Antiresonance hollow-core fiber

Abstract

An antiresonance hollow-core fiber (100) of the present disclosure is provided with: an outer cladding (120) having a pipe shape extending along a fiber center axis (AX); and a plurality of inner cladding elements (121) that each have a pipe shape and are arranged in an internal region (120b) surrounded by an inner wall surface (120a) of the outer cladding (120), in contact with the inner wall surface (120a) so as to encircle a space forming a core region (110). In a cross section of the internal region (120b), the regions that remain after excluding partial regions occupied by the plurality of inner cladding elements (121) including the internal spaces of the plurality of inner cladding elements (121) are filled with a gas that has a lower optical absorption than H2 in a wavelength band between 1 μm and 2 μm inclusive, and a diffusion coefficient smaller than that of Ne.

Description

Antiresonant hollow-core fiber

[0001] The present disclosure relates to an anti-resonant hollow-core fiber. This application claims priority to Japanese Application No. 2022-157789, filed on September 30, 2022, and incorporates by reference all of the contents of said Japanese application.

[0002] Examples of hollow-core fibers that have been studied include photonic crystal hollow-core fibers and anti-resonant hollow-core fibers. Hollow-core fibers are used for measuring gas components through spectroscopic measurements. Hollow-core fibers are manufactured by drawing a preform made of a bundle of many pipes while heating it. The holes in the hollow-core fibers obtained by this manufacturing method contain gas for controlling the pressure during drawing, air, etc.

[0003] A photonic crystal hollow-core fiber is a hollow-core fiber that controls optical confinement by utilizing a photonic band gap, as disclosed in, for example, Non-Patent Document 1. Within the cladding of a photonic crystal hollow-core fiber, a large number of holes are arranged to form a periodic structure on the order of wavelength. The cross-sectional area of ​​the space that becomes the core region surrounded by a large number of holes is several tens of μm ２ to 100 μm ２ Therefore, the injection of high pressure gas into the pores is suppressed.

[0004] In an anti-resonant hollow-core fiber, as disclosed in, for example, Patent Document 1 and Non-Patent Document 2, the inner region of the outer cladding has a continuous cross-sectional structure along the central axis of the fiber. ２ Therefore, the time required to inject gas from the fiber end face can be significantly reduced compared to the above-mentioned photonic crystal hollow-core fiber.

[0005] Special table 2017-52084 publication

[0006] OH Heckl et al., “Temporal pulse compression in a xenon-filledKagome-type hollow-core photonics crystal fiber at high average power,” OPTICSEXPRESS, Vol. 19, No. 20, 26 September 2011, p. 19142-19148.GREGORY T. JASION et al., “Fabrication of tubular anti-resonanthollow core fibers: modeling, draw dynamics and process optimization,” OPTICSEXPRESS, Vol. 27, No. 15, 22 Jul. 2019, p.20567-20581.

[0007] The anti-resonant hollow-core fiber of the present disclosure comprises an outer cladding having a pipe shape and a plurality of inner cladding elements each having a pipe shape. The pipe shape of the outer cladding extends along the central axis of the fiber. The plurality of inner cladding elements are arranged so as to surround a space that becomes a core region while being in contact with an inner wall surface of the outer cladding. In addition, in a cross section of the anti-resonant hollow-core fiber perpendicular to the central axis of the fiber, of the internal region surrounded by the inner wall surface of the outer cladding, excluding a partial region occupied by the plurality of inner cladding elements, including the internal spaces of the plurality of inner cladding elements, the remaining region exhibits H in a wavelength band of 1 μm or more and 2 μm or less. ２ The material is filled with a gas having a lower optical absorption coefficient than Ne and a lower diffusion coefficient than Ne.

[0008] FIG. 1 is a diagram showing the structure of an anti-resonant hollow-core fiber according to the present disclosure. FIG. 2 is a diagram for explaining the cross-sectional structure of a main part of an anti-resonant hollow-core fiber according to the present disclosure, together with the cross-sectional structure of a corresponding main part of a photonic crystal hollow-core fiber of a comparative example. FIG. 3 is a diagram for explaining the calculation of the area ratio in the cross section of an anti-resonant hollow-core fiber according to the present disclosure. FIG. 4 is a diagram for explaining the drawing step in the manufacturing method of an anti-resonant hollow-core fiber according to the present disclosure. FIG. 5 is a diagram for explaining the gas filling step in the manufacturing method of an anti-resonant hollow-core fiber according to the present disclosure. FIG. 6 is a table showing the molecular diameter dependence of the diffusion coefficients of various gases that are candidate filling gases. FIG. 7 is a table showing the calculation results of the penetration time from the start to the completion of gas filling for several types of gases listed in FIG. 6 into various glass pipes having different pipe inner diameters D and pipe lengths L of 25,000 m (= 25 km). FIG. 8 is a table showing the calculation results of the penetration time for CF ４ Fig. 9 is a diagram showing the structure of an experimental system for obtaining the calculation results shown in Figs. 7 and 8, and a graph showing the calculation results. Fig. 10 is a graph illustrating the temperature dependence of the penetration time for several types of gases listed in Figs. 7 and 8.

