WO2005050286A1

WO2005050286A1 - Arrangement and method for the suppression of speckle structures of a pulsed laser beam

Info

Publication number: WO2005050286A1
Application number: PCT/EP2004/012596
Authority: WO
Inventors: Fedor V. Karpushko; Vitalij Lissotschenko; Aleksei Mikhailov
Original assignee: Hentze-Lissotschenko Patentverwaltungs Gmbh & Co. Kg
Priority date: 2003-11-21
Filing date: 2004-11-06
Publication date: 2005-06-02
Also published as: DE10354582A1

Abstract

The invention relates to an arrangement for the suppression of speckle structures of a pulsed laser beam (1) on a treatment plane (7), said laser beam being used for the treatment of material (1). The arrangement comprises beam divider means (2) which are used to divide the laser beam (1) into two partial beams (3, 4), in addition to beam combining means (6) which are used to bring together both of the partial beams (3, 4). The arrangement is embodied in such a manner that one of the two partial beams (3, 4) is subjected to phase displacement between the beam divider means (2) and the beam combining means (6), such that no speckle structures arise when the partial beams (3, 4) overlap on the treatment plane (7) or such that speckle structures having a predetermined shape and/or size arise when the partial beams (3, 4) overlap on the treatment place (7). The invention also relates to a method for suppressing speckle structures.

Description

"Arrangement and method for suppressing speckle structures of a pulsed laser beam"

The present invention relates to an arrangement and a method for suppressing speckle structures of a pulsed laser beam used for material processing in a processing plane.

Coherent light, in particular laser light, can have interference phenomena when superimposed on itself, which in particular can occur as so-called speckle structures. When using laser light for material processing, the laser beams are generally superimposed on themselves by homogenizers in such a way that regular intensity fluctuations occur in the processing plane, which correspond to the aforementioned interference-related speckle structures. Such speckle structures can prove to be extremely disruptive or unacceptable in many applications. In particular, if the material processing is carried out via a thermal process that requires a certain power threshold, as is the case, for example, in welding applications or also in printing applications, locally faulty processing can result from local intensity fluctuations in the applied laser radiation.

The problem on which the present invention is based is the creation of an arrangement and the specification of a method of the type mentioned at the beginning, which can suppress comparatively simple means and / or comparatively effectively speckle structures. This is achieved according to the invention with regard to the arrangement by an arrangement of the type mentioned at the outset with the characterizing features of claim 1 and with regard to the method by a method of the type mentioned at the beginning with the characterizing features of claim 1 1.

According to claim 1, it is provided that the arrangement comprises beam splitter means for splitting the laser beam into two partial beams and beam combining means for combining the two partial beams, the arrangement being designed such that one of the two partial beams between the beam splitting means and the beam combining means undergoes a phase shift such that when the partial beams are superimposed on the processing plane, no speckle structures arise or that when the partial beams are superimposed on the processing plane, speckle structures of a predetermined shape and / or size are produced. Such a phase shift of the first partial beam with respect to the second partial beam can result in interference occurring in the processing plane at certain points in time, but this can be averaged over time in the case of partial beams that are no longer or only insignificantly phase-correlated with one another, so that no speckle structures occur ,

According to claim 2 it can be provided that the beam splitter means are designed such that the laser beam is divided into two equal partial beams.

According to claim 3 it can be provided that the partial beams are equal in terms of their intensity. According to claim 4, it can be provided that the two partial beams essentially correspond to one another with regard to their cross section.

According to claim 5, it can be provided that the two partial beams are symmetrical to one another, such that those cross-sectional sections of the partial beams that are taken from the same and corresponding cross-sectional section of the laser beam are superimposed on the processing plane. Two substantially identical and mutually symmetrical partial beams can be superimposed on one another in the processing plane in such a way that corresponding regions of the first partial beam are superimposed with corresponding regions of the second partial beam at each location of the processing plane, the regions of the partial beams superimposed on one another in each case have the same intensity. If the electromagnetic field of the second partial beam thus experiences a phase shift with respect to the electromagnetic field of the first partial beam, the speckle structures can be effectively suppressed by the same intensities of corresponding areas of the two partial beams in the overlapping area.

