Device and method for expanding a laser beam

Abstract

Methods and devices for expanding a laser beam are provided. In one aspect, a device includes a telescope arrangement having two spherical folding mirrors for expanding an incident collimated laser beam with a lens arranged in the divergent beam path downstream of the telescopic arrangement. The two spherical folding mirrors in the beam path are a first, convex-curved spherical folding mirror and a second, concave-curved spherical folding mirror, respectively. The lens has a spherical lens face for collimating the expanded laser beam from the telescope arrangement. The laser beam can be an ultraviolet (UV) laser beam.

Claims

1. Device for expanding a laser beam, comprising: a telescope arrangement having first and second spherical folding mirrors; and a transparent lens arranged downstream of the telescope arrangement and having a spherical lens face, wherein the first folding mirror comprises a convex-curved spherical folding mirror configured to expand a collimated laser beam incident on the convex-curved spherical folding mirror, and wherein the second folding mirror comprises a concave-curved spherical folding mirror arranged in a divergent beam path from the first spherical folding mirror and configured to keep the divergent beam path divergent and to act together with the lens in the divergent beam path as an optical unit to collimate the expanded laser beam.

2. Device according to claim 1, further comprising: a displacement device configured to displace the lens in a beam direction (X, Y) of the expanded collimated laser beam.

3. Device according to claim 1, wherein the lens is a planar convex lens whose convex spherical lens face faces away from the telescope arrangement.

4. Device according to claim 1, wherein a beam direction (X) of the laser beam entering the telescope arrangement and a beam direction (X) of the laser beam output from the telescope arrangement extend parallel with each other.

5. Device according to claim 1, wherein the beam path of the laser beam entering the telescope arrangement intersects with the beam path of the laser beam output from the telescope arrangement.

6. Device according to claim 1, further comprising: a frequency conversion device for converting the frequency of the laser beam from a wavelength in the infrared (IR) range to a wavelength in the ultraviolet (UV) range.

7. Device according to claim 1, further comprising: a laser for producing the laser beam.

8. Device according to claim 7, wherein the laser beam produced from the laser has a wavelength in the IR range.

9. Device according to claim 1, further comprising: a tilting device for tilting the lens relative to the beam direction (X, Y) of the expanded divergent laser beam.

10. Device according to claim 1, wherein the incident collimated laser beam comprises a UV laser beam.

11. Device according to claim 10, wherein surfaces of the first and second spherical folding mirrors comprise a material that is highly reflective for UV laser radiation.

12. Device according to claim 1, wherein radii of curvature of a plurality of optical elements including the spherical lens face, the convex-curved spherical folding mirror, and the concave-curved spherical folding mirror are mutually adapted to each other, so that an output beam from the lens is collimated and aberrations from the plurality of optical elements substantially compensate for each other to obtain an imaging limited in terms of diffraction.

13. A method of expanding a laser beam comprising: expanding a collimated laser beam incident at a first, convex-curved spherical folding mirror of a telescope arrangement; and collimating the expanded laser beam by an optical unit comprising: a second, concave-curved spherical folding mirror of the telescope arrangement arranged in a divergent beam path from the first, convex-curved spherical folding mirror, and a transparent lens arranged in the divergent beam path downstream of the telescope arrangement and having a spherical lens face, wherein the second, concave-curved spherical folding mirror is configured to keep the divergent beam path divergent and to act together with the lens in the divergent beam path to collimate the expanded laser beam.

14. The method of claim 13, wherein the incident collimated laser beam comprises a UV laser beam.

15. The method of claim 14, wherein surfaces of the first, convex-curved and second, concave-curved spherical folding mirrors comprise a material that is highly reflective for UV laser radiation.

16. The method of claim 13, wherein radii of curvature of a plurality of optical elements including the spherical lens face, the first, convex-curved folding mirror, and the second, concave-curved folding mirror are mutually adapted to each other, so that an output beam from the lens is collimated and aberrations from the plurality of optical elements substantially compensate for each other to obtain an imaging limited in terms of diffraction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of a device for expanding a laser beam in the form of a Galilei telescope.

(2) FIG. 2 is a schematic illustration of a device for expanding a laser beam in the form of a mirror telescope.

(3) FIG. 3 shows illustrations of the far-field of the expanded laser beam produced using the mirror telescope shown in FIG. 2, with different angles of incidence of the input beam.

(4) FIG. 4 is an illustration of a device for beam expansion having a convex mirror and two cylinder lenses.

