DE4341553C1 - Device for homogenising the light distribution of a laser beam - Google Patents

Abstract

Many laser-beam sources have an energy-density distribution in the beam cross-section which deviates from the desired, constant energy density and whose local peaks can lead to an impairment. In order to provide a device for homogenising the light distribution of a laser beam, in which all of the leaving partial beams have the same beam divergence as the primary beam, there is provided in the beam path of the laser beam an optical system which has a beam splitter and three deflecting mirrors. The partial beam reflected by the beam splitter and deflected by the deflecting mirrors and the partial beam transmitted by the beam splitter are combined to form an overall beam whose partial beams have an offset. The two partial beams which have an offset result in a very high degree of homogeneity of the laser-beam bundle, the overall beam having the same beam divergence as a primary laser beam. The deflecting mirrors have a low numeric aperture and are cost-effective. The device is preferably used in treating (processing, machining) materials by means of laser radiation.

Description

The invention relates to a device for homogenization ren the light distribution of a laser beam.

Many laser beam sources, such as multimode Solid-state lasers, or excimer lasers, have energy density distribution in the beam cross section that of the he wanted to deviate as constant as possible energy density and their local peaks severely impaired can lead. With these laser beams for example, material processed, can not by this constant energy density distribution the processor result can be significantly affected.

They are designed as beam homogenizers tations known that homogenization by superimposition effect individual beams of a beam ken. This is due to multiple reflection of the ray beam  dels in a mirrored channel (EP-OS 0 493 365 and EP-OS 0 230 931) or a multimode optical fiber (EP-OS 0 435 825). Through multiple reflection a large number of virtual light sources are formed, whose partial beams the homogenizer under a relative leave large divergence angle.

In another known device (DE-OS 41 03 615) the beam cross-section is divided into several ne beam segments divided. The segments are in egg ner image plane in which an aperture device is arranged is shown overlapping. A beam homogenization is therefore only in a defined level in a be agreed distance from the homogenizer.

With these devices, an increase is inevitable the beam divergence, which is why for an effekti ve use the optical energy flow thus transmitted density for the further interpretation of the beam path Op tics with high numerical apertures can be used have to. In these arrangements, the trans mission of rays due to geometric loss most or the absorption of radiation in refractive Elements to a transmittance of clearly small limited to less than 1. This is especially in the wavelengths range of deep UV radiation of the molecule or exci mer laser (for example, wavelengths of 157 nm, 193 nm and 248 nm have) critical. Beam homogenizers with refractive elements show a relatively star there ke absorption of the radiation in the optical elements, resulting in a decrease in transmitted power and to Tool life problems due to the formation of absorption centers under the influence of UV radiation (color center formation) leads.

The invention has for its object a Vorrich device for homogenizing the light distribution of a laser to create rays in which all of the exits tendency partial beams has the same beam divergence like the primary laser beam, so that optics lower nume rical aperture can be used.

This task is carried out in the generic device according to the invention with the characteristic features of Claim 1 solved.

In the device according to the invention, the primary la ser beam divided by the beam splitter, the reflected partial beam directed to the deflecting mirror becomes. The partial beam transmitting the beam splitter emerges from the device. After the circulation of the right inflected partial beam between the deflecting mirrors it is split again at the beam splitter, with the reflected partial beam with the transmitted part jet the total jet emerging from the device forms. By the two, which have an offset Partial beams result in a very high homogeneity of the Laser beam, the total beam being the same Has beam divergence like the primary laser beam. The Deflecting mirrors have a small numerical aperture and are inexpensive. The effort to interpret it is also low. With the exception of the beam splitter, which can be very thin, only reflective optical elements elements used, so that especially in the UV range problems of increased absorption occurring when using refractive elements occur, reliably avoided become. If an odd number of deflecting mirrors ver turns, then the Ge formed from the partial rays velvet ray one regarding the center of gravity of the primary  laser beam symmetrical energy distribution. Becomes uses an even number of deflecting mirrors, so about the partial beams forming the total beam overlap more or less. This results in an overall beam with a wider beam profile, the modulation of which Energy distribution through the offset at recombination place, d. H. the beam splitter can be adjusted. The device according to the invention is preferably in the Material processing used by laser radiation. This results in uniform, extensive radiation. A structured mask can also be scaled down be det. Due to the uniform energy density of the Radiation becomes a uniform degree of processing aims.

