METHOD FOR LASER WELDING OF AN OBJECTIVE FIBER IN A POLETED ELEMENT USING A PULSEMED LASER BEAM AND ASSOCIATED OPTICAL ELEMENT

Description

[0001] The invention relates to a method for welding an optical fiber, in particular a hollow-core fiber, and a hole element, in particular made of glass, by means of a pulsed laser beam, in particular a USP laser beam. The optical fiber is inserted into a hole in the hole element such that the outer circumference of the optical fiber and the inner circumference of the hole form a cylindrical joining surface. The optical fiber and the hole element are locally melted in the region of the joining surface by means of the laser beam focused into the workpiece material and are thereby welded together. The laser beam preferably has a wavelength at which at least one joining partner is transparent / partially transparent. The invention further relates to an optical element comprising an optical fiber and a hole element that are welded together.

[0002] Examples of optical fibers include step-index or multi-clad fibers, PCF (polymer cladded fiber), or hollow-core fibers. Hollow-core fibers are passive transport fibers that guide radiation through a hollow core surrounded by a fine photonic glass structure, e.g., in the Kagomé configuration. The hollow core allows glass damage thresholds to be bypassed and nonlinear effects to be suppressed, allowing higher intensities to be transmitted. The hollow core can be vacuumed for this purpose. Alternatively, the hollow interior of the core can also be selectively filled with a gas, such as argon.

[0003] Examples of perforated elements include perforated bricks, ferrules / glass sleeves, perforated discs, or perforated caps.

[0004] Welding monolithic connections to the hollow-core fiber can be achieved using CO2 splicing, for example. However, this process also melts the sensitive Kagomé structure, which significantly negatively impacts beam quality. Other connection options include gluing or clamping with an O-ring, both of which can cause distortion problems. Adhesives can also outgas or age prematurely.

[0005] Furthermore, hollow-core fibers require the core to be positioned essentially centrally over the hole of the perforated element, e.g., a perforated cap, and the fiber to be aligned flatly with the perforated element. This requires a high degree of precision for the holder while simultaneously allowing sufficient degrees of adjustment. Therefore, only very tight tolerances are required for the entire process to ensure a successful weld that maintains beam quality and achieves sufficient strength and / or gas tightness.

[0006] EP 0 905 534 A2 (disclosing the preamble of claims 1 and 12) describes the connection of a fiber to a glass sleeve. For this purpose, material from the fiber and glass sleeve is melted into a ring at the fiber end at the interface between the fiber and the glass sleeve, thus joining the two workpieces.

[0007] From JP 2017 068052 A a glass element with an optical fiber and a method for its production are known.

[0008] The present invention, in contrast, has the object of providing a fiber-friendly welding process with an improved connection between the optical fiber and the hole element.

[0009] This object is achieved according to the invention in that a pulsed laser beam is focused onto the region of the joining surface, the beam direction of which runs in the axial direction of the cylindrical joining surface, and in that the laser focus is moved in the axial direction in the region of the joining surface in or against the beam direction of the laser beam in order to produce at least one weld seam, in particular an axial or axially parallel weld seam, in the region of the joining surface. The USP laser beam preferably comprises laser radiation with pulse durations of less than 50 ps, ​​preferably less than 1 ps, in particular in the femtosecond range.

[0010] This welding process is particularly advantageous for welding hollow-core fibers. Hollow-core fibers are optical fibers consisting of a hollow fiber core and a fiber cladding.

[0011] It has been shown that the small dimensions of the hollow core fiber and the sensitive structure inside the hollow core fiber place special demands on the welding process. Although heat can only be applied very locally, the process according to the invention achieves sufficient strength to enable handling of the fiber for further processing. Furthermore, it is possible to position the weld seam precisely and keep the width of the weld seam narrow. This is crucial for maintaining the beam quality of the hollow core fiber later on. The weld seam must not destroy the sensitive internal structure of the hollow core fiber and must therefore be narrower than the fiber cladding. A laser focus diameter of approximately 5-10 µm is preferred; the resulting weld seam is then approximately 50-70 µm in diameter.

[0012] A further advantage of the welding process according to the invention is the good gas-tightness of the joint that can be achieved. This is particularly important for hollow-core fibers, since these must be evacuated for high peak intensities or specifically filled with gases, such as argon, to achieve spectral broadening and pulse shortening.

