Hollow core fiber

Abstract

To provide a porous core fiber that yields lower surface scattering loss and transmission loss. [Solution] A hollow core fiber including a hollow core, wherein at least a portion of the cladding surrounding the hollow core, adjacent to the hollow core, is made of silica glass containing an alkali metal or alkaline earth metal. The alkali metal may be potassium or sodium. The concentration of potassium or sodium may be 1 ppm or more. The concentration of potassium or sodium may be 10 ppm or more.

Description

Technical Field

[0001] The present invention relates to a hollow-core fiber.

Background Art

[0002] Optical fibers are widely used in various fields including optical communication, sensing, and laser output supply as a medium for propagating light in the core region along the longitudinal direction. Although conventional silica-core optical fibers are still the standard, there are inherent losses due to light absorption of the dielectric materials constituting the silica-core optical fibers. Hollow-core optical fibers (HCFs) provide a promising alternative to silica-core optical fibers. By guiding light mainly in a core filled with air, it is possible to significantly reduce transmission loss compared to solid-core optical fibers. Also, various characteristics that are impossible to achieve with conventional solid-core optical fibers, such as low delay and low non-linearity, can be realized, and many new applications are expected.

[0003] As a hollow-core fiber, a photonic bandgap fiber (PBGF) is known. The PBGF is known to be relatively resistant to bending. Also, the loss spectrum of the PBGF is known to be divided into a narrow low-loss window between high-loss peaks where the fundamental core mode leaks into the surface mode. In the PBGF, a low-loss characteristic of 1.2 dB / km has been realized by optimizing the structure.

[0004] On the other hand, an anti-resonant fiber (ARF) that significantly reduces light leakage based on the anti-resonance principle is advantageous in that the band is not divided. With recent progress, a record-low-loss ARF has been demonstrated.

[0005] For example, in 2021, H. Sakr et al. reported achieving lower losses than any other optical fiber (including solid-core silica fiber) at wavelengths of 850 nm and 1060 nm using Nested Antiresonant Nodeless Fiber (NANF) (Non-Patent Literature 1). Also in 2022, GT Jasion et al. reported achieving an astonishing loss value of 0.174 dB / km in the C band using a Hollow-Core Double-Nested Anti-Resonant Nodeless Fiber (DNANF) design (Non-Patent Literature 2). Furthermore, patent documents have reported results of reducing confinement loss with double or more (nested) anti-resonance ring structures (Patent Literature 1).

[0006] The optical propagation mechanism in perforated core fibers depends on a combination of factors. The central region filled with air or gas (the perforated core) facilitates the induction of light by the surrounding structure, even when the refractive index of the core is lower than the average refractive index of the cladding region.

[0007] In PBGFs, the formation of a photonic bandgap due to the regular refractive index distribution in the cladding prevents light penetration in specific wavelength bands, confining the light to the vacant core. In contrast, in ARFs, the surrounding thin capillary forms anti-resonances that inhibit lateral light transmission through the fiber at specific wavelengths. Furthermore, careful control of the overlap between air-guided and tube-guided modes further reduces propagation loss.

[0008] In all of the above types of hollow core fibers, confinement loss due to the internal structure of the optical fiber is one of the main causes of loss. Numerous hollow core fiber designs have been previously published, but for example, it has been reported that low confinement loss characteristics and low transmission loss can be achieved with a nested anti-resonant nodeless fiber structure, which has multiple anti-resonant rings (thin-walled capillaries) arranged in a circle, with a cladding tube surrounding the anti-resonant rings to hold this structure together, and where adjacent anti-resonant rings do not come into contact with each other. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 6636509 [Non-patent literature]

[0010] [Non-Patent Document 1] Hesham Sakr et al., “Hollow Core NANFs with Five Nested Tubes and Record Low Loss at 850, 1060, 1300 and 1625nm”, OFC 2021 Postdeadline Papers 1(F3A). [Non-Patent Document 2] Gregory T Jasion et al., “0.174 dB / km Hollow Core Double Nested Antiresonant Nodeless Fiber (DNANF)”, OFC 2022 Postdeadline Paper Session III(Th4C). [Non-Patent Document 3] Chemical Vapor Deposition, Typical Material Parameters, [online], Heraeus Conamic 2024, [Retrieved November 1, 2024], Internet <https: / / www.heraeus-conamic.com / products-and-solutions / products-for-key-markets / specialty-fiber-applications / chemical-vapor-deposition> [Non-Patent Document 4] PJ Roberts et al., “Ultimate low loss of hollow-core photonic crystal fibers”, OPTICS EXPRESS, Vol. 13, No. 1, pp236-244 (January 2005). [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] For porous core fibers, lower transmission loss is desired. To achieve low transmission loss, it is important to reduce both confinement loss and surface scattering loss in the glass region surrounding the porous core. However, according to the inventors' research, there is room for improvement in reducing surface scattering loss.

