Interconnecting a hollow-core fibre with a standard silica single-mode fibre
Patent Information
- Application Number
- EP2023834206
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-12-22
- Filing Date
- 2023-12-20
- Publication Date
- 2025-10-29
AI Technical Summary
Existing solutions for interconnecting single-mode and hollow-core fibers fail to adequately minimize optical reflections and are not compact or easily deployable, as they require complex adaptations to match the different optical beam diameters and precise mechanical alignment.
A coupling device with a gradient index fiber section and pure silica sections is used, featuring a radial offset and cleavage angle to reduce reflections, allowing for beam adaptation and simplified mechanical assembly through welding, which aligns the fibers without requiring precise mechanical misalignment.
The solution significantly reduces optical reflections in both directions of propagation while enabling adaptation of fiber modes, facilitating a compact and straightforward assembly process by aligning the solid and hollow core fibers along a single axis.
Smart Images
Figure 1.1
Abstract
Description
Interconnection of a hollow core fiber with a standard silica single-mode fiber 1. Field of the invention
[0001] The invention lies in the field of optical data transmission, and more particularly in that of the interconnection between a single-mode optical fiber and a hollow-core fiber. 2. State of the prior art
[0002] Single-mode optical fiber is made of silica with an optical cladding and an optical core through which the optical beam propagates. The optical index of the core is slightly higher than that of the cladding. In a multi-core fiber, a single optical cladding surrounds several spatially distributed optical cores. Hollow-core fiber consists of a silica and air microstructure with a core consisting primarily of air through which the optical beam propagates.
[0003] Interconnecting a single-mode fiber and a hollow-core fiber requires an adaptation of the optical beam size between these two types, the optical beam diameter being around 9µm for single-mode fiber and around 30µm for hollow-core fiber.
[0004] The article "Interconnecting hollow-core fibers", 2021 IEEE Photonics Society Summer Topicals Meeting Series (SUM), 2021, pp. 1-2, by M. Komanec et al., presents several devices whose aim is to achieve this adaptation while minimizing optical losses.
[0005] However, these solutions have the disadvantage of not sufficiently minimizing optical reflection in each of the transmission directions, or of not being sufficiently simple or compact to be easily produced and deployed.
[0006] One of the aims of the invention is to remedy these drawbacks of the state of the art. 3. Statement of the invention
[0007] The invention improves the situation by using a coupling device between a single-mode or multi-core optical fiber, called a solid fiber, and a hollow-core optical fiber, comprising a graded-index fiber section, where:the coupling device is coupled to a core of the solid fiber with a radial offset relative to the central axis of the graded-index fiber section,the coupling device is coupled to the hollow-core fiber with an angle relative to a plane orthogonal to the central axis, the angle of which is proportional to the radial offset and to parameters of the graded-index fiber.
[0008] In the following, the term "solid fiber" refers indifferently to a single-core single-mode optical fiber, and a multi-core optical fiber. In the direction from the solid fiber to the hollow-core fiber, the radial offset introduces an angle at the output of the optical device composed of a graded-index section and prevents optical reflection from returning to the core of the solid fiber. The so-called "radial" offset can be in a plane perpendicular to the central axis of the device, or slightly inclined relative to this plane.
[0009] In the other direction of transmission, the cleavage allows any optical reflection to be deflected away from the central axis of the hollow core.
[0010] The combination eliminates or greatly reduces optical reflections in both directions of propagation, while allowing adaptation to the diameters of the fiber modes at each end.
[0011] Furthermore, by cleaving the hollow core fiber at the same angle as the device, a weld allows the alignment of the solid fiber, the coupling device, and the hollow core fiber in a single axis, which facilitates the mechanical assembly of the different sections. Indeed, during assembly, by welding for example, it is not necessary to mechanically hold the hollow core fiber at a certain angle relative to the device, an angle whose precision would be difficult to ensure and maintain.
[0012] Achieving precise offset by mechanically misaligning the solid fiber relative to the device can be difficult, but this is not necessary if the solid fiber is a multi-core fiber. Indeed, since at least one of the cores in the multi-core fiber is already eccentric relative to the device centerline, there is no need to introduce mechanical lateral misalignment.
