Patent Publication Number: US-2020298336-A1

Title: Apparatus for Measuring a Fluid Jet Guiding a Laser Beam

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for machining a workpiece with a high-intensity laser beam coupled into a pressurized fluid jet. According to the invention, the apparatus is particularly configured to measure the fluid jet guiding the laser beam. The invention relates also to a method for measuring a fluid jet guiding a high-intensity laser beam, wherein the laser beam is suitable for machining a workpiece. The invention is specifically concerned with measuring a length and/or flow characteristics of the fluid jet based on a laser-induced secondary emission. 
     BACKGROUND 
     A conventional apparatus for machining a workpiece with a laser beam coupled into a pressurized fluid jet is generally known. In order to machine the workpiece with the laser beam, the fluid jet is usually generated with a fluid jet generation nozzle, and the laser beam is coupled into and guided in the fluid jet onto the workpiece by means of total internal reflection. 
     A problem typically encountered in the conventional apparatus is that the fluid jet is only laminar over a certain absolute length from the fluid jet generation nozzle. Beyond that length, the fluid jet becomes instable and finally disperses into droplets. Once the fluid jet becomes instable, the fluid is not anymore able to guide the laser beam such that the workpiece can be machined efficiently. When the fluid disperses into droplets, the laser beam is even scattered. 
     Notably, in this document the term “fluid jet” means the laminar fluid jet. After becoming instable, the fluid may still propagate in a continuous liquid flow, before it disperses into droplets. Further, a “usable” length of the fluid jet may be shorter than its “absolute” length, since only the free-flowing fluid jet, after being output from the apparatus, is usable for machining a workpiece. 
     Accordingly, for an efficient machining process, the workpiece has to be positioned close enough to the apparatus, so that it is impinged by the usable portion of the fluid jet. 
     If the usable length of the fluid jet becomes too short, an efficient machining process may thus not be possible. Further, a very short fluid jet, or the complete absence of a fluid jet, may indicate a graver problem with the apparatus, for instance a broken fluid jet generation nozzle. 
     Additionally, also flow characteristics of the fluid jet may influence the efficiency of the workpiece machining process. 
     In view of the above, it would be of great advantage—before actually machining a workpiece—to determine a usable length of the fluid jet. Further, it would be even more advantageous, if also flow characteristics of the fluid jet, like its laminar behavior or perturbations in the fluid, could be determined. Unfortunately, the conventional apparatus does not allow any inherent measurement of the usable length of the fluid jet. External measuring devices could be used, but are typically inefficient since not being specifically designed for the case at hand, namely for measuring a high-intensity laser beam coupled into a very thin fluid jet (15-500 μm). 
     Therefore, the present invention aims at improving the conventional apparatus and fluid jet measuring solutions. It is accordingly an object of the invention to provide an apparatus and method for measuring a fluid jet guiding a high-intensity laser beam suitable for machining a workpiece. In particular, a length of the fluid jet should be determined. Additionally, flow characteristics of the fluid jet should be derived. Another goal of the invention is to enable a simple measurement of the laser power introduced by means of the laser beam into the fluid jet. 
     Thereby, the invention aims particularly for a simple but precise and non-invasive solution for carrying out said measurements. In particular, neither a complicated measuring setup should be necessary, nor should a post-processing of the measurement results require much time and computational resources. All measurements should further be performable by the apparatus itself, wherein nevertheless a compact apparatus is desired. 
     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 invention proposes determining a usable and/or absolute length of the fluid jet, and optionally detecting flow characteristic of the fluid jet, based on a laser-induced secondary emission, i.e. based on a secondary electromagnetic radiation generated by an interaction of the laser beam with the fluid jet. 
     A first aspect of the invention provides an apparatus for machining a workpiece with a high-intensity laser beam, the apparatus being configured to provide a pressurized fluid jet and to couple the laser beam into the fluid jet, wherein the apparatus comprises a detection unit configured to receive and detect secondary radiation generated by the laser beam in the fluid jet, the detection unit including a sensing unit configured to convert secondary radiation into a detection signal, wherein the apparatus is configured to generate, with the detection unit, a plurality of detection signals at a single position or at different positions along the fluid jet. 
     A “high-intensity” laser beam is a laser beam suited for machining a workpiece, wherein the workpiece may be a made of a material including, for example, metals, ceramics, diamonds, semiconductors, alloys, superalloys, or ultra-hard materials. Thereby, “machining” the workpiece means at least cutting, drilling or shaping the workpiece. The high-intensity laser beam has a laser power of between 20-400 W or even more. The laser beam may thereby be a pulsed laser beam, but can also be a continuous laser beam. A pressure of the “pressurized” fluid jet is preferably between 50-800 bar. 
     The “length” of the fluid jet may be its usable length starting from a position where it is output from the apparatus, or may be its absolute length starting from a position where it is generated. Having the usable length directly yields the absolute length and vice versa, since the apparatus configuration is known. 
     The term “along the fluid jet” means along a propagation direction of the fluid jet, or along a direction in which the fluid jet would propagate if it was generated (i.e. its potential propagation direction). The (potential) propagation direction of the fluid jet is well determined by the configuration of the apparatus, particularly by the configuration and orientation of the parts generating the fluid jet, for example, a fluid jet generation nozzle. The generated fluid jet is pressurized enough to propagate linearly, so that the propagation direction of the fluid jet can also be extrapolated beyond its stable length. Accordingly, different positions along the fluid jet can also be selected, if there is no fluid jet present at one or more of these positions. 
     For generating the plurality of detection signals from a single position, the detection unit may be stationary relative to the parts of the apparatus that generate the fluid jet, for instance, to a fluid jet generation nozzle. Each one of the generated plurality of detection signals can emphasize secondary radiation, which is received from a different portion of the fluid jet, and thus arrives at the sensing unit at a different angle of incidence. This angle of incidence can be taken into account by the sensing unit for generating the plurality of detection signals from the stationary position. 
     For generating the plurality of detection signals from different positions along the fluid jet, the detection unit may be movable along the fluid jet relative to the parts of the apparatus that generate the fluid jet, for instance, the fluid jet generation nozzle. 
