Hollow core optical fiber

Abstract

The hollow-core optical fiber (100) includes a hollow core (110) extending along the longitudinal central axis (112) of the fiber, a substrate (130), a plurality of first cladding elements (122), and a plurality of second cladding elements (124). The plurality of first cladding elements (122) are spaced apart from each other and disposed between the hollow core and the substrate, and each first cladding element (122) extends in a direction parallel to the longitudinal central axis (112) of the fiber and includes a first capillary. The plurality of second cladding elements (124) are spaced apart from each other and disposed between the hollow core (110) and the substrate (130), and each second cladding element (124) extends in a direction parallel to the longitudinal central axis (112) of the fiber and includes a second capillary. None of the first cladding elements (122) directly contact the inner surface of the substrate (130), and none of the second cladding elements (124) directly contact the inner surface of the substrate (130).

Description

Priority

[0001] This application claims the benefit of priority under 35 U.S.C. § 120 to U.S. Provisional Patent Application No. 63 / 392,648, filed on July 27, 2022, and all disclosures of this provisional application are relied upon and incorporated herein by reference.

Technical Field

[0002] This specification generally relates to optical fibers, and more particularly to hollow-core optical fibers.

Background Art

[0003] Hollow-core optical fibers transmit light through a hollow core. However, light loss from the hollow core along the length of the optical fiber has been a potential obstacle to the practical use of hollow-core optical fibers.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Therefore, there is a need for a hollow-core optical fiber having a structure that confines light within the hollow core and suppresses light loss from the hollow core along the length of the optical fiber.

Means for Solving the Problems

[0005] According to a first aspect of the present disclosure, a hollow-core optical fiber includes a hollow core extending along the longitudinal central axis of the hollow-core optical fiber, a base material having a tubular shape and an inner surface surrounding the longitudinal central axis of the hollow-core optical fiber, a plurality of first cladding elements spaced apart from each other and disposed between the hollow core and the base material, each of the plurality of first cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of first cladding elements including a first capillary, the first capillary having an inner surface defining a first cavity, the first cavity having a first diameter and being occupied only by gas, a plurality of second cladding elements spaced apart from each other and disposed between the hollow core and the base material, each of the plurality of second cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of second cladding elements including a second capillary, the second capillary having an inner surface defining a second cavity, the second cavity having a second diameter different from the first diameter and being occupied only by gas. None of the plurality of first cladding elements is in direct contact with the inner surface of the base material, and none of the second cladding elements is in direct contact with the inner surface of the base material.

[0006] A second aspect of the present disclosure may include the first aspect, in which each first cladding element is disposed spaced apart from a circumferentially adjacent first cladding element.

[0007] A third aspect of the present disclosure may include the second aspect, in which the first cladding elements are disposed at equal circumferential intervals.

[0008] A fourth aspect of the present disclosure may include any one of the first to third aspects, in which each second capillary is in direct contact with at least one first capillary.

[0009] A fifth aspect of the present disclosure may include any one of the first to fourth aspects, in which the second cladding elements are disposed radially spaced apart from the inner surface of the base material.

[0010] The sixth aspect of the present disclosure may include any one of the first to fifth aspects, in which each second clad element is arranged at intervals from second clad elements adjacent in the circumferential direction.

[0011] The seventh aspect of the present disclosure may include any one of the first to sixth aspects, in which the second clad elements are arranged at equal intervals in the circumferential direction.

[0012] The eighth aspect of the present disclosure may include any one of the first to seventh aspects, in which a plurality of first clad elements are arranged symmetrically with respect to the longitudinal central axis of the hollow-core optical fiber.

[0013] The ninth aspect of the present disclosure may include any one of the first to eighth aspects, in which a plurality of second clad elements are arranged symmetrically with respect to the longitudinal central axis of the hollow-core optical fiber.

[0014] The tenth aspect of the present disclosure may include any one of the first to ninth aspects, in which each first capillary is directly connected to two second capillaries.

[0015] The eleventh aspect of the present disclosure may include any one of the first to tenth aspects, in which a plurality of first clad elements are composed of 3 to 8 first capillaries.

[0016] The twelfth aspect of the present disclosure may include any one of the first to eleventh aspects, in which a plurality of first clad elements are composed of 5 to 6 first capillaries.

[0017] The thirteenth aspect of the present disclosure may include any one of the first to twelfth aspects, in which a plurality of second clad elements are composed of 3 to 12 second capillaries.

[0018] The fourteenth aspect of the present disclosure may include any one of the first to thirteenth aspects, in which a plurality of second clad elements are composed of 5 to 6 second capillaries.

[0019] The 15th aspect of the present disclosure may include any one of the 1st to 14th aspects, where the number of the first cladding elements is equal to the number of the second cladding elements.

[0020] The 16th aspect of the present disclosure may include any one of the 1st to 15th aspects, where the number of the first capillaries is equal to the number of the second capillaries.

[0021] The 17th aspect of the present disclosure may include any one of the 1st to 16th aspects, where the second diameter is larger than the first diameter.

[0022] The 18th aspect of the present disclosure may include any one of the 1st to 17th aspects, where each first capillary has a first capillary longitudinal central axis parallel to the longitudinal central axis of the fiber.

[0023] The 19th aspect of the present disclosure may include any one of the 1st to 18th aspects, where each second capillary has a second capillary longitudinal central axis parallel to the longitudinal central axis of the fiber.

[0024] The 20th aspect of the present disclosure may include any one of the 1st to 19th aspects, where each first capillary has a first capillary longitudinal central axis parallel to the longitudinal central axis of the hollow-core optical fiber, the first capillary longitudinal central axis is at a first radial distance from the longitudinal central axis of the hollow-core optical fiber, each second capillary has a second capillary longitudinal central axis parallel to the longitudinal central axis of the hollow-core optical fiber, the second capillary longitudinal central axis is at a second radial distance from the longitudinal central axis of the hollow-core optical fiber, and the first radial distance is larger than the second radial distance.

[0025] The 21st aspect of the present disclosure may include any one of the 1st to 20th aspects, where each first capillary includes silica-based glass.

[0026] The 22nd aspect of the present disclosure may include any one of the 1st to 21st aspects, where each second capillary includes silica-based glass.

[0027] The 23rd aspect of the present disclosure may include any of the 1st to 22nd aspects, where the substrate includes silica-based glass.

[0028] The 24th aspect of the present disclosure may include any of the 1st to 23rd aspects, where the hollow core contains one or more gases.

[0029] The 25th aspect of the present disclosure may include any of the 1st to 24th aspects, where the hollow core consists essentially of only air.

[0030] The 26th aspect of the present disclosure may include any of the 1st to 25th aspects, where the inner diameter of the first capillary is 12 μm to 54 μm.

[0031] The 27th aspect of the present disclosure may include any of the 1st to 26th aspects, where the wall thickness of the first capillary is 0.1 μm to 4.0 μm.

[0032] The 28th aspect of the present disclosure may include any of the 1st to 27th aspects, where the inner diameter of the second capillary is 16 μm to 65 μm.

[0033] The 29th aspect of the present disclosure may include any of the 1st to 28th aspects, where the wall thickness of the second capillary is 0.1 μm to 4.0 μm.

[0034] The 30th aspect of the present disclosure may include any of the 1st to 29th aspects, where the plurality of first cladding elements and the plurality of second cladding elements are configured to provide an anti-resonance effect capable of confining an optical signal having a wavelength in the range of 350 nm to 8000 nm within the hollow core.

[0035] The 31st aspect of the present disclosure may include any of the 1st to 30th aspects, where the confinement loss of the hollow core optical fiber varies with wavelength, and the minimum confinement loss of the fundamental mode of the optical signal propagating within the hollow core optical fiber in the wavelength range of 350 nm to 8000 nm is 0.50 dB / km or less.

[0036] The 32nd aspect of the present disclosure is that a hollow-core optical fiber includes a plurality of third cladding elements arranged between a plurality of second cladding elements and the hollow core with a space therebetween, each of the plurality of third cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of third cladding elements including a third capillary, the third capillary having an inner surface defining a third cavity, the third cavity having a third diameter and being occupied only by gas, and further including the plurality of third cladding elements, and may include any one of the 1st to 31st aspects.

[0037] The 33rd aspect of the present disclosure may include the 32nd aspect, wherein each third capillary has a third capillary longitudinal central axis parallel to the longitudinal central axis of the fiber.

[0038] The 34th aspect of the present disclosure may include either the 32nd aspect or the 33rd aspect, wherein each third capillary is in direct contact with at least one second capillary.

[0039] The 35th aspect of the present disclosure may include any one of the 32nd to 34th aspects, wherein each second capillary is in direct contact with two third capillaries.

[0040] The 36th aspect of the present disclosure may include any one of the 32nd to 35th aspects, wherein each third cladding element is arranged at a distance from a circumferentially adjacent third cladding element.

[0041] The 37th aspect of the present disclosure may include the 36th aspect, wherein the third cladding elements are arranged at equal circumferential intervals.

[0042] The 38th aspect of the present disclosure may include any one of the 32nd to 37th aspects, wherein the plurality of third cladding elements are composed of 3 to 8 third capillaries.

[0043] The 39th aspect of the present disclosure may include any one of the 32nd to 38th aspects, wherein the plurality of third cladding elements are composed of 5 to 6 third capillaries.