[0013] According to one aspect of the coupling device, it further comprises at least one pure silica section adjacent to one side and / or the other of the graded index fiber section.
[0014] The pure silica section has no core, and the optical beam disperses homogeneously there. This makes it possible to modify the optical parameters of the lens system that the device thus composes, and to make this device independent of the index filter of the graded index section. A wide choice of index gradients is therefore possible, for the same solid fiber / hollow core fiber pair.
[0015] According to one aspect of the coupling device, the pure silica section is located between the solid fiber and the graded index fiber section.
[0016] Thanks to this aspect, the "object" distance (between the solid fiber and the gradient index section) of the optical system composed of the gradient index lens is configurable and allows the modification of the magnification and focusing parameters of the optical beam
[0017] According to one aspect of the coupling device, the pure silica section is located between the graded index fiber section and the hollow core fiber.
[0018] Thanks to this aspect, the "image" distance (between the gradient index section and the hollow core fiber) of the optical system composed of the gradient index lens is configurable and allows the magnification and focusing parameters of the optical beam to be modified.
[0019] According to one aspect of the coupling device, it comprises a first pure silica section located between the solid fiber and the graded index fiber section, and a second pure silica section located between the graded index fiber section and the hollow core fiber.
[0020] Thanks to this aspect, both the object distance and the image distance of the optical system composed of the gradient index lens are configurable and allow the modification of the magnification and focusing parameters of the optical beam.
[0021] According to one aspect of the coupling device, the graded index fiber section is bonded to the hollow core fiber.
[0022] According to one aspect of the coupling device, the pure silica section is welded to the hollow core fiber.
[0023] The invention also relates to a method for manufacturing a coupling device between a single-mode or multi-core optical fiber, called a solid fiber, and a hollow-core fiber, comprising:welding the solid fiber to a graded-index fiber section, or to a pure silica section itself welded to the graded-index fiber section, with a radial offset of a core of the solid fiber relative to the central axis of the graded-index fiber section,sectioning the graded-index fiber section to give it a defined length, with a cleavage relative to a plane orthogonal to the central axis if the graded-index fiber is directly welded to the hollow-core fiber, the angle of which is proportional to the offset and to parameters of the graded-index fiber,welding the hollow-core fiber to the graded-index fiber section, or to a pure silica section itself welded to the graded-index fiber section with the cleavage,
[0024] the solid fiber, the graded index section and the hollow core fiber being parallel to each other.
[0025] This process allows for coupling a solid fiber to a hollow core fiber, with a strong reduction in optical reflections, simply using electric arc welding. It also allows for a simple mechanical assembly following the natural alignment of the fiber structures. The length of the graded index fiber section is calculated to allow this fiber section to function as a lens with a focal length. The pure silica sections allow the object and image distances of the solid and hollow fibers to be adjusted throughout the graded index section. 4. Presentation of figures
[0026] Other advantages and characteristics of the invention will appear more clearly on reading the following description of a particular embodiment of the invention, given as a simple illustrative and non-limiting example, and the appended drawings, among which:
[0027] lapresents a first aspect of a first embodiment of a coupling device between a single-mode or multi-core optical fiber, called solid fiber, and a hollow-core optical fiber, comprising a section of graded-index fiber,
[0028] presents a second aspect of this first embodiment,
[0029] presents a third aspect of this first embodiment,
[0030] presents a fourth aspect of this first embodiment,
[0031] presents a first aspect of a second embodiment of the coupling device,
[0032] presents a second aspect of this second embodiment,
[0033] presents a third aspect of this second embodiment,
[0034] presents a fourth aspect of this second embodiment,
[0035] presents a fifth aspect of this second embodiment,
[0036] presents a first aspect of a third embodiment of the coupling device,
[0037] presents a second aspect of this third embodiment,
[0038] presents a third aspect of this third embodiment,
[0039] presents a fourth aspect of this third embodiment,
[0040] and Figures 14 to 23 show embodiments with ferrules.
