Anti-resonant hollow-core optical fiber

Abstract

An anti-resonant hollow-core optical fiber (10) according to one embodiment comprises an outer cladding (11) having a pipe shape, a plurality of inner cladding elements (12) each having a pipe shape and contacting an inner wall surface of the outer cladding (11), and a coating layer (14) covering the outer cladding (11), a space surrounded by the plurality of inner cladding (12) serving as a core region (11d). The average linear expansion coefficient of the outermost layer of the coating layer (14) at 20°C to 30°C is 10×10-5 [ / °C] or less, the average linear expansion coefficient of the outermost layer of the coating layer (14) at -60°C to -50°C is 6×10-5 [ / °C] or less, and the in-situ elastic modulus of the outermost layer of the coating layer (14) is 1500 MPa or more.

Description

Antiresonant hollow-core optical fiber

[0001] This disclosure relates to an anti-resonant hollow-core optical fiber. This application claims priority to Japanese Patent Application No. 2023-204514 filed on December 4, 2023, and incorporates by reference the entire disclosure of said Japanese application.

[0002] In an anti-resonant hollow-core optical fiber, the inner region of the outer cladding has a continuous cross-sectional structure along the central axis of the fiber, as described in, for example, Patent Document 1 and Non-Patent Document 1. Non-Patent Document 2 describes a hollow-core fiber in which the coating is thinned to improve thermal phase stability. Patent Document 2 describes an optical fiber including a cladding made of pure silica or fluorine-doped silica.

[0003] International Publication No. 2020 / 217052 U.S. Patent Application Publication No. 2021 / 0026065

[0004] GREGORY T. JASION et al., “Fabrication of tubular anti-resonant hollow core fibers: modeling, draw dynamics and process optimization,” OPTICS EXPRESS, Vol. 27, No. 15, 22 Jul. 2019, p.20572.Bo Shi et al., “Thinly coated hollow core fiber for improved phase thermal-stability performance” Optics Letters Vol. 46, No. 20 / 15 Oct. 2021 p.5177-5180.

[0005] An anti-resonant hollow-core optical fiber according to one aspect of the present disclosure is an anti-resonant hollow-core optical fiber having an outer cladding having a pipe shape, a plurality of inner cladding elements each having a pipe shape and in contact with an inner wall surface of the outer cladding, and a coating layer that coats the outer cladding, wherein the space surrounded by the plurality of inner claddings forms a core region. The average linear expansion coefficient of the outermost layer of the coating layer at 20°C or more and 30°C or less is 10 x 10 －５ The average linear expansion coefficient of the outermost layer of the coating layer at temperatures between −60° C. and −50° C. is 6×10 －５ The in-situ elastic modulus of the outermost layer of the coating layer is 1500 MPa or more.

[0006] 1 is a diagram showing the structure of an anti-resonant hollow-core optical fiber according to an embodiment, and FIG 2 is a graph showing an example of the relationship between temperature and the average linear expansion coefficient.

[0007] When an antiresonant hollow-core optical fiber is used, the transmission delay of the optical signal can be reduced, enabling high-speed, large-capacity communication. However, in an antiresonant hollow-core optical fiber, the coating can expand due to the influence of heat, which can increase the phase sensitivity of the optical signal. The influence of heat can cause a delay in the phase of the optical signal.

[0008] An object of the present disclosure is to provide an anti-resonant hollow-core optical fiber that can reduce the phase delay that occurs in an optical signal.

[0009] According to the present disclosure, it is possible to suppress the influence of heat and reduce the phase delay that occurs in an optical signal.

[0010] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. (1) An anti-resonant hollow-core optical fiber according to one embodiment is an anti-resonant hollow-core optical fiber having an outer cladding having a pipe shape, a plurality of inner cladding elements each having a pipe shape and in contact with the inner wall surface of the outer cladding, and a coating layer that coats the outer cladding, and the space surrounded by the plurality of inner claddings forms a core region. The average linear expansion coefficient of the outermost layer of the coating layer at 20°C or more and 30°C or less is 10 x 10 －５The average linear expansion coefficient of the outermost layer of the coating layer at temperatures between −60° C. and −50° C. is 6×10 －５ The in-situ elastic modulus of the outermost layer of the coating layer is 1500 MPa or more.

