Hole-core fiber

Abstract

To provide a hole core fiber capable of obtaining lower or stable low loss characteristics.SOLUTION: A hole-core fiber includes an outer tube and a plurality of inner tubes arranged inside the outer tube, the plurality of inner tubes are arranged in a polygonal shape in a plane perpendicular to a longitudinal direction, a hole core portion is formed in a region surrounded by the plurality of inner tubes, the plurality of inner tubes confine light in the hole core portion by an anti-resonant phenomenon, and the plurality of inner tubes are not in direct contact with the outer tube.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

[0003] Photonic bandgap air-core fibers (PBG-HCFs) are known as air-core fibers. However, PBG-HCFs face significant challenges, with higher-than-expected losses due to fabrication precision and high sensitivity to structural changes. Although 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 challenge can be resolved 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 previous optical fiber (including solid-core silica fiber) at wavelengths of 850 nm and 1060 nm using nested antiresonant nodeless fiber (NANF) (Non-Patent Document 1). Also, in 2022, GT 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 innovation 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2017-520804 [Non-patent literature]

[0008] [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). Summary of the Invention [Problem to be solved by the invention]

[0009] However, in the known AR-HCF, there is room for improvement in the confinement loss and therefore the 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 obtain lower or stable low-loss characteristics. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention comprises an outer tube and a plurality of inner tubes arranged inside the outer tube, the plurality of inner tubes being arranged in a polygonal shape on a plane perpendicular to the longitudinal direction, a hole core portion being formed in an area surrounded by the plurality of inner tubes, the plurality of inner tubes confining light to the hole core portion by the antiresonant phenomenon, and the plurality of inner tubes being hole core fibers that are not in direct contact with the outer tube.

[0012] The number of polygonal layers formed by the plurality of inner tubes may be one.

[0013] The number of polygonal layers formed by the plurality of inner tubes may be two or more.

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

[0015] The hollow-core fiber may further include an intermediate tube arranged concentrically inside the outer tube and with the multiple inner tubes arranged inside, and a support portion, and the multiple inner tubes may be supported on the outer tube by the intermediate tube and the support portion.

[0016] The hollow-core fiber may further include a plurality of sub-tubes arranged between the outer tube and the plurality of inner tubes, the plurality of sub-tubes being in contact with the outer tube but not in contact with the intermediate tube, and the plurality of sub-tubes may confine the light inside the sub-tubes by an antiresonant phenomenon.

[0017] The holey-core fiber may have a propagation loss of less than 2 dB / km at the wavelength of the light.

[0018] The multiple 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 between 2 and 50, and 2N or more of the multiple sub-tubes may be arranged to have rotational symmetry.

[0019] The symmetry of the plurality of inner tubes may be different from the symmetry of the plurality of secondary tubes.

[0020] The core diameter dcore of the core hole portion may be 10 μm or more and 100 μm or less.

[0021] The thickness tc of the intermediate tube may be 0.50 μm or more and 0.85 μm or less.

[0022] The support portion may be tubular, and a ratio d3 / d2 of an inner diameter d3 of the secondary pipe to an inner diameter d2 of the support portion may be 5% or more and less than 100%.

[0023] The ratio d3 / d2 may be 50% or more and 90% or less.

[0024] The ratio d3 / d2 may be 72% or more and 83% or less.

[0025] At least one inner tube of the plurality of inner tubes may have a different inner diameter or thickness than other inner tubes of the plurality of inner tubes.

[0026] 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. [Effects of the Invention]

