Method and system for determining the local position of at least one optical element in a machine for laser processing of materials using low-coherence optical interferometry

Abstract

A method and system for determining the local position of at least one optical element associated with an optical path of a transmitted laser beam in a work head of a machine for laser processing of materials are described, comprising: generating a corresponding measuring low-coherence optical radiation beam, directing the measuring beam toward the optical element, and directing the measuring beam reflected or diffused by the optical element toward an optical interference sensor arrangement; generating a corresponding beam of reference low-coherence optical radiation, and directing the reference beam toward an interferometric optical sensor arrangement; superimposing the measuring beam and the reference beam on a common incident area of ​​the sensor arrangement; detecting a position of an interference fringe pattern between the measuring beam and the reference beam on the incident area; and determining an optical length difference between the measuring optical path and the reference optical path based on a position of the interference pattern along an illumination axis of the incident area or a frequency of the interference pattern in the frequency domain, the optical length difference between the measuring optical path and the reference optical path representing a difference between (a) a current local position of the optical element and (b) a predetermined nominal local position of the optical element.

Description

Technical Field

[0001] The present invention relates to laser processing of materials, preferably metallic materials, and in particular to improved control of laser processing of materials, such as laser cutting, drilling or welding of said materials, or additive manufacturing of predetermined structures of said materials.

[0002] More particularly, the present invention relates to a method and system for determining the local position of at least one optical element associated with an optical path of a transmitted laser beam in a work head of a machine for laser processing of materials.

[0003] According to another aspect, the invention relates to a machine for laser processing of material according to the preamble of claim 17, comprising a system for determining the position of at least one optical element associated with a light path for transmitting the laser beam, the system being designed for implementing the above-described method.

[0004] Another aspect of the present invention relates to determining the pressure in an auxiliary gas chamber associated with an auxiliary gas flow delivery nozzle, which auxiliary gas flow delivery nozzle is carried by a work head of a machine for laser cutting, drilling or welding materials, or for laser additive manufacturing of three-dimensional structures, and determining the temperature of an optical element inserted along the transmission optical path of the laser beam in the machine for laser processing of materials or for laser additive manufacturing of three-dimensional structures.

[0005] In the following description and claims, the terms "material" and, in the preferred embodiment, "metallic material" are used to identify any manufactured article, such as a plate or elongated profile, having a closed cross-section (e.g., a hollow circular, rectangular, or square shape) or an open cross-section (e.g., a flat cross-section or a cross-section in the shape of an L, C, U, etc.). In additive manufacturing, the term material identifies a raw material, usually in powder form, that undergoes localized sintering or melting by a laser beam. Background Art

[0006] In processes for the industrial processing of materials, and in particular sheet metal and metal profiles, lasers are used as thermal tools for a wide variety of applications that depend on parameters related to the interaction between the laser beam and the material being processed, in particular on the energy density of the laser beam per unit volume of incidence on the material, and on the interaction time interval.

[0007] For example, by directing low energy density (tens of W / mm 2 The hardening process occurs by directing high energy densities (tens of MW / mm2) on the same metallic material for an extended amount of time (in the range of seconds). 2The photoablation process occurs in a range of femtoseconds or picoseconds (of the order of magnitude of the surface). Controlling these parameters within the intermediate range of increasing energy density and reducing processing time makes it possible to implement welding, cutting, drilling, engraving, and marking processes.

[0008] In several processes, including machining by drilling and cutting, it is necessary to generate an auxiliary gas flow in the processing zone where the laser beam interacts with the material, which auxiliary gas flow has a mechanical function of driving melting, or a chemical function of assisting combustion, or even a technical function of shielding the environment around the processing zone.

[0009] In the field of laser processing of materials, laser cutting, drilling and welding are processes that can be performed by the same machine, which is capable of generating a focused high-power laser beam with a preset lateral power distribution in at least one processing plane of the material, typically with a power between 1 and 10,000 kW / mm 2 The power density of the laser beam varies between different ranges, and the direction and position of the beam along the material can be controlled. The differences between the various types of processing that can be performed on a material are essentially due to the power of the laser beam used and the interaction time between the laser beam and the material being processed.

[0010] Figure 1 and Figure 2 A laser processing machine according to the prior art is shown in FIG.

[0011] Figure 1A machine for industrial CO2 laser machining is schematically illustrated. The machine has an optical path for a laser beam in air and includes an emission source 10 (such as a CO2 laser generator device), adapted to emit a single-mode or multimode laser beam B, and a plurality of mirrors 12a, 12b, and 12c. The emission source 10 is adapted to emit a single-mode or multimode laser beam B, and the plurality of mirrors 12a, 12b, and 12c are adapted to guide the laser beam emitted by the emission source along the optical path for transmission toward a work head, generally designated 14, disposed adjacent to a material WP. The work head 14 includes an optical system for focusing the laser beam 16. The optical system typically includes a focusing lens adapted to focus the laser beam along a light propagation axis incident on the metal material. A nozzle 18 is disposed downstream of the focusing lens and is passed through by the laser beam, which is directed toward a region of the material's machining plane. The nozzle is adapted to direct a flow of assist gas, injected by a corresponding device (not shown), toward the machining region on the material. The assist gas is used to control the execution of the machining process (drilling or cutting) and the achievable machining quality. For example, the auxiliary gas may include oxygen, which promotes an exothermic reaction with the metal, such as the oxidation of iron, which produces various types of iron oxides by exothermic reactions, releasing energy in the material, which energy, together with the energy released by the laser beam, helps to maintain the dynamic equilibrium of the process, thereby allowing an increase in the cutting speed, or includes an inert gas such as nitrogen, which does not contribute to the melting of the material, but performs a propulsion function of the molten material itself, protects the material (metal) from undesirable oxidation at the edges of the machined profile, protects the working head from any splashing of the melt, and can also serve to cool the sides of the grooves produced on the material, thereby limiting the extent of the thermal change zone.

[0012] Figure 2 A machine for industrial processing using a laser beam emitted into an optical fiber is schematically shown. The machine comprises an emission source 10, such as a laser generator device capable of emitting a laser beam into a transmission optical fiber, for example an ytterbium-doped fiber laser or a direct diode laser capable of emitting a single-mode or multimode laser beam, and an optical fiber guide 12d adapted to guide the laser beam emitted by the emission source toward a work head 14 arranged near a material WP. At the work head, the laser beam, which has its own controlled divergence, emerging from the optical fiber is collimated by a collimating refractive system 20 and reflected by a back reflection system 22 before being focused along the light propagation axis incident on the material WP through an emission nozzle 18 through an optical focusing system 16 (typically consisting of a focusing lens).

[0013] Figure 3A working head 14 according to the prior art is illustrated. Reference numeral 30 denotes a tubular channel having a cylindrical or conical section, within which a laser beam, denoted by B, is transmitted. The laser beam B, generated by the emission source 10 and transmitted to the working head via an optical path in air or in an optical fiber with multiple reflections, impinges on a reflective deflector element 32, which deflects the light propagation axis in the direction of incidence on the material being processed. The optical focusing system 16 is intermediate between the reflective deflector element 32 and a protective glass 34 arranged downstream, which is adapted to protect the focusing system from any splashes of melt, and comprises a lens holder unit 36 ​​to which a mechanical adjustment mechanism 38 is coupled for calibrating the positioning of the lens transversely to the propagation direction of the beam (axis XY) and in the propagation direction of the beam (axis Z).

[0014] As a first approximation, an ideal laser beam (i.e., one perfectly collimated into a parallel beam) downstream of the optical focusing system is concentrated into a focused spot of finite size at its waist. Typically, in industrial processing applications, optimal processing conditions are achieved when the transverse plane is positioned corresponding to the waist of the beam, precisely defined to even tenths of a millimeter, with respect to the wall of the material into which the beam enters and the wall of the material from which the beam exits.

[0015] The power density distribution of a generally collimated laser beam is typically of a Gaussian shape with rotational symmetry in the case of a single-mode beam, i.e., where the power is concentrated around the longitudinal axis (Z-axis) of the beam and tapers off along the peripheral skirt, or in the case of a multimode beam, it can be described as the envelope of a Gaussian profile with rotational symmetry.

[0016] In the field of high-power laser applications, single-mode or multi-mode laser radiation beams are used to respond to technological control needs. Single-mode or multi-mode laser radiation beams can be described, to a first approximation, as Gaussian. In fact, a Gaussian beam is easy to describe with a few parameters, and because it has the property of propagating without changing the power distribution, it is easily controllable during propagation along the transmission optical path from the emission source to the head of the working machine. Therefore, it can be described by the radius and divergence values ​​in the far-field propagation conditions (in this case, the geometric optics approximation can be used). In the near-field propagation conditions of the focused beam, along the processing trajectory where the geometric optics approximation is no longer valid, the beam still maintains the Gaussian shape of the power distribution in each of its cross sections.

[0017] In contrast, laser beams containing higher-order transverse modes have a non-Gaussian power distribution. These conditions are typically achieved by using a refractive system (transmissive optics, i.e., lenses) or a reflective system (reflective optics, i.e., mirrors) that shapes the beam starting from a Gaussian distribution.

[0018] Controlling the propagation direction or distribution form of the lateral power of the laser beam (which is not Gaussian) and may have a symmetry different from the rotational shape over the material processing area, for example in connection with the controlled distribution of the assist gas, or depending on the standoff distance between the work head and the material, the processing trajectory to be followed, and the type of process to be performed, can bring advantages to the processing process. For example, controlling the power distribution of the laser beam (which may violate the rotational symmetry of the beam) can allow the power distribution to be positioned or extended as desired relative to the standoff distance between the work head and the material and relative to the processing path.