[0011] In the above-mentioned anti-resonant hollow-core optical fiber, by reducing the linear expansion coefficient of the outermost layer of the coating layer, the influence of heat can be suppressed, the phase delay occurring in the optical signal can be reduced, and the temperature characteristics of the anti-resonant hollow-core optical fiber can be improved.

[0012] (2) In the above (1), the thickness of the coating layer may be 15 μm or more. In this case, the phase delay time can be more reliably reduced and external damage resistance can be improved.

[0013] (3) In the above (1) or (2), the coating layer may have a primary resin layer and a secondary resin layer that coats the outer periphery of the primary resin layer. The primary resin layer may have an in-situ elastic modulus of 10 MPa or less. In this case, the primary resin layer having an in-situ elastic modulus of 10 MPa or less is provided inside the secondary resin layer. Therefore, the lateral pressure resistance characteristics of the anti-resonant hollow core optical fiber can be improved while maintaining good temperature characteristics.

[0014] (4) In the above (3), the thickness of the primary resin layer may be 3 μm or more and 20 μm or less. In this case, the thickness of the primary resin layer is 3 μm or more, which allows the primary resin layer to more fully function. The thickness of the primary resin layer is 20 μm or less, which allows the outermost layer of the coating, which has a small linear expansion coefficient, to be relatively thick. As a result, the influence of heat can be suppressed.

[0015] (5) In any of the above (1) to (4), the outermost layer of the coating layer may contain an inorganic oxide.

[0016] [Details of the embodiments of the present disclosure] Specific examples of anti-resonant hollow-core optical fibers of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, identical or corresponding elements are given the same reference numerals, and redundant explanations are omitted as appropriate. The drawings may be appropriately simplified or exaggerated for ease of understanding, and dimensional ratios and the like are not limited to those shown in the drawings.

[0017] 1 is a diagram showing the structure of an anti-resonant hollow-core optical fiber 10 according to an embodiment. As shown in FIG. 1, the anti-resonant hollow-core optical fiber 10 comprises an outer cladding 11, a plurality of inner cladding elements 12, a jacket layer 13, and a coating layer 14.

[0018] The outer cladding 11 functions as an optical cladding. The outer cladding 11 has a pipe shape. The outer cladding 11 extends along the central axis A of the anti-resonant hollow-core optical fiber 10. The outer cladding 11 has an inner wall surface 11b. An inner region 11c defined by the inner wall surface 11b of the outer cladding 11 corresponds to the inner region of the anti-resonant hollow-core optical fiber 10.

[0019] A plurality of inner cladding elements 12 are provided in the inner region 11c of the outer cladding 11. The number of the inner cladding elements 12 is, for example, six. The inner cladding elements 12 function as trench layers. The plurality of inner cladding elements 12 are arranged in contact with the inner wall surface 11b of the outer cladding 11 to surround a space that becomes a core region 11d. The core region 11d functions as a hollow optical waveguide region. The space that becomes the core region 11d extends along the central axis A.

[0020] A jacket layer 13, which serves as, for example, a physical cladding, is provided on the outer periphery of the outer cladding 11. For example, a glass fiber 10A is configured by the outer cladding 11, the multiple inner cladding elements 12, and the jacket layer 13. The diameter of the glass fiber 10A is, for example, 170 μm.

[0021] A coating layer 14 is provided on the outer periphery of the jacket layer 13. The coating layer 14 is made of ultraviolet curable resin. The thickness of the coating layer 14 is, for example, 15 μm or more. The diameter of the anti-resonant hollow core optical fiber 10 (coating layer 14) is, for example, 300 μm. The coating layer 14 includes a primary resin layer 15 and a secondary resin layer 16. The primary resin layer 15 surrounds the glass fiber 10A (jacket layer 13) and covers the outer periphery of the glass fiber 10A. The primary resin layer 15 is in contact with the glass fiber 10A. The thickness of the primary resin layer 15 is, for example, 3 μm or more and 20 μm or less. The diameter of the primary resin layer 15 is, for example, 210 μm.

