Apparatus and optical system for coupling laser beam into multi-clad optical fiber

Abstract

The invention relates to a device (4) for coupling a laser beam (3) into a multi-clad optical fiber (5), comprising: a beam splitter (6) for splitting the laser beam (3) into a plurality of sub-laser beams (3.1, 3.2,...), where the beam splitter (6) comprises at least two birefringent optical wedges (9a, 9b,...) and at least one polarization rotation device (12a, 12b,...), at least one polarization rotation device having an adjustable polarization rotation effect and arranged between the birefringent optical wedges (9a, 9b,...); the invention relates to a multi-clad optical fiber (5) comprising a plurality of sub-laser beams (3.1, 3.2,...) emitted from a beam splitter (6), and coupling-in optics (7) for coupling the sub-laser beams (3.1, 3.2,...) emitted from the beam splitter (6) into the multi-clad optical fiber (5), the coupling-in optics (7) being designed to couple at least two of the sub-laser beams (3.1, 3.2,...) emitted from the beam splitter (6) into at least two different light guide cores (14a, 14b,...) of the multi-clad optical fiber (5). The invention also relates to an optical system (1) comprising a multi-clad optical fiber (5) and a device (4) as described above for coupling a laser beam (3) into the multi-clad optical fiber (5).

Description

Technical Field

[0001] The invention relates to a device for coupling a laser beam into a multi-clad optical fiber. The invention also relates to an optical system having such a device and a multi-clad optical fiber. Background Art

[0002] Multi-clad optical fibers 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 for generating a laser beam. The target location can be, for example, a processing optical device that shapes the laser beam and then applies the laser beam to a workpiece to process the workpiece. The processing can be, for example, a welding process or a cutting process.

[0003] Different processing processes usually have different requirements for characteristic laser beam parameters such as focal spot diameter, intensity distribution, beam profile, etc.

[0004] It is known from EP2556397 that laser beam components coupled into the inner core or outer core of a multi-clad optical fiber provide different beam characteristics and beam quality in the outcoupled laser beam. In order to adjust the beam quality, the laser beam is separated into two sub-laser beams by a mechanically retractable wedge-shaped beam splitter (Keilweiche), which are coupled into different cores of a double-clad optical fiber. However, the wedge-shaped beam splitter for adjusting the beam quality is difficult to manufacture, and the power distribution between the inner core and the outer core is affected by changes in the position of the laser beam on the wedge-shaped beam splitter (detuning sensitivity).

[0005] US 10914902 describes various variants of coupling a laser beam into a double-clad optical fiber, wherein polarization separation of the incident laser beam is achieved by means of a birefringent element and the two resulting sub-laser beams are mapped into different cores of the double-clad optical fiber.

[0006] Purpose of the Invention

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

[0008] This object is achieved by a device for coupling a laser beam, the device comprising: a beam splitter for splitting a laser beam into a plurality of sub-laser beams, wherein the beam splitter comprises at least two birefringent wedges and at least one polarization influencing device, wherein the at least one polarization influencing device has an adjustable polarization influencing effect and is arranged between the birefringent wedges; and a coupling-in optical device for coupling the sub-laser beams emitted from the beam splitter into a multi-clad optical fiber, wherein the coupling-in optical device is designed to couple at least two of the sub-laser beams emitted from the beam splitter into at least two different optical cores of the multi-clad optical fiber.

[0009] In the device described here, at least two birefringent wedges are used to separate a laser beam into a plurality of sub-laser beams. This utilizes the fact that a regular (ordentlicher) sub-laser beam and an unconventional (auβerordentlicher) sub-laser beam are formed in the birefringent wedge, which have a difference angle when emerging from the beam exit surface of the birefringent wedge. The two sub-laser beams emerging from the birefringent wedge are linearly polarized and have mutually perpendicular polarization directions. The birefringent material of the birefringent wedge is preferably a uniaxial crystal, such as calcite or quartz. The optical axis of the crystal is usually oriented perpendicularly to the optical axis of the beam splitter. The beam entrance surface or the beam exit surface of the birefringent wedge is also usually oriented perpendicularly to the optical axis of the beam splitter.

[0010] When passing through the polarization influencing device, the polarization directions of the two linearly polarized sub-laser beams emerging from the birefringent wedge are influenced or manipulated so that the weights of the s-polarization component and the p-polarization component of the sub-beams are changed. When passing through a subsequent birefringent wedge in the beam path, the corresponding sub-laser beams are thus separated again into a regular sub-laser beam and an unconventional sub-laser beam. Thus, when using, for example, two birefringent wedges in a beam splitter, a total of up to four sub-laser beams can be generated, which are oriented at a plurality of different angles relative to the optical axis of the beam splitter.

[0011] With the aid of the coupling-in optics, the angular distribution of the sub-laser beams is converted into a spatial distribution at the end face of the multi-clad optical fiber. The exit angle of the sub-laser beams from the beam splitter is matched to the coupling-in optics and causes a spatial offset in the focal plane of the coupling-in optics, which is matched to the geometry of the multi-clad optical fiber, so that the sub-laser beams are coupled into the multi-clad optical fiber at the location of the light-guiding core. The coupling-in optics can have one or more optical elements. The coupling-in optics generally include at least one focusing optical element for focusing the sub-laser beams on a focal plane, on which the end face of the multi-clad optical fiber into which the laser beam is to be coupled is located.

[0012] The polarization-influencing device can be, for example, a rotatable delay element in the form of a delay wave plate, for example a rotatable delay element in the form of a λ / 2 wave plate or a λ / 4 wave plate. A delay wave plate usually causes a phase shift between two mutually perpendicular polarization directions. A λ / 2 wave plate causes a rotation of the polarization direction of a corresponding linearly polarized sub-beam, a λ / 4 wave plate can convert a corresponding linearly polarized sub-beam into a circularly polarized sub-beam or an elliptically polarized sub-beam. In order to set the splitting ratio between 0% and 100% (see below), it is advantageous if the polarization-influencing device is designed as a polarization rotation device, for example as a rotatable λ / 2 wave plate. A rotatable λ / 4 wave plate usually only allows the splitting ratio to be set in a reduced value range, for example in a value range between 50% and 100%.

