Anti-resonant hollow-core fiber and corresponding manufacturing process

Abstract

A new hollow-core fiber design is proposed, comprising an outer sheath (1) having an inner surface and an inner sheath region (2) delimited by the inner surface. The inner sheath region comprises a plurality of anti-resonant modules distributed without touching around the longitudinal axis of the hollow-core fiber to define a hollow-core region (3). Each anti-resonant module comprises at least two tubular elements (21, 22) embedded at two distinct levels. The lower-level tubular element is embedded within the preceding upper-level tubular element and arranged in cross-section so as to be tangent at two points of the module in a direction tangential to the hollow-core fiber. Only the first-level tubular element is in contact with the inner surface of the outer sheath. Figure 2

Description

Title of the invention: Anti-resonant hollow-core fiber and corresponding manufacturing process technical field

[0001] The invention relates to hollow-core optical fibers, in particular anti-resonant optical fibers. More specifically, the invention relates to a new design of hollow-core optical fiber with improved optical properties.

[0002] The invention is particularly, but not exclusively, suited to single-mode hollow-core optical fibers operating at telecom wavelengths.

[0003] Hollow core fibers have many potential applications, such as long-distance data transmission, high-power beam guidance and routing, gas-based nonlinear optics, sensing, and various laser applications. Previous art

[0004] Hollow-core optical fibers show promise for guiding light with very low optical attenuation and reduced latency, because the light is guided through air, or a gas, instead of through a solid material. Depending on the physical mechanism of light guidance, there are two main types of hollow-core fibers: "photonic bandgap fibers" and "anti-resonance fibers".

[0005] As indicated in the publication by Francesco Poletti "Nested antiresonant nodeless hollow core fiber" Optics Express Vol. 22, Issue 20, pp. 23807-23828 (2014), antiresonant hollow core fibers can achieve lower attenuation over a wider transmission spectrum than photon bandgap fibers.

[0006] Traditionally, an anti-resonant hollow-core fiber comprises an outer sheath, which defines an inner sheath surface, and an inner sheath region delimited by the inner surface. The inner sheath region comprises several anti-resonant elements arranged, without touching, around the longitudinal axis of the fiber to define a hollow core region. These anti-resonant elements extend along the longitudinal axis of the fiber, their walls being in contact with the inner sheath surface. Patent documents EP3152607B1 and EP3766843A1 disclose well-known examples of nested anti-resonant tubular element configurations.

[0007] These anti-resonant elements (which are uniformly distributed around the hollow core region) act as a resonant cavity, reflecting the incident light and guiding it through the fiber core by making the coupling of the core modes and an anti-resonant cladding. Such inhibited coupling between the core and cladding modes results in low attenuation, a wide transmission spectrum, and low latency in data transmission. However, even with the use of anti-resonant structures, current fiber designs cannot overcome certain limitations: two particular drawbacks of conventional anti-resonant hollow fibers are the elusive nature of the core modes, which makes it difficult to design a low-loss, bend-insensitive, single-mode anti-resonant hollow fiber, and the limitations of its fabrication, especially the fabrication of anti-resonant elements at specific azimuthal locations in a sufficiently stable and reproducible manner.

[0008] With the progressive development of telecommunications and the increasing requirements for high-capacity data transmission applications, there is therefore a real need to provide an improved hollow core fiber structure and, in particular, a new hollow core fiber design that exhibits ultra-efficient single-mode operation and low bend sensitivity over a wide range of bend radii, in order to further improve optical transmission. Objectives of the invention

[0009] The invention, in at least one embodiment, aims in particular to overcome the drawbacks of the prior art. More specifically, at least one objective of at least one embodiment of the invention is to provide a hollow-core fiber that exhibits:

[0010] - an attenuation of the fundamental mode (FM) less than IdB / km at lengths of telecom waves, in particular at the wavelength of 1550 nm;

[0011] - a macro-curvature weakening in fundamental mode (FM) less than 0.1 dB per 100 turns at a radius of curvature of 30 mm at telecommunication wavelengths, in particular at the wavelength 1550 nm;

[0012] - a macro-curvature weakening of the lower fundamental mode (FM) at 1 dB / turn for a bending radius of 10 mm at telecommunications wavelengths, in particular at the wavelength 1550 nm;

[0013] - a higher mode extinction factor (HOM) greater than 50, or even greater than 100 at telecom wavelengths, in particular at the 1550 nm wavelength. Summary of the invention

[0014] According to a first aspect, the invention relates to a hollow core fiber comprising an outer sheath having an inner surface and an inner sheath region delimited by the inner surface, the inner sheath region comprising a plurality of anti-resonant modules distributed without touching each other around the longitudinal axis of the hollow core fiber to define a hollow core region. The hollow core fiber is characterized by the fact that the anti-resonance modules each comprise at least two tubular elements embedded on at least two distinct levels, the lower level tubular element being embedded in the preceding upper level tubular element and arranged in cross section so as to be tangent at two points of said module in a tangential direction of the hollow core fiber, only the first level tubular element being in contact with the inner surface of the outer cladding.

[0015] Thus, the invention proposes a hollow-core fiber design based on a novel configuration of multi-level integrated tubular elements arranged around the central core region to improve the confinement of the core fundamental mode while reducing the guidance of HOMs. Unlike prior art hollow-core fibers, the anti-resonant modules have a multi-level configuration such that the embedded elements only touch at two points of module contact in the tangential direction of the fiber, thereby creating a greater number of anti-resonant reflective surfaces oriented to further reduce confinement losses.Indeed, unlike prior art hollow core fiber designs which provide only one reflective surface per anti-resonant module (contact point on the inner surface of the cladding), the embedding structure proposed by the present invention allows the fiber to present at least two additional reflective surfaces per anti-resonant module without requiring additional embedded elements, which makes the inhibited coupling between the core and cladding modes much more efficient.

[0016] According to the invention, at least one lower-level tubular element has a cross-sectional geometry different from that of the preceding upper-level tubular element, the cross-sectional geometry belonging to the group comprising an elliptical geometry, an oval geometry and a circular geometry.

[0017] Thus, different forms of cross-sections can be envisaged, offering a wide range of design possibilities.