[0022] For example, the Young's modulus of the primary resin layer 15 is 0.05 MPa or more and 10 MPa or less. In this case, microbending loss can be sufficiently suppressed and the possibility of damage to the coating layer 14 can be reduced. The primary resin layer 15 may be made of, for example, a polyether-based or polyester-based urethane acrylate. The primary resin layer 15 may also contain a reactive diluent monomer or a photoinitiator. The Young's modulus of the primary resin layer 15 is adjusted, for example, by the molecular weight of the polyether portion of the ultraviolet-curable resin and the type of diluent monomer.

[0023] The primary resin layer 15 contains, for example, 0.3 wt % or more and 2.0 wt % or less of a phosphorus-containing photoinitiator. The primary resin layer 15 contains, for example, polypropylene glycol having a mass average molecular weight of 1000 or more and 5000 or less. This makes it possible to make peeling at the interface between the glass fiber 10A and the primary resin layer 15 and damage to the coating layer 14 less likely to occur even if the anti-resonant hollow core optical fiber 10 is subjected to an external force.

[0024] For example, the in-situ elastic modulus of the primary resin layer 15 is 10 MPa or less. The in-situ elastic modulus of the primary resin layer 15 is the Young's modulus of the primary resin layer 15 at 23°C. As an example, the in-situ elastic modulus of the primary resin layer 15 may be 0.1 MPa or more and 1 MPa or less. When the in-situ elastic modulus of the primary resin layer 15 is 0.1 MPa or more, coating cracks called voids and coating peeling (delamination) are less likely to occur in the primary resin layer 15. When the in-situ elastic modulus of the primary resin layer 15 is 1 MPa or less, excellent lateral pressure resistance characteristics are obtained. The in-situ elastic modulus of the primary resin layer 15 may be 0.5 MPa or less. In this case, microbending loss can be further reduced and lateral pressure resistance characteristics can be further improved.

[0025] In this embodiment, the secondary resin layer 16 is the outermost layer of the coating layer 14. The secondary resin layer 16 surrounds the primary resin layer 15 and covers the outer peripheral surface of the primary resin layer 15. The diameter of the secondary resin layer 16 is, for example, 250 μm or more and 300 μm or less. For example, the thickness of the secondary resin layer 16 is at least twice the thickness of the primary resin layer 15. As an example, the thickness of the secondary resin layer 16 is 15 μm or more. The Young's modulus of the secondary resin layer 16 is, for example, 1000 MPa or more and 3000 MPa or less.

[0026] The secondary resin layer 16 may be made of, for example, a polyether-based or polyester-based urethane acrylate. The secondary resin layer 16 may contain a reactive diluent monomer or a photoinitiator. The Young's modulus of the secondary resin layer 16 is adjusted by, for example, the molecular weight of the polyether portion of the ultraviolet-curable resin and the type of diluent monomer.

[0027] The in-situ elastic modulus of the secondary resin layer 16 is 1500 MPa or more. For example, the in-situ elastic modulus of the secondary resin layer 16 may be 1500 MPa or more and 2800 MPa or less, or 2000 MPa or more and 2700 MPa or less. The in-situ elastic modulus of the secondary resin layer 16 is the Young's modulus of the secondary resin layer 16 at 23°C. When the in-situ elastic modulus of the secondary resin layer 16 is 1500 MPa or more, the lateral pressure resistance can be improved. When the in-situ elastic modulus of the secondary resin layer 16 is 2800 MPa or less, the secondary resin layer 16 can be imparted with appropriate toughness and can be made less susceptible to deterioration of appearance due to external damage and cracking of the secondary resin layer 16.

