Beam position detection assembly and method for determining the spatial position of a laser beam

Abstract

The invention relates to a beam position detection assembly (100) for detecting the spatial position of a laser beam (L), comprising: - a beam position detector (10) having a, in particular a single, position detection surface (11); - an optical element (20), which is designed to separate an (input) laser beam (L) into at least two partial beams (L1, L2) by means of reflection and / or transmission in such a way that the propagation properties of the at least two partial beams (L1, L2) differ in their particular focusing and preferably in the particular propagation direction, wherein the beam position detection assembly (100) is designed such that the at least two partial beams (L1, L2) impinge on the position detection surface (11) of the beam position detector (10) and the beam position detector (10) is designed to detect positions and / or position deviations of the at least two partial beams (L1, L2) on the position detection surface (11).

Description

[0001] Beam position detection arrangement and method for determining the spatial position of a laser beam

[0002] Description

[0003] The invention relates to a beam position detection arrangement for detecting the spatial position of a laser beam according to patent claim 1 and to a method for determining the spatial position of a laser beam according to patent claim 10.

[0004] To achieve high performance in optical systems, such as laser systems or laser scanning modules, a laser beam must usually be guided with high precision through transmissive optical components or over reflective optical components. The corresponding adjustment for this is usually performed manually by a user.

[0005] In addition, the performance of optical systems (e.g. for measuring or processing arrangements) suffers from uncontrolled fluctuations in the spatial position of the respective laser beam, which occur after or during adjustment.

[0006] These can be caused, for example, by laser (pointing) drift or scanning system drift. Environmental influences, such as heating of mechanical or optical components, can also lead to beam movement.

[0007] To counteract such a movement of the laser beam, a change in the spatial position of the laser beam is usually determined in a first step, and then counteracted accordingly (either manually or automatically) in a subsequent step. Typically, two separate (image) sensors with separate detection surfaces are used to determine the spatial position, which must be calibrated to each other.

[0008] For this purpose, a portion of the laser beam under investigation is split into two beams using a beam splitter. The first beam is typically directed onto a position-sensitive sensor (PSD, position-sensitive detector) without any additional (focusing) optics. The second beam is directed onto a second position-sensitive sensor via a focusing optic, e.g., a lens.

[0009] This setup allows the spatial position of the laser beam relative to a reference point to be extracted from the relative movements of the partial beams or the relative measured values ​​of the two sensors. This takes advantage of the fact that the focused spot of the second partial beam is not sensitive to a beam offset in front of the focusing optics. This means that if the beam impinges on the focusing optics at an offset, the position of the focused spot on the sensor does not change. This can be derived from considerations of Fourier optics and the mathematical description of the optical "far field."

[0010] The first partial beam, which directly hits the sensor, is sensitive to both a relative offset and a relative tilt of the input beam. This is referred to as the optical "near field."

[0011] By exploiting these different sensitivities, the tilt in both spatial directions and then the two-dimensional offset can be separated.

[0012] Disadvantages of previously known devices that can perform such measurements include the large size, limitations in absolute accuracy and the comparatively high price, since many components have to be installed.

[0013] It is therefore an object of the invention to provide a beam position detection arrangement that enables highly precise detection of the spatial position of a laser beam in the most compact and cost-effective manner possible. Furthermore, it is an object of the invention to provide an improved method for determining the spatial position of a laser beam, by means of which the spatial position can be determined particularly advantageously, in particular comparatively quickly.

[0014] The object is achieved with regard to a beam position detection arrangement by the subject matter of claim 1 and with regard to a method by the subject matter of claim 10.

[0015] In particular, the object is achieved by a beam position detection arrangement for detecting the spatial position of a laser beam, wherein the beam position detection arrangement comprises:

[0016] - a beam position detector with one, in particular a single, position detection surface;

[0017] - an optical element designed to separate an input laser beam into at least two partial beams by means of reflection and / or transmission, such that the propagation properties of the at least two partial beams differ in their respective focusing and preferably in the respective propagation direction, wherein the beam position detection arrangement is designed such that the at least two partial beams impinge on the (same) position detection surface of the beam position detector and the beam position detector is designed to detect positions and / or position deviations of the at least two partial beams on the position detection surface.

[0018] An important idea of ​​the invention is to reduce the number of components of conventional devices for detecting or determining a spatial beam position. A core idea of ​​the invention is therefore, on the one hand, to provide one, in particular a single, optical element that combines the functionality of a beam splitter and a focusing element (which are used as separate elements in conventional devices), and, on the other hand, to design this optical element in such a way that preferably only one (single) beam position detector with one (single) position detection surface is required. According to the invention, all partial beams or their corresponding spots are measured in this way with a single beam position detector or sensor, for example a (CCD) camera. Overall, this makes it possible to construct the beam position detection arrangement in a particularly compact manner.Eliminating the need for an additional sensor also offers another major advantage in terms of cost savings and reducing the complexity of the mechanical design of the beam position detection arrangement. Furthermore, balancing or adjusting / calibrating two beam position detectors relative to each other is no longer necessary. With the inventive approach, the maximum input beam size depends solely on the size of the optical element, which can be scaled (up to certain limits), but whose cost correlates less strongly with the size than that of the beam position detectors.

[0019] Determining the beam position via the position or the position deviations of the partial beams is advantageous compared to other approaches, such as determining the power of differently diffracted partial beams, because the recording of the position or the position deviations of the partial beams is less susceptible to noise, so that the measurement accuracy can be increased. In particular, through adaptive methods in the binarization (adaptive threshold calculation) of the recorded images, fluctuations in the power diffracted into the individual partial beams have little to no influence. In addition, a sub-pixel-precise resolution is achieved, for example, through subsequent mathematical fitting (e.g. of an ellipse function), preferably for determining the spot centers. In this step, all pixels of the partial beams preferably contribute (in a weighted manner), which creates an averaging effect.