According to claim 6 it can be provided that an electro-optical modulator is arranged between the beam splitter means and the beam combining means, which can shift or change the phase of one of the two partial beams. In particular, if the coherence length of the laser beam is very long, it can be a very complex process to choose the difference in the optical path of the two partial beams larger than the coherence length. In this case, it certainly makes sense to change the phase or phase of one of the two partial beams by other means, in particular by means of an electro-optical modulator bring about. By means of an electro-optical modulator, in particular a comparatively arbitrary phase shift can be brought about, so that the speckle structures can not only be suppressed but can be changed as desired.

According to claim 7 it can be provided that the beam splitter means are designed as a partially transparent mirror. The design of the beam splitter means as a partially transparent mirror can be implemented very easily in terms of production technology.

According to claim 8 it can be provided that the arrangement comprises a mirror which can reflect the second partial beam emanating from the beam splitter means in such a way that the two partial beams run parallel to one another after the reflection of the second partial beam on the mirror. This measure allows the partial beams to be superimposed comparatively simply in the processing plane in such a way that regions of each of the partial beams removed from corresponding regions of the original laser beam are superimposed on one another. In particular, there is an even number of reflections, namely two reflections in the specific case, namely one reflection on the partially transparent mirror and one reflection on the additional mirror. There is also the possibility of choosing a different even number of reflections, namely four reflections, for example, in order to further extend the optical path of the second partial beam. An even number of reflections enables a corresponding symmetrical overlay and thus, under certain circumstances, a complete suppression of the speckle structures in the processing plane. According to claim 9 it can be provided that the beam combining means comprise a lens array and a Fourier lens.

It can be provided according to claim 10 that the lens array has a plurality of cylindrical lenses, in particular crossed cylindrical lenses on the entrance and exit surfaces. A homogenization of light beams can be achieved by means of such lens arrays. In particular, however, in connection with the Fourier lens, it is possible to achieve a largely uniform superimposition of partial beams striking it in the processing plane.

The inventive method according to claim 1 1 provides the following step: the laser beam is divided into two partial beams; one of the two partial beams experiences a phase shift relative to the other of the partial beams; the two partial beams are superimposed on the processing plane.

It can be provided according to claim 12 that the phase shift of the second partial beam with respect to the first partial beam is achieved by passing through an electro-optical modulator. Further features and advantages of the present invention will become clear from the following description of preferred exemplary embodiments with reference to the accompanying figures. Show in it

Fig. 1 is a schematic side view of an arrangement according to the invention;

2 shows a schematic side view of imaging means of the arrangement according to the invention;

3 shows the intensity distribution of laser radiation in a processing plane with interfering speckle structures;

4 shows the intensity distribution of laser radiation in a processing plane in which the speckle structures have been suppressed by an arrangement according to the invention.

Cartesian coordinate axes have been inserted in some of the figures for clarity.

An arrangement according to the invention is shown schematically in FIG. From the left in FIG. 1, a laser beam 1 falls into the arrangement according to the invention, which is part of a laser pulse. The arrangement according to the invention comprises a partially transparent mirror serving as beam splitter means 2, which allows a first partial beam 3 of the laser beam 1 to pass through unhindered. A second partial beam 4 is reflected obliquely upwards and backwards by the beam splitter means 2 in FIG. 1. The second partial beam 4 is reflected by a mirror 5, which is aligned essentially parallel to the partially reflecting mirror serving as beam splitting means 2, such that the partial beams 3, 4 after reflection of the second partial beam 4 on the mirror 5 run parallel to one another in the positive Z direction.

The arrangement according to the invention shown in FIG. 1 further comprises a beam combining means 6, which is shown only schematically in FIG. 1. An exemplary embodiment of such a beam combining means 6 can be seen in detail in FIG. 2 and is described in more detail below. The beam combining means 6 combine the partial beams 3, 4 in such a way that they are overlapped with one another in a processing plane 7.

According to the invention, there is the possibility of introducing an electroacoustic modulator 8 between the beam splitter means 2 and the beam combining means 6, which is shown in FIG. 1 is shown in dashed lines.

1, the laser beam 1 is marked with an L at its upper edge in the X direction and with an R at its lower edge in the X direction in order to clarify the position of the left and right edges of the laser beam 1. The partial beam 3 ultimately represents the extension of the laser beam 1 in the Z direction by the beam splitter means 2, so that the left and right edges of the partial beam 3 are not changed with respect to the laser beam 1, so that the left edge also in the partial beam 3 is up in the X direction and the right edge is down in the X direction. 1 that the left edge L 'is also arranged at the top in the X direction, whereas the right edge R' is arranged at the bottom in the X direction.