[0044] The 40th aspect of the present disclosure may include any one of the 32nd to 39th aspects in which the number of the first cladding elements is equal to the number of the second cladding elements and the number of the third cladding elements.

[0045] The 41st aspect of the present disclosure may include any one of the 32nd to 40th aspects in which the third diameter is 15 μm to 66 μm.

[0046] The 42nd aspect of the present disclosure may include any one of the 32nd to 41st aspects in which the wall thickness of the third capillary is 0.1 μm to 4.0 μm.

[0047] The 43rd aspect of the present disclosure may include any one of the 32nd to 42nd aspects in which the third diameter is different from the first diameter and the second diameter.

[0048] According to a 44th aspect of the present disclosure, a hollow-core optical fiber includes a hollow core extending along a longitudinal central axis of the hollow-core optical fiber, a base material having a tubular shape and an inner surface surrounding the longitudinal central axis of the hollow-core optical fiber, a plurality of first cladding elements spaced apart from each other and disposed between the hollow core and the base material, each of the plurality of first cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of first cladding elements including a first capillary, the first capillary having an inner surface defining a first cavity, the first cavity having a first diameter, none of the plurality of first cladding elements directly contacting the inner surface of the base material, a plurality of second cladding elements spaced apart from each other and disposed between the hollow core and the base material, each of the plurality of second cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of second cladding elements including a second capillary, the second capillary having an inner surface defining a second cavity, the second cavity having a second diameter different from the first diameter, none of the second cladding elements directly contacting the inner surface of the base material. Each first capillary may include a nested capillary, with each nested capillary directly contacting the inner surface of the first capillary, or alternatively, each second capillary may include a nested capillary, with each nested capillary directly contacting the inner surface of the second capillary.

[0049] A 45th aspect of the present disclosure may include the 44th aspect where the inner diameter of the nested capillary is from 6 μm to 25 μm.

[0050] A 46th aspect of the present disclosure may include the 44th or 45th aspect where the wall thickness of the nested capillary is from 0.1 μm to 4.0 μm.

[0051] A 47th aspect of the present disclosure may include any one of the 44th to 46th aspects where each nested capillary directly contacts the inner surface of the second capillary at a point close to the base material.

[0052] The 48th aspect of the present disclosure may include any one of the 44th to 47th aspects, in which the confinement loss of the hollow-core optical fiber varies with wavelength, and the minimum confinement loss of the fundamental mode of the optical signal propagating in the hollow-core optical fiber within the wavelength range of 350 nm to 8000 nm is 0.2 dB / km or less.

[0053] In the following detailed description, further features and advantages are described. The following further features and advantages will be readily understood by those skilled in the art to some extent from the description, or alternatively, will be understood by practicing the embodiments described in this specification including the following detailed description, the claims, and the accompanying drawings.

[0054] It should be understood that both the above summary description and the following detailed description are intended to describe various embodiments and provide an overview or framework for understanding the nature and characteristics of the claimed subject matter. The accompanying drawings are provided to enhance the understanding of the various embodiments and are incorporated herein and constitute a part thereof. The drawings exemplarily illustrate the various embodiments described herein and, in conjunction with the following detailed description, are for explaining the principles and operations of the claimed subject matter.

Brief Description of the Drawings

[0055]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0056] Next, various embodiments of the hollow-core optical fiber will be described in detail. Throughout the drawings, the same or similar parts are denoted by the same reference numerals as much as possible. In the embodiments, the hollow-core optical fiber can include a hollow core, a base material, and a cladding element disposed between the hollow core and the base material. The hollow-core optical fiber can include a plurality of first cladding elements and a plurality of second cladding elements. The first cladding elements are in direct contact with the inner surface of the base material, and none of the plurality of second cladding elements are in direct contact with the inner surface of the base material. Details of each embodiment of the hollow-core optical fiber will be described later.

[0057] In this specification, ranges may be expressed in the form of "about" a certain value or more, "about" a certain value to "about" another certain value, or "about" less than the other certain value. When expressing a range in such a manner, there are other embodiments that include the certain value to the other certain value. Similarly, when a value is expressed as an approximate value by prefixing "about" thereto, it will be understood that there are also other embodiments constituted by the specific value itself. It will also be understood that both endpoints of each range have meanings that are correlated with each other and also independent of each other.

[0058] In this specification, directional terms (e.g., up, down, right, left, front, back, top, bottom, etc.) are only with reference to the drawings and are not intended to mean absolute directions.

[0059] Regarding various components described in this specification, there may be cases where it is stated that they are "directly connected" or "indirectly connected". When a component is "directly connected", it means that these components are joined to each other without an intervening structure. The joining of components can be performed by appropriate attachment means such as fusion, welding, or an adhesive. When a component is "indirectly connected", it means that these components are joined to each other with an intervening structure. Examples of the intervening structure include welding aids (such as frit, solder, flux, etc.), adhesives, and joining materials. In an embodiment, the connection of indirectly connected components is performed only by a welding aid, an adhesive, or a joining material. The term "connected" means "directly connected" or "indirectly connected". Components that are "directly connected" to each other can also be said to be in direct contact with each other. Components that are "indirectly connected" to each other can also be said to be in indirect contact with each other. Components that are "connected" to each other are in direct or indirect contact with each other.

[0060] In this specification, the singular forms indicated by "a", "an", and "the" shall include references to the corresponding plural forms as well, unless it is clear from the context that the singular form does not include the plural form. Thus, for example, an expression introducing a component with the article "a" includes embodiments having two or more of that component as well, unless it is clear from the context that this is not the case.

[0061] Although not intended to be bound by theory, an optical signal (i.e., light) can pass through the hollow core of a hollow-core optical fiber. As used herein, "attenuation" refers to a decrease in the intensity of an optical signal passing through a hollow-core optical fiber. The attenuation of an optical signal guided to pass through a hollow-core optical fiber can be suppressed by various effects such as, but not limited to, an anti-resonance effect and a coupling suppression mechanism. Each of these effects can reduce the leakage of light from the hollow core of the optical fiber to the cladding element of the optical fiber, and thus suppress the attenuation of the optical signal propagating within the hollow core. That is, each of these effects can improve the confinement of light to the hollow core of the optical fiber, thereby suppressing the attenuation of the optical signal propagating within the hollow core. Embodiments of the hollow-core optical fiber described herein can be provided with a structure that suppresses the attenuation of an optical signal passing through the hollow-core optical fiber by utilizing one or more of these effects.

[0062] As used herein, "anti-resonance" or "anti-resonant effect" refers to an effect that occurs by making the thickness of a material (e.g., the material used to form a cladding element) proportional to the wavelength of light passing through a hollow-core optical fiber so that the light passing through the hollow-core optical fiber is confined within the hollow core. Although not intended to be bound by theory, the anti-resonant effect occurs when the thickness of the material satisfies the quarter-wave condition (the phase accumulated in one pass is an odd multiple of one-quarter and one-quarter of 2π). Applying this condition to the thickness of the material confines the light within the hollow core and minimizes light leakage to the cladding. In other words, satisfying this condition helps to suppress the coupling between the core mode and the cladding mode, resulting in reduced transmission loss and improved confinement of the optical signal to the hollow core. In one embodiment, the anti-resonant effect can be obtained when the material has a thickness given by Equation 1.

[0063]

Equation

[0064] In Equation 1, t AR is the thickness of a material that satisfies the anti-resonance condition, λ is the wavelength of the optical signal, m is an integer of 1 or more, and n is the refractive index of the material. Note that Equation 1 represents the ideal material thickness at which the anti-resonance effect can be obtained, and it should be noted that even a material thickness that is not exactly equal to t AR can improve the confinement of light in the hollow core. Although not limited, for example, t AR within ±10% of (t AR from 90% of t AR to 110% of) is contemplated to be able to act to confine light within the hollow core.

[0065] As used herein, the "inhibited coupling mechanism" refers to the effect that occurs when the coupling between the core mode and the cladding mode is suppressed by a cladding element having a negative curvature, and the leakage of light from the hollow core is suppressed. As used herein, "negative curvature" refers to a state in which the surface of the cladding element forms a convex shape facing the longitudinal central axis of the hollow core optical fiber. Although not intended to be bound by theory, by using a cladding element having a surface that forms a convex shape facing the longitudinal central axis of the hollow core optical fiber, the amount of light contacting the cladding element can be reduced, and the light leaking from the cladding element or the gap between the cladding elements can be reduced. Thereby, it is possible to suppress the attenuation of the optical signal due to light leakage from the cladding element or the gap between the cladding elements, and it is also possible to suppress the light scattering that may occur when light contacts the surface of the cladding element.

[0066] Next, referring to FIG. 1, the hollow core optical fiber 100 can include a hollow core 110 extending along the longitudinal central axis 112 of the fiber and a substrate 130. Generally, the hollow core 110 is the light guiding portion of the hollow core optical fiber 100. The radius of the hollow core 110 extends from the longitudinal central axis 112 in a direction orthogonal to the closest contact with one of the cladding elements 120. The diameter of the hollow core 110 is twice the radius of the hollow core 110. The diameter of the hollow core 110 is 15 micrometers or more, or 20 micrometers or more, or 25 micrometers or more, or 45 micrometers or less, or 40 micrometers or less, or 35 micrometers or less, or in the range of 15 micrometers to 45 micrometers, or in the range of 20 micrometers to 40 micrometers, or in the range of 25 micrometers to 35 micrometers.