[0018] According to a first approach, the anti-resonant modules are each made up of first-level and second-level embedded tubular elements, the second-level tubular element being embedded in the first-level tubular element, which has an elliptical cross-section geometry, an oval cross-section geometry or a circular cross-section geometry.

[0019] This first approach is therefore based on a simple fixed structure.

[0020] According to a first approach, the anti-resonant modules are each made up of of first, second and third level embedded tubular elements, the third level tubular element being embedded in the tubular element of

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[0033] second level which is itself embedded in the tubular element of the first level, which has an elliptical cross-section geometry or an oval cross-section geometry. This second approach is therefore based on a double-fixed structure. According to a first embodiment conforming to the first approach, the first and second level embedded tubular elements are defined as follows: - the first-level tubular element has an elliptical cross-section geometry defined by an ellipticity el, a principal axis radius Rtubel arranged in a radial direction of the hollow core fiber and a secondary axis radius Rtubel2 arranged in a tangential direction of the hollow core fiber, such that: Rtube 1 = (2 x sin(n / N) x Rcore - g) / (2 x ( 1 - sin(n / N))) Rtube2 = Rtube x ^(1-el 2 ) Or : N is an integer, with N > 2, corresponding to the number of anti-resonant modules included in the inner sheath region, Rcore is the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements. 3 is the azimuthal distance between two adjacent first-level tubular elements, - the second-level tubular element has a circular cross-section defined by a first radius Rtube3 arranged in a radial direction of the hollow core fiber and a second radius Rtube4 arranged in a tangential direction of the hollow core fiber, such that: Rtube3 = Rtube4 = Rtubel x ^1-el 2 ) The structural parameters are chosen to optimize low-loss single-mode operation. The inventors determined that with such an anti-resonant element design, the resulting hollow-core fiber exhibits a significant reduction in losses compared to traditional hollow-core fiber designs. According to a second embodiment conforming to the first approach, said first-level and second-level embedded tubular elements are defined as follows: - the first-level tubular element has a circular cross-section geometry, a first radius Rtube2' arranged in a radial direction of the hollow core fiber and a second radius Rtube2' arranged in a tangential direction of the hollow core fiber, such that:

[0034] Rtube2' = Rtubel' = (2 x sin(n / N') x Rcore'- g') / (2 x (1 - sin(n / N'))) - the second-level tubular element has an elliptical cross-section geometry defined by an ellipticity ©2, a main axis radius Rtube3' arranged in a tangential direction to the hollow core fiber and a secondary axis radius Rtube4' arranged in a radial direction to the hollow core fiber, such that:

[0035] Rtube3' = Rtubel' = (2 x sin(n / N') x Rcore' - g') / (2 x (1 - sin(n / N')))

[0036] Rtube4' = Rtube3' x ^(l-e2 2 )

[0037] where:

[0038] N' is an integer, with N' > 2, corresponding to the number of anti-resonant modules included in the inner sheath region,

[0039] Rcore' is the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements,

[0040] g' is the azimuthal distance between two adjacent first-level tubular elements.

[0041] The structural parameters are chosen to optimize low-loss single-mode operation. This second embodiment is particularly advantageous in that it allows the first-level anti-resonant surface to reflect the fundamental core mode, which has a circular shape. This results in an even more effective anti-resonance effect compared to traditional hollow-core fiber designs.

[0042] According to a third embodiment conforming to the second approach, said first, second and third level embedded tubular elements are defined as follows:

[0043] - the first-level tubular element has a cross-sectional geometry elliptic defined by an ellipticity el", a first ray with principal axis Rtube2' arranged in a radial direction of the hollow core fiber and a first ray with secondary axis Rtube2' arranged in a tangential direction of the hollow core fiber, such that:

[0044] Rtubel" = (2x sin(n / N")x Rcore"-g) / (2x(1 -sin(n / N")))

[0045] Rtube2" = Rtube" x ^(1-el" 2 )

[0046] where:

[0047] N” is an integer, with N > 2, corresponding to the number of tubular elements included in the inner sheath region,

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[0057] "Rcore" is the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements, g" is the azimuthal distance between two adjacent first-level tubular elements, - the second-level tubular element has a circular cross-section defined by a first radius Rtube3' arranged in a radial direction of the hollow core fiber and a second radius Rtube4" arranged in a tangential direction of the hollow core fiber, such that: Rtube4" = Rtube3" = Rtubel" x ^(1-el" 2 ) - the third-level tubular element has an elliptical cross-section geometry defined by an ellipticity e3, a second radius with secondary axis Rtube5 arranged in a radial direction of the hollow core fiber and a second radius with principal axis Rtube6 arranged in a tangential direction of the hollow core fiber, such that: Rtube6 = Rtube" x ^(1-el" 2 ) Rtube5 = Rtube6 x ^(l-e3 2 ) The structural parameters are chosen to optimize low-loss single-mode operation. The inventors determined that with such an anti-resonant element design, the resulting hollow-core fiber exhibits a significant reduction in losses compared to traditional hollow-core fiber designs. According to a fourth embodiment conforming to the second approach, the embedded tubular elements of the first, second and third levels are defined as follows: - the first-level tubular element has a circular cross-section geometry, a first radius Rtube"1 arranged in a radial direction of the fiber and a second radius Rtube2"' arranged in a tangential direction of the fiber, such that: Rtube2"' = Rtubel"1 = (2 x sin(n / N'") x Rcore1"- g1") / (2 x (1- sin(n / N"j)) - the second-level tubular element has an elliptical cross-section geometry defined by an ellipticity e2"\ a main axis radius Rtube3"" arranged in a tangential direction of the fiber and a secondary axis radius Rtube4"' arranged in a radial direction of the hollow-core fiber, such: Rtube3"'= Rtubel'" = (2x sin(n / N"jx Rcore"'-g"j / (2x(l-sin(n / N"j))