[0019] It is obvious that the control of the propagation direction and focus of the laser beam, the control of the lateral power distribution of the laser beam, and the control of the auxiliary gas flow (in the process requiring it) must be precise and reproducible in order to obtain the advantages mentioned. To this end, the position of the optical elements associated with the transmission path of the laser beam in the machine's work head (inserted along the optical path, facing the optical path, or inserted beside the optical path) must be controlled with extremely high precision and in real time. Otherwise, there is a risk of focusing the laser power into an undesired processing plane in the thickness of the material and using excessive or insufficient auxiliary gas pressure at the surface of the material. The position of the optical elements can actually vary according to the current operating conditions of the machine, but can also vary due to errors in the positioning of the elements by the operator during installation, or due to construction tolerances and undesirable assembly gaps. It is also necessary to check the operating parameters of the machine and especially the work head, including the current temperature of the area and optical elements that are affected by the high-power processing laser beam, the pressure of the auxiliary gas, possible mechanical deformations of the optical elements associated with the transmission light path of the laser beam (optical fiber, reflectors, lenses), so that these parameters do not deviate from the nominal values, thereby causing uncontrolled changes in the process or affecting the measurement of the process, such as measuring the separation distance between the work head and the material, which must be as accurate as possible in real time to be able to accurately control the movement of the work head and the position of its proximal end relative to the material, that is, the position of the laser beam output and the auxiliary gas outflow nozzle in the process where it is needed. Summary of the Invention

[0020] It is an object of the present invention to provide a method for efficiently (possibly in real time) monitoring the laser processing of a material, and in particular the status of optical components and operating parameters of a laser processing machine.

[0021] In this context, a particular object of the present invention is to accurately determine the local position of an optical element associated with the transport path of a laser beam in a work head of a machine for laser processing of materials.

[0022] A further object of the present invention is to provide a method for determining possible disturbances of optical measurements related to components and / or operating conditions of a machine for laser processing of materials, for example due to the influence of process parameters (such as pressure, temperature or mechanical deformations) on physical parameters of a transmission medium in which at least a portion of the optical measurement path extends.

[0023] It is a further object of the present invention to provide an efficient method for determining the pressure in an assist gas chamber associated with an assist gas flow delivery nozzle carried by a work head of a machine for laser cutting, drilling or welding materials, or for additive manufacturing of three-dimensional structures by laser.

[0024] Another object of the present invention is to provide an effective method for determining the temperature of an optical element or a transmission medium inserted along the transmission path of a processing laser beam in a machine for laser processing of materials or for manufacturing three-dimensional structures by laser additive manufacturing.

[0025] These objects are achieved according to the invention by a method for determining the local position of at least one optical element having the features mentioned in claim 1 .

[0026] Detailed embodiments form the subject matter of the dependent claims, whose content is intended to be an integral part of the present description.

[0027] The invention also relates to a system for determining the local position of at least one optical element having the features mentioned in claim 16 .

[0028] Another subject matter of the invention is a machine for laser processing of a material, comprising a system for determining the position of an optical element, the system being arranged to carry out the above-described method.

[0029] Another subject matter of the invention is a method for determining the pressure in an auxiliary gas chamber associated with a nozzle for supplying an auxiliary gas flow, having the features mentioned in claim 18, and a method for determining the temperature of an optical element or a transmission medium, having the features mentioned in claim 21.

[0030] In summary, the present invention is based on the application of the optical interferometry principle.

[0031] The term "optical interferometry" refers to a variety of techniques that exploit the phenomenon of interference between a measuring and reference beam, where these beams superimpose and generate interference fringes. The theory of optical interferometry using coherent light is well known and allows for relative comparisons of distances. However, it cannot provide absolute, unambiguous measurements of these distances, for example if the optical signal is temporarily interrupted.

[0032] The present invention was inspired by the consideration that absolute distance measurement can be performed in the optical domain using low-coherence interferometry, a simple technique for measuring the distance between a probe and a target with high accuracy. The technique is based on comparing the distance traveled by a measurement beam from a source to a detector assembly (the measurement beam being emitted by the probe and back-reflected by the target on this optical path) with the distance traveled by a reference beam from the source to the detector assembly (the reference beam being propagated on a reference optical path tuned to the measurement optical path) under conditions of a known nominal distance between the probe and the target.

[0033] In low-coherence interferometry, the measuring and reference beams are generated by low-coherence light sources (e.g., LEDs or superluminescent diodes), and interference fringes between these beams only appear when the corresponding optical paths or optical path lengths correspond, where the optical path is defined as the sum of the geometric distances within each section along the entire optical path and the product of their respective refractive indices. This means that the interference fringes appear when the length of the measuring path corresponds to the length of the reference path within the coherence length range. Assuming the length of the reference path is known, the length of the measuring path can be determined by detecting the envelope where the interference fringes appear. The resolution of the coherence length is typically in the micrometer range (from 5μm to 100μm).

[0034] This technique is particularly robust to optical noise, as light from other sources, such as laser processing, adds incoherently to the interference signal without altering the fringe pattern. The measurement is localized to the point where the measurement beam is directed, regardless of the morphology of the surrounding environment. This also allows for precise absolute distance measurements in a direction roughly coaxial with the axis of the instrument carried by the machine tool's workhead.

[0035] Different techniques are known for detecting interference fringe patterns in the time domain, frequency domain and spatial domain, respectively. Advantageously, low-coherence interferometry techniques for detecting interference fringe patterns in the spatial domain offer the greatest potential and are more effective in terms of operational flexibility for the purposes of the present invention, compared to detection in the time or frequency domain.

[0036] In low-coherence interferometry involving detection in the time domain, the interference fringe pattern is detected by a photodiode or photodiode array, or by a similar acquisition screen, by adapting the length of the reference optical path so that, with a tolerance on the order of the coherence length, the lengths of the reference and measurement optical paths correspond. In this case, limiting the available measurement range is associated with adapting the length of the reference optical path, which is performed, for example, by translating a back-reflecting element arranged along the aforementioned optical path. The spatial range of translation of the back-reflecting element of the reference optical path may be between a few micrometers and a few millimeters, a range that may hinder actuation speed or operational complexity.

[0037] While detection techniques in the time domain are relatively simple to implement and can easily achieve a correspondence between the absolute lengths of the measurement and reference paths, this is not suitable for applications requiring real-time measurement of industrial processes. For dynamic measurements, the length of the reference path must be continuously modulated to find the condition that corresponds to the current measurement path length, which results in the interference fringe pattern. This can be achieved using various types of control devices, including refractive index modulators or fast-acting mechanical actuators such as piezoelectric actuators; however, these types of devices are expensive and very precise, as they must operate at actuation speeds significantly faster than the sampling rate used to measure distance, typically exceeding kilohertz, a condition that is often difficult to achieve, especially over large displacements.

[0038] A different detection technique is based on the Fourier transform relationship between the spectral density function and the cross-correlation of the measuring beam and the reference beam, whereby a differential distance measurement in real space can be extracted from the spectral profile of the wavelengths of the two interfering beams. In this way, no mechanical actuator is required to align the length of the reference light path with the length of the measuring light path. A single spectrum acquisition of the superimposed measuring and reference beams can be performed using a diffraction grating and a focusing lens downstream thereof to project the spectral distribution of the interfering beams onto a linear sensor device (e.g., a camera). The spectra of the two interfering beams show a periodic modulation, and the periodicity (frequency) of this modulation in wavelength space varies with the length difference between the measuring and reference light paths. In order to extract a measurement result of the signal intensity peak with respect to the optical path difference in real space, an algorithm for calculating the Fourier transform, such as an FFT algorithm, is applied.

[0039] Unlike low-coherence interferometry techniques in the time domain and frequency domain, in the time domain, low-coherence interferometry techniques scan the reference optical path length distributed in time, while in the frequency domain, information used to compare the lengths of the measurement optical path and the reference optical path is encoded in the wavelength space. Low-coherence interferometry techniques, including detection in the spatial domain, combine the above two techniques to directly visualize the measurement results in real space, which can be quickly collected with the help of economical devices such as image sensors (e.g., linear sensors).

[0040] In a typical embodiment of a low-coherence interferometry system with detection in the frequency domain, a measuring beam and a reference beam are collinearly superimposed along the same direction of incidence toward a wavelength-dispersive optical device (such as a diffraction grating, a refractive prism, or a similar optical element for detecting spectra). The wavelength-dispersive optical device is adapted to separate the frequency components of the total beam obtained by superimposing the measuring and reference beams onto a common incidence region of an optical interferometer sensor device. In this configuration, the spectra (interference fringe patterns) of the two interfering beams exhibit periodic modulation, and the periodicity of the fringe pattern in wavelength space varies with the spatial variation of the measurement optical path relative to the optical path, and its extension in the frequency domain is determined by the coherence length of the low-coherence optical radiation. Therefore, a measure of the difference between the length of the measurement optical path and the length of the reference optical path can be extracted by processing the signal emitted by the sensor device to detect the frequency of the interference pattern between the measuring and reference beams.