[0027] The present invention has the effect of realizing a holey-core fiber that can obtain lower or stable low-loss characteristics. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic cross-sectional view of a hole-core fiber according to a first embodiment in a plane perpendicular to the longitudinal direction. [Figure 2] FIG. 2 is a schematic cross-sectional view of a hole-core fiber according to a second embodiment in a plane perpendicular to the longitudinal direction. [Figure 3] FIG. 3 is a schematic cross-sectional view of a hole-core fiber according to the third and fourth embodiments in a plane perpendicular to the longitudinal direction. [Figure 4] FIG. 4 is a schematic cross-sectional view of a hole-core fiber according to the fifth and sixth embodiments in a plane perpendicular to the longitudinal direction. [Figure 5] FIG. 5 is a schematic cross-sectional view of a hole-core fiber according to the seventh and eighth embodiments in a plane perpendicular to the longitudinal direction. [Figure 6] FIG. 6 is a schematic cross-sectional view of a hole-core fiber according to the ninth and tenth embodiments in a plane perpendicular to the longitudinal direction. [Figure 7] FIG. 7 is a schematic cross-sectional view of a hole-core fiber according to an eleventh and twelfth embodiment in a plane perpendicular to the longitudinal direction. [Figure 8] FIG. 8 is a schematic cross-sectional view of the hole-core fibers according to the thirteenth to seventeenth embodiments in a plane perpendicular to the longitudinal direction. [Figure 9] FIG. 9 is a schematic cross-sectional view of the hole-core fibers according to the eighteenth to twenty-second embodiments in a plane perpendicular to the longitudinal direction. [Figure 10] FIG. 10 is a schematic cross-sectional view of a calculation model of a hole-core fiber in a plane perpendicular to the longitudinal direction. [Figure 11] FIG. 11 is a diagram showing an example of the electric field distribution of the fundamental propagation mode. [Figure 12] FIG. 12 is a diagram showing the wavelength spectrum of the confinement loss. [Figure 13] FIG. 13 is a diagram showing the wavelength spectrum of surface scattering loss. [Figure 14] FIG. 14 shows the electric field distribution of the fundamental propagation mode for various parameter settings. DETAILED DESCRIPTION OF THE INVENTION

[0029] 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.

[0030] (Embodiment 1) 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 five inner tubes 1, one outer tube 2, one intermediate tube 3, and one support part 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 five inner tubes are an example of a plurality of inner tubes.

[0031] The intermediate pipe 3 is disposed inside the outer pipe 2 and concentric with the outer pipe 2. The support part 4 is disposed between the outer pipe 2 and the intermediate pipe 3. The support part 4 is fixed to the outer pipe 2 and the intermediate pipe 3 by, for example, welding. In this embodiment, the support part 4 is a tubular member.

[0032] The five inner pipes 1 are disposed inside the outer pipe 2 and inside the intermediate pipe 3. The five inner pipes 1 are arranged in a regular pentagonal shape on a plane perpendicular to the longitudinal direction, and are fixed to the inner wall of the intermediate pipe 3 by welding or the like. The number of layers of the polygon formed by the five inner pipes 1 is one. As a result, the five inner pipes 1 are supported on the outer pipe 2 by the intermediate pipe 3 and the support portion 4.

[0033] An air hole core section 5 is formed in the area surrounded by the five inner tubes 1. The thickness (wall thickness) and inner diameter of the five inner tubes 1 are designed so that light is confined in the air hole core section 5 by the antiresonant phenomenon. The thickness and inner diameter are set appropriately depending on the wavelength of the light to be confined. In Figure 1, field F indicates the field of light (propagating light) that is confined by the antiresonant phenomenon and propagates through the air hole core section 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.

[0034] In the hole-core fiber 10, the five inner tubes 1 are arranged to have 5-fold rotational symmetry with respect to the central axis of the hole core portion 5. 5-fold rotational symmetry is an example of N-fold rotational symmetry where N is an integer between 2 and 50.

[0035] 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 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 ultimately the propagation loss is reduced.

[0036] One method for reducing the confinement loss is to use a nested inner tube to realize the antiresonant phenomenon. However, with this method, if the inner 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. In this regard, the hole-core fiber 10 does not have such a misalignment problem and can achieve stable low-loss characteristics.

[0037] As described above, lower or stable low-loss characteristics can be obtained according to the hole-core fiber 10 according to embodiment 1. In the hole-core fiber 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.

[0038] (Embodiment 2) 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. The hole-core fiber 10A has a configuration in which five sub-tubes 6 are added to the hole-core fiber 10 according to embodiment 1 shown in Fig. 1. The five sub-tubes 6 are an example of a plurality of sub-tubes. The sub-tubes 6 are made of a material that is transparent to the light to be propagated, and are made of glass such as pure silica glass, for example.

[0039] The five secondary tubes 6 are arranged between the outer tube 2 and the five inner tubes 1, and between the outer tube 2 and the intermediate tube 3. The five secondary tubes 6 are fixed in contact with the outer tube 2, but are not in contact with the intermediate tube 3. Therefore, it is preferable that the outer diameter of the secondary tubes 6 is smaller than the outer diameter of the support part 4. Furthermore, the thickness and inner diameter of the five secondary tubes 6 are designed so that the propagating light is confined inside the secondary tubes 6 due to the antiresonant phenomenon.

