Patent Publication Number: US-2020282493-A1

Title: Apparatus for Machining a Workpiece with a Laser Beam

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for machining a workpiece with a laser beam, and to a corresponding machining method. In particular, the apparatus and method are for machining the workpiece with a laser beam coupled into a fluid jet. The present invention specifically relates to controlling the machining process of the apparatus and method based on a process emission. 
     BACKGROUND 
     A conventional apparatus for machining a workpiece with a laser beam is known. Also a conventional apparatus for machining a workpiece with a laser beam coupled into a fluid jet, wherein the fluid jet impinges onto the workpiece, is generally known. 
     A problem typically encountered during the laser beam machining process of a conventional apparatus is that, due to the lack of mechanical interactions of the apparatus with the workpiece, it is difficult to determine characteristic states of the process of machining the workpiece. This difficulty often leads to a significant increase of the machining process time. For example, when a conventional apparatus is used to cut a workpiece with the laser beam, it is problematic to accurately determine, if and when the laser beam has broken through the workpiece material. Thus, the cutting process cannot be finished in a time-optimized manner. Additionally, it would also be beneficial to have the possibility to accurately determine other states of machining the workpiece, for example, states that indicate an instability of the machining process. 
     Since the laser beam may be scattered back from the workpiece in an uncontrolled manner, a determination of the process state by visual inspection of the workpiece is difficult or even dangerous. Further, if the conventional apparatus uses a laser beam guided in a fluid jet, additional difficulties arise due to the wet workpiece environment. For instance, since the fluid jet may splatter back from the workpiece in an uncontrolled manner, the fluid may accumulate on surface regions of the workpiece, thereby making it even more difficult to determine states of the machining process. 
     Conventional solutions are largely based on visual inspection by means of a camera, for instance, disposed next to the apparatus and/or the workpiece. However, the conventional visual inspection is not very precise, due to the above-mentioned environmental difficulties, especially in wet environments. Further, there is a high risk that the inspection equipment, e.g. the camera, is damaged by splattering fluids. Moreover, the apparatus according to this conventional solution is relatively large, and is inconveniently distributed into multiple separate components. 
     In view of these problems and disadvantages, the present invention aims at improving the conventional apparatus and solutions. It is accordingly an object of the present invention to provide an apparatus and a method for machining a workpiece with a laser beam coupled into a fluid jet, which allow reducing the machining process time. In particular, the invention should allow determining more precisely a state of the machining process. The higher the accuracy of determining the state of machining the workpiece is, the better the machining process can be time-optimized. In particular, the invention desires a reliable determination of whether the laser beam has broken through the workpiece material or not. The present invention thereby aims particularly for a compact apparatus and a solution, which is not disturbed by the wet environment caused by the fluid jet. Accordingly, damages to the apparatus and equipment should be avoided. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are defined in the dependent claims. 
     In particular, the present invention proposes detecting a state of machining the workpiece based on a process emission, i.e. based on a laser-induced electromagnetic radiation that results from the machining of the workpiece with the laser beam. 
     A first aspect of the present invention provides an apparatus for machining a workpiece with a laser beam, the apparatus comprising a machining unit configured to provide a pressurized fluid jet onto the workpiece and to couple the laser beam through at least one optical element into the fluid jet towards the workpiece, a sensing unit arranged to receive a laser-induced electromagnetic radiation propagating away from the workpiece through the fluid jet and through at least one optical element, and configured to convert the received radiation into a signal, a signal processing unit configured determine a state of machining the workpiece based on the signal. 
     In the machining unit the laser beam is coupled into the pressurized fluid jet, which acts like a waveguide for the laser beam and guides the laser beam onto the workpiece by means of total internal reflection. The pressurized fluid jet is preferably provided by a fluid nozzle, and the laser beam may be directed by the at least one optical element through the fluid nozzle into the fluid jet and towards the workpiece. The fluid nozzle and the at least one optical element may be separated, for instance by an optical window, in order to prevent contact of the optical element and the fluid. 
     The sensing unit may be a photo-detecting device or light sensor, and is preferably positioned such that it can detect the laser-induced electromagnetic radiation that travels along the fluid jet in the direction opposite to the primary laser beam, i.e. propagates away from the workpiece. Notably, some electromagnetic radiation induced by the laser beam machining the workpiece may not back-propagate through the fluid jet, but may travel elsewhere. 
     The laser-induced electromagnetic radiation received by the sensing unit provides a very accurate fingerprint of different states of machining the workpiece. In particular, the signal, which is accordingly output by the sensing unit, shows characteristic behaviors depending on the different states of machining the workpiece. Therefore, the signal processing unit is able to accurately and efficiently determine states of machining the workpiece based on the signal. As a consequence, it may trigger according actions of the apparatus based on the determined state, in order to significantly improve the machining process time. 
     The fact that the sensing unit can receive the laser-induced electromagnetic radiation via the fluid jet allows arranging it safe from the wet environment of the machining process. Advantageously, the fluid jet acts as a waveguide also for back-propagating laser-induced radiation, and thus allows a more accurate detection of this radiation, and consequently an analysis or post-processing of the signal that the sensing unit supplies when sensing this radiation. Preferably, the sensing unit is housed in the apparatus, more preferably such that it is separated from any fluid circuitry and the fluid nozzle, in order to be protected from fluid contact and thus enjoy an increased lifetime. The apparatus of the first aspect can be built in a very compact manner, and all components of the apparatus may advantageously be integrated. 
