Hollow core fiber

Abstract

The present invention pertains to a hollow core fiber including a hollow core, wherein at least part of a portion, of a cladding part surrounding the hollow core, adjacent to the hollow core is made of silica glass containing chlorine, and the concentration of the chlorine is greater than or equal to 3000 ppm. The concentration may be greater than or equal to 4000 ppm. The concentration may be greater than or equal to 5000 ppm. The concentration may be less than or equal to 18,000 ppm. The hollow core fiber may confine light in the hollow core by using an anti-resonant phenomenon or a photonic band gap structure.

Description

Hollow-core fiber

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

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

[0003] As hollow-core fibers, photonic bandgap fibers (PBGfs) are known. PBGfs are known to be relatively resistant to bending. Also, the loss spectrum of PBGfs is known to be divided into a narrow low-loss window for the fundamental core mode. The low-loss window is located between high-loss peaks leaking into surface modes. In PBGfs, low-loss characteristics of 1.2 dB / km have been realized by optimizing the structure.

[0004] On the other hand, anti-resonant fibers (ARFs) that significantly reduce light leakage based on the anti-resonance principle are advantageous in that the band is not divided. Recent progress has demonstrated record-low-loss ARFs.

[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 PBGF, 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. On the other hand, in ARF, the surrounding thin capillary forms anti-resonances that inhibit lateral light transmission through the optical fiber at specific wavelengths. Furthermore, propagation loss is further reduced by carefully controlling the overlap between air-guided and tube-guided modes.

[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. For example, it has been reported that 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 in which adjacent anti-resonant rings do not come into contact with each other, achieves low confinement loss characteristics and low transmission loss.

[0009] Patent No. 6636509

[0010] 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). Chemical Vapor Deposition, Typical Material Parameters, [online], Heraeus Congress 2024, [Retrieved November 1, 2024], Internet <https: / / www.heraeus-conamic.com / products-and-solutions / products-for-key-markets / specialty-fiber-applications / chemical-vapor-deposition> P. J. Roberts et al., “Ultimate low loss of hollow-core photonic crystal fibers”, OPTICS EXPRESS, Vol. 13, No. 1, pp236-244 (January 2005).

[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 its object is to provide a porous core fiber that can obtain lower surface scattering loss and transmission loss.

[0014] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is 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 chlorine, and the concentration of chlorine is 3000 ppm or more.

[0015] The concentration of chlorine may be 4000 ppm or higher.

[0016] The concentration of chlorine may be 5000 ppm or higher.

[0017] The concentration of chlorine may be 18,000 ppm or less.

[0018] The aforementioned vacant core fiber may confine light within the vacant core by the antiresonant phenomenon.

[0019] The aforementioned vacant core fiber may confine light within the vacant core by a photonic bandgap structure.

[0020] The present invention has the effect of realizing a porous core fiber that can obtain lower surface scattering loss and transmission loss.

[0021] Figure 1 is a schematic cross-sectional view of a porous core fiber according to Embodiment 1, in a plane perpendicular to the longitudinal direction. Figure 2 is a diagram showing an example of the relationship between Cl concentration and normalized surface scattering loss. Figure 3 is a schematic cross-sectional view of a porous core fiber according to Embodiment 2, in a plane perpendicular to the longitudinal direction. Figure 4 is a schematic cross-sectional view of a porous core fiber according to Embodiment 3, in a plane perpendicular to the longitudinal direction. Figure 5 is a schematic cross-sectional view of a porous core fiber according to Embodiment 4, in a plane perpendicular to the longitudinal direction. Figure 6 is a schematic cross-sectional view of a porous core fiber according to Embodiment 5, in a plane perpendicular to the longitudinal direction. Figure 7A is a schematic cross-sectional view of a porous core fiber according to Embodiment 6, in a plane perpendicular to the longitudinal direction. Figure 7B is a schematic cross-sectional view of a porous core fiber according to Embodiment 7, in a plane perpendicular to the longitudinal direction. Figure 7C is a schematic cross-sectional view of a porous core fiber according to Embodiment 8, in a plane perpendicular to the longitudinal direction. Figure 7D is a schematic cross-sectional view of a porous core fiber according to Embodiment 9, in a plane perpendicular to the longitudinal direction. 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. Figure 9 is a schematic cross-sectional view of a porous core fiber according to Embodiment 11 in a plane perpendicular to the longitudinal direction.

