Device for coupling a laser beam into a multi-clad fibre and optical system

Abstract

The invention relates to a device (4) for coupling a laser beam (3) into a multi-clad fibre (5), comprising a beam switch (6) for dividing the laser beam (3) into a plurality of sub-laser-beams (3.1, 3.2, ...), wherein the beam switch (6) comprises at least two birefringent optical wedges (9a, 9b, ...) and at least one polarisation-rotating device (12a, 12b, ...) which has an adjustable polarisation-rotating effect and which is arranged between the birefringent optical wedges (9a, 9b, ...), as well as an in-coupling optical unit (7) for coupling the sub-laser-beams (3.1, 3.2, ...) exiting the beam switch (6) into the multi-clad fibre (5), wherein the in-coupling optical unit (7) is designed to couple at least two of the sub-laser-beams (3.1, 3.2, ...) exiting the beam switch (6) into at least two different light-conducting cores (14a, 14b, ...) of the multi-clad fibre (5). The invention also relates to an optical system (1) comprising a multi-clad fibre (5) and a device (4), as described above, for coupling the laser beam (3) into the multi-clad fibre (5).

Description

[0001] Device for coupling a laser beam into a multiclad fiber and optical system

[0002] The present invention relates to a device for coupling a laser beam into a multiclad fiber. The invention also relates to an optical system comprising such a device and a multiclad fiber.

[0003] A multi-clad fiber can be used to guide a laser beam from a source location to a target location. The source location can be, for example, a laser used to generate the laser beam. The target location can be, for example, a processing optics that shapes the laser beam and then applies the laser beam to a workpiece for processing. The processing process can be, for example, a welding process or a cutting process.

[0004] Different machining processes usually have different requirements for characteristic laser beam parameters, such as the focus diameter, the intensity distribution, the beam profile, etc.

[0005] It is known from EP2556397 that the laser beam components coupled into the inner core or outer core of a multi-clad fiber produce different beam characteristics and beam qualities in the output laser beam. To adjust the beam quality, the laser beam is split into two partial laser beams by a mechanically retractable wedge splitter, which are coupled into different cores of the double-clad fiber. However, the wedge splitter for adjusting the beam quality is difficult to manufacture, and the power distribution between the inner core and the outer core is influenced by changes in the position of the laser beam on the wedge splitter (sensitivity to misalignment).US10914902 describes various variants of coupling a laser beam into a double-clad fiber, in which a polarization splitting of an incident laser beam is achieved by means of birefringent elements and the two resulting partial laser beams are imaged into different cores of the double-clad fiber.

[0006] Object of the invention

[0007] The invention is based on the object of providing an improved device for coupling a laser beam into a multiclad fiber and a corresponding optical system.

[0008] Subject of the invention

[0009] This object is achieved by a device for coupling a laser beam, comprising: a beam splitter for splitting the laser beam into a plurality of partial laser beams, wherein the beam splitter comprises at least two birefringent optical wedges and at least one polarization-influencing device with an adjustable polarization-influencing effect, which is arranged between the birefringent optical wedges, and coupling optics for coupling the partial laser beams emerging from the beam splitter into the multi-clad fiber, wherein the coupling optics are designed to couple at least two of the partial laser beams emerging from the beam splitter into at least two different light-conducting cores of the multi-clad fiber.

[0010] In the device described here, at least two birefringent optical wedges are used to split the laser beam into several partial laser beams. This takes advantage of the fact that an ordinary and an extraordinary partial laser beam are formed in a birefringent optical wedge, which have a difference angle when exiting a beam exit surface of the birefringent optical wedge. The two partial laser beams exiting the birefringent wedge are linearly polarized and have a polarization direction perpendicular to one another. The birefringent material of the birefringent optical wedges is preferably a uniaxial crystal, e.g. calcite or quartz. The optical axis of the crystal is typically aligned perpendicular to the optical axis of the beam splitter.A beam entrance surface or a beam exit surface of the birefringent optical wedges is typically also aligned perpendicular to the optical axis of the beam splitter.

[0011] As they pass through the polarization-influencing device, the polarization direction of the two linearly polarized partial laser beams emerging from the birefringent optical wedge is influenced or manipulated, altering the weighting of the s- and p-polarized components of the partial beams. Upon passing through a subsequent birefringent optical wedge in the beam path, each partial laser beam is split again into an ordinary and an extraordinary partial laser beam. For example, using two birefringent optical wedges in the beam splitter, a maximum of four partial laser beams can be generated, aligned at several different angles relative to the optical axis of the beam splitter.

[0012] With the help of the coupling optics, the angular distribution of the partial laser beams is converted into a spatial distribution at the end face of the multiclad fiber. The exit angles of the partial laser beams from the beam splitter are matched to the coupling optics and lead to spatial offsets in the focal plane of the coupling optics, which are matched to the geometry of the multiclad fiber in such a way that the partial laser beams are coupled into the multiclad fiber at the positions of the light-guiding cores. The coupling optics can have one or more optical elements. The coupling optics typically comprise at least one focusing optical element for focusing the partial laser beams in a focal plane at the end face of the multiclad fiber into which the laser beam is to be coupled.

[0013] The polarization-influencing device can, for example, be a rotatable delay element in the form of a delay plate, e.g., a ½-wave plate or a ¼-wave plate. A delay plate generally causes a phase shift between two mutually perpendicular polarization directions. An ½-wave plate causes a rotation of the polarization direction of a respective linearly polarized partial beam, while an ¼-wave plate can convert a respective linearly polarized partial beam into a circularly or elliptically polarized partial beam. To set a splitting ratio between 0% and 100% (see below), it is advantageous if the polarization-influencing device is designed as a polarization-rotating device, e.g., a rotatable ½-wave plate. A rotatable ¼-wave plate typically only allows the setting of a splitting ratio within a reduced range of values, e.g., between 50% and 100%.

[0014] The distribution of the laser beam power using the polarization-influencing device(s) has the advantage over the use of a mechanically retractable wedge switch in that no stray light or diffraction effects occur at the edge of the mechanically retractable wedge. Furthermore, the distribution of the laser beam power among the various light-guiding cores can be varied by adjusting the polarization-influencing effect of the polarization-influencing device. For example, if the polarization-influencing device is set so that it has no polarization-influencing effect, a number of partial laser beams can be generated that is half the maximum possible number of partial laser beams that can be generated with the beam switch (e.g., two instead of four partial laser beams when using two birefringent optical wedges).This can be exploited to prevent any power or partial laser beams from being coupled into certain light-conducting cores of the multiclad fiber. For the purposes of this application, "different" light-conducting cores are understood to mean cores that are spatially differently positioned or spatially separated from one another.