[0009] [Problems to be Solved by the Present Disclosure] As a result of studying the above-mentioned conventional technologies, the inventors have discovered the following problems. That is, the above-mentioned conventional technologies have been unable to avoid the intrusion of gas from outside into a hollow-core fiber, the loss of gas filled in the pores of the hollow-core fiber due to diffusion, or the intrusion of gas filled in the pores of the hollow-core fiber into the glass. Therefore, the above-mentioned conventional technologies have had the problem that the transmission loss changes over time due to the refractive index of the pores filled with gas and the reaction between the intruding gas and the glass surface. Furthermore, the above-mentioned conventional technologies have been premised on the use of nonlinear optical effects and optical absorption, and have not actively filled gas into long fibers because of the enormous manufacturing time required.

[0010] The present disclosure has been made to solve the above-mentioned problems, and aims to provide an anti-resonant hollow-core fiber having a structure that enables longer lengths and more stable transmission losses compared to conventional techniques.

[0011] [Advantages of the Present Disclosure] The anti-resonant hollow-core fiber of the present disclosure enables longer lengths and more stable transmission loss compared to conventional techniques.

[0012] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be individually listed and described. (1) An anti-resonant hollow-core fiber includes an outer cladding having a pipe shape and a plurality of inner cladding elements each having a pipe shape. The pipe shape of the outer cladding extends along the central axis of the fiber. The plurality of inner cladding elements are disposed within the outer cladding so as to surround a space that becomes a core region, while being in contact with the inner wall surface of the outer cladding. In addition, in a cross section of the anti-resonant hollow-core fiber perpendicular to the central axis of the fiber, of the internal region surrounded by the inner wall surface of the outer cladding, excluding a partial region occupied by the plurality of inner cladding elements, which includes the internal spaces of the plurality of inner cladding elements, the remaining region is filled with H in a wavelength band of 1 μm or more and 2 μm or less. ２ The gas is filled with a gas having a lower optical absorption rate than hydrogen and a lower diffusion coefficient than neon.

[0013] According to the anti-resonant hollow-core fiber of the present disclosure, H ２ The hollow-core fiber is filled with a gas having an optical absorption rate lower than that of N and a diffusion coefficient smaller than that of Ne. This configuration suppresses the change in transmission loss over time compared to conventional techniques. In addition, in conventional hollow-core fibers that utilize nonlinear optical effects or optical absorption, the long fiber is not actively filled with gas. In contrast, the anti-resonant hollow-core fiber of the present disclosure makes it easy to secure a gas-filled space that becomes the hollow optical waveguide region, and also makes it possible to prevent the gas from being filled with H ２Since the fiber is selectively filled with a gas having an optical absorption rate lower than that of Ne and a diffusion coefficient smaller than that of Ne, a stable fiber space is obtained in which the change in transmission loss over time is suppressed. As a result, it is possible to extend the fiber space to 1 km or more. In particular, by filling the inner region of the outer cladding with gas under a pressure of 1 atmosphere or more, it is expected that deformation of the fiber due to bending or lateral pressure will be suppressed.

[0014] (2) In the above (1), the anti-resonant hollow-core fiber may have a length of 1 km or more. As a result of being filled with gas, the anti-resonant hollow-core fiber can be made long.

[0015] (3) In the above (1) or (2), the cross-sectional area ratio of the anti-resonant hollow-core fiber may be such that the ratio of the total cross-sectional area of ​​the multiple inner cladding elements to the cross-sectional area of ​​the internal region is 0.55 or more. In this case, a sufficient gas filling space is ensured. As a result, the gas filling time can be shortened and the anti-resonant hollow-core fiber can be made longer.

[0016] (4) In the above (3), the number of the inner cladding elements may be 3 to 6. Arranging three or more inner cladding elements surrounding the core region within the outer cladding provides an optical confinement effect. Furthermore, limiting the number of inner cladding elements to 6 or less improves the degree of freedom in fiber design, allowing for a sufficient gas-filling space while satisfying the above-mentioned area ratio.

[0017] (5) In the above (3) or (4), the diameter of the inner region may be 80 μm or more. If the diameter of the inner region is 80 μm or more, the gas filling time is shortened, and production can be performed efficiently.