[0013] Pulsed laser beams, especially ultrashort pulsed (USP) laser radiation with pulse durations of less than 500 ps, ​​especially in the femtosecond range, are increasingly being used for materials processing. The special feature of materials processing with USP laser radiation lies in the high peak powers / peak intensities of the laser radiation with the workpiece. This allows extreme thermal imbalances to be created in the solid state, for example, between electrons and atom / ion cores, which then lead to unique ablation or formation mechanisms.

[0014] The background is the local melting of the material using ultrashort laser pulses. If ultrashort laser pulses are focused into the volume of glass, e.g., quartz glass, the high intensity present at the laser focus leads to nonlinear absorption processes, which can induce various material modifications depending on the laser parameters. These nonlinear absorption processes generate free charge carriers, which subsequently absorb in a quasi-linear manner. This creates a local plasma that forms the melting zone. If the modification is placed near the interface between two glasses, the cooling melt generates a stable bond between the two glasses. Due to the very local joining process, the laser-induced stresses are typically low, which means that even glasses with very different thermal properties can be welded together.Other transparent materials such as crystals, which sometimes have even more different thermal and mechanical properties, can also be welded together or with glasses.

[0015] According to the invention, at least one weld seam is created in the area of ​​the cylindrical joining surface between the optical fiber and the perforated element by a feed movement of the laser focus in or against the direction of the laser beam. This feed movement of the laser focus can be achieved, for example, by a movement of the laser processing head in or against the direction of the laser beam and / or by a feed movement of the optical fiber and perforated element in or against the direction of the laser beam. A change in the focal length of the focusing optics is also possible.

[0016] The at least one weld seam has directional components in the axial direction of the optical fiber or the cylindrical joining surface and is located in the area of ​​the joining surface of the optical fiber and the perforated element. The at least one weld seam can also have directional components in the circumferential direction of the optical fiber. For this purpose, a movement in the circumferential direction of the cylindrical joining surface can be superimposed on the feed movement of the laser focus in or against the beam direction.

[0017] To achieve a reliable joint, several weld seams can be created spaced apart from each other along the circumferential direction of the joining surface. Preferably, the weld seam begins in the area with the smallest gap and then alternates from there. This allows a weld seam to be connected to an already securely joined weld seam. Experiments show that this can also bridge gaps larger than those that can be bridged with a single weld seam.

[0018] Preferably, the feed movement of the laser focus, either in or against the beam direction, is continuous, but can also be performed stepwise, with or without pauses between the individual steps. In the latter case, welding without pauses results in welding in material that is still hot from the previous step, while welding with pauses results in welding in material that has already cooled down due to the time delay from the previous step. As tests have shown, continuous laser welding against the beam direction, in particular, leads to a crack-free, continuous weld seam.

[0019] Preferably, when the feed is carried out against the direction of beam incidence, it is carried out with a continuous feed in the range of 0.5 mm / s to 100 mm / s, preferably 5 mm / s to 30 mm / s, particularly preferably with approximately 20 mm / s.

[0020] Preferably, the welding produces a weld seam with a seam diameter of 5-500µm, preferably 50-100µm.

[0021] In order to increase the tolerance range and the total weld width, several axial welds overlapping one another in the direction transverse to the joining surface are preferably produced sequentially at a circumferential position instead of a single axial weld.

[0022] It is possible to use single pulses during welding. Welding is preferably carried out using pulse packets comprising several individual pulses. A pulse train consists of many individual pulses, with the individual pulses in the pulse train having a single-pulse repetition rate from pulse to pulse. Pulse packets consist of at least two individual pulses. Here, there is also a single-pulse repetition rate. Furthermore, there is also a pulse packet-to-pulse packet repetition rate. The individual pulses in a pulse packet are preferably the same. The number of individual pulses in a pulse packet is, for example, between 2 and 20, preferably approximately 5. For example, the pulse packet-to-pulse packet repetition rates can be 50-200 kHz, preferably approximately 125 kHz, and the average pulse power can be 1-20 W, preferably approximately 10 W. The single-pulse repetition rate in the pulse packet is typically several MHz.

[0023] The method according to the invention can be used, for example, in the welding of hole caps, protective caps or end caps on optical fibers or in the alignment joining, e.g. the mirrors of a multipass amplifier.