[0067] The substrate 130 can be configured to be operable to support any cladding element included in the hollow core optical fiber 100. In an embodiment, the substrate 130 can have a tubular shape with an inner surface 132 surrounding the longitudinal central axis 112 of the fiber. In an embodiment, the outer diameter of the substrate 130 can be 100 μm to 500 μm. By way of example and not limitation, for example, the outer diameter of the substrate 130 can be 100 μm to 500 μm, 200 μm to 500 μm, 300 μm to 500 μm, 400 μm to 500 μm, 100 μm to 400 μm, 100 μm to 300 μm, 100 μm to 200 μm, or any combination or subset of these ranges. In an embodiment, the outer diameter of the substrate can be 100 μm to 250 μm.

[0068] In an embodiment, the substrate 130 has a wall thickness of 1 μm to 100 μm. By way of example and not limitation, for example, the wall thickness of the substrate 130 can be 2 μm to 80 μm, 5 μm to 60 μm, 10 μm to 50 μm, 15 μm to 40 μm, 15 μm to 30 μm, 20 μm to 30 μm, or any combination or subset of these ranges. In an embodiment, the outer diameter of the substrate can be 100 μm to 250 μm.

[0069] In an embodiment, one or more protective coatings (not shown) can be provided on the outer surface of the substrate 130. This coating can be an organic material such as, for example, plastic or polymer, and can protect the hollow-core optical fiber 100 from the physical environment. The coating can include a primary coating with a low Young's modulus (e.g., <1 MPa) that covers and is adjacent to the substrate 130, and a secondary coating with a high Young's modulus (e.g., >1000 MPa) that covers and is adjacent to the primary coating. Representative materials for the primary and secondary coatings include acrylate materials (e.g., urethane acrylate). In an embodiment, the outer diameter of the coated fiber can be 242 micrometers, 200 micrometers, or 190 micrometers. In an embodiment, the thickness of the primary coating in the radial direction is greater than 10 micrometers, or greater than 15 micrometers, or greater than 20 micrometers, or less than 45 micrometers, or less than 40 micrometers, or less than 35 micrometers, or in the range of 10 micrometers to 45 micrometers, or in the range of 15 micrometers to 40 micrometers, or in the range of 20 micrometers to 35 micrometers. In an embodiment, the thickness of the secondary coating in the radial direction is greater than 10 micrometers, or greater than 15 micrometers, or greater than 20 micrometers, or less than 45 micrometers, or less than 40 micrometers, or less than 35 micrometers, or in the range of 10 micrometers to 45 micrometers, or in the range of 15 micrometers to 40 micrometers, or in the range of 20 micrometers to 35 micrometers.

[0070] The hollow-core optical fiber 100 includes a plurality of first cladding elements 122 and a plurality of second cladding elements 124. Each of the plurality of first cladding elements 122 can extend in a direction parallel to the longitudinal central axis 112 of the fiber. Each of the plurality of first cladding elements 122 can include a first capillary having an inner surface 152 that defines a first cavity 162. In an embodiment, the first cavity 162 has a first diameter and can be occupied by only gas. Similarly, each of the plurality of second cladding elements 124 can extend in a direction parallel to the longitudinal central axis 112 of the fiber. Each of the plurality of second cladding elements 124 includes a second capillary, and each second capillary can have an inner surface 156 that defines a second cavity 164. In an embodiment, the second cavity 164 has a second diameter and can be occupied by only gas. In an embodiment, the second diameter is different from the first diameter. For example, the second diameter can be larger or smaller than the first diameter. In the embodiment shown in FIG. 1, the second diameter is larger than the first diameter.

[0071] In an embodiment, none of the plurality of first cladding elements 122 directly contact the inner surface 132 of the substrate 130, while none of the plurality of second cladding elements 124 directly contact the inner surface 132 of the substrate 130. In other words, the plurality of first cladding elements 122 can be disposed between the substrate 130 and the hollow core 110, and the plurality of second cladding elements 124 can be disposed between the plurality of first cladding elements 122 and the hollow core 110 such that the plurality of second cladding elements 124 are spaced apart from the substrate 130.

[0072] In an embodiment, the plurality of first clad elements 122 can be composed of 3 to 8 first capillaries. Although not limited, for example, the plurality of first clad elements 122 can be composed of 3, 4, 5, 6, 7, or 8 first capillaries. In an embodiment, the plurality of first clad elements 122 can be composed of 5 to 6 first capillaries. In an embodiment, the plurality of second clad elements 124 can be composed of 3 to 12 second capillaries. Although not limited, for example, the plurality of second clad elements 124 can be composed of 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 second capillaries. In an embodiment, the plurality of second clad elements 124 can be composed of 5 to 6 second capillaries. In an embodiment, the number of first clad elements can be made equal to the number of second clad elements, and the number of first capillaries can be made equal to the number of second capillaries. In an embodiment, the number of first clad elements can be made greater than or less than the number of second clad elements, and the number of first capillaries can be made greater than or less than the number of second capillaries.

[0073] In this specification, a capillary can have a closed cross-section composed of a wall having an inner surface and an outer surface. Therefore, it should be understood that a capillary can generally have a tubular shape. In an embodiment, the capillary can have any suitable cross-sectional shape such as circular, elliptical, or oval. Without intending to be bound by theory, when the capillary has a cross-sectional shape such as circular, elliptical, or oval, it is considered that the convex surface of the capillary faces the hollow core of the hollow core optical fiber. Without intending to be bound by theory, the negative curvature effect obtained by the convex surface promotes a coupling inhibition mechanism that suppresses the attenuation of the optical signal passing through the hollow core optical fiber.

[0074] In an embodiment, each first capillary of the plurality of first cladding elements 122 can be directly connected to the inner surface 132 of the substrate 130. For example, in the embodiment shown in FIG. 1, the first capillary 140 is directly connected to the inner surface 132 of the substrate 130 at point 142. In an embodiment, the plurality of first cladding elements 122 are arranged symmetrically with respect to the longitudinal central axis 112 of the hollow core optical fiber 100.

[0075] Each first capillary of the plurality of first cladding elements 122 can include a first capillary longitudinal central axis 126. In an embodiment, each first capillary longitudinal central axis 126 can be parallel to the longitudinal central axis 112 of the hollow core optical fiber 100. Each first capillary longitudinal central axis 126 can be parallel to other first capillary longitudinal central axes 126. In an embodiment, each first capillary longitudinal central axis 126 can be arranged on the circumference of a circle perpendicular to the longitudinal central axis 112 of the hollow core optical fiber 100. For example, in an embodiment, each first capillary longitudinal central axis 126 can be arranged on the circumference of a circle centered on and perpendicular to the longitudinal central axis 112 of the hollow core optical fiber 100. In such an embodiment, it should be understood that each first capillary longitudinal central axis 126 is equidistant from the longitudinal central axis 112 of the fiber.

[0076] In an embodiment, each first cladding element is arranged at a distance from a first cladding element adjacent in the circumferential direction 190. In other words, each first capillary of the plurality of first cladding elements 122 is arranged at a distance from a first capillary adjacent in the circumferential direction 190 of the plurality of first cladding elements 122. Each first capillary of the plurality of first cladding elements 122 can be configured not to be in direct contact with other first capillaries of the plurality of first cladding elements 122. For example, according to the embodiment shown in FIG. 1, both the first capillary 140 and the first capillary 144 are included in the plurality of first cladding elements 122, but the first capillary 140 and the first capillary 144 are arranged at a distance in the circumferential direction 190 and are configured not to be in direct contact with each other. Both the first capillary 140 and the first capillary 144 are in direct contact with the base material 130. Further, in the embodiment, the first cladding elements 122 are arranged at equal intervals in the circumferential direction 190. Although not intending to be bound by theory, if the first capillaries of the plurality of first cladding elements 122 are in direct contact with each other, there is a possibility that a localized mode may be supported, in which the coupling between the core mode and the cladding mode is allowed by the contact point between these first capillaries, increasing the leakage of light from the hollow core 110. Therefore, if the first capillaries are in direct contact with each other, the confinement of light to the hollow core 110 of the hollow core optical fiber 100 at the contact point may be weakened. As a result, the attenuation of the optical signal passing through the hollow core optical fiber 100 may increase.

[0077] In an embodiment, each first capillary of the plurality of first cladding elements 122 can have an inner diameter of 12 μm to 54 μm. By way of non-limiting example, for instance, the inner diameter of each first capillary of the plurality of first cladding elements 122 can be 12 μm to 54 μm, 16 μm to 54 μm, 20 μm to 54 μm, 24 μm to 54 μm, 28 μm to 54 μm, 32 μm to 54 μm, 36 μm to 54 μm, 40 μm to 54 μm, 44 μm to 54 μm, 48 μm to 54 μm, 52 μm to 54 μm, 12 μm to 50 μm, 12 μm to 46 μm, 12 μm to 42 μm, 12 μm to 38 μm, 12 μm to 34 μm, 12 μm to 30 μm, 12 μm to 26 μm, 12 μm to 22 μm, 12 μm to 18 μm, 12 μm to 16 μm, or any combination or subset of these ranges. In an embodiment, each first capillary of the plurality of first cladding elements 122 can have the same inner diameter.