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[0071] Rtube4'" = Rtube3'" x ) - the third-level tubular element has an elliptical cross-section geometry defined by an ellipticity e3'", a principal axis radius Rtube5'' arranged in a tangential direction to the fiber and a secondary axis radius Rtube6'' arranged in a radial direction to the hollow core fiber, such that: Rt.ube5"' = Rtubel'" = (2x sin(n / N"jx Rcore"'-g"j / (2 x (1- sin(n / N"'))) Rtube6'" = Rtube5'" x ^l-e3"' 2 ) Or : N'” >2, Rcore"1, the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements around the hollow core region; g'", the smallest azimuthal distance between two adjacent first-level tubular elements. Advantageously, said at least two embedded tubular elements have a thickness between [0.3 pm - 1.2 pm], an azimuthal distance-thickness ratio between [2 - 13] and an ellipticity between [0.30 - 0.95]. The geometric configuration selected according to one of the embodiments described above, combined with these ranges of structural parameters, guarantees the hollow core fiber a single-mode transmission with very low loss. According to a second aspect, the invention relates to an optical transmission system characterized in that it comprises at least one hollow core fiber as described above according to the first aspect of the invention in any of its embodiments. According to a third aspect, the invention relates to a method for manufacturing a hollow-core optical fiber as described above according to the first aspect of the invention in any of its embodiments, the method comprising the following steps: - provide several tubes of anti-resonant modules and one hollow sheath tube; - fix the anti-resonance module tubes on an inner surface of the sheath tube in a distributed manner; - stretch the entire set of tubes including the sheath tube and the anti-resonance module tubes until the outer sheath and the anti-resonance modules meet predefined dimensions. Figures

[0072] Other features and advantages of the invention will become more apparent upon reading the following description, given solely by way of illustration and not limitation, and the accompanying drawings, among which:

[0073] [Fig-1] is a schematic cross-sectional view of a core optical fiber hollow according to a first embodiment of the invention;

[0074] [Fig.2] is a schematic cross-sectional view of a core optical fiber hollow according to a second embodiment of the invention;

[0075] [Fig.3] is a schematic cross-sectional view of a core optical fiber hollow according to a third embodiment of the invention;

[0076] [Fig.4] is a schematic cross-sectional view of a core optical fiber hollow according to a fourth embodiment of the invention;

[0077] [Fig. 5] is a schematic diagram of an optical transmission system equipped with a hollow core fiber according to the invention;

[0078] [Fig.6] is a flowchart of a particular embodiment of the process of manufacturing according to the invention;

[0079] [Fig.7] and [Fig.8] graphically represent the loss of confinement of the mode fundamental (FM) and the higher optical mode extinction factor (HOM) of a hollow core fiber conforming to the first embodiment, as a function of wavelength;

[0080] [Fig.9] graphically represents the loss of confinement of the fundamental mode (FM) as a function of wavelength for different core radius values ​​of hollow core fibers conforming to the first embodiment;

[0081] [Fig. 10] and [Fig. 11] graphically represent the fundamental mode (FM) confinement loss and the higher optical mode (HOM) extinction factor of a hollow core fiber conforming to the second embodiment, as a function of wavelength;

[0082] [Fig. 12] graphically represents the fundamental mode (FM) confinement loss as a function of the radii of curvature of a hollow-core fiber conforming to the second embodiment

[0083] [Fig. 13] graphically represents the fundamental mode (FM) confinement loss as a function of wavelength for different values ​​of core radius of hollow core fibers conforming to the second embodiment;

[0084] [Fig. 14] and [Fig. 15] graphically represent the fundamental mode (FM) confinement loss and the higher optical mode (HOM) extinction factor of a hollow core fiber conforming to the third embodiment, as a function of wavelength. Detailed description of the invention

[0085] In the figures in this document, identical elements are designated by the same numerical reference. The elements in the figures are not necessarily to scale, the emphasis being on illustrating the principle of the invention. To facilitate reading the figures, the different planes and axes are represented by dashed lines.

[0086] The general principle of the present invention is based on an improved structure for a low-loss hollow core fiber, based on a redesigned embedding configuration of the anti-resonant cladding elements, in order to improve the single-mode optical transmission capacity.

[0087] In what follows, the principle of the invention is presented according to three specific embodiments based on an elliptical-circular embedded hollow core fiber particularly well suited to low loss single-mode transmission: the first and second embodiments are based on single-embedded element configurations in relation to Figures 1 and 2, while the third and fourth embodiments are based on double-embedded element configurations in relation to [Fig.3] and [Fig.4].

[0088] Figure 1 shows an anti-resonant hollow-core optical fiber HCF1 according to a first embodiment of the invention. The hollow-core optical fiber HCF1 has the general shape of a waveguide extending along a longitudinal axis L which corresponds to the propagation axis of the optical signal. The HCF1 hollow core fiber comprises an outer sheath 1 extending along a longitudinal axis and having a tubular inner surface defining an internal volume, a central hollow core region 3 and an inner sheath region 2 arranged around the central hollow core region 3, inside the internal volume and extending along the longitudinal axis L. The inner sheath region 2, which is delimited by the inner surface of the outer sheath, comprises a set of six anti-resonant 10a-lOf reflective modules distributed without touching around the longitudinal axis L of the hollow core fiber to define the hollow core region 3 of a radius 'Rcore'.As illustrated in the figure, the anti-resonant 10a-lOf modules are regularly distributed around the hollow core region 3 at azimuthal positions defined according to a uniform distance (hereafter referred to as the azimuthal distance "g"), forming a sextuple symmetry.

[0089] The anti-resonant modules 10a-lOf of the HCF1 fiber each comprise two anti-resonant tubular elements embedded on two distinct levels: a second-level tubular element 12, having a circular cross-sectional geometry, is integrated into a first-level tubular element 11 having an elliptical cross-sectional geometry and which is arranged in cross-section so as to be tangent at two points of the module in a tangential direction T of the HCF1 fiber. These two points (or "nodes") are referenced PI and P2 for the anti-resonant module 10a (illustrated here by way of example). The tubular elements of first and second level 11-12 have substantially equal and uniform wall thicknesses t, only the first level tubular element 11 being in contact with the inner surface of the outer sheath 2. The first level tubular element 11 is characterized by a major axis disposed along the radial direction R of the fiber and by a minor axis disposed along the tangential direction T of the fiber, the two directions intersecting at the center O of the module 10a (which is also the center of the tubular elements 11 and 12).

[0090] The hollow core fiber illustrated in [Fig. 1] contains, by way of example, six anti-resonant modules, but the first embodiment is not limited to this specific example and a larger or smaller number of anti-resonant modules may be provided without departing from the scope of the invention.