[0013] Splitting the power of the laser beam with the aid of one or more polarization influencing devices has the following advantages over the use of a mechanically retractable wedge-type beam splitter: no scattered light and no diffraction effects occur at the edges of the mechanically retractable wedge. By setting the polarization influencing effect of the polarization influencing device, the distribution of the power of the laser beam to the individual light-guiding cores can also be changed. For example, if the polarization influencing device is set so that it has no polarization influencing effect, a number of sub-laser beams that is half the maximum possible number of sub-laser beams that can be generated using a wedge-type beam splitter can be generated (for example, when two birefringent wedges are used, two sub-laser beams are generated instead of four sub-laser beams). This can be used to prevent any power or sub-laser beam from coupling into a specific light-guiding core of a multi-clad optical fiber. For the purposes of the present application, "different" light-guiding cores are understood to mean cores that are spatially positioned differently or spatially separated from each other.

[0014] The number of polarization-influencing devices corresponds to the number of degrees of freedom when coupling into a multi-clad optical fiber. It is advantageous if the number of polarization-influencing devices corresponds to the number of light-conducting cores of the multi-clad optical fiber minus one. In this case, typically between 0% and 100% of the power of the laser beam can be coupled into the corresponding light-conducting core of the multi-clad optical fiber.

[0015] In one embodiment, the multi-clad optical fiber is designed to be rotationally symmetric and has an internal light-guiding core and at least one annular light-guiding core, and the coupling optical device is designed to couple two sub-laser beams with different polarization states emitted from a beam splitter into the inner core of the multi-clad optical fiber, and the propagation directions of the two sub-laser beams correspond to the beam direction of the laser beam injected into the beam splitter.

[0016] In principle, multi-clad optical fibers can be designed in different ways, for example in the form of linear multi-clad optical fibers in which a plurality of light-conducting cores are arranged adjacent to one another, or in the form of grid-like multi-clad optical fibers in which a plurality of cores are arranged in rows and columns. For the sake of simplicity, the following description assumes that the multi-clad optical fiber is a rotationally symmetrical multi-clad optical fiber having an inner, usually circular light-conducting core and one or more annular light-conducting cores surrounding the inner light-conducting core. So-called intermediate claddings are arranged between the light-conducting cores, which are non-light-conducting. The radially outermost light-conducting core can also be surrounded on its outside by a non-light-conducting cladding, to which a glass layer or the like can be attached.

[0017] The two sub-laser beams coupled into the inner core of the multi-clad optical fiber are those sub-laser beams formed at the exit portion of the first birefringent wedge and maintaining their polarization states when passing through the beam splitter. Therefore, the different polarization states are usually linear polarization states of the two sub-laser beams exiting the first birefringent wedge that are oriented perpendicular to each other. In this case, the beam splitter is usually designed so that the deflection angles of the two sub-laser beams that maintain their polarization states when passing through the beam splitter just compensate each other when passing through two or more birefringent wedges.

[0018] In the case of an orientation of the laser beam parallel to the optical axis of the beam splitter, the two emerging sub-laser beams whose deflection angles exactly compensate for each other are also oriented parallel to the optical axis of the beam splitter, but are laterally offset relative to the incident laser beam. In order to couple the two sub-laser beams into the core of the multi-clad optical fiber, the coupling optics usually focus the two sub-laser beams on the optical axis of the coupling optics, on which the core of the multi-clad optical fiber is positioned. Other sub-laser beams emerging from the beam splitter that are not oriented parallel to the incident laser beam are coupled into one or more annular light-guiding cores of the multi-clad optical fiber. In principle, the optical axis of the coupling optics can coincide with the optical axis of the beam splitter. However, it is usually advantageous if the optical axis of the coupling optics is laterally offset relative to 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 usually corresponds essentially to the lateral offset of the two sub-laser beams from the incident laser beam, wherein the deflection angles exactly compensate for each other.

[0019] In a further embodiment, the device is designed to couple one or more pairs of sub-laser beams having two different polarization states into respective light-conducting cores of a multi-clad optical 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 paired coupling of sub-laser beams having mutually perpendicular polarization states allows polarization-independent coupling of the sub-laser beams into the respective light-conducting cores. This type of coupling is particularly possible when using rotationally symmetric multi-clad optical fibers, wherein the sub-laser beams can be coupled into the respective annular cores at two positions opposite to each other in the radial direction in each case. For polarization-independent coupling, it is necessary that the number of birefringent wedges corresponds at least to the number of light-conducting cores of the multi-clad optical fiber. In case polarization-independent coupling is omitted, the number of wedges can be a number which is the number of light-conducting cores of the multi-clad optical fiber minus one.

[0020] In one embodiment, the optical beam splitter has a first number of birefringent wedges oriented in the same direction and a second number of birefringent wedges oriented opposite to the first number of birefringent wedges, wherein the sum of the wedge angles of the first number of birefringent wedges is equivalent to the sum of the wedge angles of the second number of birefringent wedges.

[0021] The same orientation of two or more birefringent wedges means that their wedge tips are positioned on the same side of the optical axis or beam direction of the laser beam. Opposite orientation is to be understood such that the wedge tips of the respective birefringent wedges are arranged on opposite sides with respect to the optical axis. The birefringent wedges in this case are usually made of the same birefringent material and the optical axes of the birefringent wedges are oriented parallel to each other. It is to be understood that the birefringent wedges do not necessarily have to have a wedge tip as long as the laser beam or the sub-laser beam does not pass through the wedge tip. In this case, the wedge angle is also to be understood as the angle at which the beam entrance surface and the beam exit surface are arranged with respect to each other.

[0022] The fact that the sum of the wedge angles of the first number of birefringent wedges of the same orientation is equal to the sum of the wedge angles of the second number of birefringent wedges of the same orientation generally results in that the two sub-laser beams described above maintain their orientation when passing through the beam splitter and are coupled into the inner core of the multi-clad optical fiber. It should be understood that a small deviation of the sum of the wedge angles of the first number of birefringent wedges and the sum of the wedge angles of the second number of birefringent wedges is tolerable, provided that the sub-laser beams can be coupled into the corresponding light-conducting core of the multi-clad optical fiber despite this deviation. The coupling of the two sub-laser beams described above into the inner light-conducting core of the multi-clad optical fiber can also be achieved if different wedge angles are used. In this case, it is generally required that the birefringent materials of the birefringent wedges are different from each other and that the wedge angles of the birefringent wedges are matched to the different birefringent materials.

