Optical arrangements for processing a workpiece

Abstract

Optical arrangements for processing a workpiece include a fiber laser arrangement for emitting laser radiation, a fiber arrangement directly coupled to the fiber laser arrangement and configured to guide the laser radiation in a direction to the workpiece to be processed, the fiber arrangement including a transport fiber having a fiber core and at least one fiber cladding surrounding the fiber core, and at least one coupling device for coupling a portion of the laser radiation guided in the fiber arrangement into at least one fiber cladding of the transport fiber. The coupling device has a spectral bandwidth of at least the same magnitude as a spectral bandwidth of the laser radiation. The fiber laser arrangement is configured to change a beam profile of the guided laser radiation to shift the spectral bandwidth of the laser radiation relative to the spectral bandwidth of the coupling device.

Claims

1. An optical arrangement for processing a workpiece, comprising: a fiber laser arrangement configured to emit laser radiation; a fiber arrangement directly coupled to the fiber laser arrangement and configured to guide the laser radiation from the fiber laser arrangement to the workpiece to be processed, wherein the fiber arrangement comprises a transport fiber having a fiber core and one or more fiber claddings surrounding the fiber core; and at least one coupling device configured to couple a portion of the laser radiation guided in the fiber arrangement into at least one fiber cladding of the transport fiber, wherein a spectral bandwidth of the coupling device has at least the same magnitude as a spectral bandwidth of the laser radiation emitted by the fiber laser arrangement, wherein the fiber laser arrangement comprises a resonator having an active fiber, and the resonator is formed between a first fiber Bragg grating and a second fiber Bragg grating, and wherein a spectral bandwidth of a reflectivity of the second fiber Bragg grating is smaller than and within a spectral bandwidth of a reflectivity of the first fiber Bragg grating, such that the spectral bandwidth of the laser radiation of the fiber laser arrangement corresponds to the spectral bandwidth of the reflectivity of the second fiber Bragg grating, wherein the fiber laser arrangement is configured to change a beam profile of the laser radiation guided in the direction of the workpiece to be processed by shifting the spectral bandwidth of the laser radiation of the fiber laser arrangement relative to the spectral bandwidth of the coupling device and thereby changing the portion of the laser radiation that is coupled into the at least one fiber cladding of the transport fiber, and wherein the fiber laser arrangement is configured to shift the spectral bandwidth of the laser radiation of the fiber laser arrangement relative to the spectral bandwidth of the coupling device by spectrally shifting the spectral bandwidth of the reflectivity of the second fiber Bragg grating relative to the spectral bandwidth of the reflectivity of the first fiber Bragg grating.

2. The optical arrangement of claim 1, wherein the coupling device is configured to be a chirped long-period grating.

3. The optical arrangement of claim 2, wherein the chirped long-period grating has a spectral bandwidth of at least 0.3 nm.

4. The optical arrangement of claim 1, wherein the fiber laser arrangement is configured to generate laser radiation having a spectral bandwidth of at least 0.1 nm.

5. The optical arrangement of claim 1, wherein the spectral bandwidth of the reflectivity of the first fiber Bragg grating has a magnitude at least 1.5 times that of the spectral bandwidth of the reflectivity of the second fiber Bragg grating.

6. The optical arrangement of claim 1, further comprising: an actuator configured to act on the second fiber Bragg grating for spectrally shifting the bandwidth of the reflectivity of the second fiber Bragg grating relative to the spectral bandwidth of the reflectivity of the first fiber Bragg grating.

7. The optical arrangement of claim 6, wherein the actuator is configured to generate at least one of a tensile stress or a compressive stress on the second fiber Bragg grating.

8. The optical arrangement of claim 1, wherein the fiber laser arrangement comprises one or more fiber amplifiers for amplifying the laser radiation generated in a resonator.

9. The optical arrangement of claim 1, wherein the fiber laser arrangement comprises an active fiber comprising a multimode fiber or a large mode area fiber.

10. The optical arrangement of claim 1, wherein the fiber arrangement comprises: a plurality of coupling devices each configured to couple the laser radiation from the fiber core into a respective one of the fiber claddings.