[0041] In a typical design of a low-coherence interferometry system with detection in the spatial domain, a measuring beam and a reference beam are projected from different directions and superimposed onto a sensor device surface, which is directly adapted to detect the interference fringe pattern. In this configuration, due to the mutual tilt angle of the two beams, the spatial variation of the measuring beam path relative to the reference beam path is directly visible on the sensor device. Therefore, by detecting the position of the interference fringe pattern on the sensor device, whose extension in the linear dimension of the sensor device is on the order of the coherence length of the optical radiation of the beams, a measurement result representing the difference between the measuring and reference beam path lengths can be easily extracted.

[0042] In low-coherence interferometry techniques involving detection in the spatial domain, the optical path length of each light beam obliquely incident on a common incident area of ​​the sensor device varies linearly with position along the sensor device's illumination axis, and thus the difference between the measurement and reference paths also varies linearly. The interference fringe pattern appears within a specific linear range of the image acquired by the sensor device, corresponding to the condition where the measurement and reference paths are equal in length within the coherence length of the optical radiation. In other regions of the sensor device, the light beams superimpose incoherently. By detecting the position of the interference fringe pattern's envelope along the linear extension of the sensor device, the corresponding length of the measurement path can be extracted.

[0043] This approach is limited only by the requirement that the envelope of the interference fringe pattern be obtained within the illumination area of ​​the sensor device, i.e., within the sensitive area of ​​the photodetector arrangement forming the sensor device. The measurement range is determined by the inclination of the light beams on the incident area, or more preferably, by the angle of incidence between the light beams, and, for the same beam inclination, by the minimum between the number of photodetectors or photodetection areas (also known as pixels of the sensor device) illuminated by the superimposed light beams and the total number of photodetectors—that is, the minimum number of areas (pixels) that must be illuminated relative to the total number of available areas (pixels) on the sensor device to demodulate the interference fringe pattern. Under typical conditions, including sensor arrangements with thousands of photodetectors, a measurement range of a few tenths of a millimeter, corresponding to a subsampled state of the interference fringes, can be achieved before aliasing effects occur. However, the inventors have demonstrated that the presence of aliasing effects on the interference fringe pattern does not limit the measurement but can actually be used to expand the range of measurable distances. In effect, this subsampling system effectively demodulates the fringe pattern at a lower spatial frequency, achieving this demodulation directly in an analog fashion at the photodetector level of the interferometric sensor device without the need for additional components.

[0044] Advantageously, an interferometric measurement technique involving detection in the spatial domain is employed, and for each individual acquisition or sampling of the spatial distribution of the optical radiation of the superimposed measurement and reference beams impinging on the sensor device, a precise distance measurement can be performed using a static system of measurement and reference beam paths. To implement such a system, only standard optical components are required, and the signals emitted by the sensor device are processed based on simple calculation algorithms, thus avoiding heavy computational effort.

[0045] Applying the above considerations to a machine for laser processing of materials, in particular for laser cutting, drilling or welding, or for additive manufacturing of three-dimensional structures by laser, this is achieved by means of an arrangement of an interferometric measurement system, comprising a measuring light path at least partially integrated in a work head and an optical reference path associated with the measuring light path, the measuring light path also being integrated in the work head or external to the work head, wherein the measuring light path is reflected or diffused by at least one back-reflecting surface of an optical element associated with the light path for transmitting the laser beam (for example inserted in the work head along the light path).

[0046] This allows determining the local position of any optical element associated with the transmission path of the laser beam (for example inserted along said path in the work head of a machine for laser processing of materials) relative to predetermined nominal local positions, and the differences between these positions allow obtaining useful, temporary or permanent information related to deformations or displacements of the monitored optical element, which may be the result of incorrect installation or deviations or disturbances occurring due to specific operating conditions to which the element is subjected, for example depending on the ambient temperature in the vicinity of the element or the pressure of the assist gas impinging on the element. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Further features and advantages of the invention will become more apparent from the following detailed description of embodiments thereof, given by way of non-limiting example with reference to the accompanying drawings, in which:

[0048] Figure 1 and Figure 2 is an example of a laser processing machine according to the prior art;

[0049] Figure 3 shows a schematic example of a working head of a laser machine according to the prior art;

[0050] Figure 4a A schematic diagram showing the configuration of a low-coherence interferometry system for detection in the frequency domain;

[0051] Figure 4b shows an interference pattern F positioned on the illumination axis of a sensor arrangement of an interference fringe pattern after wavelength dispersion;

[0052] Figure 4c The relationship between the frequency of the interference fringes and the difference in optical length between the measurement and reference paths is shown;

[0053] Figure 5a is a schematic diagram of the configuration of a linear low-coherence interferometry system for detection in the spatial domain;

[0054] Figure 5b is a schematic diagram of changes in the lengths of the measurement light path and the reference light path relative to the relative incidence point of the interference fringe pattern on the illumination axis of the sensor arrangement;

[0055] Figure 5c Schematic diagram of the change in the difference between the lengths of the measurement optical path and the reference optical path with respect to the relative incidence point of the interference fringe pattern on the illumination axis of the sensor arrangement (top figure), and recognition of the interference fringe pattern on the illumination axis of the sensor arrangement in a state where the optical lengths of the measurement path and the reference path are equal (bottom figure);

[0056] Figure 6An exemplary diagram of a system for determining the local position of an optical element and the separation distance between a working head of a machine for laser processing material and the surface of the material, object of the present invention, is shown;

[0057] Figure 7 schematically illustrates the paths of a machining laser beam and a low-coherence measuring beam in a work head according to an exemplary embodiment of the work head;

[0058] Figure 8a-8e is a schematic illustration of different possible conditions for back reflection or partial diffusion of the measuring optical radiation at the optical element; and

[0059] Figure 9 is a graph showing the dependence of the interferometric reading results as a function of the local position of the surface of the optical element protecting or delimiting the assist gas chamber along the axis of the measuring beam on the trend (increase, decrease) of the assist gas pressure. DETAILED DESCRIPTION

[0060] The prior art has been described previously with reference to Figures 1 to 3 , and it is intended here to mention their contents as they are common to the implementation of a controlled processing machine for carrying out the method according to the teaching of the invention.

[0061] Figure 4a A schematic diagram of a Michelson configuration of a low-coherence interferometry system for detection in the frequency domain is shown. A collimated beam of measurement light radiation (denoted by M) from a lens T and a collimated beam of the same reference light radiation (denoted by R) from a reference reflective element RM (both originating from a source L) impinge on a diffraction grating G in an overlapping manner. Leaving the diffraction grating G, the spectral distribution of the interfering beams passes through a focusing lens and reaches a common incidence region C of a sensor arrangement S. In the common incidence region, the spectral distribution of the interfering beams forms an interference fringe pattern F, as shown in FIG. Figure 4b As shown in .

[0062] The sensor arrangement S comprises, for example, an arrangement of photodetectors along at least one illumination axis of the incident region (the x-axis in the figure). The photodetector arrangement is a linear or two-dimensional arrangement of photodetectors, preferably a linear arrangement. The illumination axis of the incident region is determined by the intersection of a plane defined by the angles of incidence of the measurement beam M and the reference beam R, and a sensor surface of the sensor arrangement.

[0063] Figure 4c The results of the interference fringe pattern acquisition by the photodetector are shown, where Figure 4bThe spectrum of the interfering beam is extracted from the intensity distribution of the measured light, and the frequency of the fringes is determined by the FFT algorithm. It is known that the frequency of the fringes depends on the phase difference of the interfering beams, that is, on the corresponding difference Δp in the optical lengths of the measurement path and the reference path.

[0064] Figure 5a Schematic diagram of the configuration of a low-coherence interferometry system with linear spatial detection. A collimated measurement beam of optical radiation, designated M, and a collimated reference beam of the same optical radiation, designated R, are projected at a predetermined angle of incidence α so as to overlap onto a common area of ​​incidence C of a sensor arrangement S. At the common area of ​​incidence C, the collimated measurement beam and the collimated reference beam form a pattern of interference fringes F, the extension of which over the common area of ​​incidence being of the order of the coherence length of the optical radiation. The widths of the collimated measurement beam and the collimated reference beam of optical radiation are preferably designed to illuminate substantially the entire sensor arrangement. To increase the intensity and contrast of the detected signal, the beams can be focused onto the sensor in a direction perpendicular to the illumination axis, for example, using a cylindrical focusing lens.

[0065] The sensor arrangement S comprises, for example, an arrangement of photodetectors along at least one illumination axis of the incident region (the x-axis in the figure). The photodetector arrangement is a linear or two-dimensional arrangement of photodetectors, preferably a linear arrangement. The illumination axis of the incident region is determined by the intersection of a plane defined by the angles of incidence of the measurement beam M and the reference beam R, and a sensor surface of the sensor arrangement.

[0066] exist Figure 5b In the diagram, the graph schematically shows the variation in the length p of the measurement and reference optical paths with reference to the initial incidence wavefronts of the respective measuring and reference beams on a common incidence area of ​​the sensor arrangement S, in a typical configuration in which the two incident beams are symmetrical on the sensor arrangement. The x-axis represents the position or x-coordinate along the illumination axis of the photodetector arrangement. Reference numeral p1 represents the additional length of a first optical path (e.g. the measurement optical path of the measuring optical radiation beam M) relative to the initial incidence point of the wavefront of the measuring beam M at a first end of the common incidence area C, x1, which is the origin of the measurement axis. Reference numeral p2 represents the additional length of a second optical path (e.g. the reference optical path of the reference optical radiation beam R) relative to the initial incidence point of the wavefront of the reference beam R at a second end of the common incidence area x2, which is opposite the first incidence area. Reference numeral Δp represents the difference between the additional lengths p1 to p2 of the two paths, the difference being zero at an intermediate coordinate of the sensor arrangement and increasing from the value Δp at the end x1 of the common incidence area. x1 Change to the value Δp at the end x2 of the common incident area x2 .

