Hollow core fiber

Abstract

This hollow core fiber comprises: an outer tube; a plurality of inner tubes that are disposed inside the outer tube; and a plurality of support tubes that are disposed inside the outer tube and that fix the plurality of inner tubes to an inner wall of the outer tube. In a plane perpendicular to the longitudinal direction, the plurality of inner tubes are disposed in a polygonal shape. A hollow core part is formed in a region surrounded by the plurality of inner tubes. The plurality of inner tubes confine light in the hollow core part via anti-resonance. The hollow core fiber further comprises a plurality of auxiliary tubes that are disposed between the outer tube and the plurality of inner tubes. The plurality of auxiliary tubes may confine light further inward than the auxiliary tubes via anti-resonance.

Description

Hole-core fiber

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

[0002] Optical fibers, which act as a longitudinal light-propagating medium within a core region, are widely used in a variety of fields, including optical communications, sensing, and laser power delivery. While conventional silica-core optical fibers remain the standard, they suffer from inherent losses due to optical absorption in the dielectric material that constitutes them. Hole-core optical fibers (HCFs) offer a promising alternative to silica-core optical fibers. By guiding light in a core primarily filled with air, HCFs have the potential to significantly reduce transmission losses compared to solid-core optical fibers. Furthermore, they offer a variety of properties not achievable with conventional solid-core optical fibers, potentially opening up many novel applications.

[0003] Photonic bandgap hole-core fibers (PBG-HCFs) are known as hole-core fibers. However, PBG-HCFs face significant challenges, including higher-than-expected losses due to their manufacturing precision and sensitivity to structural changes. While PBG-HCFs are relatively tolerant to bending, their loss spectrum is split into a narrow, low-loss window between high-loss peaks where the fundamental core mode leaks into surface modes.

[0004] On the other hand, the band-splitting problem can be solved with the advent of antiresonant air-hole-core fibers (AR-HCFs), which use carefully designed capillary structures to significantly reduce optical leakage through the principle of antiresonance. Recent advances have demonstrated AR-HCFs with record low loss.

[0005] For example, in 2021, H. Sakr et al. reported that they achieved lower loss than any other optical fiber (including solid-core silica fiber) at wavelengths of 850 nm and 1060 nm using a nested antiresonant nodeless fiber (NANF) (Non-Patent Document 1). Also, in 2022, G.T. Jasion et al. reported that they achieved an astonishing loss value of 0.174 dB / km in the C-band using a hollow-core double-nested antiresonant nodeless fiber (DNANF) design (Non-Patent Document 2). Patent literature also reports results of reducing confinement loss using double or more (nested) antiresonant ring structures (Patent Document 1).

[0006] The light propagation mechanism of AR-HCF relies on a combination of several factors. The central air- or gas-filled region (air-hole core) facilitates light guidance by the surrounding structure, even if the core's refractive index is lower than the average refractive index of the cladding region. The surrounding thin-walled capillaries create antiresonances that block lateral light transmission through the fiber at specific wavelengths. Furthermore, carefully controlling the overlap between the air-guided and tube-guided modes further reduces propagation loss. Several antiresonant air-core fiber designs have been previously published, focusing on achieving low overlap between the air-guided and tube modes while also addressing practical issues. For example, a structure with two antiresonant rings arranged in a circle, with a cladding tube holding the structure together, has been proposed. Another innovative element is the nested antiresonant nodeless fiber structure. Instead of the capillaries touching each other, the air-hole core is surrounded by multiple thin concentric capillaries at specific peripheral locations where it contacts the cladding tube.

[0007] Special table 2017-520804 publication

[0008] 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).Gregory T Jasion et al., “0.174 dB / km Hollow Core Double Nested Antiresonant Nodeless Fiber (DNANF)”, OFC 2022 Postdeadline Paper Session III(Th4C).

[0009] However, in the known AR-HCF, there is room for improvement in confinement loss and therefore propagation loss, since light leaks through the outer tube.

[0010] The present invention has been made in view of the above, and an object of the present invention is to provide a hole-core fiber that can achieve lower loss characteristics.

[0011] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention provides an optical fiber comprising an outer tube, a plurality of inner tubes arranged inside the outer tube, and a plurality of support tubes arranged inside the outer tube and fixing the plurality of inner tubes to the inner wall of the outer tube, wherein the plurality of inner tubes are arranged in a polygonal shape on a plane perpendicular to the longitudinal direction, a hole core portion is formed in an area surrounded by the plurality of inner tubes, and the plurality of inner tubes are hole core fibers that confine light in the hole core portion by the antiresonant phenomenon.