[0040] In the hole-core fiber 10A configured as above, the 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 are confined by the secondary tube 6, thereby further suppressing the light that reaches the outer tube 2. As a result, even lower loss characteristics than those of the hole-core fiber 10 according to the first embodiment can be obtained.

[0041] It is preferable that the secondary pipe 6 is arranged so that the central axis of the inner pipe 1 and the central axis of the secondary pipe 6 overlap when viewed in the radial direction from the central axis of the hole core portion 5, but they may be misaligned to the extent that the central axis of the inner pipe 1 and the internal region of the secondary pipe 6 overlap. Such a secondary pipe 6 can be arranged with higher positional accuracy than, for example, when provided inside the inner pipe 1.

[0042] (Embodiments 3 and 4) Fig. 3 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the hole core fibers according to the embodiments 3 and 4. The hole core fiber 10B according to the embodiment 3 shown in Fig. 3(a) has a configuration in which the number of supporting portions 4 in the hole core fiber 10A according to the embodiment 2 shown in Fig. 2 is increased from one to three. Moreover, the hole core fiber 10C according to the embodiment 4 shown in Fig. 3(b) has a configuration in which the number of supporting portions 4 in the hole core fiber 10A is further increased to six.

[0043] The holey-core fibers 10B and 10C configured as above can obtain the same effects as the holey-core fiber 10A, and also can obtain the effect that the inner tube 1 and the intermediate tube 3 are more firmly supported by the outer tube 2. Furthermore, by appropriately designing the thickness and inner diameter of the supporting part 4, the supporting part 4 can generate an anti-resonant phenomenon that confines the propagating light, thereby obtaining even lower loss characteristics than the holey-core fiber 10A.

[0044] (Embodiments 5 and 6) Fig. 4 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the hole-core fibers according to the embodiments 5 and 6. The hole-core fiber 10D according to the embodiment 5 shown in Fig. 4(a) has a configuration in which the intermediate tube 3 is replaced with double intermediate tubes 3D1 and 3D2, the supporting part 4 is replaced with supporting parts 4D1 and 4D2, and secondary tubes 6D1 and 6D2 are added in the hole-core fiber 10 according to the embodiment 1 shown in Fig. 1.

[0045] The intermediate tubes 3D1 and 3D2 are arranged inside the outer tube 2 and concentric with the outer tube 2. The inner diameter of the intermediate tube 3D2 is larger than the outer diameter of the intermediate tube 3D1. The five support portions 4D1 are arranged between the intermediate tubes 3D1 and 3D2. The five support portions 4D2 are arranged between the outer tube 2 and the intermediate tube 3D2. The support portions 4D1 are fixed to the intermediate tubes 3D1 and 3D2 by, for example, welding. The support portions 4D2 are fixed to the intermediate tube 3D2 and the outer tube 2 by, for example, welding.

[0046] The five inner pipes 1 are disposed inside the intermediate pipe 3D1. The five inner pipes 1 are fixed to the inner wall of the intermediate pipe 3D1 by welding or the like. As a result, the five inner pipes 1 are supported on the outer pipe 2 by the intermediate pipes 3D1, 3D2 and the support portions 4D1, 4D2.

[0047] The five secondary tubes 6D1 are disposed between the intermediate tube 3D1 and the intermediate tube 3D2. The five secondary tubes 6D1 are fixed in contact with the intermediate tube 3D2, but are not in contact with the intermediate tube 3D1. Therefore, it is preferable that the outer diameter of the secondary tubes 6D1 is smaller than the outer diameter of the support portion 4D1. Furthermore, the five secondary tubes 6D2 are disposed between the outer tube 2 and the intermediate tube 3D2. The five secondary tubes 6D2 are fixed in contact with the outer tube 2, but are not in contact with the intermediate tube 3D2. Therefore, it is preferable that the outer diameter of the secondary tubes 6D2 is smaller than the outer diameter of the support portion 4D2. Furthermore, the thickness and inner diameter of the secondary tubes 6D1 and 6D2 are designed so as to confine propagating light inside by the antiresonant phenomenon.

