US-12624990-B2 - System and method for focal position control

US12624990B2US 12624990 B2US12624990 B2US 12624990B2US-12624990-B2

Abstract

The present disclosure relates to a beam analysis device for determining a light beam state, e.g., determining the focal position of a light beam, where the device has a partial beam imaging device having at least one first selection device for forming a first partial beam from a first partial aperture region of the first measurement beam, and an imaging device for imaging the first partial beam for generating a first beam spot onto a detector unit having a spatially-resolving detector. The beam analysis device also can have an evaluation unit for processing the signals of the detector unit, for determining a lateral position (a 1 ) of the first beam spot, and for determining changes in the lateral position (a 1 , a 1 ′) of the first beam spot over time. An optical system for focal position control with a laser optics and with a beam analysis device. Additionally, the disclosure relates to a corresponding beam analysis method and methods for focal position control of a laser optics and for focal position tracking of a laser optics.

Inventors

Reinhard Kramer
Otto Märten
Stefan Wolf
Johannes Roßnagel
Roman Niedrig

Assignees

PRIMES GMBH MESSTECHNIK FÜR DIE PRODUKTION MIT LASERSTRAHLUNG

Dates

Publication Date: 20260512
Application Date: 20200616
Priority Date: 20190621

Claims (14)

1 . An optical system, comprising: a laser optics; a beam analysis device for determining a state of a laser beam, wherein the laser optics is configured to generate a laser beam focus and comprises: an interface of an optical element of the laser optics for generating a partially-reflected beam from the laser beam and for propagating the partially-reflected beam counter to a direction of the laser beam, and a partially-reflecting beam splitter for coupling out a measurement beam from the partially-reflected beam towards the beam analysis device, wherein the partially-reflected beam or the measurement beam has an intermediate focus, and wherein the beam analysis device is configured to receive the measurement beam coupled out by way of the beam splitter and comprises: a partial beam imaging device, which is configured to receive a first measurement beam, and which comprises at least a first selection device for forming a first partial beam from a first partial aperture region of the first measurement beam, and wherein the partial beam imaging device further comprises an imaging device with at least one imaging optical element, a detector unit, with at least one at least one-dimensionally spatially-resolving light-sensitive detector, which is arranged at a distance (zos) from the partial beam imaging device, and an evaluation unit that is connected to the detector unit and is configured to process signals from the detector unit, wherein the first selection device is arranged off-centre with respect to an optical axis for irradiating the first measurement beam, wherein the partial beam imaging device is configured to image the first partial beam for generating a first beam spot onto the detector unit, wherein the detector unit is configured to capture an intensity distribution of the first beam spot, wherein the evaluation unit is configured to determine a lateral position (a 1 , a 1′ ) of the first beam spot, and wherein the evaluation unit is configured to determine changes in the lateral position (a 1 , a 1 ′) of the first beam spot over time, wherein the beam analysis device constitutes a focal position sensor configured for monitoring an axial focal position of the laser beam focus by monitoring the focal position of the intermediate focus, wherein changes in an axial focal position of the laser beam focus and hence of the focal position of the intermediate focus are correlated with the changes in the lateral position of the first beam spot on the detector unit, wherein the optical system is configured such that by virtue of the partial reflection at the interface, a focal position of the intermediate focus is optically coupled to a focal position of the laser beam focus, wherein changes in the focal position of the laser beam focus simultaneously cause an alteration in the focal position of the intermediate focus, wherein the interface for generating the partially-reflected beam is the interface last transited by the laser beam before the laser beam exits the laser optics.
2 . The optical system according to claim 1 , wherein the first selection defines a first partial aperture region, wherein a centre of the first partial aperture region is located at a radial distance (r 1 ) from the optical axis in a radial direction that is perpendicular to the optical axis, wherein a value of the radial distance (r 1 ) is at least as large as a value of a width (d 1 ) of the first partial aperture region along the radial direction.
3 . The optical system according to claim 1 , wherein the evaluation unit is configured to determine the lateral position of the first beam spot by one or more of: calculating a centroid of the intensity distribution of the beam spot, determining an edge or a peripheral contour of the beam spot, determining a geometric centre of the beam spot, and adapting a setpoint intensity distribution to the captured intensity distribution of the beam spot.
4 . The optical system according to claim 1 , wherein the partial beam imaging device furthermore comprises at least a second selection device for forming a second partial beam from a second partial aperture region of the first measurement beam, and wherein the partial beam imaging device is configured to image the second partial beam onto the detector unit so as to generate a second beam spot.
5 . The optical system according to claim 4 , wherein the second selection device defines a second partial aperture region, wherein a centre of the second partial aperture region is located at a radial distance (r 2 ) from the optical axis in a radial direction that is perpendicular to the optical axis, wherein a value of the radial distance (r 2 ) is at least as large as a value of a width (d 2 ) of the second partial aperture region along the radial direction.
6 . The optical system according to claim 4 , wherein the first partial aperture region selected by the first selection device, and the second partial aperture region selected by the second selection device, are not contiguous, and wherein a distance (r 1 +r 2 ) from a centre of the first partial aperture region to a centre of the second partial aperture region is at least as large as a sum of the widths (d 1 +d 2 ) of the first and second partial aperture regions.
7 . The optical system according to claim 6 , wherein the detector unit is configured to capture an intensity distribution of the first beam spot and the second beam spot, wherein the evaluation unit is configured to identify at least the first beam spot and the second beam spots in the intensity distribution captured by the detector unit, and to determine the lateral positions of the first beam spot and the second beam spot.
8 . The optical system according to claim 7 , wherein the evaluation unit is configured to determine changes in the lateral positions of the first beam spot and the second beam spot or a beam spot separation distance between the lateral positions of the first beam spot and the second beam spot over time.
9 . The optical system according to claim 1 , wherein a partially-reflecting beam splitter is arranged in front of the partial beam imaging device for generating the measurement beam, wherein the partially-reflecting beam splitter is configured to couple out a defined beam component from a light beam or laser beam directed onto the beam splitter.
10 . The optical system according to claim 1 , wherein the laser optics comprises at least one further interface of an optical element of the laser optics for generating at least one further partially-reflected beam from the laser beam, wherein the partially-reflecting beam splitter is configured for coupling out the first measurement beam from the partially-reflected beam, and, in addition, an at least one second measurement beam from the at least one further partially-reflected beam towards the beam analysis device, wherein the beam analysis device is configured to receive the first measurement beam and the at least one second measurement beam coupled out by way of the beam splitter.
11 . The optical system according to claim 10 , wherein the partial beam imaging device is configured to receive the first measurement beam and the at least one second measurement beam, wherein the measurement beams are superimposed on the optical axis.
12 . The optical system according to claim 11 , comprising an element, which is reflecting or partially-reflecting and which is arranged at an exit of the beam splitter opposite to the beam analysis device, wherein the element is configured for reflecting a beam, which has been partially-reflected from the laser beam by the beam splitter towards the element, for transiting the beam splitter and being received as the at least one second measurement beam by the beam analysis device.
13 . The optical system according to claim 1 , wherein the laser optics is connected to a guiding machine, which is configured to adjust an axial position of a laser beam focus of the laser optics, and wherein a controller of the guiding machine is coupled to the evaluation unit for receiving data determined from the position of the first beam spot on the detector unit.
14 . The optical system according to claim 1 , wherein the laser optics includes an axially movable lens or lens group, and a translation device, wherein the position of the lens, or lens group is adjustable, and wherein the translation device is configured to control the position of the lens, or lens group, as a function of a value provided by the evaluation unit, which value is determined from the position of the first beam spot on the detector unit.