(5) FIG. 5 shows illustrations of the far-field of the device of FIG. 4 with different angles of incidence of the input beam.

(6) FIG. 6 shows illustrations of the far-field of the device of FIG. 4 with one of the cylinder lenses being tilted.

(7) FIG. 7 shows an embodiment of a device according to the invention for beam expansion having a mirror telescope in a Z-folding arrangement and having a spherical output lens.

(8) FIG. 8 shows another embodiment of the device according to the invention for beam expansion having a mirror telescope in an X-folding arrangement.

(9) FIG. 9 is an illustration of the device of FIG. 7 with a frequency conversion device and an IR laser.

(10) FIG. 10 is an illustration of the far-field of the device of FIG. 7 with different input angles of the laser beam.

DETAILED DESCRIPTION OF THE DRAWINGS

(11) FIG. 7 shows a device 11 for beam expansion of an input collimated laser beam 4, which is expanded on a first convex folding mirror 7 of a telescope arrangement 6 and converted at a second, concave folding mirror 8 of the telescope arrangement 6 and at a lens 3 which is arranged downstream of the second folding mirror 8 in the divergent beam path into a collimated output laser beam 5 which is expanded 6 times in the present example. The folding mirrors 6, 7 are spherical mirrors, the lens 3 is a planar convex lens whose lens face 3a facing away from the second folding mirror 8 is curved in a spherical manner.

(12) The arrangement of the folding mirrors 7, 8 shown in FIG. 7 enables a parallel orientation of the input-side laser beam 4 relative to the output-side laser beam 5 along a common beam axis (X direction) which is also referred to as a Z folding arrangement.

(13) Owing to the combination of the two spherical folding mirrors 7, 8 in or close to the Z folding arrangement with the spherical lens 3 as a divergence-correcting output element in the already expanded, divergent beam path, with relatively large folding angles 2, 2 which can be freely selected over a wide range at the two folding mirrors 7, 8 and with a relatively short structural form, beam expansions with very small beam deformation or imaging errors substantially below the diffraction limit can be achieved. Since the lens 3 is arranged in the expanded beam path, and the laser radiation therefore strikes it only with reduced intensity, damage to the lens 3 can be prevented or substantially reduced so that the lens material, for example, quartz glass is damaged by the laser radiation only insignificantly.

(14) In order to size the device 11 in an appropriate manner, in a first step the required expansion of the input laser beam 4 is first determined (for example, 1.42 times, 2 times, 3 times, 6 times, . . . , 10 times, . . . , 20 times, etc.). If the expansion is determined (in this instance: 6 times), an appropriately sized structural space is defined for the device 11 and a suitable positioning of the folding mirrors 7, 8 is selected in the structural space. In this instance, there is a comparatively high degree of freedom in terms of the selection of the folding angles 2, 2 and the spacings between the optical elements (that is to say, between the folding mirrors 7, 8 and the lens 3).

(15) Together with the spacings, the spherical curvature of the folding mirrors 7, 8 and the spherical lens face 3a of the lens 3 is determined in such a manner that, on the one hand, the desired expansion is achieved and, on the other hand, the aberrations of the folding mirrors 7, 8 and the aberrations of the folding mirrors 7, 8 and the lens 3 substantially compensate for each other, and the output laser beam 5 is collimated as desired. The folding angle 2 (in this instance: approx. 20) at the first folding mirror 7 and the folding angle 2 (in this instance: approx. 20) at the second folding mirror 8 may but do not have to be selected to be the same in this instance. The spacing between the folding mirrors 7, 8 is in this instance approximately 150 mm, but this may naturally also be selected to be larger or smaller.

(16) A displacement device 12, for example, in the form of a (linear) motor which enables axial displacement of the lens 3 in the beam direction (X direction) in order to adjust the divergence of the output laser beam 5, without the occurrence of lateral beam displacement or a correction by means of other optical elements being required, is indicated in FIG. 7 by means of a double-headed arrow.

(17) As an alternative to the device 11 shown in FIG. 7, in which the folding mirrors 7, 8 are arranged in a Z folding arrangement, it is also possible to arrange the folding mirrors 7, 8 in a so-called X folding arrangement in which the incident laser beam 4 extends in a first beam direction (X direction) and the output laser beam 5 extends in a second beam direction (Y direction), the two laser beams 4, 5 intersecting, of, the device 13 shown in FIG. 8, in which the incident and output laser beams 4, 5 define an angle of 90 relative to each other. The folding angle 2 (in this instance: 20) at the first folding mirror 7 and the folding angle 2 (in this instance: 70) at the second folding mirror 8 are in this instance selected to be different. The folding mirrors 7, 8 comprise at the surface a material which is highly reflective for UV laser radiation, for example, dielectric layer systems based on quartz glass. Of course, other material combinations with good UV reflection properties and high UV resistance can also be used.