[0015] The number of polarization-influencing devices corresponds to the number of degrees of freedom when coupling into the multiclad fiber. It is advantageous if the number of polarization-influencing devices corresponds to the number of light-conducting cores of the multiclad fiber minus one. In this case, between 0% and 100% of the laser beam power can generally be coupled into a respective light-conducting core of the multiclad fiber. In one embodiment, the multiclad fiber is rotationally symmetrical and has an inner light-conducting core and at least one annular light-conducting core. The coupling optics are designed to couple two partial laser beams emerging from the beam splitter with different polarization states, whose propagation direction corresponds to a beam direction of the laser beam entering the beam splitter, into the inner core of the multiclad fiber.

[0016] In principle, the multiclad fiber can be designed in different ways, for example in the form of a linear multiclad fiber in which several light-conducting cores are arranged next to one another, or in the form of a grating multiclad fiber in which several cores are arranged in rows and columns. For the sake of simplicity, the following description assumes that the multiclad fiber is a rotationally symmetrical multiclad fiber that has an inner, typically circular, light-conducting core and one or more ring-shaped light-conducting cores surrounding the inner light-conducting core. So-called intermediate claddings, which are not light-conducting, are arranged between the light-conducting cores. The radially outermost light-conducting core can also be surrounded on its outer side by a non-light-conducting cladding, which may be followed by a layer of glass or the like.

[0017] The two partial laser beams coupled into the inner core of the multiclad fiber are those partial laser beams formed at the exit of the first birefringent optical wedge and retain their polarization state as they pass through the beam splitter. The different polarization states are therefore typically the mutually perpendicular linear polarization states of the two partial laser beams emerging from the first birefringent optical wedge. In this case, the beam splitter is typically designed such that the deflection angles of the two partial laser beams, which retain their polarization state as they pass through the beam splitter, just compensate each other as they pass through the two or more birefringent optical wedges.If the laser beam is aligned parallel to the optical axis of the beam splitter, the two exiting laser beams, whose deflection angles just compensate, are also aligned parallel to the optical axis of the beam splitter, but are laterally offset from the incident laser beam. To couple the two laser beams into the inner core of the multi-clad fiber, the coupling optics typically focus these two laser beams on the optical axis of the coupling optics, where the inner core of the multi-clad fiber is positioned. The other laser beams exiting the beam splitter, which are not aligned parallel to the incident laser beam, are coupled into the ring-shaped light-guiding core(s) of the multi-clad fiber. The optical axis of the coupling optics can generally coincide with the optical axis of the beam splitter.However, it is generally advantageous if the optical axis of the coupling optics is laterally offset from the optical axis of the beam splitter. The lateral offset of the optical axis of the coupling optics from the optical axis of the beam splitter typically corresponds essentially to the lateral offset of the two partial laser beams, whose deflection angles just compensate each other, from the incident laser beam.

[0018] In a further embodiment, the device is designed to couple one or more pairs of partial laser beams with two different polarization states into a respective light-guiding core of the multi-clad fiber. The two polarization states are typically linear, mutually perpendicular polarization states, which for simplicity are referred to below as s-polarization and p-polarization. The pairwise coupling of partial laser beams with mutually perpendicular polarization states enables polarization-independent coupling of the partial laser beams into the respective light-guiding core. This type of coupling is particularly possible when using a rotationally symmetric multi-clad fiber, in which a partial laser beam can be coupled into each of the respective ring-shaped cores at two radially opposite positions.For polarization-independent coupling, the number of birefringent optical wedges must be at least equal to the number of light-guiding cores of the multiclad fiber. If polarization-independent coupling is not used, the number of wedges can be one less than the number of light-guiding cores of the multiclad fiber.

[0019] In one embodiment, the beam switch comprises a first number of equally oriented birefringent optical wedges and a second number of birefringent optical wedges oriented opposite to the first number of birefringent optical wedges, wherein the sum of the wedge angles of the first number of birefringent optical wedges corresponds to the sum of the wedge angles of the second number of birefringent optical wedges.

[0020] An identical orientation of two or more birefringent optical wedges means that their wedge tips are positioned on the same side of the optical axis or the beam direction of the laser beam. An opposite orientation means that the wedge tips of the respective birefringent optical wedges are arranged on opposite sides with respect to the optical axis. In this case, the birefringent optical wedges are typically made of the same birefringent material, and the optical axes of the birefringent optical wedges are aligned parallel to one another. It is understood that the birefringent optical wedges do not necessarily have to have a wedge tip, provided that the laser beam or the partial laser beams do not pass through it.In this case, the wedge angle is also understood to be the angle at which the beam entry surface and the beam exit surface are arranged to each other.

[0021] The fact that the sum of the wedge angles of the first and second number of equally oriented birefringent optical wedges is equal typically results in the two partial laser beams described above maintaining their alignment upon passing through the beam splitter and being coupled into the inner core of the multiclad fiber. It is understood that a slight deviation between the sum of the wedge angles of the first number of birefringent optical wedges and the sum of the wedge angles of the second number of birefringent optical wedges is tolerable, provided that, despite this deviation, the partial laser beams can be coupled into the respective light-guiding cores of the multiclad fiber. The coupling of the two partial laser beams into the inner light-guiding core of the multiclad fiber described above can also be achieved if different wedge angles are used.In this case, it is typically required that the birefringent materials of the birefringent optical wedges are different from each other and the wedge angles of the birefringent optical wedges are matched to the different birefringent materials.

[0022] In one embodiment, the beam splitter has exactly two birefringent optical wedges that are oppositely oriented and have the same wedge angle. In this embodiment, the multiclad fiber is typically a generally rotationally symmetric double-clad fiber that has an inner light-guiding core and exactly one ring-shaped light-guiding core. As described above, in this case, the two partial laser beams emerging from the beam splitter, whose propagation direction corresponds to the beam direction of the laser beam, are coupled into the light-guiding inner core, and the other two partial laser beams emerging from the beam splitter are coupled into the light-guiding ring-shaped core of the double-clad fiber.The two birefringent optical wedges are typically identical in design and rotated by 180° relative to each other, so that a beam exit surface of the first birefringent optical wedge in the beam path and a beam entrance surface of the second birefringent optical wedge in the beam path are aligned parallel to each other.