[0023] In one embodiment, the beam splitter has exactly two birefringent wedges oriented in opposite directions and having the same wedge angle. In this embodiment, the multi-clad optical fiber is typically a substantially rotationally symmetrical double-clad optical fiber having an inner light-guiding core and exactly one annular light-guiding core. As described above, in this case, two sub-laser beams emerging from the beam splitter, whose propagation directions correspond to the beam directions of the laser beam, are coupled into the light-guiding inner core, and the other two sub-laser beams emerging from the beam splitter are coupled into the light-guiding annular core of the double-clad optical fiber. The two birefringent wedges are typically identical in design and are twisted 180° relative to each other, so that the beam exit surface of the first birefringent wedge in the beam path and the beam entrance surface of the second birefringent wedge in the beam path are oriented parallel to each other.

[0024] In an alternative embodiment, the beam splitter has a number N of birefringent wedges of identical orientation, which are preferably arranged one after the other in the beam path of the laser beam. The number N of birefringent wedges of identical orientation is such that N>1, i.e. there are two or more birefringent wedges of identical orientation. It is generally possible that the beam splitter comprises a number N of birefringent wedges of identical orientation, or that the beam splitter has no further birefringent wedges. In this case, a polarization-influencing device can be arranged between every two birefringent wedges that follow one another in the beam path and, if necessary, can additionally be arranged upstream of the first birefringent wedge in the beam path.

[0025] In a further development of this embodiment, the beam splitter has oppositely oriented birefringent wedges upstream of the number N of equally oriented wedges in the beam path of the laser beam. The oppositely oriented birefringent wedges can follow the above described condition that the sum of the wedge angles should be equal.

[0026] In the case where the laser beam incident on the beam splitter has a fixed polarization state, the provision of oppositely oriented wedges can be omitted, since in this case the separation is not affected, i.e., in this case a maximum of 2 N Alternatively, the oppositely oriented wedge may be replaced by a wedge made of a non-birefringent material, such as an amorphous material. If its wedge angle satisfies the conditions specified above, after passing through the beam splitter, one of the sub-laser beams is oriented parallel to the beam direction of the laser beam incident into the beam splitter and is coupled into the core of the multi-clad optical fiber.

[0027] In another study, a number N of birefringent wedges of the same orientation have the same wedge angle, and the birefringent wedges of the opposite orientation have a wedge angle equivalent to N times the same wedge angle. In this way, the condition of the sum of the wedge angles of the birefringent wedges described above can be met. Typically, in this embodiment, a polarization-affecting device is arranged between every two birefringent wedges in the birefringent wedges.

[0028] In other studies, the beam splitter has a number N = 2 birefringent wedges with the same orientation, and the coupling-in optics are designed to couple at least three of the sub-laser beams into different light-guiding cores of a triple-clad fiber, or the beam splitter has a number N = 3 birefringent wedges with the same orientation, and the coupling-in optics are designed to couple at least four of the sub-laser beams into different light-guiding cores of a quad-clad fiber. The maximum number of sub-laser beams that can be generated using the number N of birefringent wedges is 2 N In the case of a number N of birefringent wedges of the same orientation arranged upstream of birefringent wedges of opposite orientation, a maximum number of 2 N+1 The invention provides a plurality of sub-laser beams, wherein half of the sub-laser beams have a first linear polarization state (s-polarization) and the other half of the sub-laser beams have a second polarization state (p-polarization) oriented perpendicular to the first polarization state.

[0029] In the case of a triple-clad optical fiber, a maximum of eight sub-laser beams can be generated when three birefringent wedges are used, wherein two sub-laser beams with different polarization states are coupled into the inner light-guiding core, two pairs of sub-laser beams with each pair of different polarization states are coupled into the first annular light-guiding core, and two sub-laser beams with different polarization states are coupled into the second annular light-guiding core surrounding the first annular light-guiding core.

[0030] In the case of a quad-clad optical fiber, a maximum of sixteen sub-laser beams can be generated when four birefringent wedges are used, wherein two sub-laser beams with different polarization states are coupled into the inner light-guiding core, three pairs of sub-laser beams with each pair of different polarization states are coupled into the first annular light-guiding core, three pairs of sub-laser beams with each pair of different polarization states are coupled into the second annular light-guiding core surrounding the first annular light-guiding core, and two sub-laser beams with different polarization states are coupled into the third annular light-guiding core surrounding the second annular light-guiding core.

[0031] Therefore, both triple-clad and quad-clad fibers achieve polarization-independent coupling of sub-laser beams into the corresponding light-guiding cores. It should be understood that a beam splitter designed in the manner described above can also be used to couple sub-laser beams into multi-clad fibers having more than three or four light-guiding cores.

[0032] In an alternative embodiment, the beam splitter has a number N=2 birefringent wedges of identical orientation, wherein the first birefringent wedge in the beam path has a wedge angle that is twice as large as the wedge angle of the second birefringent wedge in the beam path. This embodiment is particularly suitable for coupling a sub-laser beam into a quad-clad fiber. In this case, the beam splitter can have only two birefringent wedges of identical orientation, wherein in this case a maximum of four sub-laser beams can be coupled into four light-conducting cores of a quad-clad fiber. In this case, a polarization-influencing device is arranged between the two birefringent wedges. If necessary, a polarization-influencing device can additionally be provided upstream of the first birefringent wedge.

[0033] In a further development of this embodiment, an oppositely oriented birefringent wedge is provided in the beam path of the beam splitter laser beam upstream of the same oriented birefringent wedge, the wedge angle of the oppositely oriented birefringent wedge being equal to three times the wedge angle of the second birefringent wedge in the same oriented birefringent wedge in the beam path. In this way, the conditions described above regarding the wedge angle of the birefringent wedge can be met. In the case where the incident laser beam has a polarization direction oriented perpendicularly or parallel to the optical axis of the birefringent wedge, the oppositely oriented birefringent wedge can be omitted or replaced by a wedge made of non-birefringent material.

[0034] In a further development of this embodiment, the coupling-in optics is designed to couple at least one sub-laser beam, preferably at least one pair of sub-laser beams having two different polarization states, into the corresponding light-conducting cores of the rotationally symmetric four-clad fiber. Even with a beam splitter designed in the manner described above, polarization-independent coupling of the sub-laser beams into the light-conducting core of the four-clad fiber can be achieved.

[0035] In the embodiments described here, the beam splitter generally has two polarization-influencing devices, which are arranged between two birefringent wedges that follow one another in the beam path. In principle, it is possible to couple 100% of the power of a laser beam into each of the light-conducting cores using the beam splitter described here. However, due to the fact that there are only two degrees of freedom for adjusting the splitting ratio between the light-conducting cores, the power of the laser beam cannot be distributed arbitrarily to the four light-conducting cores of a quadruple-clad fiber.