[0018] (6) In any of the above (1) to (5), the gas filled in the anti-resonant hollow-core fiber is Ar (argon), Kr (krypton), Xe (xenon), N ２ (nitrogen), O ２ (oxygen), CF ４ (tetrafluoromethane), C ２ F ６ (hexafluoroethane), CCl ２ F ２(Dichlorodifluoromethane, ), CClF ３ (chlorotrifluoromethane). That is, the inner region of the outer cladding, which corresponds to the inner region of the anti-resonant hollow-core fiber, may be filled with one type of gas, such as Ar, listed here, or multiple types of gases may coexist in the inner region. With either gas, more stable transmission loss is achieved than in the past. In particular, in a configuration in which multiple types of gases coexist in the inner region of the outer cladding, different gas composition distributions can be formed along the longitudinal direction of the anti-resonant hollow-core fiber, and improvements in transmission characteristics can be expected by utilizing changes in refractive index along the longitudinal direction.

[0019] (7) In any of (1) to (6) above, the pressure of the gas filled in the anti-resonant hollow-core fiber may be greater than 0.101 MPa and less than 70 MPa at a temperature of 25 degrees Celsius. This allows for maintaining stable transmission characteristics in which changes in transmission loss over time are suppressed.

[0020] [Details of the embodiments of the present disclosure] Specific examples of the anti-resonant hollow-core fiber of the present disclosure will be described in detail below with reference to the accompanying drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. Furthermore, in the description of the drawings, identical elements are given the same reference numerals, and duplicate explanations will be omitted.

[0021] Fig. 1 is a diagram showing the structure of an anti-resonant hollow-core fiber according to the present disclosure. Fig. 2 is a diagram for explaining the cross-sectional structure of a main portion of the anti-resonant hollow-core fiber according to the present disclosure, together with the cross-sectional structure of a corresponding main portion of a photonic crystal hollow-core fiber as a comparative example (denoted as "cross-sectional structure" in Fig. 2). The upper part of Fig. 2 (denoted as "anti-resonant type" in Fig. 2) shows a cross-sectional view of the main portion of the anti-resonant hollow-core fiber taken along line II shown in Fig. 1. The lower part of Fig. 2 (denoted as "photonic crystal type" in Fig. 2) shows a cross-sectional view of the main portion of a photonic crystal hollow-core fiber as a comparative example, corresponding to the cross section taken along line II shown in Fig. 1.

[0022] As shown in FIG. 1 , the anti-resonant hollow-core fiber 100 of the present disclosure comprises an outer cladding 120, multiple inner cladding elements 121, a jacket layer 130, and a resin coating 140. The outer cladding 120 functions as an optical cladding and has a pipe shape extending along the fiber central axis AX. An internal region 120b surrounded by an inner wall surface 120a of the outer cladding 120 corresponds to the internal region of the anti-resonant hollow-core fiber 100, and multiple inner cladding elements 121 functioning as trench layers are provided in the internal region 120b. These multiple inner cladding elements 121 are arranged to surround a space that becomes a core region 110 that functions as a hollow optical waveguide region, with all of the inner cladding elements 121 in contact with the inner wall surface 120a of the outer cladding 120. The space that becomes the core region 110 extends along the fiber central axis AX. A jacket layer 130 that serves as a physical cladding is provided on the outer periphery of the outer cladding 120. Furthermore, a resin coating 140 is provided on the outer periphery of the jacket layer 130 .

[0023] 2, on a cross section of the outer cladding 120 perpendicular to the fiber central axis AX, i.e., in a cross section of the inner region 120b perpendicular to the fiber central axis AX, the inner region 120b surrounded by the inner wall surface 120a of the outer cladding 120, excluding a partial region occupied by the inner cladding elements 121, including the internal spaces 121b of the inner cladding elements, is filled with a specific gas. Hereinafter, this region filled with gas will be referred to as the gas-filled region, and the filled gas will be referred to as the fill gas. This fill gas is H ２ The fill gas is selected from, for example, rare gases, general gases, halogenated carbons, etc. The rare gas may be any of Ar, Kr, and Xe. The general gas may be N ２ and O ２ In addition, halogenated carbons include CF ４ , C ２ F ６ , CCl ２ F ２ , CClF ３Either of the above is acceptable.

[0024] 2 as a comparative example comprises a common cladding 220, in which a large number of air holes 221 are arranged so as to surround a hollow optical waveguide region that becomes the core region 210. On a cross section of the common cladding 220 perpendicular to the fiber central axis AX, i.e., on a cross section of the photonic crystal hollow-core fiber, the large number of air holes 221 are arranged so as to form a periodic structure on the order of wavelength. Light is confined within the core region 210 by a photonic band gap created by this periodic structure.

[0025] The gas-fillable space of the anti-resonant hollow-core fiber 100 shown in the upper part of Fig. 2 is the gas-filled region described above. On the other hand, the gas-fillable space of the photonic crystal hollow-core fiber shown in the lower part of Fig. 2 is only the core region 210 surrounded by a large number of air holes 221 arranged inside the common cladding 220. As described above, there is a significant difference in the cross-sectional area of ​​the gas-fillable space between the anti-resonant hollow-core fiber 100 of the present disclosure and the photonic crystal hollow-core fiber of the comparative example. This difference in cross-sectional area manifests itself as a difference in the penetration time from the start to the completion of gas filling, which will be described later.