[0091] More generally, considering a number N of anti-resonant modules included in the internal sheath region, the structural parameters of an embedded structure according to the first embodiment are defined as follows (for a given anti-resonant module):

[0092] - the first-level tubular element has a cross-sectional geometry elliptic defined by an ellipticity el, a principal axis radius Rtubel arranged in the radial direction R of the fiber and a secondary axis radius arranged in the tangential direction T of the fiber, such that:

[0093] Rtubel = (2 x sin(n / N) x Rcore-g) / (2 x (1 - sin(n / N)))

[0094] j^ tu j 3e 2 = Rtubel x ^1-el 2 )

[0095] with:

[0096] N > 2;

[0097] Rcore, the radius of the hollow core region is defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements around the hollow core region;

[0098] 3, the smallest azimuthal distance between two adjacent tubular elements of first level; - the second-level tubular element has a circular cross-section geometry defined by a first radius Rtube3 arranged in the radial direction R and a second radius Rtube4 arranged in the tangential direction T, such that:

[0099] _ Rt u b e 4 = Rtube 1 x ^1-el 2 )

[0100] - the first and second level tubular elements are tangent in two points of the module in said tangential direction T of the fiber.

[0101] In other words, Rtube2 is half the longest distance between the outer edges of the first-level tubular element in the radial direction R of the fiber, Rtube2 is half the longest distance between the outer edges of the first-level tubular element in the tangential direction T of the fiber, Rtube3 is half the longest distance between the outer edges of the second-level tubular element in the radial direction R of the fiber, and Rtube4 is half the longest distance between the outer edges of the second-level tubular element in the tangential direction T of the fiber.

[0102] Examples of structural parameters of embedded element modules are presented below:

[0103] - a wall thickness value t of the first and second tubular elements levels within the range 0.3 - 1.2 pm, and more particularly within the range 0.3 - 0.8 pm, and even more particularly within the range 0.4 - 0.5 pm;

[0104] - a value of the azimuthal distance-thickness ratio 9 / t within the range 2 - 13, more particularly in the range 5-10, and even more particularly in the range 6-9;

[0105] - an ellipticity value el in the range 0.30 - 0.95, and more particularly in the range 0.40 - 0.90, and even more particularly in the range 0.50 - 0.80, for the elliptical tubular element having an elliptical shape.

[0106] The HCF1 hollow core fibre has an Rcore value between 15 and 25 pm, and more particularly between 17 and 20 pm.

[0107] An example of a hollow-core fiber conforming to the first embodiment and having the following structural characteristics exhibits efficient single-mode operation with low confinement losses, particularly at a wavelength of 1550 nm: Rcore = 20 pm, N = 6, t = 0.47 pm, g / t = 3, el = 0.635, e2 = 0, ncore = 1, λ-claddinq = 1 > 45. Figures 7 and 8 illustrate the FM confinement losses (expressed in dB / m) and the HOM extinction factor for this example of a hollow-core fiber as a function of wavelength (expressed in pm). The HOM extension factor is the ratio of the HOM attenuation to the FM attenuation. As these graphs show, this example of HCF1 fiber exhibits significantly reduced confinement losses at 1550 nm, with FM attenuation less than about 1 dB / km and an HOM extension factor greater than 100.

[0108] The graph in Figure 9 represents the FM confinement losses (expressed in dB / m) as a function of wavelength (expressed in pm) for different core radius values ​​(reference curves A, B, C) of hollow core fibers conforming to the first embodiment having the following structural characteristics: N = 6, t = 0.45 pm, g / t = 3, el = 0.225, e2 = 0, ncore = 1, nciadding = 1.45. This graph This demonstrates the impact of the core region radius, delimited by the ring of anti-resonant modules, on FM confinement loss. Three core radii were tested: 15 pm (curve A), 18 pm (curve B), and 20 pm (curve C). The inventors found that the fiber with a core radius of 20 pm offers the best compromise between confinement loss and the HOM extinction factor at 1550 nm.

[0109] Figure 2 represents an anti-resonant hollow-core optical fiber HCF2 according to A second embodiment of the invention. The HCF2 hollow-core fiber differs from the HCF1 hollow-core fiber in that the inner cladding region 2 comprises a set of six anti-resonant reflecting modules 20a-20f, each consisting of two tubular elements embedded at two distinct levels: a second-level tubular element 22 with an elliptical cross-sectional geometry, which is embedded within a first-level tubular element 21 with a circular cross-sectional geometry. The first- and second-level tubular elements 21-22 are arranged in cross-section so as to be tangent to two points (nodes) Pl'-P2' of the anti-resonant module in the tangential direction T of the HCF2 fiber. The first- and second-level tubular elements 21-22 have substantially equal and uniform wall thicknesses t', and only the first-level tubular element 21 is in contact with the inner surface of the outer cladding 2.The second-level tubular element 22 is characterized by a secondary axis disposed along the radial direction R of the fiber and by a main axis disposed along the tangential direction T of the fiber, the two directions intersecting at the center O of the module 20a.

[0110] Thus, as with the HCF1 fiber, the HCF2 fiber comprises a set of six anti-resonant modules uniformly distributed without touching each other on the inner circumference of the outer sheath according to a sextuple symmetry, each consisting of two anti-resonant tubular elements embedded on two distinct levels (i.e. according to a single-embedded configuration) but differs from the HCF1 fiber in that, for each module, the elliptical tubular element 22 is embedded inside the circular element 21.

[0111] This second embodiment is particularly advantageous because the configuration of the anti-resonant elements 21-22 allows for increased negative curvature at the boundary of the core region as well as an anti-resonant multi-reflection configuration, all without compromising the core size. This further increases the confinement of the fundamental mode while promoting the leakage of higher modes, since the core mode experiences stronger inhibited coupling than with the sheath modes residing in the inner region of the sheath.

[0112] The hollow-core fiber illustrated in [Fig. 2] contains six anti-resonant modules by way of example, but the second embodiment is not limited to this example specific and a larger or smaller number of anti-resonant modules can be provided without going outside the scope of the invention.