[0048] On the other hand, the hole-core fiber 10E according to the sixth embodiment shown in FIG. 4(b) has a configuration in which the number of supporting portions 4D1, 4D2 and sub-tubes 6D1, 6D2 in the hole-core fiber 10D shown in FIG. 4(a) is reduced to three.

[0049] In the hole-core fibers 10D and 10E configured as above, the secondary tubes have a two-stage structure of secondary tubes 6D1 and 6D2, and therefore, even lower loss characteristics can be obtained than in the hole-core fiber 10A.

[0050] (Embodiments 7 and 8) Fig. 5 is a schematic cross-sectional view of a hole-core fiber according to Embodiments 7 and 8 in a plane perpendicular to the longitudinal direction. A hole-core fiber 10F according to Embodiment 7 shown in Fig. 5(a) has a configuration in which the inner tube 1 in the hole-core fiber 10 according to Embodiment 1 shown in Fig. 1 is replaced with inner tubes 1Fa and 1Fb having a double nested structure. Moreover, a hole-core fiber 10G according to Embodiment 8 shown in Fig. 5(b) has a configuration in which the inner tube 1 in the hole-core fiber 10A according to Embodiment 2 shown in Fig. 2 is replaced with inner tubes 1Fa and 1Fb having a double nested structure, and further the secondary tube 6 is replaced with secondary tubes 6Fa and 6Fb having a double nested structure.

[0051] In the holey-core fibers 10F, 10G configured as above, the inner tubes 1Fa, 1Fb and further the sub-tubes 6Fa, 6Fb have a double structure, so that even lower loss characteristics can be obtained.

[0052] (Embodiments 9 and 10) Fig. 6 is a schematic cross-sectional view of a hole-core fiber according to the ninth and tenth embodiments in a plane perpendicular to the longitudinal direction. The hole-core fiber 10H according to the ninth embodiment shown in Fig. 6(a) has a configuration in which each of the five sub-tubes 6 in the hole-core fiber 10B according to the third embodiment shown in Fig. 3(a) is replaced with a set of two sub-tubes 6H. The hole-core fiber 10I according to the tenth embodiment shown in Fig. 6(b) has a configuration in which each of the five sub-tubes 6 in the hole-core fiber 10C according to the fourth embodiment shown in Fig. 3(b) is replaced with a set of two sub-tubes 6H. The set of sub-tubes 6H are lined up along the inner wall of the outer tube 2 and fixed to the inner wall of the outer tube 2 by welding or the like. Similar to the sub-tube 6, the thickness and inner diameter of the set of sub-tubes 6H are designed so that propagating light is confined inside the sub-tubes 6H by the antiresonant phenomenon. The air-hole core fiber 10I is an example in which the inner tube 1 is arranged to have N-fold rotational symmetry with respect to the central axis of the air-hole core portion 5, and 2N or more sub-tubes are arranged to have 5-fold rotational symmetry, where N is 5.

[0053] The holey-core fibers 10H and 10I configured as above can provide the same effects as the holey-core fibers 10B and 10C.

[0054] (Embodiments 11 and 12) Fig. 7 is a schematic cross-sectional view of a hole-core fiber according to an eleventh or twelfth embodiment in a plane perpendicular to the longitudinal direction. The hole-core fiber 10J according to the eleventh embodiment shown in Fig. 7(a) has a configuration in which each of the five sets of sub-tubes 6H in the hole-core fiber 10H according to the third embodiment shown in Fig. 6(a) is replaced with a set of three sub-tubes 6J. The hole-core fiber 10K according to the tenth embodiment shown in Fig. 7(b) has a configuration in which each of the five sets of sub-tubes 6 in the hole-core fiber 10I according to the tenth embodiment shown in Fig. 6(b) is replaced with a set of three sub-tubes 6J. The set of sub-tubes 6J is arranged along the inner wall of the outer tube 2 and is fixed to the inner wall of the outer tube 2 by welding or the like. The thickness and inner diameter of the sub-tube 6J are designed so that the propagating light is confined inside the sub-tube 6J by the antiresonant phenomenon, similar to the sub-tubes 6 and 6H. The air-hole core fiber 10K is an example in which the inner tube 1 is arranged to have N-fold rotational symmetry with respect to the central axis of the air-hole core portion 5, and 2N or more sub-tubes are arranged to have 5-fold rotational symmetry, where N is 5.