[0012] The hollow-core fiber may further include a plurality of sub-tubes arranged between the outer tube and the plurality of inner tubes, and the plurality of sub-tubes may confine the light inside the sub-tubes by an antiresonant phenomenon.

[0013] The plurality of inner tubes may include inner tubes having two or more different thicknesses or diameters.

[0014] The plurality of support tubes may include support tubes having two or more different thicknesses or diameters.

[0015] One of the inner tubes may be supported by two of the support tubes.

[0016] The plurality of secondary pipes may include secondary pipes having two or more different thicknesses or diameters.

[0017] The N inner tubes may be arranged to have N-fold rotational symmetry with respect to the central axis of the hole core portion, where N is an integer greater than or equal to 2, and the N support tubes may be arranged to have N-fold rotational symmetry with respect to the central axis.

[0018] The propagation loss at the wavelength of the light may be less than 2 dB / km.

[0019] The core diameter dcore of the core hole portion may be 10 μm or more and 100 μm or less.

[0020] The ratio d3 / d2 of the inner diameter d3 of the secondary pipe to the inner diameter d2 of the support pipe may be 10% or more and 90% or less.

[0021] The ratio t3 / t2 of the thickness t3 of the secondary pipe to the thickness t2 of the support pipe may be 40% or more and 200% or less.

[0022] The plurality of support tubes are configured in two support tube groups, and the support tubes included in the same support tube group may have the same inner diameter and thickness, while the support tubes included in different support tube groups may have different inner diameters or thicknesses.

[0023] Two of the plurality of inner tubes may have a different inner diameter or thickness than other inner tubes of the plurality of inner tubes.

[0024] The present invention has the effect of realizing a holey-core fiber that can obtain lower or stable low-loss characteristics.

[0025] FIG. 1 is a schematic cross-sectional view of a hole-core fiber according to embodiment 1 in a plane perpendicular to the longitudinal direction. FIG. 2 is a schematic cross-sectional view of a hole-core fiber according to embodiment 2 in a plane perpendicular to the longitudinal direction. FIG. 3A is a schematic cross-sectional view of a hole-core fiber according to embodiment 3 in a plane perpendicular to the longitudinal direction. FIG. 3B is a schematic cross-sectional view of a hole-core fiber according to embodiment 4 in a plane perpendicular to the longitudinal direction. FIG. 4 is a schematic cross-sectional view of a calculation model of a hole-core fiber in a plane perpendicular to the longitudinal direction. FIG. 5 is a diagram showing an example of the electric field distribution of the fundamental propagation mode. FIG. 6 is a diagram showing the wavelength spectrum of the confinement loss. FIG. 7 is a diagram showing the wavelength spectrum of the surface scattering loss. FIG. 8 is a diagram showing the electric field distribution of the fundamental propagation mode for various parameter settings.

[0026] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below. Furthermore, in each drawing, the same or corresponding components are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. Furthermore, terms not specifically defined in this specification shall follow the definitions and measurement methods in G.650.1 and G.650.2.

[0027] 1 is a schematic cross-sectional view of a hole-core fiber according to embodiment 1 in a plane perpendicular to the longitudinal direction. The hole-core fiber 10 comprises six inner tubes 1, one outer tube 2, six support tubes 3, and six sub-tubes 4. All of these components are made of a material that is transparent to the light to be propagated, for example, glass such as pure silica glass. Here, the six inner tubes 1, support tubes 3, and sub-tubes 4 are examples of the multiple inner tubes, multiple support tubes, and multiple sub-tubes, respectively.

[0028] The inner tube 1 has the same inner diameter and thickness. The inner tube 1 is disposed inside the outer tube 2, and is arranged in a regular hexagonal shape (an example of a polygonal shape) on a plane perpendicular to the longitudinal direction.

[0029] The support tubes 3 have the same inner diameter and thickness. The support tubes 3 are arranged inside the outer tube 2 in a regular hexagonal shape on a plane perpendicular to the longitudinal direction. The support tubes 3 fix the inner tube 1 to the inner wall of the outer tube 2. Specifically, the support tubes 3 are fixed to the inner wall of the outer tube 2 by welding or the like. Furthermore, each inner tube 1 is fixed to any two of the support tubes 3 by welding or the like. In other words, one inner tube 1 is supported by two support tubes 3. In this way, the support tubes 3 fix the inner tube 1 to the inner wall of the outer tube 2.