     In a preferred implementation form of the apparatus, the signal processing unit is configured to determine, as the state of machining the workpiece, whether the laser beam has broken through the workpiece. 
     The apparatus of the invention can particularly determine very accurately and quickly, whether the laser beam has broken through the workpiece. Accordingly, the machining process time can be significantly reduced, particularly when the apparatus is used for cutting a workpiece with the laser beam. This is due to the fact that the machining process can be stopped immediately, once the state that the laser beam has broken through the workpiece is determined. The signal processing unit may be configured to automatically provide such an instruction. 
     In a further preferred implementation form of the apparatus, the sensing unit is arranged to receive the laser-induced electromagnetic radiation propagating through the fluid jet through the at least one optical element, which is configured to couple the laser beam into the fluid jet. 
     In this specific way, the apparatus can be built particularly compact. Furthermore, the least amount of optical components are required. 
     The apparatus may comprises an optical head including the machining unit and the sensing unit. Accordingly, in the optical head the laser beam can be coupled by the machining unit into the fluid jet, and the sensing unit may sense the laser-induced radiation back-propagating from the workpiece. The signal processing unit may in this case be arranged in the apparatus, e.g. in a housing thereof, but outside the optical head. A laser unit comprising laser source and/or laser controller may be part of the apparatus or not, but is at least separate from the optical head. 
     In a further preferred implementation form of the apparatus, the apparatus comprises an optical head including the machining unit, and a laser unit including the sensing unit. 
     The laser unit may comprise laser source and/or laser controller. In this implementation form, the laser beam may be provided by the laser unit to the optical head, where it is coupled into the fluid jet by the machining unit. Laser-induced radiation back-propagating from the workpiece may be guided through the optical head to the laser unit, where it is received by the sensing unit. The optical head can be optically connected to the laser unit, for instance, by an optical fiber. 
     It was surprisingly found that the state of machining the workpiece can be determined precisely by the signal processing unit based on the signal, which is provided by the sensing unit based on the received laser-induced radiation, even when the sensing unit is arranged in the laser unit, i.e. distanced from the optical head. An advantage of having the sensing unit  107 , and optionally associated optical elements like a beam splitter, optical separation unit, and/or focusing optics etc., provided in the laser unit is that the handling of these elements, and e.g. their maintenance or replacement, becomes easier. A further advantage is that the optical head can be exchanged without having to exchange the sensing unit and associated optical elements. The optical head can also be built smaller, and the sensing unit and associated optical elements can be built larger, due to more space available in the laser unit. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises an optical connection element, particularly an optical fiber, for optically connecting the optical head and the laser unit, wherein the machining unit in the optical head is configured to receive the laser beam via the optical connection element, and the sensing unit in the laser unit is configured to receive the laser-induced electromagnetic radiation via the optical connection element. 
     Accordingly, the same optical connection element, e.g. optical fiber, is used to transport the laser light for the laser beam and the laser-induced radiation, respectively, within the apparatus, i.e. between the optical head and the laser unit. Nevertheless, the state of machining the workpiece can be accurately determined by the processing unit based on the signal produced accordingly by the sensing unit. 
     In a further preferred implementation form of the apparatus, the at least one optical element includes a lens for coupling the laser beam into the fluid jet, and the sensing unit is arranged to receive the back-propagating laser-induced electromagnetic radiation through said lens. 
     This specific implementation form therefore allows using the lens in a double manner and allows positioning the sensing unit such that it is protected from fluid contact. Further, the apparatus can be built in a compact manner. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises a spectral separation unit, preferably an optical filter unit, configured to isolate only electromagnetic radiation of interest including at least a part of the laser-induced electromagnetic radiation on the sensing unit and/or to prevent initial laser light to reach the sensing unit. 
     In particular, the spectral separation unit is arranged and configured to receive a radiation, which is or includes the laser-induced electromagnetic radiation propagating away from the workpiece, may separate electromagnetic radiation of interest from the received radiation, and may provide the radiation of interest including at least a part of said laser-induced electromagnetic radiation to the sensing unit. The spectral separation unit has the effect that the state of machining the workpiece can be determined even more accurately and more reliably. The spectral separation unit prevents that unwanted radiation reaches the sensing unit. The sensing unit can therefore more sensitively and accurately detect the radiation of interest. The radiation of interest may be the laser-induced electromagnetic radiation with unwanted other radiation removed, or may be a part of the laser-induced electromagnetic radiation. It may, for instance, be that the laser beam induces different kinds of electromagnetic radiation caused by different mechanisms. In this case, the radiation of interest may be only laser-induced electromagnetic radiation caused by one specific mechanism. 
     In a further preferred implementation form of the apparatus, the laser-induced electromagnetic radiation includes secondary radiation emitted from a portion of the workpiece that is machined with the laser beam. 