[0012] For example, Non-Patent Document 3 discloses numerous silica glass tubes that serve as manufacturing materials for porous core fibers, each containing different components and concentrations. However, it does not disclose which of these silica glass tubes is suitable for reducing surface scattering loss. Furthermore, Non-Patent Document 4 suggests that optimizing the glass material is also effective in reducing surface scattering loss, but it does not provide specific studies or reports on the optimal glass material, taking manufacturability into consideration.

[0013] The present invention has been made in view of the above, and an object thereof is to provide a hollow-core fiber that can achieve lower surface scattering loss and transmission loss.

Means for Solving the Problems

[0014] In order to solve the above-described problems and achieve the object, one aspect of the present invention is a hollow-core fiber including a hollow core, wherein at least a part of a portion adjacent to the hollow core in a clad portion surrounding the hollow core is a hollow-core fiber made of silica glass containing an alkali metal or an alkaline earth metal.

[0015] The alkali metal may be potassium or sodium.

[0016] The concentration of the potassium or sodium may be 1 ppm or more.

[0017] The concentration of the potassium or sodium may be 10 ppm or more.

[0018] The concentration of the potassium or sodium may be 5000 ppm or less.

[0019] The hollow-core fiber may confine light in the hollow core by an anti-resonant phenomenon.

[0020] The hollow-core fiber may confine light in the hollow core by a photonic bandgap structure.

Effects of the Invention

[0021] According to the present invention, there is an effect that a hollow-core fiber capable of achieving lower surface scattering loss and transmission loss can be realized.

Brief Description of the Drawings

[0022] [Figure 1] FIG. 1 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the hollow-core fiber according to Embodiment 1. [Figure 2] Figure 2 is a diagram showing an example of the relationship between the K concentration and the normalized surface scattering loss. [Figure 3] Figure 3 is a schematic cross-sectional view of the hole-core fiber according to Embodiment 2 in a plane perpendicular to the longitudinal direction. [Figure 4] Figure 4 is a schematic cross-sectional view of the hole-core fiber according to Embodiment 3 in a plane perpendicular to the longitudinal direction. [Figure 5] Figure 5 is a schematic cross-sectional view of the hole-core fiber according to Embodiment 4 in a plane perpendicular to the longitudinal direction. [Figure 6] Figure 6 is a schematic cross-sectional view of the hole-core fiber according to Embodiment 5 in a plane perpendicular to the longitudinal direction. [Figure 7] Figure 7 is a schematic cross-sectional view of the hole-core fiber according to Embodiments 6 to 9 in a plane perpendicular to the longitudinal direction. [Figure 8] Figure 8 is a schematic cross-sectional view of the hole-core fiber according to Embodiment 10 in a plane perpendicular to the longitudinal direction. [Figure 9] Figure 9 is a schematic cross-sectional view of the hole-core fiber according to Embodiment 11 in a plane perpendicular to the longitudinal direction.

Embodiments for Carrying Out the Invention

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited by the embodiments described below. Also, in each drawing, the same or corresponding components are appropriately assigned the same reference numerals, and redundant explanations are omitted as appropriate. In addition, for terms not specifically defined in this specification, the definitions and measurement methods in ITU-T G.650.1 and G.650.2 of the International Telecommunication Union (ITU) shall be followed.

[0024] (Embodiment 1) Figure 1 is a schematic cross-sectional view of a hollow core fiber according to Embodiment 1, in a plane perpendicular to the longitudinal direction. The hollow core fiber 10 is an optical fiber that confines light in a hollow core by the antiresonant phenomenon. The hollow core fiber 10 comprises one outer tube 11 and five inner tubes 12. The five inner tubes 12 are an example of multiple inner tubes.

[0025] The five inner tubes 12 are arranged in a regular pentagonal shape in a plane perpendicular to the longitudinal direction and are fixed to the inner wall of the outer tube 11 by welding or the like.