[0041] 5. Detailed description of at least one embodiment of the invention
[0042] One solution is to use a welded assembly of fiber sections or segments of different types to achieve mode matching and a reduction in optical reflections. The welded assembly of fiber sections of different types also produces a compact coupling device. Welding remains the preferred assembly method, but it is possible to glue or mechanically assemble the sections by juxtaposing them. If necessary, it is possible to include an index liquid between the sections to limit or eliminate a fiber / air / fiber interface.
[0043] In one embodiment, the coupling device comprises a section of graded index fiber Fgi juxtaposed by welding (or any other means) to a single-mode fiber Fms on one side and to a hollow core fiber Fcc on the other side. The length of the section of graded index fiber Fgi is calculated to adapt the optical beams.
[0044] The principle is to adapt the diameter of the optical beam by inserting a section of gradient index fiber Fgi from a few microns to a few millimeters. One or two sections of fiber composed of pure silica Fsp1, Fsp2, can be inserted to modify the optical parameters of the lens system that thus composes the coupling device.
[0045] Figures 1 to 4 illustrate 4 variants of this embodiment of such a coupling device, respectively:without any section of pure silica fiber (): the coupling device is then composed of an interface with a solid fiber, a section of graded index fiber, and an interface with a hollow core fiber,with a single section of pure silica Fsp1, arranged between the single-mode fiber Fms and the section of graded index fiber Fgi (): the coupling device is then composed of an interface with a solid fiber, a section of pure silica (Fsp1), a section of graded index fiber, and an interface with a hollow core fiber,with a single section of pure silica Fsp2 arranged between the section of graded index fiber Fgi and the hollow core fiber Fcc (): the coupling device is then composed of an interface with a solid fiber, a section of graded index fiber, a section of pure silica (Fsp2),and an interface with a hollow core fiber, with a section of pure silica Fsp1 and a section of pure silica Fsp2 arranged on either side of the section of gradient index fiber Fgi (): the coupling device is then composed of an interface with a solid fiber, a section of pure silica (Fsp1), a section of gradient index fiber, a section of pure silica (Fsp2), and an interface with a hollow core fiber.,
[0046] Such a coupling device makes it possible to adapt the optical beam of approximately 9µm of the Fms silica single-mode fiber to that of 30µm of the Fcc hollow-core fiber. However, the silica / air diopter introduces a reflection of approximately -14.5 dB.
[0047] In one embodiment this principle is improved by introducing an angle at the output of the coupling device. For this it is necessary to offset the core of the single-mode fiber Fms relative to the optical axis of the device, preferably perpendicularly. The angle is proportional to the offset of the core of the single-mode fiber and the parameters of the graded index fiber. The angle Th is defined by the formula:
[0048] Th = e n0 g0
[0049] With e being a measure of the offset of the core axis of the single-mode fiber Fms from the optical axis, n0 being the index at the center of the graded-index fiber and g0 being the quadratic coefficient of the graded-index.
[0050] Figures 5 and 6 illustrate the effect of such a zero and non-zero offset respectively on the optical beam at the output of the coupling device.
[0051] The advantage of this offset is that it limits the optical reflection of the optical beam coming from the Fms single-mode fiber. Indeed, as illustrated in the, the reflected beam will propagate again in the gradient index section Fgi to be focused in an inversely offset zone in the core of the Fms single-mode fiber.
[0052] As seen previously, the core offset required at the output angle of the coupling device can be achieved by using a standard single-mode fiber that is spliced with a lateral misalignment (perpendicular to the fiber axis) relative to the graded-index fiber.
[0053] In another embodiment, this effect is achieved with a multi-core fiber, spliced without any lateral misalignment. Indeed, as illustrated by the, none of the cores Co of the 2-core multi-core fiber Fmc1, or of the 4-core multi-core fiber Fmc2, are in the central axis of the free, in other words they are all already offset from this axis.
[0054] The various embodiments can be applied to a single-mode fiber or to a multi-core fiber, and the term "full fiber", designated by Fp, is used to designate indifferently one or the other type of fiber.