[0024] In a further aspect, the invention also relates to an optical element assembled from an optical fiber and a hole element by laser welding according to the invention, wherein the optical fiber is arranged in a hole of the hole element and the outer circumference of the optical fiber and the inner circumference of the hole of the hole element form a cylindrical joining surface and wherein the optical fiber and the hole element are connected in the region of the cylindrical joining surface by means of at least one weld seam which extends in the axial direction of the cylindrical joining surface.

[0025] The optical fiber is preferably formed from a glass structure, e.g., a Kagomé structure, surrounding a hollow interior. The hollow fiber core, made of a glass structure, is surrounded by a fiber cladding. The perforated element is preferably made of glass, e.g., a perforated brick, ferrule / glass sleeve, or even a perforated disc or perforated cap.

[0026] The optical element produced by the method according to the invention has a weld seam in the region of the outer circumference of the cladding of the optical fiber. The weld seam extends, for example, in an axial or axially parallel direction to the optical fiber or to the cylindrical joining surface. The weld seam can also have directional components in the circumferential direction of the outer circumference of the fiber cladding.

[0027] Further advantages and advantageous embodiments of the subject matter of the invention will become apparent from the description, the claims, and the drawings. Likewise, the above-mentioned and further listed features can be used individually or in combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather as examples for describing the invention. They show: Fig. 1 schematically shows a laser processing machine for the inventive laser welding of an optical fiber in a perforated element; Fig. 2 schematically shows a longitudinal section through the optical fiber welded to the perforated element with an axial weld seam in the region of a cylindrical joining surface; Figs. 3, 4 schematically show a plan view of the end face of the optical fiber and the perforated element with several axial weld seams spaced apart from one another in the circumferential direction of the cylindrical joining surface ( Fig. 3 ) and with additional sequentially generated axial welds ( Fig. 4 ).

[0028] The Fig. 1 shown laser processing machine 1 used to weld the outer circumference of an optical fiber 10 with the inner circumference of the hole 14 a hole element 12 using a laser beam 16Preferably, the laser beam 16 has a wavelength at which at least one joining partner is transparent / partially transparent. The optical fiber 10 is made of glass, in particular quartz glass, for example, and the hole element 12 is made of glass, in particular quartz glass, polymer, glass ceramic, crystalline, or combinations thereof and / or with opaque materials.

[0029] The laser processing machine 1 comprises a UKP laser 18 for generating the laser beam 16 in the form of UKP laser pulses 20 with pulse durations of less than 50 ps, ​​preferably less than 1 ps, in particular in the form of femtosecond pulses, and a laser processing head movable in the XY direction 22, from which the laser beam 16 emerges. Alternatively or additionally, the assembly to be welded, consisting of optical fiber 10 and perforated element 12, can also be moved.

[0030] After the optical fiber 10 has been inserted and positioned in the hole 14 of the hole element 12, the laser beam 16 is directed into the area of ​​the cylindrical common joining surface 24 of optical fiber 10 and hole element 12 in order to locally melt the optical fiber 10 and the hole element 12 in the region of the joining surface 24 and thus weld them together. The laser focus of the laser beam 16, which is aligned axially parallel to the optical fiber 10 or to the cylindrical joining surface 24, is moved in or against the beam direction of the laser beam 16, i.e. in the axial direction of the cylindrical joining surface 24, in order to create an axial weld seam extending in the beam direction in the region of the joining surface 24. 26 (Fig. 2b). The laser focus FThe focused laser beam 16 is located at the joining surface 24 or close to this joining surface 24 in the volume of one of the two components 10, 12. Preferably, the vertical weld seam 26 extends almost across the entire thickness of the perforated element 12 and ends before the top and bottom surfaces of the perforated element 12, preventing material leakage and ensuring a clean process. This depends on the size of the weld seam 26 or weld bubble.

[0031] In the illustrated embodiment, the optical fiber 10 is merely exemplary as a hollow core fiber with a core 28 and a coat 30 trained.

[0032] Fig. 2 shows schematically a longitudinal section through the optical fiber 10 welded to the hole element 12 with an axial weld seam 26 formed in the region of the joining surface 24. The laser beam 16 strikes the joining surface 24 between the outer circumference of the optical fiber 10 and the inner circumference of the hole 14 of the hole element 12 in the axial direction of the optical fiber 10. The laser focus F is in or, as in Fig. 2 shown, moves against the beam direction (Z-direction) of the laser beam 16, so that an axial feed direction 32 of the laser focus F or a welding direction opposite to the beam direction of the laser beam 16. The optical element thus formed is provided with 34 designated.