(18) FIG. 9 shows the device 11 of FIG. 7 on an optical module 14 which defines the structural space. There are fitted to the optical module 14, in addition to the optical elements described in FIG. 7, that is to say, the lens 3 and the two folding mirrors 7, 8 arranged in a Z folding arrangement, three additional planar folding mirrors 15a to 15c, in order to make optimum use of the structural space available. In addition to the displacement direction 12, for displacing the lens 3 in a beam direction (X direction), a tilting device 16 indicated in FIG. 9 can also be fitted to the lens 3, in order to tilt it relative to the X direction or relative to the divergent laser beam. Owing to the tilting at an appropriate angle, the optical errors can be further reduced, and/or larger folding angles at the folding mirrors 7, 8, smaller radii of curvature of the folding mirrors 7, 8 or smaller structural lengths can be achieved. It is self-evident, however, that a tilting of the lens 3 is not absolutely necessary and an imaging which is limited in terms of diffraction can also be achieved without tilting.

(19) The radii of curvature of the folding mirrors 7, 8 and the spherical lens face 3a, with an approximate 3-fold to approximately 6-fold expansion, are typically approximately 100-300 mm for the first folding mirror 7, approximately 1000-2000 mm for the second folding mirror 8, or approximately 200-400 mm for the lens 3, in order to achieve the desired refractivity for the expansion. The spacing between the second folding mirror 8 and the lens 3 may in this instance, for example, be in the range between approximately 100 mm and 150 mm. It is self-evident that it is possible to deviate from the above-mentioned value ranges, in particular when an expansion is intended to be carried out which is outside the prescribed range (3 times to 6 times).

(20) It is also possible to see in FIG. 9 a frequency conversion device 17 which is arranged on the module 14 and which is for converting the frequency of an irradiated laser beam 18 whose wavelength is in the infrared spectral range into a wavelength in the ultraviolet spectral range. The frequency conversion device 17 has non-linear crystals in order to achieve the frequency conversion in a manner conventional for the person skilled in the art. FIG. 9 also shows an infrared laser 19, in the present example in the form of a Yb:YAG laser, which produces IR laser radiation having a wavelength of 1030 nm, which is converted in the frequency conversion device 17 into a UV laser beam with high radiation intensity and a wavelength of approximately 343 nm. Of course, other laser types, for example, an Nd:YVO.sub.4 laser can also be used to produce the IR laser beam 18. With the Nd:YVO.sub.4 laser, the laser wavelength is 1064 nm and the third harmonic produced in the frequency conversion device 17 is approximately 355 nm.

(21) A spot diagram of the far-field of the output collimated laser beam 5 obtained by means of the device 11 shown in FIG. 9 is illustrated in FIG. 10, the input beam properties and the pupil being selected to be identical to FIG. 3, FIG. 5 and FIG. 6 and only the scale S.sub.3 being reduced to 100 rad. As can clearly be seen in FIG. 10, the spots are located in the example shown far within the Airy disc 9, that is to say, the RMS radius of the beam distribution is substantially smaller than the Airy radius (approx. 42 rad) so that the optical unit shown in FIG. 9 is limited in terms of diffraction for the laser beam 4, 5 or for the expanded imaging. However, this does not apply to an incidence of the laser beam 4 in the direction of the beam axis (X direction), as shown in FIG. 10 at the top left-hand side, but also for the spot diagrams shown in FIG. 10 at the top right-hand side, bottom left-hand side and bottom right-hand side in which the field angle was changed in various spatial directions by approx. 0.2, the input aperture with the simulation being arranged 150 mm in front of the first folding mirror 7. In the case of non-axial incidence of the laser beam 4, an imaging which is limited in terms of diffraction can consequently also be achieved using the device 11.

(22) In summary, using the above-described devices 11, 13, an expansion of the laser beam 4 can be achieved, whilst at the same time avoiding the degradation of the optical units 3, 7, 8 owing to intensive laser radiation, with the imaging errors on the whole being able to be kept small owing to mutual compensation of the imaging errors of the individual optical elements 3, 7, 8 and an imaging which is limited in terms of diffraction thus being able to be obtained.