     The secondary radiation is in this case caused by the processing of the workpiece with the laser beam, for instance, because the machined surface region of the workpiece is transformed into a plasma. This plasma emits a characteristic radiation, which can be easily isolated on or by the sensing unit, for instance, on the sensing unit by means of the above-described spectral separation unit, in order to allow a particularly accurate determination of the state of machining the workpiece, especially of whether the laser beam broke through the workpiece. 
     In a further preferred implementation form of the apparatus, the laser-induced electromagnetic radiation includes primary laser radiation that is reflected from the workpiece. 
     This provides a simple way of implementing the present invention. The laser light reflected back from the workpiece includes at least a part of the light of the primary laser beam, and back-propagates through the fluid jet towards the sensing unit. The corresponding signal converted by the sensing unit provides an accurate fingerprint of the state of machining the workpiece, especially of whether the laser beam broke through the workpiece. 
     In a further preferred implementation form of the apparatus, the laser-induced electromagnetic radiation includes secondary radiation generated by scattering, preferably Raman scattering, of the laser beam in the fluid jet. 
     Laser-induced scattering within the fluid jet has the advantage that it is not created directly at the workpiece surface, where the environmental conditions are less controlled, but in the laminar and/or better controlled fluid jet. Nevertheless, this scattering-induced radiation provides an accurate fingerprint of the state of machining the workpiece, especially of whether the laser beam broke through the workpiece. 
     It may be the case that secondary radiation emitted from a portion of the workpiece that is machined with the laser beam, primary laser radiation that is reflected from the workpiece, and/or secondary radiation generated by scattering of the laser beam in the fluid jet propagates away from the workpiece through the fluid jet and through the at least one optical element. In this case, the above-described spectral separation unit may be used to filter out any unwanted laser-induced electromagnetic radiation, and to focus only the laser-induced electromagnetic radiation of interest, e.g. the secondary radiation emitted from a portion of the workpiece that is machined with the laser beam, onto the sensing unit. 
     In a further preferred implementation form of the apparatus, the signal processing unit is configured to process the signal with a temporal resolution of below 0.5 s, preferably of below 0.1 s. 
     For instance, the temporal resolution of the signal processing unit may be between 0.2-0.5 s, or between 0.1-0.5 s, or between 0.1-0.2 s, or even between 0.01 s-0.1 s. These preferred temporal resolutions provided by the signal processing unit allow an accurate detection of the state of machining the workpiece, and thus a particularly stable control of the machining process. In particular, the machining process time can be reduced even further with such a signal processing unit. 
     In a further preferred implementation form of the apparatus, the laser beam is a pulsed laser beam, the sensing unit is configured to convert the received radiation into a signal for each laser pulse, and the signal processing unit is configured to integrate a plurality of signals over time to produce an integrated signal, and to determine the state of machining the workpiece based on a pattern or a change of a pattern in the integrated signal. 
     Integrating the signal over time improves further the reliability of determining the state of machining the workpiece. In particular, patterns that occur in a single (non-integrated) signal can be enhanced in the integrated signal. Furthermore, additional patterns that appear only in the integrated signal may allow determining further states, or determining the states more precisely. Also the sensitivity of the determination is generally enhanced, since the signal noise impact is reduced. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises at least one peripheral device, preferably a laser controller, fluid supply controller, gas supply controller, and/or movement axes controller, wherein the signal processing unit is configured to provide the at least one peripheral device with an instruction signal based on the determined state of machining the workpiece, in order to start, break, stop and/or restart an action of the peripheral device. 
     Accordingly, the machining process time can be optimized. In particular, based on the determined state of machining the workpiece, the apparatus can react quickly to take the necessary actions. For instance, if the laser beam broke through the workpiece, the apparatus can stop the machining process. As a further advantage, the machining process quality can be improved. 
     A second aspect of the present invention provides a method of machining a workpiece with a laser beam, the method comprising providing a pressurized fluid jet onto the workpiece and coupling the laser beam through at least one optical element into the fluid jet towards the workpiece, receiving a laser-induced electromagnetic radiation propagating away from the workpiece through the fluid jet and through at least one optical element, and converting the received radiation into a signal, and determining a state of machining the workpiece based on the signal. 
     With the method of the second aspect, the same advantages and effects can be achieved as with the apparatus of the first aspect. 
     In a preferred implementation form of the method, the method further comprises, for determining the state of machining the workpiece, recording a plurality of signals, comparing the recorded signals with predetermined reference signals, and determining the state of machining the workpiece based on similar or matching reference signals. 
     By providing reference signals and carrying out the comparison with these reference signals, an easy implementation with very accurate results can be provided. 
     In a further preferred implementation form of the method, the method further comprises, for determining the state of machining the workpiece, integrating a plurality of signals over time to produce an integrated signal, evaluating a pattern or a change of a pattern in the integrated signal, and determining the state of machining the workpiece based on the pattern or the change of the pattern. 
     Thereby, the same advantages and effects as with the respective above-described implementation form of the apparatus are achieved. 