[0026] Figure 3 is a diagram for explaining the calculation of the area ratio in the cross section of the anti-resonant hollow-core fiber 100 of the present disclosure (marked "Area Ratio Calculation" in Figure 3). The upper part of Figure 3 (marked "Cross-Section Model" in Figure 3) shows a cross-sectional model corresponding to the cross section of the anti-resonant hollow-core fiber 100 shown in the upper part of Figure 2. The lower part of Figure 3 (marked "Enlarged View" in Figure 3) is an enlarged view conceptually showing the actual contact state between the outer cladding 120 and one of the inner cladding elements 121.

[0027] In FIG. 3 , r1 is the radius of the inner region 120b of the outer cladding 120. That is, r1 is the distance from the central axis AX to the inner wall surface 120a of the outer cladding 120. r2 is the radius of the inner cladding element 121 from the center 121a to the outer peripheral surface. However, as shown in the lower part of FIG. 3 , the inner cladding element 121 is recessed into the inner wall surface 120a of the outer cladding 120. Therefore, r3 is used as a correction value in calculating the area ratio. d is the distance between adjacent inner cladding elements 121. θ is the angle between a reference line set on the fiber cross section and a line segment connecting the fiber central axis AX and the center 121a of the inner cladding element 121. The correction value r3 for the radius r2 of the inner cladding element 121 is calculated using the formula "r1 / (1+1 / sin(π / n))-d / 2." n is the number of inner cladding elements 121. Therefore, the ratio of the total cross-sectional area of ​​the inner cladding elements 121 to the cross-sectional area of ​​the inner region 120b of the outer cladding 120 (hereinafter referred to as "area ratio") is n × (r3 / r1). ２ is given by

[0028] Specifically, the radius r1 of the inner region 120b of the outer cladding 120 is fixed at 40 μm, and the distance d between the inner cladding elements 121 is fixed at 2 μm as common parameters. In this case, qualitatively, the radius of each inner cladding element 121 given by the correction value r3 decreases as the number of inner cladding elements 121 in contact with the inner wall surface 120a of the outer cladding 120 increases. Similarly, the area ratio also decreases as the number increases.

[0029] For example, when the condition for the number of inner cladding elements 121 is n=3, the correction value r3 of the radius r2 and the radius ratio: r3 / r1 are 17.6 μm and 0.44, respectively, and the area ratio: n×(r3 / r1) ２ is 0.58. When n<3, the inner cladding elements 121 no longer function as a trench layer, so three or more inner cladding elements 121 are required. When n=4, r3=15.6 μm and r3 / r1=0.39, and n×(r3 / r1) ２ When n=5, r3=13.8 μm and r3 / r1=0.35, and n×(r3 / r1) ２When n=6, r3=12.3 μm and r3 / r1=0.31, and n×(r3 / r1) ２ When n=7, r3=11.1 μm and r3 / r1=0.28, and n×(r3 / r1) ２ Furthermore, when n=8, r3=10.1 μm and r3 / r1=0.25, and n×(r3 / r1) ２ is 0.51. In the cases of n=8 and n=7, the area ratio: n×(r3 / r1) is smaller than that in the case of n=6. ２ is small, and the cross-sectional area of ​​the region surrounded by two adjacent inner cladding elements 121 and the inner wall surface 120a of the outer cladding 120 becomes too small. In these cases, it becomes difficult to replace the gas injected at high pressure into the inner region 120b of the outer cladding 120 with the residual gas. In other words, when the number of inner cladding elements 121 is seven or more, it becomes difficult to uniformly fill the anti-resonant hollow-core fiber with gas. As a result, it becomes impossible to achieve stabilization of optical characteristics such as transmission loss along the fiber central axis of the anti-resonant hollow-core fiber. Therefore, in order to ensure sufficient space for gas filling and to achieve stable optical characteristics such as transmission loss along the longitudinal direction, it is sufficient that the number of inner cladding elements 121 is six or less, and the above area ratio: n × (r3 / r1) ２ The area ratio r1 may be 0.55 or greater. The above radii required for calculating the area ratio can be measured by microscopic observation of the end face. The number of inner cladding elements 121 may be 3 or greater and 6 or less, so the area ratio may be 0.57 or greater and 0.61 or less. When the radius r1 of the inner region 120b of the outer cladding 120 is greater than 40 μm, the area ratio may be 0.61 or greater.

[0030] Next, the manufacturing method of the antiresonant hollow core fiber 100 of the present disclosure will be described divided into a first half process and a second half process. Fig. 4 is a diagram for explaining the drawing process, which corresponds to the first half process, in the manufacturing method of the antiresonant hollow core fiber 100 of the present disclosure. Fig. 5 is a diagram for explaining the gas filling process, which corresponds to the second half process, in the manufacturing method of the antiresonant hollow core fiber 100 of the present disclosure.