[0036] In a further embodiment, the apparatus comprises a control device for adjusting the polarization influencing effect of at least one polarization influencing device in order to adjust the splitting ratio of the laser beam when coupled into at least two different light-guiding cores of the multi-clad optical fiber. The control device is used to electronically control the polarization rotation device. The polarization influencing device enables the polarization state of each incident sub-laser beam to be changed. The change can be, for example, a rotation of the electric field intensity vector of the corresponding incident sub-laser beam around the corresponding propagation direction of the sub-laser beam by a certain rotation angle.

[0037] Typically, the polarization influencing device is designed for continuous adjustment of the polarization influencing effect. As described above, a rotatably mounted retarding wave plate can be used as a polarization influencing device. If the device has a polarization rotation effect, the rotation angle can be continuously changed or adjusted, for example. In this case, the polarization influencing device can be, for example, a retarding wave plate in the form of a rotatably mounted λ / 2 wave plate, which can be rotated around the optical axis of the beam splitter by means of a control device acting on a suitable actuator. The polarization influencing device can alternatively be a Pockels cell, which can also be electronically controlled and which influences the polarization state of the incident sub-laser beam, in particular the rotation of the direction of polarization.

[0038] In a further embodiment, the control device is designed to adjust the polarization influencing effect of at least one polarization influencing device, wherein no power of the laser beam is coupled into at least one of the light-guiding cores of the multi-clad optical fiber, and / or wherein the entire power of the laser beam is coupled into at least one of the light-guiding cores of the multi-clad optical fiber.

[0039] In the case where the number of polarization influencing devices in the form of polarization rotators corresponds to the number of light-conducting cores of the multi-clad optical fiber minus one, any splitting ratio can generally be set between the light-conducting cores, i.e. the power of the laser beam can be arbitrarily distributed to the light-conducting cores. Thus, between 0% and 100% of the power of the laser beam can be coupled into the respective light-conducting cores. In the first case (0% coupling), no sub-laser beams are coupled into the respective light-conducting cores; in the second case, all sub-laser beams emerging from the beam splitter are coupled into the respective light-conducting cores. For example, in the case of a double-clad optical fiber, if the polarization influencing device does not produce a polarization influencing effect, the full power of the laser beam can be coupled into the inner light-conducting core. In the case where the polarization influencing device causes a polarization rotation of 90°, the full power of the laser beam can be coupled into the annular light-conducting core of the double-clad optical fiber.

[0040] In a further embodiment, the laser beam incident on the beam splitter is linearly polarized and the beam splitter has a λ / 4 delay element in the beam path upstream of the first birefringent wedge. For example, the λ / 4 delay element can be designed as a λ / 4 wave plate, whose preferred axis is oriented at 45° with respect to the optical axis of the birefringent wedge of the beam splitter. In this case, the power of the laser beam is oriented parallel or perpendicular to the optical axis of the birefringent wedge through the λ / 4 delay element with a splitting ratio of 50:50, regardless of the orientation of the linear polarization of the incident laser beam. This results in that the two sub-laser beams coupled into the same light-guiding core always have the same power.

[0041] The invention also relates to an optical system comprising: a multi-clad optical fiber, preferably a double-clad optical fiber, a triple-clad optical fiber or a quad-clad optical fiber; and a device as described above designed for coupling a laser beam into the multi-clad optical fiber. The optical system generally also comprises a laser for generating the laser beam coupled into the beam splitter. The laser wavelength of the laser beam is essentially arbitrary and adapted to the materials of the optical elements of the beam splitter, the coupling-in optics and the multi-clad optical fiber. In the case of a laser beam with a wavelength of 1030 nm, the birefringent wedge can be made, for example, of crystalline quartz transparent to this wavelength.

[0042] In the optical system described above, the polarization influencing device can be used to adjust the power distribution in the multi-clad fiber independently of the polarization of the laser beam injected into the beam splitter. The power distribution in the multi-clad fiber is also independent of the spatial intensity distribution of the incident laser beam and is largely independent of the pointing direction of the incident laser beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Further advantages of the invention are derived from the description and the drawings. Likewise, the features mentioned above and those yet to be presented can be used alone or in any desired combination in each case. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather as having an exemplary character for describing the invention.

[0044] In the attached picture:

[0045] Figure 1 shows a schematic representation of an optical system having a multi-clad optical fiber and a device for coupling a laser beam into the multi-clad optical fiber,

[0046] Figure 2 Shown through Figure 1 Schematic representation of the device beam splitter birefringent wedge laser beam,

[0047] Figure 3 Shown with Figure 1 A similar schematic representation, where the device is designed to couple a laser beam into a double-clad optical fiber,

[0048] Figures 4a to 4c Shown with Figure 1 A similar schematic representation, where the device is designed to couple a laser beam into a triple-clad optical fiber,

[0049] Figure 5 Shown with Figures 4a to 4c A similar schematic representation of a device designed to couple a laser beam into a quad-clad fiber, and

[0050] Figure 6 Shown with Figure 1 Similarly there is a schematic representation of another device designed to couple a laser beam into a quad-clad optical fiber.

[0051] In the following description of the figures, the same reference numerals are used for identical or functionally identical components. DETAILED DESCRIPTION

[0052] Figure 1 An optical system 1 is shown with a laser 2 for generating a laser beam 3. The laser beam 3 emitted from the laser 2 is incident on a device 4 designed to couple the laser beam 3 into a multi-clad optical fiber 5. The device 4 has a beam splitter 6 and a coupling optical device 7. The beam splitter 6 is used to split the laser beam 3 into a usually even number of sub-laser beams 3.1, ..., 3.M, which emerge from the beam splitter 6. Figure 1 In the example shown, two of the sub-laser beams 3.1, . . . , 3.M extend parallel to the optical axis 8 of the beam splitter 6, while the remaining sub-laser beams 3.1, . . . , 3.M are each oriented at an angle relative to the optical axis 8 of the beam splitter 6.

[0053] By means of the coupling-in optical device 7, the angular distribution of the sub-laser beams 3.1, 3.2, ... is converted into a spatial distribution at the end face of the multi-clad optical fiber 5, the end face of which is located in the focal plane of the coupling-in optical device 7. The exit angles of the sub-laser beams 3.1, 3.2, ... relative to the beam splitter 6 are matched to the coupling-in optical device 7 and cause a spatial offset in the focal plane of the coupling-in optical device 7, which is matched to the multi-clad optical fiber 5, so that the sub-laser beams 3.1, 3.2, ... are coupled into the multi-clad optical fiber 5 at the position of two or more light-guiding cores. In the simplest case, the coupling-in optical device 7 can be a focusing lens, but the coupling-in optical device 7 can also have a plurality of transmitting or reflecting optical elements.