[0023] In an alternative embodiment, the beam splitter has a number N of birefringent optical wedges with the same orientation, which are preferably arranged one after the other in the beam path of the laser beam. The following applies to the number N of similarly oriented birefringent optical wedges: N > 1 , i.e. there are two or more birefringent optical wedges with the same orientation. It is fundamentally possible for the beam splitter to consist of the number N of similarly oriented birefringent optical wedges or for the beam splitter to have no further birefringent optical wedges. In this case, a polarization-influencing device can be arranged between each two birefringent optical wedges following one another in the beam path and, if appropriate, additionally in front of the first birefringent optical wedge in the beam path.

[0024] In a further development of this embodiment, the beam switch has an oppositely oriented birefringent optical wedge in front of the number N of optical wedges with the same orientation in the beam path of the laser beam. The oppositely oriented birefringent wedge enables compliance with the condition described above that the sums of the wedge angles should be equal.

[0025] In the event that the laser beam entering the beam splitter has a fixed polarization state, the provision of the oppositely oriented optical wedge can be omitted, since in this case it does not cause any splitting, ie in this case, with the N + 1 wedges of the beam splitter, a maximum of 2 NSub-laser beams are generated. Alternatively, the oppositely oriented optical wedge can be replaced by an optical wedge made of a non-birefringent material, such as an amorphous material. If the wedge angle satisfies the condition specified above, one of the sub-laser beams, after passing through the beam splitter, is aligned parallel to the beam direction of the laser beam entering the beam splitter and is coupled into the core of the multiclad fiber.

[0026] In a further development, the number N of similarly oriented birefringent optical wedges has an identical wedge angle, and the oppositely oriented birefringent optical wedge has a wedge angle that corresponds to N times the identical wedge angle. In this way, the condition described above for the sum of the wedge angles of the birefringent optical wedges can be met. Typically, in this embodiment, a polarization-influencing device is arranged between each two of the birefringent optical wedges.

[0027] In a further development, the beam splitter comprises a number N = 2 of equally oriented birefringent optical wedges, and the coupling optics are designed to couple at least three of the partial laser beams into different light-guiding cores of a triple-clad fiber, or the beam splitter comprises a number N = 3 of equally oriented birefringent optical wedges, and the coupling optics are designed to couple at least four of the partial laser beams into different light-guiding cores of a quadruple-clad fiber. The maximum number of partial laser beams that can be generated with a number N of birefringent optical wedges is 2 N . In the case that an inversely oriented birefringent wedge is arranged in front of the number N of birefringent optical wedges with the same orientation, a maximum number of 2 N+1Partial laser beams are generated, half of which have a first linear polarization state (s-polarization) and the other half a second polarization state (p-polarization) aligned perpendicular to the first.

[0028] In the case of a triple-clad fiber, a maximum of eight partial laser beams can be generated using three birefringent optical wedges, of which two partial laser beams with a different polarization state are coupled into the inner light-guiding core, two pairs of partial laser beams with each pairwise different polarization state are coupled into a first ring-shaped light-guiding core, and two partial laser beams with different polarization state are coupled into a second ring-shaped light-guiding core surrounding the first.

[0029] In the case of a quadruple clad fiber, a maximum of sixteen partial laser beams can be generated using four birefringent optical wedges, of which two partial laser beams with different polarization states are coupled into the inner light-guiding core, three pairs of partial laser beams with pairwise different polarization states are coupled into a first annular light-guiding core, three pairs of partial laser beams with pairwise different polarization states are coupled into a second annular light-guiding core surrounding the first, and two partial laser beams with different polarization states are coupled into a third annular light-guiding core surrounding the second.

[0030] Both triple-clad and quadruple-clad fibers therefore allow polarization-independent coupling of the partial laser beams into the respective light-guiding cores. It goes without saying that the beam splitter constructed as described above can also be used to couple the partial laser beams into multi-clad fibers that have more than three or four light-guiding cores.

[0031] In an alternative embodiment, the beam splitter has a number N = 2 of similarly oriented birefringent optical wedges, wherein a first birefringent optical wedge in the beam path has a wedge angle that is twice as large as a wedge angle of the second birefringent optical wedge in the beam path. This embodiment is particularly suitable for coupling the partial laser beams into a quadruple-clad fiber. In this case, the beam splitter can have only the two similarly oriented birefringent optical wedges, whereby in this case a maximum of four partial laser beams can be coupled into the four light-guiding cores of the quadruple-clad fiber. In this case, a polarization-influencing device is arranged between the two birefringent optical wedges. An additional polarization-influencing device can optionally be provided upstream of the first birefringent optical wedge.

[0032] In a further development of this embodiment, the beam switch has an oppositely oriented birefringent optical wedge in the beam path of the laser beam, upstream of the two similarly oriented birefringent optical wedges, the wedge angle of which corresponds to three times the wedge angle of the second of the similarly oriented birefringent optical wedges in the beam path. In this way, the above-described condition regarding the wedge angles of the birefringent optical wedges can be met. In the event that the incident laser beam has a polarization direction that is oriented perpendicular or parallel to the optical axis of the birefringent optical wedges, the oppositely oriented birefringent optical wedge can be omitted or replaced with an optical wedge made of a non-birefringent material.

[0033] In a further development of this embodiment, the coupling optics are designed to couple at least one partial laser beam, preferably at least one pair of partial laser beams with two different polarization states, into a respective light-guiding core of a rotationally symmetric quadruple-clad fiber. Even with the beam splitter designed in the manner described above, polarization-independent coupling of the partial laser beams into the light-guiding cores of the quadruple-clad fiber is possible.

[0034] In the embodiment described here, the beam splitter typically comprises two polarization-influencing devices, each arranged between two consecutive birefringent optical wedges in the beam path. In principle, the beam splitter described here allows 100% of the laser beam power to be coupled into each of the light-guiding cores. However, due to the fact that only two degrees of freedom are available for adjusting the splitting ratios between the light-guiding cores, the laser beam power cannot be distributed arbitrarily among the four light-guiding cores of the quadruple-clad fiber.