[0031] In order to carry out the first half of the process, the drawing apparatus shown in FIG. 4 includes a pressure device 300 that applies pressure to the inside of the optical fiber preform 10 to be drawn, a heater 400 that heats one end of the optical fiber preform 10, a resin application device 500 that applies resin to the surface of the hollow glass fiber drawn from the optical fiber preform 10, a winding device 600 that winds up the fiber intermediate member 150, and a roller 610 that adjusts the traveling direction of the fiber intermediate member 150.

[0032] The optical fiber preform 10 is composed of a pipe-shaped outer cladding portion 12 that will become the outer cladding 120 after drawing, multiple pipe-shaped inner cladding portions 12b that will become inner cladding elements 121 after drawing, and a jacket portion 13 that will become the jacket layer 130 after drawing. In the internal region surrounded by the inner wall surface 12a of the outer cladding portion 12, multiple inner cladding portions 12b are arranged so as to surround the center of the outer cladding portion 12 while each being in contact with the inner wall surface 12a. In addition, the jacket portion 13 is provided on the outer periphery of the outer cladding 120.

[0033] One end of the optical fiber preform 10 is heated and softened by the heater 400, and the drum of the winding device 600 rotates in the direction indicated by arrow S, thereby drawing a hollow glass fiber from the end of the optical fiber preform 10. At this time, a pressure-control gas or air is supplied by a pressurizing device 300 to the inner region of the outer cladding portion 12 and the inner regions of each of the multiple inner cladding portions 12b, and these inner regions are pressurized to prevent deformation of the pipe shape. A resin is applied to the surface of the drawn glass fiber by a resin application device 500, resulting in a fiber intermediate member 150. In this fiber intermediate member 150, the inner region of the post-drawing portion corresponding to the outer cladding 120 and the inner region of the post-drawing portion corresponding to each of the multiple inner cladding elements 121 are hollow. The obtained fiber intermediate member 150 is finally wound around the drum of the winding device 600 via a roller 610.

[0034] Furthermore, the fiber intermediate member 150 obtained by the drawing apparatus having the above-described structure is set in an apparatus for performing a gas filling process shown in FIG. 5 , where the latter half of the process is performed. That is, the apparatus shown in FIG. 5 includes a high-pressure gas supply system 710, a vacuum pump 720, and a gas analyzer 730. One end face of the fiber intermediate member 150 is connected to the high-pressure gas supply system 710 via an on-off valve 741. When a fill gas is supplied from the high-pressure gas supply system 710 to one end face of the fiber intermediate member 150 via the on-off valve 741, the fill gas is supplied to the gas filling region. Note that, because the internal space 121b of the portions corresponding to the multiple inner cladding elements 121 is in a vacuum or reduced pressure state, the fill gas does not enter the portions corresponding to these multiple inner cladding elements 121. Furthermore, since the cross-sectional area of ​​the internal space 121b corresponding to the multiple inner cladding elements 121 is small, even if gas is filled into the internal space 121b corresponding to the multiple inner cladding elements 121, the gas filling will not be completed within the time it takes to fill the gas filling area with gas.

[0035] A vacuum pump 720 is connected to the other end face of the fiber intermediate member 150 via an on-off valve 742, and this vacuum pump 720 evacuates residual gas from the internal region of the fiber intermediate member 150, i.e., the region corresponding to the internal region 120b of the outer cladding 120. The type of gas evacuated by the vacuum pump 720 is analyzed by a gas analyzer 730. When the gas analyzer 730 detects the filled gas, it is confirmed that the gas filling is complete and the residual gas has been replaced with the filled gas in the internal region of the fiber intermediate member 150 corresponding to the internal region 120b of the outer cladding 120. After the gas filling, if both ends of the anti-resonant hollow-core fiber 100 are hermetically sealed, the gas pressure at the time of sealing is maintained. The pressure of the gas filled in the anti-resonant hollow-core fiber 100 at the time of sealing is greater than 0.101 MPa and less than 70 MPa at a temperature of 25°C. The pressure of the gas during filling can be measured by a pressure gauge provided in the high-pressure gas supply system 710. The type and pressure of the gas filled in the anti-resonant hollow-core fiber 100 after filling can be measured by reconnecting the anti-resonant hollow-core fiber 100 to the gas analyzer 730 and the high-pressure gas supply system 710. Furthermore, the type and pressure of the gas filled in the anti-resonant hollow-core fiber 100 after filling can also be estimated from the area intensity or peak intensity of the spectrum obtained by spectroscopic measurement such as Raman spectroscopy or stimulated Raman scattering.

[0036] In the gas filling step, the fiber intermediate member 150 may be housed in a thermostatic chamber 800 whose interior is maintained at a constant temperature, and the gas filling may be performed in that state. The thermostatic chamber 800 includes a housing 810 that houses the fiber intermediate member 150, a cooling source 820, a heating source 830, a temperature sensor 840, and a temperature control unit 850. The temperature control unit 850 controls the cooling source 820 or the heating source 830 while monitoring the internal temperature of the housing 810 with the temperature sensor 840 in order to maintain the internal temperature of the housing 810 at a desired set temperature.

