WO2009000500A1 - Method and apparatus for measuring the wavefront of laser radiation - Google Patents

Abstract

In the case of a method and an apparatus for measuring the wavefront of laser radiation, - partial laser beams (4) are coupled out of the laser beam (1) to be measured in temporal succession with the aid of an aperture (3) which can be displaced relative to the cross section of the laser beam (1) to be measured and is small in comparison with the latter, - the partial laser beams (4) are detected using a position-resolving detector (5) and wavefront-specific measurement data are generated therefrom, and - the wavefront (6) of the laser beam (1) is determined from the wavefront-specific measurement data and the position coordinates of the respective partial laser beam (4), which has been coupled out, inside the laser beam (1) to be measured.

Description

 Method and device for wavefront measurement of laser radiation

The invention relates to a method and a device for wave front measurement of laser radiation.

In the background of the invention, it should be noted that the measurement of an optical wavefront of a laser beam is useful for various purposes, for example allowing conclusions to be drawn about the quality of optical components with regard to their surface and transmission properties, in laser technology for characterizing the optical properties used in astronomy to measure the deviation of the front rays of light also stars on their way through the atmosphere. The present invention relates in particular to the characterization of the wavefront of laser beams, from which it is possible, for example, to draw conclusions about the alignment accuracy and the degree of soiling of the optical components in the beam path of a laser processing machine.

The measurement of the wavefront of laser radiation can take place, for example, by means of interferometric methods with the aid of a reference wave. In practice, however, the main method used is the Hartmann-Shack measuring principle, in which a segmentation of the laser beam by means of pinhole apertures or lens fields and an imaging or focusing of the individual partial beams on a position-sensitive detector are measured. The wavefront can be reconstructed by detecting the lateral deviation of the sub-beams to a predetermined position or referenced by a plane wave. With commercially available systems laser radiation in a wavelength range of about 400 nm to 1400 nm can be measured. This can be done with very high accuracy due to the availability of high resolution CCD chips operating in this waveband.

Variants of the Hartmann-Shack measuring principle are described in various patents, such as DE 39 43 518 C2, DE 40 07 321 C2, DE 197 35 096 A1 or DE 102 43 838 B3.

The above-mentioned Hartmann-Shack measuring sensors are problematic in their use for the in-situ wavefront measurement of high-power lasers, since the laser beam is to be attenuated for a measured value recording in order not to damage the sensor element. For this purpose, special optical components are used , For example, transmit a small proportion of the laser radiation and reflect the greater part. Disadvantage of this solution is the heating of the optical components by absorption of the laser radiation, which can lead to a change in the wavefront of the laser beam due to the temperature dependence of the refractive index of the medium. An accurate measurement of the wavefront is thereby at least made difficult, if not impossible.

Another problem is the wavefront measurement with laser radiation in the middle and far infrared range, ie at a wavelength of about 1.5 to 15 microns. The available sensor elements, such as for example micro-bolometer arrays or pyroelectric detectors, are currently available in production only in a low resolution compared to CCD or CMOS chips. In addition, they are extremely expensive. Another disadvantage of these detectors lies in their small detector surface. When measuring a laser beam whose cross-section is larger than the detector surface, then additional optical elements for cross-sectional conversion must be used, which in turn can cause a distortion of the wavefront to be measured by aberrations. Finally, the reduction of the cross-section by optical elements leads to a decrease in the resolution of the wavefront sensor due to larger angles of the beams in the optical design of the measuring device.

Based on the described problems of the prior art, the present invention seeks to provide a method and a corresponding device for measuring the wavefront of laser radiation, with the aid of which high-power laser can be measured directly with respect to their wavefront.

The solution of this problem is solved in procedural terms by the specified in the characterizing part of claim 1 process steps and under device-technical aspects by the construction specified in the characterizing part of claim 5.

Accordingly, the invention provides a scanning head with a small aperture relative to the laser beam cross-section, which decouples partial laser beams from the laser beam to be measured in temporal succession. The respective partial laser beams are detected by means of a position-resolving detector, which is arranged in the beam path of the decoupled partial laser beam, which generates wavefront-specific measurement data. With the aid of an evaluation unit, the determination of the wave- front of the wavefront-specific measurement data of the individual partial laser beams in conjunction with the position coordinates of the decoupled partial laser beams within the laser beam to be measured reconstructed the wavefront to be measured.