11. The optical arrangement of claim 10, wherein there is no spectral overlap among spectral bandwidths of the coupling devices.

12. The optical arrangement of claim 1, wherein the fiber laser arrangement is configured to couple a larger portion of the laser radiation into the at least one fiber cladding of the transport fiber when processing a thick workpiece than when processing a thin workpiece.

13. A method of processing a workpiece by an optical arrangement, the method comprising: guiding a laser radiation generated from a laser arrangement in a direction to the workpiece to be processed by a fiber arrangement directly coupled to the laser arrangement, wherein the fiber arrangement comprises a transport fiber having a fiber core and one or more fiber claddings surrounding the fiber core; coupling, by at least one coupling device, a portion of the laser radiation guided in the fiber arrangement into at least one fiber cladding of the transport fiber; and changing a beam profile of the laser radiation guided in the direction of the workpiece to be processed by shifting a spectral bandwidth of the laser radiation of the fiber laser arrangement relative to a spectral bandwidth of the coupling device and thereby changing the portion of the laser radiation that is coupled into the at least one fiber cladding of the transport fiber, wherein the spectral bandwidth of the coupling device has at least the same magnitude as the spectral bandwidth of the laser radiation, wherein the laser arrangement comprises a resonator formed between a first fiber Bragg grating and a second fiber Bragg grating, and wherein a spectral bandwidth of a reflectivity of the second fiber Bragg grating is smaller than and within a spectral bandwidth of a reflectivity of the first fiber Bragg grating, such that the spectral bandwidth of the laser radiation of the fiber laser arrangement corresponds to the spectral bandwidth of the reflectivity of the second fiber Bragg grating, and wherein shifting the spectral bandwidth of the laser radiation relative to the spectral bandwidth of the coupling device comprises: spectrally shifting the spectral bandwidth of the reflectivity of the second fiber Bragg grating relative to the spectral bandwidth of the reflectivity of the first fiber Bragg grating.

14. The method of claim 13, wherein spectrally shifting the spectral bandwidth of the reflectivity of the second fiber Bragg grating relative to the spectral bandwidth of the reflectivity of the first fiber Bragg grating comprises: generating at least one of a tensile stress or a compressive stress on the second fiber Bragg grating.

15. The method of claim 13, wherein coupling the portion of the laser radiation guided in the fiber arrangement into at least one fiber cladding of the transport fiber comprises: coupling, by each of a plurality of coupling devices, the laser radiation from the fiber core into a respective one of the fiber claddings, wherein there is no spectral overlap among spectral bandwidths of the coupling devices.

16. The method of claim 13, further comprising amplifying the laser radiation generated in a resonator by one or more fiber amplifiers in the laser arrangement.

17. The method of claim 13, wherein the laser arrangement comprises an active fiber comprising a multimode fiber or a large mode area fiber.

18. The method of claim 13, wherein the laser arrangement comprises a resonator formed between a highly reflective mirror and a blazed grating, and wherein shifting the spectral bandwidth of the laser radiation comprises changing an orientation of the blazed grating relative to the laser radiation.

Description

DESCRIPTION OF DRAWINGS

(1) FIGS. 1A, 1B, and 1C show schematic illustrations of first, second, and third examples of optical arrangements as described herein for processing a workpiece, which optical arrangements have a fiber laser arrangement and a fiber arrangement.

(2) FIG. 2 is a graphical illustration of a spectral distribution of the laser radiation generated by the fiber laser arrangement from FIGS. 1A, 1B, and 1C, and of a spectral distribution of a coupling device of the fiber arrangement.

(3) FIG. 3 shows an illustration of a transport fiber in a form of a multiple cladding fiber with a plurality of chirped long-period gratings.

