation between the mode field diameter (hereinafter referred to as MFD) of the hollow region Rh and the MFD of the solid region Rs is controlled to be below a certain value, thereby suppressing the increase in loss caused by the mismatch of the MFD. When the glass structure 11 is solidified, the hollow region Rh has substantially the same structure as the solid region Rs.

[0076] In this embodiment, the hollow-core optical fiber 1A includes a plurality of hollow regions Rh and a plurality of solid regions Rs. The plurality of hollow regions Rh and the plurality of solid regions Rs are alternately connected along the length direction. The solid regions Rs are arranged at intervals of 3 km or more and 20 km or less in the length direction of the hollow-core optical fiber 1A. If the interval is too short, the additional loss increases. If the interval is too long, the effect of limiting the characteristic degradation region at the time of breakage, which is brought about by solidification, becomes weak.

[0077] It can be said that the interval formed by a pair of adjacent solid regions Rs separated by the hollow region Rh is greater than 3 km and less than 20 km. It can also be said that the two closest solid regions Rs are arranged with a hollow region Rh of a length of greater than 3 km and less than 20 km in between. In other words, the length of a hollow region Rh in the longitudinal direction is greater than 3 km and less than 20 km. The length of the solid region Rs in the longitudinal direction is greater than 0.5 mm.

[0078] The plurality of solid regions Rs include a pair of solid regions Rs1 and one or more solid regions Rs2. The pair of solid regions Rs1 are disposed at both ends of the hollow-core optical fiber 1A. A hollow region Rh is disposed at one end of each solid region Rs1. An optical fiber having a core and a cladding (so-called universal optical fiber) is connected to the other end of the solid region Rs1, for example. Solid regions Rs (solid regions Rs1 or solid regions Rs2) are disposed at both ends of each hollow region Rh. Hollow regions Rh are disposed at both ends of the solid region Rs2. The solid region Rs2 is disposed between a pair of hollow regions Rh. It should be noted that the hollow-core optical fiber 1A only needs to have at least a hollow region Rh and a solid region Rs, and there is no limitation on the number of hollow regions Rh and the number of solid regions Rs. The hollow-core optical fiber 1A may also be composed of, for example, a hollow region Rh and a pair of solid regions Rs1 disposed at both ends thereof.

[0079] Next, a method for producing the hollow-core optical fiber 1A will be described. The method for producing the hollow-core optical fiber 1A is different from the method for producing the hollow-core optical fiber 1 in that the drawing step includes a solidification step. Figure 6 It is a diagram for explaining the wire drawing process including the solidification process.

[0080] Figure 6 The wire drawing device 10A shown is Figure 3 The drawing device 10 shown is different in that it is provided with a heating and melting device 410. The heating and melting device 410 heats a portion of the length direction of the hollow glass optical fiber 2 drawn from the optical fiber base material 100 from the side to solidify it. The heating and melting device 410 can intermittently form solidified areas in the length direction of the glass optical fiber 2. The heating and melting device 410, for example, irradiates CO 2 Laser heats and melts the glass optical fiber 2. By adjusting CO 2 The laser irradiation pattern can adjust the interval and length of the solid area Rs. The heating and melting device 410 can also irradiate microwaves or arc discharge instead of irradiating CO. 2 The glass optical fiber 2 is heated and melted by the laser.

[0081] As described above, the hollow glass optical fiber 2 is pulled out from one end of the optical fiber mother material 100 softened by heating. The solidification process is performed on a portion of the drawn glass optical fiber 2 in the longitudinal direction. In the solidification process, the glass optical fiber 2 is solidified by heating from the side by the heating and melting device 410 to melt a portion of the glass optical fiber 2 in the longitudinal direction. The solidification process is implemented as follows: by controlling the heating and melting device 410, the solidified area reaches a specified length and is arranged at intervals of more than 3 km and less than 20 km. As described above, the resin is applied to the surface of the glass optical fiber 2 in which a portion of the longitudinal direction is solidified by the resin coating device 500 to obtain a hollow-core optical fiber 1A. The obtained hollow-core optical fiber 1A is finally wound on the reel of the winding device 600 via the roller 610.

[0082] Since the hollow-core optical fiber 1A has a solid region Rs, even if a portion of the fiber is broken in the longitudinal direction, moisture and CO 2 Or fine dust particles intruding from the fractured part into the hollow region Rh can also suppress the influence on other hollow regions Rh. Therefore, only the hollow region Rh including the fractured part can be discarded, and the rest can be used as a product. In the case of a hollow-core optical fiber 1A that has been laid and used, only the hollow region Rh including the fractured part can be replaced with a new hollow-core optical fiber 1A.