Further features of the invention result from the white ter claims, the description and the drawings.

The invention is illustrated by some in the drawings illustrated embodiments explained in more detail. It demonstrate

Fig. 1 shows a schematic representation of a first form from a guide according to the invention before direction,

Fig. 2 different forms of laser beams after exiting the apparatus of the invention,

Fig. 3 shows a schematic representation of a second embodiment of an inventive pre direction,

Fig. 4 shows a schematic representation of another exporting the invention Vorrich approximately form tung,

Fig. 5 shows a schematic representation of the arrangement of the device according to the invention in a processing system.

The apparatus according to Fig. 1 serves to homogenize the energy density of a laser radiation beam, without increasing the maximum angle of divergence of the beam to increase. The device has three deflecting mirrors 1 to 3 , which are arranged so that the impingement points 12 to 14 of the laser beams on the mirror gel surfaces 15 to 17 of the deflecting mirrors 1 to 3 lie in a common plane. In the illustrated embodiment, the deflection mirrors 1 to 3 and a beam splitter 4 , which is arranged in the area between the deflection mirrors 1 and 3, are arranged in one plane. The laser beam 5 coming from the laser (not shown) is split by the beam splitter 4 into two partial beams 6 and 8 . The partial beam 8 passes through the beam divider 4 unhindered. The other partial beam 6 is created by reflection on the beam splitter 4 and strikes the mirror surface 17 of the deflecting mirror 1 . There the partial beam 6 is reflected to the deflecting mirror 2 . The re reflected partial beam 6 strikes at point 13 on the mirror surface 16 of the deflecting mirror 2 , at which the partial beam is reflected toward the deflecting mirror 3 . The partial beam 6 hits the mirror surface 15 at the point of incidence 12 , on which this partial beam is reflected towards the beam splitter 4 . At it the partial beam 6 is partially reflected, whereby the partial beam 7 is created, while a partial beam 11 passes through the beam splitter 4 and at the point of impact 14 'strikes the mirror surface 17 of the deflecting mirror 1 . At the impingement points 12 'and 13 ', the partial beam 11 is reflected in the same way as the partial beam 6 until it reaches the beam splitter 4 again. On it, the partial beam 11 is again split into two partial beams, which accordingly take the partial beams 6 and 8 the way described above.

The degree of reflection of the beam splitter 4 is designed such that the summation of all the partial beams running according to the partial beam 8 and the summation of all the corresponding partial beams 7 partial beams 7 give the same possible values of the radiation intensities or energy densities. The transmission of the partial beams results from the following relationship:

Here mean:

From these relationships, there is a reflection RTS of the beam splitter 4 of 0.666 for a beam splitting of 50% for loss-free steering mirrors.

The beam offset between the partial beams 7 and 8 can be adjusted by a lateral displacement of one of the deflection mirrors 1 to 3 . In Fig. 1, for example, the deflecting mirror 3 in the X and / or in the Z direction is adjustable ver. An offset between the partial beams 7 and 8 can be achieved by an opposite tilting of two deflecting mirrors or a deflecting mirror and the beam splitter 4 .

In Fig. 1, the intensity distribution of the laser beam les 5 is shown on the basis of the intensity curve 9 . This intensity curve 9 runs similar to a bell curve. By an appropriate arrangement of the deflecting mirrors 1 to 3 and the beam splitter 4 , this intensity curve ve 9 can be changed into the intensity curve 10 (dashed line) by dividing the laser beam 5 into the two partial beams 7 and 8 in the manner described. Since these two partial beams 7 , 8 have a beam offset, an intensity curve 10 (dashed line in FIG. 1) results from the laser beam formed in the partial beams. Thus, as a comparison of the intensity curves 9 and 10 shows by way of example, a plateau-like distribution corresponding to the intensity curve 10 can be generated from an energy density distribution of the laser beam 5 according to the curve 9 . The size of the suitable beam offset between the two partial beams 7 and 8 depends on the structure of the primary beam 5 . Due to the described adjustments of the deflection mirrors 1 to 3 and / or the beam splitter 4 , different energy density distributions can be easily adjusted in this way.

