Abstract:
Disclosed is a system ( 10 ) for regulating the power of a laser beam ( 12 ). The system ( 10 ) comprises a first transparent plate ( 16 ) that is arranged in the light path of the laser beam ( 12 ) and can rotate about a first axis ( 18 ), that is perpendicular to the light path, a first drive device ( 20 ) for rotating the first transparent plate ( 16 ) about the first axis ( 18 ), a measurement device ( 34 ) for detecting the power of the laser beam ( 12 ′) downstream of the first transparent plate ( 16 ) and for generating an actual power value, and a regulating device ( 44 ) with an input ( 46   a ) that is connected to the measurement device ( 34 ), and an output ( 46   b ) that is connected to the first drive device ( 20 ), the regulating device ( 44 ) receiving the actual power value and a desired power value and generating a control value which it outputs, wherein the first drive device ( 20 ) rotates the first transparent plate ( 16 ) depending on the control value, in order to minimize the difference between the actual power value and the desired power value.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a system and a method for regulating the power of a laser beam. The invention particularly relates to a system and method of regulating the power of a laser beam in a laser scanning system having a laser source and a deflection device having at least one deflection mirror that can be rotated by a galvanometric motor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Laser beams are used in a wide range of applications for machining work-pieces, for example for cutting, labeling or inscribing them. In some of these applications the power of the laser beam must be regulated. For example, one of the greatest difficulties in the use of CO 2 -lasers, which are widely used in the machining of work-pieces, is the inherent instability of their output power. This instability is caused by many different factors, for example by a change in the coolant water temperatures or the expansion and contraction of the laser cavity. Systems are therefore needed that can regulate the power at a constant value. 
         [0003]    In other applications the power is not to be regulated at a constant value but rather according to a pre-defined power profile. This is the case for example when marking with different grey levels. Adaptation of the laser beam intensity can also be necessary in order to obtain uniform lines or cutting widths under varying scan speeds on the work-piece surface, for example when marking or cutting corners or tight curves. 
         [0004]    In the prior art methods are disclosed for using optical filters or acousto-optical modulators in order to modulate the laser output power. 
         [0005]    In German utility model DE 20 2004 009 U1 by the same applicant, a system is further disclosed for regulating the power of a laser beam, which uses a rotatable Brewster-element that is arranged at the Brewster angle to the light path. In this known method the laser light impinging on the Brewster-element is polarized. The Brewster-element can additionally be rotated around an axis parallel to the direction of the laser beam. When the Brewster-element is rotated into a position in which the polarization vector lies in the plane of incidence, according to Brewster&#39;s law all the light is transmitted through the Brewster-element and no light is reflected. When on the other hand the Brewster-element is rotated into a position in which the polarization vector is perpendicular to the plane of incidence, virtually all the incident light is reflected and virtually none is transmitted through the Brewster-element. By rotating the Brewster-element between these two extreme positions, the proportion of the transmitted light and thereby the intensity of the emitted laser beam can be adjusted. 
         [0006]    U.S. Pat. No. 6,004,487 discloses a method and apparatus for a disk texturing operation in which laser pulses are successively bombarded against a delimited surface area on the face of rotating disk. A pulsed laser beam from a laser light source is adjusted to an optimum power level for an aimed bump diameter using an attenuation device having two rotatable transmission plates which are arranged to turn about a rotational axis perpendicular to the light path. In the preferred embodiment, one of the transmission plates of the light attenuation means is fixedly retained in a predetermined inclined position and the light attenuation means of the second stage is rotatable continuously or step by step in a fine pitch for fine adjustment of the output laser power and at the same time for preventing fluctuations in the output power by feedback control. The system, however, is not adapted for a rapid control of the laser power, as is for example needed in real time regulation of laser power when marking different ray levels during laser scanning. 
         [0007]    WO 03/065102 A1 discloses an attenuator for linear polarized laser pulses by rotating or tilting a plate with respect to the light path. A second plate may be arranged in a symmetrical manner to compensate for offset of the pulse from the optical axis. This system is similar to the German utility model DE 20 2004 009 U1. 
         [0008]    U.S. Pat. No. 4,747,673 discloses an attenuator for high power laser beams in which the beam passes successively through a pair of pivoting transmissive and reflective elements. The elements are individually mounted on intermeshed gears so that the elements are adjustable through equal and opposite angles. The elements are preferably elements commonly used as edge filters, that is, interference filters having an abrupt monotonic transition with wavelength from reflection to transmission. Both of the intermeshed gears carrying the transmissive and reflective elements are driven by a single motor. The momentum of inertia of the assembly is therefore too high to be useful in laser scanning systems where the laser power is to be controlled in real time, for example for marking at different gray levels. 