[0078] In an embodiment, each first capillary of the plurality of first cladding elements 122 can have a wall thickness of 0.1 μm to 4.0 μm. As used herein, this wall thickness refers to the distance between the outer surface 154 of each first capillary of the plurality of first cladding elements 122 and the inner surface 152 of the first capillary of the plurality of first cladding elements 122. By way of non-limiting example, for instance, the wall thickness of each first capillary of the plurality of first cladding elements 122 can be 0.1 μm to 4.0 μm, 0.5 μm to 4.0 μm, 1 μm to 4.0 μm, 1.5 μm to 4.0 μm, 2.0 μm to 4.0 μm, 2.5 μm to 4.0 μm, 3.0 μm to 4.0 μm, 3.5 μm to 4.0 μm, 0.1 μm to 3.5 μm, 0.1 μm to 3.0 μm, 0.1 μm to 2.5 μm, 0.1 μm to 2.0 μm, 0.1 μm to 1.5 μm, 0.1 μm to 1.0 μm, 0.1 μm to 0.5 μm, or any combination or subset of these ranges. In an embodiment, each first capillary of the plurality of first cladding elements 122 can have the same wall thickness. In an embodiment, each first capillary can have a wall thickness that promotes confinement of an optical signal to the hollow core 110, such as a wall thickness determined by Equation 1, due to an anti-resonance effect. Without intending to be bound by theory, when the wall thickness of each first capillary is assumed to have an anti-resonance effect, attenuation of an optical signal passing through the optical fiber can be suppressed.

[0079] Each second capillary of the plurality of second cladding elements 124 can be directly connected to at least one first capillary of the plurality of first cladding elements 122. In an embodiment, each capillary of the plurality of second cladding elements 124 can be directly connected to two or more first capillaries of the plurality of first cladding elements 122. For example, in the embodiment shown in FIG. 1, the second capillary 146 of the plurality of second cladding elements 124 can be directly connected to the first capillary 140 of the plurality of first cladding elements 122 at point 147 and can be directly connected to the first capillary 144 of the plurality of first cladding elements 122 at point 148. Without intending to be bound by theory, by directly connecting the capillaries of the plurality of second cladding elements 124 to two capillaries of the plurality of first cladding elements 122, the structural integrity of the plurality of second cladding elements 124 can be improved. Thereby, the occurrence of defects in the plurality of second cladding elements 124 during the manufacture of the hollow core optical fiber 100 can be reduced.

[0080] The first capillaries of the plurality of first cladding elements 122 can be disposed in the gaps between the second capillaries of the plurality of second cladding elements 124. Without limitation, for example, in the embodiment shown in FIG. 1, the first capillary 144 of the plurality of first cladding elements 122 is disposed in the gap between the second capillary 146 and the second capillary 149 of the plurality of second cladding elements 124. Without intending to be bound by theory, by disposing the first capillaries of the plurality of first cladding elements 122 in the gaps between the second capillaries of the plurality of second cladding elements 124, the plurality of first cladding elements 122 can support the plurality of second cladding elements 124. Further, by disposing the first capillary in the gap between the second capillaries, it becomes possible to confine light that might otherwise have passed through the gap between the second capillaries further within the hollow core 110. As a result, the attenuation of the optical signal passing through the hollow core optical fiber 100 can be suppressed.

[0081] In an embodiment, each second capillary of the plurality of second cladding elements 124 has a second capillary longitudinal central axis 128. Each second capillary longitudinal central axis 128 can be parallel to the longitudinal central axis 112 of the hollow core optical fiber 100. In an embodiment, each second capillary longitudinal central axis 128 can be parallel to other second capillary longitudinal central axes 128. In an embodiment, each second capillary longitudinal central axis 128 can be parallel to each first capillary longitudinal central axis 126. In an embodiment, the longitudinal central axis 112 of the hollow core optical fiber 100 can be parallel to both the first capillary longitudinal central axis 126 and the second capillary longitudinal central axis 128. In an embodiment, each second capillary longitudinal central axis 128 can be arranged on the circumference of a circle perpendicular to the longitudinal central axis 112 of the hollow core optical fiber 100. For example, in an embodiment, each second capillary longitudinal central axis 128 can be arranged on the circumference of a circle centered on and perpendicular to the longitudinal central axis 112 of the fiber. In such an embodiment, it should be understood that each second capillary longitudinal central axis 128 is equidistant from the longitudinal central axis 112 of the fiber. In an embodiment, the plurality of second cladding elements 124 are arranged symmetrically with respect to the longitudinal central axis 112 of the hollow core optical fiber 100.

[0082] In an embodiment, each first capillary longitudinal central axis 126 is at a first radial distance from the longitudinal central axis 112 of the hollow core optical fiber 100. Similarly, each second capillary longitudinal central axis 128 is at a second radial distance from the longitudinal central axis 112 of the hollow core optical fiber 100. In an embodiment, the first radial distance can be a distance greater than the second radial distance.

[0083] Each second capillary of the plurality of second cladding elements 124 can be arranged at a radial distance 192 from the inner surface 132 of the substrate 130. That is, each capillary of the plurality of second cladding elements 124 can be configured not to be in direct contact with the inner surface 132 of the substrate 130. For example, according to the embodiment shown in FIG. 1, by arranging the second capillaries 146 of the plurality of second cladding elements 124 at a radial distance 192 from the inner surface 132 of the substrate 130, the second capillaries 146 are configured not to be in direct contact with the inner surface 132 of the substrate 130.

[0084] In an embodiment, each second capillary of the plurality of second cladding elements 124 is arranged at a distance from a second capillary adjacent in the circumferential direction 190 of the plurality of second cladding elements 124. Each second capillary of the plurality of second cladding elements 124 can be configured not to be in direct contact with other second capillaries of the plurality of second cladding elements 124. For example, according to the embodiment shown in FIG. 1, both the second capillary 146 and the second capillary 149 are included in the plurality of second cladding elements 124, but the second capillary 146 and the second capillary 149 are arranged at a distance in the circumferential direction 190, and the second capillary 146 and the second capillary 149 are configured not to be in direct contact with each other. Also, in an embodiment, the second cladding elements 124 are arranged at equal intervals in the circumferential direction 190. Although not intending to be bound by theory, if the second capillaries of the plurality of second cladding elements 124 are in direct contact with each other, there is a possibility that a localized mode that allows coupling between the core mode and the cladding mode and increases the leakage of light from the hollow core 110 may be supported at the contact point between these second capillaries. Therefore, when the second capillaries are in direct contact with each other, the confinement of light to the hollow core 110 of the hollow core optical fiber 100 at that contact point may be weakened. As a result, the attenuation of the optical signal passing through the hollow core optical fiber 100 may increase.

[0085] In an embodiment, each second capillary of the plurality of second cladding elements 124 has an inner diameter of 16 μm to 65 μm. Although not limited, for example, the inner diameter of each second capillary of the plurality of second cladding elements 124 is 16 μm to 65 μm, 20 μm to 65 μm, 25 μm to 65 μm, 30 μm to 65 μm, 35 μm to 65 μm, 40 μm to 65 μm, 45 μm to 65 μm, 50 μm to 65 μm, 55 μm to 65 μm, 60 μm to 65 μm, 16 μm to 60 μm, 16 μm to 55 μm, 16 μm to 50 μm, 16 μm to 45 μm, 16 μm to 40 μm, 16 μm to 35 μm, 16 μm to 30 μm, 16 μm to 25 μm, 16 μm to 20 μm, or any combination or subset of these ranges.

[0086] In an embodiment, each second capillary of the plurality of second cladding elements 124 has a wall thickness of 0.1 μm to 4.0 μm. As used herein, this wall thickness refers to the distance between the outer surface 158 of each second capillary of the plurality of second cladding elements 124 and the inner surface 156 of the second capillary of the plurality of second cladding elements 124. Although not limited, for example, the wall thickness of each second capillary of the plurality of second cladding elements 124 can be 0.1 μm to 4.0 μm, 0.5 μm to 4.0 μm, 1 μm to 4.0 μm, 1.5 μm to 4.0 μm, 2.0 μm to 4.0 μm, 2.5 μm to 4.0 μm, 3.0 μm to 4.0 μm, 3.5 μm to 4.0 μm, 0.1 μm to 3.5 μm, 0.1 μm to 3.0 μm, 0.1 μm to 2.5 μm, 0.1 μm to 2.0 μm, 0.1 μm to 1.5 μm, 0.1 μm to 1.0 μm, 0.1 μm to 0.5 μm, or any combination or subset of these ranges. In an embodiment, each second capillary can have a wall thickness that promotes confinement of an optical signal to the hollow core 110, such as the wall thickness determined by Equation 1, due to an anti-resonance effect. Without intending to be bound by theory, when the wall thickness of each second capillary is assumed to have an anti-resonance effect, attenuation of an optical signal passing through the optical fiber can be suppressed.

[0087] Referring now to FIG. 2, in an embodiment, the hollow core optical fiber 100 can further include a plurality of third cladding elements 210. The plurality of third cladding elements 210 can be spaced apart from each other and disposed between the plurality of second cladding elements 124 and the hollow core 110. Each of the plurality of third cladding elements 210 can extend in a direction parallel to the longitudinal central axis 112 of the hollow core optical fiber 100. Each third cladding element can include a third capillary. The third capillary has an inner surface 252 that defines a third cavity 262 having a third diameter. In an embodiment, the third cavity 262 can be occupied only by a gas.