[0067] exist Figure 5cThe top view graph shows the corresponding Figure 5b The curve Δp in the graph in FIG and the bottom graph shows the identification of the pattern of interference fringes F on the illumination axis (x) of the sensor arrangement S that occurs when the optical lengths of the measurement path and the reference path are equal. The envelope of the pattern of interference fringes F is indicated by hatching, and the corresponding difference Δp between the additional lengths of the paths of the measurement and reference beams p With the help of the upper curve and the coordinate x of the envelope peak p associated.

[0068] P M and P R Represents the measurement path and reference path, and its total length can be expressed as P M =P1+p1 and P R = P2 + p2, where P1 is the optical length of the measurement optical path from the low-coherence optical radiation source to the first wavefront incident on the sensor device, and P2 is the optical length of the reference optical path from the same low-coherence optical radiation source to the first wavefront incident on the sensor device, and is preferably constant. P1 can be considered as the sum of P nom +d, among which P nom is the nominal length of the optical path, the optical path comprising a first section between the low-coherence optical radiation source and a predetermined back-reflection surface of the optical element for determining the position at a predetermined nominal position of the optical element, and a second section between the aforementioned back-reflection surface and the sensor arrangement S, these sections having respective predetermined and constant geometric lengths. d represents the positional offset of the optical element relative to its nominal position. P2 is the optical length of the reference optical path, which is equal to the optical length P of the measurement optical path under nominal operating conditions. nom , where the optical elements are in their predetermined nominal positions.

[0069] The optical length difference between the measurement optical path and the reference optical path is mathematically expressed as:

[0070] P M -P R

[0071] The interference fringes appear under the condition of zero, that is:

[0072] P M -P R =0

[0073] It can be broken down into:

[0074] P1+p1-(P2+p2)=0

[0075] It can be rewritten as:

[0076] P nom+d+p1-P2-p2=0

[0077] From this we can deduce the following:

[0078] P nom +d-P2+Δp=0

[0079] P nom +dP nom +Δp=0

[0080] Δp=-d

[0081] That is, the current position of the optical element is equal to the difference between the additional lengths of the measurement and reference optical paths.

[0082] Therefore, the current local position of the optical element relative to its nominal local position, determined by the optical length difference between the measurement optical path and the reference optical path, can be attributed to the difference between the additional lengths of the measurement optical path and the reference optical path, and therefore to the movement of the interference fringe pattern along the illumination axis x of the sensor arrangement S relative to the nominal position (e.g., the median plane of the sensor arrangement S).

[0083] In the application that is the subject of the present invention, the length of the reference optical path is established so as to correspond to the length of the measuring optical path at a predetermined nominal position of the optical element, and the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measuring beam is generated by the length difference between the measuring optical path and the reference optical path, if an interferometric measurement technique for detecting the interference fringe pattern in the spatial domain is used, the length difference can be used to identify the position of the interference fringe pattern according to the illumination axis along the incident area of ​​the sensor arrangement S, or, if an interferometric measurement technique for detecting the interference fringe pattern in the frequency domain is used, the length difference can be used as a frequency according to the interference fringe pattern.

[0084] It should be noted that the local position is the position of a region of the optical element along the axis z relative to a predetermined reference system (e.g., an axial reference system along the propagation axis of the machining laser beam or a Cartesian reference system of the work head) that is local to the axis of the measuring beam affecting the element. The back-reflecting surface of the optical element can be the first surface of the element encountered by the measuring beam, or a surface opposite thereto, depending on the amount of reflected optical radiation, with the measurement being preferably performed based on a higher amount of back-reflected radiation.

[0085] The "local position" therefore represents the absolute position of the rigid component or carries information about the position of the area of ​​the component where deformation occurs. The axis of the measurement beam can thus be dynamically controlled near the axis of the machining beam in order to explore the surface of the optical component in the xy plane.

[0086] Advantageously, in the preferred case of an interferometry technique in which the interference fringe pattern is detected in the spatial domain, the median position of the interference fringe pattern along the illumination axis of the sensor arrangement corresponds to the nominal position of the optical element. Alternatively, the end positions of the interference fringe pattern along the illumination axis may correspond to the nominal position of the optical element, if this position is variable in only one direction, such that the interference pattern is shifted only toward the opposite end of the illumination axis.

[0087] refer to Figure 5c In the bottom-up graph, the interference fringe pattern is located along the illumination axis x p is the inherent position of the envelope of the light radiation intensity of the interference fringe pattern, and this inherent position of the envelope of the light radiation intensity of the interference fringe pattern is, for example, the position of the peak or maximum intensity of the envelope of the light radiation, or the average position of the photodetector weighted by the light intensity of the fringe envelope.

[0088] The detection of the fringe envelope can be performed by light intensity distribution demodulation techniques, for example by applying a bandpass spatial filter, or by applying a high-pass filter and a low-pass filter in sequence, so as to reveal only the unique signal component corresponding to the spatial frequency of the interference fringes. For example, in a first step of processing the light intensity data, the light intensity detected by the sensor matrix is ​​integrated in a direction perpendicular to the expansion direction of the interference fringes, for example, for the columns of the sensor matrix oriented to receive vertically aligned interference fringe patterns (this operation is not required if the sensor arrangement is a linear arrangement of photodetectors (the light beam is focused on the photodetectors by means of cylindrical lenses)). Subsequently, the signals generated by these photodetectors are normalized relative to the background signal (for example, extracted from an image without interference fringes). Therefore, a high-pass spatial filter is applied to, for example, 1 / 5 of the spatial frequency of the photodetectors in order to remove the baseline and maintain the interference fringe pattern. Since, in this way, a signal oscillating around zero is obtained, the absolute value of the signal is extracted and therefore a low-pass spatial filter is applied to, for example, 1 / 25 of the spatial frequency of the photodetectors in order to extract the envelope of the interference fringe pattern. The position of the interference fringe pattern is ultimately obtained by detecting the position of the envelope of the interference fringe pattern by finding the maximum value of the interference fringe pattern or by comparing the envelope with a predetermined model function (eg, Gaussian function) and extracting the peak of the model function.

[0089] Figure 6 An exemplary illustration of the system object of the present invention is shown for determining the local position of at least one optical element integrated in the system in relation to the optical path of the laser beam transmission in the work head 14 of a machine for laser processing of material WP and - according to a currently preferred embodiment - integrated in a system for determining the distance between the work head and the material.

[0090] In the figure, 100 denotes a source of low-coherence optical radiation, suitably with linear polarization, such as an LED or a superluminescent diode, which operates, for example, in the visible or near-infrared wavelength range. The optical radiation emitted by the source 100 downstream of a suitable optical frequency isolator 120 is injected into an optical waveguide (for example an optical fiber 140) and conveyed to a beam splitter 160 adapted to generate a beam in the measuring optical path P. M The optical radiation sent on the measuring beam M and the reference optical path P R The reference light radiation beam R is sent on.

[0091] Measurement optical path P M and reference optical path P R A light guide is a guiding path and comprises light guides (eg optical fibers) adapted to maintain the same polarization of the light beam along the entire path.

[0092] Measurement optical path P M The measuring beam M is directed to a work head 14 of a machine for laser processing materials as described above, and may emerge from the work head toward the material WP to be processed. The measuring beam M may impinge on the material WP to be processed. The region where the measuring beam M is output corresponds to a cross section of the measuring head (e.g., an opening in a nozzle for supplying an assist gas flow or an output end of a laser beam) for measuring the distance thereto.

[0093] Alternatively, the optical reference path P R The light is directed to the retroreflective element 180, preferably passing through an interposed optical density filter 200, an optical dispersion compensation element 220, a λ / 4 plate 240, and a focusing lens 260. The reflective optical element 180 is arranged along a reference optical path such that the optical length of the path from the beam splitter 160 to the reflective optical element 180 corresponds to the optical length of the measurement optical path from the beam splitter 160 to the (reflecting) surface of the optical element to be monitored (i.e., its position is to be determined in its predetermined nominal position). The reflective optical element can be axially moved and arranged to determine different optical reference path lengths or one of a plurality of reference optical paths, optionally including a corresponding reflective optical element 180 and having different optical lengths, in order to switch between methods for determining the local position of different optical elements and, possibly, to a method for determining the separation distance between the processed material WP and the work head (i.e., the end of the work head close to the material, such as the opening of the assist gas nozzle or the beam output end).

[0094] Specifically, when the positions of a plurality of optical elements inserted along an optical path for transmitting a laser beam are determined, a plurality of optical measurement paths associated with a plurality of corresponding reference optical paths are provided by extracting a plurality of corresponding measurement beams associated with each of the plurality of optical elements downstream of reflection or diffusion from at least one back-reflection surface of each of the plurality of optical elements. The plurality of reference optical paths are arranged to guide the respective separated or superimposed reference beams, i.e., are determined by means of a continuous variation in the length of the basic reference optical path through the optical elements for deflection and separation of the reference beams.