[0030] A hole core portion 5 is formed in the region surrounded by the inner tube 1. The thickness (wall thickness) and inner diameter of the inner tube 1 are designed so as to confine light in the hole core portion 5 by the antiresonant phenomenon. The thickness and inner diameter are set appropriately depending on the wavelength of the light to be confined. In FIG. 1 , field F indicates the field of light (propagating light) that is confined by the antiresonant phenomenon and propagates through the hole core portion 5. In field F, the intensity of the field is indicated by the density of the dot pattern, and the higher the dot density, the higher the field intensity.

[0031] In the hole-core fiber 10, the inner tube 1 is arranged so as to have six-fold rotational symmetry with respect to the central axis of the hole core portion 5. Six-fold rotational symmetry is an example of N-fold rotational symmetry where N is an integer of 2 or more. N is, for example, 50 or less. In the hole-core fiber 10, the support tube 3 is also arranged so as to have six-fold rotational symmetry with respect to the central axis of the hole core portion 5.

[0032] The secondary tube 4 has the same thickness and inner diameter as the secondary tube 2. The secondary tube 4 is disposed between the outer tube 2 and the inner tube 1. In other words, the secondary tube 4 is disposed closer to the outer tube 2 than the inner tube 1, so as to surround the air hole core portion 5. The secondary tube 4 is fixed to the inner wall of the outer tube 2 by welding or the like. The thickness and inner diameter of the secondary tube 4 are designed so that the propagating light is confined inside the secondary tube 4 by the antiresonant phenomenon.

[0033] It is preferable that the secondary tubes 4 are arranged so that the central axis of the inner tube 1 and the central axis of the secondary tube 4 overlap when viewed in the radial direction from the central axis of the hole core portion 5. In this case, the secondary tubes 4 are arranged in a regular hexagonal shape on a plane perpendicular to the longitudinal direction and are arranged so as to have six-fold rotational symmetry with respect to the central axis of the hole core portion 5. However, the arrangement of the secondary tubes 4 is not limited to this, and they may be shifted to such an extent that the central axis of the inner tube 1 and the internal region of the secondary tube 4 overlap. Such secondary tubes 4 can be arranged with higher positional accuracy than, for example, when provided inside the inner tube 1.

[0034] In the hole-core fiber 10 configured as above, the inner tube 1 that confines propagating light in the hole core region 5 by the antiresonant phenomenon is not in direct contact with the outer tube 2. As a result, components of the propagating light that cannot be completely confined by the inner tube 1 and pass through the inner tube 1 to leak to the outside, and components of the propagating light that leak from between the two inner tubes 1 to the outside, are prevented from directly reaching the outer tube 2 and leaking out. As a result, the confinement loss is reduced, and therefore the propagation loss is also reduced. Furthermore, in the hole-core fiber 10, the sub-tube 4 also confines propagating light in the hole core region 5 that is inside the sub-tube 4 by the antiresonant phenomenon, so that the confinement loss and therefore the propagation loss are further reduced. Note that in the hole-core fibers according to this embodiment or the other embodiments described below, the propagation loss at the wavelength of the propagating light is preferably less than 2 dB / km.

[0035] 2 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of a hole-core fiber according to embodiment 2. A hole-core fiber 10A has a configuration in which the number of inner tubes 1, support tubes 3, and sub-tubes 4 in the hole-core fiber 10 according to embodiment 1 shown in FIG. 1 is changed from six to five.

[0036] The inner tube 1 and the support tube 3 are arranged in a regular pentagonal shape on a plane perpendicular to the longitudinal direction. Therefore, the inner tube 1 is arranged so as to have five-fold rotational symmetry with respect to the central axis of the hole core portion 5. The support tube 3 is also arranged so as to have five-fold rotational symmetry with respect to the central axis of the hole core portion 5. Furthermore, it is preferable that the secondary tubes 4 are also arranged in a regular pentagonal shape on a plane perpendicular to the longitudinal direction and so as to have five-fold rotational symmetry with respect to the central axis of the hole core portion 5.

[0037] The hole-core fiber 10A configured as above can achieve low loss characteristics, similar to the hole-core fiber 10 according to the first embodiment.

[0038] 3A and 3B are schematic cross-sectional views of hole-core fibers according to third and fourth embodiments taken along a plane perpendicular to the longitudinal direction.

[0039] The hole-core fiber 10B according to embodiment 3 shown in Figure 3A has a configuration in which the support tube 3 in the hole-core fiber 10 according to embodiment 1 shown in Figure 1 is replaced with support tubes 3Ba and 3Bb, and the secondary tube 4 is replaced with a secondary tube 4B.