DETAILED DESCRIPTION

(4) FIGS. 1A, 1B, and 1C each show an example of a construction of an optical arrangement 1 for processing a plate shaped workpiece 2, for example a metal sheet. The optical arrangement has a fiber laser arrangement 3 for emitting laser radiation 4 and a fiber arrangement 5. In the example shown, the fiber laser arrangement 3 includes a fiber laser oscillator (or fiber laser resonator) 12, which generates the emitted laser radiation 4. The fiber arrangement 5 is directly (monolithically) coupled (e.g., fiber-to-fiber) to the fiber laser arrangement 3, to put it more precisely, to the fiber laser oscillator 12. The fiber arrangement 5 has a transport fiber 6 having a lengthwhich is comparatively long in the example shownof usually significantly more than 2 m, to guide the laser radiation 4 from the fiber laser arrangement 3 in the direction of the workpiece 2. The combination of fiber laser arrangement 3 and fiber arrangement 5 is configured in a fully monolithic fashion, that is to say, that the fibers of the fiber laser oscillator 12 and of the fiber arrangement 5 are connected to one another, e.g., by splicing, and that the laser radiation 4 emitted by the fiber laser arrangement 3 is fiber guided and leaves the fiber arrangement 5 only at the exit end 6a of the transport fiber 6. At the exit end 6a of the transport fiber 6, the laser radiation 4 emerges from the fiber arrangement 5 and impinges on a focusing device 7, for example, in the form of a lens, which focuses the freely propagating laser radiation 4 onto the workpiece 2.

(5) The optical arrangement 1 may form, in particular, a part of a laser processing machine (not illustrated in more specific detail) for the laser processing of workpieces 2. The transport fiber 6 may serve for guiding the laser radiation 4 from the fiber laser arrangement 3, to put it more precisely from the fiber laser oscillator 12, which serves as a beam source of the laser processing machine, to a laser processing head, in which the focusing optical unit 7 is accommodated and which is moved relative to the workpiece 2 in order to process the latter.

(6) The fiber laser oscillator 12 of the fiber laser arrangement 3, as shown in FIG. 1A and FIG. 1B, has an active fiber 8, which is configured as a large mode area fiber and which has a fiber core 9a and a fiber cladding 9b surrounding the fiber core 9a in a ring shaped manner. In the example shown, the fiber core 9a has a diameter dK of approximately 20 m. The diameter of the cladding 9b of the active fiber 8 is 400 m. In the example shown, the numerical aperture NA of the fiber core 9a is approximately 0.06 and the numerical aperture of the cladding 9b is approximately 0.46. From these parameters, the so-called normalized frequency V (V parameter) of the active fiber 8 can be calculated as follows: V=2 (dK/2) NA, wherein denotes the wavelength of the laser radiation 4 generated in the active fiber 8.

(7) Under the assumptionfulfilled herethat the active fiber 8 is a step index fiber, the normalized frequency V in accordance with the formula below represents an (approximate) measure of the number of modes M guided in the active fiber 8: MV.sup.2/2.

(8) For V=2.405, it follows from the formula above that only one mode (per polarization direction) can be guided. In the example shown, the normalized frequency of the active fiber 8 at a wavelength of 1065 nm is approximately 4.5, that is to say, approximately ten transverse modes are guided in the active fiber 8. With the use of an active fiber 8 in the form of a multimode fiber, the normalized frequency V would be greater and the number M of guided modes would be correspondingly higher.

(9) To amplify the modes in the active fiber 8, the fiber laser oscillator 12 has two pump light sources 10a, b in the form of laser diodes having a pump wavelength of 975 nm, for example, which, via respectively associated pump fibers 11a,b couple the pump radiation into the fiber cladding 9b of the active fiber 8, from which the pump radiation crosses into the fiber core 9a. The pump fibers 11a, 11b are spliced on the active fiber 8 with the aid of a pump coupler. The active fiber 8 of the fiber laser oscillator 12 has a resonator section formed between a first fiber Bragg grating 13a and a second fiber Bragg grating 13b, which are written in each case into the fiber core 9a of the active fiber 8. In the example shown, the first fiber Bragg grating 13a is configured as a highly reflective grating, that is to say, the first fiber Bragg grating 13a forms the end mirror of the fiber laser oscillator 12. The second fiber Bragg grating 13b is configured as an output coupler, e.g., as a partially transmissive grating having a typical reflectance of 2% to 10%, that is to say, at the second fiber Bragg grating 13b, the laser radiation 4 generated in the active fiber 8 is coupled out from the fiber laser oscillator 12.