     The detection unit is preferably positioned such that the sensing unit can detect at least a part of the secondary electromagnetic radiation that is induced by the laser beam in the fluid jet, and that propagates away from the fluid jet in all directions. Notably, some of the laser-induced secondary radiation may travel elsewhere and not into the detection unit. 
     The secondary radiation received by the detection unit provides an accurate indication of whether a laminar fluid jet is present at a given position along the fluid jet or not. In particular, the signal produced by the sensing unit shows a characteristic behavior depending on whether a laminar fluid jet exists at the given position or not. In fact, the secondary radiation is preferably only generated in such a fluid jet, but not in any continuous flow of fluid or even in fluid droplets. Thus, the secondary radiation can be used to accurately determine the length of the usable fluid jet. Additionally, the secondary radiation may also allow accurately determining flow characteristics of the fluid jet. 
     Of note, the secondary radiation, which is able to provide the indication of the length of the fluid jet and optionally the fluid jet flow characteristics, is only generated with a high-intensity laser beam, as necessarily used in an apparatus for machining a workpiece. For instance, a conventional laser pointer device would not generate this secondary radiation in the fluid jet. 
     The idea of employing the secondary radiation to measure the fluid jet, leads particularly to a simple but precise solution. Further, the apparatus can be compact, although all of its components may advantageously be integrated. The apparatus can carry out the measurements itself, i.e. without requiring external equipment. 
     In a preferred implementation form of the apparatus, the detection unit further includes a spectral separation unit configured to isolate at least a part of the received secondary radiation onto the sensing unit. 
     Thus, radiation of interest, which is or includes the at least part of the secondary radiation, can be separated from undesired radiation that would potentially also impinge on the sensing unit, if no spectral separation unit was present. In particular, the spectral separation unit is arranged and configured to receive radiation, which includes the secondary radiation propagating away from the fluid jet, may isolate the radiation of interest from the received radiation, and may provide the radiation of interest to the sensing unit. The spectral separation unit thus prevents that undesired radiation reaches the sensing unit. Such undesired radiation could be ambient light, laser light or other laser-induced secondary radiation not of interest (or of any higher order). When using the spectral separation unit, the detection signals more accurately reflect the radiation of interest, and can thus provide an even more precise indication of whether and where (i.e. at what position(s)) the fluid jet is present. 
     In a further preferred implementation form of the apparatus, the spectral separation unit includes an optical filter, a prism, a dielectric mirror, a diffraction grating, or a multiple aperture optical setup. 
     In a further preferred implementation form of the apparatus, the detection unit is stationary and is configured to observe, from its stationary position, a determined length section along the fluid jet, and the apparatus is configured to generate, with the detection unit, the plurality of detection signals at the stationary position of the detection unit. 
     This specific implementation form allows measuring the fluid jet without a relative movement between the parts of the apparatus that generate the fluid jet and the detection unit. This makes the setup of the apparatus easier. The detection unit has preferably a large or even an unlimited aperture, so as to be able to receive radiation coming from the fluid jet over a large range of angles of incidence. Thus, the detection unit is able to observe at least the determined length section along the fluid jet, preferably even the entire length of an ideal fluid jet (i.e. the maximum length possible for the fluid jet). The sensing unit can generate the plurality of detection signals, for instance, with a plurality of sensing elements arranged at different positions, preferably different positions along the fluid jet. These pluralities of detection signals provide the indication, where along the fluid jet the secondary radiation is generated. Thus, a length of the fluid jet can be determined with high precision. 
     In a further preferred implementation form of the apparatus, the sensing unit is a charge-coupled device or a spatial array of multiple photodiodes, thermal diodes or avalanche diodes (or any other photo detector suitable). 
     The spatial arrangement of multiple such diodes allows generating the plurality of detections signals. For instance, one detection signal per diode may be generated, such that the detection signal provides an indication about the secondary radiation generated along the fluid jet, specifically over the determined length section along the fluid jet, which can be observed by the sensing unit. The sensing unit of this implementation form is advantageous for a stationary detection unit. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises a motion unit configured to move the detection unit along the fluid jet, wherein the detection unit includes an observation unit arranged to admit secondary radiation propagating towards the sensing unit, and the apparatus is configured to generate, with the detection unit, the plurality of detection signals at different positions along the fluid jet. 
     This specific implementation form allows measuring the fluid jet with a relative movement between the parts of the apparatus that generate the fluid jet and the detection unit, namely by moving the detection unit. Notably, the detection unit being movable along the fluid jet does not mean that its direction of movement is parallel to the propagation direction of the fluid jet. The direction of movement of the detection unit can also be at an angle to the propagation direction of the fluid jet. The movement direction of the detection unit must not even be straight. This is because any angular displacement to the propagation direction of the fluid jet can be easily corrected, e.g. by signal processing of the plurality of detection signals. Of course, the direction of movement of the detection unit can also be parallel to the propagation direction of the fluid jet. Again, as mentioned above, the propagation direction of the fluid jet does not depend on or require the presence of a fluid jet, but is determined by the configuration of the apparatus. 
     The motion unit is preferably configured to generate one detection signal for each different position along the fluid jet. However, it is also possible that it is configured to generate multiple detection signals for one and the same position along the fluid jet. 
     The observation unit preferably limits the aperture of the detection unit, in order to increase the spatial resolution of sensing radiation along the fluid jet. A detection signal can thus more precisely reflect the secondary radiation generated in the fluid jet at a given position along the fluid jet. 
     In a further preferred implementation form of the apparatus, the detection unit is configured to continuously or repeatedly detect secondary radiation and thereby generate the plurality of detection signals, while being moved by the motion unit along the fluid jet. 
     In this manner, a precise measurement of the fluid jet, i.e. of the secondary radiation generated in the fluid jet along the fluid jet, can be carried out. 
     In a further preferred implementation form of the apparatus, the motion unit is configured to move the detection unit over at least a determined distance between a first reference point and a second reference point along the fluid jet. 