     In a further preferred implementation form of the method, the laser-induced electromagnetic radiation is secondary radiation emitted from a portion of the workpiece that is machined with the laser beam, and the method comprises determining, as the state of machining the workpiece, that the laser beam has broken through the workpiece, when a value of a single signal or the integrated signal, which increased after a start of machining the workpiece, decreases again below a determined threshold value. 
     In a further preferred implementation form of the method, the laser-induced electromagnetic radiation is primary laser radiation reflected from the workpiece, and the method comprises determining, as the state of machining the workpiece, that the laser beam has broken through the workpiece, when a value of a single signal or the integrated signal, which decreased after a start of machining the workpiece, increases again above a determined threshold value. 
     The above-mentioned characteristic fingerprints in the single signal or integrated signal, i.e. the patterns followed by the signal or integrated signal, have been found to provide very accurate determination results. Moreover, the processing load is low, so that the method can be carried out very quickly. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above-described aspects and preferred implementation forms of the present invention are explained in the following description of specific embodiments in relation to the enclosed drawings, in which 
         FIG. 1  shows an apparatus according to an embodiment of the present invention. 
         FIG. 2  shows an apparatus according to an embodiment of the present invention. 
         FIG. 3  shows an apparatus according to an embodiment of the present invention. 
         FIG. 4  shows a method according to an embodiment of the present invention. 
         FIG. 5  schematically shows characteristic signal patterns used in a method according to an embodiment of the present invention. 
         FIG. 6  shows an apparatus according to an embodiment of the present invention. 
         FIG. 7  shows an apparatus according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows an apparatus  100  according to an embodiment of the present invention. In particular, the apparatus  100  is configured to machine a workpiece  101  with a laser beam  102 . The workpiece  101  may be a made of a material including, for example, metals, ceramics, diamonds, semiconductors, carbides, alloys, superalloys, or ultra-hard materials. The workpiece  101  is notably not a part of the apparatus  100 . However, the workpiece  101  can be positioned on a machining surface, which may or may not be part of the apparatus  100 . In either case, the apparatus  100  can be arranged such that it can machine the workpiece  101  disposed on the machining surface. The apparatus  100  may also control movements of the machining surface in up to three dimensions. The apparatus  100  comprises a machining unit  103 , a sensing unit  107 , and a signal processing unit  109 . These units  103 ,  107  and  109  of the apparatus  100  are preferably all integrated into the apparatus  100 , i.e. they are preferably disposed within a housing or enclosure of the apparatus  100 . The apparatus  100  may also comprise further units also disposed within the same housing or enclosure. 
     The machining unit  103  is configured to provide a pressurized fluid jet  104  onto the workpiece  101 , and to couple the laser beam  102  into the fluid jet  104 . In particular, the laser beam  102  is coupled into the fluid jet  104  by means of at least one optical element  105 . This at least one optical element  105  may include, for example, one or more lenses, lens assemblies, light guiding optics, beam splitters, mirrors, filters, or polarizers. The laser beam  102  is guided by the preferably thin (i.e. having a diameter in the μm range) fluid jet  104 , in principle like it would be guided in an optical fiber. The laser beam  102  is, for instance a pulsed laser beam  102  and is directed towards and onto the workpiece  101 , and can thus be used to precisely machine the workpiece  101 , while the fluid jet  104  continually cools the workpiece  101  and potentially removes debris. For instance, the apparatus  100  may specifically be configured to accurately cut or shape the workpiece  101 . 
     The laser beam  102  is provided by a laser, which may be a part of the apparatus or which may be external but couple the laser beam  102  into a laser supply port of the apparatus  100 . The laser beam  102  may be visible, and is preferably from the green spectrum. For instance, the laser beam  102  may have a wavelength of 532 nm. 
     The sensing unit  107  is arranged to receive laser-induced electromagnetic radiation  106 , i.e. an electromagnetic emission that occurs when the workpiece  101  is machined with the laser beam  102 . Thus, the laser-induced electromagnetic radiation  106  may also be referred to as “process emission”. The sensing unit  107  is arranged such that it can receive, and thus sense, the laser-induced electromagnetic radiation  106  propagating away from the workpiece  101  through the fluid jet  104  and through at least one optical element. Accordingly, the back-propagating process emission  106  can reach the sensing unit  107  guided in and by the fluid jet  104 . The at least one optical element, through which the sensing unit  107  can receive the process emission  106  can be the at least one optical element  105 , which is used to couple the laser beam  102  into the fluid jet  104 . This is exemplarily shown in  FIG. 1 . 
     The sensing unit  107  is further configured to convert the received laser-induced electromagnetic radiation  106  into a signal  108 , for instance, it provides a photo current as output signal. The sensing unit  107  may thus, for example, be a photo-detector, but it can also be any other device or light sensor that is able to convert at least the electromagnetic radiation of interest into an electrical signal. The signal  108  is provided to the signal processing unit  109  for further analysis and/or processing. 