(10) The first and second fiber Bragg gratings 13a, 13b have a wavelength dependent reflectivity R, which is illustrated in FIG. 2 (in arbitrary units). The first fiber Bragg grating 13a has a reflectivity R having a comparatively large spectral bandwidth .sub.FBG1 of approximately 4 nm that extends over a wavelength range of between 1063 nm and 1067 nm. The second fiber Bragg grating 13b has a wavelength dependent reflectivity R which is comparatively narrowband and has a spectral bandwidth .sub.FBG2 of approximately 1.8 nm and which, in the example shown in FIG. 2, extends over a wavelength range of between 1064.6 nm and 1066.4 nm.

(11) The laser radiation 4 generated by the fiber laser oscillator 12 or the fiber laser arrangement 3 has a spectral bandwidth .sub.L which corresponds to the intersection of the two spectral bandwidths .sub.FBG1, .sub.FBG2 of the first and second fiber Bragg gratings 13a, 13b. In the example shown, in which the spectral bandwidth .sub.FBG2 of the second fiber Bragg grating 13b lies completely within the wavelength range of the spectral bandwidth .sub.FBG1 of the first fiber Bragg grating 13a, the spectral bandwidth .sub.L of the fiber laser arrangement 3 corresponds to the spectral bandwidth .sub.FBG2 of the second fiber Bragg grating 13b. The fiber laser arrangement 3 or the fiber laser oscillator 12 emits laser radiation 4 whose laser wavelength .sub.L in the case of the symmetrical curve profileshown in FIG. 2of the reflection of the second fiber Bragg grating 13b lies in the center of the spectral bandwidth .sub.FBG2 thereof, e.g., at .sub.L=1065.5 nm. The intensity I or the power of the laser radiation 4 generated by the fiber laser arrangement 3 has a spectral distribution having its maximum at the laser wavelength .sub.L.

(12) The spectral distribution of the reflectivity R of the second fiber Bragg grating 13b can be shifted in wavelength by means of an actuator 15. For this purpose, the actuator 15 acts on two fiber holders 16a, b, between which a section of the active fiber 8 is arranged, the second fiber Bragg grating 13b being formed in said section. In the example shown, the actuator 15 is configured to change, e.g., to increase or to decrease, a distance A between the two fiber holders 16a,b. The actuator 15, which may be configured as a piezoactuator, for example, generates a compressive or tensile stress on the active fiber 8 and compresses or expands the section of the active fiber 8 arranged between the two fiber holders 16a, 16b in the axial direction (i.e., in the fiber longitudinal direction). In this way, the period length of the second fiber Bragg grating 13b written into the fiber core 9a of the active fiber 8 changes, as a result of which the wavelength dependent reflectivity R of the second fiber Bragg grating 13b as shown in FIG. 2 is shifted spectrally, as is indicated by a double headed arrow in FIG. 2. The spectral shift of the reflectivity R or of the spectral bandwidth .sub.FBG2 of the second fiber Bragg grating 13b along the horizontal axis in FIG. 2 results in a corresponding shift of the spectral distribution of the laser radiation 4 coupled out from the fiber laser arrangement 3, that is to say, the laser wavelength .sub.L of the laser radiation 4 can be tuned with the aid of the actuator 15.

(13) In the example shown, the actuator 15 is configured to shift the laser wavelength .sub.L between approximately 1064.5 nm and approximately 1066.5 nm. Larger shifts of the laser wavelength .sub.L are also possible if the actuator 15 is dimensioned suitably. As an alternative to an axial compression or expansion, the active fiber 8 can be bent in the section between the two fiber holders 16a, 16b to apply a tensile and/or compressive stress to the second fiber Bragg grating 13b and to bring about a shift of the laser wavelength .sub.L of the fiber laser arrangement 3 in this way. In this case, the actuator 15 can act on the active fiber 8 in a radial direction, for example, in the section between the two fiber holders 16a, 16b.