[0083] Figure 7 This figure explains the method of identifying the abnormality occurrence site in the hollow core optical fiber. Figure 7As shown in the figure, in the case of a hollow-core optical fiber without a solid area, the light intensity P detected by the OTDR (Optical Time Domain Reflectometer) is approximately constant and has nothing to do with the distance L from one end of the hollow-core optical fiber. In the case of a hollow-core optical fiber with a solid area, the light intensity P of the hollow area is also approximately constant and has nothing to do with the distance L. The light intensity P of the solid area decreases linearly as the distance L increases. In the case of an abnormality, the light intensity P of the solid area changes significantly before and after the abnormality occurs. In this way, the area where the abnormality occurs can be determined.

[0084] (Third Embodiment)

[0085] Figure 8 FIG. 4 is a cross-sectional view of the hollow region of the hollow core optical fiber according to the third embodiment. Figure 8 As shown in FIG. 1 , the hollow-core optical fiber 1B according to the third embodiment is different from the hollow-core optical fiber 1 according to the first embodiment and the hollow-core optical fiber 1A according to the second embodiment in that the glass structure 11 has a plurality of second inner cladding units 17 and a plurality of third inner cladding units 18. Although not shown in the figure, when the hollow-core optical fiber 1B has a solid region Rs, the solid region Rs of the hollow-core optical fiber 1B has the same structure as the solid region Rs of the hollow-core optical fiber 1A. In the present embodiment, the glass structure 11 includes five first inner cladding units 16, five second inner cladding units 17, and five third inner cladding units 18.

[0086] The second inner cladding unit 17 is a thin-walled tube having a tube shape extending along the central axis AX. One second inner cladding unit 17 is disposed inside each first inner cladding unit 16. The number of the second inner cladding units 17 is the same as the number of the first inner cladding units 16. The second inner cladding unit 17 is made of, for example, the same material (glass material) as that of the first inner cladding unit 16.

[0087] The third inner cladding unit 18 is a thin-walled tube having a tube shape extending along the central axis AX. One third inner cladding unit 18 is disposed inside each second inner cladding unit 17. The number of the third inner cladding units 18 is the same as the number of the second inner cladding units 17. The third inner cladding unit 18 is made of, for example, the same material (glass material) as that of the first inner cladding unit 16.

[0088] The second inner cladding unit 17 and the third inner cladding unit 18 are arranged in a state of being welded to the welded portion of the first inner cladding unit 16 and the outer cladding 12. That is, the first inner cladding unit 16, the second inner cladding unit 17, and the third inner cladding unit 18 are arranged in a state of being welded to the inner peripheral surface 12a of the outer cladding 12 at a common point. The diameter, wall thickness, and other dimensions of the second inner cladding unit 17 and the third inner cladding unit 18 are designed so as to be able to propagate light in the communication band with low loss.

[0089] Hereinafter, experimental examples will be described. It should be noted that the present disclosure is not limited to the examples.

[0090] (Example 1)

[0091] As Example 1, a hollow-core optical fiber having a configuration corresponding to the hollow-core optical fiber 1B according to the third embodiment was manufactured. Specifically, a hollow-core optical fiber consisting only of a hollow region was manufactured in the same manner as the method for manufacturing the hollow-core optical fiber 1 according to the first embodiment. Then, CO was irradiated to the end of the obtained hollow-core optical fiber. 2 The solid area is formed by laser. The length of the solid area is 10 mm. The outer diameter of the outer cladding is 150 μm, the inner diameter of the outer cladding is 79.7 μm, the diameter of the first inner cladding unit is 28.1 μm, the thickness of the first inner cladding unit is 0.5 μm, the diameter of the second inner cladding unit is 22.6 μm, the thickness of the second inner cladding unit is 0.5 μm, the diameter of the third inner cladding unit is 7.6 μm, and the thickness of the third inner cladding unit is 0.5 μm. The total area of ​​the cross section of the glass structure perpendicular to the length direction is 458.6 μm 2 , the diameter of a circle with the same area as that of the quartz crystal is 24.2 μm.