[0022] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below. Furthermore, in each drawing, the same or corresponding components are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate. In addition, terms not specifically defined in this specification shall be defined and measured according to the definitions and measurement methods in ITU-T G. 650.1 and G. 650.2 of the International Telecommunication Union (ITU).

[0023] (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 a plurality of inner tubes.

[0024] 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.

[0025] 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.

[0026] 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 chlorine. The concentration of chlorine in the inner tube 12 is 3000 ppm or more. Furthermore, the inner tube 12 does not need to substantially contain dopants that change the refractive index, such as germanium or fluorine.

[0027] With the porous core fiber 10 configured as described above, lower surface scattering loss and transmission loss can be obtained.

[0028] In other words, by making the portion adjacent to the porous core 13, such as the inner tube 12, out of silica glass containing chlorine at a concentration of 3000 ppm or more, 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. Furthermore, the amount of OH groups becomes relatively low due to the dehydrating effect of chlorine, increasing the surface tension (γ) of the inner tube 12, thus further reducing the surface roughness. As a result, surface scattering loss is reduced, and transmission loss, including surface scattering loss, is reduced.

[0029] Furthermore, by using a large amount of chlorine as a dopant to lower the virtual temperature of the glass, it is easier to control the shape of the inner tube 12 made of that glass to a distortion-free state. 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.

[0030] Furthermore, the constituent material of the outer tube 11 is not particularly limited as long as it is silica-based glass.

[0031] Furthermore, the concentration of chlorine contained in the inner tube 12 can be confirmed, for example, by electron probe microanalyzer (EPMA) or refractive index distribution measurement.

[0032] (Experimental Example) The inventors manufactured hollow core fibers with the configuration shown in Figure 1, each with a different chlorine concentration in the inner tube, and investigated their surface scattering loss. 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 tubes. Each set of silica glass tubes had a different chlorine concentration. Next, each set of silica glass tubes was temporarily fixed in place 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. These base materials were then drawn while controlling the internal pressure to produce 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 hollow core fiber was set to 400 μm, and the coating diameter was set to 600 μm.

[0033] 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.

[0034] Figure 2 shows an example of the relationship between Cl concentration and normalized surface scattering loss. Here, normalized surface scattering loss is the surface scattering loss normalized by the surface scattering loss when the Cl concentration in the inner tube is 2000 ppm (the data point indicated by point P1 in Figure 2).

[0035] As shown in Figure 2, it was confirmed that when the Cl concentration is 3000 ppm or higher, the effect of reducing surface scattering loss is observed compared to when the Cl concentration is 2000 ppm or lower, and when it is 4000 ppm or higher, and even more significantly when it is 5000 ppm or higher. Furthermore, it was confirmed that the normalized surface scattering loss can be reduced to 0.7 or less when the Cl concentration is, for example, around 9000 ppm to 15000 ppm. However, it was confirmed that when the Cl concentration exceeds a certain level, the scattering loss due to structural defects increases, so it is preferable for the Cl concentration to be 18000 ppm or lower.

[0036] (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.

[0037] The inner tube 12A has the same inner diameter and wall thickness as the inner tube 12, but is made of silica glass with a chlorine concentration of 2000 ppm or less.

[0038] 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 chlorine at a concentration of 3000 ppm or more, thereby achieving lower surface scattering loss and transmission loss.

[0039] 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 one to three.

[0040] (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.

[0041] The inner tube 12B has the same inner diameter and wall thickness as the inner tube 12, but only the portion 12Ba contains chlorine at a concentration of 3000 ppm or more, and the portions other than the portion 12Ba are made of silica glass having a chlorine concentration of 2000 ppm or less, for example.