[0028] In this embodiment, the secondary resin layer 16 contains an inorganic oxide. For example, the inorganic oxide is nanosilica. For example, the content of the inorganic oxide is 1 wt % or more and 50 wt % or less based on the total amount of the resin composition of the secondary resin layer 16. The inorganic oxide may be hydrophobic inorganic oxide particles. By using a resin composition containing inorganic oxide particles in a specific range, it is possible to reduce the cure shrinkage rate and form a secondary resin layer 16 with excellent toughness. The inorganic oxide particles may be at least one selected from the group consisting of silicon dioxide (silica), zirconium dioxide (zirconia), aluminum oxide (alumina), magnesium oxide (magnesia), titanium oxide (titania), tin oxide, and zinc oxide, because they have excellent dispersibility in the resin composition and are easy to form a hard coating. The inorganic oxide particles according to this embodiment may include spherical hydrophobic silica particles, from the viewpoints of being inexpensive, easy to surface treat, UV-transmitting, and easily imparting appropriate hardness to the resin layer.

[0029] From the viewpoint of imparting appropriate toughness to the secondary resin layer 16, the average primary particle size of the inorganic oxide particles may be 500 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less. From the viewpoint of reducing the cure shrinkage rate of the secondary resin layer 16, the average primary particle size of the inorganic oxide particles may be 5 nm or more, or 10 nm or more. As an example, the average primary particle size of the inorganic oxide particles is 10 nm. The average primary particle size can be measured, for example, by image analysis of an electron microscope photograph, a light scattering method, or a BET method.

[0030] The secondary resin layer 16 contains the inorganic oxide described above, which reduces the linear expansion coefficient of the secondary resin layer 16. The average linear expansion coefficient of the secondary resin layer 16 at temperatures between 20° C. and 30° C. is 5×10 －５ [ / °C] or more and 10 x 10 －５ The average linear expansion coefficient of the secondary resin layer 16 at 20°C or higher and 30°C or lower is 6×10 －５ [ / ℃] or more, or 7 × 10 －５ The average linear expansion coefficient of the secondary resin layer 16 at 20°C or higher and 30°C or lower may be 9 x 10 －５ [ / ℃] or less, or 8 × 10 －５ [ / °C] or less.

[0031] The average linear expansion coefficient of the secondary resin layer 16 at temperatures between −60° C. and −50° C. is 1×10 －５ [ / °C] or more and 6 x 10 －５ The average linear expansion coefficient of the secondary resin layer 16 at temperatures between −60° C. and −50° C. is 2×10 －５ [ / ℃] or more, or 3 × 10 －５ The average linear expansion coefficient of the secondary resin layer 16 at temperatures between −60° C. and −50° C. may be 4×10 －５ [ / °C] or less, or 3 x 10 －５ [ / °C] or less.

[0032] As described above, in the anti-resonant hollow-core optical fiber 10 according to the embodiment, the secondary resin layer 16, which is the outermost layer of the coating layer 14, has a small linear expansion coefficient, and therefore it is possible to reduce the phase sensitivity of an optical signal passing through the anti-resonant hollow-core optical fiber 10. As a result, in the anti-resonant hollow-core optical fiber 10, the influence of the expansion of the coating layer 14 can be reduced and the phase sensitivity of an optical signal can be kept low.

[0033] When the secondary resin layer 16 contains nanosilica, the linear expansion coefficient of the secondary resin layer 16 and the phase sensitivity of the anti-resonant hollow-core optical fiber 10 can be reduced. For example, when the secondary resin layer 16 does not contain nanosilica, the phase sensitivity is 2.8 [rad / K / m], whereas when the secondary resin layer 16 contains nanosilica, the phase sensitivity can be reduced by 5% or more depending on the amount of nanosilica. For applications requiring high-precision management of optical phase, such as interferometry or sensing, it is necessary to reduce the phase sensitivity. In the case of a double coating having a primary resin layer 15 and a secondary resin layer 16, if the linear expansion coefficient of the secondary resin layer 16 is small, the linear expansion coefficient of the entire anti-resonant hollow-core optical fiber 10 can be reduced. By having a primary resin layer 15 with a low Young's modulus, it is possible to reduce transmission loss due to lateral pressure applied to the anti-resonant hollow-core optical fiber 10. By having a secondary resin layer 16 with a small linear expansion coefficient, it is possible to reduce the phase sensitivity of the anti-resonant hollow-core optical fiber 10.

