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EP-4488615-B1 - OPTICAL COHERENCE TOMOGRAPHY DEVICE FOR A LASER MACHINING SYSTEM AND LASER MACHINING SYSTEM HAVING SAME

EP4488615B1EP 4488615 B1EP4488615 B1EP 4488615B1EP-4488615-B1

Inventors

  • Shkarban, Igor
  • MOSER, Rüdiger
  • CHANG, LI WEN

Dates

Publication Date
20260513
Application Date
20240619

Claims (15)

  1. An optical coherence tomography device (50) for a laser machining system (1000) for measuring the distance to an object (20, 25) in a predetermined distance range (O), comprising: a measuring light source (53) for generating measuring light; an optical element (54) for splitting the measuring light into the measuring light beam (525) and a reference beam; a measuring arm (52) for directing the measuring light beam (525) onto said object (20, 25); a reference arm (51) for guiding the reference beam with a plurality of reference sections (511), each of which has a measuring range (MB); a controllable switching element (55) for switching between the reference sections (511) of said reference arm (51); and a detector (57) for detecting an interference signal between said measuring arm (52) and said reference arm (51); wherein the measuring range (MB) of each reference section (511) comprises a negative active measuring range (nMB) and a positive active measuring range (pMB), between which a dead zone (TT) is located; wherein the dead zone (TT) of one of the reference sections (511) is overlapped by a negative or positive active measuring range (nMB, pMB) of at least one other reference section (511); wherein the positive and negative active measuring ranges (nMB, pMB) of the reference sections (511) together cover the predetermined distance range (O); and wherein the controllable switching element (55) is configured and connected to the reference sections so as to switch back and forth or change between the positive active measuring range (pMB) of one of the reference sections (511) and the negative active measuring range (nMB) of another of the reference sections (511).
  2. The optical coherence tomography device according to claim 1, wherein said reference arm comprises N reference sections, and: a negative active measuring range of an nth reference section corresponds to smaller distance values than a negative active measuring range of an (n+1)th reference section, where N is a natural number greater than 1 and n is a natural number with 1 ≤ n ≤ N; and/or a positive active measuring range of an nth reference section corresponds to smaller distance values than a positive active measuring range of an (n+1)th reference section, where N is a natural number greater than 1 and n is a natural number with 1 ≤ n ≤ N.
  3. The optical coherence tomography device according to claim 1 or 2, wherein said reference arm comprises N reference sections, and the reference sections comprise at least one jth reference section the negative active measuring range of which is directly adjacent to a positive active measuring range of an ith reference section, where i, j are natural numbers with 1 ≤ i < j ≤ N.
  4. The optical coherence tomography device according to one of the preceding claims, wherein said reference arm comprises a group of M reference sections, where M is a natural number with 1 ≤ M ≤ N; and wherein the following applies to each group: the negative measuring ranges of (M-m) other reference sections and/or the positive measuring ranges of (m-1) other reference sections are arranged between a negative active measuring range and a positive active measuring range of an m-th reference section, where m is a natural number with 1 ≤ m ≤ M.
  5. The optical coherence tomography device according to claim 4, wherein said reference arm comprises K groups of reference sections which comprise the same number M of reference sections or different numbers Mk of reference sections; and wherein a measuring range of a (k+1)-th group corresponds to larger distance values than a measuring range of a k-th group, where K is a natural number with 1 < K and k is a natural number with 1 ≤ k ≤ K.
  6. The optical coherence tomography device according to one of the preceding claims, wherein the measuring range (MB) of each reference section (511) further comprises: a positive tolerance range (pTB) which borders on one or both sides of the positive active measurement range (pMB) of said reference section; and/or a negative tolerance range (nTB) which borders on one or both sides of the negative active measurement range (nMB) of said reference section.
  7. The optical coherence tomography device according to claim 6, wherein the negative or positive tolerance range (nTB, pTB) of a reference section overlaps with a negative or positive active measurement range (nMB, pMB) of at least one other reference section.
  8. The optical coherence tomography device according to one of the preceding claims, further comprising: a control (58) for controlling said switching element (55) of said optical coherence tomography device; wherein said control (58) is configured to select a reference section (511) with a positive active or negative active measurement range (nMB, pMB) according to a predetermined sub-range of the distance range (O) and to switch to the selected reference section (511) by means of the switching element (55).
  9. The optical coherence tomography device according to claim 8, wherein said control (58) is configured to switch to a shorter reference section (511) for a distance measurement in a sub-range of the predetermined distance range (O) with larger distances by means of said switching element (55).
  10. A laser machining system (1000), comprising: a laser machining head (100) for radiating a laser beam (10) onto a workpiece (20); an optical coherence tomography device (50) according to one of the preceding claims; and a control device (1100) for controlling said laser machining system.
  11. The laser machining system according to claim 10, wherein said laser machining head (100) comprises a scanner device (30) with at least one scanning element for deflecting the laser beam (10) to a plurality of positions on said workpiece (20), and wherein said control device (1100) is configured to select one of the reference sections (511) for distance measurement in accordance with a scan command for adjusting said scanning element of said scanner device (30).
  12. The laser machining system according to claim 11, further comprising: an evaluation device (59) configured to determine the distance based on the scan command and based on a superposition of a portion of the measuring light beam (525) from said measuring arm (52) reflected by said object and the reference beam from said reference arm (51).
  13. The laser machining system according to claim 10, 11 or 12, wherein said control device (1100) is configured to determine an alignment and/or inclination of said laser machining head (100) with respect to a working plane and/or said workpiece (20) based on distance values that were determined by said optical coherence tomography device (50) during a scanning of the measuring light beam (525) over said workpiece (20) by means of said scanner device (30).
  14. The laser machining system according to claim 10, 11, 12 or 13, wherein said control device is configured to assign reference height values to corresponding positions in a scan area of said scanner device (30) and to determine the distance to said workpiece at a predetermined scanner position based on this assignment and a measurement at said scanner position.
  15. The laser machining system according to claim 10, 11, 12, 13 or 14, wherein said control device is configured to adjust system parameters and/or machining parameters based on the determined distance.