[0033] In the Fig. 3 In the optical element 34 shown, a plurality of axial weld seams 26 spaced apart from one another in the circumferential direction of the cylindrical joining surface 24 have been produced in the region of the cylindrical joining surface 24, as described above, whereby the optical fiber 10 is welded in the hole element 12.

[0034] As in Fig. 4 As shown, at a circumferential position, instead of a single axial weld seam, several axial weld seams 26 can be produced sequentially next to each other, which are aligned with each other in the direction 36 overlap transversely to the joining surface 24 in order to increase the total weld width and thus the tolerance range.

Claims

1. A method for welding an optical fiber (10), in particular a hollow-core fiber, and a perforated element (12), in particular made of glass, by means of a pulsed laser beam (16), in particular a USP laser beam, wherein the optical fiber (10) is inserted into a hole (14) of the perforated element (12) so that the outer circumference of the optical fiber (10) and the inner circumference of the hole (14) form a cylindrical joining surface (24), wherein the optical fiber (10) and the perforated element (12) are locally melted in the region of the joining surface (24) by means of the laser beam (16) focused into the workpiece material and thereby welded to one another, characterized in that a pulsed laser beam (16) whose beam direction extends in the axial direction of the cylindrical joining surface (24) is focused on the region of the joining surface (24), and in that the laser focus (F) in the region of the joining surface (24) is moved in the axial direction in or opposite to the beam direction of the laser beam (16) in order to produce at least one weld seam (26), in particular an axial weld seam, in the region of the joining surface (24).

2. The method according to claim 1, characterized in that a plurality of weld seams (26) spaced apart from one another in the circumferential direction of the cylindrical joining surface (24) are produced in the region of the joining surface (24).

3. The method according to one of the preceding claims, characterized in that the feed movement (32) of the laser focus (26) in or opposite to the beam direction is continuous or stepwise.

4. The method according to claim 3, characterized in that the feed movement (32) takes place continuously at a feed rate of between 0.5 mm / s and 100 mm / s, preferably between 5 mm / s and 30 mm / s, in particular preferably at approximately 20 mm / s.

5. The method according to one of the preceding claims, characterized in that a weld seam (26) is produced with a seam diameter of 5-500 µm, preferably 50-100 µm.

6. The method according to one of the preceding claims, characterized in that a plurality of axial weld seams overlapping one another in the direction (36) transverse to the joining surface (24) are produced sequentially.

7. The method according to one of the preceding claims, characterized in that the pulsed laser beam (26) comprises laser pulse packets which consist of individual pulses.

8. The method according to claim 7, characterized in that the individual pulses in a pulse packet are the same.

9. The method according to claim 7 or 8, characterized in that the number of individual pulses in a pulse packet is between 1 and 20.

10. The method according to one of the preceding claims, characterized in that the laser pulses (20) of the laser beam (16) have an individual pulse repetition rate in the pulse packet of 50-200 kHz and / or an average pulse power of 1-20 W and / or a laser focus (F) at the welding point of 5-10 µm.

11. The method according to one of the preceding claims, characterized in that the USP laser beam emits laser radiation with pulse durations of less than 50 ps, preferably less than 1 ps, in particular in the femtosecond range.

12. An optical element (34) made of an optical fiber (10), in particular a hollow-core fiber, and a perforated element (12), in particular made of glass, wherein the optical fiber (10) is arranged in a hole (14) of the perforated element (12) and the outer circumference of the optical fiber (10) and the inner circumference of the hole (14) of the perforated element (12) form a cylindrical joining surface (24), characterized in that: the optical fiber (10) and the perforated element (12) are connected in the region of the cylindrical joining surface (24) by means of at least one weld seam (26) which extends in the axial direction of the cylindrical joining surface (24), and wherein the at least one weld seam (26) is produced by the method according to one of claims 1 to 11.

13. The optical element according to claim 12, characterized in that the at least one weld seam (26) also extends in the circumferential direction of the cylindrical joining surface (24).

14. The optical element according to claim 12 or 13, characterized by a plurality of weld seams (26) spaced apart from one another in the circumferential direction of the cylindrical joining surface (24) in the region of the cylindrical joining surface (24).