[0037] 6 is a table showing the molecular diameter dependence of the diffusion coefficient of various gases that are candidates for filler gases applicable to the anti-resonant hollow-core fiber 100 of the present disclosure. The table in FIG. 6 shows the molecular diameter (nm) and diffusion coefficient (cm) of various gases.２ The diffusion coefficient at 25°C is calculated from the known values ​​of the diffusion coefficient and activation energy at 1000°C.

[0038] As can be seen from the table in Figure 6, the larger the molecular diameter, the smaller the diffusion coefficient. In particular, when focusing on the diffusion coefficient at a temperature of 25°C, the diffusion coefficients of Ar, Kr, and Xe among rare gases are 1 x 10 －２０ cm ２ / s, both of which are smaller than the diffusion coefficient of Ne, and the light absorption rate is also H ２ Therefore, from the group of noble gases, Ar, Kr, and Xe are suitable fill gases for the anti-resonant hollow-core fiber 100 of the present disclosure. －２０ cm ２ A diffusion coefficient of less than 1 / s means that the diffusion distance is 0.1 μm or less in 15 years. He and Ne have large diffusion coefficients and are therefore not suitable for filling gases. In addition, N is a common gas. ２ , O ２ The diffusion coefficient of －２０ cm ２ / s, both of which are smaller than the diffusion coefficient of Ne, ２ Its light absorption rate is lower than that of other common gases. ２ , O ２ is suitable for filling gas. ２ , O ２ Although the diffusion coefficient of CF is smaller than that of Ne, it is not suitable as a filling gas because it absorbs light in the wavelength range of 1 μm or more and 2 μm or less. ４ , CCl ２ F ２ , CClF ３ The diffusion coefficient of －２０ cm ２ / s, both of which are smaller than the diffusion coefficient of Ne, ２ Therefore, any of the gases listed in the table in Figure 6 are suitable as fill gases. Other examples of gases include CH ４ , CO ２ , CO, C２ H ４ The diffusion coefficient of －２０ cm ２ However, these gases all have a wavelength range of 1 μm or more and 2 μm or less. ２ It has a higher optical absorption rate than SiO2 and is therefore not suitable as a fill gas.

[0039] Although not shown in the table of FIG. 6, C ２ F ６ Also 1 x 10 －２０ cm ２ / s and H ２ It has a lower optical absorption rate than H, making it suitable as a filling gas. ２ , He, Ne, etc. have small molecular diameters and diffuse through glass, so they may escape in the radial direction of the hollow-core fiber. Therefore, these gases are not suitable as filling gases for the anti-resonant hollow-core fiber 100 of the present disclosure. Furthermore, CH ４ , C ２ H ６ , CO ２ These gases are not suitable for filling because they absorb light due to vibration of C-H bonds and C-O bonds, which increases transmission loss.

[0040] Next, calculation results of the relationship between the pipe inner diameter of the fiber model and the penetration time for several types of gases among the various gases listed in Fig. 6 will be explained below with reference to Figs. 7 to 9. Fig. 7 is a table showing calculation results of the penetration time from the start to the end of gas filling into various glass pipes having different pipe inner diameters D and pipe lengths L of 25,000 m for several types of gases among the various gases listed in Fig. 6. Here, the upper row of the table shown in Fig. 7 shows the results of calculations of the penetration time from the start to the end of gas filling into various glass pipes having different pipe inner diameters D and pipe lengths L of 25,000 m for several types of gases among the various gases listed in Fig. 6. －５ The bottom row of the table in FIG. 7 shows the calculation results of the penetration time from the start to the end of gas filling into a glass pipe having an inner diameter D of 1.0×10 m and a pipe length L of 25,000 m. －５ 8 shows the calculation results of the penetration time from the start to the end of gas filling into a glass pipe having a diameter of 1000 m and a pipe length of 25,000 m. ４FIG. 9 is a table showing the calculation results of the penetration time from the start to the completion of gas filling for various glass pipes with different pipe inner diameters D and pipe lengths L of 25,000 m. FIG. 9 is a diagram showing the structure of the experimental system used to obtain the calculation results shown in FIGS. 7 and 8 , and a graph showing the calculation results (denoted as "Experimental System and Calculation Results" in FIG. 9 ). The upper part of FIG. 9 (denoted as "Experimental System" in FIG. 9 ) shows a schematic structure of a glass pipe prepared as an experimental fiber model. The middle part of FIG. 9 (denoted as "Viscosity-Pressure Characteristics" in FIG. 9 ) shows the relationship between the fluid viscosity μ (Pa s) and pressure ΔP (MPa) of the gases listed in the upper part of the table shown in FIG. 7 . The lower part of FIG. 9 (denoted as "Pipe Inner Diameter-Time Characteristics" in FIG. 9 ) shows the CF (F) analysis results based on the table shown in FIG. 8 . ４ 7 and 8. Furthermore, FIG. 10 shows the relationship between the pipe inner diameter D (m) and the penetration time (h) for the gases Ar, N ２ , and C.F. ４ 1 is a graph illustrating the temperature dependency of the penetration time for the above-mentioned compound.