[0092] Fig. 9 is a cross-sectional view showing the connection portion between the hollow region and the universal optical fiber. Fig. 9 As shown, the end portion of the hollow-core optical fiber 1B of Example 1 before being solidified is directly connected to the general-purpose optical fiber 60 by fusion splicing. Fig.10 is a cross-sectional view showing the connection portion of the solid area and the general optical fiber. Fig.10 As shown, the solidified end of the hollow core optical fiber 1B of Example 1 is directly connected to the general optical fiber 60 by fusion splicing. Fig. 9 and Fig.10 In the figure, the hollow core optical fiber 1B is shown with the same reference numerals as those in the third embodiment. Fig. 9 and Fig.10 In FIG. 1 , the structure of the inner region of the hollow-core optical fiber 1B is omitted and shown, and the outer cladding, the jacket, and the resin coating are simply represented by one layer.

[0093] The MFD of the hollow region Rh of the hollow core optical fiber 1B is 25.0 μm at a wavelength of 1550 nm, and the MFD of the general-purpose optical fiber 60 is 10.0 μm at a wavelength of 1550 nm. Fig. 9 In this case, the loss involved in a connection is an average of 0.6dB. Fig.10 In the case of , the loss involved in one connection portion varies according to the v value of equation (2). The v value is varied by adjusting the relative refractive index difference Δ between the glass structure and the outer cladding.

[0094] Table 1 shows the relative refractive index difference Δ[%], v value and Fig.10 The losses involved in a connection part of the case [dB].

[0095] [Table 1]

[0096] Δ[%] v-value Loss / connection section [dB] 1.501 12.35 0.36 1.166 10.84 0.36 0.963 9.83 0.37 0.759 8.71 0.37 0.552 7.43 0.37 0.346 5.88 0.39 0.277 5.26 0.40 0.208 4.56 0.42 0.138 3.72 0.46 0.069 2.63 0.58 0.035 1.86 0.81

[0097] Fig.11 This is a graph showing the relationship between the v value and the loss involved in one connection portion. Fig.11 The horizontal axis represents the v value of the hollow-core optical fiber 1B of Example 1. Fig.11 The vertical axis represents Fig.10 The loss involved in a connection part in the case of [dB]. Fig.11 The graph of is used to find an approximate curve, and the v value at which the approximate curve is 0.6 dB is found. As a result, it can be confirmed that if the v value is 2.56 or more, the loss involved in one connection portion can be made less than 0.6 dB.

[0098] (Example 2)

[0099] As Example 2, a hollow-core optical fiber having a configuration corresponding to the hollow-core optical fiber 1B according to the third embodiment was manufactured. Specifically, in the same manner as the method for manufacturing the hollow-core optical fiber 1A according to the second embodiment, CO was irradiated at intervals of 5 km during the drawing process. 2 Laser was used to manufacture a 50 km long hollow core optical fiber. The length of the solid area was 10 mm. The outer diameter and inner diameter of the outer cladding, the diameter and thickness of the first inner cladding unit, the diameter and thickness of the second inner cladding unit, the diameter and thickness of the third inner cladding unit, and the total area of ​​the cross section of the glass structure orthogonal to the length direction were the same as those of the hollow core optical fiber involved in Example 1. The loss involved in a solid area (solid point) varies according to the v value of formula (2). The v value is changed by adjusting the relative refractive index difference Δ between the glass structure and the outer cladding.

[0100] Table 2 shows the relative refractive index difference Δ [%], the v value, and the loss [dB] involved in one solid area.

[0101] [Table 2]

[0102] Δ[%] v-value Loss / Solid area [dB] 1.501 12.35 0.32 1.166 10.84 0.30 0.963 9.83 0.28 0.759 8.71 0.26 0.552 7.43 0.24 0.346 5.88 0.18 0.277 5.26 0.16 0.208 4.56 0.12 0.138 3.72 0.06 0.069 2.63 0.00 0.035 1.86 0.14

[0103] Fig.12 is a graph showing the relationship between the v value and the loss involved in a solid area. Fig.12 The horizontal axis represents the v value of the hollow-core optical fiber of Example 2. Fig.12 The vertical axis represents the loss involved in a solid area (the loss of each solid point) [dB]. Fig.12 It was confirmed that when the v value was 11 or less, the loss was suppressed to 0.30 dB or less, when the v value was 8.0 or less, the loss was suppressed to 0.25 dB or less, and when the v value was 6.0 or less, the loss was suppressed to 0.20 dB or less.

[0104] Although the embodiments have been described above, the present disclosure is not necessarily limited to the above embodiments, and various modifications can be made within the scope not departing from the gist of the present disclosure. The above embodiments can also be combined as appropriate.