[0088] In an embodiment, the plurality of third cladding elements 210 can be composed of 3 to 8 third capillaries. By way of example and not limitation, for example, the third cladding element 210 can be composed of 3, 4, 5, 6, 7, or 8 third capillaries. In an embodiment, the plurality of third cladding elements 210 can be composed of 5 to 6 third capillaries. In an embodiment, the plurality of first cladding elements 122, the plurality of second cladding elements 124, and the plurality of third cladding elements 210 can be composed of the same number of capillaries. For example, according to the embodiment shown in FIG. 2, the plurality of first cladding elements 122, the plurality of second cladding elements 124, and the plurality of third cladding elements 210 are all composed of 6 capillaries.

[0089] Each third capillary of the plurality of third cladding elements 210 can be directly connected to at least one second capillary of the plurality of second cladding elements 124. In an embodiment, each third capillary of the plurality of third cladding elements 210 can be directly connected to two or more second capillaries of the plurality of second cladding elements 124. For example, in the embodiment shown in FIG. 2, the capillary 212 of the plurality of third cladding elements 210 can be directly connected to the capillary 222 of the plurality of second cladding elements 124 at point 214 and can be directly connected to the capillary 223 of the plurality of second cladding elements 124 at point 215. In an embodiment, each second capillary can be in direct contact with two third capillaries. Without intending to be bound by theory, by directly connecting the capillaries of the third cladding elements 210 to two capillaries of the plurality of second cladding elements 124, the structural integrity of the plurality of third cladding elements 210 can be improved. Thereby, the occurrence of defects in the plurality of third cladding elements 210 during the manufacture of the hollow core optical fiber 100 can be reduced.

[0090] In an embodiment, each third capillary of the plurality of third clad elements 210 is arranged at a distance from a third capillary adjacent in the circumferential direction 190 of the third clad element 210. Each third capillary of the plurality of third clad elements 210 can be configured not to be in direct contact with other third capillaries of the plurality of third clad elements 210. For example, according to the embodiment shown in FIG. 2, both the third capillary 212 and the third capillary 216 are clad elements of the plurality of third clad elements 210, but the third capillary 212 and the third capillary 216 are arranged at a distance in the circumferential direction 190 and are configured not to be in direct contact with each other. Although not intending to be bound by theory, if the third capillaries of the plurality of third clad elements 210 are in direct contact with each other, there is a possibility that a localized mode that allows coupling between the core mode and the clad mode and may increase light leakage from the hollow core is supported due to the contact point between these capillaries. Therefore, when the third capillaries are in direct contact with each other, the confinement of light to the hollow core 110 of the hollow core optical fiber 100 at the contact point may be weakened. As a result, the attenuation of the optical signal passing through the hollow core optical fiber 100 may increase.

[0091] Each third capillary of the plurality of third cladding elements 210 has a third capillary longitudinal central axis 218. Each third capillary longitudinal central axis 218 can be parallel to the longitudinal central axis 112 of the hollow core optical fiber 100. In an embodiment, each third capillary longitudinal central axis 218 can be parallel to other third capillary longitudinal central axes 218. In an embodiment, each third capillary longitudinal central axis 218 can be parallel to each first capillary longitudinal central axis 126. In an embodiment, each third capillary longitudinal central axis 218 can be parallel to each second capillary longitudinal central axis 128. In an embodiment, the longitudinal central axis 112 of the hollow core optical fiber 100 can be parallel to any of the first capillary longitudinal central axis 126, the second capillary longitudinal central axis 128, and the third capillary longitudinal central axis 218. In an embodiment, each third capillary longitudinal central axis 218 can be arranged on the circumference of a circle perpendicular to the longitudinal central axis 112 of the hollow core optical fiber 100. For example, in an embodiment, each third capillary longitudinal central axis 218 can be arranged on the circumference of a circle centered on and perpendicular to the longitudinal central axis 112 of the fiber. In such an embodiment, it should be understood that each third capillary longitudinal central axis 218 is equidistant from the longitudinal central axis 112 of the fiber.

[0092] In an embodiment, each third capillary of the plurality of third cladding elements 210 has an inner diameter of 15 μm to 66 μm. By way of non-limiting example, for instance, the inner diameter of each third capillary of the plurality of third cladding elements 210 can be 15 μm to 66 μm, 20 μm to 66 μm, 25 μm to 66 μm, 30 μm to 66 μm, 35 μm to 66 μm, 40 μm to 66 μm, 45 μm to 66 μm, 50 μm to 66 μm, 55 μm to 66 μm, 60 μm to 66 μm, 15 μm to 60 μm, 15 μm to 55 μm, 15 μm to 50 μm, 15 μm to 45 μm, 15 μm to 40 μm, 15 μm to 35 μm, 15 μm to 30 μm, 15 μm to 25 μm, 15 μm to 20 μm, or any combination or subset of these ranges.

[0093] In an embodiment, each third capillary of the plurality of third cladding elements 210 has a wall thickness of 0.1 μm to 1.5 μm. As used herein, this wall thickness refers to the distance between the outer surface 254 and the inner surface 252 of each third capillary. By way of non-limiting example, for instance, the wall thickness of each third capillary of the plurality of third cladding elements 210 can be 0.1 μm to 1.5 μm, 0.3 μm to 1.5 μm, 0.5 μm to 1.5 μm, 0.7 μm to 1.5 μm, 0.9 μm to 1.5 μm, 1.1 μm to 1.5 μm, 1.3 μm to 1.5 μm, 0.1 μm to 1.3 μm, 0.1 μm to 1.1 μm, 0.1 μm to 0.9 μm, 0.1 μm to 0.7 μm, 0.1 μm to 0.5 μm, 0.1 μm to 0.3 μm, or any combination or subset of these ranges. In an embodiment, each third capillary can have a wall thickness that promotes confinement of an optical signal to the hollow core 110, such as a wall thickness determined by Equation 1, due to an anti-resonance effect. Without intending to be bound by theory, when the wall thickness of each third capillary is assumed to have an anti-resonance effect, attenuation of an optical signal passing through the optical fiber can be suppressed.

[0094] In an embodiment, each first capillary of the plurality of first clad elements 122 or each second capillary of the plurality of second clad elements 124 can also include a nested capillary 310. As used herein, a "nested capillary" refers to a capillary that is disposed within another capillary such that its outer surface is directly connected to the inner surface of the other capillary. According to the embodiment of FIG. 3, the outer surface 354 of each nested capillary 310 is directly connected to the inner surface 156 of the capillary within which the nested capillary 310 is disposed. For example, according to the embodiment shown in FIG. 3, the nested capillary 312 is in direct contact with the inner surface 322 of the second capillary 320. In an embodiment, each first capillary of the plurality of first clad elements 122 can include a nested capillary (not shown). Also, in an embodiment, as shown in FIG. 3, each second capillary of the plurality of second clad elements 124 can also include a nested capillary 310. Note that the nested capillary can be occupied only by gas, but it should be noted that the capillary within which the nested capillary is disposed is not configured as such. For example, according to the embodiment shown in FIG. 3, the nested capillary 312 is occupied only by gas, but since a part of the second capillary 320 is occupied by the nested capillary 312, the second capillary 320 is of course not configured to be occupied only by gas.

[0095] Each nested capillary 310 can directly contact the inner surface of the first capillary or the inner surface of the second capillary at a point close to the substrate 130. In this specification, a point on the inner surface of the capillary that is evaluated to be closest to the substrate 130 in the radial direction 192 is defined as a point on the inner surface of the capillary close to the substrate 130. For example, according to the embodiment shown in FIG. 3, the nested capillary 312 is directly connected to the inner surface 322 of the second capillary 320 at a point 324 close to the substrate 130, and this point 324 is defined as a point close to the substrate 130 because it is the point on the inner surface 322 of the second capillary 320 that is evaluated to be closest to the substrate 130 in the radial direction 192.

[0096] Note that in the embodiment shown in FIG. 3, the capillary including the nested capillary is not in direct contact with the inner surface of the substrate 130. Rather, a gap is provided. For example, when the second capillary 320 includes the nested capillary 312, the second capillary 320 is spaced from the inner surface of the substrate 130. Without wishing to be bound by theory, in some embodiments, providing a gap is thought to improve the confinement of the optical signal into the hollow core 110 (reduce the confinement loss). In other embodiments, the capillary including the nested capillary is in direct contact with the inner surface of the substrate tube 130.

[0097] In an embodiment, each nested capillary 310 can have an inner diameter of 6 μm to 25 μm. By way of non-limiting example, the inner diameter of each nested capillary 310 can be 6 μm to 25 μm, 9 μm to 25 μm, 12 μm to 25 μm, 15 μm to 25 μm, 18 μm to 25 μm, 21 μm to 25 μm, 6 μm to 22 μm, 6 μm to 19 μm, 6 μm to 16 μm, 6 μm to 13 μm, 6 μm to 10 μm, or any combination or subset of these ranges.

[0098] In an embodiment, each nested capillary 310 can have a wall thickness of 0.1 μm to 1.5 μm. As used herein, this wall thickness refers to the distance between the outer surface 354 and the inner surface 352 of the nested capillary 310. By way of non-limiting example, for instance, the wall thickness of each nested capillary 310 can be 0.1 μm to 1.5 μm, 0.3 μm to 1.5 μm, 0.5 μm to 1.5 μm, 0.7 μm to 1.5 μm, 0.9 μm to 1.5 μm, 1.1 μm to 1.5 μm, 1.3 μm to 1.5 μm, 0.1 μm to 1.3 μm, 0.1 μm to 1.1 μm, 0.1 μm to 0.9 μm, 0.1 μm to 0.7 μm, 0.1 μm to 0.5 μm, 0.1 μm to 0.3 μm, or any combination or subset of these ranges. In an embodiment, each nested capillary can have a wall thickness having an anti-resonance effect, such as the wall thickness obtained by Equation 1. Without intending to be bound by theory, if the wall thickness of each nested capillary is assumed to have an anti-resonance effect, attenuation of an optical signal passing through the optical fiber can be suppressed.