[0054] The multi-clad optical fiber 5 may be a linear multi-clad optical fiber in which a plurality of light-guiding cores are arranged adjacent to each other and into which the sub-laser beams 3.1, 3.2, ... are coupled. Alternatively, the multi-clad optical fiber may be a grid-like multi-clad optical fiber in which a plurality of light-guiding cores are arranged in rows and columns, or a rotationally symmetric multi-clad optical fiber.

[0055] In order to split the laser beam 3 into a plurality of laser sub-beams 3.1, 3.2, ..., the beam splitter 6 has at least two birefringent wedges. Figure 2The effect of the first wedge 9a of the beam splitter 6 in the beam path on the laser beam 3 is described in more detail. The laser beam 3 has two polarization components s, p, wherein the electric field intensity vector is oriented parallel to the plane of the figure or perpendicular to the plane of the figure, which corresponds to the XZ plane of the XYZ coordinate system. The birefringent wedge 9a is made of a uniaxial crystal such as quartz or calcite. The optical axis O (in the X direction) of the birefringent wedge 9a is oriented perpendicularly to the optical axis 8 of the beam splitter 6. When passing through the optically anisotropic birefringent material of the birefringent wedge 9a, the laser beam 3 is separated into a first s-polarized sub-laser beam 3.1 and a second p-polarized sub-laser beam 3.2. The first sub-laser beam 3.1 forming a regular beam is oriented at a first angle β to the optical axis 8. The second sub-laser beam 3.2 forming an unconventional beam is oriented at a second angle γ to the optical axis 8. When passing through the birefringent wedge 8a, an angular difference δ=γ-β is thus generated between the first sub-laser beam 3.1 and the second sub-laser beam 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 wedge 9a. The angular difference δ together with the focal length f of the coupling-in optical system 7 determines the lateral distance Δx between the first sub-laser beam 3.1 and the second sub-laser beam 3.2 in the focal plane of the coupling-in optical system 7, whereby the following applies approximately: Δx=fδ.

[0056] exist Figure 2 In the example shown, the beam entrance surface 10a of the birefringent wedge 9a is oriented perpendicularly to the optical axis 8 of the beam splitter 6. The beam exit surface 10b of the birefringent wedge 8a is oriented at a wedge angle α relative to the XY plane perpendicular to the optical axis 8.

[0057] Figure 3 An optical system 1 is shown with a beam splitter 6 having a beam path in the beam path as shown in FIG. Figure 2 . A second birefringent wedge 9b in the beam path has the same construction as or is identical to the first birefringent wedge 9a and is rotated 180° relative to the first birefringent wedge in the XZ plane corresponding to the plane of the figure. Here, the beam exit surface 10b of the first birefringent wedge 9a is oriented parallel to the beam incident surface 11a of the second birefringent wedge 9b. The beam exit surface 11b of the second birefringent wedge 9b is oriented perpendicular to the optical axis 8 of the beam splitter 6. The wedge angles α of the two birefringent wedges 9a, 9b are equal in size.

[0058] The laser beam 3 incident on the beam splitter 6 passes through the first birefringent wedge 9a and is separated into two sub-laser beams 3.1 and 3.2. Figure 2As described. The two sub-laser beams 3.1 and 3.2 are incident on the polarization influencing device 12, which is designed in the form of a rotatable λ / 2 wave plate. The polarization influencing device 12 in the form of a rotatable λ / 2 wave plate has a polarization rotation effect and is therefore referred to as the polarization rotation device 12 below. The control device 13 is used to electronically control the polarization rotation device 12. In the example shown, the control device 13 is used to rotate the λ / 2 wave plate around the optical axis 8 of the beam splitter 6. In this way, the polarization rotation effect of the polarization rotation device 12 can be continuously adjusted, more precisely, the rotation angle when rotating the direction of polarization can be continuously adjusted. By rotating the direction of polarization, an outgoing first sub-laser beam 3.1' is formed by the s-polarized first sub-laser beam 3.1 incident on the polarization rotation device 12, and the outgoing first sub-laser beam has a polarization component of s polarization and a polarization component of p polarization. Therefore, an outgoing second sub-laser beam 3.2' is formed by the p-polarized second sub-laser beam 3.2, and the outgoing second sub-laser beam has a polarization component of s polarization and a polarization component of p polarization.

[0059] The two sub-laser beams 3.1', 3.2' emerging from the polarization rotation device 12 pass through the second birefringent wedge 9b in the beam path and are separated there into a total of four sub-laser beams 3.1 to 3.4. Two of the sub-laser beams 3.1, 3.2 emerge from the second birefringent wedge 9b oriented parallel to the optical axis 8. Two other sub-laser beams 3.3, 3.4 emerge from the second birefringent wedge 9b at an angle to the optical axis 8 of the beam splitter 6.

[0060] As in Figure 3 It can also be seen that the first sub-laser beam 3.1 and the second sub-laser beam 3.2, which are oriented parallel to the incident laser beam 3 or to the optical axis 8 of the beam splitter 6 when emerging from the beam splitter 6, are coupled into the inner light-guiding core 14a of the multi-clad optical fiber 5, which is designed as a rotationally symmetric double-clad optical fiber. The third sub-laser beam 3.3 and the fourth sub-laser beam 3.4 are coupled into the second light-guiding core 14b of the double-clad optical fiber 5, which is designed in an annular shape and surrounds the inner light-guiding core 14a, as shown in FIG. Figure 3 A cross section of the double-clad optical fiber 5 shown on the right in FIG. 7 can be seen. Here, the optical axis 8a of the coupling-in optical device 7 and the central axis of the double-clad optical fiber 5 are laterally offset relative to the optical axis 8 of the beam splitter 6, i.e., the amount of the offset is the amount of the offset of the two sub-laser beams 3.1, 3.2 at the exit from the beam splitter 6 relative to the incident laser beam 3 parallel to the optical axis 8. This allows the sub-laser beams 3.1, 3.2, ... to be incident on the incident end of the double-clad optical fiber 5 at the smallest possible angle of incidence.