[0113] More generally, considering a number N' of anti-resonant modules included in the internal sheath region, the structural parameters of an embedded structure according to the second embodiment are defined as follows (for a given anti-resonant module):

[0114] - the first-level tubular element has a cross-sectional geometry circular, a first radius Rtube2' arranged in the radial direction R of the fiber and a second radius Rtube2' arranged in the tangential direction T of the fiber, such that:

[0115] Rtube2' = Rtubel' = (2 x sin(n / N') x Rcore' - g') / (2 x (1 - sin(n / N'))) - the second-level tubular element has an elliptical cross-section geometry defined by an ellipticity e2', a main axis radius Rtube3' arranged in the tangential direction T of the fiber and a secondary axis radius Rtube4' arranged in the radial direction R of the hollow-core fiber, such that:

[0116] Rtube3' = Rtubel1 = (2 x sin(n / N') x Rcore'- g') / (2 x (1 - sin(n / N')))

[0117] Rtube4' = Rtube3' x ^(1 - e2 2 )

[0118] where:

[0119] N'>2

[0120] Rcore', the radius of the hollow core region is defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements around the hollow core region;

[0121] g', the smallest azimuthal distance between two adjacent tubular elements of first level.

[0122] In other words, for a given anti-resonant modulus, Rtube2' is half the longest distance between the outer edges of the first-level tubular element in the radial direction R of the fiber, Rtube2' is half the longest distance between the outer edges of the first-level tubular element in the tangential direction T of the fiber, Rtube3' is half the longest distance between the outer edges of the second-level tubular element in the radial direction R of the fiber, Rtube4' is half the longest distance between the outer edges of the second-level tubular element in the tangential direction T of the fiber.

[0123] Examples of structural parameters of integrated element modules are presented below:

[0124] - a wall thickness value t' of the first and second tubular elements levels within the range 0.3 - 1.2 pm, and more particularly within the range 0.3 - 0.8 pm, and even more particularly within the range 0.4 - 0.5 pm;

[0125] - a value of the azimuthal distance-thickness ratio çj' / t' within the range 2 - 13, and more particularly in the range 5 - 10, and even more particularly in the range 6 - 9;

[0126] - an ellipticity value in the range of 0.30 - 0.95, more particularly in the range 0.40 - 0.90, and even more particularly in the range 0.50 - 0.80.

[0127] The HCF2 hollow core fibre has an Rcore' value between 15 and 25 pm, and more particularly between 17 and 20 pm.

[0128] An example of a hollow-core fiber conforming to the second embodiment and having the following structural characteristics exhibits efficient single-mode operation with low confinement losses, particularly at a wavelength of 1550 nm: Rcore' = 17 pm, N' = 5, t = 0.47 pm, g / t' = 12, e2' = 0.87, el' = 0, ncore = 1, Ilcladding = 1.45. Figures 10 and 11 illustrate the FM confinement losses (expressed in dB / m) and the HOM extinction factor of this example of a hollow-core fiber as a function of wavelength (expressed in pm). The HOM extension factor is the ratio of the HOM attenuation to the FM attenuation. As these graphs show, this fiber example exhibits a significant reduction in confinement losses at 1550 nm, with FM attenuation between 0.1 and 1 dB / km and an HOM extension factor greater than 100. The graph in [Fig.

[12] represents the FM bending loss (in dB / turn) obtained for different bending radii for this example of a hollow core fiber at a wavelength of 1550 nm. For a bending radius of 10 mm, the FM macro-bending loss obtained for this fiber is 0.076 dB / turn.

[0129] The graph in Figure 13 represents the FM confinement loss (expressed in dB / m) as a function of wavelength (expressed in pm) for different core radii (reference curves D, E, F) of hollow-core fibers conforming to the second embodiment having the following structural characteristics: N' = 5, t' = 0.47 pm, g' / r = 12, e2' = 0.87, el' = 0, ncore = 1, λcladding = 1.45. This graph shows the impact of the radius of the hollow-core region, delimited by the ring of anti-resonant modules, on the FM confinement losses. Three core radii were tested: 17 pm (curve D), 16 pm (curve E), and 15 pm (curve F). The inventors found that the fiber having an Rcore' of 17 pm corresponds to the best compromise between confinement losses and the HOM extinction factor at 1550 nm.

[0130] Figure 3 represents an anti-resonant hollow-core optical fiber HCF3 according to a third embodiment of the invention. The HCF3 hollow-core fiber differs from the HCF1 and HCF2 fibers in that the tubular elements of which the anti-resonant modules are made are arranged in a double-embedded element configuration.

[0131] The inner cladding region 2 of the HCF3 fiber comprises a set of six anti-resonant reflecting modules 30a-30f, each consisting of three tubular elements embedded at three distinct levels: the third-level tubular element 33, which has an elliptical cross-sectional geometry, is embedded in the preceding second-level tubular element 32, which has a circular cross-sectional geometry, itself embedded in the preceding first-level tubular element 31, which has an elliptical cross-sectional geometry. The first, second, and third-level tubular elements 31-32-33 are arranged in cross-section so as to be tangent at two points (nodes) P1”-P2” of the anti-resonant module in the tangential direction T of the HCF3 fiber. The first, second, and third-level tubular elements 31-32-33 have substantially equal and uniform wall thicknesses t”.

[0132] The first tubular element 31 is characterized by a main axis disposed along the radial direction R of the fiber and by a secondary axis disposed along the tangential direction T of the fiber, the two directions intersecting at the center O of the module 30a. The third tubular element 33 is characterized by a secondary axis disposed along the radial direction R of the fiber and by a main axis disposed along the tangential direction T of the fiber.

[0133] Thus, as with HCF1 and HCF2 fibers, the hollow core fiber according to this third embodiment comprises a set of six anti-resonant modules uniformly distributed without touching each other on the surface of the inner sheath according to a sextuple symmetry but differs in that each of the anti-resonant modules comprises three tubular elements embedded on three distinct levels.

[0134] The hollow core fiber illustrated in [Fig.3] contains six anti-resonant modules by way of example, but the third embodiment is not limited to this specific example and a larger or smaller number of anti-resonant modules may be provided without departing from the scope of the invention.