     The signal processing unit  109  is, for example, realized by a microprocessor or computer, and is specifically configured to determine a state of machining the workpiece  101  based on the signal  108 , which it receives from the sensing unit  107 . In particular, the signal processing unit  109  is configured to determine, as the state of machine the workpiece  101 , whether the laser beam  102  has broken through the workpiece  101 . In other words, it can determine that and when the laser beam  102  breaks through the workpiece  101 . To this end, the signal processing unit  109  may apply signal processing on the signal  108 . Signal processing may include, for example, scaling, averaging, recording over time, integrating over time, or converting the signal  108 , and may include comparing the signal  108  or an integrated signal with one or more reference signals. For instance, the signal processing unit  109  may be configured to record a plurality of signals  108 , and to compare the recorded signals  108  with pre-stored reference signals. The signal processing unit  109  may alternatively or additionally be configured to integrate a plurality of signals  108  over time to produce an integrated signal, and to evaluate a pattern or a change of a pattern in the integrated signal. The plurality of signals  108  may in each case arise from laser-pulse induced electromagnetic radiation  106  sensed by the sensing unit  107 , if the laser beam  102  is a pulsed laser beam. 
     The signal processing unit  109  may then be configured to determine the state of machining the workpiece  101  based on similar or matching reference signals (in the first case) or based on the pattern or the change of the pattern (in the second case). Preferably, the signal processing unit  109  provides a temporal resolution of below 0.5 s, preferably of below 0.2 s, more preferably of below 0.1 s, since preferably the sensing unit  107  is operated with at least 10 Hz, more preferably with at least 15 Hz. 
       FIG. 2  shows an apparatus  200  according to an embodiment of the present invention, which builds on the apparatus  100  shown in  FIG. 1 . Identical elements in  FIG. 2  and  FIG. 1  are labeled with the same reference signs and function likewise. Accordingly, the apparatus  200  of  FIG. 2  includes the machining unit  103 , which is configured to provide the pressurized fluid jet  104  onto the workpiece  101 , and to couple the laser beam  102  into the fluid jet  104 . It also includes the sensing unit  107 , which is configured to receive a back-propagating laser-induced electromagnetic radiation  106  via the fluid jet  104  and through the machining unit  103 , preferably through the at least one optical element  105 , and to convert it into the signal  108 . The signal processing unit  109  (here referred to as “Digital signal processing unit+I PC”, i.e. the signal processing unit  109  may also provide inter-process communications (IPC)) is again configured to determine a state of machine the workpiece  101  based on the signal  108 .  FIG. 2  additionally shows further details about the preferred overall layout of the apparatus  200 . The apparatus  200  of  FIG. 2  especially includes several peripheral devices. 
     The apparatus  200  may include a laser source  221  and a laser controller  207  for controlling the laser source  221 . The laser source  221  is configured to supply  204  the laser light for the laser beam  102 . The laser source  221  may also be an external device not included in the apparatus  200  but for supplying  204  the laser light to a laser supply port of the apparatus  200 . In the apparatus  200  the laser beam  102  is preferably directed by an optical unit  201  of the apparatus  200 , which is preferably a beam splitter and preferably arranged in the optical path between the machining unit  103  and the sensing unit  107 , towards the machining unit  103 . In the machining unit  103 , the laser beam  102  is coupled into the fluid jet  104 . Notably, also the back-propagating laser-induced electromagnetic radiation  106  passes preferably through this optical unit  201 , but is directed towards the sensing unit  107 . 
     The apparatus  200  includes preferably also a spectral separation unit  202 , which is configured to isolate only electromagnetic radiation of interest  203  including laser induced-electromagnetic radiation  106  on the sensing unit  107 . The spectral separation unit  202  is preferably arranged in the optical path between the machining unit  103  and the sensing unit  107 , such that it can receive laser-induced electromagnetic radiation  106  travelling away from the workpiece  101  through the fluid jet  104  and through the at least one optical element  105 , and can output the radiation of interest  203 , which includes at least a part of the received laser-induced electromagnetic radiation  106 , onto the sensing unit  107 . The spectral separation unit  202  may be an optical filter unit, which may consist of one or more optical filters and is configured to filter out unwanted electromagnetic radiation, i.e. to prevent electromagnetic radiation of certain (unwanted) wavelengths from reaching the sensing unit  107 . The electromagnetic radiation of interest  203  may particularly be only the laser-induced electromagnetic radiation  106 , wherein other electromagnetic radiation that accidently reaches the spectral separation unit  202  (and would reach the sensing unit  107  without the spectral separation unit  202 ) is filtered out. The electromagnetic radiation of interest  203  may also be a part of the laser-induced electromagnetic radiation  106  that reaches the spectral separation unit  202  (and would reach the sensing unit  107  without the spectral separation unit  202 ), wherein laser-induced electromagnetic radiation that is not of interest is filtered out. If the laser-induced electromagnetic radiation is or includes secondary radiation, the spectral separation unit  202  may be configured to prevent laser light reaching the sensing unit  107 . That is, the spectral separation unit  202  may be configured to filter out light of the same wavelength than provided by the laser source  221 . There are specifically three mechanisms envisaged by the invention, which produce laser-induced electromagnetic radiation  106  of interest that can provide an accurate fingerprint of the state of machining the workpiece  101 . 