[0095] Measurement optical path P M and reference optical path P R The optical radiation passes through the measuring optical path and the reference optical path in two directions and returns towards the beam splitter 160 after reflection, and returns to at least part of the back reflection surface of the optical element and the reflective optical element 180 respectively. R In the embodiment, the double passage of the reference beam R through the λ / 4 plate 240 causes a 90° rotation of the linear polarization of the beam, thereby presenting a linear polarization orthogonal to the linear polarization of the measuring beam M. The beam splitter 160 then performs a recombination of the measuring beam and the reference beam, and the beam is transmitted along the detection optical path P. D The superimposed measuring beam and reference beam are directed toward the sensor arrangement S (a portion of the measuring optical path and a portion of the reference optical path are common).

[0096] Both the measuring and reference beams are directed through a cylindrical focusing lens 280, which can focus the collimated beams in only one direction, specifically orthogonal to the illumination axis of the sensor arrangement, in order to concentrate the signal along this axis and thus optimize the illumination of the photodetector, and then to a polarization beam splitter 300, which separates the measuring and reference beams M and R based on their polarization. In the latter case, a first portion of the beam is directed toward a first reflective element M1, and a second portion toward a second reflective element M2, by inserting a λ / 2 plate 320 that restores the original polarization. Due to this configuration, the first and second reflective elements M1, M2, respectively, direct the measuring and reference beams toward the sensor arrangement S at an angle of incidence α, and more precisely, toward a common area of ​​incidence of the sensor arrangement. In an embodiment of the system, the angle of incidence α can advantageously be controlled within a predetermined range of values, wherein the reflective elements M1 and M2 are respectively movable translationally relative to the beam propagation axis and rotationally about an axis perpendicular to the plane of incidence (shown as a dashed line in the figure).

[0097] Of course, in embodiments based on interferometry techniques that detect interference fringe patterns in the frequency domain, the optical detection path P DNo longer provides separation of measurement and reference beams, but includes Figure 4a Spectrometer with the architecture described in .

[0098] As described above, the sensor arrangement S includes a plurality of photodetector devices, each of which is adapted to emit a specific signal representing the intensity of light incident thereon, and these signals are transmitted as a whole to the processing device 350, which is configured to identify the pattern of interference fringes F formed on the common incident area C of the sensor arrangement by obtaining the total incident light power of the superimposed measuring beam and the reference beam.

[0099] Preferably, the measurement and reference optical paths include corresponding optical elements, and in particular, the reference optical path includes a retroreflective element whose reflective and optical diffusion properties correspond as much as possible to those of the monitored optical element inserted in the measurement optical path. Optionally, an optical attenuation device may be provided, adapted to balance the intensity of the reference light radiation reflected by the retroreflective element with the intensity of the measurement light radiation reflected by the monitored optical element.

[0100] With the help of Figure 6 A system or an equivalent system implements a method for determining the local position of at least one optical element.

[0101] The method includes generating a low-coherence measurement light radiation beam M, which is directed toward an optical element that is associated with a transmission light path of a laser beam in a work head of a machine for laser processing a material (for example, inserted along the transmission light path), and the low-coherence measurement light radiation beam is reflected or scattered by at least one back-reflecting surface of the optical element and directed through the work head 14 toward a sensor arrangement S.

[0102] In the case of a reflective optical element, it can be assumed that reflection or scattering of the measuring beam occurs at a first surface of the element, whereas in the case of a reflective optical element, it can be assumed that reflection or scattering of the measuring beam occurs at both surfaces of the element.

[0103] The measuring optical radiation beam M specifically travels along an optical measurement path from the source 100 to the sensor arrangement S, the sensor arrangement S comprising two sections having corresponding predetermined and invariant geometric lengths when the optical element is in predetermined nominal positions corresponding respectively to predetermined operating conditions, the first section being comprised between the source 100 and the back reflection surface of the optical element, and the second section being comprised between the back reflection surface of the optical element and the sensor arrangement S.

[0104] Said beam of reference low-coherence optical radiation R is generated from the same source 100 and is directed towards the sensor arrangement S. The reference beam R follows a reference optical path P R Travel, reference optical path P R The optical length is equal to the optical path P measured under nominal operating conditions when the position of the optical element is the predetermined nominal position. M optical length.

[0105] The measuring beam M and the reference beam R are superimposed on a common area of ​​incidence C of the sensor arrangement S along a preset illumination axis.

[0106] Applying the interferometry technique of detecting the interference fringe pattern in the spatial domain, the position of the pattern of interference fringes F between the measuring beam M and the reference beam R along the illumination axis on the common incidence area C is detected by the processing device 350, and its position allows, as described above, to determine the measuring optical path P M and reference optical path P R , which represents the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measuring beam.

[0107] By applying the interferometry technique of detecting the interference fringe pattern in the frequency domain, the frequency of the interference fringe pattern F between the measuring beam M and the reference beam R, obtained by diffusing said beams in wavelength, is detected by the processing device 350, and its frequency allows, as described above, to determine the measurement optical path P M and reference optical path P R , which indicates the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measuring beam.

[0108] The method can be implemented in real time during material processing, but can also be implemented before or after processing, for example, to determine the state of an optical element of a working head. During the monitoring step of the optical element, which is performed separately from the processing step, for example, using low-coherence optical radiation at a wavelength that is maximally reflected by the optical element, the processed material may also be absent. In the presence of material when monitoring the optical element occurs during the processing step, a portion of the measurement signal is obtained that is back-reflected onto the surface of interest of the optical element, and a corresponding reference beam path is selected for that surface.

[0109] See also Figure 7 , schematically shows an exemplary embodiment of the paths of the machining laser beam B and the measuring beam M within the work head.

[0110] Figure 7A reflective element for deflecting the laser beam, such as a dichroic mirror designated DM, is shown. This element deflects the propagation axis of the processing laser beam B from the head entry direction to the direction of incidence on the material WP being processed. This is the configuration employed in one embodiment of a work head including a transverse laser beam input. In this embodiment, the measurement radiation beam M is directed toward the downstream optical element to be monitored (and toward the material measurement area) by means of a reflective optical scanning system SM or a folding mirror. The measurement radiation beam M passes through the dichroic mirror DM without significant deflection. The tilt of the measurement radiation beam M is controlled, for example, by piezoelectrics, as needed to monitor different areas of the element and control the position at which the measurement point intercepts the element's surface. In the non-limiting example shown, the primary element is the focusing lens FL. As can be seen in the figure, the propagation direction of the measurement beam can be controlled by tilting the reflective optical scanning system SM, resulting in non-coaxial superposition of the measurement beam B and a difference between the two. Those skilled in the art will also appreciate that a "dual" or "opposite" configuration is possible, in which a dichroic mirror is provided that is transparent to the processing laser beam but reflects the measurement beam from the transverse input.

[0111] Figure 8a 、 Figure 8b and Figure 8c Schematically shows a first condition for measuring back reflection or partial diffusion of light irradiated to the first surface S1 or the second surface S2 of the optical element E, with the possibility of inserting the material WP to be processed. T represents the portion of the measurement light radiation beam that passes through the optical element E, M R1 and M R2 represents the portion of the measuring radiation beam reflected at the surface S1 or S2 of the optical element E. Figure 8c In, M TR represents the portion of the measurement light radiation beam that passes through the optical element E but is reflected by the material WP on the return path, where M TR2 is the portion of the measurement light radiation beam that is further reflected by the surface S2 of the optical element E, and M TR2R is the part of the measuring light radiation beam that is further reflected by the material WP, and M TR2RT It is the part of the measuring light radiation beam that passes through the optical element E.

[0112] Figure 8d , wherein the measuring beam path comprises at least one third intermediate section between a first section from the light source to the back reflection surface S2 of the optical element E and a second section between the back reflection surface S2 of the optical element E and the sensor arrangement S. The third section is included between the first back reflection and the second back reflection to the back reflection surface S2 of the optical element and comprises at least one at least partial back reflection to the back reflection surface S1 of the optical element E. R2represents the portion of the measurement light radiation beam reflected by the surface S2 of the optical element E, M R21 represents the portion of the measurement light radiation beam that is further reflected by the surface S1 of the optical element E, M R212 represents the portion of the measurement light radiation beam that is reflected again by the surface S2 of the optical element E. When the optical element is in a predetermined nominal position and / or predetermined operating condition, the aforementioned third section has a corresponding predetermined nominal geometric and optical length under nominal conditions.

[0113] Figure 8e Another possible operating condition is shown in , in which the optical measurement path includes at least one third intermediate section between a first section from the light source to the back reflection surface S1 of the optical element E and a second section between the back reflection surface S1 of the optical element E and the sensor arrangement S. The third section is included between the first and second back reflections from the back reflection surface S1 of the optical element and includes at least one at least partial back reflection from a different optical element E' also interposed along the transmission optical path of the laser beam to the back reflection surface S2'. When the optical element E is in a predetermined nominal position and / or predetermined operating condition, the aforementioned third section has a corresponding predetermined nominal geometric and optical length.

[0114] The predetermined operating condition is a stationary state of the machine or a processing condition associated with predetermined processing parameters.

[0115] Furthermore, the first section and the second section of the measuring beam path may comprise at least one at least partial back reflection at a back reflection surface of different optical elements interposed along the transmission beam path for the laser beam or the material being processed.