[0061] exist Figure 3In FIG. 1 , s-polarized laser beams 3.1 and 3.4 are shown using white arrows, and p-polarized laser beams 3.2 and 3.3 are shown using black arrows. Figure 3 As can be seen in FIG. 1 , a first pair of sub-laser beams 3.1, 3.2 having different polarization states (s-polarization or p-polarization) are coupled into the inner light-guiding core 14a of the double-clad optical fiber 5, and a second pair of sub-laser beams 3.3, 3.4 having different polarization states (s-polarization or p-polarization) are coupled into the annular light-guiding core 14b of the double-clad optical fiber 5. In the inner light-guiding core 14a and the annular light-guiding core 14b, the number of s-polarized sub-laser beams or p-polarized sub-laser beams 3.1, 3.2 and 3.3, 3.4 is the same, i.e., coupling of the laser beam 3 into the respective cores 14a, 14b of the double-clad optical fiber 5 occurs independently of polarization.

[0062] The splitting ratio, i.e. the respective ratio of the power of the laser beam 3 coupled into the inner light-guiding core 14a and the power coupled into the annular light-guiding core 14b, can be adjusted by controlling the polarization-influencing device 12 by means of the control device 13. In principle, the power of the laser beam 3 can be distributed between the two light-guiding cores 14a, 14b as required, i.e. between 0% and 100% of the power of the laser beam 3 can be coupled into the respective light-guiding cores 14a, 14b. The polarization rotation effect of the polarization rotation device 12 can be specifically selected so that no power of the laser beam 3 is coupled into the respective light-guiding cores 14a, 14b, while the full power of the laser beam 3 is coupled into the respective other light-guiding core 14b, 14a.

[0063] Figures 4a to 4c An optical system 1 is shown in which a multi-clad optical fiber 5 is designed as a triple-clad optical fiber and has an inner light-guiding core 14a and two annular light-guiding cores 14b, 14c surrounding the inner light-guiding core 14a. Figure 4a As can be seen in FIG. 1 , the beam splitter 6 in this case is designed to split the incident laser beam 3 into a total of eight sub-laser beams 3.1 to 3.8. For this purpose, the beam splitter 6 has three birefringent wedges 9a, 9b, 9c. The second birefringent wedge 9b and the third birefringent wedge 9c in the beam path have the same orientation relative to the optical axis 8, and the first birefringent wedge 9a in the beam path is oriented opposite to the second birefringent wedge 9b and the third birefringent wedge 9c in the beam path.

[0064] Second birefringent wedge 9b and third birefringent wedge 9c in the beam path have the same wedge angle α. First birefringent wedge 9a in the beam path has a wedge angle 2α that is twice as large as the wedge angle α of second birefringent wedge 9b and third birefringent wedge 9c. Three birefringent wedges 9a to 9c are made of the same birefringent optical material.

[0065] Two sub-laser beams 3.1, 3.2 of a total of eight sub-laser beams 3.1 to 3.8 emerging from the beam splitter 6 are oriented parallel to the optical axis 8 of the beam splitter 6 or to the incident laser beam 3 and are coupled via the coupling-in optical device 7 and coupled into the inner light-guiding core 14a of the multi-clad optical fiber 5, as combined with Figure 3 To simplify the diagram, Figures 4a to 4c The lateral offset between the optical axis 8 of the beam splitter 6 and the optical axis 8a of the coupling-in optical device 7 is not shown in the following illustrations. Of the remaining six sub-laser beams 3.3 to 3.8, four sub-laser beams 3.3 to 3.6 are coupled into the second light-guiding core 14b of the triple-clad optical fiber 5, and two sub-laser beams 3.7, 3.8 are coupled into the third light-guiding core 14c of the triple-clad optical fiber 5. The sub-laser beams 3.1, 3.2, 3.3 to 3.6, 3.7, 3.8 coupled into the corresponding cores 14a to 14c are polarized differently in pairs, as shown in FIG. Figure 4a As indicated by the circles drawn in the cross section of the triple-clad optical fiber 5. Here, the white circle corresponds to the s-polarized sub-laser beam, and the black circle corresponds to the p-polarized sub-laser beam.

[0066] like Figure 4a It can also be seen in FIG. 1 that the beam splitter 6 has two polarization rotation devices 12a, 12b, which are electronically controlled by means of a control device 13 in order to distribute the power of the laser beam 3 to the three light-conducting cores 14a to 14c of the triple-clad optical fiber. Figure 4a In the device 4 shown in FIG. 4 , between 0% and 100% of the power of the laser beam 3 can be coupled into the respective light guiding core 14 a to 14 c.

[0067] Figure 4b The following situation is shown: the control device 13 controls the first polarization rotation device 12a in the beam path so that it has no polarization rotation effect, i.e. the optical axis of the λ / 2 wave plate is oriented in the X direction or in the Y direction. This has the following consequence: the separation of the two sub-laser beams 3.1, 3.2 formed at the first birefringent wedge 9a does not occur at the second birefringent wedge 9b. Therefore, in this case, the beam splitter 6 generates only four sub-laser beams 3.1 to 3.4, which are coupled into the first light guiding core 14a and the second light guiding core 14b.

[0068] By adjusting the polarization rotation effect of the second polarization rotation device 12b, the splitting ratio during coupling to the first light guiding core 14a and the second light guiding core 14b can be adjusted. If the second polarization rotation device 12b does not have a polarization rotation effect due to its optical axis being oriented in the X direction or in the Y direction, only the first sub-laser beam 3.1 and the second sub-laser beam 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 rotation device 12b is oriented at 45° relative to the X direction and the Y direction, two sub-laser beams 3.3, 3.4 are generated, which are coupled into the first annular light guiding core 14b.

[0069] Figure 4c The following situation is shown: the control device 13 controls the second polarization rotation device 12a in the beam path so that it has no polarization rotation effect, that is, the optical axis of the λ / 2 wave plate is oriented in the X direction or in the Y direction. In this case, the separation of the four sub-laser beams 3.1, 3.2, 3.7, 3.8 formed when passing through the second birefringent wedge 9b does not occur at the third birefringent wedge 9c in the beam path. Therefore, in this case, the beam splitter 6 only generates four sub-laser beams 3.1, 3.2, 3.7, 3.8, which are coupled into the first light guiding core 14a and the third light guiding core 14c. By adjusting the polarization rotation effect of the first polarization rotation device 12a, the splitting ratio during coupling into the first light guiding core 14a and the third light guiding core 14c can also be adjusted in this case. If the first polarization rotation device 12a does not have a polarization rotation effect, only the first sub-laser beam 3.1 and the second sub-laser beam 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 rotation device 12a is oriented at 45° with respect to the X-direction or the Y-direction, two sub-laser beams 3.7, 3.8 are generated, which are coupled into the second annular light-guiding core 14c.