[0035] In a further embodiment, the device comprises a control device for adjusting the polarization-influencing effect of the at least one polarization-influencing device in order to adjust a splitting ratio of the laser beam when coupled into the at least two different light-conducting cores of the multi-clad fiber. The control device serves to electronically control the polarization-rotating device. The polarization-influencing device enables a change in the polarization state of a respective incident partial laser beam. The change can, for example, be a rotation of the electric field strength vector of a respective incident partial laser beam by a rotation angle about the respective propagation direction of the partial laser beam.

[0036] Typically, the polarization-influencing device is designed for continuous adjustment of the polarization-influencing effect. As described above, a rotatably mounted retardation plate can be used as the polarization-influencing device. If the device has a polarization-rotating effect, the angle of rotation can be continuously changed or adjusted. In this case, the polarization-influencing device can, for example, be a retardation plate in the form of a rotatably mounted Å / 2 plate, which can be rotated about the optical axis of the beam splitter with the aid of the control device acting on a suitable actuator.The polarization-influencing device may alternatively be a Pockels cell, which is also electronically controllable and which influences the polarization state, in particular a rotation of the polarization direction, of an incident partial laser beam.

[0037] In a further embodiment, the control device is designed to adjust a polarization-influencing effect of the at least one polarization-influencing device, in which no power of the laser beam is coupled into at least one of the light-conducting cores of the multi-clad fiber and / or in which the entire power of the laser beam is coupled into at least one of the light-conducting cores of the multi-clad fiber.

[0038] If the number of polarization-influencing devices in the form of polarization-rotating devices corresponds to the number of light-guiding cores of the multiclad fiber minus one, it is typically possible to set any desired splitting ratio between the light-guiding cores, i.e., the laser beam power can be distributed arbitrarily among the light-guiding cores. Thus, between 0% and 100% of the laser beam power can be coupled into a respective light-guiding core. In the first case (coupling of 0%), no partial laser beams are coupled into the respective light-guiding core; in the second case, all partial laser beams emerging from the beam splitter are coupled into the respective light-guiding core.For example, in the case of a double-clad fiber, the entire power of the laser beam can be coupled into the inner light-guiding core if the polarization-influencing device does not produce a polarization-influencing effect. If the polarization-influencing device causes a 90° polarization rotation, the entire power of the laser beam can be coupled into the annular light-guiding core of the double-clad fiber.

[0039] In a further embodiment, the laser beam entering the beam splitter is linearly polarized, and the beam splitter has a 4X delay element in the beam path upstream of the first birefringent optical wedge. The 4X delay element can, for example, be designed as an 4X plate whose easy axis is aligned at 45° to the optical axis of the birefringent optical wedges of the beam splitter. In this case, the power of the laser beam is aligned by the 4X delay element with a splitting ratio of 50:50 parallel or perpendicular to the optical axis of the birefringent optical wedges, regardless of the orientation of the linear polarization of the entering laser beam. This means that two partial laser beams coupled into the same light-guiding core always have the same power.

[0040] The invention also relates to an optical system comprising: a multiclad fiber, preferably a double-clad fiber, a triple-clad fiber, or a quadruple-clad fiber, and a device configured as described above for coupling the laser beam into the multiclad fiber. The optical system typically also includes a laser used to generate the laser beam that is coupled into the beam splitter. The laser wavelength of the laser beam is fundamentally arbitrary and is adapted to the material of the optical elements of the beam splitter, the coupling optics, and the multiclad fiber. If the wavelength of the laser beam is 1030 nm, the birefringent optical wedges can be made, for example, from crystalline quartz, which is transparent to this wavelength.

[0041] In the optical system described above, the power distribution in the multiclad fiber can be adjusted using polarization-influencing devices, independent of the polarization of the laser beam entering the beam splitter. The power distribution in the multiclad fiber is also independent of the spatial intensity distribution of the incoming laser beam and largely independent of the pointing of the incoming laser beam.

[0042] Further advantages of the invention will become apparent from the description 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 intended to be exhaustive, but rather serve as examples for describing the invention.

[0043] They show:

[0044] Fig. 1 is a schematic representation of an optical system comprising a multiclad fiber and a device for coupling a laser beam into the multiclad fiber,

[0045] Fig. 2 is a schematic representation of a laser beam passing through a birefringent optical wedge of a beam splitter of the device of Fig. 1,

[0046] Fig. 3 is a schematic representation analogous to Fig. 1 , in which the device for coupling the laser beam into a double-clad fiber is designed,

[0047] Fig. 4a-c are schematic representations analogous to Fig. 1 , in which the device for coupling the laser beam into a triple clad fiber is designed,

[0048] Fig. 5 is a schematic representation analogous to Fig. 4a-c, in which the device for coupling the laser beam into a quadruple clad fiber is designed, and

[0049] Fig. 6 is a schematic representation analogous to Fig. 1 with a further device designed to couple the laser beam into a quadruple clad fiber.

[0050] In the following description of the drawings, identical reference numerals are used for identical or functionally equivalent components. Fig. 1 shows an optical system 1 having a laser 2 for generating a laser beam 3. The laser beam 3 emerging from the laser 2 enters a device 4 designed to couple the laser beam 3 into a multi-clad fiber 5. The device 4 has a beam splitter 6 and coupling optics 7. The beam splitter 6 serves to split the laser beam 3 into a typically even number of partial laser beams 3.1, ..., 3.M, which emerge from the beam splitter 6. In the example shown in Fig. 1, two of the partial laser beams 3.1, ..., 3.M run parallel to an optical axis 8 of the beam switch 6, while the remaining partial laser beams 3.1, ..., 3.M are each aligned at an angle to the optical axis 8 of the beam switch 6.

[0051] With the help of the coupling optics 7, the angular distribution of the partial laser beams 3.1, 3.2, ... is converted into a spatial distribution on an end face of the multi-clad fiber 5, which lies in the focal plane of the coupling optics 7. The exit angles of the partial laser beams 3.1, 3.2, ... from the beam switch 6 are matched to the coupling optics 7 and lead to spatial offsets in the focal plane of the coupling optics 7, which are matched to the multi-clad fiber 5 in such a way that the partial laser beams 3.1, 3.2, ... are coupled into the multi-clad fiber 5 at the positions of two or more light-conducting cores. In the simplest case, the coupling optics 7 can be a focusing lens, but the coupling optics 7 can also have several transmitting or reflecting optical elements.