The method and apparatus according to the invention have the advantage that the proportion of the laser power passing through the aperture of the arrangement is very small. Thus, no attenuation of the partial laser beam is necessary, corresponding optical elements, which can lead to a distortion of the wavefront to be detected, omitted. Should a power attenuation be required in exceptional cases, the thermal heating of the corresponding optical elements can be neglected due to the low power of the decoupled partial beam. If, after all, thermal effects nevertheless occur, they can be avoided by better cooling of the comparatively smaller optical components.

It should be noted that an intensity measurement of a laser beam using a so-called pinhole arrangement known from DE 199 09 595 Al is known. This method thus allows a representation of the beam cross section in the measured plane. By recording a measured value in various planes, this would at best make it possible to recalculate the wavefront actually present in the laser beam. By contrast, the invention obtains additional information about the laser beam, since, for example, aberrations of the radiation on optical components can be detected and quantified by the knowledge of the wavefront of the laser beam. DE 199 09 595 A1 gives no indication of this. Furthermore, DE 10 2005 038 587 A1 shows a measuring system and method for measuring a laser beam, in which the laser beam to be measured is guided by means of a deflection system over a pinhole of the measuring system, from where the decoupled partial beam falls onto a corresponding measuring sensor. A wavefront survey is also not addressed in this document.

The method according to the invention and the corresponding device make it possible to directly measure the wavefront in the measurement plane by evaluating the propagation direction of the partial radiation passing through the aperture on a position-sensitive detector. Thus, this measurement method uses a completely different physical effect from the previously discussed prior art to describe the wavefront. Advantageously, a wavefront determination with the aid of the invention is possible by a direct measurement of the propagation direction of the partial laser beams in a measurement plane for arbitrary wavefronts without costly reconstruction from a power distribution in several planes. The inventive method is thus much faster and more accurate than the power measurement, as disclosed in DE 199 09 595 Al or DE 10 2005 038 587 Al.

In the dependent claims advantageous developments of the method according to the invention or the corresponding device are given, which are explained in more detail in the following description with reference to the accompanying drawings. Show it:

1 and 2 are schematic representations of a hollow needle measuring device for the wavefront of a laser beam in two different embodiments, 3 and 4 is a plan view and an axial section of a measuring device with aperture stops in a first embodiment,

5 shows an axial section of a measuring device in a further embodiment with aperture diaphragm and absorber diaphragm,

6 to 8 are plan views of this surveying device with differently shaped aperture diaphragms,

9 is an axial section of the arrangement of FIG. 8,

10 shows a top view of a measuring device in a further embodiment with aperture diaphragm and absorber diaphragm, and FIG

11 shows an axial section of an aperture stop of the surveying device according to FIG. 10.

As is clear from FIG. 1, the laser beam 1 to be measured is measured with the aid of a hollow needle-like scanning head 2. The latter has, at its end pointing counter to the beam direction S, an aperture 3 small in relation to the laser beam cross section in the form of a circular opening with a diameter of, for example, 1 mm. In principle, the size of the aperture 3 is preferably of the order of the intended lateral resolution. At very high power densities of the laser beam to be measured, the opening can also be reduced, whereby Care must be taken that the aperture size does not become so small that the diffraction produced at the aperture significantly affects the resolution of the measuring arrangement. The aperture diameter is thus to be kept well above the size of the wavelength of the laser radiation to be measured.

By means of the aperture 3, a partial laser beam 4 is coupled out of the laser beam 1 to be measured. A position-sensitive detector 5 is arranged in the scanning head in the beam path of the partial laser beam 4, onto which the partial laser beam 4 impinges in a position significant for the course of the wavefront 6 in the region of the coupled-out partial laser beam 4. Thus, the phase angle of the partial laser beam 4 can be determined via its impact location on the position-resolving detector 5. The scanning head 4 is scanned for detecting the entire wavefront 6 with the aid of an x-y positioning system 7, for example in a two-dimensional grid arrangement with a certain number of measuring points in the x and y directions. With the aid of an evaluation unit 8, the corresponding measurement data of the detector 5 are assigned to the corresponding position of the aperture 3 in the laser beam 1 and thus the laser front 6 of the laser beam to be measured is determined from the wave front-specific position data of the individual partial laser beams 4 in conjunction with the position coordinates of the positioning system 7.