     Firstly, the laser-induced electromagnetic radiation  106  may be or include secondary radiation  206   a  that is emitted from the portion of the workpiece  101  that is machined with the laser beam  102 . The machined surface portion of the workpiece  101  may, for instance, be transformed into a plasma by the laser beam  102 , which plasma is the source of the secondary radiation  206   a . Typically, this secondary radiation  206   a  is from the yellow and/or orange spectrum. Accordingly, the spectral separation unit  202  may in this case be configured to allow light from the yellow and/or orange spectrum to reach the sensing unit  107 , while it blocks light from other parts of the spectrum, especially blocks the laser light e.g. from the green spectrum. Thus, the radiation of interest  203  may include only the secondary radiation  206   a . The sensing unit  107  can in this case be configured to be particularly sensitive to the yellow and/or orange spectrum. 
     Secondly, the laser-induced electromagnetic radiation  106  may be or include primary laser radiation  206   b  reflected from the workpiece  101 , especially from the workpiece surface. Since the laser light is preferably from the green spectrum, the spectral separation unit  202  may in this case be configured to allow light from the green spectrum to reach the sensing unit  107 , while it blocks light from other parts of the spectrum. Thus, the radiation of interest  203  may include only the secondary radiation  206   b . The sensing unit  107  can in this case be configured to be particularly sensitive to the green spectrum. 
     Thirdly, the laser-induced electromagnetic radiation  106  may be or include secondary radiation  206   c  generated by scattering of the laser beam  102  in the fluid jet  104 . Particularly, this secondary radiation  206   c  can be caused by Raman scattering of the laser beam  102 . Typically, this secondary radiation  206   c  is from the red spectrum. Accordingly, the spectral separation unit  202  may in this case be configured to allow light from the red spectrum to reach the sensing unit  107 , while it blocks light from other parts of the spectrum, especially the laser light e.g. from the green spectrum. Thus, the radiation of interest  203  may include only the secondary radiation  206   c . The sensing unit  107  can in this case be configured to be particularly sensitive to the red spectrum. 
     Notably, both the secondary radiation  206   a  and the laser reflection  206   b  originate from the workpiece  101 , particularly from the machined workpiece surface portion, while the secondary radiation  206   c  originates from the fluid jet  104 , particularly from one or several different locations along the fluid jet  104 . 
     The apparatus  200  can further include several peripheral devices, and the signal processing unit  109  is preferably configured to provide one or more of the peripheral devices with instruction signals based on the determined state of machining the workpiece  101 . In this manner, the signal processing unit  109  can control the peripheral devices in dependence of the determined state, and can for example instruct these peripheral devices to start, break, stop and/or restart their respective actions. 
     For example, the apparatus  200  shown in  FIG. 2  includes as peripheral devices a laser controller  207 , a fluid supply controller (here optionally integrated with a fluid pump)  205 , a gas supply controller  223  (here a “Protection gas controller”, since the gas is preferably used to protectively surround the fluid jet  104 ), and a movement axes controller  208  (here a “Computer Numerical Control (CNC)”), which may be configured to move in perpendicular X, Y and Z directions and/or rotational a, b, c directions a machining surface, onto which the workpiece  101  is put, or alternatively the workpiece  101  itself. 
     The signal processing unit  109  is configured to provide respective instruction signals to the different peripheral devices. Preferably, the signal processing unit  109  can control the laser controller  207  via a signal  217 , the fluid supply controller  205  via a signal  214 , the gas supply controller  223  via a signal  216 , and the movement axes controller  208  via a signal  213 . The signal processing unit  109  preferably controls the peripheral devices independently from another, and without any outside input, but only based on the signal  108  and the determined state of machining the workpiece  101 . Nevertheless, it is additionally possible that the apparatus  200  includes a human machine interface  210 , which can receive human/script input  204 , in order to provide additional instructions to the signal processing unit  109  via a signal  212 . 
     According to the instructions provided by the signal processing unit  109  based on the determined state of machining the workpiece  101 , the peripheral devices can take instructed actions. For instance, the laser controller  207  may start, pause or stop the supply of laser light for the laser beam  102 . The fluid supply controller  205  may control a fluid pressure control valve  215 , in order to start, break or stop a supply  222  of fluid, which is preferably water, to the machining unit  103 . The gas supply controller  223  can control a protection gas control valve  211 , in order to start, break or stop a supply  219  of gas, which is preferably helium, to the machining unit  103 . The movement axes controller  208  may provide specific movement of the workpiece  101 , i.e. it can control a machining surface, onto which the workpiece  101  is disposed. 
       FIG. 3  shows an apparatus  300  according to an embodiment of the present invention, which builds on the apparatus  100  shown in  FIG. 1  and the apparatus  200  shown in  FIG. 2 . Identical elements in  FIG. 3  and in  FIG. 1  and/or  FIG. 2  are labeled with the same reference signs and function likewise. In particular,  FIG. 3  illustrates more details about the optical arrangement and fluid circuitry of the apparatus  300 , particularly as provided in the machining unit  103 . The machining unit  103  may particularly include a lens  305  for coupling the laser beam  102  into the fluid jet  104 . Also back-propagating laser-induced electromagnetic radiation  106  is preferably transferred through the fluid jet  104  via this lens  305 , and then further towards the sensing unit  107 . Thereby, the radiation  106  preferably travels from the lens  305  through the optical unit  201  to the spectral separation unit  202 , and further as radiation of interest  203  including at least a part of the radiation  106  to the sensing unit  107 . 