[0052] The multiclad fiber 5 can be a linear multiclad fiber in which several light-conducting cores are arranged side by side, into which the partial laser beams 3.1, 3.2, ... are coupled. Alternatively, it can be a grating multiclad fiber in which several light-conducting cores are arranged in rows and columns, or a rotationally symmetric multiclad fiber.

[0053] For splitting the laser beam 3 into the plurality of partial laser beams 3.1, 3.2, ..., the beam splitter 6 has at least two birefringent optical wedges. The effect of a first optical wedge 9a of the beam splitter 6 in the beam path on the laser beam 3 is described in more detail below with reference to Fig. 2. The laser beam 3 has two polarization components s, p, for which the electric field strength vector is aligned parallel to the plane of the drawing and perpendicular to the plane of the drawing, which corresponds to the XZ plane of an XYZ coordinate system. The birefringent optical wedge 9a is made of a uniaxial crystal, for example quartz or calcite. The optical axis 0 of the birefringent optical wedge 9a is aligned perpendicular to the optical axis 8 of the beam splitter 6 (in the X direction).Upon passing through the optically anisotropic birefringent material of the birefringent optical wedge 9a, the laser beam 3 is split into a first, s-polarized partial laser beam 3.1 and a second, p-polarized partial laser beam 3.2. The first partial laser beam 3.1, which forms the ordinary beam, is aligned at a first angle β to the optical axis 8. The second partial laser beam 3.2, which forms the extraordinary beam, is aligned at a second angle γ to the optical axis 8. Upon passing through the birefringent optical wedge 8a, an angular difference δ = γ - β is thus generated between the first and second partial laser beams 3.1, 3.2. The size of the angular difference δ depends on the type of birefringent material used and on the wedge angle α of the birefringent optical wedge 9a.The angular difference ö together with the focal length f of the coupling optics 7 determines a lateral distance Ax of the first and second partial laser beams 3.1, 3.2 in the focal plane of the coupling optics 7, whereby approximately: Ax = f ö.

[0054] In the example shown in Fig. 2, a beam entrance surface 10a of the birefringent optical wedge 9a is oriented perpendicular to the optical axis 8 of the beam splitter 6. A beam exit surface 10b of the birefringent optical wedge 8a is oriented perpendicular to the optical axis 8 with respect to the XY plane at the wedge angle α.

[0055] Fig. 3 shows an optical system 1 with a beam splitter 6, which has a first birefringent optical wedge 9a in the beam path, which is designed as in Fig. 2. A second birefringent optical wedge 9b in the beam path is of the same construction or identical to the first birefringent optical wedge 9a and is rotated by 180° relative to it in the XZ plane, which corresponds to the plane of the drawing. The beam exit surface 10b of the first birefringent optical wedge 9a is aligned parallel to a beam entrance surface 11a of the second birefringent optical wedge 9b. A beam exit surface 11b of the second birefringent optical wedge 9b is perpendicular to the optical axis.

[0056] 8 of the beam splitter 6. The wedge angles a of the two birefringent optical wedges 9a, 9b are equal.

[0057] The laser beam 3 entering the beam splitter 6 passes through the first birefringent optical wedge 9a and is split into the two partial laser beams 3.1, 3.2, as described in connection with Fig. 2. The two partial laser beams 3.1, 3.2 impinge on a polarization-influencing device 12, which is designed in the form of a rotatable X / 2 plate. The polarization-influencing device 12 in the form of the rotatable X / 2 plate has a polarization-rotating effect and is therefore referred to below as the polarization-rotating device 12. A control device 13 serves to electronically control the polarization-rotating device 12. In the example shown, the control device 13 serves to rotate the X / 2 plate about the optical axis 8 of the beam splitter 6.In this way, the polarization-rotating effect of the polarization-rotating device 12, or more precisely, the angle of rotation during the rotation of the polarization direction, can be continuously adjusted. By rotating the polarization direction, an outgoing first partial laser beam 3.1, which has both an s-polarized and a p-polarized polarization component, is formed from the s-polarized first partial laser beam 3.1 entering the polarization-rotating device 12. Accordingly, an outgoing second partial laser beam 3.2', which has s-polarized and p-polarized polarization components, is formed from the p-polarized second partial laser beam 3.2.

[0058] The two partial laser beams 3.1, 3.2' emerging from the polarization-rotating device 12 pass through the second birefringent optical wedge 9b in the beam path and are split into a total of four partial laser beams 3.1 to 3.4. Two of the partial laser beams 3.1, 3.2 emerge from the second birefringent optical wedge 9b aligned parallel to the optical axis 8. The other two partial laser beams 3.3, 3.4 emerge from the second birefringent optical wedge 9b aligned at an angle to the optical axis 8 of the beam splitter 6. As can also be seen in Fig. 3, the first and the second partial laser beam 3.1, 3.2, which are aligned parallel to the incident laser beam 3 and to the optical axis 8 of the beam switch 6 when exiting the beam switch 6, are coupled into an inner light-conducting core 14a of the multi-clad fiber 5, which is designed as a rotationally symmetrical double-clad fiber.The third and fourth partial laser beams 3.3, 3.4 are coupled into a second light-guiding core 14b of the double-clad fiber 5, which is ring-shaped and surrounds the inner light-guiding core 14a, as can be seen from the cross-section of the double-clad fiber 5 shown on the right in Fig. 3. An optical axis 8a of the coupling optics 7 and the central axis of the double-clad fiber 5 are laterally offset from the optical axis 8 of the beam splitter 6, specifically by the amount by which the two partial laser beams 3.1, 3.2 were offset parallel to the optical axis 8 with respect to the incident laser beam 3 at the exit from the beam splitter 6. This enables the partial laser beams 3.1, 3.2, ... to impinge on the entrance-side end of the double-clad fiber 5 with the smallest possible angle of incidence.