(14) The shift of the laser wavelength .sub.L of the laser radiation 4 emitted by the fiber laser arrangement 3 can be used to change or to set the beam profile 19a, 19b of the laser radiation 4 guided by the fiber arrangement 5 in the direction of the workpiece 2. For this purpose, a coupling device 17 is fitted in the fiber arrangement 5. The coupling device 17 is configured in the form of a chirped long-period grating which is written into the fiber core 9a of a fiber adapter 18 that connects the active fiber 8 of the fiber laser arrangement 3 to the transport fiber 6. It goes without saying that, alternatively, the coupling device 17 can also be formed in the transport fiber 6, to put it more precisely in the fiber core 9a of the transport fiber 6. The fiber adapter 18 and the transport fiber 6 of the fiber arrangement 5 are substantially constructed like the active fiber 8, just that there is no doping with an active medium in the fiber core 9a. To facilitate the writing in the coupling device in the form of the chirped long-period grating 17, the fiber adapter 18 may have a doping with germanium.

(15) The coupling device in the form of the chirped long-period grating 17 has a wavelength dependent reflectivity R which has a maximum in the region of approximately 1065.5 nm (>90%) and which has a spectral bandwidth .sub.C of 2 nm. That portion of the laser radiation 4 which is reflected by the chirped long-period grating 17 is coupled into the fiber cladding 9b of the fiber adapter 18, where no reversal of direction takes place at the chirped long-period grating 17, that is to say, the reflected portion of the laser radiation 4 does not change its direction of propagation and propagates in the transport fiber 6. In contrast to the illustration shown in FIG. 2, the transmission or the decrease in the transmission of the chirped long-period grating 17 for laser radiation 4 guided in the fiber core 9a, 9a is often represented instead of the reflectivity R. The transmission of the chirped long-period grating 17 has a spectral bandwidth .sub.C corresponding to the reflectivity R and having a minimum likewise at a wavelength of approximately 1065.5 nm. As can be discerned in FIG. 2, the spectral distribution or the spectral bandwidth .sub.L of the laser radiation 4 generated by the fiber laser arrangement 3 lies within the spectral bandwidth .sub.C of the chirped long-period grating 17.

(16) In the case of the spectral position of the laser wavelength .sub.L or the spectral bandwidth .sub.FBG2 of the fiber laser 3 as shown in FIG. 2, therefore, the laser radiation 4 emerging from the fiber laser arrangement 3, at the coupling device 17 in the form of the chirped fiber Bragg grating, is practically completely coupled from the fiber core 9a into the fiber cladding 9b of the fiber adapter 18. In this way, the beam profile of the laser radiation 4 is changed, specifically from a first, Gaussian beam profile 19a of the laser radiation 4 guided in the fiber core 9a or 9a into a second, different beam profile 19b of the laser radiation 4 guided in the fiber cladding 9b.

(17) By shifting the spectral distribution of the reflectivity R of the second fiber Bragg grating 13b and thus the laser wavelength .sub.L of the fiber laser arrangement 3 relative to the spectral bandwidth .sub.C (not able to be shifted) of the reflectivity R of the coupling device 17, it is possible to change or set that portion of the laser radiation 4 which is coupled from the fiber core 9a into the fiber cladding 9b. In particular, the spectral bandwidth .sub.L of the laser radiation 4 generated by the fiber laser arrangement 3 can be spectrally shifted to an extent such that it no longer overlaps the spectral bandwidth .sub.C of the coupling device 17. In this case, laser radiation 4 is practically no longer coupled into the fiber cladding 9b, such that the Gaussian beam profile 19a generated in the fiber core 9a of the fiber laser 3 is not changed in the fiber arrangement 5. To ensure that in the case of such a shift of the laser wavelength .sub.L of the fiber laser arrangement 3 laser radiation 4 continues to be generated, the spectral bandwidth .sub.FBG1 of the first fiber Bragg grating 13a should have a magnitude at least 1.5 times, preferably at least twice, that of the spectral bandwidth .sub.FBG2 of the second fiber Bragg grating 13b, that is to say, .sub.FBG1>1.5 .sub.FBG2 or .sub.FBG1>2 .sub.FBG2 can hold true.