[0026] A hollow core 13 is formed in the region surrounded by the five inner tubes 12. The thickness (wall thickness) and inner diameter of the inner tubes 12 are designed so that light is confined in the hollow core 13 by the anti-resonant phenomenon. The thickness and inner diameter are set appropriately according to the wavelength of the light to be confined. The wavelength of the light to be confined is, for example, a wavelength included in the wavelength band used for optical communication, for example, 1.55 μm.

[0027] Here, the outer tube 11 and the inner tube 12 constitute a cladding portion surrounding the void core 13 in the void core fiber 10, which includes the void core 13. The inner tube 12, which constitutes the portion of the cladding portion adjacent to the void core 13, is made of silica glass containing an alkali metal or alkaline earth metal. Furthermore, the inner tube 12 does not need to substantially contain dopants that change the refractive index, such as germanium or fluorine.

[0028] With the porous core fiber 10 configured as described above, lower surface scattering loss and transmission loss can be obtained.

[0029] In other words, by making the portion adjacent to the porous core 13, such as the inner tube 12, out of silica glass containing an alkali metal or alkaline earth metal, the virtual temperature (Tg) of that portion is lower than that of pure silica glass. Here, pure silica glass is an extremely high-purity silica glass that substantially does not contain dopants that change the refractive index and has a refractive index of approximately 1.444 at a wavelength of 1550 nm. As a result, the surface roughness of the inner tube 12, which forms the boundary between the porous core 13 and the silica glass, is reduced. Consequently, surface scattering loss is reduced, as is transmission loss, including surface scattering loss. Potassium or sodium is preferred as the alkali metal, but it is not particularly limited.

[0030] Furthermore, by using alkali metals or alkaline earth metals as dopants to lower the virtual temperature of the glass, the shape of the inner tube 12 made of that glass can be easily controlled to be distortion-free. As a result, the increase in confinement loss caused by the shape of the inner tube 12 can be suppressed, which is preferable from the viewpoint of reducing transmission loss.

[0031] Furthermore, when manufacturing the porous core fiber 10, it is preferable to control the manufacturing conditions so that the amount of hydroxyl groups (OH groups) on the surface of the inner tube 12 is relatively low, thereby making the surface tension (γ) of the inner tube 12 equal to or greater than that of pure silica glass. This allows the effect of reducing the surface roughness of the inner tube 12 due to the inclusion of alkali metals or alkaline earth metals to be more effectively exhibited.

[0032] Furthermore, the constituent material of the outer tube 11 is not particularly limited as long as it is silica-based glass.

[0033] Furthermore, the inner tube 12 may contain chlorine (Cl). The presence of chlorine also lowers the virtual temperature (Tg), and furthermore, the dehydrating effect of chlorine reduces the amount of OH groups relatively, increasing the surface tension (γ) of the inner tube 12, thus further reducing surface roughness. The chlorine concentration is not particularly limited, but for example, it is 2000 ppm or less.

[0034] Furthermore, the concentrations of alkali metals, alkaline earth metals, chlorine, etc., contained in the inner tube 12 can be confirmed, for example, by electron probe microanalyzer (EPMA) or refractive index distribution measurement.

[0035] (Example of experiment) The inventors of the present invention manufactured hollow core fibers with the configuration shown in Figure 1, but with different concentrations of alkali metal (potassium (K) in this experiment) in the inner tube, and investigated their surface scattering losses. Specifically, five silica glass tubes with a diameter of 35 mm and a wall thickness of 0.4 mm were prepared as a set to serve as the inner tube. Each set of silica glass tubes had a different K concentration. However, all silica glass tubes contained chlorine at a concentration of 2000 ppm. Next, each set of silica glass tubes was temporarily fixed with a jig, and multiple types of base materials were fabricated by thermal bonding to the inner wall of the silica glass tube that would serve as the outer tube using an oxyhydrogen flame. Then, these base materials were drawn while controlling the internal pressure to manufacture multiple types of hollow core fibers with the configuration shown in Figure 1. The manufactured hollow core fibers were coated with a coating equivalent to that normally applied to communication optical fibers. Furthermore, in order to minimize the effects of microbend loss, the outer diameter of the outer tube of the porous core fiber was set to 400 μm, and the coating diameter to 600 μm.

[0036] Next, the transmission loss of the manufactured porous core fibers was measured in a bundle state, without being wound on a bobbin. Then, the surface scattering loss was evaluated by subtracting the confinement loss, calculated based on the cross-sectional structure of the porous core fibers, from the measured transmission loss.

