DE102017010698A1 - Arrangements for shaping pulsed laser beams - Google Patents

Abstract

In this patent application, optical arrangements for shaping a beam emitted by a pulsed laser, in particular an ultrashort pulse laser, are disclosed, in which at least one optic which spatially divides the beam into at least two partial beams in cross section and different optical beams for the partial beams Paths generated, and an optical imaging unit, which images the partial beams so that the cross sections of the partial beams are superposed in a common plane, are used. It is decisive that the differences in the optical paths are so great that the resulting relative temporal offsets of the partial beams become equal to or greater than the coherence length of the beam. This eliminates the interference of sub-beams in the superposition plane.

Description

State of the art

Lasers are becoming increasingly important in material processing. In many cases, the lasers have rotationally symmetric gain volumes so that most laser beams have a round beam cross section. For surface processing, such as removal and marking, a round beam cross-section is ineffective for surface filling. In order to enable a surface treatment often high percentage overlaps of the processing zones are required.

In addition, the intensity profile of high quality beams is Gaussian. Due to the swelling behavior of different processes, the energy / power below the threshold intensity does not contribute to the processes and represents a loss. In addition, the energy contents above the threshold intensity also do not contribute to the removal. On the contrary, intensities above the threshold intensity can lead to component damage. The optimal beam cross section with respect to the surface filling is rectangular or square. Optimal intensity distribution in terms of effective use of laser energy / output is a top-hat distribution.

To generate top-hat intensity distribution, there are different optical arrangements. One often uses integrators such as light waveguides with a round or rectangular cross-section. To others, microlens array is used to homogenize the intensity. A disadvantage of the arrangements is the strong loss of beam quality after beam forming.

description

The present invention relates to optical arrangements for shaping of rays, the z. B. of pulsed lasers, in particular ultrashort pulse lasers are emitted. The optical arrangement consists of an optical system which divides a beam into at least 2 partial beams in cross section and generates different optical paths for the partial beams and an imaging unit which images the partial beams such that the cross sections of the partial beams are superposed in a common plane. This results in a beam whose intensity distribution corresponds to the sum of the intensity distribution of the partial beams.

The above object is achieved in an arrangement for shaping and guiding a beam of the type described above in that the beam is spatially divided into at least two partial beams according to a specification by means of optics in cross section and the optics used is chosen so that behind the optics the sub-beams have a relative delay to one another which is comparable to or greater than the coherence length of the laser pulses. Another optics is used with which the cross sections of the partial beams are superimposed in a common plane. Thus, a beam with an intensity distribution corresponding to the sum of the partial beams can be generated.

In the following, the present invention will be described by means of beams with Gaussian profiles and with reference to the preferred embodiments.

shows a first embodiment of this present invention. A one-dimensional beam ( 1 ) with a Gaussian beam profile ( 11 ) meets an optic ( 8th ). In the illustrated optics ( 8th ) is a transmissive optic which has two zones of different thicknesses. Behind the optics ( 8th ) create two sub-beams ( 101 ) and (102), each with half a Gaussian beam profile. Due to the different thicknesses of the partial beam ( 102 ) to the sub-beam ( 101 ) a time delay. The difference in thickness is chosen such that the time delay is equal to or greater than the coherence length of the beam (FIG. 1 ) to avoid interference from the two sub-beams.

Another optical unit made of lenses ( 261 ) 262 ) and ( 266 ) is used to superimpose the sub-beams ( 101 ) and ( 102 ) used. The lenses ( 261 ) and ( 262 ) are added to the partial beams ( 101 ) and ( 102 ). In cooperation with the lens ( 266 ), the two partial beams ( 101 ) and ( 102 ) in a common plane ( 910 ). If necessary, the lens ( 266 ) are replaced by a lens group. Ideally, the lenses ( 261 ) and ( 266 ) arranged in the form of a telescope. The same applies to the lenses ( 262 ) and ( 266 ). In the plane ( 910 ) overlap the cross sections of the two partial beams ( 111 ) and ( 112 ). Due to the relative time delay of the two partial beams, which is comparable to or greater than the coherence length of the beam, the intensities add up to the two partial beams ( 111 ) and ( 112 ) with half-Gaussian beam profile (cf. to a beam with a top-hat-like beam profile ( 99 ) (see. ,