[0055] To obtain good coupling of the device with the hollow core fiber Fcc, it is possible to align this fiber according to the angle Th of the device, as illustrated by the. This device has the disadvantage of not maintaining perfect alignment between the fiber Fp, the fiber Fcc and the coupling device, which prevents mechanical maintenance by welding for example.
[0056] According to an embodiment illustrated by the, the coupling device is improved by introducing a cleavage angle Tc at its end, proportional to the angle Th of the optical beam. This makes it possible to have an optical beam at the output of the coupling device which is parallel and centered on the axis of the coupling device.
[0057] The relationship between the value of Tc and the value of Th is governed by the Snell-Descartes laws:
[0058] n(fgi) sin(Tc) = n(ffc) sin(Th)
[0059] with n(fgi) being the index of the core of the gradient section of index Fgi,
[0060] and n(ffc) being the index of the hollow core fiber Fcc (approximately = 1).
[0061] In this embodiment, it is advantageous to cleave the hollow core fiber Fcc at the same angle Tc, as illustrated by the. This allows assembly by simple welding of the different elements that are the fiber Fp, the coupling device and the fiber Fcc, while facilitating their alignment along the same axis. The coupling device is then composed of an interface without cleavage with a solid fiber, a section of graded index fiber, and an interface with cleavage with a hollow core fiber.
[0062] This embodiment further allows the adaptation of the optical beam to the mode diameter of the fibers at the ends. As illustrated by the, this embodiment also allows the reduction of optical reflections in each of the two directions of propagation of the optical beam.
[0063] In one embodiment illustrated by the, pure silica fiber sections Fsp1 and Fsp2 are added to modify the magnification or focusing parameters of the optical beam in the coupling device. The coupling device is then composed of a cleavage-free interface with a solid fiber, a pure silica section (Fsp1), a graded index fiber section, a pure silica section (Fsp2), and a cleavage interface with a hollow core fiber.
[0064] La illustrates the case where a section of pure silica fiber is added to both the right and left of the graded index fiber section Fgi, but it is possible to add one on only one side. The cleavage device can be composed of one or two sections of pure silica fiber (or none, as illustrated by la). If a section of pure silica fiber Fsp2 is interposed between the fiber Fgi and the fiber Fcc, the fiber cleavage at angle Tc is arranged at this location. The calculation of Tc as a function of Th is also governed by the Snell-Descartes laws:
[0065] n(sp) sin(Tc) = n(ffc) sin(Th)
[0066] with n(sp) being the core index of the pure silica section Fsp2,
[0067] and n(ffc) being the index of the hollow core fiber Fcc (approximately = 1).
[0068] Such a coupling device between a single-mode or multi-core optical fiber, called solid fiber, and a hollow-core optical fiber, is simple to manufacture.
[0069] An example of a manufacturing process consists of interposing a section of graded index fiber Fgi between a solid fiber Fp and a hollow core fiber Fcc.
[0070] At least one core of the solid fiber Fp must be offset from the central axis of the graded index fiber Fgi.
[0071] The solid fiber Fp must be welded or glued, with this core offset, to the gradient index fiber Fgi, or possibly to a section of pure silica fiber Fsp1 inserted between the fibers Fp and Fgi.
[0072] The Fgi graded index fiber or Fsp1 pure silica fiber section must be fractured (sectioned) at a defined length from the joint. This length is calculated to allow the graded index section to function as a lens with the desired focal length and the pure silica sections as the object and image distances from this lens.
[0073] The contact surfaces between the Fgi graded index fiber and the Fcc hollow core fiber must be cleaved at an angle and then joined by welding or gluing. If a section of Fsp2 pure silica fiber is inserted between the Fgi and Fcc fibers, the cleavage at an angle is between the Fsp2 and Fcc fibers.
[0074] In the case of welding, the welding is carried out under electric arc in a standard fiber optic welding machine. This assembly requires the optimization of welding programs.
[0075] In all stages of the manufacturing process, the different fibers and fiber sections are held end to end in the same rectilinear alignment.