[0135] More generally, considering a number N” of anti-resonant modules included in the internal sheath region, the structural parameters of an embedded structure according to the third embodiment are defined as follows (for a given anti-resonant module):

[0136] - the first-level tubular element has a cross-sectional geometry elliptic defined by an ellipticity el", a first radius of principal axis Rtubel" arranged in the radial direction of the hollow core fiber and a first secondary axis radius Rtube2" arranged in said tangential direction of the hollow core fiber, such that:

[0137] Rtubel" = (2 x sin(n / N") x Rcore" - g") / (2 x (1 - sin(n / N")))

[0138] Rtube2" = Rtube" x ^(l-el" 2 )

[0139] where:

[0140] N” >2,

[0141] Rcore", the radius of the hollow core region is defined as the smallest distance between the center of the hollow core fiber and the outer edge of the tubular elements of first level around the hollow heart region;

[0142] g", the smallest azimuthal distance between two adjacent tubular elements of first level. - the second-level tubular element has a circular cross-section geometry defined by a first radius Rtube3" arranged in the radial direction of the hollow core fiber and a second radius Rtube4" arranged in said tangential direction of the hollow core fiber, such that:

[0143] Rtube4" = Rtube3" = Rtube" x - the third-level tubular element has an elliptical cross-section geometry defined by an ellipticity e3, a second radius with secondary axis Rtube5 arranged in the radial direction of the hollow core fiber and a second radius with principal axis Rtube6 arranged in said tangential direction of the hollow core fiber, such that:

[0144] Rtube6 = Rtube" x ^(l-el" 2 ) ^ 0145 ] Rtube5 = Rtube6 x ^(l-e3 2 )

[0146] In other words, for a given anti-resonant modulus, Rtube1" is half the longest distance between the outer edges of the first-level tubular element in the radial direction R of the fiber; Rtube2" is half the longest distance between the outer edges of the first-level tubular element in the tangential direction T of the fiber; Rtube3" is half the longest distance between the outer edges of the second-level tubular element in the radial direction R of the fiber; Rtube4" is half the longest distance between the outer edges of the second-level tubular element in the tangential direction T of the fiber; Rtube5 is half the longest distance between the outer edges of the third-level tubular element in the radial direction R of the fiber; Rtube6 is half the longest distance between the outer edges of the third-level tubular element in the tangential direction T of the hollow core fiber.

[0147] Examples of structural parameters of embedded element modules are presented below:

[0148] - a wall thickness value t” of the first and second tubular elements levels within the range 0.3 - 1.2 pm, and more particularly within the range 0.3 - 0.8 pm, and even more particularly within the range 0.4 - 0.5 pm;

[0149] - a value of the azimuthal distance-thickness ratio g" / t" within the range 2 - 13, and more particularly in the range 5 - 10, and even more particularly in the range 6 - 9;

[0150] - an ellipticity value in the range 0.30 - 0.95, more particularly in the range 0.40 - 0.90, and even more particularly in the range 0.50 - 0.80.

[0151] The HCF3 hollow core fibre has an Rcore value " between 15 and 25 pm, and more particularly between 17 and 20 pm.

[0152] An example of a hollow-core fiber conforming to the third embodiment and having the following structural characteristics exhibits efficient single-mode operation with low confinement losses, particularly at a wavelength of 1550 nm: Rcore = 20 pm, N = 6, t = 0.47 pm, g' / t = 3, e1 = 0.635, e2 = 0, e3 = 0.800, ncore = 1, mciaading = 1 / 45. Figures 14 and 15 illustrate the FM confinement losses (expressed in dB / m) and the HOM extinction factor of this example of a core fiber as a function of wavelength (expressed in pm). The HOM extension factor is the ratio of the HOM attenuation to the FM attenuation. As these graphs show, this fiber example exhibits a significant reduction in confinement losses at 1550 nm, with an FM attenuation close to 0.1 dB / km and an HOM extension factor greater than 100.

[0153] Figure 4 represents an anti-resonant hollow-core optical fiber HCF4 according to a fourth embodiment of the invention. As with the HCF3 fiber, this HCF4 hollow-core fiber comprises tubular elements arranged in a double-embedded configuration.

[0154] The inner cladding region 2 of the HCF4 fiber comprises a set of six anti-resonant reflecting modules 40a-40f, each consisting of three tubular elements embedded on three distinct levels: the third-level tubular element 43, which has an elliptical cross-sectional geometry, is embedded inside the preceding second-level tubular element 42, which has an elliptical cross-sectional geometry, itself embedded inside the preceding first-level tubular element 41, which has a cross-sectional geometry circular. The first, second, and third level tubular elements 41-42-43 are arranged in cross-section so as to be tangent to two points (nodes) Pl'”-P2'” of the anti-resonant module in the tangential direction T of the HCF4 fiber. The first, second, and third level tubular elements 41-42-43 have substantially equal and uniform wall thicknesses t'”.

[0155] The second and third tubular elements 42-43 are both characterized by a main axis disposed along the tangential direction T of the fiber and by a secondary axis disposed along the radial direction R of the fiber, the two directions intersecting at the center O of the module 40a. As with the HCF1 and HCF2 fibers, this hollow core fiber HCF4 comprises a set of six anti-resonant modules uniformly distributed without touching each other on the surface of the inner cladding in a sextuple symmetry, but differs in that each of the anti-resonant modules comprises three tubular elements embedded on three distinct levels.

[0156] The hollow core fiber illustrated in [Fig.4] contains six anti-resonant modules by way of example, but the fourth embodiment is not limited to this specific example and a larger or smaller number of anti-resonant modules may be provided without departing from the scope of the invention.