     The determined distance should be at least as large as the length of a fluid jet that is necessary to machine a workpiece efficiently. The first reference point is preferably as close as possible to the parts of the apparatus that generate and/or output the fluid jet. Most preferably, the first reference point is at a fluid exit aperture or nozzle of the part of the apparatus, in which the fluid jet is generated. 
     In a further preferred implementation form of the apparatus, the determined distance is between 0-25 cm, preferably is between 0-15 cm. 
     This allows a large enough measuring range, longer even than the length of an ideal fluid jet. 
     In a further preferred implementation form of the apparatus, the motion unit is configured to move the detection unit stepwise along the fluid jet with a spatial resolution of less than 2 mm, preferably of between 10 μm-2 mm. 
     In this manner, a very precise and high-resolution measurement of the fluid jet, i.e. of the secondary radiation generated along the fluid jet, is possible. 
     In a further preferred implementation form of the apparatus, the observation unit is an opening or tele-centric lens defining an aperture. 
     The opening is, for example, realized as a slot, which preferably extends perpendicular to the propagation direction of the fluid jet. That is, a horizontal slot if the fluid jet propagates along the vertical direction. The (limited) aperture improves the spatial resolution of the measurements of the secondary radiation generated along the fluid jet. 
     In a further preferred implementation form of the apparatus, an optical resolution of the detection unit along the fluid jet is defined by the size of the aperture and a distance between the observation unit and the fluid jet, and the size of the aperture and said distance are selected such that the optical resolution of the detection unit is equal to or higher than the spatial resolution of the motion unit. 
     Thus, the accuracy of the measurements along the fluid jet is not limited by the optical resolution, and can be carried out very accurately with a precise linear motion unit, for instance, having the above-mentioned spatial resolution of less than 2 mm. 
     In a further preferred implementation form of the apparatus, the sensing unit includes a photodiode, thermal diode or an avalanche diode (or any other photo detector suitable). 
     Accordingly, simple and rather inexpensive parts can be used for the sensing unit, in order to realize the detection unit. The sensing unit of this implementation form is advantageous for a movable detection unit. 
     In a further preferred implementation form of the apparatus, the detection unit further includes a protection unit for protecting the observation unit from ingress of fluid, humidity, dust and further products of laser beam machining. 
     Accordingly, the lifetime of the detection unit is increased, the detection unit has to be cleaned less often, and is able to provide more reliable measurements. 
     In a further preferred implementation form of the apparatus, the protection unit includes a unit configured to produce an overpressure within at least the observation unit of the detection unit. 
     The overpressure prevents that unwanted machining process products and/or fluid enter the observation unit. Even if some unwanted products or fluid should enter, the overpressure produced by the unit will again expel them from the observation unit. 
     In a further preferred implementation form of the apparatus, the protection unit includes a transparent window covering the observation unit towards the fluid jet. 
     Notably, the window is preferably transparent at least for the secondary radiation that is of interest. It may not be transparent to all incoming radiation, and can thus additionally act as a spectral separation unit (similar as described above). Preferably, the transparent window is provided with at least one flap, in order to selectively open and close it for access to the detection unit. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises a movable machining unit configured to provide the pressurized fluid jet and to couple the laser beam into the fluid jet, wherein the detection unit is stationary and includes the sensing unit and an observation unit arranged to admit secondary radiation propagating towards the sensing unit, and the apparatus is configured to move the machining unit, in order to generate, with the detection unit, the plurality of detection signals at different positions along the fluid jet. 
     This specific implementation form allows measuring the fluid jet with a relative movement between the parts of the apparatus that generate the fluid jet and the detection unit, namely by moving said parts, for instance, the fluid jet generation nozzle or a machining unit including said nozzle. Otherwise, this implementation form works in a similar manner as the specific implementation form with a movable detection unit described above. Of course, it is also possible to make both the detection unit and the machining unit movable. 
     In a further preferred implementation form of the apparatus, the detection unit further includes at least one optical element or assembly arranged between the observation unit and the sensing unit. 
     This element or assembly can be used to shape or change the direction of the admitted secondary radiation. For instance, if the aperture of the observation unit is relatively small, in order to increase the optical resolution of the detection unit, said element or assembly can disperse the received radiation onto a spectral separation unit or the sensing unit. Alternatively, the element or assembly can focus the received radiation if necessary. Accordingly, the measurement efficiency and performance of the detection unit can be further improved. 
     In a further preferred implementation form of the apparatus, the secondary radiation is electromagnetic radiation generated by inelastic scattering and/or fluorescence of the laser beam in the fluid jet. 
     Inelastic scattering is particularly Raman scattering of the laser beam in the fluid jet, and is the preferred laser-induced secondary radiation for measuring the fluid jet. 
     In a further preferred implementation form of the apparatus, the secondary radiation is laser light scattered in the fluid jet. 
     Scattering of the laser beam is possible, if the total internal reflection condition is not fulfilled, due to any fluid jet imperfection. Accordingly, an indication for the length of the fluid jet able to provide internal reflection is provided by this secondary radiation. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises a processing unit configured to determine a length of the fluid jet based on the plurality of detection signals received from the sensing unit. 
     The processing unit can process the generated detection signals, and can evaluate where (i.e. at which position(s)) secondary radiation is generated along the fluid jet and preferably also in what amount (i.e. its intensity). From this information, the processing unit can precisely determine a fluid jet length, particularly the usable fluid jet length. The processing unit may then use the obtained information to instruct other units of the apparatus to perform specific actions. For instance, the processing unit could control a laser unit generating the laser beam to stop, if there is no fluid jet or it could control a pressure of the fluid supplied for generating the pressurized fluid jet, if the fluid jet length is not sufficient. Further, it could send a signal to the operator. 
     In a further preferred implementation form of the apparatus, the apparatus further comprises a processing unit configured to determine, based on the plurality of detection signals received from the sensing unit, a power of the laser beam coupled into the fluid jet and/or at least one flow characteristic of the fluid jet. 
     The amount and distribution of the secondary radiation along the fluid jet provides information about the laser power that is coupled by the laser beam into the fluid jet. Typically, not all of the nominal laser power provided by a laser unit for the laser beam necessarily couples into the fluid jet. However, it is advantageous for providing an efficient machining process to determine, how much of the nominal laser power is guided in the fluid jet onto the workpiece. Usually, such measurements are conducted with external power meters or the like. In comparison, the measurement of the secondary radiation and the further determination of the laser power in the fluid jet from the secondary radiation is faster and more efficient. 