[0020] In principle, it is possible to spatially separate the partial beams (directly) using the optical element in such a way that they differ in their respective propagation directions. However, it is equally possible for the two partial beams to propagate co-linearly and differ (only) in the respective position of the focal planes. For this variant, the optical element could possibly be designed in such a way that it generates the two partial beams with different polarizations, e.g. in such a way that the first partial beam has a polarization that is essentially perpendicular to the polarization of the second partial beam. A polarizer can then be used to separate the two co-linearly propagating partial beams. Alternatively, the co-linear partial beams could be separated by diaphragms.Furthermore, it would also be conceivable to design the optical element in such a way that the two partial beams have (slightly) different wavelengths, so that they could be separated from each other by (spectral) filters.

[0021] In the context of this application, a beam position is understood to mean the position or orientation of the (input) laser beam in space, in particular in three-dimensional space.

[0022] In the context of this application, the "position of the focal plane" is understood to mean a point in the beam direction (of the respective beam or partial beam) at which the beam diameter has been reduced to a minimum by appropriate focusing.

[0023] The beam position detector and / or the beam position detection arrangement can comprise a computing unit or be connected to a computing unit, for example an external computing unit, in order to carry out a corresponding data evaluation by evaluating and / or image processing the detected beam profiles of the partial beams.

[0024] In one embodiment, the optical element comprises a diffractive or holographic optical element. Other terms considered equivalent for the diffractive or holographic optical element are: diffractive optical element (DOE for short), holographic optical element (HOE for short), computer-generated hologram (CGH for short), or hologram. By appropriately designing the optical (holographic) element, both the optical function of a beam splitter and a focusing optical element, such as a lens, can be implemented. This allows the number of components and installation space to be reduced. This reduces both the complexity of the beam position detection arrangement and its production and maintenance costs.In addition, the beam position detection arrangement is made more flexible because, through appropriate design of the holographic element, not just two partial beams (as with a conventional beam splitter), but several partial beams can be generated. The diffractive or holographic optical element can be designed specifically for one wavelength (e.g., 1064 nm). Alternatively or additionally, the diffractive or holographic optical element can be designed for several wavelengths (e.g., 1064 nm and 532 nm) or for a wavelength range (e.g., 950 nm to 1150 nm). Analyzing the spots of several, i.e., more than two, partial beams can, under certain circumstances, serve to achieve greater accuracy as well as greater stability and / or low noise.

[0025] By averaging the movement of the similar beams, noise (of the laser, the evaluation process, the image sensor, etc.) and manufacturing-related errors of the optical element (manufacturing tolerance of the "pixels" / the optical structure) are suppressed.

[0026] On the other hand, it is also possible that all partial beams have different propagation properties (e.g., with regard to their respective focal lengths). This leads to a different type of averaging, since more relative movements of different beam types can be determined.

[0027] In an alternative embodiment, the optical element can comprise a bifocal or multifocal lens or a corresponding lens arrangement comprising at least one bifocal or multifocal lens. This also makes it possible to combine both the optical function of a beam splitter and that of a focusing optical element, such as a lens, in a (single) optical element. This also allows the number of components and installation space to be reduced. This reduces both the complexity of the beam position detection arrangement and its production and maintenance costs.

[0028] In one possible embodiment, the lens arrangement comprises at least one refractive lens and / or at least one metamaterial lens. A metamaterial lens is understood to mean that a preferably flat substrate is used into which corresponding nanostructures are incorporated, which act, for example, as nanoantennas. In particular, these nanostructures are usually periodic, microscopically fine structures (cells, individual elements) made of electrically or magnetically active materials. This design enables a particularly compact design of the beam position detection arrangement.

[0029] In one embodiment, the optical element is designed and arranged such that the at least two partial beams have the same mode as the (input) laser beam, in particular such that, given a round input beam profile of the (input) laser beam, partial beams with round (output) beam profiles are generated. In this way, the determination of the positions of the partial beams is simplified because the beam profiles to be evaluated are homogeneous (and not, for example, frayed, crescent-shaped, or the like). The homogeneity and symmetry of the partial beams simplify fitting and are also possible in a short time with little computational effort.

[0030] In one embodiment, the optical element is designed such that a plurality of partial beams can be generated, and a first partial beam runs in the propagation direction of the input laser beam and the further partial beams run at an angle (deflection angle) to the first partial beam.

[0031] The fact that a plurality of partial beams "can be generated" or "are generated" is to be understood here in such a way that the input laser beam is separated into a plurality of partial beams or is divided or broken down into corresponding portions.

[0032] A corresponding deflection angle should be at least 0.5°, preferably at least 1.0°, and more preferably at least 1.5°. This ensures that the beams are clearly separated from one another, simplifying analysis. The large number of partial beams also increases measurement accuracy (possibly through various types of averaging). In other words, the optical element can be designed such that the partial beams impinge on the position detection surface separately from one another.

[0033] In a possible further development of this embodiment, the optical element is designed and arranged such that the additional partial beams, in particular at least five additional partial beams, are arranged on the position detection surface on a circular path around the first partial beam. This can further increase the accuracy of the measurement.

[0034] Particularly preferably, the partial beams can be arranged on a circular path whose center is formed by the first partial beam.

[0035] In one embodiment, the optical element has at least two different focal lengths fi, f2 for the respective focusing of the at least two partial beams, wherein the absolute value (abs) of the difference between the respective focal length fi, f2 and the distance D of the optical element from the position detection surface is at least twice as large for a / the first partial beam as for at least a second or (the) further partial beams.

[0036] In particular, the following condition has proven to be advantageous:

[0037] This results in particularly high sensitivity, accuracy and advantageous evaluability of the measurement.

[0038] In one possible embodiment (of the beam position detection device or the corresponding method), the optical element comprises an active optical element (or is designed as such), which is designed in particular to actively modulate the intensity and / or the phase of the laser beam in space. For example, an SLM (spatial light modulator) is possible here as the active optical element, such as a DMD (digital mirror device) or an LCoS (liquid crystal on silicon). This increases the flexibility of the arrangement. For example, the number of partial beams can be varied (depending on the application or the desired measurement accuracy). The active optical element can be controlled accordingly, for example, by software. It is also possible to vary a deflection angle of the further partial beams or corresponding focal lengths (depending on the application or the desired measurement accuracy).