[0040] The six support tubes 3Ba have the same thickness and inner diameter. The support tubes 3Ba are arranged so that they have a regular hexagonal shape in a plane perpendicular to the longitudinal direction and have six-fold rotational symmetry with respect to the central axis of the hole core portion 5, and are fixed to the inner tube 1 by welding or the like. The twelve support tubes 3Bb have the same thickness and inner diameter. The support tubes 3Bb are arranged so that they have a regular dodecagonal shape in a plane perpendicular to the longitudinal direction and have twelve-fold rotational symmetry with respect to the central axis of the hole core portion 5, and are fixed to the six support tubes 3Ba and the inner wall of the outer tube 2 by welding or the like. In this way, the support tubes 3Ba and 3Bb fix the inner tube 1 to the inner wall of the outer tube 2.

[0041] The six sub-tubes 4B have the same thickness and inner diameter. The sub-tubes 4B are arranged in a regular hexagonal shape in a plane perpendicular to the longitudinal direction and are fixed to the support tube 3Bb by welding or the like. The thickness and inner diameter of the sub-tubes 4B are designed so that the propagating light is confined inside the sub-tubes 4B due to the antiresonant phenomenon.

[0042] In the air-hole core fiber 10B configured as described above, the inner tube 1 and the outer tube 2 are separated by two layers of support tubes 3Ba and 3Bb, so that lower loss characteristics are obtained than in the air-hole core fiber 10 according to embodiment 1.

[0043] The hole-core fiber 10C according to the fourth embodiment shown in FIG. 3B has a configuration in which a sub-tube 4C is added to the hole-core fiber 10B according to the third embodiment shown in FIG. 3A.

[0044] The twelve sub-tubes 4C have the same thickness and inner diameter. The sub-tubes 4C are arranged in a regular dodecagonal shape on a plane perpendicular to the longitudinal direction and are fixed to the inner wall of the outer tube 2 by welding or the like. The thickness and inner diameter of the sub-tubes 4C are designed so that the propagating light is confined inside the sub-tubes 4C due to the antiresonant phenomenon.

[0045] In the hole-core fiber 10C configured as above, the secondary tube has a two-layer structure of secondary tubes 4B and 4C, and therefore lower loss characteristics than the hole-core fiber 10B according to the third embodiment can be obtained.

[0046] In the above embodiment, the support tubes 3Ba and the support tubes 3Bb may have different thicknesses or inner diameters. In this case, six support tubes 3Ba and twelve support tubes 3Bb form support tube groups, respectively, and the support tubes in the same support tube group have the same inner diameter and thickness, while the support tubes in different support tube groups have different inner diameters or thicknesses.

[0047] (Calculation Example) In the following, a calculation example based on simulation calculations will be described regarding preferable design parameters of a holey-core fiber. The simulation calculations were performed using the finite element method and advanced modeling techniques for surface roughness.

[0048] 4 is a schematic cross-sectional view of a calculation model of a hole-core fiber in a plane perpendicular to the longitudinal direction. The hole-core fiber 1000 is made of silica glass and includes an inner tube 100, an outer tube 200, a support tube 300, a secondary tube 400, and a hole core portion 500. The central axis of the hole core portion 500 is central axis C.

[0049] The radius of the air hole core region 500 is represented by the radius R of a circle inscribed around the central axis C and inscribed in the six inner tubes 100. The diameter (core diameter) of the air hole core region 500 is denoted as d. The larger the core diameter of the air hole core region 500, the smaller the confinement loss value. However, if the core diameter of the air hole core region 500 is too large, microbending loss increases. Furthermore, if the core diameter is too large, the fiber diameter also increases, which may cause problems such as a loss of mechanical reliability. From this perspective, in this calculation example, d was set to 32 μm (R = 16 μm). This core diameter is a proven size as reported in the past and is a desirable example.

[0050] The inner diameter of the inner tube 100 is d1, which is, for example, 22.42 μm. The thickness of the inner tube 100 is t1, which is, for example, 0.46 μm, provides the minimum total loss (the sum of the confinement loss and the scattering loss) for the propagating light of wavelength 1550 nm.

[0051] If the inner diameter of the support tube 300 is d2 and the thickness is t2, then d2 is preferably 21.93 μm, for example, and t2 is preferably 0.64 μm, for example. If the inner diameter of the secondary tube 400 is d3 and the thickness is t3, then d3 is preferably 11.50 μm, for example, and t3 is preferably 0.41 μm, for example. tclad is preferably 3.82 μm, for example.