     The machining unit  103  may also include an optically transparent protection window  301 , in order to separate the optical arrangement, here exemplarily the lens  305 , from the fluid circuitry and the region of the machining unit  103  where the fluid jet  104  is produced. For producing the fluid jet  104 , the machining unit  103  preferably includes a fluid nozzle  302 . This fluid nozzle  302  is provided with the fluid via the fluid supply  222 , which may be implemented as a channel through the housing or enclosure of the apparatus  300 . To this end, the fluid nozzle  302  includes a fluid aperture, which defines the width of the fluid jet  104 . The aperture has preferably a diameter of 10-200 μm, and the fluid jet  104  has preferably a diameter of about 0.6-1 times the fluid aperture. The pressure for the pressurized fluid jet  104  is provided via the fluid supply  222 . Preferably, the pressure is between 50-800 bar. 
     It can also be seen in  FIG. 3  that a protection gas, preferably helium, can be supplied via gas supply  219  into the machining unit  103 , particularly into a space provided within the machining unit  103 , through which the generated fluid jet  104  passes after leaving the fluid nozzle  302 . Here, the protection gas can protectively envelop the fluid jet  104 , before the fluid jet  104  exits the machining unit  103  through a lower exit aperture, and then travels further towards and onto the workpiece  101 . 
     The apparatus  300  shown in  FIG. 3  also includes focusing optics  300  preferably arranged in the optical path between spectral separation unit  202  and sensing unit  107  and for directing light on the sensing unit  107 . In particular, the laser-induced electromagnetic radiation  106 , which back-propagates through the fluid jet  104  through the lens  305 , preferably passes the optical unit  201 , then the optical separation unit  202 , and the remaining filtered electromagnetic radiation of interest  203  is focused by the focusing optics  300  onto the sensing unit  107 , particularly onto a light-sensitive area of the sensing unit  107 . The focusing optics  300  preferably includes or is at least one lens, but may also include or be another optical element like a parabolic mirror. The sensing unit  107  is configured to convert the received filtered radiation of interest  203  into the signal  108  for further analysis and/or processing by the signal processing unit  109  (which is not shown here). 
       FIG. 3  also shows that the above-described elements of the apparatus  300  may be provided within an enclosure, particularly within an optical head  303 . That is, the apparatus  300  may further comprises the optical head  303 , which may include the machining unit  103  and the sensing unit  107 . The optical head  303  may also include the optical unit  201 , the spectral separation unit  202 , and/or the focusing optics  300 . 
       FIG. 4  shows a method  400  according to an embodiment of the present invention. The method  400  includes a step  401  of providing a pressurized fluid jet  104  onto a workpiece  101  and coupling a laser beam  102  through at least one optical element  105  into the fluid jet  104  towards the workpiece  101 . Further, the method  400  includes a step  402  of receiving a laser-induced electromagnetic radiation  106  propagating away from the workpiece  101  through the fluid jet  104  and through at least one optical element, preferably the at least one optical element  105  used for coupling the laser beam  102  into the fluid jet  104 . Further, the method  400  includes a step  403  of converting the received radiation into a signal  108 . Finally, the method  400  includes a step  404  of determining a state of machining the workpiece based on the signal  108 . 
     The method  400  may be carried out by the apparatus  100 ,  200  or  300  shown in the  FIGS. 1, 2 and 3 , respectively. In particular, step  401  may be carried out by the machining unit  103 , the steps  402  and  403  by the sensing unit  107 , and step  404  by the signal processing unit  109 . 
     The method  400  may particularly include an automated time-recording of multiple signals  108 , wherein each signal  108  may be caused by one of a plurality of laser pulses. That is, the laser beam  102  is a pulsed laser beam. The laser pulses may be provided regularly and may be in the order of ns. Each signal  108  may be represented by a single numeric value or by a sequence of values as a function of time. Multiple obtained signals  108  may also be integrated over time to produce an integrated signal. The method  400  may further include an automated comparison of the obtained signals  108  with reference signals, which are read out from a reference storage e.g. from a memory of the signal processing unit  109 . The method  400  then may include evaluating a pattern or a change of a pattern in the obtained signals  108  or integrated signal, and may determine the state of machining the workpiece  101  based on said pattern or change of the pattern. Optionally, the method  400  may include the automated generation of instruction signals or instruction codes and subsequent transmission to one or more peripheral devices, so that these peripheral devices start, break, stop or restart their respective actions. 
       FIG. 5  schematically shows two specific signal patterns  500   a  and  500   b , which are preferably used in the method  400  (or by the signal processing unit  109  of the apparatus  100 ,  200 ,  300 ), in order to determine the state of machining the workpiece  101 . 