[0039] In one embodiment, the optical element can be designed and arranged such that the focal plane of the second partial beam lies on or near the position detection surface. For example, it is possible for the optical element to be configured such that the focal plane of the second (or a further) partial beam lies approximately 1 mm to 10 mm in front of or behind the position detection surface. However, these values ​​are intended only to provide a rough understanding of a possible configuration. It goes without saying that the principle also works for focal plane positions of, for example, 500 mm in front of or behind the position detection surface. By positioning the focal plane of the second partial beam, in particular essentially directly, on the position detection surface, an evaluation of the spatial position can be simplified or accelerated, since no degrees of freedom, namely tilt and offset, are "mixed up."This is because, if the focal plane lies directly on the position detection surface, there is no sensitivity to an offset of the laser beam in front of the optical element.

[0040] However, both partial beams can also exhibit propagation properties such that their respective focal planes lie far away from the position detection surface. While this "mixes the degrees of freedom," potentially complicating the algorithms for such evaluation, it also offers advantages for the detection setup. For example, it allows for higher laser power of the beam under investigation, as the resulting intensity is lower due to the larger, unfocused spot diameter, thus preventing damage to the position detection surface.

[0041] In one embodiment, the optical element is designed such that, in addition to the first and second partial beams, at least one, preferably at least two, more preferably at least three, further partial beams can be generated, or such that a plurality of partial beams can be generated. The use of multiple partial beams and thus the analysis of multiple spots on the position detection surface has a beneficial effect on the noise and the achievable accuracy of the beam position detection arrangement.

[0042] In one embodiment, the optical element is designed such that a plurality of partial beams have different propagation properties with respect to the respective position of the focal plane, and / or that at least two partial beam groups are generated, wherein a first partial beam group comprises first partial beams with a first position of the respective focal planes and a second partial beam group comprises second partial beams with a second position of the respective focal planes (which differs from the position of the first partial beam group).

[0043] This provides an effective approach to noise reduction in measurements. Different spot movements can be compared with each other. The multitude of relative spot movements allows for redundant determination of the degrees of freedom of the incident laser beam and averaging of the results. This accelerates evaluation and increases accuracy.

[0044] The design of this embodiment, according to which several or all (possibly except for the first partial beam) partial beams have the same propagation properties, such as different propagation directions but essentially the same focal length, offers a further effective approach to noise reduction in the measurement. In this way, the relative movement (deviation) of identical spots of the partial beams with the same propagation properties can be averaged. This can significantly increase accuracy.

[0045] In one embodiment, the optical element is designed such that the propagation properties of the first partial beam remain substantially unaffected by the optical element and / or such that the optical element (also) slightly focuses the first partial beam. "Substantially unaffected" is understood in particular to mean that the optical element influences the corresponding partial beam only in the same way as, for example, a plane-parallel (glass) plate would be arranged in the beam. "Slight focusing" can be understood in particular to mean an effect similar to the effect of a lens with a comparatively large difference between the focal length and the distance between the optical element and the sensor.

[0046] For example, a configuration has proven particularly advantageous in which the distance of the focal plane of the "slightly focused" partial beams to the detector surface is approximately 130 Rayleigh lengths, and the distance of the focal plane of the "more focused" partial beams is approximately 13 Rayleigh lengths.

[0047] Advantageously, the optical element can therefore have at least two focal lengths (for focusing corresponding partial beams), whose respective refractive power differs by a factor of 10, preferably by at least a factor of 10. In this way, the beam position detection arrangement offers sensitivity with respect to a relative offset and also with respect to a relative tilt of the input beam. Slightly focusing the first partial beam may offer the advantage (see also above) that larger input beams can be measured, or smaller beam position detectors can be used, since the resulting beam size on the position detection surface is smaller.

[0048] In one embodiment of the invention, the beam position detector comprises an image sensor, in particular a single one. The image sensor can be embodied, for example, as a (CCD) camera or the like. This makes it possible to read images and evaluate them using image processing.

[0049] Compared to other detectors such as quadrant diodes, this allows for greater absolute accuracy in the evaluation, especially since this approach does not require calibration or scaling of the measurements, but is based on counting from a known pixel size. This advantageously eliminates the dependence of the measurement on a spot diameter, a gap between the quadrants, or an intensity distribution.

[0050] In particular, the object is also achieved by a method for determining the spatial position of a laser beam, preferably using a beam position detection arrangement according to the invention, the method comprising the following steps: a) separating (dividing) the laser beam into at least two partial beams in reflection and / or transmission of an optical element, in particular a diffractive and / or active optical element, such that the propagation properties of the first partial beam and the second partial beam differ in the position of their focal plane and preferably in the respective propagation direction; b) detecting the at least two partial beams of the laser beam on a position detection surface, in particular a common one, of a beam position detector, preferably with an image sensor; c) determining the positions and / or position deviations of the partial beams on the position detection surface of the beam position detector.This results in the same or similar advantages as those already described in connection with the beam position detection arrangement.

[0051] In one embodiment, the separation is performed in such a way that, given a round input beam profile of the (input) laser beam, partial beams with round output beam profiles are generated. This simplifies the determination of the positions of the partial beams, since the beam profiles to be evaluated are homogeneous (and not, for example, frayed, crescent-shaped, or the like). The homogeneity and symmetry of the partial beams simplify fitting and, moreover, can be performed quickly with little computational effort.

[0052] In one embodiment, step c) comprises determining an offset and a spatial tilt of the (input) laser beam, in particular absolute, from position deviations of the at least two partial beams on the position detection surface.

[0053] This allows the spatial position of the laser beam to be clearly determined. This allows the spatial position of the laser beam to be corrected in a downstream process, for example using controllable mirrors, so that drift movements and fluctuations can be compensated for. The process and the beam position detection arrangement (since the design and mechanical structure are known) enable these four degrees of freedom to be extracted as absolute values ​​without further calibration. In conventional processes or arrangements, only relative values ​​are extracted, which must be corrected by additional calibration because, for example, a shift in the x-direction leads to a deflection of the offset measured value in the same x-direction; however, an actual offset value then does not correspond to the measured values.

[0054] The following is preferably understood as "absolute values" and / or an "absolute" determination of the values.

[0055] For example, if an initial position of the laser beam is defined as a "reference position" (offset = 0 mm / 0 mm, tilt = 0 rad / 0 rad), a new absolute position of the laser beam can be determined due to the different movement of the various partial beams on the position detection surface, without the need to apply calibration factors that have to be determined experimentally.