[0076] In an embodiment illustrated by the, optical connectors, called ferrules, or connector ferrules, of standard dimensions, are used in order to facilitate connection and disconnection actions between on one side a single-mode or multi-core fiber, called solid fiber, Fp, and on the other side a hollow core fiber, Fcc. The solid fiber and the hollow fiber are inserted into their respective ferrule FF1 and FF2. The connection between the two fibers (solid and hollow) is made mechanically by aligning and bringing together the two ferrules FF1 and FF2 until the two fibers are in near mechanical contact.
[0077] Each ferrule also serves as an easily manipulated protection element when the solid fiber and the hollow core fiber are not connected together. Typically, with a graded-index fiber section having a quadratic constant of the graded-index section (or quadratic coefficient g0) of 4.3 mm-1, an offset e of the order of 32 µm gives a cleavage angle Tc of 8 degrees, for a multi-core fiber with a standard outer diameter of 125 µm. To achieve low optical reflection, the typical angle of the ferrules for a solid single-mode fiber is 8 degrees.
[0078] A relation between e and Tc is:
[0079] e = Pi * Tc / (g * 180)
[0080] It is then possible to insert this device into a ferrule drilled to approximately the same diameter of 125 µm with a cleavage angle of 8° at the end. The hollow core fiber can also be inserted into an identical connector with a cleavage angle of 8°. During connection, the two ferrules come into mechanical contact.
[0081] In an embodiment illustrated by Figures 15 and 16, the end of the hollow core single-mode fiber is protected by a section of pure silica fiber, Cap. This section acts as a cap to mechanically protect the end of the hollow fiber during implementation in the ferrule (with a possible polishing operation) and limit the insertion of impurities into the hollow part. This section of pure silica can be welded to the hollow fiber with a cleavage angle (), or without a cleavage angle ().
[0082] In an embodiment illustrated by the, the hollow core fiber Fcc is inserted into a ferrule FF2b without any cleavage angle, i.e. with an angle of 0°. It is then necessary to adapt the index gradient sections to allow for a working distance of the optical beam in the free space between the two connectors.
[0083] In an embodiment illustrated by the, the assembly by welding of fiber sections of different natures uses a standard single-mode fiber Fp with an offset e relative to the axis of the sections of graded index fiber Fgi and possibly pure silica. However, the offset does not allow the assembly to be maintained within a diameter of 125 µm, each section having this same diameter. A special ferrule FFs is then necessary, with a guide of a diameter of 125 µm only over a length less than that of the graded index section, and a guide of a larger diameter along the single-mode fiber. The length of the 125 µm guide is typically of the order of 380 µm (i.e. Pi / (2 * g)).
[0084] However, this implementation uses a non-standard ferrule. It is possible to use a standard ferrule by increasing the length of the graded-index fiber section. This latter length can be a multiple of 760 µm (i.e., Pi / g), for example, 1.14 mm, as illustrated with the FFn1 ferrule by the, or 1.90 mm as illustrated with the FFn2 ferrule by the.
[0085] The use of ferrules in these embodiments allows the insertion of a Dcm device between the two connectors such as an anti-reflection treatment or any thin-film type device to provide optical filtering or additional mechanical protection (water-repellent or hydrophobic coatings, protection against scratches and abrasions, etc.). A thin glass plate can also be inserted between the two ferrules. This glass plate can have an optical function such as anti-reflection, filtering and also a mechanical protection function. This glass plate can be inserted between the two ferrules, each with a polishing angle of 8°. It is also possible to insert this glass plate between two ferrules with different angles, such as a ferrule at 8° for solid fiber and 0° for hollow fiber.Figures 21, 22 and 23 illustrate 3 variations of the mechanical alignment of the glass slide Dcm between ferrules: with an identical angle or with different angles.