     The secondary radiation may show also a characteristic behavior depending on flow characteristics of the fluid jet. For instance, the less perturbed the fluid jet, the more homogeneous the secondary radiation may be generated along the fluid jet. Accordingly, detecting the secondary radiation provides also a useful tool for evaluating these characteristics within the fluid jet, in addition to the length measurement. 
     A second aspect of the present invention provides a method for measuring a pressurized fluid jet guiding a high-intensity laser beam for machining a workpiece, the method comprising providing the fluid jet and coupling the laser beam into the fluid jet, receiving and detecting, with a detection unit, secondary radiation generated by the laser beam in the fluid jet, wherein the detecting includes, converting, with a sensing unit, secondary radiation into a detection signal, and generating, with the detection unit, a plurality of detection signals at a single position or at different positions along the fluid jet. 
     With the method of the second aspect, the same advantages and effects can be achieved as described above for the apparatus of the first aspect. 
     Notably, a method step carried out “with” some unit particularly means that the method step is carried out “by” that unit. 
     In a preferred implementation form of the method, the method further comprises moving the detection unit along the fluid jet, in order to generate the plurality of detection signals at different positions along the fluid jet. 
     This implementation form accordingly achieves the same advantages as described above for the apparatus with movable detection unit. As for the apparatus, of course also for the method it is alternatively possible to relatively move the detection unit along the fluid jet, by moving the fluid jet, i.e. moving a component that generates the fluid jet. 
     In a further preferred implementation form of the method, the method further comprises, defining, with a processing unit, a first reference value, generating, with the detection unit, a first detection signal at a first position along the fluid jet, comparing, with the processing unit, the first detection signal with the first reference value, and generating an alarm and/or interrupting the method, if the first detection signal is below the first reference value. 
     The first position is preferably a referenced position, i.e. its distance to the point of generation of the fluid jet is known. Preferably, the first position coincides with the first reference point mentioned above. The first reference value is thus used as an emergency alarm or stop. The situation that the first detection signal, which is preferably obtained from as close to the exit aperture or exit nozzle for outputting the fluid jet as possible, is therefore capable to serve as an indicator of one and/or several problems, e.g. a broken fluid jet generation nozzle. The fluid jet does in this case not have any usable length. Notably, those implementation forms of the apparatus, in which the plurality of detection signals are obtained at different positions along the fluid jet, are configured to perform this implementation form of the method. 
     In a further preferred implementation form of the method, the method further comprises defining, with the processing unit, a second and/or third reference value, generating, with the detection unit, a further detection signal at a further position along the fluid jet, comparing, with the processing unit, the further detection signal with a first product of the first detection signal and the second reference value and/or comparing the further detection signal with a second product of the first detection signal and the third reference value, determining the length of the fluid jet based on the distance between the first position and the further position, if the further detection signal is smaller than the first product or larger than the second product, and repeating the obtaining and comparing steps, if the further detection signal is equal to or larger than the first product and/or equal to or smaller than the second product. 
     If the further detection signal is smaller than the first product or larger than the second product, the fluid jet cannot be longer than a distance of the further position from the origin of the fluid jet, for instance, from the fluid jet generation nozzle. Since the first positions is preferably a known position, for instance coinciding with the above-mentioned first reference point of which the distance to the origin of the fluid jet is known, the usable fluid jet length can be determined. In this way, by using the second and/or third reference values, a precise length measurement of particularly the usable fluid jet length is enabled. The measurement and processing of the results is simple and fast. Notably, those implementation forms of the apparatus, in which the plurality of detection signals are obtained at different positions along the fluid jet, are configured to perform this implementation form of the method. 
     In a further preferred implementation form of the method, the second reference value is between 5-95%, preferably between 20-80% and/or the third reference value is between 105-300%, preferably between 140-260%. 
     These optimized reference values provide a robust but precise method. 
    
    
     
       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 an apparatus according to an embodiment of the present invention. 
         FIG. 5  shows an apparatus according to an embodiment of the present invention. 
         FIG. 6  shows an apparatus according to an embodiment of the present invention. 
         FIG. 7  shows a method 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. The apparatus  100  is configured to machine a workpiece with a high-intensity laser beam  101  coupled into a pressurized fluid jet  102 . To this end, the apparatus  100  is configured to provide the fluid jet  102 , and to couple the laser beam  101  into the fluid jet  102 . The laser beam  101  may be pulsed or continuous. During the machining process, the workpiece may 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 is able to machine the workpiece disposed on the machining surface. The apparatus  100  may thereby control movements of the machining surface in up to three dimensions. 
     However, the apparatus  100  of the invention is particularly for measuring the fluid jet  102  guiding the laser beam  101 . Accordingly, the components of the apparatus  100  required for this purpose are shown in  FIG. 0.1 . In particular, the apparatus  100  comprises a detection unit  103 , which includes a sensing unit  105 . 
     The detection unit  103  is configured to receive and detect secondary radiation  104  generated by the laser beam  101  in the fluid jet  102 . The laser beam  101  induces the secondary radiation  104  particularly by interacting with the fluid of the fluid jet  102 , and advantageously only in the laminar fluid jet  102  but not in an unstable liquid flow or droplets. That is, the secondary radiation  104  is generated along the entire length of the fluid jet  102 . The generated secondary radiation  104  propagates away from the fluid jet  102  in all directions, as is indicated in  FIG. 1 . The detection unit  103  is accordingly arranged to receive at least a part of all the secondary radiation  104  generated in the fluid jet  102 . 