         [0009]    This known system has proven itself extremely well in practice. It would nevertheless be advantageous to reduce the manufacturing costs of this system and increase the speed with which the power can be regulated. A problem addressed by the present invention therefore is to disclose a system of the type described above, that is cheaper to manufacture, and to disclose a system and a method that enable faster regulation. 
       SUMMARY OF THE INVENTION 
       [0010]    The system of the invention comprises a first light-transparent plate that is arranged in a section of the light path of the laser beam and can be rotated about a first axis that is perpendicular to the said section of the light path. The system comprises a first drive device for rotating the first transparent plate about the first axis and a measurement device for detecting the power of the laser beam downstream of the first transparent plate and for generating an actual power value. The system further comprises a regulating device with an input that is connected to the measurement device, and an output that is connected to the first drive device, the regulating device receiving the actual power value and a desired power value and generating a control value which it outputs. The first drive device rotates the first transparent plate depending on the control value, in order to minimize the difference between the actual power value and the desired power value. 
         [0011]    Whereas therefore in the above cited prior art the Brewster-element (which is also a transparent plate) is always positioned at the Brewster angle to the laser beam and only the plane of incidence is adjusted relative to the polarization direction of the laser light, in the system of the invention, by rotating the transparent plate about the first axis, the angle of incidence of the laser light onto said transparent plate is changed. According to Fresnel&#39;s laws, the proportion of the light reflected by the transparent plate and of the light transmitted thereby changes, so that by rotating the transparent plate the intensity of the transmitted laser light can be adjusted. 
         [0012]    It has been found that this way of rotating the transparent plate is simpler and cheaper to implement than the rotation of a transparent plate at the Brewster angle about an axis parallel to the laser beam, as is done in the above cited prior art. In the prior art mentioned, each Brewster-element is held in a ball-bearing and is mounted on an inner ring of the ball-bearing. The Brewster-elements have a lever connected to them, which is rotated on the spindle of a motor. In comparison to this prior art the inventive system requires fewer parts and is therefore cheaper. Moreover, the combination of the transparent plate and the associated drive device in embodiments of the invention tends to have a lower moment of inertia than can be achieved with the rotatable Brewster-elements from the prior art, so that the response time of the system is lower than in the prior art. 
         [0013]    A further important advantage of the system according to the invention is that the system is very flexible and in particular can be adapted with very little expense to different beam diameters. In the system of the invention essentially only the size of the transparent plate needs to be adapted to the beam diameter. The conventional Brewster-elements from the prior art are by contrast designed for specific beam diameters, to which the size of the ball-bearing used are matched, so that these are only just as large as required for the intended application. This means however that a system designed for a specific beam diameter can not, or at least not optimally, used for other diameters. 
         [0014]    The system preferably also includes a second transparent plate, which is arranged in the light path of the laser beam between the first transparent plate and the measurement device and can be rotated about a second axis that is perpendicular to the light path, and a second drive system for rotating the second transparent plate about the second axis. The first and the second drive systems are thereby preferably controlled by the regulating device in such a way that the first and the second transparent plates turn synchronously in opposite directions by the same angular amount. 
         [0015]    By the use of two rotatable transparent plates the intended effects are multiplied, that is a certain increase or reduction in the transmission can be achieved by two smaller movements of the two transparent plates, instead of by a larger one, which causes the response time of the system to increase. Controlling both of the drive devices synchronously in opposite directions allows any offset generated due to light refraction on passing through the first transparent plate to be compensated by an offset in the opposite direction when passing through the second transparent element, as will be explained below in more detail with reference to an exemplary embodiment. This is important so that the laser beam is not displaced during the intensity regulation. 
         [0016]    The angular region, within which the first and possibly the second transparent plate are turned, preferably includes the Brewster angle with respect to the light path. When the transparent plates form the Brewster angle, all the light polarized parallel to the plane of incidence is transmitted. This position thus represents the maximal transmissivity of the system. On adjusting the transparent plates away from the Brewster angle, the reflection of the incident light increases and the transmission drops. The laser beam which is incident on the first transparent plate is preferably linearly polarized. Additionally the first and possibly the second axis are perpendicular to the polarization plane of the incident laser beam. In this setup a transmission of almost 100% is produced when the two transparent plates are at the Brewster angle to the incident laser beam. 