[0116] Advantageously, the described method allows verification of the (temporary (in-process) or permanent) positioning and possible deformations or displacements of optical elements (such as lenses or mirrors) associated with the transmission path of the laser beam. In practice, the shape of an optical element may change due to the temperature of the environment in which it is located, or (if it is flexible) due to the pressure to which it is exposed, or more preferably due to the pressure difference established between the spaces in which it divides the environment. If these physical parameters affect the receptacle of the optical element, such as a translatable deformable ring, the position of the optical element may also change due to the influence of temperature or pressure.

[0117] The following describes specific application cases.

[0118] In machines for laser cutting, drilling, or welding, or for additive manufacturing of three-dimensional structures by laser, the machine comprises a work head carrying a nozzle for supplying an assist gas flow. It is desirable to control the assist gas pressure in the nozzle chamber without having to install a dedicated sensor. Knowledge of the assist gas pressure also helps to determine the effect of the assist gas pressure on the propagation characteristics of a measured optical radiation beam passing through the nozzle, for example, to determine the distance of the work head (of the nozzle) from the material being processed.

[0119] The pressure of the auxiliary gas in the auxiliary gas chamber of the nozzle can be indirectly derived from measuring changes in the local position of the surface of a protective optical element inserted along the transmission optical path of the processing laser beam or the boundary of the auxiliary gas chamber, or the local position of an auxiliary optical element facing the auxiliary gas chamber (which may be located outside the axis of the processing laser beam) according to a predetermined reference model, which indicates a predetermined nominal relationship between the position of the surface of the optical element relative to a corresponding predetermined nominal position corresponding to a predetermined reference pressure value of the auxiliary gas and the pressure of the auxiliary gas.

[0120] This reference model can be constructed starting from direct pressure measurements and from detection of the position of the optical elements during a calibration step.

[0121] Figure 9 The dependence of the interferometry readings on the local position of the surface of the optical element protecting or delimiting the auxiliary gas chamber along the axis of the measurement beam is shown as a function of the auxiliary gas pressure in the chamber (increase, decrease). Curve A shows the change in the local position of the surface protecting the optical element or the boundary of the auxiliary gas chamber as the pressure in the chamber increases. Curve B shows the change in the local position of the surface protecting the optical element or the boundary of the auxiliary gas chamber as the pressure in the chamber decreases. The hysteresis between these two curves is likely due to inelastic deformation of the materials involved.

[0122] In this embodiment, the reference optical path advantageously includes an optical element corresponding to the optical protection element or the optical auxiliary element, which is arranged in a position along the reference optical path corresponding to the nominal position of the optical protection element or the auxiliary optical element in the measuring optical path and is subjected to a controlled pressure value, which is the above-mentioned predetermined reference pressure value of the auxiliary gas in the measuring optical path.

[0123] Similar to indirect pressure measurement, another specific application of the present invention involves determining the temperature of an optical element or a transmission medium inserted along the transmission path of a processing laser beam, or the temperature of the environment in which the element is located, if this determines a local deformation or displacement. The state of the optical element is determined by using the method for determining its local position as described above, in which a reference beam travels along a reference beam path whose optical length is equal to the optical length of the measurement beam path under nominal operating conditions, which includes partial back reflection of the measurement beam at the surface of the optical element when the optical element is in a predetermined nominal position corresponding to a predetermined reference temperature value. The operating temperature of the optical element is then determined based on a predetermined reference model that indicates the nominal relationship between the position of the optical element relative to the predetermined nominal position and the temperature of the element.

[0124] Another specific application case of the inventive method for determining the position of an optical element target involves determining a perturbation of the current optical length of at least a portion of the measuring beam path relative to the current optical length of the corresponding portion of the reference beam path.

[0125] The propagation characteristics of the measured optical radiation beam are influenced by the physical parameters of the transmission medium in which it propagates (temperature, pressure, mechanical deformation), essentially because the refractive index of the transmission medium varies depending on these parameters. The method according to the invention can therefore be used to measure refractive index variations of the intersecting transmission medium, for example, caused by the pressure of the auxiliary gas, assuming that the position of the back-reflecting optical element is static.

[0126] In a machine for laser cutting, drilling or welding of materials or for additive manufacturing of three-dimensional structures by laser, the machine comprises a work head carrying a nozzle for supplying a flow of assisting gas, the measuring optical radiation beam propagating through the nozzle and having its propagation characteristics influenced by the pressure of the assisting gas.

[0127] In order to improve the accuracy of the process goals of the present invention, the determination of the optical length difference between the measurement optical path and the reference optical path can preferably be based on a normalized optical length of the measurement optical path, which is calculated starting from the geometric length of the portion of the optical measurement path passing through the auxiliary gas chamber (i.e., the nozzle) and the normalized refractive index of the portion of the optical measurement path passing through the auxiliary gas chamber (i.e., the nozzle). The normalized refractive index is calculated according to a predetermined nominal relationship between the refractive index of the transmission medium filled with auxiliary gas and the pressure of the gas, and according to the pressure of the auxiliary gas in the chamber.

[0128] More generally, since the optical length of the measuring path depends on the geometric length of the path and on the refractive index of the transmission medium, the determination of the optical length difference between the measuring path and the reference path can be based on a normalized optical length of the optical measuring path, which is calculated starting from the geometric length of the transmission medium of the measuring path or of a part of the measuring path, and the normalized refractive index of the transmission medium of the measuring path or of a part of the measuring path, which is calculated as a function of a change in at least one physical parameter of the transmission medium, such as the temperature, and as a function of a predetermined nominal relationship between the refractive index or the reflectivity and the temperature of the element itself.

[0129] Alternatively or in combination with the above, the determination of the optical length difference between the measuring optical path and the reference optical path can be based on a normalized optical length of the measuring optical path, the normalized optical length being calculated starting from a geometric length of a material transmission medium normalizing a portion of the measuring optical path and from a refractive index of the material transmission medium of the portion of the measuring optical path, wherein the normalized geometric length is calculated based on a mechanical deformation or mechanical deformation (e.g., lengthening or shortening) of the material transmission medium according to a predetermined nominal relationship.

[0130] Advantageously, the technical object of the present invention allows determining a disturbance of the current optical length of at least a portion of the measurement beam path relative to the current optical length of the corresponding portion of the reference beam path, and correcting, based on the determined disturbance, a value determined of the local current position of the optical element along the axis of the measurement beam relative to the nominal local position, for example by subtracting the measurement of the disturbance from the measurement of the current local position of the element (possibly after applying a correction factor). The disturbance occurs, for example, due to a change in at least one physical parameter of the transmission medium in which the measurement beam path extends.

[0131] For these purposes, the measuring beam incident on the sensor arrangement S comprises at least one calibrated measuring beam generated by a course of a calibrated measuring beam path, wherein the measuring beam is reflected or diffused by at least one back-reflecting surface of a static optical element inserted along the measuring beam path, and wherein the reference beam incident on the sensor arrangement S comprises a corresponding calibrated reference beam generated by a course of the calibrated reference beam path, said calibrated reference beam having an optical length equal to the optical length of the calibrated measuring beam path under calibrated nominal operating conditions, wherein the geometric length and the refractive index of the transmission medium of the calibrated measuring beam path are equal to the geometric length and the refractive index of the transmission medium of the calibrated reference beam path within a predetermined tolerance range. The static optical element may be, for example, an optical focusing system 16 for a laser beam.

[0132] Determining a perturbation of a current optical length of at least a portion of the measuring optical path comprises the following operations:

[0133] - superimposing the calibration measuring beam and the calibration reference beam along the illumination axis onto a common area of ​​incidence of the sensor arrangement S;

[0134] - if an interferometry technique with frequency domain detection is applied, detecting the position of the interference fringe pattern between the calibration measuring beam and the calibration reference beam on the common incidence area along the illumination axis, or the frequency of the interference fringe pattern between the calibration measuring beam and the calibration reference beam; and

[0135] -Determining the optical length difference between the calibration measurement light path and the calibration reference light path based on any position of the interference fringe pattern along the illumination axis of the incident area, or the frequency of the interference fringe pattern in the frequency domain, the optical length difference indicating (a) the difference between the geometric length of the calibration measurement light path and the geometric length of the calibration reference light path, and / or (b) the difference between the refractive index of the calibration measurement light path and the refractive index of the calibration reference light path.

[0136] The optical length difference between the calibrated measurement light path and the calibrated reference light path is indicative of the aforementioned perturbation of the current optical length of at least a portion of the measurement light path.

[0137] Correction of the determined value of the current local position of the optical element along the axis of the measuring beam relative to the nominal local position based on the determined disturbance is performed, for example, by subtracting the calibration value from the main measured value.

[0138] Improvements of the present invention will be described in the following description.

[0139] Specifically, a solution to increasing the difference interval between the lengths of the measurement path and the reference path measurable by the technology of the present invention is to utilize partial back reflection at the surface of at least one different optical element inserted along the path of the processing laser beam and the measuring optical radiation beam, or to utilize a reference optical path of a predetermined length different from the length of the main reference optical path.

[0140] In an embodiment, the measuring beam incident on the sensor arrangement S comprises a main measuring beam resulting from a run of a main measuring beam path, wherein the main measuring beam has reflections from a back-reflecting surface being measured and is transmitted through any other optical element interposed along the optical path of the machining laser beam upstream of the optical element being measured, and at least one additional multiplexed measuring beam resulting from a run of an additional measuring beam path, wherein the at least one additional multiplexed measuring beam has reflections from a back-reflecting surface of said optical element being measured and has a geometric length that is greater than the geometric length of the main measuring beam path, for example because it comprises at least partial back-reflections on surfaces of different optical elements interposed along the optical paths of the machining laser beam and the measuring optical radiation beam.