     The sensing unit  105  is configured to convert secondary radiation  104  into a detection signal  106 . The converted secondary radiation  104  may be all secondary radiation  104  received by the detection unit  103 , or may be a part of the received secondary radiation  104 . The detection signal  106  is preferably an electrical signal. The sensing unit  105  is able to produce a plurality of detection signals  106 , for instance, every time it receives secondary radiation  104 . This could be the case, if the laser beam  101  is pulsed. The sensing unit  105  may in this case convert the secondary radiation  104  generated by each laser pulse into at least one detection signal  106 . However, the sensing unit  105  may also be able to generate multiple detection signals  106  in determined time intervals. That is, even when the laser beam  101  is not pulsed but continuous, the sensing unit  105  may constantly receive secondary radiation  104  and convert it into a plurality of detection signals  106 , each detection signal  106  at a different point in time. The sensing unit  105  may also produce multiple detection signals  106  concurrently, for instance, with a plurality of sensing elements it includes, wherein each sensing element is configured to convert secondary radiation  104  into a detection signal  106 . 
     The apparatus  100  is specifically configured to generate, with the detection unit  103 , a plurality of detection signals  106  at a single position or at different positions along the fluid jet  102 . That is, the detection unit  103  may be movable relative to the fluid jet  102 , and the sensing unit  105  may produce at least one detection signal  106  each determined time interval and/or after each step of movement. The detection unit  103  may also be stationary with respect to the fluid jet  102 , and the sensing unit  105  may produce a plurality of detection signals  106  each determined time interval and/or simultaneously with a plurality of sensing elements. 
     In each case, the plurality of detection signals  106  derived from the secondary radiation  104  provide an indication of the usable length of the fluid jet  102  and potentially the flow characteristics of the fluid jet  102 . 
       FIG. 2  shows an apparatus  100  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  100  of  FIG. 2  also comprises the detection unit  103 , which is configured to receive and detect secondary radiation  104  coming from the fluid jet  102  guiding the laser beam  101 . Particularly, it also comprises the sensing unit  105 , which is configured to convert the received secondary radiation  104  into the plurality of detection signals  106 . 
     The apparatus  100  shown in  FIG. 2  further includes a motion unit  201 , which is configured to move the detection unit  103  along the fluid jet  102 . In particular, the motion unit  201  is configured to move the detection unit  103  over at least a determined distance A, preferably between a first reference point A 0  and a second reference point A 1 , along the fluid jet  102 . The determined distance A is preferably 0-25 cm, more preferably between 0-15 cm. The determined distance A is obtained in the propagation direction of the fluid jet  102 . Notably, the motion unit  201  is also configured to move the detection unit  103  along that determined distance A, if there is no fluid jet  102 . The movement direction applied by the motion unit  201  is indicated in  FIG. 2 , and is schematically shown to be parallel to the determined distance A, i.e. to the propagation direction of the fluid jet  102 . However, such a parallel alignment is in practical implementations possible, but not necessary. Notably, the propagation direction of the fluid jet  102  is in use of the apparatus  100  often along the vertical direction to be directed onto an at least horizontally movable workpiece. However, since the fluid jet  102  is pressurized, it can also propagate at an angle to the vertical direction, or even along the horizontal direction, without becoming significantly non-linear. The motion unit  201  may be particularly configured to move the detection unit  103  stepwise along the fluid jet  102 , with a spatial resolution of less than 2 mm, preferably of between 10 μm-2 mm. Alternatively, the motion unit  201  may also be instructed to move the detection unit  103  continuously along the fluid jet  102 . 
     The apparatus  100  of  FIG. 2  is accordingly configured to generate, with the detection unit  103  including the sensing unit  105 , the plurality of detection signals  106  at different positions along the fluid jet  102 . Preferably, the detection unit  103  is configured to continuously or repeatedly detect secondary radiation  104  and thereby generate the plurality of detection signals  106 , while being moved by the motion unit  201  along the fluid jet  102 . The sensing unit  105  is or includes preferably a photodiode, thermal diode or an avalanche diode. It is also possible for the sensing unit  105  to be a power-meter or a spectrometer. 
     Beneficial for the movable detection unit  103  is that it also includes an observation unit  200 , which is configured to admit secondary radiation  104  received from the fluid jet  102  propagating towards (in direction of) the sensing unit  105 . The observation unit  200  may be an opening, like a slot or a tele-centric lens which define an aperture  202 . The aperture  202  limits the angle of incidence at which the detection unit  103  can receive secondary radiation  104  from the fluid jet  102 . Accordingly, the observation unit  200  increases the optical resolution along the fluid jet  102 . 
     For instance, the aperture  202  may have a size (diameter or a slot with an opening) d along the fluid jet  102 . An optical resolution of the detection unit  103  along the fluid jet  102  is then defined by the size d of the aperture  202  and a distance l between the observation unit  200  and the fluid jet  102 . The size d of the aperture  202  and said distance l are preferably selected such that the optical resolution of the detection unit  103  is equal to or higher than the spatial resolution of the motion unit  201 . As an example, the size d may be a slot with 1-5 mm width, preferably 1.5 mm width, and 5-10 mm length. Alternatively, it may be a diameter of 1-5 mm, preferably 1.5 mm. The distance l may be between 5-30 mm, preferably between 10-15 mm. 
       FIG. 3  shows an apparatus  100  according to an embodiment of the present invention, which builds on the apparatus  100  shown in  FIG. 2 . Identical elements in  FIG. 3  and in  FIG. 2  are labeled with the same reference signs and function likewise. In particular, the apparatus  100  in  FIG. 3  has also a motion unit  201 , and thus also a movable detection unit  103 . Compared to  FIG. 2 , further advantageous components of the apparatus  100  are shown in  FIG. 3 . These additional components are optional, and can be added to the apparatus of  FIG. 2  individually or in any combination. 
     In particular, the apparatus  100  of  FIG. 3  further includes a spectral separation unit  303 , which is configured to isolate at least a part of the secondary radiation  104  received by the detection unit  103  onto the sensing unit  105 . The spectral separation unit  303  is thus preferably arranged in the optical path between the fluid jet  102  and the sensing unit  105 , such that it receives secondary radiation  104  travelling away from the fluid jet  102  through the observation unit  200 , and outputs radiation of interest—which includes at least the part of the secondary radiation  104 —onto the sensing unit  105 . The spectral separation unit  303  may be an optical filter unit, which may consist of one or more optical filters and is configured to filter out undesired electromagnetic radiation. That is, it is configured to prevent electromagnetic radiation of certain (undesired) wavelengths from reaching the sensing unit  105 . Other electromagnetic radiation that accidently reaches the spectral separation unit  303 —and would reach the sensing unit  105  without the spectral separation unit  303 —is accordingly filtered out. 