[0037] Figure 2 shows an example of the relationship between K concentration and normalized surface scattering loss. Figure 2(a) shows the K concentration on the horizontal axis on a linear scale, and Figure 2(b) shows the K concentration on the horizontal axis on a logarithmic scale. Here, normalized surface scattering loss is the surface scattering loss normalized by the surface scattering loss when the K concentration in the inner tube is zero (the data point indicated by point P1 in Figure 2(a)). That is, when the K concentration is zero, the normalized surface scattering loss is 1.

[0038] As shown in Figure 2, a reduction in surface scattering loss is observed when the K concentration is 1 ppm or higher. While the normalized surface scattering loss is 0.997 at a K concentration of 1 ppm, the reduction is even more pronounced at 10 ppm or higher. Furthermore, it was confirmed that the normalized surface scattering loss can be reduced to 0.7 or less when the K concentration is around 100 ppm to 1000 ppm. However, since scattering loss due to structural defects increases above a certain K concentration, it was confirmed that a K concentration of 5000 ppm or lower is preferable.

[0039] (Embodiment 2) Figure 3 is a schematic cross-sectional view of a hollow core fiber according to Embodiment 2 in a plane perpendicular to the longitudinal direction. The hollow core fiber 10A has a configuration in which four of the five inner tubes 12 are replaced with inner tubes 12A in the hollow core fiber 10 according to Embodiment 1 shown in Figure 1.

[0040] The inner tube 12A has the same inner diameter and wall thickness as the inner tube 12, but does not contain alkali metals or alkaline earth metals, and is made of silica glass with a chlorine concentration of 2000 ppm or less, for example.

[0041] In the porous core fiber 10A, the inner tube 12, which constitutes at least a portion of the part adjacent to the porous core 13, is made of silica glass containing alkali metals or alkaline earth metals, thus lower surface scattering loss and transmission loss can be obtained.

[0042] In the case of the hollow core fiber 10A, four of the five inner tubes 12 of the hollow core fiber 10 are replaced with inner tubes 12A, but the number of tubes replaced can be any of 1 to 3.

[0043] (Embodiment 3) Figure 4 is a schematic cross-sectional view of a hollow core fiber according to Embodiment 3 in a plane perpendicular to the longitudinal direction. The hollow core fiber 10B has a configuration in which the inner tube 12 is replaced with an inner tube 12B in the hollow core fiber 10A according to Embodiment 2 shown in Figure 3.

[0044] The inner tube 12B has the same inner diameter and wall thickness as the inner tube 12, but only the portion 12Ba contains alkali metals or alkaline earth metals, while the portion other than portion 12Ba is made of silica glass with a chlorine concentration of 2000 ppm or less. The concentration of alkali metals or alkaline earth metals in portion 12Ba is, for example, 1 ppm or more or 10 ppm or more, and for example, 5000 ppm or less.

[0045] In the porous core fiber 10B, at least a portion 12Ba adjacent to the porous core 13 is made of silica glass containing alkali metals or alkaline earth metals, thus lower surface scattering loss and transmission loss can be obtained.

[0046] Furthermore, one to four of the other four inner tubes 12A in the hollow core fiber 10B may be replaced with inner tubes 12B.

[0047] (Embodiment 4) Figure 5 is a schematic cross-sectional view of a hollow core fiber according to Embodiment 4 in a plane perpendicular to the longitudinal direction. The hollow core fiber 10C has a configuration in which a sub-tube 14 is provided inside each inner tube 12 of the hollow core fiber 10 according to Embodiment 1 shown in Figure 1.

[0048] The wall thickness and inner diameter of the sub-tube 14 are designed according to the wavelength of light to be confined in the porous core 13 by the anti-resonant phenomenon. The sub-tube 14 does not contain alkali metals or alkaline earth metals and is made of silica glass with a chlorine concentration of 2000 ppm or less, for example.

[0049] In the perforated core fiber 10C, as with the perforated core fiber 10, the inner tube 12 is made of silica glass containing alkali metals or alkaline earth metals, resulting in lower surface scattering loss. Furthermore, in the perforated core fiber 10C, components of light that leak into the inner tube 12 because they cannot be contained by the inner tube 12 are contained by the sub-tube 14, thus reducing the containment loss. As a result, even lower transmission loss can be obtained in the perforated core fiber 10C.

[0050] (Embodiment 5) Figure 6 is a schematic cross-sectional view of a hollow core fiber according to Embodiment 5 in a plane perpendicular to the longitudinal direction. The hollow core fiber 10D has a configuration in which each of the sub-tubes 14 of the hollow core fiber 10 according to Embodiment 4 shown in Figure 5 is replaced with a sub-tube 14D.