[0141] In this embodiment, the method of the invention is based on the detection of the position of an additional pattern of interference fringes on a common incidence area C of the sensor arrangement S, as determined by the interference between the additional measuring beam and the reference beam. In an interferometry technique for detecting the interference fringe pattern in the spatial domain, the additional pattern of interference fringes has (i) a peak or maximum intensity of the envelope of the optical radiation of the main pattern of interference fringes that is different from, for example, lower than, or (ii) an intrinsic position of the envelope of the optical radiation intensity that is different from the intrinsic position of the optical radiation intensity of the main interference pattern, if it occurs simultaneously with the main interference pattern.

[0142] Under the above conditions, a difference in optical length is thus determined between the additional measurement optical path and the reference optical path, the difference indicating the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measurement beam, the difference being determined based on the position of the additional pattern of interference fringes along the illumination axis of the incident area, or the frequency of the interference fringe pattern in the frequency domain.

[0143] In a different embodiment, the reference beam incident on the sensor arrangement S comprises a main reference beam generated by a run of a main reference light path and at least one additional multiplexed reference beam generated by a run of an additional reference light path having a geometric length different from the geometric length of the main reference light path.

[0144] In this embodiment, the method of the invention is based on the detection of the position of an additional pattern of interference fringes on a common area of ​​incidence of the sensor arrangement S, the position being determined by the interference between the measuring beam and an additional reference beam.

[0145] Also in this case, in the interferometry technique of detecting the interference fringe pattern in the spatial domain, the additional pattern of interference fringes (i) has a peak or maximum intensity of the envelope of the optical radiation of the main pattern of interference fringes that is, for example, different from, for example lower than, the peak value of the envelope of the optical radiation of the main pattern of interference fringes between the measuring beam and the main reference beam, or (ii) has an intrinsic position of the envelope of the optical radiation intensity that is offset from the intrinsic position of the optical radiation intensity of the main interference pattern if it occurs simultaneously with the main interference pattern.

[0146] The selection of the interference fringe pattern occurs by selecting additional reference paths in case several different operating intervals are not beside or superimposed on the sensor arrangement, but are sufficiently separated to show these corresponding interference fringes alternately.

[0147] Under the above conditions, therefore, a difference in optical length is determined between the measuring optical path and the additional reference optical path, the difference indicating the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measuring beam, the difference being determined based on the position of the additional pattern of interference fringes along the illumination axis of the incident area, or the frequency of the interference fringe pattern in the frequency domain.

[0148] Conveniently, the position of the optical element is accurately determined, and the position of each optical element of the transmission path of the laser beam is better determined. The transmission path can be selected by considering a corresponding reference optical path that can be associated with the measurement optical path, allowing the control unit of the laser processing machine to operate by feedback correction or control of certain operating parameters (such as the pressure of the assist gas), or to issue an alarm signal and stop each working process if it is determined that the optical element is in an abnormal position. This is particularly useful for improving the safety of the manufacturing process, for example.

[0149] It should be noted that the embodiments proposed for the present invention in the foregoing discussion are purely illustrative and non-restrictive of the present invention. Those skilled in the art can easily implement the present invention in different embodiments, however, these embodiments do not depart from the principles outlined herein and are therefore included in this patent.

[0150] This applies in particular to the possibility of using wavelengths of low-coherence optical radiation other than those cited or measuring and reference optical paths with interposed optical elements other than those Figure 6 The optical elements shown therein are shown by way of non-limiting example only.

[0151] Of course, it should be understood that the principle, manufacturing details and embodiments of the invention may vary widely compared to what has been described and illustrated purely by way of non-limiting examples, without departing from the scope of the present invention as defined in the appended claims.

Claims

1. A method for determining the local position of at least one optical element associated with a transmission path of a laser beam in a working head of a machine for laser processing of materials, characterized in that The method comprises the following steps: generating a respective measuring low-coherence optical radiation beam, directing the measuring beam towards the optical element and directing the measuring beam reflected or diffused by at least one back-reflecting surface of the optical element, the measuring beam impinging on the at least one back-reflecting surface of the optical element with at least a partial back-reflection towards an optical interferometry sensor device, wherein the measuring beam travels from a respective source to the optical interferometry sensor device along a measuring optical path, the measuring optical path comprising a first section between the source and the back-reflecting surface of the optical element and a second section between the back-reflecting surface of the optical element and the optical interferometry sensor device, the first section and the second section having respective predetermined nominal geometric lengths when the optical element is in a predetermined nominal position corresponding to a predetermined operating condition, generating a corresponding reference beam of the low-coherence optical radiation and directing the reference beam towards the optical interferometry sensor device, wherein the reference beam travels along a reference optical path having an optical length equal to the optical length of the measurement optical path under nominal operating conditions with the position of the optical element in the predetermined nominal position; superimposing the measuring beam and the reference beam on a common incident area of ​​the optical interferometric sensor device along a predetermined illumination axis; detecting a position of an interference fringe pattern between the measuring beam and the reference beam on the common incident area along the illumination axis, wherein an extension of the interference fringe pattern along the illumination axis corresponds to a coherence length of the low-coherence optical radiation, or detecting a frequency of a fringe pattern in a wavelength spectrum, the fringe pattern being obtained by interference between the measuring beam and the reference beam through wavelength dispersion of the beams, the extension of the fringe pattern in the frequency domain being determined by the coherence length of the low-coherence optical radiation; and The optical length difference between the measuring optical path and the reference optical path is determined correspondingly according to the position of the interference fringe pattern along the illumination axis of the incident area, or the frequency of the interference fringe pattern in the frequency domain, and the optical length difference between the measuring optical path and the reference optical path indicates the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measuring beam.

2. The method according to claim 1, wherein The measurement optical path comprises at least one third segment, which is interposed between the first segment and the second segment and included between the first back reflection and the second back reflection at the back reflection surface of the optical element, the third segment including at least one at least partial back reflection at the second back reflection surface of the optical element, and when the optical element is in the predetermined nominal position and / or the predetermined operating condition, the third segment has a corresponding predetermined nominal geometric and optical length.

3. The method according to claim 1 or 2, wherein: The measurement optical path includes at least one third segment, which is inserted between the first segment and the second segment and includes a first back reflection and a second back reflection at the back reflection surface of the optical element. The third segment includes at least one at least partial back reflection at the back reflection surface of different optical elements inserted along the transmission optical path of the laser beam. When the optical element is in the predetermined nominal position and / or the predetermined operating condition, the third segment has a corresponding predetermined nominal geometric and optical length.

4. The method according to claim 1, wherein The first section and the second section of the measuring optical path comprise at least one at least partial back reflection at a back reflection surface of a different optical element interposed along the transmission optical path for the laser beam or the material being processed.

5. The method according to claim 1 or 2, wherein: The measuring beam is directed onto the common incident area of ​​the optical interferometer sensor device along a first incident direction, and the reference beam is directed onto the common incident area of ​​the optical interferometer sensor device along a second incident direction at a predetermined incident angle relative to the first incident direction.

6. The method according to claim 1 or 2, wherein: The measuring beam and the reference beam are collinearly superimposed along the same incident direction toward a wavelength dispersive optical device adapted to separate frequency components of the beams obtained by superimposing the measuring beam and the reference beam on the common incident area of ​​the optical interference type sensor device.

7. The method according to claim 1 or 2, wherein: The predetermined operating condition is a machine standby condition or an operating condition related to pre-established working parameters.

8. The method according to claim 1 or 2, wherein: The position of the interference fringe pattern along the illumination axis is the intrinsic position of the envelope of the optical radiation intensity of the interference fringe pattern.

9. The method according to claim 8, wherein The intrinsic position of the envelope of the light radiation intensity of the interference fringe pattern is the position of the peak or maximum value of the envelope of the light radiation intensity.

10. The method according to claim 1 or 2, wherein: The optical interferometric sensor device comprises a linear or two-dimensional arrangement of photodetectors along the illumination axis.

11. The method according to claim 1 or 2, wherein: the measuring beam incident on the optical interferometric sensor device comprises a main measuring beam and at least one additional multiplexed measuring beam, the main measuring beam resulting from a course of a main measuring optical path, wherein the main measuring beam has reflections from the at least one back-reflecting surface of the optical element being measured and transmission through any other optical element interposed along the optical path of the machining laser beam upstream of the optical element being measured, the at least one additional multiplexed measuring beam resulting from a course of an additional measuring optical path, wherein the at least one additional multiplexed measuring beam has reflections from the at least one back-reflecting surface of the optical element being measured and has a geometric length greater than the geometric length of the main measuring optical path, the at least one additional multiplexed measuring beam including at least partial back-reflections on surfaces of different optical elements interposed along the optical paths of the machining laser beam and the measuring beam, The method comprises the following steps: detecting a position of an additional interference fringe pattern on the common incidence area, the additional interference fringe pattern (i) having a peak or maximum value of optical radiation intensity that is different from a peak or maximum value of optical radiation intensity of a main interference fringe pattern between the main measuring beam and the reference beam, or (ii) having an envelope of optical radiation intensity whose inherent position is shifted from an inherent position of the envelope of the optical radiation intensity of the main interference fringe pattern; and The optical length difference between the additional measurement optical path and the reference optical path is determined correspondingly according to the position of the interference fringe pattern along the illumination axis of the incident area, or the frequency of the interference fringe pattern in the frequency domain, and the optical length difference between the additional measurement optical path and the reference optical path indicates the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measurement beam.