[0056] In other words: If the laser beam deviates, for example, by 1 mm in the x-direction and 0.5 rad in the y-direction, an offset of 1 mm in the x-direction and 0.5 rad in the y-direction can be determined according to the invention without further calibration.

[0057] This is primarily due to the fact that the design of the optical element (DOE) and / or the deflection angle of the partial beams, as well as the mechanical structure (distance between the optical element and the beam position detector), are known. Since the pixel size or similar of the detector is also known, the movement of the partial beams on the position detection surface can be determined (converted), preferably in SI units.

[0058] Based on this knowledge, a change in the laser beam's position can be determined absolutely. If, as described, the laser beam's coordinate system is considered the reference coordinate system, the (new) beam position can also be determined absolutely.

[0059] Alternatively or additionally, it is also possible to determine the relative position of the laser beam to the beam position detection arrangement (or its change) absolutely. This means that absolute values ​​can be determined (without further calibration) by comparing the determined position of the partial beams on the position detection surface with the theoretical position of the partial beams when the input beam is incident centrically and perpendicularly (offset = 0 mm / 0 mm, tilt = 0 rad / 0 rad) on the sensor.

[0060] This allows a spatial position of the laser beam with respect to a reference position (in particular the initial position of the laser beam or the initial relative position to the detection arrangement) to be clearly determined.

[0061] Overall, the proposed method simplifies both the method and the beam position detection arrangement itself, and increases measurement accuracy. In one embodiment, the offset and / or tilt of the laser beam is determined by iteratively minimizing a difference value 8min between

[0062] - theoretical relative position deviations of the partial beams on the position detection surface, and

[0063] - measured relative position deviations of the partial beams on the position detection surface, determined by recalculating the theoretical relative position deviations with changed parameters for offset (Ax, Ay) and / or tilt (a, ß).

[0064] Preferably, the offset and tilt can be carried out taking into account calculated (theoretical) simulation values ​​with "randomly" selected offsets and tilts and the resulting calculated (relative) position deviations can be compared with the measured (relative) position deviations.

[0065] Overall, the iterative minimization of the difference value results in a fast and reliable method for determining the spatial position of the input laser beam.

[0066] The term "relative" is to be understood here in particular in such a way that at least two different partial beams with different propagation properties (different position of the focal plane) are used to extract offset and tilt from the respective position deviations.

[0067] In one embodiment, the difference between calculated position deviations of the partial beams is determined using a previously determined or measured reference point on the detector surface.

[0068] The use of a reference point also increases the accuracy of the method and accelerates the determination of the spatial position of the input laser beam. In one embodiment, the iterative minimization is carried out using support value tables, each containing support values ​​of an equidistant step size, wherein in a first iteration step S1 a first support value table is used which has a first step size, and the difference value is minimized based on the corresponding support values ​​of the first support value table, and wherein in a second iteration step S2 a best-fit support value determined in iteration step S1, for which the difference value is minimal in iteration step S1 oris used as the center point in a second support value table, and support value limits are selected according to the support values ​​of the first support value table adjacent to the best-fit support value, and the support values ​​of the second support value table between these support value limits are used to minimize the difference value, wherein a second step size of the second support value table is smaller than the first step size of the first support value table.

[0069] This approach significantly reduces the required computing power and thus drastically accelerates the process for determining the spatial position of the input laser beam. This is achieved because corresponding spot positions only need to be calculated for a few offsets and tilts. A possible alternative would be a significantly larger support value table, which has the same resolution as the final, fine support value table, but covers the same wide range of values ​​as the first, coarse support value table.

[0070] In particular, the object is also achieved by a computer-readable (storage) medium which contains instructions which, when executed by a computer, cause the computer to carry out the method (in particular step c) of the method as described above and / or further developments of the method according to the invention described below) as described above.

[0071] Advantageous further developments emerge from the subclaims. The invention is described below with regard to further details, features, and advantages, which are explained in more detail with reference to the figures.

[0072] The described features and combinations of features, as shown below in the figures of the drawing and described with reference to the drawing, are applicable not only in the respective combination specified, but also in other combinations or in isolation, without thereby departing from the scope of the invention.

[0073] Here we show:

[0074] FIGS. 1A to 1C show geometric definitions regarding tilt and offset of a laser beam;

[0075] FIG. 2A shows an embodiment of a beam positioner according to the invention.

[0076] Detection arrangement for detecting the spatial position of a laser beam using two partial beams, wherein the optical element is used in transmission;

[0077] FIG. 2B shows an embodiment of a beam positioner according to the invention.

[0078] Detection arrangement for detecting the spatial position of a laser beam using two partial beams, wherein the optical element is used in reflection;

[0079] FIG. 3 is a schematic plan view of a position

[0080] Detection surface of a beam position detector according to an embodiment in which two spots of two partial beams with different propagation properties are detected;

[0081] FIG. 4 shows a further embodiment of an inventive

[0082] Beam position detection arrangement for detecting the spatial position of a laser beam using several partial beams;

[0083] FIG. 5A is a schematic plan view of a position

[0084] Detection surface of a beam position detector according to an embodiment in which six spots of six partial beams are detected, wherein five partial beams have substantially the same propagation properties (optical element with two focal lengths);

[0085] FIG. 5B is a schematic plan view of a position

[0086] Detection surface of a beam position detector according to an embodiment in which six spots of several partial beams are detected, wherein all partial beams have different propagation properties (optical element with several focal lengths);

[0087] FIGS. 6A to 6C show a schematic sequence for determining the spot positions or spot movements according to an embodiment of the method according to the invention for determining the spatial position of a laser beam;

[0088] FIG. 7 shows a schematic sequence for determining the offset and tilt of the laser beam according to an embodiment of the method according to the invention for determining the spatial position of a laser beam using support value tables.

[0089] FIGS. 1A to 1C show geometric definitions regarding the terminology used for a tilt and an offset of a laser beam L (schematically indicated by the arrow with a larger line width) propagating in three-dimensional space with the coordinates x, y and z.