The partial beams 3, 4 are superimposed in the working plane 7 by the beam combining means 6 such that the left edge L of the Partial beam 3 with the left edge L 'of the partial beam 4 and the right edge R of the partial beam 3 with the right edge R' of the partial beam 4 is superimposed.

FIG. 2 shows an example of beam combining means 6, which can be used both for superimposing two partial beams 3, 4 spaced apart in the X direction and for superimposing directly adjacent partial beams. Furthermore, the beam combining means 6 shown in FIG. 2 can also overlay a beam extended in the X direction with itself such that the laser beam is homogenized.

The beam combining means 6 shown in FIG. 2 comprise a lens array 9 and a Fourier lens 10. The lens array 9 has on its exit side a plurality of cylindrical lenses 11 arranged parallel to one another. The cylinder axes of the cylindrical lenses 11 extend in the Y direction and thus into the imaging plane of FIG. 2 or out of it.

There is certainly the possibility that a plurality of cylindrical lenses arranged parallel to one another are also arranged on the entry surface of the lens array 9, but whose cylinder axes extend in the X direction. In this way, the lens array 9 can comprise crossed cylindrical lenses on the entry surface and the exit surface, so that the laser beams or partial beams 3, 4 passing through them can be deflected both with respect to the X direction and with respect to the Y direction.

The Fourier lens 10 is designed in the exemplary embodiment shown as a plano-convex spherical lens. Partial beams of the individual partial beams 3, 4 are deflected in different directions by each of the cylindrical lenses 11 or, if appropriate, also by the cylindrical lenses (not shown) on the entry surface of the lens array 9. This multiplicity of partial beams are then superimposed by the Fourier lens 10 in the processing plane 7, partial beams emerging from the cylindrical lenses 11 arriving at the same locations on the processing plane 7 at the same angle. In this way it can be ensured that, for example, the left edges L, L 'of the partial beams 3, 4 are superimposed on one another in the processing plane at the same location.

FIG. 3 shows the intensity distribution of a laser beam in a processing plane, which did not run through an arrangement according to the invention, but only superimposed on itself in processing plane 7 by means of a lens array 9 and a Fourier lens 10, as can be seen in FIG. 2 has been. Because of the coherence of the laser light, so-called speckle structures 12 arise in the processing plane 7. These speckle structures arise due to the interference of the laser light with itself.

4 shows the intensity distribution of the laser radiation in the processing plane 7 after passing through an arrangement according to the invention. It can be clearly seen that the speckle structures according to FIG. 3 are suppressed in FIG. 4. The reason for this is that in the arrangement according to the invention according to FIG. 1, the partial beam 4 has undergone a phase shift with respect to the partial beam 3 which is large enough that when the partial beams 3, 4 are superimposed in the processing plane 7 there is no or hardly noticeable speckle. Structures emerge. This can be the case with the arrangement 1 can be achieved in particular in that the additional optical path which the partial beam 4 experiences with respect to the partial beam 3 due to the partial back reflection is greater than the coherence length of the laser beam 1.

Due to the additional optical path that the partial beam 4 travels, the partial beam 4 is shifted in time with respect to the partial beam 3. In the event of a path difference of the partial beam 4 greater than the coherence length of the laser beam 1, the partial beams 3, 4 ultimately superimposed in the processing plane at a specific time thus originate from wave trains of the laser beam 1 which are not coherent with one another and are temporally successively emitted. In a laser pulse of a length of approximately 10 ns, 10 or 100 wave trains, each of which are consecutively not coherent, may be present. For example, in the case of an excimer laser, the maximum coherence lengths that can be achieved today are approximately in the range of 0.3 m, so that a laser pulse of a duration of 10 ns, which has a spatial extent of approximately 3 m, is therefore approximately 10 in each case, but not includes mutually coherent wave trains. Thus, if the additional optical path that the partial beam 4 travels with respect to the partial beam 3 is more than 0.3 m, the partial beams 3, 4 will almost certainly be taken from non-coherent wave trains of the laser beam 1 when they are superimposed in the processing plane 7 , so that the partial beams 3, 4 in the processing plane 7 are not coherent with one another. For this reason, no speckle structures arise because interference effects can occur at a certain point in time, but these are averaged out over time over the duration of the laser pulse. This results in an intensity distribution in the processing plane 7, which corresponds approximately to FIG. 4. In the case of more coherent laser sources, such as, for example, an Nd: YAG laser, the coherence length can be 10 m and more. It has proven to be extremely complex and costly to design an arrangement according to the invention in such a way that the optical path length additionally traveled by the partial beam 4 compared to the partial beam 3 is 10 m and more. Therefore, as indicated by dashed lines in FIG. 1, an electro-optical modulator 8 can be introduced into the beam path of the partial beam 4. With this electro-optical modulator, the phase of the partial beam 4 can be shifted or changed in such a way that the phase correlation between the partial beams 3 and 4 is eliminated. In this case, even for path differences between the partial beams 3, 4, which are smaller than the coherence length of the laser beam 1, it can be achieved that no speckle structures occur when the partial beams 3, 4 are superimposed in the working plane 7.