     The spectral separation unit  303  may, for instance, be configured to prevent laser light reaching the sensing unit  105 . That is, the spectral separation unit  303  may be configured to filter out light of the same wavelength than provided by a laser unit generating the laser beam  101 . Further, also laser-induced secondary radiation that is not of interest may be filtered out. There may even be different mechanisms producing secondary radiation  104  that in principle provides an indication about the usable fluid jet length, but only secondary radiation  104  attributed to one specific mechanism is currently of interest. In this case, the spectral separation unit  303  may filter out secondary radiation currently not of interest. 
     The secondary radiation  104  may be electromagnetic radiation generated by inelastic scattering of the laser beam  101  in the fluid jet  102 . That is, it may be radiation caused by Raman scattering of the laser beam  101 , which is typically shifted to longer wavelengths compared to the wavelength of the initiating laser light. For instance, this secondary radiation  104  is from the red spectrum, if the laser light is from the green spectrum. Accordingly, the spectral separation unit  303  may in this case be configured to allow light from the red spectrum to reach the sensing unit  105 , while it blocks light from other parts of the spectrum, especially the laser light from the green spectrum. Thus, only the secondary radiation  104  may reach the sensing unit  105 . Also the sensing unit  105  can in this case be configured to be particularly sensitive to the red spectrum. As an example, the laser light may be at 532 nm, and the bandpass of the spectral separation unit  303  may here be 600-700 nm, preferably 630-670 nm. 
     Secondly, the secondary radiation  104  may be fluorescence of the laser beam  101  in the fluid jet  102 . Accordingly, the spectral separation unit  303  may be configured to allow light from the fluorescence spectrum to reach the sensing unit  105 , while it blocks light from other parts of the spectrum, especially the laser light e.g. from the green spectrum or a secondary radiation generated from Raman scattering of the laser light. The sensing unit  105  can in this case be configured to be particularly sensitive to the fluorescence spectrum. The fluorescence spectrum may, for instance, be in the yellow range in case of a green laser, particularly between 560-640 nm. 
     Thirdly, the secondary radiation  104  may be laser light scattered in the fluid jet  102 . Since the laser light is preferably from the green spectrum, the spectral separation unit  303  may in this case be configured to allow light from the green spectrum to reach the sensing unit  105 , while it blocks light from other parts of the spectrum. The sensing unit  105  can in this case be configured to be particularly sensitive to the green spectrum. For example, for laser light at 532 nm, the bandpass of the spectral separation unit  303  may here be 500-600 nm, preferably 510-550 nm. 
     The detection unit  103  of the apparatus  100  of  FIG. 3  has further a protection unit  301  for protecting the observation unit  200  from ingress of fluid, humidity, dust and/or further products of laser beam machining. The protection unit  301  may be or include a unit configured to produce an overpressure within at least the observation unit  200  of the detection unit  103 . The protection unit  301  may also be or include a transparent window covering the observation unit  200  towards the fluid jet  102 . The window may have a movable protection unit. 
     The detection unit  103  of the apparatus  100  of  FIG. 3  is further equipped with at least one optical element or assembly  302  arranged between the observation unit  200  and the sensing unit  105 . This optical element or assembly  302  may be a lens, filter, prism, grating or a combination thereof, and may function to shape or influence the radiation admitted into the detection unit  103  by the observation unit  200 , and/or the radiation isolated by the spectral separation unit  303 . 
     The apparatus  100  of  FIG. 3  also includes a processing unit  300 , which is configured to receive the detection signals  106  from the sensing unit  105 . The processing unit  300  is configured to process the detection signals  106 , and to determine an absolute and/or usable length of the fluid jet  102  based on the plurality of detection signals  106  received from the sensing unit  105 . To this end, the processing unit  300  may particularly be configured to carry out, by controlling the apparatus  100 , a method of measuring the fluid jet  102  according to an embodiment of the present invention. This method is described further below. The processing unit  300  may also (or alternatively) be configured to determine, based on the plurality of detection signals  106  received from the sensing unit  105 , a power of the laser beam  101  coupled into the fluid jet  102  and/or at least one flow characteristic of the fluid jet  102 . 
     The processing unit  300  is, for example, realized by a microprocessor or computer, and may apply signal processing on the detection signals  106 . Signal processing may include, for example, scaling, averaging, recording over time, integrating over time, or converting the detection signals  106 , and may include comparing the detection signals  106 —or an averaged or integrated signal—with one or more reference values. The processing unit  300  is also configured to set and change reference values, with which the detection signals  106  can be compared. The processing unit  300  may also be configured to record a plurality of detection signals  106 , and to compare the recorded signals  106  with pre-stored reference values. The processing unit  300  may alternatively or additionally be configured to integrate a plurality of detection signals  106  over time, in order to produce an integrated signal, and to evaluate a pattern or a change of a pattern in the integrated signal  106 . The plurality of detection signals  106  may arise from laser-pulse induced secondary radiation  104  sensed by the sensing unit  105 , if the laser beam  102  is a pulsed laser beam. 
     Specifically, the processing unit  300  may be configured to define a first reference value, and compare a first detection signal  106  generated by the detection unit  103  at a first position along the fluid jet  102  with the first reference value. It may further generate an alarm and/or shut down the apparatus  100 , or may at least instruct another unit of the apparatus  100  to do so, if the first detection signal  106  is below the first reference value. 
     The processing unit  300  may be further configured to define a second and/or third reference value, and compare a further detection signal  106  generated by the detection unit  103  at a further position along the fluid jet  102  with a first product of the first detection signal  106  and the second reference value, and/or with a second product of the first detection signal  106  and the third reference value. The processing unit  300  may further be configured to determine that the distance between the first position and the further position is the length of the fluid jet  102 , if the further detection signal  106  is smaller than the first product or larger than the second product. The processing unit  300  may also be configured to instruct the detection unit  103  to repeat the steps of obtaining of the detection signals  106 , and to repeat the comparing steps, if the further detection signal  106  is equal to or larger than the first product and/or equal to or smaller than the second product. 