[0090] To determine the beam position of the laser beam in three-dimensional space with respect to a reference point, the following four degrees of freedom are preferably determined:

[0091] - Offset in x-direction (Ax),

[0092] - Offset in y-direction (Ay),

[0093] - Tilt in x-direction (a),

[0094] - Tilt in the y-direction (ß). Tilt of the laser beam L is understood as a change in the spatial position of the laser beam along the angles α and β shown (FIG. 1A).

[0095] An offset in the x-direction (Ax) means that the laser beam is shifted in space essentially parallel in the x-direction by the amount Ax (FIG. 1B).

[0096] An offset in the y-direction (Ay) means that the laser beam is shifted in space essentially parallel in the y-direction by the amount Ay (FIG. 1C).

[0097] FIG. 2A shows an embodiment of a beam position detection arrangement 100 according to the invention for detecting the spatial position of an (input) laser beam L using two partial beams Li and L2.

[0098] According to one embodiment, the laser beam L to be examined can be attenuated accordingly before being fed to the beam position detection arrangement 100. For this purpose, for example, a corresponding beam splitter BS can be used, which allows a major portion of the laser power to be transmitted and reflects a reflection of the laser beam L in the direction of the beam position detection arrangement 100.

[0099] However, it may also be conceivable (for example at low laser powers) to couple the laser beam L directly or with the aid of mirrors (without beam splitter BS) into the beam position detection arrangement 100.

[0100] The beam position detection arrangement 100 for detecting the spatial position of a laser beam L comprises a beam position detector 10 with a position detection surface 11. The beam position detector 10 is preferably designed as an image sensor, for example, as a (CCD) camera.

[0101] Furthermore, the beam position detection arrangement 100 has an optical element 20 which is designed such that when the laser beam L (or the branched reflection) strikes an entrance plane 21, the laser beam L passes through the optical element 20 and is thereby separated into a first partial beam Li and a second partial beam L2.

[0102] The optical element 20 is specifically designed such that the propagation properties of the first partial beam Li and the second partial beam L2 differ after passing through the optical element 20 in the respective propagation direction and the position of their focal plane.

[0103] Subsequently, the two partial beams Li, L2 (spatially separated from each other) impinge on the position detection surface 11 of the beam position detector 10.

[0104] The optical element 20 combines the functionality of a beam splitter and a focusing element, so that the beam position detection arrangement 100 can be constructed compactly and only a single beam position detector 10 is required.

[0105] In one embodiment, the optical element 20 may be a holographic (diffractive) optical element. In an alternative embodiment, the optical element 20 may be configured as a bifocal lens.

[0106] The optical element 20 is designed such that the focal plane of the second partial beam L2 lies on or near the position detection surface.

[0107] FIG. 2B shows an embodiment similar to that of FIG. 2A, wherein in FIG. 2B, the optical element 20 is configured to be used in reflection, while in FIG. 2A it is configured to be used in transmission. Otherwise, the statements regarding FIG. 2A apply equally to FIG. 2B.

[0108] FIG. 3 shows a schematic plan view of a position detection surface 11 of a beam position detector 10 according to an embodiment in which two spots of two partial beams Li, L2 with different propagation properties are detected. In this example, the two partial beams Li, L2 are focused (differently). The first partial beam Li is only slightly focused by means of the optical element 20, while the second partial beam L2 is more strongly focused, so that its focal plane lies near (in the z-direction) the position detection surface.

[0109] Furthermore, according to this embodiment of FIG. 3, the first partial beam Li is not influenced in its propagation direction (or in that of the incident beam L), but rather passes through the optical element 20 in the z-direction (see FIG. 2A) and remains essentially unaffected in the x- and y-directions.

[0110] According to this exemplary embodiment, the second partial beam L2 is influenced by the optical element 20 in both the x and y directions. Alternatively, the second partial beam L2 could also be influenced only in the x or y direction.

[0111] FIG. 4 shows a further embodiment of a beam position detection arrangement 100 according to the invention for detecting the spatial position of a laser beam L using a plurality of partial beams.

[0112] In this embodiment, the optical element 20 is designed as a holographic element 20 with multiple focal lengths and is configured such that multiple partial beams are generated.

[0113] In FIG. 4, a first partial beam Li and five further partial beams L2 to Le are shown. It is understood that this is merely an example, and a different number of further partial beams L2 to L n can be implemented.

[0114] Similar to the previously described embodiment, the first partial beam Li should either not be focused or should be focused slightly by the optical element 20 and its propagation direction should remain essentially unaffected.

[0115] According to this exemplary embodiment, the further partial beams L2 to Le are each deflected differently by the optical element 20 (in the x and y directions). The optical element 20 can be designed such that all further partial beams L2 to Le are influenced such that their respective focal planes lie on or near the position detection surface 11 (FIG. 5A). All spots of the partial beams L2 to Le thus have essentially the same size. The optical element 20 is designed here, in particular, as an optical element with two focal lengths.

[0116] According to one example, in the configuration according to FIG. 5A, the position detection surface 11 is, for example, a camera chip, e.g. a 2" chip (approx.

[0117] 6.4 mm x 4.8 mm). A distance D (see FIG. 2A) of the optical element 20 to the position detection surface 11 is, for example, between 50 mm and 60 mm (in particular, approximately 53.5 mm). The focal length fi of the optical element for the first partial beam Li is, for example, 54 mm, and the focal length f2 for the partial beams L2 to Le is, for example, 60 mm.

[0118] Regarding the distance D, it should be noted in particular that the distance D (e.g., in the case of a holographic optical element 20) preferably refers to the distance between an active surface of the optical element 20 and the position detection surface 11. The active surface can, for example, be formed on the front or back side of a substrate of the optical element 20.

[0119] The partial beams L2 to Le lie on a circular path around the first partial beam Li. The deflection angle of the partial beams L2 to Le is approximately 1.95° (from the first partial beam). Alternative focal lengths, distances, and deflection angles are possible and can be adjusted if necessary (depending on the size of the position detection area 11). In particular, this allows a balance to be struck between the required resolution and the measuring range.