[0034] 2 is a graph showing the relationship between the temperature and the average linear expansion coefficient in the secondary resin layer 16 for each content of nanosilica. The average primary particle size of this nanosilica is 10 nm. As shown in FIG. 2, the higher the content of nanosilica in the secondary resin layer 16, the smaller the average linear expansion coefficient of the secondary resin layer 16. For example, when no nanosilica is contained, the average linear expansion coefficient at 30°C is 11×10 －５ [ / °C], whereas the average linear expansion coefficient at 30°C when nanosilica is added at 18 wt% is 10 × 10 －５ When nanosilica is contained in an amount of 18 wt %, the phase sensitivity can be reduced by about 5% compared to when nanosilica is not contained.

[0035] When nanosilica is not contained, the average linear expansion coefficient at -50°C is 7 x 10 －５ [ / °C], whereas the average linear expansion coefficient at -50°C when nanosilica is added at 18 wt% is 6 × 10 －５ When nanosilica is contained at 28 wt%, the average linear expansion coefficient at -50°C is about 5 × 10 －５ When nanosilica is contained at 38 wt%, the average linear expansion coefficient at -50°C is about 4 × 10 －５ By increasing the content of nanosilica, the average linear expansion coefficient of the secondary resin layer 16 can be reduced, and the phase sensitivity of the anti-resonant hollow-core optical fiber 10 can be reduced by about 7%.

[0036] The above describes embodiments of the anti-resonant hollow core optical fiber according to the present disclosure. However, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible within the scope of the claims.

[0037] In the above-described embodiment, an example has been described in which the coating layer 14 includes the primary resin layer 15 and the secondary resin layer 16, and the number of layers in the coating layer 14 is two. However, the number of layers in the coating layer 14 may be one. The secondary resin layer 16 may be provided with a colored layer (ink layer) surrounding the secondary resin layer 16.

[0038] In the above-described embodiment, an anti-resonant hollow-core optical fiber 10 has been described, which has an outer cladding 11, six inner cladding elements 12, a jacket layer 13, and a coating layer 14. However, for example, the number of inner cladding elements 12 does not have to be six, and the configuration of the anti-resonant hollow-core optical fiber is not limited to the above example.

[0039] REFERENCE SIGNS LIST 10... Anti-resonant hollow-core optical fiber 10A... Glass fiber 11... Outer cladding 11b... Inner wall surface 11c... Internal region 11d... Core region 12... Inner cladding element 13... Jacket layer 14... Coating layer 15... Primary resin layer 16... Secondary resin layer A... Central axis

Claims

1. An antiresonant hollow-core optical fiber having an outer cladding having a pipe shape, a plurality of inner cladding elements each having a pipe shape and contacting an inner wall surface of the outer cladding, and a coating layer coating the outer cladding, wherein a space surrounded by the plurality of inner claddings forms a core region, and the average linear expansion coefficient of the outermost layer of the coating layer at 20°C or more and 30°C or less is 10 x 10 －５ [ / °C] or less, and the average linear expansion coefficient of the outermost layer of the coating layer at −60°C or more and −50°C or less is 6×10 －５ [ / °C] or less, and an in-situ elastic modulus of the outermost layer of the coating layer is 1500 MPa or more.

2. The antiresonant hollow core optical fiber according to claim 1, wherein the coating layer has a thickness of 15 μm or more.

3. An anti-resonant hollow core optical fiber as described in claim 1 or claim 2, wherein the coating layer has a primary resin layer and a secondary resin layer coating the outer periphery of the primary resin layer, and the in-situ elastic modulus of the primary resin layer is 10 MPa or less.

4. The antiresonant hollow core optical fiber according to claim 3, wherein the thickness of said primary resin layer is not less than 3 μm and not more than 20 μm.

5. An anti-resonant hollow-core optical fiber according to any one of claims 1 to 4, wherein the outermost layer of the coating layer comprises an inorganic oxide.

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