[0042] In the hole-core fiber 10B as well, since the portion 12Ba that constitutes at least a part of the portion adjacent to the hole core 13 is made of silica glass containing chlorine at a concentration of 3000 ppm or more, lower surface scattering loss and transmission loss can be obtained.

[0043] Note that one to four of the other four inner tubes 12A in the hole-core fiber 10B may be replaced with the inner tube 12B.

[0044] (Embodiment 4) FIG. 5 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the hole-core fiber according to Embodiment 4. The hole-core fiber 10C has a configuration in which auxiliary tubes 14 are further provided inside each inner tube 12 of the hole-core fiber 10 according to Embodiment 1 shown in FIG. 1.

[0045] The auxiliary tubes 14 are designed in terms of their wall thickness and inner diameter according to the wavelength of the light to be confined in the hole core 13 by the anti-resonance phenomenon. The auxiliary tubes 14 are made of silica glass having a chlorine concentration of 2000 ppm or less, for example.

[0046] In the hole-core fiber 10C as well, similar to the hole-core fiber 10, since the inner tubes 12 are made of silica glass containing chlorine at a concentration of 3000 ppm or more, lower surface scattering loss can be obtained. Furthermore, in the hole-core fiber 10C, the components of the light that leak into the inside of the inner tubes 12 and cannot be confined by the inner tubes 12 are confined by the auxiliary tubes 14, so that the confinement loss is reduced. As a result, in the hole-core fiber 10C, even lower transmission loss can be obtained.

[0047] (Embodiment 5) FIG. 6 is a schematic cross-sectional view in a plane perpendicular to the longitudinal direction of the hole-core fiber according to Embodiment 5. The hole-core fiber 10D has a configuration in which each auxiliary tube 14 of the hole-core fiber 10 according to Embodiment 4 shown in FIG. 5 is replaced with an auxiliary tube 14D.

[0048] The auxiliary tube 14D has the same inner diameter and wall thickness as the auxiliary tube 14, and is made of silica glass containing chlorine at a concentration of 3000 ppm or more.

[0049] In the hollow core fiber 10D, since the surface scattering loss is also reduced in the auxiliary tube 14D, a lower transmission loss than that of the hollow core fiber 10C can be obtained.

[0050] In the hollow core fiber 10D, the five auxiliary tubes 14 of the hollow core fiber 10C are replaced by the auxiliary tube 14D, but the number of tubes to be replaced may be any one of 1 to 4.

[0051] (Embodiments 6 to 9) FIGS. 7A to 7D are schematic cross-sectional views in a plane perpendicular to the longitudinal direction of the hollow core fiber according to Embodiments 6 to 9. The hollow core fibers 10E to 10H according to Embodiments 6 to 9 all have a double structure of the auxiliary tube.

[0052] The hollow core fiber 10E according to Embodiment 6 shown in FIG. 7A has a configuration in which an auxiliary tube 15 is further provided inside each auxiliary tube 14 of the hollow core fiber 10C shown in FIG. 5.

[0053] The wall thickness and inner diameter of the auxiliary tube 15 are designed according to the wavelength of the light to be confined in the hollow core 13 by the anti-resonance phenomenon. The auxiliary tube 15 is made of, for example, silica glass with a chlorine concentration of 2000 ppm or less.

[0054] In the hollow core fiber 10E, the components of the light that leak into the inside of the inner tube 12 and the auxiliary tube 14 and cannot be confined by the inner tube 12 and the auxiliary tube 14 are confined by the auxiliary tube 15, so the confinement loss is reduced. As a result, in the hollow core fiber 10E, a lower transmission loss than that of the hollow core fiber 10C can be obtained.

[0055] The hollow core fiber 10F according to Embodiment 7 shown in FIG. 7B has a configuration in which three of the five inner tubes 12 of the hollow core fiber 10E shown in FIG. 7A are replaced by the inner tube 12A. Even in such a hollow core fiber 10E, lower surface scattering loss and transmission loss can be obtained.