[0105] In the method for manufacturing the hollow core optical fiber 1A according to the second embodiment, a portion of the length direction of the hollow glass optical fiber 2 pulled out from the optical fiber preform 100 is solidified by the heating and melting device 410, but the glass optical fibers 2 may be fused to each other to form a solid region Rs at the fusion point.

[0106] The hollow region Rh may have any configuration other than the above-described embodiment as long as it has the glass structure 11 and the outer cladding 12 that constitute the light transmission path provided with the hollow portion. The hollow region Rh may also be a photonic crystal hollow core fiber.

[0107] Description of Reference Numerals

[0108] 1, 1A, 1B hollow-core optical fiber; 2 glass optical fiber; 10, 10A drawing device; 11 glass structure; 12 outer cladding; 12a inner circumference; 12b internal area; 13 jacket layer; 14 resin coating; 15 core area; 16 first inner cladding unit; 17 second inner cladding unit; 18 third inner cladding unit; 21 core; 22 cladding; 23 jacket layer; 24 resin coating; 60 general optical fiber; 100 optical fiber mother material; 120 outer cladding part; 120a inner circumference; 120b internal area; 130 jacket part; 160 first inner cladding unit part; 300 pressurizing device; 400 heater; 410 heating and melting device; 500 resin coating device; 600 winding device; 610 roller; AX center axis; L distance; P light intensity; Rh hollow area; Rs, Rs1, Rs2 solid area; S arrow.

Claims

1. A hollow core optical fiber, comprising a hollow region, wherein the hollow region is a region extending in a length direction, The hollow area has: A glass structure constituting a light transmission path provided with a hollow portion; and an outer cladding surrounding the glass structure, When the diameter of a circle having the same area as the total area of ​​the cross-section of the glass structure orthogonal to the longitudinal direction is set to 2a, the average refractive index of the glass structure is set to n1, the average refractive index of the outer cladding is set to n2, the wavelength of light propagating in the optical transmission path is set to λ, and the relative refractive index difference between the glass structure and the outer cladding is set to Δ represented by formula (1), n1>n2, and the normalized frequency v represented by formula (2) is greater than or equal to 2.56, 2. The hollow core optical fiber according to claim 1, in, The hollow core optical fiber further comprises a solid region, wherein the solid region is a region extending in the length direction. The solid area has: a core composed of the same material as that of the glass structure; and a cladding layer, composed of the same material as the outer cladding layer, and surrounding the core, The area of ​​the cross section of the core perpendicular to the longitudinal direction is equal to the total area of ​​the glass structure. An area of ​​a cross section of the cladding orthogonal to the longitudinal direction is equal to an area of ​​a cross section of the outer cladding orthogonal to the longitudinal direction.

3. The hollow core optical fiber according to claim 2, in, A pair of the solid areas are disposed at both ends of the hollow area.

4. The hollow core optical fiber according to claim 2 or 3, in, A pair of the hollow areas are arranged at both ends of the solid area.

5. The hollow core optical fiber according to any one of claims 2 to 4, in, The solid area is arranged at the end of the hollow core optical fiber and is connected to the optical fiber having a core and a cladding.

6. The hollow core optical fiber according to any one of claims 2 to 5, in, The plurality of hollow areas and the plurality of solid areas are arranged alternately.

7. The hollow core optical fiber according to any one of claims 2 to 6, in, The two closest solid areas are provided with the hollow area having a length of 3 km to 20 km interposed therebetween.

8. The hollow core optical fiber according to any one of claims 2 to 7, in, The length of the solid area is greater than 0.5 mm.

9. The hollow core optical fiber according to any one of claims 2 to 8, in, The hollow area and the solid area are coaxially arranged.

10. The hollow core optical fiber according to any one of claims 2 to 9, in, The normalized frequency v is less than 11.

0.

11. The hollow core optical fiber according to any one of claims 2 to 9, in, The normalized frequency v is less than 8.

0.

12. The hollow core optical fiber according to any one of claims 2 to 9, in, The normalized frequency v is less than 6.

0.

13. The hollow core optical fiber according to any one of claims 2 to 12, in, The outer diameter of the solid region is smaller than the outer diameter of the hollow region.

14. The hollow core optical fiber according to any one of claims 1 to 13, in, The glass structure includes a plurality of inner cladding units.

15. A method for manufacturing a hollow core optical fiber, comprising the following steps: A glass optical fiber is solidified by melting a portion of the glass optical fiber in the longitudinal direction by heating from the side. The glass optical fiber has a glass structure constituting a light transmission path provided with a hollow portion and an outer cladding surrounding the glass structure.

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