(18) Switching between the first beam profile 19a from the fiber core 9a and the second beam profile 19b from the fiber cladding 9b makes it possible to optimize the processing (e.g., cutting or welding) of the workpiece 2. By way of example, the first beam profile 19a can be set for the cutting of thin plate shaped workpieces 2 (e.g., metal sheets), while the second beam profile 19b can be chosen for the cutting of thicker workpieces 2. It goes without saying that it is also possible to set beam profiles whose radial intensity distribution lies between the two beam profiles 19a, 19b, by suitably setting the proportion of the laser radiation 4 coupled into the fiber cladding 9b by means of the actuator 15. The setting of a beam profile 19a, 19b that is suitable or optimized for the processing can be carried out with the aid of a control device which drives the actuator 15 and which may be, for example, a part of the laser processing machine described further above.

(19) The optical arrangement 1 shown in FIG. 1B differs from the optical arrangement 1 shown in FIG. 1A merely in that a fiber amplifier 20 is formed in addition to the fiber laser oscillator 12 on the active fiber 8, into which fiber amplifier, from further pump light sources 21a, 21b, e.g., in the form of fiber coupled diode lasers via corresponding pump fibers 22a, b, additional pump radiation is coupled into the fiber cladding 9b of the active fiber 8 to increase the power of the laser radiation 4 guided in the fiber core 9a. The fiber laser resonator 12 forms a seed laser (oscillator), which together with the fiber amplifier 20 forms a master oscillator power amplifier (MOPA) (also called master oscillator fiber amplifier (MOFA)). As an alternative to the example shown in FIG. 1B, in which only one fiber amplifier 20 is arranged in the fiber laser arrangement 3, it is also possible for a plurality of fiber amplifiers 20 to be connected in series in the fiber laser arrangement 3.

(20) As in FIG. 1A, the laser wavelength .sub.L of the fiber laser arrangement 3 is set by means of the second fiber Bragg grating 13b being acted on with the aid of the actuator 15. It goes without saying that, in the case of the optical arrangements 1 shown in FIG. 1A and FIG. 1B, the magnitudes of the spectral bandwidths .sub.FBG1, .sub.FBG2 of the two fiber Bragg gratings 13a, 13b as shown in FIG. 2 can also be interchanged, that is to say, the spectral bandwidth .sub.FBG1 of the first, highly reflective fiber Bragg grating 13a can be smaller than the spectral bandwidth .sub.FBG2 of the second fiber Bragg grating 13b. In this case, the actuator 15 acts on the first fiber Bragg grating 13a to shift the wavelength of the fiber laser arrangement 3.

(21) The fiber laser arrangement 3 shown in FIG. 1C has a fiber amplifier 20 like the fiber laser arrangement 3 shown in FIG. 1B. In contrast to FIG. 1A and FIG. 1B, however, the fiber laser arrangement 3 from FIG. 1C has a resonator 12 in which a laser rod 8 (doped crystal) is arranged as laser active medium. The rod laser resonator 12 as shown in FIG. 1C has a highly reflective mirror 14a and a grating 24 as output coupler. The laser radiation 4 generated in the resonator 12 propagates in free space downstream of the resonator 12 and is coupled into the fiber amplifier 20 via an optical element, in the example shown via a focusing lens 23, said fiber amplifier being directly fiber-to-fiber coupled to the fiber arrangement 5 as in the examples shown in FIG. 1A and FIG. 1B.

(22) In the example shown in FIG. 1C, the setting of the wavelength .sub.L of the laser radiation 4 generated by the resonator 12 is effected with the aid of a blazed grating 24, the orientation of which relative to the impinging laser radiation 4 can be changed (rotated in the example shown) by means of an actuator 25 indicated by a double-headed arrow. Given a suitable design of the blazed grating 24, the wavelength .sub.L of the laser radiation 4 can be spectrally shifted in an interval similar to that in the case of the fiber laser resonator (oscillator) 12 described further above in association with FIG. 1A and FIG. 1B.