Description

Technical field The present disclosure relates to an optical coherence tomography device for a laser processing system for measuring the distance to an object or a workpiece, and to a laser processing system with the same. Technical background In a laser material processing system, i.e., a laser processing system, the laser beam exiting a laser light source or the end of a laser fiber is focused or bundled onto the workpiece using beam guidance and focusing optics. Typically, a laser processing head with collimator optics and focusing optics is used, with the laser light supplied via an optical fiber. Optical coherence tomography (OCT) can be used in laser material processing to measure various process parameters, such as the distance to the workpiece, edge position during the initial phase, weld penetration depth during a welding process, or surface topography afterward. In this process, a measuring light beam is directed by a scanner device to a desired position on the workpiece. Optical coherence tomography (OCT) uses interference effects to determine distance differences relative to a reference distance. The predefined reference distance of an OCT device specifies the absolute distance to the object being measured. Measurements within a defined range are possible at this distance. The measurement range of the OCT device is determined by the characteristics of the light source used (e.g., a superluminescent diode) and the detector. Typically, the measurement range is a few millimeters (<20 mm). In other words, an OCT device measures the difference between the optical path length of a measuring arm and that of a reference arm to determine a distance. In a so-called scanner laser processing system, the laser beam (i.e., the processing laser beam) can be directed to different positions on the workpiece using a scanner device. The scanner device typically includes at least one scanning element, such as a scan mirror, which can be pivoted around one or two axes to deflect the laser beam. Therefore, in a scanner laser processing system, the optical path length (OWL) changes with the deflection of the mirror. To enable consistent measurements across the entire working or scan area, this OWL change must be taken into account during measurement, thus reducing the effectively usable OCT measurement range. In many cases, the increase in optical path length exceeds the OCT measurement range. In such cases, distance measurements, especially at the edge of the scan field, are no longer possible. For this reason, the use of multiple reference paths with different lengths or different measurement ranges, or the use of a single reference path with variable length, is necessary to enable measurements across the entire scan area. Known methods for increasing the measurement range of an OCT measurement system, for example as in the DE 10 2013 008 269 A1 The described methods are based on the synchronous adjustment of the optical path length in the reference arm. This adjustment is achieved by mechanically changing the optical path length of the reference arm, for example, by altering the position of an end mirror or prism on a linear axis. Disadvantages of these methods lie in the very complex control of the described mechanics. In the EP 3 830 515 B1 An OCT measurement system is described in which the reflected measurement light is simultaneously guided into a multitude of reference sections of the reference arm. Another OCT measurement system with a multitude of reference sections is described. US2020/001395 A1 known. These established methods utilize the one-sided measurement range of the OCT measurement system, i.e., a positive or negative measurement range (i.e., either the positive or negative solution of the Fourier transform that outputs the distance signal). Here, for example, a pre-adjustment determines which part of the measurement range is used to ensure measurement accuracy and avoid the singularity at the zero point. This results in the number of reference sections required to cover the distance range ΔM: N = ΔM/Δm, where Δm is the maximum possible measurement range for a given reference arm setting, i.e., for a single reference section. The maximum possible measurement range here refers to the one-sided measurement range within which a detectable measurement signal can be observed. A large ΔM specification, for example in a scanner-laser processing system, necessitates the construction of numerous reference paths. Therefore, for use in a scanner-laser processing system, a very high number of reference paths is required, or other costly components must be used, resulting in high manufacturing costs, a large footprint, and significant adjustment effort. Summary of the invention It is an object of the present invention to provide an optical coherence tomography device for a laser processing system for measuring the distance to an object or to a workpiece, which has an extended measuring range or enables better