[0070] In the case where the same power is to be coupled into all three light-guiding cores 14a to 14c, the first polarization rotator 12a in the form of a λ / 2 wave plate is oriented at an angle of 22.5° relative to the X direction or the Y direction. The second polarization rotator 12b in the form of a λ / 2 wave plate is oriented at an angle of 17.63° relative to the X direction or the Y direction. Figures 4a to 4c In the device 4 described in , it is fundamentally advantageous if the distances between the light guiding cores 14a to 14c or the distances between the sub-laser beams 3.1, 3.2, . . . coupled into adjacent light guiding cores 14a to 14c are approximately of equal size.

[0071] In the case where the incident laser beam 3 is linearly polarized, a Figure 4a. In this case, the optical axis of the λ / 4 delay element 15 is oriented at 45° with respect to the X direction or with respect to the optical axis of the first birefringent wedge 9a. The power of the linearly polarized laser beam 3 is split by the λ / 2 delay element 15 with a splitting ratio of 50:50 into two polarization components oriented in the X direction or in the Y direction, respectively, wherein the splitting is independent of the orientation of the electric field intensity vector of the incident linearly polarized laser beam 3. This generates two sub-laser beams 3.1, 3.2, ... with different polarization states, which are coupled into the same light guiding cores 14a to 14c of the triple-clad optical fiber 5 and thus have the same power.

[0072] In the case where the incident laser beam 3 is linearly polarized and its electric field intensity vector is oriented in the X direction or in the Y direction, a maximum of four sub-laser beams 3.1, 3.2, ... can be generated in the beam splitter 6. In this case, the first birefringent wedge 9a can be omitted. Alternatively, instead of the first birefringent wedge 9a, a wedge can be arranged in the beam splitter 6, which is not made of a birefringent material and has a wedge angle 2α.

[0073] Figure 5 A device 4 is shown which is designed for coupling a laser beam 3 into a radially symmetrical quadruple-clad optical fiber 5 having an inner light-conducting core 14 a and three annular light-conducting cores 14 b to 14 d surrounding the inner light-conducting core 14 a . Figure 5 The device 4 shown in Figures 4a to 4c 4 differs in that it has three birefringent wedges 9b, 9c, 9d of identical orientation and three polarization rotators 12a to 12c which are arranged in the beam path between two consecutive birefringent wedges 9a, 9b, between two birefringent wedges 9b, 9c, between two birefringent wedges 9c, 9d. The first birefringent wedge 9a in the beam path has a wedge angle 3α which corresponds to three times the (identical) wedge angle α of the three wedges 9b to 9c of identical orientation.

[0074] Figure 5 The beam splitter 6 of the device 4 shown in FIG. 4 can couple up to sixteen sub-laser beams 3.1 to 3.16 into the four-clad optical fiber 5. Figures 4a to 4c The separation into sub-laser beams 3.1 to 3.16 is performed in the manner described. Two of the sub-laser beams 3.1, 3.2 emerge from 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 optical fiber 5. Respective six of the sub-laser beams 3.1 to 3.16 are coupled into the first annular light-guiding core 14b and the second annular light-guiding core 14c, and two of the sub-laser beams 3.1 to 3.16 are coupled into the third annular light-guiding core 14d. By means of three polarization rotation devices 12a to 12c in combination Figures 4a to 4c The beam splitting ratio is adjusted in the manner described.

[0075] In the case where the incident laser beam 3 is linearly polarized and its electric field intensity vector is oriented in the X direction or in the Y direction, a maximum of eight sub-laser beams 3.1, 3.2, ... can be generated in the beam splitter 6. In this case, the first birefringent wedge 9a can be omitted, or, instead of the first birefringent wedge 9a, a wedge which is not made of a birefringent material and has a wedge angle 3α can be arranged in the beam splitter 6. The material of the wedge can be an amorphous (glass) material, such as quartz glass.

[0076] Figure 6 Shown with Figure 5 The device 4 shown in FIG. 4 is similarly designed for coupling a laser beam 3 into a quad-clad optical fiber 5. The device 4 has three birefringent wedges 9a to 9c. The second birefringent wedge 9b and the third birefringent wedge 9c in the beam path have the same orientation, and the first birefringent wedge 9a in the beam path is oriented opposite to the other two wedges 9b, 9c. The second birefringent wedge 9b has a wedge angle 2α that is twice as large as the wedge angle α of the third birefringent wedge 9c. The first birefringent wedge 9a has a wedge angle 3α that is three times as large as the wedge angle α of the third birefringent wedge 9c.

[0077] like Figure 6 As can be seen in FIG. 1 , the beam splitter 6 is designed to generate at most eight sub-laser beams 3.1 to 3.8. Two differently polarized sub-laser beams 3.1, 3.2, ... are coupled in each case into one of the light-conducting cores 14a to 14d of the quad-clad optical fiber 5. Two of the sub-laser beams 3.1, 3.2 which emerge from the beam splitter 6 and are oriented parallel to the optical axis 8 or to the beam direction Z of the incident laser beam 3 are coupled here into the inner light-conducting core 14a of the quad-clad optical fiber 5.

[0078] In the case where the incident laser beam 3 is linearly polarized and its electric field intensity vector is oriented in the X direction or in the Y direction, a maximum of four sub-laser beams 3.1, 3.2, ... can be generated in the beam splitter 6. In this case, the first birefringent wedge 9a can be omitted, or alternatively, instead of the first birefringent wedge 9a, a wedge made of an amorphous material can be arranged in the beam splitter 6. The wedge should have a wedge angle 3α.

[0079] Figure 6The device 4 shown in FIG. 4 has two polarization rotation devices 12a, 12b. Therefore, only two degrees of freedom are available for distributing the power of the laser beam 3 to the four light guiding cores 14a to 14d of the quad-clad optical fiber 5. Therefore, unlike the above-described example, it is not possible to arbitrarily distribute the power of the laser beam 3 between the four light guiding cores 14a to 14d, i.e., it is not possible to set an arbitrary splitting ratio. However, it is still possible to couple the entire power of the laser beam 3 into one of the light guiding cores 14a to 14d.