         [0017]    In an advantageous embodiment the first and/or second drive device is a galvanometric motor which is also referred to as “galvanometric scanner” or “galvo” in short. In this case the combination of transparent plate and galvanometric motor is very similar to a deflection element in a X-Y deflection unit. This constructional similarity is extremely advantageous because the components are well matched to one another. If for example the intensity of the laser beam during X-Y scanning is to be varied depending on the location of incidence of the laser beam on a target area, it is advantageous if the system for adjusting the intensity has a dynamic behavior similar to that of the deflection device, and is for instance not slower than the latter. 
         [0018]    The system preferably comprises an energy absorber which is so arranged and designed that it can receive the portion of the light reflected from the first and/or second transparent plate and can absorb at least a part of the light energy. It should be noted that the reflected light which is removed from the working beam is reflected in different directions depending on the current position of the first and second transparent element. The energy absorber must therefore be dimensioned so that it can pick up reflected light in every single one of these positions. The energy absorber is preferably a liquid-cooled metal element. 
         [0019]    In an advantageous embodiment the measurement device comprises a beam splitter, preferably a half-mirror, which diverts a defined part of the laser beam as a measurement beam on to a power measurement device. Between the beam splitter and the power measurement device a Brewster-element is preferably arranged, which is at the Brewster angle relative to the measurement beam. With this Brewster-element the intensity of the measurement beam can be further reduced, which allows a light sensor to be used that has a high temporal resolution but typically only withstands low beam intensities. In an advantageous extension the Brewster-element can be rotated about an axis parallel to the measurement beam, so that the intensity of the part of the measurement beam incident on the light sensor can be adjusted. 
         [0020]    The regulation device preferably comprises a PID-regulator. The transparent plates preferably consist of ZnSe and are coated with an anti-reflection layer. 
         [0021]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates. 
     
    
     
       SHORT DESCRIPTION OF DRAWINGS 
         [0022]    The figures show an exemplary embodiment of the invention, namely: 
           [0023]      FIG. 1  is a plan view of essential components of a system for regulating the power of a laser beam, 
           [0024]      FIG. 2  is a side view of two transparent plates, as are used in the system of  FIG. 1 , 
           [0025]      FIG. 3  is a block diagram of a laser scanning system comprising a CO 2 -laser source for generating a laser beam, a system for regulating the power of the laser beam and a deflection device, 
           [0026]      FIG. 4  is a schematic perspective view of the deflection device of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    In  FIG. 1  a system  10  for regulating the power of a laser beam  12  is shown, which laser beam  12  runs from right to left in the illustration of  FIG. 1 . The system  10  comprises a housing, only one base plate  14  of which is shown in the illustration of  FIG. 1 . The system  10  comprises a first transparent plate  16 , which is mounted rotatably about a first axis  18  and can be rotated about the first axis  18  by a first drive device  20 . In the embodiment shown the first drive device  20  is formed by a galvanometric motor, which in the prior art is also termed a “Galvo-scanner” or a “Galvanometer scanner”. 
         [0028]    Behind the first transparent plate  16  with respect to the propagation direction of the laser beam  12 , a second transparent plate  22  is disposed, that can rotate about a second axis  24  and can be driven by a second galvanometric motor  26  for rotation about the second axis  24 . The laser beam  12  emerging from the second transparent plate  22  is a damped laser beam  12 ′, the degree of damping depending on the transmission of the transparent plates  16 ,  22  in their current position. 
         [0029]    Following the course of the laser beam  12 ′, there is next disposed a half-mirror  28 , which allows the majority of the laser beam  12 ′ (e.g. 99%) to pass through as a working beam and diverts a small but defined proportion of the laser beam  12 ′ as a measurement beam on to a power measuring device  34 . 
         [0030]    The power measuring device  34  comprises a Brewster-element  36 , which is always positioned at the Brewster angle to the measurement beam  32  but can be rotated about an axis parallel to the measurement beam  32 . The power measurement device  34  additionally comprises a light sensor (not shown), which is hidden by a cooling element  38  in  FIG. 1 , and a lens arrangement  40  which focuses the measurement beam  32  onto the light sensor. 
         [0031]    The system  10  further comprises an energy absorber  42 , which consists of metal and is cooled by a coolant liquid. The absorber  42  shown is located underneath the transparent plates  16  and  22 . A similar absorber is also arranged above the transparent plates  16 ,  22 , omitted from  FIG. 1  however, to allow a clear view of the transparent plates  16 ,  22 . 
         [0032]    Finally the system  10  comprises a regulation unit  44 , which is connected via a signal lead  46   a  to the power measurement device  34  and via signal leads  46   b ,  46   c  to the first and second galvanometric motor  20  and  26 , respectively. Finally the regulation unit  44  is connected to a signal lead  46   d  by which it is connected to an external device (not shown), for example a computer. 