[0059] In Fig. 3, s-polarized partial laser beams 3.1, 3.4 are represented by white arrows, and p-polarized partial laser beams 3.2, 3.3 are represented by black arrows. As can be seen in Fig. 3, a first pair of partial laser beams 3.1, 3.2 with different polarization states (s- and p-polarization, respectively) are coupled into the inner light-guiding core 14a of the double-clad fiber 5, and a second pair of partial laser beams 3.3, 3.4 with different polarization states (s- and p-polarization, respectively) are coupled into the annular light-guiding core 14b of the double-clad fiber 5. Both in the inner light-guiding core 14a and in the ring-shaped light-guiding core 14b, the number of s- or p-polarized partial laser beams 3.1, 3.2 and 3.3, 3.4 is the same, ie the coupling of the laser beam 3 into the respective cores 14a, 14b of the double-clad fiber 5 is polarization-independent.

[0060] The splitting ratio, i.e. the respective proportion of the power of the laser beam 3 that is coupled into the inner light-guiding core 14a and into the annular light-guiding core 14b, can be adjusted by controlling the polarization-influencing device 12 with the aid of the control device 13. In principle, the power of the laser beam 3 can be divided as desired between the two light-guiding cores 14a, 14b, i.e. between 0% and 100% of the power of the laser beam 3 can be coupled into a respective light-guiding core 14a, 14b. The polarization-rotating effect of the polarization-rotating device 12 can, in particular, be selected such that no power of the laser beam 3 is coupled into a respective light-guiding core 14a, 14b, while the entire power of the laser beam 3 is coupled into the respective other light-guiding core 14b, 14a.

[0061] Fig. 4a-c show an optical system 1 in which the multi-clad fiber 5 is designed as a triple-clad fiber and has an inner light-guiding core 14a and two ring-shaped light-guiding cores 14b, 14c that surround the inner light-guiding core 14a. As can be seen in Fig. 4a, the beam splitter 6 in this case is designed to split the incident laser beam 3 into a total of eight partial laser beams 3.1 to 3.8. For this purpose, the beam splitter 6 has three birefringent optical wedges 9a, 9b, 9c. The second and third birefringent optical wedges 9b, 9c in the beam path have the same orientation with respect to the optical axis 8, and the first birefringent optical wedge 9a in the beam path is oriented opposite to the second and third birefringent optical wedges 9b, 9c in the beam path.

[0062] The second and third birefringent optical wedges 9b, 9c in the beam path have the same wedge angle α. The first birefringent optical wedge 9a in the beam path has a wedge angle 2α, which is twice the wedge angle α of the second and third birefringent optical wedges 9b, 9c. The three birefringent optical wedges 9a-c are made of the same birefringent optical material.

[0063] Two partial laser beams 3.1, 3.2 of the total of eight partial laser beams 3.1 to 3.8 emerging from the beam splitter 6 are aligned parallel to the optical axis 8 of the beam splitter 6 or to the incident laser beam 3 and are coupled by the coupling optics 7 and into the inner light-guiding core 14a of the multi-clad fiber 5, as described in connection with Fig. 3. To simplify the illustration, the lateral offset between the optical axis 8 of the beam splitter 6 and the optical axis 8a of the coupling optics 7 is not shown in Fig. 4a-c and in the following illustrations. Of the remaining six partial laser beams 3.3 to 3.8, four partial laser beams 3.3 to 3.6 are coupled into the second light-guiding core 14b of the triple-clad fiber 5, and two partial laser beams 3.7, 3.8 are coupled into the third light-guiding core 14c of the triple-clad fiber 5. The partial laser beams 3.1, 3.2; 3.3 to 3.6; 3.7, 3.8 coupled into the respective cores 14a-c.8 are each polarized differently in pairs, as indicated in Fig. 4a by the circles drawn in the cross-section of the triple-clad fiber 5. White circles correspond to s-polarized partial laser beams, while black circles correspond to p-polarized partial laser beams.

[0064] As can also be seen in Fig. 4a, the beam switch 6 has two polarization-rotating devices 12a, 12b, which are electronically controlled by the control device 13 to distribute the power of the laser beam 3 to the three light-conducting cores 14a-c of the triple-clad fiber. In the device 4 shown in Fig. 4a, between 0% and 100% of the power of the laser beam 3 can be coupled into a respective light-conducting core 14a-c.

[0065] Fig. 4b shows the case where the control device 13 controls the first polarization-rotating device 12a in the beam path such that it has no polarization-rotating effect, i.e., the optical axis of the X / 2 plate is aligned either in the X direction or in the Y direction. This results in no splitting of the two partial laser beams 3.1, 3.2, which were formed at the first birefringent optical wedge 9a, at the second birefringent optical wedge 9b. Accordingly, in this case, the beam splitter 6 generates only four partial laser beams 3.1 to 3.4, which are coupled into the first and second light-conducting cores 14a, 14b.

[0066] By adjusting the polarization-rotating effect of the second polarization-rotating device 12b, the splitting ratio during coupling into the first and second light-guiding cores 14a, 14b can be adjusted. If the second polarization-rotating device 12b has no polarization-rotating effect because its optical axis is aligned in the X-direction or the Y-direction, only the first and second partial laser beams 3.1, 3.2 emerge from the beam splitter 6 and are coupled into the inner light-guiding core 14a. If the optical axis of the second polarization-rotating device 12b is aligned at 45° to the X-direction and Y-direction, two partial laser beams 3.3, 3.4 are generated, which are coupled into the first annular light-guiding core 14b.

[0067] Fig. 4c shows the case in which the control device 13 controls the second polarization-rotating device 12a in the beam path such that it has no polarization-rotating effect, i.e., the optical axis of the X / 2 plate is aligned either in the X direction or in the Y direction. In this case, the four partial laser beams 3.1, 3.2, 3.7, 3.8, which were formed upon passing through the second birefringent optical wedge 9b, are not split at the third birefringent optical wedge 9c in the beam path. Accordingly, in this case, the beam splitter 6 generates only four partial laser beams 3.1, 3.2, 3.7, 3.8, which are coupled into the first and third light-conducting cores 14a, 14c. By adjusting the polarization-rotating effect of the first polarization-rotating device 12a, the division ratio during coupling into the first and third light-conducting cores 14a, 14c can also be adjusted in this case.If the first polarization-rotating device 12a has no polarization-rotating effect, only the first and second partial laser beams 3.1, 3.2 emerge from the beam splitter 6 and are coupled into the inner light-guiding core 14a. If the optical axis of the first polarization-rotating device 12a is aligned at 45° to the X-direction or Y-direction, two partial laser beams 3.7, 3.8 are generated, which are coupled into the second annular light-guiding core 14c.