[0052] Figure 5 shows an example of the electric field distribution of the fundamental propagation mode in a calculation example. The design parameters are: dcore = 32 μm, d1 = 22.42 μm, t1 = 0.46 μm, d2 = 21.93 μm, t2 = 0.64 μm, d3 = 11.50 μm, and t3 = 0.41 μm. The hole-core fiber is wound with a bending radius of 16 cm. This bending radius is an example of the radius of a typical fiber drum. As shown in Figure 5, with the above combination of design parameters, light with a wavelength of 1550 nm is confined in the fundamental propagation mode even when the hole-core fiber is bent. In this case, the total loss (propagation loss) is the sum of the confinement loss and the scattering loss. The total loss, confinement loss, and surface scattering loss at a wavelength of 1550 nm were 0.3627 dB / km, 0.252 dB / km, and 0.1107 dB / km, respectively.

[0053] Figure 6 shows the wavelength spectrum of the confinement loss, and Figure 7 shows the wavelength spectrum of the surface scattering loss for the hole-core fibers of calculation models #1 to #4, in which dcore was fixed at 32 μm and the combinations of other design parameters were changed as shown in Table 1. Here, calculation model #1 is the hole-core fiber whose electric field distribution is shown in Figure 5. As shown in Figure 6, calculation model #1 exhibited low loss over a wavelength range of 150 nm or more, from 1400 nm to 1570 nm. In particular, the confinement loss was low in the S-band and C-band, being less than 0.4 dB / km, and at a wavelength of 1550 nm it was 0.26 dB / km. Furthermore, as shown in Figure 7, calculation models #1 and #2 exhibited 0.15 dB / km or less in the wavelength region longer than 1470 nm.

[0054]

[0055] Table 2 shows examples of maximum and minimum values ​​for the design parameters dcore, d1, t1, d2, t2, d3, t3, and tclad, as well as the resulting confinement loss (CL) and surface scattering loss (SSL) when the design parameters are varied within the constraint of a total loss of 1 ± 0.01 dB / km. tclad is the thickness of the outer tube. Note that CL and SSL are values ​​at a wavelength of 1550 nm. As shown in Table 2, dcore is preferably in the range of 29.45 μm to 36.36 μm, for example. d1 is preferably in the range of 18.41 μm to 25.66 μm, for example. d2 is preferably in the range of 20.88 μm to 24.03 μm, for example. d3 is preferably in the range of 3.18 μm to 12.83 μm, for example. t1 is preferably in the range of 0.37 μm to 0.59 μm, for example. t2 is preferably in the range of 0.63 μm to 0.71 μm. t3 is preferably in the range of 0.23 μm to 0.74 μm, and tclad is preferably in the range of 0.27 μm to 24.64 μm.

[0056]

[0057] Fig. 8 shows the electric field distribution of the fundamental propagation mode for various parameter settings shown in Table 2. The hole-core fiber is wound with a bending radius of 16 cm. Fig. 8(a) shows the case where d3 is minimum, Fig. 8(b) shows the case where d1 is minimum, Fig. 8(c) shows the case where dcore is maximum, Fig. 8(d) shows the case where dcore is minimum, Fig. 8(e) shows the case where t3 is maximum, and Fig. 8(e) shows the case where t2 is minimum.

[0058] Here, in the hole-core fiber according to the embodiment, a hole-core fiber having n-fold rotationally symmetric orders has eigenmodes of the same order or degenerate double-order. Therefore, modes without perfect symmetry orders are provided by pairs with two identical eigenvalues ​​(e.g., effective refractive index). Because the fundamental modes of a polarization-maintaining fiber must have substantially different effective refractive indices to prevent power coupling between them if the fiber is not perfectly straight, a polarization-maintaining fiber cannot have n≧3 rotationally symmetric orders. In the case of a solid core, a polarization-maintaining fiber with a symmetric order of n=2 is typically achieved by introducing two stress rods arranged laterally 180° apart from each other. The hole-core fiber according to the embodiment as a polarization-maintaining fiber can be realized by having at least one inner tube of the multiple inner tubes have a different inner diameter or thickness from the other inner tubes of the multiple inner tubes. As an example, when n is an even number, two of the inner tubes may be selected to have different diameters or thicknesses from the other inner tubes to achieve the hole-core fiber as a polarization-maintaining fiber. More specifically, when n=6, two inner tubes may be selected to have a different diameter or a different thickness than the other four inner tubes.