[0099] In an embodiment, each capillary can include silica-based glass. The silica-based glass includes pure silica and silica doped with one or more dopants to change the refractive index. Each first capillary of the plurality of first cladding elements 122 can include silica-based glass. Each second capillary of the plurality of second cladding elements 124 can include silica-based glass. In an embodiment, either the first capillary, the second capillary, or both can consist essentially of only silica-based glass or can consist of silica-based glass. In an embodiment, each third capillary of the plurality of third cladding elements 210 can include silica-based glass. Each third capillary of the plurality of third cladding elements 210 can consist essentially of only silica-based glass or can consist of silica-based glass. In an embodiment, each nested capillary 310 can include silica-based glass. Each nested capillary can consist essentially of only silica-based glass or can consist of silica-based glass. In an embodiment, the substrate 130 can include silica-based glass. In an embodiment, the substrate 130 can consist essentially of only silica-based glass or can consist of silica-based glass.

[0100] The hollow core 110 can contain one or more gases. In an embodiment, the hollow core 110 can contain one or more inert gases. In an embodiment, the hollow core 110 can consist essentially of only air or can consist of air.

[0101] In the embodiments described in this specification, the cladding element can be configured to confine the fundamental mode of an optical signal (i.e., light) propagating in the hollow core 110 of the hollow core optical fiber 100 within the hollow core 110 by one or more of the anti-resonance effect and the coupling suppression mechanism. In an embodiment, the confinement is performed without utilizing the photonic bandgap effect due to the periodic structure. In an embodiment, the fundamental mode of the optical signal can have a wavelength in the range of 350 nm to 8000 nm. Although not limited, for example, the wavelength of the fundamental mode of the optical signal can be 350 nm to 8000 nm, 500 nm to 8000 nm, 1000 nm to 8000 nm, 1500 nm to 8000 nm, 2000 nm to 8000 nm, 2500 nm to 8000 nm, 3000 nm to 8000 nm, 3500 nm to 8000 nm, 4000 nm to 8000 nm, 4500 nm to 8000 nm, 5000 nm to 8000 nm, 5500 nm to 8000 nm, 6000 nm to 8000 nm, 6500 nm to 8000 nm, 7000 nm to 8000 nm, 7500 nm to 8000 nm, 350 nm to 7500 nm, 350 nm to 7000 nm, 350 nm to 6500 nm, 350 nm to 6000 nm, 350 nm to 5500 nm, 350 nm to 5000 nm, 350 nm to 4500 nm, 350 nm to 4000 nm, 350 nm to 3500 nm, 350 nm to 3000 nm, 350 nm to 2500 nm, 350 nm to 2000 nm, 350 nm to 1500 nm, 350 nm to 1000 nm, 350 nm to 500 nm, or any combination or subset of these ranges. In an embodiment, the plurality of first cladding elements and the plurality of second cladding elements can be configured to obtain the anti-resonance effect at a wavelength in the range of 350 nm to 8000 nm. This anti-resonance effect can act to confine the optical signal propagating in the hollow core optical fiber at a wavelength in the range of 350 nm to 8000 nm within the hollow core 110.

[0102] Although not intended to be bound by theory, in the hollow-core optical fibers described herein, confinement loss is considered to be the main attenuation factor. Confinement loss may occur due to light leaking from the hollow core 110 to the cladding 120. Confinement loss can be calculated using Equations 2 and 3.

[0103]

Number

[0104]

Number

[0105] In Equations 2 and 3, n eff is the effective refractive index of the mode propagating in the hollow-core fiber, with its real part being n r and its imaginary part being n im . The real part of the effective refractive index is related to the propagation speed of the mode, and the imaginary part is related to the confinement loss of the mode. In the case of an anti-resonant hollow-core fiber where the core and cladding have a predetermined structure, the effective refractive index can be obtained using a fiber modeling tool such as COMSOL Multiphysics (registered trademark). The confinement loss is calculated using Equation 3.

[0106] In an embodiment, the confinement loss of the hollow-core optical fiber 100 varies with wavelength, and the minimum confinement loss of the fundamental mode of an optical signal propagating in the hollow core 110 of the hollow-core optical fiber within the wavelength range of 350 nm to 8000 nm is 0.50 dB / km or less. Although not limited, for example, within the wavelength ranges of 350 nm to 8000 nm, 500 nm to 8000 nm, 1000 nm to 8000 nm, 1500 nm to 8000 nm, 2000 nm to 8000 nm, 2500 nm to 8000 nm, 3000 nm to 8000 nm, 3500 nm to 8000 nm, 4000 nm to 8000 nm, 4500 nm to 8000 nm, 5000 nm to 8000 nm, 5500 nm to 8000 nm, 6000 nm to 8000 nm, 6500 nm to 8000 nm, 7000 nm to 8000 nm, 7500 nm to 8000 nm, 350 nm to 7500 nm, 350 nm to 7000 nm, 350 nm to 6500 nm, 350 nm to 6000 nm, 350 nm to 5500 nm, 350 nm to 5000 nm, 350 nm to 4500 nm, 350 nm to 4000 nm, 350 nm to 3500 nm, 350 nm to 3000 nm, 350 nm to 2500 nm, 350 nm to 2000 nm, 350 nm to 1500 nm, 350 nm to 1000 nm, 350 nm to 500 nm, or any combination or subset of these wavelength ranges, the minimum confinement loss of the hollow-core optical fiber 100 can be 0.5 dB / km or less, 0.4 dB / km or less, 0.3 dB / km or less, 0.2 dB / km or less, 0.1 dB / km or less, or 0.5 dB / km or less.

[0107] In an embodiment where the hollow-core optical fiber includes a cladding element containing nested capillaries, the confinement loss of the hollow-core optical fiber varies with wavelength, and the minimum confinement loss of the fundamental mode of an optical signal propagating in the hollow-core optical fiber within a wavelength range of 350 nm to 8000 nm is 0.2 dB / km or less. Without limitation, for example, within a wavelength range of 350 nm to 8000 nm, 500 nm to 8000 nm, 1000 nm to 8000 nm, 1500 nm to 8000 nm, 2000 nm to 8000 nm, 2500 nm to 8000 nm, 3000 nm to 8000 nm, 3500 nm to 8000 nm, 4000 nm to 8000 nm, 4500 nm to 8000 nm, 5000 nm to 8000 nm, 5500 nm to 8000 nm, 6000 nm to 8000 nm, 6500 nm to 8000 nm, 7000 nm to 8000 nm, 7500 nm to 8000 nm, 350 nm to 7500 nm, 350 nm to 7000 nm, 350 nm to 6500 nm, 350 nm to 6000 nm, 350 nm to 5500 nm, 350 nm to 5000 nm, 350 nm to 4500 nm, 350 nm to 4000 nm, 350 nm to 3500 nm, 350 nm to 3000 nm, 350 nm to 2500 nm, 350 nm to 2000 nm, 350 nm to 1500 nm, 350 nm to 1000 nm, 350 nm to 500 nm, or any combination or subset of these wavelength ranges, the minimum confinement loss of the hollow-core optical fiber 100 can be 0.2 dB / km or less, 0.15 dB / km or less, 0.1 dB / km or less, 0.06 dB / km or less, 0.05 dB / km or less, 0.04 dB / km or less, 0.03 dB / km or less, 0.02 dB / km or less, or 0.01 dB / km or less.

[0108] Embodiments of the hollow-core optical fiber can be manufactured by the following method. First, cladding elements such as a plurality of first cladding elements 122 and a plurality of second cladding elements 124 can be sleeve-attached to a substrate 130 in a desired arrangement. Then, joining of the cladding elements can be performed as needed, such as joining the cladding elements to the substrate or joining the cladding elements to each other, to form a parent material assembly. The joining of the cladding elements to the substrate can be performed by any suitable means, such as, for example, fusion, welding, an adhesive, etc. Examples of welding techniques include laser welding, flame welding, and plasma welding. Then, the parent material assembly can be redrawn by a conventional fiber drawing technique to obtain a fiber parent material. Thereafter, the fiber parent material can be drawn by a conventional fiber drawing technique to obtain an optical fiber.

Example

[0109] The embodiments described in this specification will be further clarified by the following examples.