[0068] In the event that the same power is to be coupled into all three light-conducting cores 14a-c, the first polarization-rotating device 12a in the form of the X / 2 plate is aligned at an angle of 22.5° to the X-direction or the Y-direction. The second polarization-rotating device 12b in the form of the X / 2 plate is aligned at an angle of 17.63° to the X-direction or the Y-direction. In the device 4 described in Fig. 4a-c, it is generally advantageous if the distances between the light-conducting cores 14a-c or between the partial laser beams 3.1 coupled into adjacent light-conducting cores 14a-c,

[0069] 3.2, ... are approximately the same size.

[0070] If the incident laser beam 3 is linearly polarized, a 4X delay element 15, indicated in Fig. 4a, can be arranged in the beam path in front of the first birefringent optical wedge 9a. In this case, the optical axis of the 4X delay element 15 is aligned at 45° to the X direction or to the optical axis of the first birefringent optical wedge 9a. The power of the linearly polarized laser beam 3 is split by the 2X delay element 15 at a splitting ratio of 50:50 into two polarization components oriented in the X direction and in the Y direction, respectively, whereby the splitting is independent of the orientation of the electric field strength vector of the incident linearly polarized laser beam 3. This results in two partial laser beams 3.1, 3.2, ... with different polarization states, which are coupled into the same light-guiding core 14a-c of the triple clad fiber 5, having the same power.

[0071] If the incident laser beam 3 is linearly polarized and its electric field strength vector is oriented in the X-direction or the Y-direction, a maximum of four partial laser beams 3.1, 3.2, ... can be generated in the beam splitter 6. In this case, the first birefringent optical wedge 9a can be omitted. Alternatively, instead of the first birefringent optical wedge 9a, an optical wedge can be arranged in the beam splitter 6 that is not made of a birefringent material and has a wedge angle of 2α.

[0072] Fig. 5 shows a device 4 configured for coupling the laser beam 3 into a radially symmetric quadruple-clad fiber 5 having an inner light-conducting core 14a and three annular light-conducting cores 14b-d surrounding the inner light-conducting core 14a. The device 4 shown in Fig. 5 differs from the device 4 shown in Fig. 4a-c in that it has three similarly oriented birefringent optical wedges 9b, 9c, 9d and three polarization-rotating devices 12a-c, which are arranged between each two birefringent optical wedges 9a, 9b; 9b, 9c; 9c, 9d that follow one another in the beam path. A first birefringent optical wedge 9a in the beam path has a wedge angle 3a which corresponds to three times the (identical) wedge angles a of the three similarly oriented wedges 9b-c.

[0073] The beam splitter 6 of the device 4 shown in Fig. 5 allows a maximum of sixteen partial laser beams 3.1 to 3.16 to be coupled into the quadruple-clad fiber 5. The distribution among the partial laser beams 3.1 to 3.16 occurs in the manner described above in connection with Figs. 4a-c. Two of the partial laser beams 3.1, 3.2 exit the beam splitter 6 parallel to the incident laser beam 3 and are coupled into the inner light-guiding core 14a of the quadruple-clad fiber 5. Six of the partial laser beams 3.1 to 3.16 are each coupled into the first and second annular light-guiding cores 14b, 14c, and two of the partial laser beams 3.1 to 3.16 are coupled into the third annular light-guiding core 14d. The division ratio is adjusted using the three polarization-rotating devices 12a-c in the manner described in connection with Fig. 4a-c.

[0074] In the event that the incident laser beam 3 is linearly polarized and its electric field strength vector is oriented in the X-direction or in the Y-direction, a maximum of eight partial laser beams 3.1, 3.2, ... can be generated in the beam splitter 6. In this case, the first birefringent optical wedge 9a can be omitted, or instead of the first birefringent optical wedge 9a, an optical wedge can be arranged in the beam splitter 6 that is not made of a birefringent material and that has a wedge angle of 3α. The material of the optical wedge can be an amorphous (glass) material, for example quartz glass.

[0075] Fig. 6 shows a device 4 which, like the device 4 shown in Fig. 5, is designed for coupling the laser beam 3 into a quadruple-clad fiber 5. The device 4 has three birefringent optical wedges 9a-c. The second and third birefringent optical wedges 9b, 9c in the beam path are oriented the same, while the first birefringent optical wedge 9a in the beam path is oriented opposite to the other two wedges 9b, 9c. The second birefringent optical wedge 9b has a wedge angle 2a that is twice as large as the wedge angle a of the third birefringent optical wedge 9c. The first birefringent optical wedge 9a has a wedge angle 3a that corresponds to 3 times the wedge angle a of the third birefringent optical wedge 9c.

[0076] As can be seen in Fig. 6, the beam splitter 6 is designed to generate up to eight partial laser beams 3.1 to 3.8. Two differently polarized partial laser beams 3.1, 3.2, ... are coupled into one of the light-guiding cores 14a-d of the quadruple-clad fiber 5. Two of the partial laser beams 3.1, 3.2, which emerge from the beam splitter 6 and are aligned parallel to the optical axis 8 or the beam direction Z of the incident laser beam 3, are coupled into the inner light-guiding core 14a of the quadruple-clad fiber 5.

[0077] If the incident laser beam 3 is linearly polarized and its electric field strength vector is oriented in the X-direction or the Y-direction, a maximum of four partial laser beams 3.1, 3.2, ... can be generated in the beam splitter 6. In this case, the first birefringent optical wedge 9a can be omitted, or an optical wedge made of an amorphous material can be arranged in the beam splitter 6 instead of the first birefringent optical wedge 9a. The optical wedge should have a wedge angle of 3α.

[0078] The device 4 shown in Fig. 6 has two polarization-rotating devices 12a, 12b. Therefore, only two degrees of freedom are available for distributing the power of the laser beam 3 among the four light-guiding cores 14a-d of the quadruple-clad fiber 5. Unlike the examples described above, an arbitrary distribution of the power of the laser beam 3 among the four light-guiding cores 14a-d is not possible, i.e., arbitrary division ratios cannot be set. However, it is still possible to couple the entire power of the laser beam 3 into one of the light-guiding cores 14a-d.