[0056] The hollow core fiber 10G according to Embodiment 8 shown in Figure 7C has a configuration in which each of the sub-tubes 14 of the hollow core fiber 10E shown in Figure 7A 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 chlorine at a concentration of 3000 ppm or more, similar to the inner tube 12. 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.

[0057] The hollow core fiber 10H according to Embodiment 9 shown in Figure 7D has a configuration in which each of the sub-tubes 15 of the hollow core fiber 10G shown in Figure 7C 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 chlorine at a concentration of 3000 ppm or more, similar to the inner tube 12 and sub-tube 14G. 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.

[0058] (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.

[0059] The porous core fiber 10I is provided with a cladding portion 16 made of glass. The cladding portion 16 is provided with a microporous region 16a. Fine porous structures 16aa and 16ab are formed in the microporous region 16a, 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.

[0060] 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.

[0061] The size of the void core 13 is approximately equivalent to 19 of the void structures 16aa and 16ab. Such a void core 13 is sometimes referred to as a 19-cell type.

[0062] In the porous core fiber 10I, the porous structure 16ab is made of silica glass containing chlorine at a concentration of 3000 ppm or more. The porous structure 16aa is made of silica glass with a chlorine concentration of, for example, 2000 ppm or less.

[0063] In the porous core fiber 10I configured as described above, the porous structure 16ab in the portion adjacent to the porous core 13 (region A) is made of silica glass containing chlorine at a concentration of 3000 ppm or more, thereby reducing surface scattering loss and transmission loss including surface scattering loss.

[0064] In addition, in the porous core fiber 10I, the porous structure 16aa may be replaced with a porous structure 16ab containing chlorine at a concentration of 3000 ppm or more.

[0065] (Embodiment 11) Figure 9 is a schematic cross-sectional view of a porous core fiber according to Embodiment 11 in a plane perpendicular to the longitudinal direction. The porous core fiber 10J is also an optical fiber that confines light in a porous core by a photonic bandgap structure.

[0066] The porous core fiber 10J is provided with a cladding portion 16J made of glass. The cladding portion 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 portion 17 is provided approximately in the center of the microporous region 16Ja, and the inside of the tubular portion 17 is the porous core 13. The size of the tubular portion 17 is approximately 19 times the size of the porous structure 16aa.

[0067] In this case, the hollow core fiber 10J has a tubular section 17 made of silica glass containing chlorine at a concentration of 3000 ppm or more.

[0068] 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 chlorine at a concentration of 3000 ppm or more, thereby reducing surface scattering loss and transmission loss, including surface scattering loss.

[0069] In addition, in the porous core fiber 10J, the porous structure 16aa may be replaced with a porous structure containing chlorine at a concentration of 3000 ppm or more.

[0070] Furthermore, in the above embodiment, the portion composed of silica glass containing chlorine at a concentration of 3000 ppm or more may also contain fluorine, or alkali metals or alkaline earth metals. This can further reduce surface scattering loss.

[0071] 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.

[0072] 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-tube 16, 16J: Cladding 16a, 16Ja: Micro-hollow region 16aa, 16ab: Hollow structure 17: Tube section A: Region P1: Point

Claims

1. A hollow core fiber comprising 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 chlorine, and the concentration of the chlorine is 3000 ppm or more.

2. The porous core fiber according to claim 1, wherein the concentration of chlorine is 4000 ppm or more.

3. The porous core fiber according to claim 1, wherein the concentration of chlorine is 5000 ppm or more.

4. The porous core fiber according to claim 1, wherein the concentration of chlorine is 18,000 ppm or less.

5. The vacant core fiber according to claim 1, wherein the vacant core fiber confines light in the vacant core by an antiresonant phenomenon.

6. The vacant core fiber according to claim 1, wherein the vacant core fiber confines light in the vacant core by a photonic bandgap structure.

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