Preferably, the lenses are dimensioned and arranged so that the lenses ( 261 ) and (266) for the sub-beam ( 101 ) an off-axis telescope and the lenses ( 261 ) and (266) for the sub-beam ( 102 ) form an off-axis telescope. The two off-axis telescopes have a common image plane ( 910 ). Thus, the partial beams are telecentric in the common image plane ( 910 ) superimposed.

For a two-dimensional beam, the optics ( 8th ) from 4 segments ( 801 ) 802 ) 803 ) and ( 804 ), as in is shown formed. The segments have different thickness. This results in 4 partial beams. Due to the different thicknesses of the segments, the 4 sub-beams have relative time delays to one another. In the illustrated case, the thickness of the segments increases in the order of ( 801 ) 802 ) 801 ) and ( 804 ). This will cause the sub-beam passing through the segment ( 804 ) goes through the most delayed. If the minimum delay of adjacent clockwise sub-beams is chosen to be greater than the coherence length, then the sub-beams may be superimposed without interference. Thus the intensity of the superimposed steel equals the sum of the intensity of the 4 partial beams.

As in In order to superimpose the 4 partial beams, a lens array of 4 lenses ( 221 ) 222 ) 223 ) and ( 224 ) in combination with the common lens ( 266 ) be used. The 4 lenses ( 221 ) 222 ) 223 ) and ( 224 ) and the lens ( 266 ) work like 4 off-axis telescopes and form each of the 4 partial beams telecentric and superimposing in a common plane.

Starting from a rotationally symmetric beam with a Gaussian beam profile, it is thus possible to produce a superimposed beam with a square cross-section and with a top-near-intensity distribution.

The time delay may be further generated by optics having zones of different refractive indices. A further embodiment of the optics for the time delay results when an optic is formed by mirror segments. The mirror segments are arranged at different axial positions.

To eliminate the interference in the beam superposition, polarization effects can also be used.

shows a ray ( 1 ) with a linear polarization. A lambda / 2 retardation plate ( 7 ) is used to control the beam ( 1 ) divided into two sub-beams with polarizations perpendicular to each other.

As in and is shown in the beam path, a lambda / 2-Verzörgerungsplatte ( 7 ) used. The lambda / 2 retardation plate ( 7 ) is placed so that about half of the beam passes through the λ / 2 retarder plate. This means that half of the beam cross-section through the lambda / 2-Verzörgerungsplatte ( 7 ) is covered (cf. ). Behind the lambda / 2 retardation plate, the beam is split into two sub-beams ( 81 ) and ( 82 ) divided with different polarization. The polarization of the sub-beam passed through the lambda / 2-retardation plate is rotated by 90 ° while the polarization of the other sub-beam remains unchanged. This is indicated by the symbols circle with a dot and an arrow (cf. and ).

shows an exemplary embodiment for the coaxial superposition of two partial beams, in which a beam offset ( 61 ) is used. Behind the lambda / 2-delay plate (7) arise from the linearly polarized input beam ( 1 ) two partial beams ( 81 . 82 ) with perpendicular polarization. The two partial beams pass through the beam offset ( 61 ). Behind the beam offset the two partial beams are spatially superimposed, with the same or substantially the same direction of propagation. How the spatial overlap should look like can be determined simply by the length of the beam displacer along the propagation direction. Since the two partial beams have mutually perpendicular polarizations, the intensity of the total output beam ( 78 ) of the sum of the intensities of the two partial beams (cf. , Thus, the interference and an associated, strong intensity modulation are suppressed.

• YVO₄, alpha-BBO, Quarz, LiNbO₃.