     In particular, the left-hand side signal pattern  500   a  is preferably used, if the laser-induced electromagnetic radiation  106  is or includes the above-mentioned secondary radiation  206   a  emitted from a portion of the workpiece  101  that is machined with the laser beam  102 . The signal pattern  500   a  indicates that the laser beam has broken through the workpiece  101 , when the signal  108  or the integrated signal first increases after the “Start” of machining the workpiece  101  towards an “In-Process” time section, and then decreases again after the “In-Process” time section. When the signal  108  or integrated signal falls below a determined threshold value, the “Finish” of the process is reached, i.e. it can be determined that the laser beam broke through the workpiece  101 . 
     Likewise, the right-hand signal pattern  500   b  is preferably used, if the laser-induced electromagnetic radiation  106  is or includes primary laser radiation  206   b  reflected from the workpiece  101 . The signal pattern  500   b  indicates that the laser beam has broken through the workpiece  101 , when the signal  108  or the integrated signal first decreases after the “Start” of machining the workpiece  101  towards an “In-Process” time section, and then increases again from the “In-Process” time section. When the signal  108  or integrated signal rises above a determined threshold value, the “Finish” is reached, i.e. the laser beam broke through the workpiece  101 . 
       FIG. 6  shows an apparatus  200  according to an embodiment of the present invention, which builds on the apparatus  200  shown in  FIG. 2 . Identical elements in  FIG. 2  and  FIG. 6  are labeled with the same reference signs and function likewise. 
       FIG. 6  shows in particular that the apparatus  200  may further include an enclosure, particularly and optical head  600 , which may include the machining unit  103  and the sensing unit  107 . The optical head  600  may consequently include also the optical unit  201  and the spectral separation unit  202 . As described already with respect to  FIG. 2 , the laser light for the laser beam  102  may be provided  204  by a laser unit  601  (comprising laser source  201  and/or laser controller  207 ), for instance via an optical fiber and to a laser supply port of the apparatus  200  (if the laser unit  601  is not part of the apparatus  200 ) or to the optical head  600 , e.g. to a laser supply port of the optical head  600  (if the laser unit  601  is part of the apparatus  200 ). 
       FIG. 7  shows an apparatus  700  according to an embodiment of the invention, which builds on the apparatus  100  shown in  FIG. 1  and has elements in common with the apparatus  200  of  FIG. 2  and  FIG. 6 . Identical elements in  FIG. 7  and in  FIGS. 1, 2 and/or 6  are labeled with the same reference signs and function likewise. 
     The apparatus  700  of  FIG. 7  has a different configuration than the apparatus  200  shown in  FIG. 6 . In particular, the apparatus  700  comprises an optical head  701 , which includes the machining unit  103 . However, the optical head  701  does not comprise the sensing unit  107 . The sensing unit  107  is provided in a laser unit  703  (including laser source  221  and/or laser controller  207 ). Notably, the laser unit  703  may be formed by the laser controller  207  being provided in the laser source  221  or vice versa. The laser unit  703  is part of the apparatus  700 . The machining unit  103  in the optical head  701  is configured to couple the laser beam  102  into the fluid jet  104 . The sensing unit  107  in the laser unit  703  is configured to receive the laser-induced electromagnetic radiation  106  (e.g. secondary radiation  206   a  emitted from a portion of the workpiece  101  that is machined with the laser beam  102 , or primary laser radiation  206   b  reflected from the workpiece  101 , or secondary radiation  206   c  generated by scattering of the laser beam  102  in the fluid jet  104 ). 
     The apparatus  700  comprises also an optical connection element  702 , particularly an optical fiber  702 , optically connecting the optical head  701  and the laser unit  703 . The laser unit  703  is configured to provide the laser light for the laser beam  102 , which is transported through the optical connection element  702  to the optical head  701 , e.g. to a supply port of the optical head  701 , where it is further provided to the machining unit  103 , which couples it into the fluid jet  104 . Laser-induced electromagnetic radiation  106  is received via the fluid jet  104  by the machining unit  103 , and is further provided through the optical head  701  and through the optical connection element  702  to the laser unit  703 , e.g. to a supply port of the laser unit  703 , where it is further provided to the sensing unit  107 , which converts it into the signal  108 . 
     The laser unit  703  may accordingly comprise also the optical unit  201 , which is preferably a beam splitter and is preferably arranged in the optical path between laser source  222  and optical connection element  702 , in order to provide the laser light for the laser beam  102  from the laser source  221  to the optical connection element  702 . Also the back-propagating laser-induced electromagnetic radiation  106  may pass through this optical unit  201 , but is then directed towards the sensing unit  107 . The laser unit  703  may also include the spectral separation unit  202 , which is configured to isolate only electromagnetic radiation of interest  203  including laser induced-electromagnetic radiation  106  on the sensing unit  107 . The spectral separation unit  202  is preferably arranged in the optical path between the optical unit  201  and the sensing unit  107 , such that it receives the laser-induced electromagnetic radiation  106  from the optical unit  201  and provides the radiation of interest  203  onto the sensing unit  107 . 
     In summary, the present invention provides an apparatus  100 ,  200 ,  300  and  700 , and a method  400 , that enable machining a workpiece  101  with a laser beam  102  coupled into a fluid jet  104 , wherein due to accurate determination of a state of the machining process, the machining process time can significantly be reduced. 
     The present invention has been described in conjunction with various embodiments as examples as well as implementation forms. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, the description and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.