In order to increase the resolution of the measuring arrangement, an additional optical element 9 for focusing the partial laser beam 4 onto the surface of the detector 5 can be installed in the beam path of the partial laser beam 4. Incidentally, the surface 10 of the scanning head 2 has a highly reflective design so that a large part of the laser radiation impinging thereon is deflected and does not lead to heating of the scanning head 2.

The measuring principle according to the invention is distinguished in comparison to known measuring principles by a high spatial resolution and a large measuring range. The resolution of the detector arrays used for the known Hartmann-Shack method is low, especially for the measurement of IR radiation in the range of 8 to 14 microns. The current state is a maximum of 640 x 480 pixel VGA resolution. In the Hartmann-Shack method, the total available detector area of nxm pixels is subdivided into k sub-areas and assigned to a subaperture. As a result, the measuring range and the resolution of the measuring system are drastically reduced, since for a subaperture that detects a section of the wavefront, therefore, only nxm / k ² pixels are available. By contrast, in the case of the method according to the invention, the partial laser beam 4 coupled in via the aperture is evaluated with the aid of the entire surface of the detector 5 with n × m pixels. This allows dramatically higher resolution or the ability to use smaller, low-pixel detectors or simpler position-sensitive semiconductors, such as four-quadrant diodes or PSD detectors, which then achieve similar resolution compared to prior art Hartmann-Shack systems but are significantly less expensive ,

The embodiment shown in FIG. 2 differs from that of FIG. 1 only in that the aperture 3 of the scanning head 2 is associated with a deflecting mirror 11 which deflects the coupled-out partial laser beam 4 transversely to the beam direction S of the laser beam 1 to be measured onto the detector 5 , This allows a reduction in the height of the Ab- probe. As a result, the mechanical properties can be improved in certain design variants. Incidentally, reference may be made to the embodiment of FIG. 2 to the corresponding description of FIG. 1. Matching components are identified by identical reference numbers.

In Fig. 3 and 4, an alternative embodiment for forming the laser beam 1 scanning aperture 3 is shown, which is constructed on the model of the so-called Nipkov disc. This construction has a first disk element 12 with an aperture slot 13 occupying a radius line, which is combined with a second disk element 14. The latter has at uniform angles of rotation W in each case with increasing distance a arranged aperture openings 16. If the second disk element 14 is rotated relative to the first disk element 12, one of the aperture openings 16 successively comes into coverage in the area of the aperture slot 13 successively. The laser beam 1 can thus be scanned on a chord line relative to the laser cross section. For complete detection of the laser beam 1, the two disk elements 12, 14 are moved together either linearly to the laser beam 1, so that the Aper- turschlitz 13 sweeps over the complete cross section of the laser beam 1 and correspondingly the entire laser beam 1 is scanned. A rotation of the disk elements 12, 14 about the center of the aperture 14 is also possible for detecting the entire laser beam cross-section, in which case a fan-shaped scanning of the laser beam 1 takes place.

As is clear from the axial section according to FIG. 4, below the disc element 14 with the aperture slit 13, the optical element 9 is arranged in the form of a lens, with the aid of which the sub-beams which are hidden in time succession through the aperture openings 16 4 in turn be imaged onto the detector 5 in the form of a four-quadrant diode.

In the embodiment illustrated in FIGS. 5 and 6, the aperture stop is designed as a rotating perforated mirror 17 with an aperture opening 16 in the region of a circumferential groove recess 18 which is conical in cross-section. The hole mirror 17 is thus formed as a W-axicon. About this hole mirror 17 with the single aperture 16, an absorber ring aperture 19 is arranged, the central opening is concentric with the traversed by the aperture 16 peripheral line of the hole mirror 17. The absorber ring diaphragm 19 defines with its central opening 20, which has the diameter A, the measuring aperture of the device through which the laser beam 1 passes.