     The processing unit  300  may advantageously be configured to set the second reference value is between 5-95%, preferably between 20-80% and/or the third reference value is between 105-300%, preferably between 140-260%. 
       FIG. 4  shows an apparatus  100  according to an embodiment of the present invention, which builds on the apparatus  100  shown in  FIG. 1 . Identical elements in  FIG. 4  and  FIG. 1  are labeled with the same reference signs and function likewise. Accordingly, the apparatus  100  of  FIG. 4  includes the detection unit  103 , which includes the sensing unit  105 , and is configured to receive and detect secondary radiation  104  coming from the fluid jet  102  guiding the laser beam  101 , particularly to convert the detected secondary radiation  104  into the plurality of detection signals  106 . 
     The apparatus  100  shown in  FIG. 4  is configured to generate, with the detection unit  103 , the plurality of detection signals  106  at a single position of the detection unit  103 . Preferably, the detection unit  103  is therefore stationary, and the apparatus  100  is configured to generate the detection signals  106  at the stationary position of the detection unit  103 . In this case, as shown in  FIG. 4 , the apparatus  100  does not include a motion unit  201 . However, it is also possible that the apparatus  100  of  FIG. 4  includes a motion unit  201  and accordingly a movable detection unit  103 , but is nevertheless configured to generate the detection signals  106  at a single (in this case preferably a pre-determined) position. The apparatus  100  may also be configured to generate the plurality of detection signals  106  at a single position (in one mode of operation), and generate the plurality of detection signals  106  at different positions along the fluid jet  102  (in another mode of operation). 
     Since the detection unit  103  in  FIG. 4  is stationary, it is further configured to observe, from its stationary position, a determined length section B along the fluid jet  102  between reference points B 0  and B 1 . This length section B may be identical to the predetermined distance A, over which the motion unit  201  is able to move the detection unit  103  in the apparatus  100  of  FIG. 2 . The reference points B 0  and B 1  may also be the same as A 0  and A 1 , respectively. Accordingly, the length section B is preferably between 0-25 cm, and is preferably between 0-15 cm. To this end the detection unit  103  may have a large enough aperture along the fluid jet  102  (larger than the aperture  202  shown in  FIG. 2 ) or even an unlimited aperture. That is, the detection unit  103  preferably does not include an observation unit  200 . And if it does include an observation unit  200 —which is possible—it includes an observation unit  200  with an aperture  202  of a size d, which is large enough to observe B. 
     The sensing unit  105  of the apparatus  100  in  FIG. 4  is preferably a charge-coupled device or a spatial array of multiple photodiodes, thermal diodes or avalanche diodes, in order to produce the plurality of detection signals  106 . Preferably, each diode or sensing element of the charge-coupled device produces one detection signal  106 . These detection signals  106  arise from the sensing unit  105  receiving secondary radiation  104  stemming from anywhere in the fluid jet  102  within the determined length section B. From each position along the fluid jet  102 , at which secondary radiation  104  is generated by the laser beam  101 , the secondary radiation  104  reaches the sensing unit  105  under a different angle and over a different distance. Accordingly, the sensing unit  105 , especially when it has multiple diodes or sensing elements, produces the detection signals  106  with a characteristic pattern (e.g. relation between detection signals  106 ), which provides an indication of the length of the usable fluid jet  102 . 
     Of note, the apparatus  100  shown in  FIG. 4  may further include one or more of the features introduced in  FIG. 3 . That is, also the apparatus  100  of  FIG. 4  may have the processing unit  300  for processing the detection signals  106 . Further, it may have the protection unit  301 , spectral separation unit  303  and/or optical element or assembly  302  in the detection unit  103 . 
       FIG. 5  shows an apparatus  100  according to an embodiment of the present invention, which builds on the apparatus  100  shown in  FIG. 1 ,  FIG. 2  or  FIG. 4 . Identical elements in  FIG. 1, 2 or 4  and in  FIG. 5  are labeled with the same reference signs and function likewise. The motion unit  201  is optional and thus shown in dashed lines. 
       FIG. 5  shows more details about the optical arrangement and fluid circuitry of the apparatus  100 , which may be provided in a machining unit  503 . The machining unit  503  may particularly include an optical element, like a lens  504 , for coupling the laser beam  101  into the fluid jet  102 . The laser beam  101  is produced by a laser unit  505  located outside of the machining unit  503 , and is injected into the machining unit  503 . The machining unit  503  may also include an optically transparent protection window (not shown), in order to separate the optical arrangement, here exemplarily the optical element  504 , from the fluid circuitry and the region of the machining unit  503  where the fluid jet  102  is produced. For producing the fluid jet  102 , the machining unit  503  may include a fluid jet generation nozzle  506  having a fluid nozzle aperture  507 . The fluid jet generation nozzle  506  is disposed within the machining unit  503  to produce the fluid jet  102  in a protected environment. The fluid nozzle aperture  507  defines the width of the fluid jet  102 . The fluid nozzle aperture  507  has preferably a diameter of 10-200 μm, and the fluid jet  102  has preferably a diameter of about 0.6-1 times the fluid nozzle aperture  507 . The pressure for the pressurized fluid jet  102  is provided via an external fluid supply. Preferably, the pressure is between 50-800 bar. For outputting the fluid jet  102  from the apparatus  100 , the machining unit  503  preferably includes an exit nozzle  501  with an exit aperture  502 . The exit aperture  502  is preferably wider than the fluid nozzle aperture  507 . 