[0120] The following relationship between the distance D and the focal lengths fi, f2 has proven to be particularly advantageous in terms of sensitivity, accuracy and evaluability: Alternatively, the optical element 20 can also be designed such that all further partial beams L2 to Le are influenced such that their respective focal planes lie at different distances (in the z-direction) from the position detection surface 11 (FIG. 5B). All spots of the partial beams L2 to Le thus have different sizes. The optical element 20 is then designed, in particular, as an optical element with multiple focal lengths (six in the specific case shown).

[0121] In both FIGS. 5A and 5B, a round (e.g., Gaussian) input beam profile is maintained, ie, the sub-beams Li to Le show the same or a similar mode with round beam profiles.

[0122] Averaging when evaluating the relative movements of "similar" spots, ie partial beams with the same focal length (or the same position of the focal planes, FIG. 5A), can have an advantageous effect on the noise and the achievable accuracy of the beam position detection arrangement 100.

[0123] The configuration shown in FIG. 5B offers a different approach to noise reduction by averaging multiple spots with different focal lengths (different positions of the focal planes).

[0124] In principle, different spot movements can be compared with each other. The multitude of relative spot movements allows for redundant determination of the degrees of freedom of the incident laser beam L and averaging of the results.

[0125] As already described above, the position and movement of all partial beams Li to L generated by the optical element 20 nrecorded and measured with a beam position detector 10, for example a (CCD) camera.

[0126] A suitable camera can be selected depending on the requirements for accuracy and speed. Likewise, the distance between the optical element 20 and the beam position detector 10 or its position detection surface 11 can be selected almost freely. Furthermore, according to a further aspect of the invention, it is possible to infer a (weighted central) wavelength of the incident light from a (mean) distance between the partial beams L2 to Le, which are preferably arranged in an orbit (see, for example, FIG. 5A or 5B) around partial beam Li. When the optical element is irradiated with a wavelength different from the design wavelength (e.g., DOE designed for 1064 nm, irradiation with a laser source at 1074 nm), the diffraction strength of the optical element changes. The partial beams L2 to Le are deflected to a lesser or greater extent and hit the camera closer or further away from the center.In particular, a deviation from the design wavelength of the optical element 20 can be determined via the deviation in the position of the partial beams L2 to Le. Therefore, according to one possible embodiment of the invention, the beam position detection arrangement 100 can also be used as (a type of) spectrometer and can be used for wavelength monitoring.

[0127] FIGS. 6A to 6C show a schematic sequence for determining the spot positions or spot movements according to an embodiment of the method according to the invention for determining the spatial position of a laser beam L.

[0128] In a first step, in order to determine the spatial position of the laser beam L, information (images or image data) from the beam position detector is read in by means of the computing unit (the beam position detector or an external computing unit connected to the beam position detector) - e.g. as individual images.

[0129] This information from the beam position detector can be evaluated using an evaluation program in the computing unit.

[0130] In one embodiment, the information is read in or displayed as a grayscale image (see FIG. 6A).

[0131] The corresponding resolution depends on the beam position detector and can, for example, be 8 bits, which corresponds to a division into 256 different values.

[0132] For highly accurate determination of the spot positions of the individual partial beams, image processing is applied to the grayscale image. For this purpose, the images are first binarized, i.e., converted into a black-and-white image according to a fixed or adaptive threshold (see FIG. 6B).

[0133] Subsequently, an ellipse detection is performed and the center of gravity of the spots is extracted (see FIG. 6C).

[0134] This is done, for example, through automated weighting of the pixels contributing to the respective spot.

[0135] This approach makes it possible to determine the ellipse's centroid with subpixel precision. Therefore, this approach is advantageous over simply detecting the brightest point / pixel to determine the spot position.

[0136] By averaging multiple camera images for noise reduction and two-dimensional interpolation in the areas of the spots to increase the resolution, the accuracy of the center of gravity determination can be further increased.

[0137] For example, this method enables a resolution / accuracy of 0.005 pixels, which corresponds to approximately 23.25 nm for a typical pixel edge length of 4.65 pm.

[0138] With reference to FIGS. 6A to 6C, it was described how spot positions of the partial beams on the position detection surface 11 of the beam position detector 10, or their movement, can be determined by recording and evaluating a camera image.

[0139] In the following, it is described how this evaluation can be used to determine which part of the spot movements results from an offset Ax and / or Ay and which part results from a tilt a and / or ß of the input laser beam L.

[0140] In particular, the following assumptions are made:

[0141] All partial beams Li ... L generated by the optical element 20 n, or the corresponding spots on the position detection surface 11 of the beam position detector 10, have the same sensitivity for a tilt a and / or ß of the input laser beam L, ie for a given tilt angle of the input laser beam L to be examined, all spots of the partial beams Li ... L n (essentially) to the same extent.

[0142] The sensitivity of the partial beams Li ... L generated by the optical element 20 n , or that of the corresponding spots on the position detection surface 11 of the beam position detector 10, with respect to an offset Ax and / or Ay of the input laser beam L to be examined in front of the optical element 20 depends, according to the ray theorem, on their respective focal length (position of the respective focal plane relative to the position detection surface 11).

[0143] The influence of offset Ax and / or Ay and tilt a and / or ß of the input laser beam L on the positions of the respective spots is independent of each other.

[0144] A tilt a and / or ß or an offset Ax and / or Ay of the input laser beam L result in a spot movement in the same direction (e.g. tilt of the input laser beam L in the x-direction causes spot movement in the x-direction).

[0145] These assumptions allow us to set up and solve the following system of equations:

[0146] Eq. (1) AXdisplacement,spotl + AXtilt,spotl — AXtotal,spotl

[0147] GL (2) AXOffset,spot2 + AXTilt,spot2 — AXtotal,spot2

[0148] Equation (3) is obtained from the difference between equation (2) and equation (1) and the assumption Axtilt,spot1 - AXtilt,spot2.

[0149] Eq. (3) AXOffset,spotl _ AXOffset,spot2 — AXtotal,spotl - AXtotal,spot2

[0150] "spoti" or "spot2" (... spotn) in the equations denotes spots of partial beams Li ... L n with different focal lengths (or different diameters on the position detection surface 11). Axial offset and axial displacement PP ung denotes the measure for a movement of the respective spot on the position detection surface in the x-direction, which is caused by a beam offset Ax or by the tilt a.