[0055] The holey-core fibers 10J and 10K configured as above can provide the same effects as those of the holey-core fibers 10B, 10C, 10H, and 10I.

[0056] (Embodiments 13 to 17) Fig. 8 is a schematic cross-sectional view of a hole-core fiber according to any one of the thirteenth to seventeenth embodiments in a plane perpendicular to the longitudinal direction. The hole-core fiber 10L1 according to the thirteenth embodiment shown in Fig. 8(a) has a configuration in which, in the hole-core fiber 10E according to the sixth embodiment shown in Fig. 4(b), each of the five secondary tubes 6D1 is replaced with a set of two secondary tubes 6L1, and each of the five secondary tubes 6D2 is replaced with a set of two secondary tubes 6L2. The set of secondary tubes 6L1 is aligned along the inner wall of the outer tube 2 and fixed to the inner wall of the outer tube 2 by welding or the like. The set of secondary tubes 6L2 is aligned along the inner wall of the intermediate tube 3D2 and fixed to the inner wall of the intermediate tube 3D2 by welding or the like.

[0057] A hole core fiber 10L2 according to embodiment 14 shown in Fig. 8(b) has a configuration in which five sets of secondary tubes 6L2 in the hole core fiber 10L1 are replaced with a set of three secondary tubes 6L2a. A hole core fiber 10L3 according to embodiment 15 shown in Fig. 8(c) has a configuration in which five sets of secondary tubes 6L2a in the hole core fiber 10L2 are replaced with a set of four secondary tubes 6L2b. A hole core fiber 10L4 according to embodiment 16 shown in Fig. 8(d) has a configuration in which five sets of secondary tubes 6L1 in the hole core fiber 10L3 are replaced with a set of three secondary tubes 6L1a. A hole core fiber 10L5 according to embodiment 17 shown in Fig. 8(e) has a configuration in which five sets of secondary tubes 6L1 in the hole core fiber 10L3 are replaced with a set of three secondary tubes 6L1a.

[0058] In the hole-core fibers 10L1 to L6 configured as above, the same effects as in the hole-core fiber 10E can be obtained.

[0059] (Embodiments 18 to 22) Fig. 9 is a schematic cross-sectional view of a hole core fiber according to any one of the eighteenth to twenty-second embodiments in a plane perpendicular to the longitudinal direction. A hole core fiber 10L6 according to the eighteenth embodiment shown in Fig. 9(a) has a configuration in which the supporting portions 4L1 and 4L2 in a hole core fiber 10L1 are arranged like the supporting portions 4L1 and 4L2 in the hole core fiber 10D of Fig. 4(a). A hole core fiber 10L7 according to the nineteenth embodiment shown in Fig. 9(b) has a configuration in which the supporting portions 4L1 and 4L2 in a hole core fiber 10L2 are arranged like the supporting portions 4L1 and 4L2 in the hole core fiber 10D. A hole core fiber 10L8 according to the twenty-first embodiment shown in Fig. 9(c) has a configuration in which the supporting portions 4L1 and 4L2 in a hole core fiber 10L3 are arranged like the supporting portions 4L1 and 4L2 in the hole core fiber 10D. A hole-core fiber 10L9 according to embodiment 21 shown in Fig. 9(d) has a configuration in which the supporting portions 4L1 and 4L2 in a hole-core fiber 10L4 are arranged like the supporting portions 4L1 and 4L2 in the hole-core fiber 10D. A hole-core fiber 10L10 according to embodiment 22 shown in Fig. 9(e) has a configuration in which the supporting portions 4L1 and 4L2 in a hole-core fiber 10L5 are arranged like the supporting portions 4L1 and 4L2 in the hole-core fiber 10D.

[0060] In the hole-core fibers 10L7 to L10 configured as above, the same effects as in the hole-core fiber 10D can be obtained.

[0061] (calculation example) Below, we explain the preferred design parameters of the holey-core fiber by performing simulation calculations using the finite element method and advanced surface roughness modeling techniques.

[0062] 10 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, an intermediate tube 300, a support portion 400, a hole core portion 500, and a sub-tube 600. The central axis of the hole core portion 500 is central axis C.

[0063] The radius of the air hole core region 500 is expressed by the radius R of a circle inscribed in the six inner tubes 100 with the central axis C as its center. The diameter (core diameter) of the air hole core region 500 is denoted as dcore. 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, the 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, dcore was set to 32 μm (R=16 μm). This core diameter is a proven size as previously reported and is a desirable example.