[0041] To calculate the penetration time (h), we assume a glass pipe with a single narrow hole as shown in the upper part of Figure 9 as the experimental fiber model. This glass pipe has a circular cross section, an inner diameter D, and a length L. The pressure loss when a fluid such as a gas flows through this glass pipe is expressed by the following equation (1). Furthermore, if the fluid is a laminar flow, the condition of the following equation (2) holds true.

[0042] where λ is the friction coefficient of the glass pipe against the gas, L is the pipe length (m), D is the pipe inner diameter (m), and ρ is the fluid density (kg / m ３ ) u is the mean flow velocity (m / s) μ is the fluid viscosity Re is the Reynolds number

[0043] For example, if the pipe inner diameter D is 8.0 x 10 －５When a filler gas pressure of about 50 MPa is applied to one end of a glass pipe having a pipe length L of 25,000 m, it takes about 350 hours for the filler gas to reach the other end of the glass pipe. The calculation results are shown in the upper part of the table in Figure 7. In both the upper and lower parts of the table in Figure 7, the gases that are the subject of calculations are He, H ２ , N ２ , Ar, Kr, Xe, air, O ２ , and C.F. ４ For these gases, the temperature T (K), molecular weight (g / mol), fluid viscosity μ (Pa s), and fluid density ρ (kg / m ３ 9 shows values ​​of the fluid viscosity μ (Pa s) and the pressure ΔP (MPa), the Reynolds number Re, the pressure ΔP (Pa), the pressure ΔP (MPa), and the penetration time (h). As can be seen from the graph shown in the middle of Fig. 9, the gases listed in the top row of the table in Fig. 7 show a correlation between the fluid viscosity μ (Pa s) and the pressure ΔP (MPa).

[0044] On the other hand, the pipe inner diameter D is 1.0 × 10 －５ When a filler gas pressure of 50 MPa is applied to one end of a glass pipe having a pipe length L of 25,000 m and a pipe diameter L of 1000 m, it takes more than 20,000 hours for the filler gas to reach the other end of the glass pipe. The calculation results are shown in the bottom row of the table in Figure 7.

[0045] Furthermore, the table shown in FIG. 8 lists CF as the target gas. ４ , pipe inner diameter D (m), temperature T (K), molecular weight (g / mol), fluid viscosity μ (Pa・s), fluid density ρ (kg / m ３), mean flow velocity (m / s), Reynolds number Re, pressure ΔP (Pa), pressure ΔP (MPa), and permeation time (h) are shown. As can be seen from the graph in the lower part of Figure 9, there is also a correlation between the pipe inner diameter D (m) and permeation time (h). This trend can be inferred for other gases as well. Furthermore, from the above formulas (1) and (2), it can be seen that the fluid viscosity μ affects the gas filling speed. The fluid viscosity μ varies depending on the type of gas and its temperature. Temperature has a particularly large effect, with fluid viscosity μ tending to decrease at lower temperatures. Therefore, cooling the fiber intermediate member 150 in Figure 5 using a thermostatic bath 800 or the like is effective in reducing the permeation time. This time dependency of permeation time is shown in Figure 10, which shows that permeation time is reduced by filling the gas at low temperatures, regardless of the type of gas. 6 and the like, the internal region of the fiber intermediate member 150 may be filled with a single type of gas, or multiple types of gases may coexist in the internal region of the fiber intermediate member 150. In particular, in a configuration in which multiple types of gases coexist in the internal region of the fiber intermediate member 150, different gas composition distributions can be formed along the longitudinal direction of the obtained anti-resonant hollow-core fiber 100, and in the gas-filled region that essentially becomes the core region 110, improvements in transmission characteristics can be expected by utilizing changes in refractive index along the longitudinal direction.