[0079] Instead of the polarization-rotating devices 12, 12a, 12b described above in the form of rotatable Å / 2 plates, polarization-influencing devices 12, 12a, 12b, ... can also be used. These devices have a retarding effect that changes the polarization state of the laser beam 3 or the partial beams 3.1, 3.2, ... but do not cause a rotation of the polarization direction. For example, the polarization-influencing devices 12, 12a, 12b, ... can be rotatable Å / 4 plates.

Claims

Patent claims Device (4) for coupling a laser beam (3) into a multiclad fiber (5), comprising: a beam splitter (6) for splitting the laser beam (3) into a plurality of partial laser beams (3.1, 3.2, ...), wherein the beam splitter (6) comprises at least two birefringent optical wedges (9a, 9b, ...) and at least one polarization-influencing device (12; 12a, 12b, ...) with an adjustable polarization-influencing effect, which is arranged between the birefringent optical wedges (9a, 9b, ...), and a coupling optics (7) for coupling the partial laser beams (3.1, 3.2, ...) emerging from the beam splitter (6) into the multiclad fiber (5), wherein the coupling optics (7) is designed to couple at least two of the partial laser beams (3.1, 3.2, ...) emerging from the beam splitter (6) into at least two different light-conducting cores (14a, 14b, ...) of the multi-clad fiber (5). Device according to claim 1, in which the multi-clad fiber (5) is rotationally symmetrical and has an inner light-conducting core (14a) and at least one annular light-conducting core (14b, 14c, 14d), and in which the coupling optics (7) are designed to couple two partial laser beams (3.1, 3.2) emerging from the beam splitter (6) with different polarization states (s, p), the propagation direction of which corresponds to a beam direction (X) of the laser beam (3) entering the beam splitter (6), into the inner core (14a) of the multi-clad fiber (5). Device according to claim 1 or 2, which is designed to introduce into a respective light-conducting core (14a-d) of the multi-clad fiber (5) one or more pairs of partial laser beams (3.1, 3.2; ...) with two different polarization states (s, p). Device according to one of the preceding claims, in which the beam splitter (6) has a first number of equally oriented birefringent optical wedges (9a) and a second number of birefringent optical wedges (9b; 9b, 9c; 9b-d) that are oriented opposite to the first number of birefringent optical wedges (9a), wherein the sum of the wedge angles (α; 2α; 3α) of the first number of birefringent optical wedges (9a) corresponds to the sum of the wedge angles (α; 2α; 3α) of the second number of birefringent optical wedges (9b; 9b, 9c; 9b-d). Device according to one of the preceding claims, in which the beam splitter (6) has exactly two birefringent optical wedges (9a, 9b) that are oppositely oriented and have the same wedge angle (α).Device according to one of claims 1 to 4, wherein the beam splitter (6) has a number N of birefringent optical wedges (9b, 9c; 9b-d) with the same orientation, which are preferably arranged successively in the beam path of the laser beam (3). Device according to claim 6, wherein the beam splitter (6) has an oppositely oriented birefringent optical wedge (9a) in front of the number N of optical wedges (9b, 9c; 9b-d) with the same orientation in the beam path of the laser beam (3). Device according to claim 7, wherein the number N of similarly oriented birefringent optical wedges (9b, 9c; 9b-d) have an identical wedge angle (α), and wherein the oppositely oriented birefringent optical wedge (9a) has a wedge angle (N a) that corresponds to N times the identical wedge angle (α).Device according to one of claims 6 to 8, in which the beam switch (6) has a number N = 2 of equally oriented birefringent optical wedges (9b, 9c) and the coupling optics (7) for coupling at least three of the partial laser beams (3.1, 3.2, ...) into different light-conducting cores (14a-c) of a. Triple-clad fiber (5) is formed, or in which the beam splitter (6) has a number N = 3 of equally oriented birefringent optical wedges (9b-d) and the coupling optics (7) are designed to couple at least four of the partial laser beams (3.1, 3.2, ...) into different light-guiding cores (14a-c) of a quadruple-clad fiber (5). Device according to claim 6, in which the beam splitter (6) has a number N = 2 of equally oriented birefringent optical wedges (9b, 9c), wherein a first birefringent optical wedge (9b) in the beam path has a wedge angle (2a) that is twice as large as a wedge angle (a) of the second birefringent optical wedge (9c) in the beam path.Device according to claim 10, wherein the beam switch (6) in the beam path of the laser beam (3) has, upstream of the two equally oriented birefringent optical wedges (9b, 9c), an oppositely oriented birefringent optical wedge (9a), the wedge angle (3a) of which corresponds to three times the wedge angle (α) of the second of the equally oriented birefringent optical wedges (9b) in the beam path. Device according to claim 10 or 11, wherein the coupling optics (7) are designed to couple at least one partial laser beam (3.1, 3.2, ...), preferably at least one pair of partial laser beams (3.1, 3.2; ...) with two different polarization states (s, p), into a respective light-conducting core (14a-d) of a rotationally symmetric quadruple-clad fiber (5).Device according to one of the preceding claims, further comprising: a control device (13) for adjusting the polarization-influencing effect of the at least one polarization-influencing device (12;. 12a, b; 12a-c) to adjust a splitting ratio of the laser beam (3) when coupling into the at least two different light-conducting cores (14a, 14b, ...) of the multiclad fiber (5). Device according to claim 13, wherein the control device (13) for Adjustment of a polarization-influencing effect of the at least one polarization-influencing device (12; 12a,b; 12a-c) is configured such that no power of the laser beam (3) is coupled into at least one of the light-conducting cores (14a, 14b, ...) of the multi-clad fiber (5) and / or such that the entire power of the laser beam (3) is coupled into at least one of the light-conducting cores (14a, 14b, ...) of the multi-clad fiber (5). Device according to one of the preceding claims, wherein the laser beam (3) entering the beam splitter (6) is linearly polarized, and the beam splitter (6) has a λ / 4 delay element (15) in the beam path upstream of the first birefringent optical wedge (9a).Optical system (1) comprising: a multi-clad fiber (5), preferably a double-clad fiber, a triple-clad fiber or a quadruple-clad fiber, and a device (4) according to one of the preceding claims for coupling the laser beam (3) into the multi-clad fiber (5).

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