12. The method according to claim 1 or 2, wherein: the reference beam incident on the optical interferometry sensor device comprises a main reference beam and at least one additional multiplexed reference beam, the main reference beam being generated by a run of a main reference optical path, the at least one additional multiplexed reference beam being generated by a run of an additional reference optical path, the additional reference optical path having a geometric length different from that of the main reference optical path, The method comprises the following steps: detecting a position of an additional interference fringe pattern on the common incidence area, the additional interference fringe pattern (i) having a peak or maximum value of optical radiation intensity that is different from a peak or maximum value of optical radiation intensity of a main interference fringe pattern between the measuring beam and the main reference beam, or (ii) having an envelope of optical radiation intensity whose inherent position is shifted from an inherent position of the envelope of the optical radiation intensity of the main interference fringe pattern; and The optical length difference between the measuring optical path and the additional reference optical path is determined correspondingly according to the position of the interference fringe pattern along the illumination axis of the incident area, or the frequency of the interference fringe pattern in the frequency domain, and the optical length difference between the measuring optical path and the additional reference optical path indicates the difference between (a) the current local position of the optical element and (b) the predetermined nominal local position of the optical element along the axis of the measuring beam.

13. The method according to claim 1 or 2, comprising determining the positions of a plurality of optical elements inserted along the transmission optical path of the high-power laser beam, characterized in that The method comprises implementing a plurality of measurement beam paths associated with a plurality of corresponding reference beam paths by extracting a respective plurality of measurement beams associated with each of the plurality of optical elements downstream of reflection or diffusion from at least one back-reflection surface of each of the plurality of optical elements, the plurality of reference beam paths being arranged for guiding corresponding separated or superimposed reference beams or being determined by means of a continuous variation of the length of a base reference beam path passing through an optical element for deflection and separation of the reference beams.

14. The method according to claim 1 or 2, comprising: determining a perturbation of a current optical length of at least a portion of the measurement optical path relative to a current optical length of a corresponding portion of the reference optical path, and correcting a determined value of a current local position of the optical element along the axis of the measurement beam relative to the nominal local position based on the perturbation, wherein the measuring beams incident on the optical interferometry sensor device include at least one calibration measuring beam, the at least one calibration measuring beam resulting from a travel of a calibration measuring optical path, wherein the measuring beams are reflected or diffused by at least one back-reflecting surface of a static optical element interposed along the measuring optical path, and wherein the reference beams incident on the optical interferometry sensor device include corresponding calibration reference beams resulting from a travel of a calibration reference optical path, the optical length of the calibration reference optical path being equal to the optical length of the calibration measuring optical path under calibrated nominal operating conditions, wherein the geometric length and the refractive index of a transmission medium of the calibration measuring optical path are equal to the geometric length and the refractive index of a transmission medium of the calibration reference optical path within a predetermined tolerance range, and wherein determining a perturbation of a current optical length of at least a portion of the measurement optical path comprises: superimposing the calibration measurement beam and the calibration reference beam on a common incidence area of ​​the optical interferometry sensor device along a predetermined illumination axis; detecting a position of an interference fringe pattern between the calibration measurement beam and the calibration reference beam on the common incidence area along the illumination axis, or detecting a frequency of the interference fringe pattern between the calibration measurement beam and the calibration reference beam obtained by wavelength dispersion of the beams; and determining an optical length difference between the calibration measurement light path and the calibration reference light path based on any position of the interference fringe pattern along the illumination axis of the incident region or a frequency of the interference fringe pattern in the frequency domain, wherein the optical length difference between the calibration measurement light path and the calibration reference light path indicates (a) a difference between a geometric length of the calibration measurement light path and a geometric length of the calibration reference light path, and / or (b) a difference between a refractive index of the calibration measurement light path and a refractive index of the calibration reference light path, The optical length difference between the calibration measurement optical path and the calibration reference optical path is indicative of the perturbation in the current optical length of at least a portion of the measurement optical path.

15. The method according to claim 11, comprising controlling the propagation axis of the measuring beam to be within a predetermined vicinity of the propagation axis of the machining laser beam.

16. A system for determining the local position of at least one optical element associated with a transmission path of a laser beam in a working head of a machine for laser processing of materials, characterized in that The system comprises: means for generating a corresponding measuring beam of low-coherence optical radiation; means for propagating a measuring light beam adapted to direct the measuring light beam towards the optical element and to direct the measuring light beam reflected or diffused by at least one back-reflecting surface of the optical element, the measuring light beam impinging on the at least one back-reflecting surface of the optical element with at least a partial back-reflection towards an optical interferometry sensor device, wherein the measuring light beam travels from a respective source to the optical interferometry sensor device along a measuring light path comprising a first section between the source and the back-reflecting surface of the optical element and a second section between the back-reflecting surface of the optical element and the optical interferometry sensor device, the first section and the second section having respective predetermined nominal geometric lengths when the optical element is in a predetermined nominal position corresponding to a predetermined operating condition; means for generating a corresponding reference beam of low-coherence optical radiation; means for propagating a reference beam adapted to direct the reference beam towards the optical interferometry sensor device, wherein the reference beam travels along a reference optical path having an optical length equal to the optical length of the measurement optical path under nominal operating conditions in which the position of the optical element is in a predetermined nominal position; wherein the means for propagating a measuring beam and the means for propagating a reference beam are arranged to superimpose the measuring beam and the reference beam on a common incidence area of ​​the optical interferometric sensor device along a predetermined illumination axis; means for detecting the position of an interference fringe pattern between the measuring beam and the reference beam on a common incidence area along the illumination axis, wherein the extension of the interference fringe pattern along the illumination axis corresponds to the coherence length of the low-coherence optical radiation, or for detecting the frequency of a fringe pattern in a wavelength spectrum, the fringe pattern being obtained by interference between the measuring beam and the reference beam by wavelength dispersion of the beams, the extension of the fringe pattern in the frequency domain being determined by the coherence length of the low-coherence optical radiation; and A processing device is arranged to determine an optical length difference between the measurement optical path and the reference optical path based on the position of the interference fringe pattern along the illumination axis of the incident area or the frequency of the interference fringe pattern in the frequency domain, the optical length difference between the measurement optical path and the reference optical path indicating a difference between (a) a current local position of the optical element and (b) a predetermined nominal local position of the optical element along the axis of the measurement beam.

17. A machine for laser processing of materials by means of a high-power processing laser beam, said high-power processing laser beam being guided along a transmission optical path of the laser beam, said machine comprising at least one optical element, characterised in that The machine comprises a system for determining the position of the at least one optical element, the system being arranged to perform the method according to any one of claims 1 to 15.

18. A method for determining the pressure in an assist gas chamber associated with a nozzle for supplying an assist gas flow, the nozzle being carried by a working head of a machine for laser cutting, drilling or welding materials, or for additive manufacturing of three-dimensional structures by laser, characterized in that The method comprises the following steps: determining the local position of an optical protection element for protection or limitation of the auxiliary gas chamber inserted along a transmission light path for a machining laser beam, or the local position of an optical auxiliary element facing the auxiliary gas chamber and capable of being located outside the axis of the machining laser beam, by a method for determining the local position of at least one optical element associated with a transmission light path for a laser beam in a work head of a machine for laser machining of materials according to any one of claims 1 to 15, wherein the reference beam travels along a reference light path having an optical length equal to the optical length of the measuring light path under nominal operating conditions, the nominal operating conditions comprising that the measuring beam is partially back-reflected at a surface of the optical protection element or the optical auxiliary element when the optical element is in a predetermined nominal position corresponding to a predetermined reference pressure value of the auxiliary gas in the auxiliary gas chamber; and The pressure of the auxiliary gas in the auxiliary gas chamber is determined according to a predetermined reference model indicating a nominal relationship between the position of an optical protection element or an optical auxiliary element of the auxiliary gas chamber relative to the predetermined nominal position and the auxiliary gas pressure.

19. The method according to claim 18, wherein The reference optical path includes an optical element corresponding to the optical protection element or to the optical auxiliary element, arranged along the reference optical path in a position corresponding to a nominal position of the optical protection element or the optical auxiliary element in the measuring optical path.

20. The method according to claim 18 or 19, wherein The reference optical path includes an optical element corresponding to the optical protection element or corresponding to the optical auxiliary element, which is subjected to a controlled pressure value, the controlled pressure value being a predetermined reference pressure value of the auxiliary gas in the measuring optical path.

21. A method for determining the temperature of an optical element inserted along the transmission path of a processing laser beam of a machine for laser processing of materials or for additive manufacturing of three-dimensional structures by laser, characterized in that The method comprises the following steps: determining the local position of at least one optical element associated with a transmission optical path of a laser beam in a work head of a machine for laser processing of materials by means of a method for determining the local position of said optical element according to any one of claims 1 to 15, wherein a reference beam travels along a reference optical path having an optical length equal to the optical length of the measuring optical path under nominal operating conditions, said nominal operating conditions comprising a partial back reflection of the measuring beam at a surface of the optical element when the optical element is in a predetermined nominal position corresponding to a predetermined reference temperature value; The operating temperature of the optical element is determined according to a predetermined reference model indicating a nominal relationship between the position of the optical element relative to the predetermined nominal position and the temperature of the element.

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