In addition, the resulting reflection of the partial beam 7 on the YZ plane, as shown in Fig. 2, in conjunction with the described translational displacement of the partial beams 7 and 8 against each other, a rotation of the partial beams 7 , 8 about the axis of the incident Primary beam 5 are generated. The symmetry level of the mirroring is defined by the direction of propagation of the primary beam 5 and by the normal vector which is defined by the points of incidence 12 to 14 of the partial beam 6 on the deflecting mirrors 1 to 6 on the tensioned plane.

For an odd number of deflecting mirrors (i = 3, 5...), The partial beams are mirror images. This case is chosen when it is important to generate a symmetrical beam profile from an asymmetrical beam profile by precise and slightly offset superposition of the two partial beams 7 and 8 .

A single deflecting mirror (i = 1) comes from geometri reasons out of the question.

Fig. 2 shows the orientation of a beam 10 with a rectangular cross-section after leaving the homogenizer depending on the orientation of the rectangular cross-section to the plane of symmetry of the reflection. The exemplary embodiments of beam bundles shown in FIGS. 2a to 2d arise with an odd number of deflecting mirrors. The beam cross section shown schematically in FIG. 2a is typical, for example, for excimer lasers in which different beam divergences occur in two spatial directions due to the beam geometry. When focusing the radiation, this leads to elliptical beam diameters. In an embodiment of the beam according to FIGS. 2b and 2d, averaging over both divergences in the overlap area of the partial beams 7 and 8 is possible, so that a spatially isotropic distribution of the beam divergence can be achieved. As shown in FIGS. 2a and 2c, the two partial beams 7 and 8 lie one above the other, while in the exemplary embodiment according to FIGS. 2b and 2d they cross each other.

In the manner described, the rays formed from the part rays 7 , 8 can be adjusted so that the desired energy density distribution is produced. As has been shown by way of example in FIG. 1, the beam can be designed with a wider and / or higher maximum than the partial beams 7 or 8 .

With an even number of deflecting mirrors, the partial beams remain unshielded. Such an arrangement is selected, for example, if a broader partial beam is to be generated from a beam with a sufficiently symmetrical profile by displacing the two partial beams 7 , 8 . An exemplary embodiment of this is shown in FIG. 3. In this embodiment, the device has four deflecting mirrors 18 to 21 . The beam splitter 4 is located in the area between the two deflecting mirrors 18 and 19 . As in the embodiment according to FIG. 1, the primary laser beam 5 strikes the beam splitter 4 and is divided there into the two partial beams 6 and 8 . The partial beam 6 is reflected on the beam splitter 4 to the deflecting mirror 19 , during which the beam splitter 4 passes on the other beam 8 . At the deflecting mirror 19 , the partial beam is deflected to the deflecting mirror 20 , at which it is in turn reflected to the deflecting mirror 21 and from there to the deflecting mirror 18 . On At steering mirror 18 of the partial beam 6 is reflected to the beam splitter. 4 There, the partial beam 6 is in turn divided into two partial beams, the reflected partial beam 7 diverging to the partial beam 8 . The other, the beam splitter 4 passing beam 6 then runs again in the manner described within the op table system. A beam 7 , 8 , which is wider than the primary laser beam 5 , is thus formed from the primary laser beam 5 .

With the devices described, it is possible to carry out averaging over a single laser pulse during its pulse duration. The prerequisite for this is that the pulse duration is greater than the transit time between the deflection mirrors 1 to 3 or 18 to 21 . For an excimer laser with its typical pulse duration in the 10 ns range and with beam cross sections of a few cm², this condition is easy to meet. If a laser radiation with a shorter pulse duration is used, the transit time of the laser beam between the deflecting gels must be reduced, because otherwise the partial rays 7 , 8 appear as two separate pulses. The adjustment of the runtime is possible for example by reducing the overall arrangement.