[0059] The hole-core fiber according to the above embodiment is suitable for optical communications, optical power transmission, ultra-low latency transmission, sensors, and the like.

[0060] In the above embodiment, the multiple inner tubes may include inner tubes with two or more different thicknesses or diameters. Also, the multiple support tubes may include support tubes with two or more different thicknesses or diameters. Also, the multiple secondary tubes may include secondary tubes with two or more different thicknesses or diameters.

[0061] In the above embodiment, dcore may be 10 μm or more and 100 μm or less, d3 / d2 may be 10% or more and 90% or less, and t3 / t2 may be 40% or more and 200% or less.

[0062] Furthermore, in the above embodiment, the cross sections of the inner tube, outer tube, support tube, secondary tube, etc. are circular, but even if distortion occurs in these during the manufacturing process of the air-hole core fiber, and they are not completely circular but are approximately circular, the above-mentioned effects can be obtained.

[0063] Furthermore, in the above-described embodiment, the inner tube or the secondary tube may have a nested structure. The adoption of a nested structure can reduce confinement loss. However, when a nested structure is adopted, if the tubes constituting the nested structure are misaligned with each other due to manufacturing errors or the like, the light confinement effect may not be achieved as designed. Therefore, if a nested structure is not adopted, such a misalignment problem does not occur, and stable low-loss characteristics can be obtained.

[0064] Furthermore, the present invention is not limited to the above-described embodiments. Configurations in which the above-described components are appropriately combined are also included in the present invention. Furthermore, further effects and modifications can be easily derived by those skilled in the art. For example, a configuration in which the sub-pipe is omitted from the above-described embodiments is also included in the present invention. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

[0065] 1, 100: Inner tube 2, 200: Outer tube 3, 3Ba, 3Bb, 300: Support tube 4, 4B, 4C, 400: Sub-tube 5, 500: Hole core portion 10, 10A, 10B, 10C, 1000: Hole core fiber C: Central axis F: Field

Claims

1. A hole-core fiber comprising: an outer tube; a plurality of inner tubes arranged inside the outer tube; and a plurality of support tubes arranged inside the outer tube and fixing the plurality of inner tubes to the inner wall of the outer tube, wherein the plurality of inner tubes are arranged in a polygonal shape on a plane perpendicular to the longitudinal direction, a hole core portion is formed in an area surrounded by the plurality of inner tubes, and the plurality of inner tubes confine light in the hole core portion by the antiresonant phenomenon.

2. The air-core fiber according to claim 1, further comprising a plurality of sub-tubes arranged between the outer tube and the plurality of inner tubes, wherein the plurality of sub-tubes confine the light inside the sub-tubes by an antiresonant phenomenon.

3. The air-core fiber according to claim 1, wherein said plurality of inner tubes include inner tubes of two or more different thicknesses or diameters.

4. The air-core fiber according to claim 1, wherein said plurality of support tubes include support tubes of two or more different thicknesses or diameters.

5. The air-core fiber according to claim 1, wherein one of said inner tubes is supported by two of said support tubes.

6. The air-core fiber according to claim 2, wherein the plurality of sub-tubes include sub-tubes having two or more different thicknesses or diameters.

7. The air-core fiber according to claim 1, wherein N inner tubes are arranged so as to have N-fold rotational symmetry about the central axis of the air-hole core portion, where N is an integer of 2 or greater, and N support tubes are arranged so as to have N-fold rotational symmetry about the central axis.

8. The hole-core fiber of claim 1, wherein the propagation loss at the wavelength of said light is less than 2 dB / km.

9. The air-core fiber according to claim 1, wherein the core diameter dcore of said air-core portion is 10 μm or more and 100 μm or less.

10. The air-core fiber according to claim 2, wherein the ratio d3 / d2 of the inner diameter d3 of said secondary tube to the inner diameter d2 of said support tube is 10% or more and 90% or less.

11. The hollow-core fiber according to claim 2, wherein the ratio t3 / t2 of the thickness t3 of said secondary tube to the thickness t2 of said support tube is 40% or more and 200% or less.

12. The hollow-core fiber according to claim 1, wherein the plurality of support tubes are configured into two support tube groups, and the support tubes included in the same support tube group have the same inner diameter and thickness, while the support tubes included in different support tube groups have different inner diameters or thicknesses.

13. The air-core fiber of claim 1, wherein two of said plurality of inner tubes have a different inner diameter or thickness than the other inner tubes of said plurality of inner tubes.

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