[0110] Example 1: Modeling of the confinement loss of a hollow-core optical fiber The confinement loss of the hollow-core optical fiber 100 was determined by modeling a hollow-core optical fiber 100 having a plurality of first cladding elements 122 and a plurality of second cladding elements 124. FIG. 4 shows a cross section of the hollow-core optical fiber model of Example 1. The plurality of first cladding elements 122 were composed of six capillaries. Each capillary of the plurality of first cladding elements 122 had an inner diameter of 30 μm and a wall thickness of 560 nm. The plurality of first cladding elements 122 were arranged symmetrically with respect to the longitudinal central axis of the hollow core 110. The plurality of second cladding elements 124 were also composed of six capillaries. Each capillary of the plurality of second cladding elements 124 had an inner diameter of 27 μm and a wall thickness of 560 nm. The plurality of second cladding elements 124 were arranged symmetrically with respect to the longitudinal central axis of the hollow core 110. The inner diameter of the substrate 130 was 132 μm and the wall thickness of the substrate 130 was 2 μm. The diameter of the hollow core 110 was approximately 34.5 μm. The diameter of the hollow core 110 was defined as the diameter of a circle that contacts the plurality of second cladding elements 124 at each point closest to the longitudinal central axis of the hollow core 110. The substrate 130, the plurality of first cladding elements 122, and the plurality of second cladding elements 124 were made of pure silica. As shown in FIG. 4, the structure of the hollow-core optical fiber was 60° rotationally symmetric, that is, six-fold rotationally symmetric.

[0111] For the hollow-core optical fiber of Example 1, the confinement of the optical signal into the hollow core 110 was modeled using the modeling software "COMSOL Multiphysics". According to this model, the hollow-core optical fiber of Example 1 was able to effectively confine the optical signal within the hollow core of the fiber. In particular, the confinement loss of this fiber for the fundamental mode of the optical signal with a wavelength of 1550 nm was 0.46 dB / km.

[0112] Example 2: Modeling of the Confinement Loss of a Hollow-Core Optical Fiber Containing Concentric Capillaries By modeling a hollow-core optical fiber 100 having a plurality of first cladding elements 122 and a plurality of second cladding elements 124, the confinement loss of the hollow-core optical fiber 100 was determined. FIG. 5 shows a cross-section of the model of the hollow-core optical fiber 100 of Example 2. The plurality of first cladding elements 122 were composed of six capillaries. Each capillary of the plurality of first cladding elements 122 had an inner diameter of 20 μm and a wall thickness of 570 nm. The plurality of first cladding elements 122 were arranged symmetrically with respect to the longitudinal central axis of the hollow core 110. The plurality of second cladding elements 124 were composed of six capillaries each having a nested capillary 310. Each capillary of the plurality of second cladding elements 124 had an inner diameter of 27.5 μm and a wall thickness of 520 nm. The plurality of second cladding elements 124 were arranged symmetrically with respect to the longitudinal central axis of the hollow core 110. Each nested capillary 310 had an inner diameter of 12.8 μm and a wall thickness of 520 nm. The plurality of nested cladding elements were arranged symmetrically with respect to the longitudinal central axis of the hollow core 110. The inner diameter of the substrate 130 was 110 μm and the wall thickness of the substrate 130 was 2 μm. The diameter of the hollow core 110 was approximately 34.5 μm. The substrate 130, the plurality of first cladding elements 122, and the plurality of second cladding elements 124 were made of pure silica. As shown in FIG. 5, the structure of the hollow-core optical fiber 100 was 60° rotationally symmetric, that is, six-fold rotationally symmetric.

[0113] Regarding the hollow-core optical fiber of Example 2, the confinement of the optical signal into the hollow core was modeled using the "COMSOL Multiphysics" modeling software. According to this model, the hollow-core optical fiber of Example 2 was able to confine the optical signal very well within the hollow core of the fiber. In particular, the confinement loss of this fiber for the fundamental mode of the optical signal with a wavelength of 1550 nm varied depending on the polarization of the light, but was 0.035 to 0.055 dB / km.

[0114] Example 3: Modeling the confinement loss of a hollow-core optical fiber as a function of wavelength One hollow-core optical fiber (Fiber 1) having a plurality of first clad elements and a plurality of second clad elements, and four hollow-core optical fibers (Fibers 2 to 5) having a plurality of first clad elements, a plurality of second clad elements, and a plurality of third clad elements were modeled, and the confinement loss of the hollow-core optical fiber was analyzed as a function of wavelength. Fig. 6 shows a schematic structure of four hollow-core fibers (Fibers 2 to 5 in Table 1) having a plurality of first clad elements, a plurality of second clad elements, and a plurality of third clad elements. The schematic structure of Fiber 1 is shown in Fig. 4. In each of these fibers, the plurality of first clad elements 122 had six capillaries, the plurality of second clad elements 124 had six capillaries, and the plurality of third clad elements 210 had six capillaries. The plurality of clad elements 122, 124, 210 were arranged symmetrically with respect to the longitudinal central axis of the hollow core 110. The clad elements and the substrate of Fibers 1 to 5 were made of pure silica. The dimensions of these five optical fibers are shown in Table 1.

[0115]

Table 1

[0116] For wavelengths of 1300 nm to 1700 nm, the confinement loss of each fiber was calculated. The results are shown in FIG. 7. Also shown in FIG. 7 is the confinement loss of the hollow-core optical fiber 800 of the comparative example shown in FIG. 8 as a function of wavelength. The hollow-core optical fiber 800 of the comparative example includes six capillaries 822, and nested capillaries 850 are provided one by one in each capillary 822. Each nested capillary 850 is attached to the capillary 822 at a point close to the base material 830. Each capillary 822 has a diameter of 27.5 μm and a wall thickness of 0.5 μm, and each nested capillary 850 has a diameter of 13 μm and a wall thickness of 0.53 μm. The diameter of the hollow core 810 was about 34.5 μm. The inner diameter of the base material 830 was 165 μm and the wall thickness was 2 μm. The base material 830, the capillary 822, and the nested capillary 850 were made of pure silica. As shown in FIG. 8, the structure of the hollow-core optical fiber 800 of the comparative example was 60° rotationally symmetric, that is, six-fold rotationally symmetric. Referring to FIG. 7 again, it was demonstrated from FIG. 7 that by adjusting the wall thickness of the capillary and the number of a plurality of cladding elements, the confinement loss (including the wavelength at which the confinement loss is minimized) of the hollow-core optical fiber can be appropriately adjusted.

[0117] The present disclosure is directed to various embodiments of a hollow-core optical fiber. In an embodiment, the hollow-core optical fiber includes a hollow core extending along a longitudinal central axis of the fiber, a base material, a plurality of first cladding elements, and a plurality of second cladding elements. The plurality of first cladding elements are arranged at intervals between the hollow core and the base material, and the plurality of second cladding elements are arranged at intervals between the hollow core and the plurality of first cladding elements. Such a hollow-core optical fiber can be configured to function to transmit an optical signal, and the cladding element can suppress the attenuation of the optical signal by one or more of an anti-resonance effect and a negative curvature effect.

[0118] It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the subject matter of the claims. Accordingly, it is intended that the modifications and variations to the various embodiments described herein also be included within the scope of this specification, as long as they do not depart from the scope of the appended claims and their equivalents.

[0119] Hereinafter, preferred embodiments of the present invention will be described item by item.

[0120] Embodiment 1 A hollow-core optical fiber, wherein the hollow-core optical fiber has a hollow core extending along the longitudinal central axis of the hollow-core optical fiber, a base material having a tubular shape and an inner surface surrounding the longitudinal central axis of the hollow-core optical fiber, a plurality of first cladding elements arranged at intervals between the hollow core and the base material, each of the plurality of first cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of first cladding elements including a first capillary, the first capillary having an inner surface defining a first cavity, the first cavity having a first diameter and being occupied only by gas, the plurality of first cladding elements, a plurality of second cladding elements arranged at intervals between the hollow core and the base material, each of the plurality of second cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of second cladding elements including a second capillary, the second capillary having an inner surface defining a second cavity, the second cavity having a second diameter different from the first diameter and being occupied only by gas, the plurality of second cladding elements, and none of the plurality of first cladding elements is in direct contact with the inner surface of the base material, a hollow-core optical fiber in which none of the second cladding elements is in direct contact with the inner surface of the base material.

[0121] Embodiment 2 The hollow-core optical fiber according to Embodiment 1, wherein each first cladding element is arranged at a distance from a first cladding element adjacent in the circumferential direction.

[0122] Embodiment 3 The hollow-core optical fiber according to Embodiment 1 or 2, wherein each second capillary is in direct contact with at least one first capillary.

[0123] Embodiment 4 The hollow-core optical fiber according to any one of Embodiments 1 to 3, wherein the second cladding element is arranged at a radial distance from the inner surface of the base material.

[0124] Embodiment 5 The hollow-core optical fiber according to any one of Embodiments 1 to 4, wherein each second cladding element is arranged at a distance from a second cladding element adjacent in the circumferential direction.

[0125] Embodiment 6 The hollow-core optical fiber according to any one of Embodiments 1 to 5, wherein each first capillary is directly connected to two second capillaries.

[0126] Embodiment 7 The hollow-core optical fiber according to any one of Embodiments 1 to 6, wherein the second diameter is larger than the first diameter.

[0127] Embodiment 8 Each first capillary has a first capillary longitudinal central axis parallel to the longitudinal central axis of the hollow-core optical fiber, and the first capillary longitudinal central axis is at a first radial distance from the longitudinal central axis. Each second capillary has a second capillary longitudinal central axis parallel to the longitudinal central axis of the hollow-core optical fiber, and the second capillary longitudinal central axis is at a second radial distance from the longitudinal central axis. The hollow-core optical fiber according to any one of Embodiments 1 to 7, wherein the first radial distance is greater than the second radial distance.

[0128] Embodiment 9 The hollow-core optical fiber according to any one of Embodiments 1 to 8, wherein the inner diameter of the first capillary is 12 μm to 54 μm.