In the embodiment according to FIG. 3, the modulation of the energy distribution can be set by the offset of the partial beams 7 , 8 at the recombination location, ie at the beam splitter 4 .

To generate a so-called top hat energy distribution, several devices according to FIG. 3 can be connected in series. If two-dimensional top hat profiles are to be generated, these devices are used for directions perpendicular to one another, as a result of which the energy distribution is symmetrized in both directions of the beam profile.

Also, in the apparatus of Fig. 1, several devices can be used one after the other for mutually perpendicular directions to a symmetrization of the energy distribution of the beam profiles in both directions to obtain.

FIG. 4 shows the device corresponding to FIG. 3 with the four deflecting mirrors 18 to 21 and the beam splitter 4 . The device is oriented such that the points of incidence of the beams on the deflecting mirrors 18 to 21 are oriented in the YZ plane. The laser beam 5 is reflected in the manner described with reference to FIG. 3 and divided into partial beams. The intensity distribution of the laser beam 5 in the two mutually perpendicular directions X and Y is shown in the window 22 .

The shop window 23 shows the intensity distribution of the laser beam after the homogenization in the Y direction.

In Fig. 4, the device with the four Umlenkspie gels 18 to 21 and the beam splitter 4 is shown in such a position that the points of incidence of the rays on the deflecting mirrors 18 to 21 lie in the XZ plane. This is how the shop window 24 shows the laser beam homogenized in the X direction.

The devices described are suitable for use in material processing, in medical technology and the like.

Fig. 5 shows an example of an arrangement of the apparatus in a processing system. A schematically presented laser beam source 25 emits the laser beam 5 . The device 26 is shown in accordance with the embodiment shown in FIG. 1, but may also have an embodiment corresponding to FIG. 3. The device 26 is arranged rotatably about the axis of the laser beam 5 . In the device 26 , the laser beam 5 is homogenized in the manner described and emerges as a homogenized laser beam 27 from the device 26 and strikes a deflecting mirror 28 which deflects the homogenized laser beam 27 to a processing optics or lens 29 . Through this optics 29 , the laser beam is directed onto a workpiece 30 which it is processing.

Claims (8)

1. Device for homogenizing the light distribution of a laser beam, characterized in that in the beam path of the laser beam ( 5 ) there is at least one optical system which has at least one beam splitter ( 4 ) and at least three deflecting mirrors ( 1 to 3 ; 18 to 21 ) , and that the partial beam ( 6 , 11 ) reflected by the beam splitter ( 4 ) and deflected by the deflecting mirrors ( 1 to 3 ; 18 to 21 ) and the partial beam ( 8 ) transmitted by the beam splitter ( 4 ) to a total beam ( 7 , 8 ) are brought together, the partial beams ( 7 , 8 ) have an offset. 2. Device according to claim 1, characterized in that the impingement points ( 12 to 14 ; 12 'to 14 ') of the partial beam ( 6 , 11 ) rotating between the deflecting mirrors ( 1 to 3 ; 18 to 21 ) on the mirror surfaces ( 15 to 17 ) the deflecting mirror lie in a common plane. 3. Apparatus according to claim 1 or 2, characterized in that at least one of the order steering mirror ( 1 to 3 ; 18 to 21 ) for adjusting the offset between the partial beams ( 7 , 8 ) is adjustable in the X or Z direction. 4. Apparatus according to claim 1 or 2, characterized in that the deflecting mirror ( 3 ) is adjustable in the X and Z directions. 5. Device according to one of claims 1 to 4, characterized in that the beam splitter ( 4 ) is tiltable. 6. Device according to one of claims 1 to 5, characterized in that two mutually adjacent te deflecting mirror ( 1 to 3 ; 18 to 21 ) are tiltable against each other. 7. Device according to one of claims 1 to 6, characterized in that for the symmetrization of the Energy distribution in two mutually perpendicular Directions two optical systems in a row are switched. 8. Device according to one of claims 1 to 7, characterized in that an even number of deflection mirrors ( 18 to 21 ) is provided for generating a top hat energy distribution.