[0046] In FIG. 10, the line G1010 shows the temperature dependence of the penetration time when Ar is selected as the fill gas, and the line G1020 shows the temperature dependence of the penetration time when N is selected as the fill gas. ２ The line G1030 shows the temperature dependence of the penetration time when CF is selected as the fill gas. ４ The graph shows the temperature dependence of the penetration time when the filling gas is selected as the filling gas. The pipe sample prepared for the measurement has the same structure as the glass pipe shown in the upper part of FIG. 9, and the pipe inner diameter D is 8.0 × 10 ―５m, and the pipe length L is 25,000 m. Furthermore, for each of the lines G1010 to G1030, the permeation time when the temperature T is 298 K, i.e., 25°C, has been adjusted to facilitate comparison. Specifically, for the line G1010 showing the temperature characteristics of Ar, the various parameters of the fluid viscosity μ, fluid density ρ, mean flow velocity u, Reynolds number Re, and pressure ΔP have been adjusted so that the permeation time when the temperature T is 25°C is 385 h. Similarly, for N ２ The line segment G1020 showing the temperature characteristics of CF is adjusted so that the permeation time is 305 h when the temperature T is 25° C., and various parameters such as the fluid viscosity μ are adjusted. ４ For the line segment G1030 showing the temperature characteristics of N, various parameters such as fluid viscosity have been adjusted so that the permeation time is 225 hours when the temperature T is 25°C. As can be seen from FIG. 10, the slope of the line segment G1010 showing the degree of temperature dependency is greater than the remaining line segments G1020 and G1030. In other words, N ２ and C.F. ４ It can be seen that the temperature dependence of Ar is greater than that of

[0047] In the anti-resonant hollow-core fiber 100 of the present disclosure, the area ratio, that is, the ratio of the total cross-sectional area of ​​the multiple inner cladding elements 121 to the cross-sectional area of ​​the inner region 120b of the outer cladding 120, is set to be 0.55 or greater. In this way, in the case of the anti-resonant hollow-core fiber 100, a sufficient gas-filled space that serves as the hollow optical waveguide region is ensured, and it becomes possible to actually manufacture a fiber having a length of 1 km or more.

[0048] Furthermore, in the anti-resonant hollow-core fiber 100 of the present disclosure, the diameter of the inner region 120b of the outer cladding 120 may be 80 μm or greater. If the diameter of the inner region 120b is 80 μm or greater, fiber lengths of 1 km or greater can be efficiently manufactured.

[0049] As can be seen from the description of the above-mentioned embodiments, the present specification includes the disclosure of the following aspects: (Supplementary Note 1) An anti-resonant hollow-core fiber comprising: an outer cladding having a pipe shape extending along a fiber central axis; and a plurality of inner cladding elements each having a pipe shape and arranged to surround a space that becomes a core region while in contact with an inner wall surface of the outer cladding, wherein the anti-resonant hollow-core fiber has a length of 1 km or more, and on a cross section of the outer cladding that is orthogonal to the fiber central axis, H ２ An anti-resonant hollow-core fiber filled with a gas having an optical absorption rate lower than that of Ne and a diffusion coefficient lower than that of Ne.

[0050] DESCRIPTION OF SYMBOLS 10...Optical fiber preform 12...Outer cladding portion 12a...Inner wall surface 12b...Inner cladding portion 13...Jacket portion 100...Anti-resonant hollow-core fiber 110...Core region 120...Outer cladding 120a...Inner wall surface 120b...Inner region 121...Inner cladding element (partial region) 121a...Center 121b...Inner space (partial region) 130...Jacket layer 140...Resin coating 150...Fiber intermediate member 210...Core region 220...Common cladding 221...Hole 300...Pressure device 400...Heater 500...Resin application device 600...Winding device 610...Roller 710...High-pressure gas supply system 720...Vacuum pump 730...Gas analyzer 741, 742...Opening and closing valves 800...Constant-temperature bath 810...Housing 820: Cooling source 830: Heating source 840: Temperature sensor 850: Temperature control section AX: Fiber central axis S: Arrow.

Claims

1. an outer cladding having a pipe shape extending along a central axis of the fiber; A plurality of pipe-shaped inner cladding elements are arranged in an internal region surrounded by an inner wall surface of the outer cladding so as to be in contact with the inner wall surface and surround a space that becomes a core region; Equipped with In a cross section of the inner region perpendicular to the central axis of the fiber, a region including an internal space of the inner cladding elements and excluding a partial region occupied by the inner cladding elements has a wavelength band of 1 μm or more and 2 μm or less. ２ and a gas having a lower optical absorption coefficient than Ne. Antiresonant hollow-core fiber.

2. Has a length of 1 km or more, 10. The anti-resonant hollow-core fiber of claim 1 .

3. A ratio of a total cross-sectional area of ​​the inner cladding elements to a cross-sectional area of ​​the internal region on the cross-section is 0.55 or greater.

10. The anti-resonant hollow-core fiber of claim 1 .

4. the number of the inner cladding elements is 3 or more and 6 or less; 4. The anti-resonant hollow-core fiber of claim 3.

5. The diameter of the inner region is 80 μm or more.

4. The anti-resonant hollow-core fiber of claim 3.

6. The gas is Ar, Kr, Xe, N ２ , O ２ , C.F. ４ , C ２ F ６ , CCl ２ F ２、 CClF ３ At least one of 10. The anti-resonant hollow-core fiber of claim 1 .

7. The pressure of the gas is greater than 0.101 MPa and less than 70 MPa at a temperature of 25 degrees Celsius; 7. An anti-resonant hollow-core fiber according to any one of claims 1 to 6.