In the region of the aperture opening of the perforated mirror 17, an optical element 9 in the form of a lens is again arranged underneath it, with the aid of which the hidden partial laser beam 4 is imaged onto the detector 5 in the form of a four-quadrant diode.

As is clear from FIG. 5, the laser radiation 21 reflected by the perforated mirror 17 in the region of the groove recess 18 is absorbed in a defined manner by the absorber ring diaphragm 19 with its absorber section 22 which widens conically downwards.

The laser beam 1 can be completely scanned in the direction R by a displacement of the measuring device according to FIGS. 5 and 6. In each case arcuate tendon lines of the laser beam cross section are scanned. In Fig. 7, a multi-hole mirror 23 is shown with a plurality of aperture openings 16, which are arranged analogously to the nipkovscheibenartigen arrangement of FIG. 3 and 4 on a spiral line. Thus, during one rotation of the multiple-hole mirror 23, a plurality of arc-shaped chord sections of the laser beam 1 can be scanned one after the other without displacement of the surveying device, resulting overall in a shorter scanning time for the measurement of the entire laser beam 1.

In the embodiment shown in FIG. 8, a plurality of aperture openings 16 are arranged on a single circumferential line in the multiple-hole mirror 23 '. Their distance D must be greater than the diameter A of the central opening 20 of the absorber ring diaphragm 19. When shifting this multiple-hole mirror 23 'in the direction R, a substantially arcuate chord line of the laser beam 1 is scanned with each aperture 16.

As is apparent from Fig. 9, the aperture openings 16 at the lowest point of the groove recesses 18 are arranged. Thus, the distance of the aperture openings 16 from the lens 9 is constant and minimal. The inclined flanks of the groove recesses 18 cause the main laser beam 1 is controlled in the absorber ring diaphragm 19. The groove recesses 18 should be as pointed as possible at their lowest point, ie have the greatest possible curvature in order to produce as little diffuse backscatter as possible. In that regard, in the case of a perforated mirror 17 with only one aperture opening 16 (FIG. 6) or a multiple-hole mirror 23 'with aperture openings 16 at a constant distance from the center of the mirror (FIG. 8), it is advantageous to use only one groove recess 18. The upwardly facing ridges 24 between the groove recesses 18 are less problematic in terms of scattering, since they can be made quasi acute with an extremely small radius of curvature.

Incidentally, a helical groove structure is conceivable for the multiple-hole mirror 23 according to FIG. 7.

FIG. 10 shows a variant of a multiple-hole mirror 25, in which the aperture openings 16 are each surrounded by conically extending zones 26. The aperture openings 16 are in each case at the downward-pointing cone tip. According to the number of aperture openings a plurality of conical zones 26 are arranged distributed over the circumference of the multi-hole mirror 25.

Depending on the pitch angle of the conical zones 26, the disk of the multi-hole mirror 25 can be made relatively thick. For better cooling then - as can be seen from FIG. 11 - cooling louvers 27 may be provided on the rear side of the multiple-hole mirror 25.

If this is necessary for technical reasons, the multiple-hole mirror 23 (FIG. 9) or 25 (FIG. 11) can be provided at the rear with a plane-parallel absorber 28 in front of each aperture 16. Due to the contact with the heat-dissipating disk of the multi-hole mirror 23 or 25, heating of the absorber 28 is limited. The attenuation thus also takes place in the region of the parallel beam of the partial laser beam 4 and not in the focused section. As an alternative to the absorber 28, a partially reflective coating of the focusing lens 9 is also possible. For measuring the laser beam 1, the aperture openings 16 are moved uniformly in circular paths over the laser beam 1 by the rotation of the respective mirrors 17, 23, 23 'or 25. The detector 5 experiences a comparatively slow change in intensity. The integration time of the detector 5 determines the spatial resolution of the surveying system in this direction at a constant rotational speed. It is thus electronically adjustable. By a method of the mirrors 17, 23, 23 'and 25 with fixed lens 9 and detector 5, a rastering of the entire beam cross section is effected, from which the wavefront of the laser beam 1 can be determined.