[0151] Axtotai refers to the total movement of a spot or spot type on the camera in the x-direction.

[0152] The system of equations can be set up analogously for the y-direction.

[0153] It also applies to any relative relationship between partial beams Li ... L nwith different focal lengths (and therefore especially for the consideration of a system with a large number of partial beams with different focal lengths and all the resulting relative relationships).

[0154] A spot position or a deviation from a spot position can theoretically be calculated for a given offset and tilt of the input beam L. However, it should be noted that the calculation is not necessarily reversible and may require significant computational effort. This means that the spot position cannot be used to directly determine the offset and tilt (with a finite amount of time).

[0155] Instead, according to the invention, the expected spot positions on the position detection surface 11 are determined for randomly or specifically selected values ​​of tilts a and / or ß or offsets Ax and / or Ay of the input laser beam L.

[0156] These (theoretical) values ​​are stored in look-up tables LUTi, LUT2, LUT3, ... , LUTn, which contain this correlation in tabular form.

[0157] To solve the above equation (Eq. (3), the spot positions are calculated for various offsets relative to a reference point (e.g., relative to the previous measurement) in the x-direction. Subsequently, the difference in the relative change in the spot position, or the relative spot movement, or the difference in spot movements, is determined. This corresponds to the left-hand part of Eq. (3).

[0158] The right-hand part of equation (3) can be determined directly from the determination of the relative spot movements recorded by the beam position detector 10 and determined by image processing.

[0159] The offset Ax is considered to be determined when a difference value 8min, which is calculated from a difference between the left and right sides of the equation (3), is minimal.

[0160] Eq. (3a) 8min — (AX offset,spotl “ AX offset,spot2) — (AXtotal,spotl “ AXtotal,spot2)

[0161] The offset-dependent movement of the spots can then be subtracted from the total movement, leaving the tilt-dependent part:

[0162] Eq. (4) AXdisplacement,spotl + AXtilt,spotl — AXtotal,spotl | “ AXdisplacement,spotl

[0163] Eq. (5) AXtilt,spotl — AXtotal,spotl “ AXoffset,spotl

[0164] Equation (5) applies analogously to "other types of spots" (spot2 ... spotn), as well as to the y-direction.

[0165] Again, the right part of equation (5), equivalent to equation (3), can be determined by measuring and evaluating the data from the beam position detector, while the left part of the equation is approximated by varying the tilt angle a.

[0166] The problem with the usual solution of this approach is the combination of the high desired resolution in the pm / prad and sub-pm / sub-prad range on the one hand and a large measuring range on the other.

[0167] Assume that the offset in the x-direction Ax is to be determined with a resolution of 0.1 pm and a measuring range of ± 2 mm. This would require a support table with 40,000 entries for the equidistantly distributed steps.

[0168] However, since the determination of the four degrees of freedom of the beam position must be carried out live and within a few hundred microseconds to a few seconds for many applications, this presents a problem.

[0169] Therefore, the invention proposes a novel approach of an "iterative look-up table".

[0170] The mode of operation of the method according to the invention for determining the spatial position of a laser beam using an iterative approach is shown schematically in FIG. 7.

[0171] At the beginning of the method, in a first step S1, a comparatively coarse, first support value table LUTi is created with widely spaced, equidistant support values ​​of a first step size ASi, which covers the entire value range (measurement range, size / area of ​​the position detection area 11).

[0172] According to equation (3), the minimum deviation of the calculated relative spot movement from the measured relative spot movement is determined, in other words the difference value 8min (cf. equation (3a)) for the support values ​​from the first support value table LUTi is minimized.

[0173] The (first) best-fit support value Xi determined in this way serves as the center point for a second support value table LUT2, whose boundaries are the adjacent support points from LUTi xi and x+i.

[0174] The second support value table has equidistant support values ​​of a second step size AS2, which is smaller (finer) than the first step size ASi of the first support value table LUTi.

[0175] According to equation (3), the minimum deviation of the calculated relative spot movement from the measured relative spot movement is determined again in a second step S2, in other words the difference value 8min (cf.

[0176] Equation (3a)) is minimized for the support values ​​from the second support value table LUT2. The (second) best-fit support value XH determined in this way serves as the center point for another (third) support value table LUT3, whose boundaries are the adjacent support points from LUTi XH and X+H.

[0177] The third support value table has equidistant support values ​​of a second step size ASs, which is smaller (finer) than the second step size AS2 of the second support value table LUT2.

[0178] According to equation (3), the minimum deviation of the calculated relative spot movement from the measured relative spot movement is determined again in a third step S3, in other words the difference value 8min (cf.

[0179] Equation (3a)) for the support values ​​from the third support value table LUT3 is minimized.

[0180] The (third) best-fit support value x determined in this way can now be used as the actual solution or alternatively as the center point for another support value table LUTn, whose boundaries are the adjacent support points from LUT3 x-iü and x+iü.

[0181] The value range thus becomes significantly smaller, but the resolution increases due to the fixed division into a certain number of support values.

[0182] The same principle is continued until the resolution in the final LUT3 corresponds to the required resolution (or better). The number of iteration steps and the number of LUTs can, of course, vary. For example, only two steps and LUTs (instead of the three shown) are conceivable, or even more than three.

[0183] The resolution of 0.1 pm required above for a measuring range of ± 2 mm can be achieved using this method with a fixed division of the iterative support value tables, for example in 11 steps, after just 65 instead of 40,001 calculations.

[0184] In a possible extension of the invention, it is possible to also consider the wavelength of the incident light when calculating the spatial position of the laser beam, so that expected beam positions can be calculated depending on the wavelength of the irradiation. This can be done, for example, with irradiation at 532 nm for an optical element designed for 1064 nm. By means of such a calculation, a (theoretical) useful wavelength range of the beam position detection arrangement according to the invention is significantly increased.

[0185] The beam position detection arrangement 100 described in the above embodiments, as well as the corresponding method for determining the spatial position of a laser beam, can be used, for example, in a beam stabilization device. For this purpose, the beam position detection arrangement can determine a spatial position as well as a deviation from the position of an input laser beam. Using appropriately controllable mirrors or other beam-shaping or deflecting optical elements, the laser beam can be adjusted back to its desired position (if a deviation from a desired position is detected).