       FIG. 5  also illustrates the determined distance/length A/B between the reference points A 0 /B 0  and A 1 /B 1  (compare  FIGS. 2 and 4 , respectively) in relation to a length L of the fluid jet  102 . Here in  FIG. 5 , the first reference point A 0 /B 0  is preferably directly at the exit nozzle  501  of the machining unit  503 , i.e. as close to the machining unit  503  as possible. The length L shown in  FIG. 5  is the usable length of the fluid jet  102  between the exit nozzle  501 —here coinciding with the first reference point A 0 /B 0 —and the position where the fluid jet  102  becomes unstable and disperses into droplets. Notably, also the absolute length of the fluid jet  102  starting from the fluid jet&#39;s origin, i.e. the fluid jet generation nozzle  506 , may be determined, because the distance between the fluid jet generation nozzle  506  and the exit nozzle  501  is known. If the first reference point A 0 /B 0  does not coincide with the exit nozzle  501 , then preferably the distance of the first reference point A 0 /B 0  relative to the exit nozzle  501  is known. The distance between the first reference point A 0 /B 0  and the second reference point A 1 /B 1  is also known. Accordingly, the usable length L of the fluid jet  102  can be derived from a relative length of the fluid jet  102  to these reference points A 0 /B 0  and A 1 /B 1 , respectively. 
       FIG. 6  shows an apparatus  100  according to an embodiment of the present invention, which builds on the apparatus  100  shown in  FIG. 3  and  FIG. 5 . Identical elements in  FIG. 3  or  FIG. 5  and  FIG. 6  are labeled with the same reference signs and function likewise. Accordingly, also the apparatus  100  of  FIG. 6  includes the detection unit  103  configured to convert the secondary radiation  104  coming from the fluid jet  102 , into the plurality of signals  106 . The fluid jet  102  carries the laser beam  101 , which is exemplarily generated by the laser unit  505  and coupled into the fluid jet  102  in a machining unit  503 . The detection unit  103  may be movable by the motion unit  201  (optional, thus shown with dashed line). 
     The apparatus  100  in  FIG. 6  shows in more detail the processing unit  300 , which receives the detection signals  106  from the detection unit  103 . As mentioned above with respect to  FIG. 3 , the processing unit  300  may be configured to determine a first reference value and a second and/or third reference value, in order to compare with the detection signals  106 . The apparatus  100  may to this end also include a human-machine interface (HMI)  600 , which a user of the apparatus  100  can provide with an input  602 , like a script. The HMI  600  may be configured to transmit, based on the user input  602 , the first, second and/or third reference value to the processing unit  300  via signal  601 . 
     The processing unit  300  may be further configured to instruct the motion unit  201 , if present, via instruction signal  603 , to move the detection unit  103  along the fluid jet  102 . 
       FIG. 7  shows a method  700  according to an embodiment of the invention. The method  700  may be carried out by the apparatus  100  as shown in one of the  FIGS. 1-6 , respectively. The method includes a step  701  of providing a fluid jet  102  and coupling a high-intensity laser beam  101  into the fluid jet  102 . This can, for example, be done with the machining unit  503  and laser unit  505 , respectively, as shown in  FIG. 4  Further, it includes a step  702  of receiving and detecting, with a detection unit  103 , secondary radiation  104  generated by the laser beam  101  in the fluid jet  102 . The detecting step  702  may include a step  702   a  of converting, with a sensing unit  105 , secondary radiation  104  into a detection signal  106 . The method  700  then further includes a step  703  of generating, with the detection unit  103 , a plurality of detection signals  106  at a single position or at different positions along the fluid jet  102 . 
     The method  700  may further comprise a step of moving the detection unit  103  along the fluid jet  102 , in order to generate the plurality of detection signals  106  at different positions along the fluid jet  102 . This implementation of the method  700  may be carried out with an apparatus  100  including a motion unit  201 . The method  700  can also include applying, with a processing unit  300 , an algorithm to the detection signals  106 , in order to determine a length of the fluid jet  102  or to determine flow properties of the fluid jet  102 . This can, for example, be carried out with an apparatus  100  including the processing unit  300 . 
     An algorithm for determining the length of the fluid jet  102  may be implemented as follows. All steps may be carried out by the processing unit  300 .
     Step 1: Instruct e.g. a laser unit  505  to provide the laser beam  101 .   Step 2: Define the reference points A 0  and A 1 , and define a first reference value R 0  and a second reference value R 1  and/or third reference value R 2 , e.g. by reading them out from a datasheet or over HMI  600 .   Step 3: Control the motion unit  201  to a first position A 0 .   Step 4: Instruct the detection unit  103  to measure a first detection signal  106 , and record it as signal S 0 .   Step 5: Compare the detection signal S 0  to the first reference value R 0 .
       If signal S 0 &lt;R 0 , generate and alarm and/or stop.   Else, proceed.   
       Step 6: Control the motion unit  201  to a further position A n .   Step 7: Instruct the detection unit  103  to measure a further detection signal  106 , and record it as signal S n      Step 8: Compare the further signal S n  to the first signal S 0  multiplied by the second reference value R 1  and/or compare the further signal S n  to the first signal S 0  multiplied by the third reference value R 2 ,
       If S n &lt;S 0 *R 1  or S n &gt;S 0 *R 2 , determine the absolute and/or usable length of the fluid jet  102  based on A n .   Else, increment A n .
           If A n ≥A 1 , stop.   Else, return to step 6.   
           
       

     According to the above algorithm, for every position A n , a signal S n  is obtained. If the detection unit has an observation unit  200  with an aperture  202  of size d, each signal S n  is obtained with a resolution of ±D/2, wherein D(d, l) is a function of the size d and the distance  1  shown in  FIG. 2 . Each time Sn is above S 0 *R 0  and/or below S 0 *R 2 , the signal is considered positive, and below that limit as negative. Since the reference points A 0  and A 1  have a known distance from the origin of the fluid jet  102 , like from the fluid jet generation nozzle  506 , the length of the fluid jet  102  can be determined based on the different between A n  and A 0 . 
     The signals Sn may be further evaluated, in order to qualify the fluid jet  102 , i.e. to determine a laminar behavior of the fluid jet  102 , perturbation characteristics of the fluid jet  102 . This can be done, for instance, by the length over which secondary radiation  104  is generated in the fluid jet  102 . Further, the signals S n  may be issued to determine a laser power of the laser light coupled as laser beam  101  into the fluid jet  102 . This can be done on the amount (intensity) of the secondary radiation  104  detected. 
     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.