In the case of the beam displacer ( 61 ) is a birefringent medium in which the rays of different polarization are refracted differently as they enter the medium and exiting the medium. In the example, a beam containing both s- and p-polarized components falls vertically into the beam displacer 61 on. The beam displacer is configured so that, as it enters, the s-polarized component passes uninterrupted while the p-polarized component is broken up. Upon exit, the s-component is not broken as it did on entry while the p-polarized component is broken down. Refraction upon entry and exit creates a lateral offset between the two components. With the Beam Displacer 61 with parallel entrance and exit surfaces, the two beams of different polarizations propagate parallel after passage with a lateral offset. Among the birefringent media are:

• YVO ₄ , alpha-BBO, quartz, LiNbO ₃ .

Instead of a retardation plate for changing the polarization can also be a rotator made of quartz, a Faraday rotator made of TGG or YIG, or a rotator of reflecting surfaces, etc., are used. It has the property that rays of different polarization propagate at different speeds in the element, so that after passing through the element, the phases of different polarization undergo an unequal delay, thus changing the relative relationship between the different polarization components and the polarization state. For example, in a λ / 4 retardation plate, a linearly polarized beam becomes a circularly or elliptically polarized beam. For a lambda / 2 retardation plate, the polarization rotates at an angle twice the angle between the input polarization and the optical axis of the plate.

The retarder plate may be made of quartz, YVO ₄ , alpha-BBO, etc.

An advantageous embodiment for shaping a two-dimensional beam results from a succession of rows in and illustrated embodiments. In doing so, the in illustrated arrangement before in arranged arrangement arranged. As in the grouping and the coaxial superimposition in the xz-plane is done over the polarizations and in the yz-plane the grouping is done by the optics ( 8th ) and the superposition by the lenses ( 261 ) 262 ) and (266) realized. Behind the overall arrangement arises a superimposed beam with a rectangular or square cross section and with an almost top hat intensity distribution.

Claims (8)

Optical arrangement for shaping of rays, characterized in that at least one optical system which divides a beam in cross section into at least 2 partial beams and generates different optical paths for the partial beams and an imaging unit which images the partial beams so that the cross sections of the partial beams superimposing in a common plane, the differences of the optical paths being chosen such that the resulting relative time delay of partial beams becomes equal to or greater than the coherence length of the beam, so that the intensity distribution of the superposed beam is the sum of the intensity distribution of the beam Partial beams resembles. Optical arrangement for shaping of rays after Claim 1 , characterized in that the optics (8) of a transmissive optics, which has at least two zones of different thicknesses, wherein the thickness differences are chosen so that the consequent relative time delays of partial beams are equal to or greater than the coherence length of the beam , Optical arrangement for shaping of rays after Claim 1 or 2 , characterized in that the imaging unit of a lens array with at least two lens elements (261) and (262), which are arranged side by side, and a further common lenses (266), wherein the lenses are arranged to each other that the partial beams in each case so shows that the cross sections of the partial beams overlap in a common plane. Optical arrangement for shaping of rays after Claim 1 , characterized in that the optic (8) consists of a transmissive optic having 4 zones (801), (802), (803) and (804) of different thickness, the thickness differences being chosen so that the consequent relative time delays of adjacent sub-beams become equal to or greater than the coherence length of the beam. Optical arrangement for shaping of rays after Claim 4 characterized in that the imaging unit consists of a lens array having 4 lens elements (221), (222), (223) and (224) associated with the respective zone of optics (8) and another common lens (266) , wherein the lenses are arranged to one another such that the partial beams in each case images such that the cross sections of the partial beams are superimposed in a common plane. Optical arrangement for shaping beams according to one of Claims 1 to 5 , characterized in that each lens element (261) or (262) or (221) or (222) or (223) or (224) and the common lens (266) form an off-axis telescope, so that the partial beams are each telecentrically imaged superimposed in a common image plane. Optical arrangement for shaping beams according to one of Claims 1 to 6 , characterized in that a λ / 2 retardation plate (7) is used for splitting the beam into two partial beams (81) and (82). Optical arrangement for shaping beams according to one of Claims 1 to 7 , characterized in that for coaxial superposition, a beam displacer (61) is used.

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