(23) Instead of a resonator 12 in the form of a rod laser, alternatively it is also possible to provide a diode laser or a disk laser as seed laser in the fiber laser arrangement 3, where the laser wavelength .sub.L can be shifted with the aid of suitable devices, e.g., in the form of gratings. It is also possible to use a resonator 12 which is configured as in FIG. 1C and in which, instead of the laser rod 8, an active fiber, e.g., a fiber doped with rare earths, serves as laser-active medium. In this case, the resonator 12 is configured as a fiber laser oscillator 12 as in the examples in FIG. 1A and FIG. 1B to the setting of the laser wavelength .sub.L, however, is not effected via a fiber Bragg grating 13a, 13b, but rather by means of the blazed grating 24 in free space propagation. In this case, a further optical element is typically arranged between the active fiber and the blazed grating 24 to collimate the laser radiation 4 emerging from the active fiber before it impinges on the blazed grating 24.

(24) What is important in the case of all the fiber laser arrangements 3 shown in FIG. 1A, FIG. 1B and FIG. 1C is that they are directly coupled to the fiber arrangement 5 at their exit end, e.g., at the end at which the laser radiation 4 has its maximum power.

(25) In FIG. 1A, FIG. 1B and FIG. 1C, a fiber arrangement 5 having a fiber core 9a and just a single fiber cladding 9b is used. It goes without saying however, that it is also possible to use fiber adapters 18 and/or transport fibers 6 which have more than one fiber cladding. FIG. 3 shows a transport fiber 6 having a fiber core 9a and three fiber claddings 9b-d surrounding the fiber core 9a in a ring-shaped fashion, in each of which fiber claddings laser radiation 4 can be guided. In the example shown there are fitted in the transport fiber 6 three coupling devices 17a-c in the form of chirped long-period gratings, the first of which couples a portiondependent on the laser wavelength .sub.Lof the laser radiation 4 from the fiber core 9a into the first fiber cladding 9b, situated radially furthest on the inside. The second and third coupling devices 17b and 17c, respectively, correspondingly couple a portiondependent on the laser wavelength .sub.Lof the laser radiation 4 guided in the fiber core 9a into the second fiber cladding 9c and into the third fiber cladding 9d, respectively. The fixedly predetermined spectral bandwidths .sub.C1, .sub.C2, .sub.C3 of the three coupling devices 17a-c in the form of the chirped long-period gratings are typically chosen such that they do not overlap spectrally. What can be achieved in this way is that, given a suitably set laser wavelength .sub.L, the laser radiation 4 is coupled only into one of the three fiber claddings 9b-d. In the example shown in FIG. 3, the active fiber 8 may likewise have more than one fiber cladding, but this is not absolutely necessary. By means of the transport fiber 6 shown in FIG. 3, different focus diameters can be generated on the workpiece 2 using one and the same focusing device 7, which may be configured as a zoom optical unit, if appropriate. It goes without saying that, in contrast to the illustration shown in FIG. 3, the coupling devices 17 a-c can also be fitted in a fiber adapter 18 of the fiber arrangement 5, which fiber adapter is formed between the fiber laser arrangement 3 and the transport fiber 6 or, if appropriate, is adjacent to the exit end of the transport fiber 6. Such a fiber adapter 18 typically has geometrical parameters identical to those of the transport fiber 6 and it is generally configured as a photosensitive fiber (doped with germanium) to facilitate the writing in of the gratings.

(26) Different types of fibers 8, 18, 6 than those described above can also be used in the optical arrangement 1. To generate a high power of the laser radiation 4, e.g., more than 500 W or 1000 W, a plurality of modes to propagate in the active fiber 8 of the fiber laser 3 can be used, that is to say, the use of a single-mode fiber in the fiber laser arrangement 3 may be not desired.

(27) To summarize, in the manner described above, laser radiation 4 can be coupled from the fiber core 9a, 9a into at least one fiber cladding 9b-d, without having to leave the fibers 8, 18, 6 for this purpose, that is to say, the fully monolithic, both optically and mechanically robust and compactly realizable construction of the combination of fiber laser arrangement 3 and fiber arrangement 5 is maintained.

OTHER EMBODIMENTS

(28) A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.