[0129] Embodiment 10 The hollow-core optical fiber according to any one of Embodiments 1 to 9, wherein the wall thickness of the first capillary is 0.1 μm to 4.0 μm.

[0130] Embodiment 11 The hollow-core optical fiber according to any one of Embodiments 1 to 10, wherein the inner diameter of the second capillary is 16 μm to 65 μm.

[0131] Embodiment 12 The hollow-core optical fiber according to any one of Embodiments 1 to 11, wherein the wall thickness of the second capillary is 0.1 μm to 4.0 μm.

[0132] Embodiment 13 The hollow-core optical fiber according to any one of Embodiments 1 to 12, wherein the plurality of first cladding elements and the plurality of second cladding elements are configured to provide an anti-resonance effect capable of confining an optical signal having a wavelength in the range of 350 nm to 8000 nm within the hollow core.

[0133] Embodiment 14 The confinement loss of the hollow-core optical fiber varies with wavelength, and the minimum confinement loss of the fundamental mode of the optical signal propagating in the hollow-core optical fiber within the wavelength range of 350 nm to 8000 nm is 0.50 dB / km or less. The hollow-core optical fiber according to any one of Embodiments 1 to 13.

[0134] Embodiment 15 A plurality of third cladding elements disposed at intervals from each other between the plurality of second cladding elements and the hollow core, each of the plurality of third cladding elements extending in a direction parallel to the longitudinal central axis of the hollow core optical fiber, each of the plurality of third cladding elements including a third capillary, the third capillary having an inner surface defining a third cavity, the third cavity having a third diameter and being occupied only by the gas, the hollow core optical fiber according to any one of Embodiments 1 to 14 further comprising a plurality of third cladding elements.

[0135] Embodiment 16 The hollow core optical fiber according to Embodiment 15, wherein each third capillary is in direct contact with at least one second capillary.

[0136] Embodiment 17 The hollow core optical fiber according to Embodiment 15 or 16, wherein the third diameter is different from the first diameter and the second diameter.

[0137] Embodiment 18 A hollow core optical fiber, the hollow core optical fiber comprising: A hollow core extending along the longitudinal central axis of the hollow core optical fiber; A base material having a tubular shape and an inner surface surrounding the longitudinal central axis of the hollow core optical fiber; A plurality of first cladding elements disposed at intervals from each other between the hollow core and the base material, each of the plurality of first cladding elements extending in a direction parallel to the longitudinal central axis of the hollow core optical fiber, each of the plurality of first cladding elements including a first capillary, the first capillary having an inner surface defining a first cavity, the first cavity having a first diameter, and all of the plurality of first cladding elements being in direct contact with the inner surface of the base material; a plurality of first cladding elements; A plurality of second cladding elements spaced apart from each other and disposed between the hollow core and the substrate, each of the plurality of second cladding elements extending in a direction parallel to the longitudinal central axis of the hollow core optical fiber, each of the plurality of second cladding elements including a second capillary, the second capillary having an inner surface defining a second cavity, the second cavity having a second diameter different from the first diameter, and none of the second cladding elements being in direct contact with the inner surface of the substrate. comprising A hollow core optical fiber, wherein each second capillary includes a nested capillary, and each nested capillary is in direct contact with the inner surface of the second capillary.

[0138] Embodiment 19 The hollow core optical fiber according to Embodiment 18, wherein the inner diameter of the nested capillary is 6 μm to 25 μm.

[0139] Embodiment 20 The hollow core optical fiber according to Embodiment 18 or 19, wherein the wall thickness of the nested capillary is 0.1 μm to 4.0 μm.

Description of Reference Numerals

[0140] 100 Hollow core optical fiber 110 Hollow core 112 Longitudinal central axis 120 Cladding, cladding element 122 First cladding element 124 Second cladding element 126 First capillary longitudinal central axis 128 Second capillary longitudinal central axis 130 Substrate, substrate tube 132 Inner surface of the substrate 140, 144 First capillary 146, 149, 222, 223, 320 Second capillary 152 Inner surface of the first capillary 154 Outer surface of the first capillary 156, 322 Inner surface of the second capillary 158 Outer surface of the second capillary 162 First cavity 164 Second cavity 210 Third cladding element 212, 216 Third capillary 218 Central longitudinal axis of the third capillary 252 Inner surface of the third capillary 254 Outer surface of the third capillary 262 Third cavity 310, 312 Concentric capillary 352 Inner surface of the concentric capillary 354 Outer surface of the concentric capillary 800 Hollow-core optical fiber of the comparative example 810 Hollow core of the hollow-core optical fiber of the comparative example 822 Capillary of the hollow-core optical fiber of the comparative example 830 Substrate of the hollow-core optical fiber of the comparative example 850 Concentric capillary of the hollow-core optical fiber of the comparative example

Claims

**Claim 1** A hollow-core optical fiber, wherein the hollow-core optical fiber has a hollow core extending along a longitudinal central axis of the hollow-core optical fiber, a base material having a tubular shape and an inner surface surrounding the longitudinal central axis of the hollow-core optical fiber, a plurality of first cladding elements spaced apart from each other and disposed between the hollow core and the base material, each of the plurality of first cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of first cladding elements including a first capillary, the first capillary having an inner surface defining a first cavity, the first cavity having a first diameter and being occupied only by gas, and the plurality of first cladding elements, a plurality of second cladding elements spaced apart from each other and disposed between the hollow core and the base material, each of the plurality of second cladding elements extending in a direction parallel to the longitudinal central axis of the hollow-core optical fiber, each of the plurality of second cladding elements including a second capillary, the second capillary having an inner surface defining a second cavity, the second cavity having a second diameter different from the first diameter and being occupied only by gas, and the plurality of second cladding elements, and comprising none of the plurality of first cladding elements directly contacting the inner surface of the base material, A hollow-core optical fiber in which none of the second cladding elements directly contacts the inner surface of the base material. **Claim 2** The hollow-core optical fiber according to claim 1, wherein each first cladding element is disposed spaced apart from a circumferentially adjacent first cladding element. **Claim 3** The hollow-core optical fiber according to claim 1, wherein each second capillary directly contacts at least one first capillary. **Claim 4** The hollow-core optical fiber according to any one of claims 1 to 3, wherein the second cladding element is disposed radially spaced apart from the inner surface of the base material. **Claim 5** The hollow-core optical fiber according to any one of claims 1 to 3, wherein each second cladding element is disposed spaced apart from a circumferentially adjacent second cladding element. **Claim 6** The hollow-core optical fiber according to any one of claims 1 to 3, wherein each first capillary is directly connected to two second capillaries. **Claim 7** Each first capillary has a first capillary longitudinal central axis parallel to the longitudinal central axis of the hollow core optical fiber, and the first capillary longitudinal central axis is at a first radial distance from the longitudinal central axis, Each second capillary has a second capillary longitudinal central axis parallel to the longitudinal central axis of the hollow core optical fiber, and the second capillary longitudinal central axis is at a second radial distance from the longitudinal central axis, The hollow core optical fiber according to any one of claims 1 to 3, wherein the first radial distance is greater than the second radial distance.

8. The hollow core optical fiber according to any one of claims 1 to 3, wherein the plurality of first cladding elements and the plurality of second cladding elements are configured to provide an anti-resonance effect capable of confining an optical signal having a wavelength in the range of 350 nm to 8000 nm within the hollow core.

9. The hollow core optical fiber according to any one of claims 1 to 3, wherein the confinement loss of the hollow core optical fiber varies with wavelength, and the minimum confinement loss of the fundamental mode of the optical signal propagating within the hollow core optical fiber within the wavelength range of 350 nm to 8000 nm is 0.50 dB / km or less.

10. A plurality of third cladding elements disposed between the plurality of second cladding elements and the hollow core at intervals from each other, each of the plurality of third cladding elements extending in a direction parallel to the longitudinal central axis of the hollow core optical fiber, each of the plurality of third cladding elements including a third capillary, the third capillary having an inner surface defining a third cavity, the third cavity having a third diameter and being occupied only by the gas, further comprising a plurality of third cladding elements. The hollow core optical fiber according to any one of claims 1 to 3.

11. A hollow core optical fiber, the hollow core optical fiber comprising: a hollow core extending along the longitudinal central axis of the hollow core optical fiber; a base material having a tubular shape and an inner surface surrounding the longitudinal central axis of the hollow core optical fiber; A plurality of first cladding elements spaced apart from each other and disposed between the hollow core and the substrate, each of the plurality of first cladding elements extending in a direction parallel to the longitudinal central axis of the hollow core optical fiber, each of the plurality of first cladding elements including a first capillary, the first capillary having an inner surface defining a first cavity, the first cavity having a first diameter, and none of the plurality of first cladding elements directly contacting the inner surface of the substrate, and a plurality of first cladding elements; A plurality of second cladding elements spaced apart from each other and disposed between the hollow core and the substrate, each of the plurality of second cladding elements extending in a direction parallel to the longitudinal central axis of the hollow core optical fiber, each of the plurality of second cladding elements including a second capillary, the second capillary having an inner surface defining a second cavity, the second cavity having a second diameter different from the first diameter, and none of the second cladding elements directly contacting the inner surface of the substrate, and a plurality of second cladding elements; comprising; A hollow core optical fiber, wherein each second capillary includes a nested capillary, and each nested capillary directly contacts the inner surface of the second capillary.

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