[0051] Sub-tube 14D has the same inner diameter and wall thickness as sub-tube 14, but is made of silica glass containing alkali metals or alkaline earth metals. The concentration of alkali metals or alkaline earth metals in sub-tube 14D is, for example, 1 ppm or more or 10 ppm or more, and for example, 5000 ppm or less.

[0052] In the porous core fiber 10D, surface scattering loss is also reduced in the sub-tube 14D, resulting in even lower transmission loss than that of the porous core fiber 10C.

[0053] In the case of the hollow core fiber 10D, the five sub-tubes 14 of the hollow core fiber 10C are replaced with sub-tubes 14D, but the number of sub-tubes replaced can be any of 1 to 4.

[0054] (Embodiments 6-9) Figure 7 is a schematic cross-sectional view of a hollow core fiber according to embodiments 6 to 9 in a plane perpendicular to the longitudinal direction. In all hollow core fibers 10E to 10H according to embodiments 6 to 9, the secondary tube has a double-layered structure.

[0055] The hollow core fiber 10E according to Embodiment 6 shown in Figure 7(a) has a configuration in which a sub-tube 15 is provided inside each sub-tube 14 of the hollow core fiber 10C shown in Figure 5.

[0056] The wall thickness and inner diameter of the sub-tube 15 are designed according to the wavelength of light to be confined in the porous core 13 by the anti-resonant phenomenon. The sub-tube 15 does not contain alkali metals or alkaline earth metals and is made of silica glass with a chlorine concentration of 2000 ppm or less, for example.

[0057] In the perforated core fiber 10E, light components that leak into the inner tube 12 and sub-tube 14 because they cannot be completely contained by the inner tube 12 and sub-tube 14 are contained by the sub-tube 15, thus reducing the containment loss. As a result, the perforated core fiber 10E achieves even lower transmission loss than the perforated core fiber 10C.

[0058] The hollow core fiber 10F according to Embodiment 7 shown in Figure 7(b) has a configuration in which three of the five inner tubes 12 of the hollow core fiber 10E shown in Figure 7(a) are replaced with inner tubes 12A. Even with such a hollow core fiber 10E, lower surface scattering loss and transmission loss can be obtained.

[0059] The hollow core fiber 10G according to Embodiment 8 shown in Figure 7(c) has a configuration in which each of the sub-tubes 14 of the hollow core fiber 10E shown in Figure 7(a) is replaced with a sub-tube 14G. The sub-tube 14G has the same inner diameter and wall thickness as the sub-tube 14, but is made of silica glass containing alkali metal or alkaline earth metal, similar to the inner tube 12. The concentration of alkali metal or alkaline earth metal in the sub-tube 14G is, for example, 1 ppm or more or 10 ppm or more, and for example, 5000 ppm or less. In such a hollow core fiber 10G, the effect of reducing surface scattering loss can be obtained even in the sub-tube 14G compared to the hollow core fiber 10E, so an even lower transmission loss can be obtained than that of the hollow core fiber 10E.

[0060] The hollow core fiber 10H according to Embodiment 9 shown in Figure 7(d) has a configuration in which each of the sub-tubes 15 of the hollow core fiber 10G shown in Figure 7(c) is replaced with a sub-tube 15H. The sub-tube 15H has the same inner diameter and wall thickness as the sub-tube 15, but is made of silica glass containing alkali metal or alkaline earth metal, similar to the inner tube 12 and sub-tube 14G. The concentration of alkali metal or alkaline earth metal in the sub-tube 15H is, for example, 1 ppm or more or 10 ppm or more, and for example, 5000 ppm or less. In such a hollow core fiber 10H, the effect of reducing surface scattering loss can be obtained even in the sub-tube 15H compared to the hollow core fiber 10G, so an even lower transmission loss can be obtained than that of the hollow core fiber 10G.

[0061] (Embodiment 10) Figure 8 is a schematic cross-sectional view of a porous core fiber according to Embodiment 10 in a plane perpendicular to the longitudinal direction. The porous core fiber 10I is an optical fiber that confines light in a porous core by a photonic bandgap structure.

[0062] The porous core fiber 10I includes a cladding portion 16 made of glass. The cladding portion 16 is provided with a microporous region 16a. Microporous regions 16a have fine porous structures 16aa and 16ab arranged in a triangular lattice pattern. A porous core 13 is provided approximately in the center of the microporous region 16a. The porous structure 16ab is a porous structure included in region A surrounding the porous core 13, and the porous structure 16aa is a porous structure other than the porous structure 16ab.