[0064] If the inner diameter of the inner tube 100 is d1, d1 is, for example, 22.64 μm. If the thickness of the inner tube 100 is t1, for example, when t1 is 0.39 μm, the minimum total loss (the sum of the confinement loss and the scattering loss) is obtained for the propagating light with a wavelength of 1550 nm.

[0065] The inner diameter dc of the intermediate tube 300 is expressed by the following formula. dc=2R+2d1+4t1 If the thickness of the intermediate tube 300 is tc, then tc is preferably 0.67 μm, for example.

[0066] If the inner diameter of the support portion 400 is d2 and the thickness is t2, then d2 is preferably 21.85 μm, for example, and t2 is preferably 0.53 μm, for example. If the inner diameter of the secondary tube 600 is d3 and the thickness is t3, then d3 is preferably 17.09 μm, for example, and t3 is preferably 0.41 μm, for example. tclad is preferably 11.15 μm, for example.

[0067] Figure 11 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 = 22.64 μm, t1 = 0.39 μm, tc = 0.67 μm, d2 = 21.85 μm, t2 = 0.53 μm, d3 = 17.09 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 11, 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 scattering loss. The propagation loss, confinement loss, and scattering loss at a wavelength of 1550 nm were 0.1024 dB / km, 0.0064 dB / km, and 0.096 dB / km, respectively.

[0068] Figure 12 shows the wavelength spectrum of the confinement loss, and Figure 13 shows the wavelength spectrum of the surface scattering loss for the hole-core fibers of calculation models #1 to #4, in which the dcore was fixed at 32 μm and the combinations of other design parameters were changed as shown in Table 1. Here, calculation model #3 is the hole-core fiber whose electric field distribution is shown in Figure 11. As shown in Figure 12, calculation model #3 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 0.01 dB / km. For example, the confinement loss at a wavelength of 1550 nm was 0.00656 dB / km. Furthermore, as shown in Figure 13, calculation model #3 exhibited less than 0.1 dB / km in the wavelength region longer than 1470 nm. In particular, the surface scattering loss was low in the S-band and C-band, being less than 0.1 dB / km.

[0069] [Table 1]

[0070] Table 2 shows examples of maximum and minimum values ​​for the design parameters dcore, d1, t1, tc, d2, t2, d3, t3, and tclad, as well as the resulting confinement loss (CL) and surface scattering loss (SSL) when the total loss is within the constraint of 1 ± 0.01 dB / km. tclad is the thickness of the outer tube. CL and SSL are values ​​at a wavelength of 1550 nm. As shown in Table 2, dcore is preferably in the range of 24.28 μm to 68.57 μm, for example. d1 is preferably in the range of 13.77 μm to 30.36 μm, for example. d2 is preferably in the range of 17.93 μm to 28.73 μm, for example. d3 is preferably in the range of 4.32 μm to 21.04 μm, for example. t1 is preferably in the range of 0.31 μm to 0.66 μm, for example. t2 is preferably in the range of 0.20 μm to 0.74 μm. t3 is preferably in the range of 0.15 μm to 0.74 μm, tc is preferably in the range of 0.54 μm to 0.81 μm, and tclad is preferably in the range of 0.80 μm to 120.80 μm.

[0071] [Table 2]

[0072] Figure 14 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. Figure 14(a) shows the case where d3 is minimum, Figure 14(b) shows the case where d1 is minimum, Figure 14(c) shows the case where dcore is maximum, Figure 14(d) shows the case where dcore is minimum, Figure 14(e) shows the case where tc is maximum, and Figure 14(e) shows the case where t3 is maximum.

[0073] 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 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 when the fiber is not perfectly straight, a polarization-maintaining fiber cannot have rotationally symmetric orders of n≧3. 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 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.

[0074] 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.

[0075] In the above embodiment, the number of polygonal layers formed by the multiple inner tubes is one, but it may be two or more. Also, the multiple inner tubes may include inner tubes with two or more different thicknesses or diameters.

[0076] In the above embodiment, the symmetry of the plurality of inner tubes may be different from the symmetry of the plurality of sub-tubes.