[0157] More generally, considering a number N'” of anti-resonant modules included in the internal sheath region, the structural parameters of a structure embedded according to the fourth embodiment are defined as follows (for a given anti-resonant module):

[0158] - the first-level tubular element has a cross-sectional geometry circular, a first radius Rtube"1 arranged in the radial direction R of the fiber and a second radius Rtube2" arranged in the tangential direction T of the fiber, such that:

[0159] Rtube2'" = Rtubel'" = (2 x sin(n / N1") x Rcore"'- g'") / (2 X (1 - sin(n / N"1))) - the second-level tubular element has an elliptical cross-section geometry defined by an ellipticity e2"\ a principal axis radius Rtube3"' arranged in the tangential direction T of the fiber and a secondary axis radius Rtube4"' arranged in the radial direction R of the hollow-core fiber, such that:

[0160] Rtube3"'= Rtubel"' = (2x sin(n / N"')xRcore'"-g"') / (2x(l-sin(n / N"')))

[0161] Rtube4"' = Rtube3'" x ^l-e2'" 2 ) - the third-level tubular element has an elliptical cross-sectional geometry defined by an ellipticity e3'", a principal axis radius Rtube5'" arranged in the tangential direction T of the fiber and a secondary axis radius Rtube6"' arranged in the radial direction R of the hollow core fiber, such that:

[0162] Rtube5"' = Rtubel"' = (2 x sin[n / N'")

[0163] Rtube6'" = Rtube5"' x - e3'" 2 )

[0164] where:

[0165] N'” >2,

[0166] Rcore"1, the radius of the hollow core region is defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements around the hollow core region;

[0167] g1", the smallest azimuthal distance between two adjacent tubular elements of first level.

[0168] Examples of structural parameters of embedded element modules are presented below:

[0169] - a wall thickness value t”' of the first and second tubular elements levels within the range 0.3 - 1.2 pm, and more particularly within the range 0.3 - 0.8 pm, and even more particularly within the range 0.4 - 0.5 pm;

[0170] - a value of the azimuthal distance-thickness ratio g"7t"' within the range 2 - 13, and more particularly in the range 5 - 10, and even more particularly in the range 6 - 9;

[0171] - an ellipticity value e2"' and e3"" within the range 0.30 - 0.95, plus particularly in the range 0.40 - 0.90, and even more particularly in the range 0.50 - 0.80.

[0172] The HCF4 hollow core fibre has an Rcore'" value between 15 and 25 pm, and more particularly between 15 and 20 pm.

[0173] The anti-resonant elements referred to above (in any of the embodiments described herein) refer to any hollow wave-guiding element extending along the length of the hollow-core fiber and arranged circumferentially around the longitudinal axis of the fiber to reflect the incident light and guide it by anti-resonant reflection through the hollow-core region. The anti-resonant elements are uniformly distributed tubes with substantially equal and uniform wall thicknesses, so as to create the desired anti-resonance effects.They may have a polygonal cross-section or an elliptical, oval, or circular geometry, provided that the lower-level tubular element has a cross-sectional geometry different from that of the preceding upper-level tubular element and that it is arranged in cross-section so as to be tangent at two points to the module under consideration in the tangential direction of the fiber. Anti-resonant elements are preferably made of glass, particularly silica, but may be made of [material not specified in the original text]. The material may be plastic (polymer or composite) or a crystalline material suitable for anti-resonant guidance. The anti-resonant elements may be empty (air) or filled with gas or liquid, for example, like the hollow core region. An even number of anti-resonant elements, for example 4 or 6, or an odd number, for example 3 or 5, may be provided, arranged with even or odd symmetry to define the hollow core region.

[0174] Figure 5 schematically illustrates an example of an optical transmission system equipped with an anti-resonant hollow-core optical fiber according to the invention. In this example, the optical transmission system comprises a data transmitter Tx, a data receiver Rx, and at least one OF hollow-core optical fiber according to one of the embodiments described above, which is connected between the transmitter Tx and the receiver Rx and is suitable for high-capacity data transmission, for example, in a passive optical network. Such a transmission system may also include other equipment necessary for the operation of the system (not shown in the figure), such as a multiplexer / multiplexer, an optical modulator, a connector, a photodetector, etc.

[0175] Figure 6 schematically illustrates the main stages of manufacturing a hollow core fiber according to a particular embodiment of the invention.

[0176] In the SI ("STR") stage, several glass anti-resonant module tubes and a hollow glass sheathing tube are provided. For this purpose, a large hollow glass tube is drawn out to form what are called capillaries (for example, the capillaries have a diameter of about 1 to a few mm and a length of about 1 m or more). These capillaries will form the anti-resonant elements of the fibers.

[0177] In step S2 ("ATT"), the tubes of the anti-resonant module are attached to an inner surface of the sheath tube in a distributed manner. To this end, the glass capillaries are inserted into a first hollow sheath tube, which is larger than the glass capillaries, and then they are welded, for example, one by one to the inner surface of the sheath tube, in order to form a large-scale fiber structure. The term "distributed" refers to a non-contact arrangement of the glass capillaries around the longitudinal axis of the structure.

[0178] In step S3 ("DRW"), the sheath tube and the tubes of the anti-resonant module are stretched until the outer sheath and the anti-resonant modules conform to predefined dimensions. To this end, the structure is heated and stretched to form a micro-structured rod. This sheath is then inserted into a second hollow glass tube. The resulting new tubular structure is called a preform and is subsequently stretched in the presence of gas. Gases are injected into the preformed tubes and capillaries of the tubular structure at precisely controlled pressure and temperature to keep the air holes intact and adjust the dimensions. cavities in the process of formation. The thermal formation process and associated parameters are not detailed here, as they can be derived from prior art methods.

Claims

Demands

1. Hollow core fibre comprising an outer sheath (1) having an inner surface and an inner sheath region (2) delimited by the inner surface, the inner sheath region comprising a plurality of anti-resonance modules distributed without touching around the longitudinal axis of the hollow core fibre to define a hollow core region (3), characterized in that the anti-resonance modules each comprise at least two tubular elements embedded on at least two distinct levels, the lower level tubular element being embedded in the preceding upper level tubular element and arranged in cross section so as to be tangent at two points of said module in a tangential direction of the hollow core fibre, only the first level tubular element being in contact with the inner surface of the outer sheath.

2. Hollow core fibre according to claim 1, wherein at least one lower level tubular element has a cross section geometry different from that of the preceding upper level tubular element, the cross section geometry belonging to the group comprising an elliptical geometry, an oval geometry and a circular geometry.

3. Hollow core fibre according to claim 2, wherein the anti-resonant modules (10a-10of; 20a-20f) are each made up of first and second level embedded tubular elements (11, 12; 21, 22), the second level tubular element being embedded in the first level tubular element, which has an elliptical cross-section geometry, an oval cross-section geometry or a circular cross-section geometry.