[0186] List of reference symbols

[0187] 100 beam position detection arrangement

[0188] L (Input) laser beam or laser beam to be examined

[0189] 10 Beam position detector

[0190] 11 Position detection area

[0191] 20 optical element

[0192] 21 Entrance plane / reflection plane of the optical element

[0193] Li first partial beam

[0194] L2 second partial beam

[0195] L n further partial beam

Claims

Claims 1. Beam position detection arrangement (100) for detecting the spatial position of a laser beam (L), comprising: - a beam position detector (10) with one, in particular a single, position detection surface (11); - an optical element (20) designed to separate an input laser beam (L) into at least two partial beams (Li, L2) by means of reflection and / or transmission, such that the propagation properties of the at least two partial beams (Li, L2) differ in their respective focusing and preferably in their respective propagation direction, wherein the beam position detection arrangement (100) is designed such that the at least two partial beams (Li, L2) impinge on the position detection surface (11) of the beam position detector (10) and the beam position detector (10) is designed to detect positions and / or position deviations of the at least two partial beams (Li, L2) on the position detection surface (11).

2. Beam position detection arrangement (100) according to claim 1, characterized in that the optical element (20) comprises a diffractive (holographic) optical element (20).

3. Beam position detection arrangement according to claim 1, characterized in that the optical element (20) comprises a bifocal or multifocal lens arrangement (20), wherein the lens arrangement preferably has at least one refractive lens and / or at least one metamaterial lens.

4. Beam position detection arrangement according to one of claims 1 to 3, characterized in that the optical element (20) is designed and arranged in such a way that the at least two partial beams (Li, L2, ...) have the same mode as the (input) laser beam (L), in particular in such a way that in a round input beam profile of the (input) laser beam (L) partial beams (Li, 1_2, ...) with round output beam profiles are generated.

5. Beam position detection arrangement according to one of the preceding claims, characterized in that the optical element (20) is designed such that a plurality of partial beams can be generated and a first partial beam (Li) runs in the propagation direction of the (input) laser beam (L) and the further partial beams (l_2, L3, ..., L n ) are deflected at an angle to the first partial beam, in particular by at least 0.5°, preferably by at least 1.0°, more preferably by at least 1.5°.

6. Beam position detection arrangement according to claim 5, characterized in that the optical element (20) and / or the beam position detection arrangement is designed and arranged such that the further partial beams (L2, L3, ..., Ln), in particular at least five partial beams (L2, L3, L4, Ls), are arranged on the position detection surface (11) on a circular path around the first partial beam (Li).

7. Beam position detection arrangement according to one of the preceding claims, characterized in that the optical element (20) has at least two different focal lengths (f1, f2) for the respective focusing of the at least two partial beams (L1, L2), wherein the absolute value of the difference between the respective focal length (f1, f2) and the distance (D) of the optical element (20) from the position detection surface (11) for a / the first partial beam (L1) is at least twice as large as for at least a second or the further partial beams (L2, L3, ..., L n ), in particular such that the following condition is met:

8. Beam position detection arrangement according to one of the preceding claims, characterized in that the beam position detector (10) comprises one, in particular a single, image sensor.

9. Beam position detection arrangement according to one of the preceding claims, characterized in that the optical element (20) comprises an active optical element which is designed in particular to spatially actively modulate the intensity and / or the phase of the laser beam (L).

10. A method for determining the spatial position of a laser beam (L), preferably using a beam position detection arrangement according to one of claims 1 to 9, comprising the following steps: a) separating the laser beam (L) into at least two partial beams (Li, L2) in reflection and / or transmission of a, in particular diffractive and / or active, optical element (20), such that the propagation properties of the first partial beam (Li) and the second partial beam (L2) differ in the respective position of their focal plane and preferably in the respective propagation direction; b) detecting the at least two partial beams (Li, L2) of the laser beam (L) on a, in particular common, position detection surface (11) of a beam position detector (10), preferably with an image sensor (10); c) determining the positions and / or position deviations of the partial beams (Li, L2) on the position detection surface (11) of the beam position detector (10).

11. Method according to claim 10, characterized in that the separation is carried out in such a way that, given a round input beam profile of the (input) laser beam (L), partial beams (Li, L2, ...) with round output beam profiles are generated.

12. Method according to claim 10 or 11, characterized in that Step c) comprises determining an offset from position deviations of the at least two partial beams (Li, L2) on the position detection surface (11) (Ax, Ay) as well as a spatial tilt (a, ß) of the laser beam (L) to be determined absolutely.

13. Method according to one of claims 10 to 12, characterized in that the offset (Ax, Ay) and / or the tilt (α, β) of the laser beam (L) is determined by iteratively minimizing a difference value (θmin) between - theoretical relative position deviations of the partial beams (Li, L2) on the position detection surface (11), and - measured relative position deviations of the partial beams (Li, L2) on the position detection surface (11), by recalculating the theoretical relative position deviations with changed parameters for offset (Ax, Ay) and / or tilt (a, ß).

14. Method according to one of claims 10 to 13, in particular according to claim 13, characterized in that the difference between calculated position deviations of the partial beams (Li, L2) is determined on the basis of a respective previously determined or measured reference point on the position detector surface (11).

15. Method according to claim 13 or 14, characterized in that the iterative minimization is carried out using support value tables (LUT1, LUT2, LUT3), which each contain support values of an equidistant step size (ASi, AS2, AS3), wherein in a first iteration step S1 a first support value table (LUT1) is used which has a first step size (ASi), and based on the corresponding support values of the first support value table (LUT1) the difference value (θmin) is minimized, wherein in a second iteration step S2 a best-fit support value (Xi) determined in iteration step S1, for which the difference value (θmin) is or becomes minimal in iteration step S1, is used as the center point in a second support value table (LUT2) is used, and support value limits are selected according to the support values (xi, x+i) of the first support value table (LUT1) adjacent to the best-fit support value (xi), and the support values of the second support value table (LUT2) between these support value limits are used to minimize the difference value (8min), wherein a second step size (AS2) of the second support value table (LUT2) is smaller than the first step size (ASi) of the first support value table (LUT1).

Images