[0063] These void structures 16aa and 16ab are formed by welding together numerous capillaries having approximately the same inner diameter and wall thickness, forming a photonic bandgap in a wavelength band that includes a predetermined wavelength. As a result, light in the wavelength band that includes the predetermined wavelength is confined to the void core 13. The predetermined wavelength is, for example, a wavelength included in the wavelength band used for optical communication, for example, 1.55 μm.

[0064] The size of the void core 13 is approximately equivalent to 19 void structures 16aa and 16ab. Such a void core 13 is sometimes referred to as a 19-cell type.

[0065] In the porous core fiber 10I, the porous structure 16ab is made of silica glass containing an alkali metal or alkaline earth metal. The concentration of the alkali metal or alkaline earth metal in the porous structure 16ab is, for example, 1 ppm or more or 10 ppm or more, and for example, 5000 ppm or less. The porous structure 16aa does not contain alkali metals or alkaline earth metals and is made of silica glass with a chlorine concentration of, for example, 2000 ppm or less.

[0066] In the porous core fiber 10I configured as described above, the portion adjacent to the porous core 13 (porous structure 16ab in region A) is made of silica glass containing alkali metals or alkaline earth metals, thereby reducing surface scattering loss and transmission loss, including surface scattering loss.

[0067] In addition, in the porous core fiber 10I, the porous structure 16aa may be replaced with a porous structure 16ab containing an alkali metal or alkaline earth metal.

[0068] (Embodiment 11) Figure 9 is a schematic cross-sectional view of a hollow core fiber according to Embodiment 11 in a plane perpendicular to the longitudinal direction. The hollow core fiber 10J is also an optical fiber that confines light in a hollow core by a photonic bandgap structure.

[0069] The porous core fiber 10J is equipped with a glass cladding 16J. The cladding 16 is provided with a microporous region 16Ja. A fine porous structure 16aa arranged in a triangular lattice is formed in the microporous region 16Ja. A tubular section 17 is provided approximately in the center of the microporous region 16Ja, and the inside of the tubular section 17 is the porous core 13. The size of the tubular section 17 is approximately 19 times the size of the porous structure 16aa.

[0070] In the porous core fiber 10J, the tubular portion 17 is made of silica glass containing an alkali metal or alkaline earth metal. The concentration of the alkali metal or alkaline earth metal in the tubular portion 17 is, for example, 1 ppm or more or 10 ppm or more, and for example, 5000 ppm or less.

[0071] In the porous core fiber 10J configured as described above, the tubular portion 17 adjacent to the porous core 13 is made of silica glass containing alkali metals or alkaline earth metals, thereby reducing surface scattering loss and transmission loss, including surface scattering loss.

[0072] In addition, in the porous core fiber 10J, the porous structure 16aa may be replaced with a porous structure containing an alkali metal or alkaline earth metal.

[0073] Furthermore, in the above embodiment, the portion composed of silica glass containing an alkali metal or alkaline earth metal may also contain fluorine, or more than 2000 ppm of chlorine, for example, 3000 ppm or more. This can further reduce surface scattering loss.

[0074] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of symbols]

[0075] 10, 10A, 10B, 10C, 10D, 10E, 10F, 10G, 10H, 10I, 10J: Hollow core fiber 11:Outer tube 12,12A,12B: Inner tube 12Ba: Part 13: Hollow core 14,14D,14G,15,15H: Sub pipe 16,16J: Clad section 16a,16Ja: Micropore region 16aa, 16ab: Hollow structure 17: Pipe part A:Area P1: point

Claims

1. A hollow core fiber containing a hollow core, At least a portion of the cladding surrounding the void core, specifically the portion adjacent to the void core, is made of silica glass containing an alkali metal or alkaline earth metal. Hollow core fiber.

2. The alkali metal is potassium or sodium. The porous core fiber according to claim 1.

3. The concentration of potassium or sodium is 1 ppm or more. The porous core fiber according to claim 2.

4. The concentration of potassium or sodium is 10 ppm or higher. The porous core fiber according to claim 2.

5. The concentration of potassium or sodium is 5000 ppm or less. The porous core fiber according to claim 2.

6. The aforementioned void core fiber confines light within the void core through the antiresonant phenomenon. The porous core fiber according to claim 1.

7. The aforementioned vacant core fiber confines light within the vacant core by a photonic bandgap structure. The porous core fiber according to claim 1.

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