Description

CROSS REFERENCE TO RELATED APPLICATIONS The present application is a U.S. National Phase Application pursuant to 35 U.S.C. § 371 of International Application No. PCT/DE2020/000134 filed Jun. 16, 2020, which claims priority to German Patent Application No. 10 2019 004 337.5 filed Jun. 21, 2019. The entire disclosure contents of these applications are herewith incorporated by reference into the present application. TECHNICAL FIELD The present disclosure provides a focal position control system for a light beam, and more particularly a focal position control system for a laser beam in laser material processing applications. The present disclosure relates to a focal position sensor. The present disclosure also relates to a laser optics with a focal position sensor for monitoring the focal position of the laser optics in real time, that is to say, during the application of the laser beam, and to a laser optics with a focal position sensor for controlling and/or regulating the focal position of the laser optics. The present disclosure further relates to a method for determining a focal position of a light beam, as well as a method for monitoring the focal position of a laser optics in real time, and a method for controlling and/or regulating the focal position of a laser optics. BACKGROUND A central task in laser materials processing is the adjustment of the axial focal position of the laser beam relative to the material or workpiece to be processed. For optimal process management, the focus of the laser beam is not necessarily located directly on the surface of the workpiece. Rather, the optimal positioning of the laser beam focus relative to the workpiece depends on a plurality of factors. The focus can, for example, be located within the workpiece, that is to say, below the workpiece surface, in particular when processing workpieces of high material thickness. The machining result is often sensitively dependent on the exact focal position of the laser beam, which is why it is desirable or necessary that the positioning of the laser beam focus relative to the workpiece does not change during machining. Modern laser processing systems use lasers with a high brilliance and a high power, often of the order of several kilowatts. Due to the material properties in the optical elements of laser processing optics, the high laser power leads to heating of the optical elements. This creates a radial temperature gradient in the optical elements, which results in an alteration in the refractive power of the optical elements, due to the temperature dependence of material parameters such as the refractive index. This effect is called thermal focus shift. Although this thermal focus shift can be minimised by suitable material selection for the optical elements, for example by using high-purity, low-absorption grades of quartz glass, it is nevertheless always present in practice. The effect is amplified by the gaseous reaction products generated during laser material processing, which can deposit on the laser optics or on the protective glass of the laser optics and lead to an increased absorption. Thus the protective glasses, in particular, often contribute to an undesired, change in the focal position of the laser optics over time. To solve this problem, various devices have already been described in the prior art, which aim to determine the actual focal position of an optical system, and thus also enable the focal position to be tracked. DE 10 2011 054 941 B3, for example, shows a device for correcting the thermal displacement of the focal position of a laser beam managed by way of optical elements. In this case, a back reflection from one of the surfaces of one of the last optical elements in front of the material to be processed is used, and a sensor is arranged at the location of the focus of the back reflection. According to the concepts propounded by the disclosure, the sensor itself can be any focus sensor that determines the location of the focus with sufficient accuracy. As an example, the publication refers to a focus sensor as disclosed in DE 198 23 951 A1. The latter publication propounds a focus sensor, in which an input beam is split into a reference beam and a sample beam, whereby the reference beam is modulated with a high-frequency dither signal, the sample beam and reference beam are recombined to generate an interference pattern, and the interference pattern is reproduced with a detection device. A circuit generates a focus change correction signal from the signals of the detection device. The cited focus sensor is thus a highly complex optical device with moving components and relies on the split beams having sufficient coherence. A further difficulty arises from the fact that when using a back reflection from a surface of an optical element of the laser optics, in particular from a surface of a protective glass, as is of known art from DE 10 2011 054 941 B3, it is usually not possible to ensure that only