[0077] In the above embodiment, dcore may be 10 μm or more and 100 μm or less. tc may be 0.50 μm or more and 0.85 μm or less. d3 / d2 may be 5% or more and less than 100%, 50% or more and 90% or less, or 72% or more and 83% or less. d2 is an important parameter that indicates how far the inner tube (inner capillary) is from the outer tube (outer tube). d3 is also an important parameter that determines the position of the capillary layer (sub-tube) that enhances the antiresonance confinement of the inner tube. Therefore, the ratio of the index (d2) that indicates the distance between the inner tube and the outer tube and the index (d3) that is closely related to the position of the enhancing antiresonance layer is one of the important parameters that affect the confinement loss.

[0078] Furthermore, in the above embodiment, the cross sections of the inner tube, outer tube, intermediate tube, support portion, 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.

[0079] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible. [Explanation of symbols]

[0080] 1, 1Fa, 1Fb, 100: Inner tube 2, 200: Outer tube 3, 3D1, 3D2: Intermediate tube 4, 4D1, 4D2, 4L1, 4L2: Support part 5, 500: Hole core 6, 6D1, 6D2, 6Fa, 6Fb, 6H, 6J, 6L1, 6L1a, 6L2, 6L2a, 6L2b, 600: Secondary pipe 10, 10A, 10B, 10C, 10D, 10E, 10F, 10G, 10H, 10I, 10J, 10K, 10L1, 10L2, 10L3, 10L4, 10L5, 10L6, 10L7, 10L8, 10L9, 10L10, 1000: Hole-core fiber C: Central axis F : Field

Claims

1. An outer tube; a plurality of inner tubes disposed inside the outer tube; Equipped with the plurality of inner tubes are arranged in a polygonal shape in a plane perpendicular to the longitudinal direction, a hole core portion is formed in a region surrounded by the plurality of inner tubes; the plurality of inner tubes confine light in the air hole core portion by an antiresonant phenomenon; The inner tubes are not in direct contact with the outer tube. Hole-core fiber.

2. The number of polygonal layers formed by the plurality of inner tubes is 1. The hole-core fiber of claim 1 .

3. The number of polygonal layers formed by the plurality of inner tubes is two or more. The hole-core fiber of claim 1 .

4. The plurality of inner tubes include inner tubes having two or more different thicknesses or diameters. The hole-core fiber of claim 1 .

5. an intermediate tube disposed concentrically inside the outer tube and having the plurality of inner tubes disposed therein; A support part; Furthermore, The plurality of inner pipes are supported on the outer pipe by the intermediate pipe and the support portion. The hole-core fiber of claim 1 .

6. further comprising a plurality of secondary pipes disposed between the outer pipe and the plurality of inner pipes; the plurality of secondary pipes are in contact with the outer pipe but are not in contact with the intermediate pipe; The plurality of sub-tubes confine the light inside the sub-tubes by an antiresonant phenomenon. The hole-core fiber of claim 5 .

7. The propagation loss at the wavelength of the light is less than 2 dB / km. The hole-core fiber of claim 6 .

8. The plurality of inner tubes are arranged to have N-fold rotational symmetry with respect to the central axis of the hole core portion, where N is an integer of 2 or more and 50 or less, and 2N or more of the plurality of sub-tubes are arranged to have rotational symmetry. The hole-core fiber of claim 6 .

9. The symmetry of the plurality of inner tubes is different from the symmetry of the plurality of secondary tubes. The hole-core fiber of claim 8 .

10. The core diameter dcore of the hole core portion is 10 μm or more and 100 μm or less. The hole-core fiber of claim 6 .

11. The thickness tc of the intermediate tube is 0.50 μm or more and 0.85 μm or less. The hole-core fiber of claim 6 .

12. The support portion is tubular, and a ratio d3 / d2 of an inner diameter d3 of the secondary pipe to an inner diameter d2 of the support portion is 5% or more and less than 100%. The hole-core fiber of claim 6 .

13. The ratio d3 / d2 is 50% or more and 90% or less. The hole-core fiber of claim 12.

14. The ratio d3 / d2 is 72% or more and 83% or less. The hole-core fiber of claim 12.

15. At least one of the plurality of inner tubes has an inner diameter or thickness that is different from the other inner tubes of the plurality of inner tubes. The hole-core fiber of claim 1 .

16. Two of the plurality of inner tubes have different inner diameters or thicknesses than the other inner tubes of the plurality of inner tubes. The hole-core fiber of claim 1 .

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