Claims
Coupling device between a single-mode or multi-core optical fiber, called solid fiber (Fp), and a hollow-core optical fiber (Fcc), comprising the solid fiber, a graded-index fiber section (Fgi), and the hollow-core fiber, where:the graded-index fiber section (Fgi) is coupled to a core of the solid fiber (Fp) with a radial offset (e) relative to the central axis of the graded-index fiber section (Fgi),the graded-index fiber section (Fgi) is coupled to the hollow-core fiber (Fcc), the contact surface of the hollow-core fiber being cleaved at an angle (Tc) relative to a plane orthogonal to the central axis, the value of which is proportional to the radial offset (e) and to parameters of the graded-index fiber (Fgi). Coupling device according to claim 1, further comprising at least one pure silica section (Fsp1, Fsp2) adjacent on one side and / or the other of the graded index fiber section (Fgi). Coupling device according to claim 2, wherein the pure silica section (Fsp1) is between the solid fiber (Fp) and the graded index fiber section (Fgi). Coupling device according to claim 2, wherein the pure silica section (Fsp2) is between the graded index fiber section (Fgi) and the hollow core fiber (Fcc). Coupling device according to claim 2, comprising a first pure silica section (Fsp1) between the solid fiber (Fp) and the graded index fiber section (Fgi), and a second pure silica section (Fsp2) between the graded index fiber section (Fgi) and the hollow core fiber (Fcc). Coupling device according to one of claims 1 to 3, wherein the graded index fiber section (Fgi) is welded to the hollow core fiber (Fcc). Coupling device according to one of claims 4 or 5, wherein the pure silica section (Fsp2) is welded to the hollow core fiber (Fcc).A method of manufacturing a coupling device according to claim 1, between a single-mode or multi-core optical fiber, called a solid fiber (Fp), and a hollow core fiber (Fcc), comprising:welding the solid fiber (Fp) to a graded index fiber section (Fgi), or to a first pure silica section (Fsp1) itself welded to the graded index fiber section (Fgi), with a radial offset (e) of a core of the solid fiber (Fp) relative to the central axis of the graded index fiber section (Fgi),cutting the graded index fiber section (Fgi) in order to give it a defined length, with a cleavage relative to a plane orthogonal to the central axis, according to an angle (Tc) whose value is proportional to the offset (e) and to parameters of the graded index fiber (Fgi),cutting the hollow core fiber (Fcc) with the cleavage according to the angle (Tc), weld the hollow core fiber (Fcc) to the graded index fiber section (Fgi),with the cleavage (Tc) according to the angle (Tc), the solid fiber (Fp), the gradient index section (Fgi) and the hollow core fiber (Fcc) being parallel to each other., A method of manufacturing a coupling device according to claim 1, between a single-mode or multi-core optical fiber, called a solid fiber (Fp), and a hollow core fiber (Fcc), comprising:welding the solid fiber (Fp) to a graded index fiber section (Fgi), or to a first pure silica section (Fsp1) itself welded to the graded index fiber section (Fgi), with a radial offset (e) of a core of the solid fiber (Fp) relative to the central axis of the graded index fiber section (Fgi),welding the graded index fiber section (Fgi) to a second pure silica section (Fsp2),cutting the second pure silica section (Fsp2) to give it a defined length, with a cleavage relative to a plane orthogonal to the central axis, according to an angle (Tc) whose value is proportional to the offset (e) and to parameters of the graded index fiber (Fgi), cut the hollow core fiber (Fcc) with the cleavage at the angle (Tc),welding the hollow core fiber (Fcc) to the second pure silica section (Fsp2), with the cleavage (Tc) at the angle (Tc), the solid fiber (Fp), the graded index section (Fgi), the second pure silica section (Fsp2) and the hollow core fiber (Fcc) being parallel to each other., Coupling device according to claim 1, wherein the graded index fiber section (Fgi) and the solid fiber (Fp) are inserted into a first connection ferrule (FF1, FFs, FFn1, FFn2), and the hollow core fiber (Fcc) is inserted into a second connection ferrule (FF2, FF2b), the first ferrule and the second ferrule being adapted and configured to connect to and disconnect from each other. Coupling device according to claim 10, further comprising a thin film on blade type device (Dcm), between the first ferrule and the second ferrule. Coupling device according to claim 11, wherein the thin film type device (Dcm) is an anti-reflective treatment.