[0080] Instead of the polarization rotation means 12, 12a, 12b described above in the form of a rotatable λ / 2 wave plate, it is also possible to use polarization influencing means 12, 12a, 12b, ... which have a delay effect which changes the polarization state of the laser beam 3 or the sub-beams 3.1, 3.2, ..., but does not cause a rotation of the polarization direction. For example, the polarization influencing means 12, 12a, 12b, ... may be a rotatable λ / 4 wave plate.

Claims

1. A device (4) for coupling a laser beam (3) into a multi-clad optical fiber (5), the device comprising: A beam splitter (6) for splitting the laser beam (3) into a plurality of sub-laser beams (3.1, 3.2, ...), wherein the beam splitter (6) comprises at least two birefringent wedges (9a, 9b, ...) and at least one polarization influencing device (12; 12a, 12b, ...), the at least one polarization influencing device having an adjustable polarization influencing effect and being arranged between the birefringent wedges (9a, 9b, ...); and A coupling-in optical device (7) for coupling the sub-laser beams (3.1, 3.2, ...) emitted from the beam splitter (6) into the multi-clad optical fiber (5), wherein the coupling-in optical device (7) is designed to couple at least two of the sub-laser beams (3.1, 3.2, ...) emitted from the beam splitter (6) into at least two different light-guiding cores (14a, 14b, ...) of the multi-clad optical fiber (5).

2. The device according to claim 1, wherein: The multi-clad optical fiber (5) is designed to be rotationally symmetrical and has an inner light-guiding core (14a) and at least one annular light-guiding core (14b, 14c, 14d), wherein the coupling optical device (7) is designed to couple two sub-laser beams (3.1, 3.2) with different polarization states (s, p) emitted from the beam splitter (6) into the inner core (14a) of the multi-clad optical fiber (5), and the propagation directions of the two sub-laser beams correspond to the beam direction (X) of the laser beam (3) emitted into the beam splitter (6).

3. The device according to claim 1 or 2, wherein the device is designed to couple one or more pairs of sub-laser beams (3.1, 3.2; ...) into the corresponding optical cores (14a to 14d) of the multi-clad optical fiber (5), each pair of sub-laser beams having two different polarization states (s, p).

4. The device according to any one of the preceding claims, wherein: The optical beam splitter (6) comprises a first number of birefringent wedges (9a) oriented in the same direction and a second number of birefringent wedges (9b; 9b, 9c; 9b to 9d) oriented opposite to the first number of birefringent wedges (9a), wherein the sum of the wedge angles (α; 2α; 3α) of the first number of birefringent wedges (9a) corresponds to the sum of the wedge angles (α; 2α; 3α) of the second number of birefringent wedges (9b; 9b, 9c; 9b to 9d).

5. Apparatus according to any one of the preceding claims, wherein: The beam splitter (6) has exactly two birefringent wedges (9a, 9b) oriented in opposite directions and having the same wedge angle (α).

6. The device according to any one of claims 1 to 4, wherein: The beam splitter (6) has a number N of identically oriented birefringent wedges (9b, 9c; 9b to 9d) which are preferably arranged one behind the other in the beam path of the laser beam (3).

7. The device according to claim 6, wherein: The beam splitter (6) has oppositely oriented birefringent wedges (9a) upstream of the number N of identically oriented wedges (9b, 9c; 9b to 9d) in the beam path of the laser beam (3).

8. The device according to claim 7, wherein: The number N of birefringent wedges (9b, 9c; 9b to 9d) of the same orientation have the same wedge angle (α), wherein the birefringent wedges (9a) of the opposite orientation have a wedge angle (Nα) that is N times the same wedge angle (α).

9. The apparatus according to any one of claims 6 to 8, wherein: The beam splitter (6) has a number N=2 birefringent wedges (9b, 9c) of the same orientation and the coupling-in optical device (7) is designed to couple at least three of the sub-laser beams (3.1, 3.2, ...) into different light-conducting cores (14a to 14c) of a triple-clad optical fiber (5), or wherein the beam splitter (6) has a number N=3 birefringent wedges (9b to 9d) of the same orientation and the coupling-in optical device (7) is designed to couple at least four of the sub-laser beams (3.1, 3.2, ...) into different light-conducting cores (14a to 14c) of a quad-clad optical fiber (5).

10. The device according to claim 6, wherein: The beam splitter (6) has a number N=2 birefringent wedges (9b, 9c) of identical orientation, wherein a first birefringent wedge (9b) in the beam path has a wedge angle (2α) that is twice as large as a wedge angle (α) of a second birefringent wedge (9c) in the beam path.

11. The device according to claim 10, wherein: The beam splitter (6) has an oppositely oriented birefringent wedge (9a) upstream of the two identically oriented birefringent wedges (9b, 9c) in the beam path of the laser beam (3), the wedge angle (3α) of the oppositely oriented birefringent wedge corresponding to three times the wedge angle (α) of the second birefringent wedge (9b) in the identically oriented birefringent wedges in the beam path.

12. The device according to claim 10 or 11, wherein: The coupling-in optical device (7) is designed to couple at least one sub-laser beam (3.1, 3.2, ...), preferably at least one pair of sub-laser beams (3.1, 3.2; ...) having two different polarization states (s, p), respectively, into corresponding light-conducting cores (14a to 14d) of a rotationally symmetric four-clad optical fiber (5).

13. The apparatus according to any one of the preceding claims, further comprising: A control device (13) for adjusting the at least one polarization influencing device (12; 12a, 12b; 12a to 12c) so as to adjust the splitting ratio of the laser beam (3) when it is coupled into the at least two different light-guiding cores (14a, 14b, . . . ) of the multi-clad optical fiber (5).

14. The device according to claim 13, wherein: The control device (13) is designed to adjust the polarization influencing effect of the at least one polarization influencing device (12; 12a, 12b; 12a to 12c), wherein no power of the laser beam (3) is coupled into at least one of the light-guiding cores (14a, 14b, ...) of the multi-clad optical fiber (5), and / or wherein the entire power of the laser beam (3) is coupled into at least one of the light-guiding cores (14a, 14b, ...) of the multi-clad optical fiber (5).

15. Apparatus according to any one of the preceding claims, wherein: The laser beam (3) incident on 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 wedge (9a).

16. An optical system (1), comprising: A multi-clad optical fiber (5), wherein the multi-clad optical fiber is preferably a double-clad optical fiber, a triple-clad optical fiber or a quad-clad optical fiber; as well as The device (4) according to any of the preceding claims, for coupling the laser beam (3) into the multi-clad optical fiber (5).

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