The phase shift or phase change of the partial beam 4 caused by the electro-optical modulator can also be selected such that speckle structures of a desired shape are created in the processing plane 7. For example, 7 speckle structures could arise in the processing plane, which have approximately the shape of several rectangular pulses or sawtooth pulses or the like.

Reference numerals list

Laser beam center of beam splitter first partial beam second partial beam mirror beam combining means processing plane electro-optical modulator lens array Fourier lens cylindrical lens speckle structures

Claims

claims:

Arrangement for suppressing speckle structures of a pulsed laser beam (1) used for material processing in a processing plane (7), characterized in that the arrangement beam splitter means (2) for splitting the laser beam (1) into two partial beams (3, 4) and Beam combining means (6) for bringing the two partial beams (3, 4) together, the arrangement being designed such that one of the two partial beams (3, 4) experiences such a phase shift between the beam splitting means (2) and the beam combining means (6), that when the partial beams (3, 4) are superimposed in the processing plane (7) there are no speckle structures or that when the partial beams (3, 4) are superimposed in the processing plane (7) there are speckle structures of a predetermined shape and / or size.

0 Arrangement according to claim 1, characterized in that the beam splitter means (2) are designed such that the laser beam (1) is divided into two equal partial beams (3, 4).

J. Arrangement according to claim 2, characterized in that the partial beams (3, 4) are equal in terms of their intensity.

1. Arrangement according to one of claims 2 or 3, characterized in that the two partial beams (3, 4) essentially correspond to one another with regard to their cross section.

5. Arrangement according to one of claims 2 to 4, characterized in that the two partial beams (3, 4) are symmetrical to each other, such that in the processing plane (7) those cross-sectional sections of the partial beams (3, 4) that are taken from the same and corresponding cross-sectional section of the laser beam (1) are superimposed.

6. Arrangement according to one of claims 1 to 5, characterized in that between the beam splitter means (2) and the beam combining means (6) an electro-optical modulator (8) is arranged, which shift the phase of one of the two partial beams (3, 4) or can change.

7. Arrangement according to one of claims 1 to 6, characterized in that the beam splitter means (2) are designed as a partially transparent mirror.

8. Arrangement according to one of claims 1 to 7, characterized in that the arrangement comprises a mirror (5) which can reflect the second partial beam (4) emanating from the beam splitter means (2) such that the two partial beams (3, 4 ) run parallel to each other after the reflection of the second partial beam (4) on the mirror (5).

9. Arrangement according to one of claims 1 to 8, characterized in that the beam combining means (6) comprise a lens array (9) and a Fourier lens (10).

10. The arrangement according to claim 9, characterized in that the lens array (9) has a plurality of cylindrical lenses (1 1), in particular on the entrance and exit surface crossed cylinder lenses (1 1).

1 1. Method for suppressing speckle structures of a pulsed laser beam used for material processing (1) in a processing plane (7), characterized by the following method steps: the laser beam (1) is divided into two partial beams (3, 4); one of the two partial beams (3, 4) experiences a phase shift relative to the other of the partial beams (3, 4); the two partial beams (3, 4) are superimposed on the processing plane (7).

12. The method according to claim 1 1, characterized in that the phase shift of the second partial beam (4) compared to the first partial beam (3) is achieved by passing through an electro-optical modulator (8).