4. Hollow core fibre according to claim 2, wherein the anti-resonant modules (30a-30f) are each made up of first, second and third level embedded tubular elements (30, 31, 32), the third level tubular element being embedded in the second level tubular element which is itself embedded in the first level tubular element, which has an elliptical cross-section geometry or an oval cross-section geometry.

5. Hollow core fiber according to claim 3, wherein the embedded first- and second-level tubular elements are defined as follows: - the first-level tubular element has an elliptical cross-sectional geometry defined by an ellipticity el, a principal axis radius Rtubel disposed in a radial direction of the hollow core fiber, and a secondary axis radius Rtube2 disposed in a tangential direction of the hollow core fiber, such that: Rtubel = (2 x sin(n / N) x Rcore - g) / (2 x (1 - sin(n / N))) Rtube2 = Rtubel x ^(1 - el2) where N is an integer, with N > 2, corresponding to the number of anti-resonant modules included in the inner cladding region, Rcore is the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements, and α is the azimuthal distance between two elements adjacent first-level tubular- the second-level tubular element has a circular cross-section defined by a first radius Rtube3 arranged in a radial direction of the hollow core fiber and a second radius Rtube4 arranged in a tangential direction of the hollow core fiber, such that: Rtube3 = Rtube4 = Rtube x - el2 ),

6. Hollow core fiber according to claim 3, wherein said first-level and second-level embedded tubular elements are defined as follows: - the first-level tubular element has a circular cross-section geometry, a first radius Rtube' disposed in a radial direction of the hollow core fiber and a second radius Rtube2' disposed in a tangential direction of the hollow core fiber, such that: Rtube2' = Rtube1 = (2 x sin(n / N') x Rcore' - g') / (2 x (1 - sin(n / N'))) - the second-level tubular element has an elliptical cross-section geometry defined by an ellipticity e2, a main axis radius Rtube3' arranged in a tangential direction of the hollow core fiber and a secondary axis radius Rtube4' arranged in a radial direction of the hollow core fiber, such that: Rtube3' = Rtubel' = (2x sin(n / N')xRcore'-g,) / (2x(l-sin(n / N1))) Rtube4' = Rtube3' x ^(1 - e22 ) where: N' is an integer, with N' > 2, corresponding to the number of anti-resonant modules included in the inner cladding region, Rcore' is the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements, g' is the azimuthal distance between two adjacent first-level tubular elements.

7. Hollow core fiber according to claim 4, wherein said embedded tubular elements of the first, second, and third levels are defined as follows: - the first-level tubular element has an elliptical cross-sectional geometry defined by an ellipticity el", a first radius of principal axis Rtubel" disposed in a radial direction of the hollow core fiber and a first radius of secondary axis Ptube?' disposed in a tangential direction of the hollow core fiber, such that: Rtubel" = (2 x sin(n / N") x Rcore" - g) / (2 x (1 - sin(n / N"))) Rtube2" = Rtubel" x ^1 - el"2) where: N" is an integer, with N > 2, corresponding to the number of tubular elements included in the inner cladding region, Rcore" is the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements,

8. g" is the azimuthal distance between two adjacent first-level tubular elements, - the second-level tubular element has a circular cross-section defined by a first radius Rtube3' arranged in a radial direction of the hollow core fiber and a second radius Rtube4" arranged in a tangential direction of the hollow core fiber, such that: Rtube4" = Rtube3" = Rtubel" x ^(1-el" 2 ) - the third-level tubular element has an elliptical cross-section geometry defined by an ellipticity e3, a second radius with secondary axis Rtube5 arranged in a radial direction of the hollow core fiber and a second radius with principal axis Rtube6 arranged in a tangential direction of the hollow core fiber, such that: Rtube6 = Rtube" x ^1-el" 2 ) Rtube5 = Rtube6 x ^(l-e3 2 ) Hollow core fiber according to claim 4, wherein the first, second and third level embedded tubular elements are defined as follows: - the first-level tubular element has a circular cross-section geometry, a first radius Rtube'" arranged in a radial direction of the fiber and a second radius Rtube2'" arranged in a tangential direction of the fiber, such that: Rtube2"' = Rtubel'" = (2 x sin(n / N"') x Rcore'"- g'") / (2 x (1- sin(n / N'"))) - the second-level tubular element has an elliptical cross-section geometry defined by an ellipticity e2"\ a principal axis radius Rtube3"' arranged in a tangential direction of the fiber and a secondary axis radius Rtube4"' arranged in a radial direction of the hollow-core fiber, such: Rtube3'" = Rtubel'" = (2 x sm(n / N"') x Rcore'"- g"') / (2 x (1 - sin(n / N'"))) Rtube4'" = Rtube3"' x ^(1 - e2"2 ) - the third-level tubular element has an elliptical cross-section geometry defined by an ellipticity e3'", a main axis radius Rtube5'" disposed in a tangential direction of the fiber and a secondary axis radius Rtube6"' disposed in a radial direction of the hollow core fiber, such that: Rtube5'" = Rtube1" = (2 x sin(n / N") x Rcore"' - g1") / (2 x (1 - sin(n / N'"))) Rtube6'" = Rtube5'" x ^(1 - e3 2 ) where: N'" >2, Rcore"1, the radius of the hollow core region defined as the smallest distance between the center of the hollow core fiber and the outer edge of the first-level tubular elements around the hollow core region; g1", the smallest azimuthal distance between two adjacent first-level tubular elements.

9. Hollow core fibre according to any one of claims 2 to 8, wherein said at least two embedded tubular elements have a thickness between [0.3 pm - 1.2 pm], an azimuthal distance-thickness ratio between [2 - 13] and an ellipticity between [0.30 - 0.95].

10. Optical transmission system characterized in that it comprises at least one hollow core fiber according to any one of claims 1 to 9.

11. A method for manufacturing a hollow core fiber according to any one of claims 1 to 9, characterized in that it comprises the following steps: - providing (S1) several tubes of anti-resonant modules and a hollow sheath tube; - fixing (S2) the tubes of anti-resonant modules on an inner surface of the sheath tube in a distributed manner; - stretch (S3) the entire set of tubes including the sheath tube and the anti-resonance module tubes until the outer sheath and the anti-resonance modules meet predefined dimensions.

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