In summary, in contrast to the previously known solutions, the invention has a multiplicity of advantages:

The measurement of the wavefront can be realized at high laser powers without prior power attenuation. As a result, the subject

Determination of the wavefront no distortion by thermo-optical effects.

The lateral resolution of the measuring system can be increased by scanning the laser beam cross-section with the aid of a high-precision positioning system.

The measurement of large laser beam cross sections can be carried out without cross section conversion.

Low cost position sensitive detectors such as four quadrant diodes or P SD detectors may be used.

The detectors are more easily interchangeable so that laser beam sources with different wavelengths can be measured flexibly.

Claims

claims 1. A method for wavefront measurement of laser radiation, characterized in that - with the aid of a relative to the cross section of the laser beam to be measured (1) movable, contrast small aperture (3, 16) temporally successive partial laser beams (4) from the laser beam to be measured (1) be disconnected the partial laser beams (4) are detected by means of a position-resolving detector (5) and from this wavefront-specific measurement data are generated, and - The wavefront (6) of the laser beam (1) from the wavefront-specific measurement data and the position coordinates of each decoupled partial laser beam (4) within the laser beam to be measured (1) is determined. 2. The method according to claim 1, characterized in that the partial laser beams (4) in a two-dimensional grid arrangement of the laser beam to be measured (1) are coupled out. 3. The method according to claim 1 or 2, characterized in that each decoupled partial laser beams (4) is focused on the detector (5). 4. The method according to any one of the preceding claims, characterized in that each of the decoupled partial laser beams (4) in the scanning head (2) transversely to the beam direction (S) of the laser beam to be measured (1) is deflected. 5. Apparatus for wavefront measurement of laser radiation, characterized by a scanning head (2) having an aperture (3, 16) which is movable relative to the laser beam cross-section relative to the cross-section of the laser beam (1) for sequentially hiding partial laser beams (4) from the laser beam (1) to be measured, a position-resolving detector in the beam path of the coupled partial laser beam for generating wavefront-specific measurement data of this partial laser beam, and an evaluation unit for determining the wavefront to be measured from the wavefront-specific measurement data of the individual partial laser beams ) in conjunction with the position coordinates of the decoupled partial laser beams (4) within the laser beam (1) to be measured. 6. Apparatus according to claim 5, characterized in that the scanning head (2) is arranged on a scanner positioning system (7). 7. Apparatus according to claim 5 or 6, characterized in that the scanning head (2) is designed as a hollow needle. 8. The device according to claim 5, characterized in that in the scanning head (2) for the realization of the laser beam (1) scanning aperture (3) at least one moving, disc-shaped aperture diaphragm (12, 14, 17, 23, 23 ', 25), is preferably provided in the manner of Nipkovscheiben, which the respective partial laser beam (4) on a laser beam (1) stationary detector array (5) hides. 9 Device according to one of claims 5 to 8, characterized by a in the beam path of the partial laser beam (4) arranged optical element (9) for focusing the partial laser beam (4) on the detector (5). 10. Device according to one of claims 5 to 9, characterized in that in the scanning head (2) a deflecting mirror (11) for deflecting the decoupled partial laser beam (4) transversely to the beam direction (S) of the laser beam to be measured (1) is arranged. 11. Device according to one of claims 5 to 10, characterized in that the scanning head (2) at least on its the laser beam (1) exposed surface (10) is highly reflective for the laser beam (1). 12. Device according to one of claims 5 to 11, characterized in that as a position-resolving detector (5) a four-quadrant diode or a PSD detector is used. 13. Device at least according to claim 8, characterized in that the reflective aperture diaphragm (17, 23, 23 ', 25) is designed as a W-axicon with reflective elements (18). 14. The apparatus according to claim 13, characterized in that above the for the laser beam (1) reflecting the aperture diaphragm (17, 23, 23 ', 25), an absorber ring (19) for the reflected laser radiation (21) is arranged. 15. Device at least according to claim 8, characterized in that the aperture diaphragm (17, 23, 23 ', 25) is formed as a single or multiple-hole mirror.