         [0033]    In the following the functioning of the system  10  will be described with reference to  FIGS. 1 and 2 . In the illustration of  FIG. 1  the laser beam  12  enters the system  10  at its right-hand end. In the exemplary embodiment shown, the laser beam  12  is linearly polarized in a plane perpendicular to the paper plane. This linear polarization can either be inherent in the laser source (e.g. a CO 2 -laser source), or achieved by polarizer (not shown) connected upstream. The laser beam  12  first impinges at an angle α on the first transparent plate  16 , which consists of ZnSe and is coated with an anti-reflective layer. A part  12   a  of the incident laser beam  12  is reflected by the first transparent plate  16  (see  FIG. 2 ) and is deflected on to the energy absorber  42 , which absorbs the light energy. The reflected part of the lights corresponds to the part of the power which is to be removed from the laser beam  12  in the process of power regulation. A part  12   b  of the laser beam  12  is transmitted through the transparent plate  16 . this transmitted part  12   b  is refracted upon entering into and passing out of the transparent plate  16 , so that the propagation direction of the transmitted beam  12   b  is the same as that of the incident laser beam  12 , but the transmitted laser beam  12   b  is shifted by an offset d (see  FIG. 2 ). 
         [0034]    The transmitted beam  12   b  then impinges on the second transparent plate  22 , the absolute value of the angle of incidence β of the beam  12   b  is equal to that of the angle of incidence α, but the angles α and β lie on different sides of a respective vertical line at the first and second transparent plates  16 ,  22  and therefore have different signs (α=−β). A part of the beam  12   b  incident on the second transparent plate  22  is reflected by the latter as beam  12   c  and absorbed by the energy absorber  42 . The other part of the light beam  12   b  is transmitted by the second transparent plate  22  as damped beam  12 ′. Upon entry and exit of the beam  12   b  into and out of the transparent plate  22 , the beam  12 ′ is refracted in turn, and because of the symmetric arrangement of the transparent plates  16 ,  22  (i.e. α=−β), the offset d is compensated by this refraction. It should be noted that the offset d is dependent on the angle α and in an asymmetric arrangement would therefore be difficult to compensate for. 
         [0035]    The ratio between transmitted and reflected light, i.e. the ratio of the intensities of the beams  12   b  to  12  and  12 ′ to  12   b  depends on the respective angle of the transparent plate  16 ,  22 , the galvanometric motors  20 ,  26  are constantly controlled via the signal leads  46   b ,  46   c  in such a way that the first and second transparent plate  16 ,  22  rotate synchronously in opposite directions, so that α=−β holds at all times. By adjusting the angles α and β the intensity of the laser beam  12 ′ that has passed through both, the first and second plates  16 ,  22  can thus be adjusted. In particular, if α and β are equal to the Brewster angle, no light is reflected (i.e. the intensity of the reflected light beams  12   a ,  12   c  is zero) and the intensity of the emerging light beam  12 ′ is equal to the intensity  12  of the incident laser beam. In other words, the arrangement formed by the transparent plates  16 ,  22  is adjusted to maximum transmission, when the angles α and β are equal to the Brewster angle. 
         [0036]    If the transparent plates  16 ,  22  are turned away from the Brewster angle, however, the reflected portion increases and the transmitted portion decreases, which allows the power of the emerging laser beam  12 ′ to be made arbitrarily small. It should be noted that the effects of the first and second transparent plates  16 ,  22  are multiplied together. This means that in order to achieve a specific change in the damping of the transmitted laser beam  12 ′, smaller movement of the individual transparent plates  16 ,  22  is necessary than if the same change in the damping were to be achieved by adjusting only one transparent plate. This allows in turn a shorter response time of the system  10  and a more rapid regulation of the power. 
         [0037]    As can furthermore be seen in  FIG. 1 , the laser beam  12 ′ transmitted by the transparent plates  16 ,  22  is split up at the beam splitter  28  into a working beam  30  and a measurement beam  32 . The intensity of the measurement beam  32  is a small, but firmly defined fraction of the intensity of the beam  12 ′, for example 1%. Even this relatively small proportion of the laser beam  12 ′ however, when using high-power lasers such as a CO 2 -laser, often still has too great an intensity for a light sensor to withstand. For the light sensor (not shown), for example a CMOS-element could be used, which is characterized by a very fast response time, which although advantageous with regard to a fast regulation time nevertheless would be damaged by excessive light energies. In order to damp the measurement beam  32  further, it is passed through a Brewster element  36 , which can be rotated about a measurement axis parallel to the measurement beam  32 . By rotation of the Brewster element  36 , an adjustable part of the measurement beam  32  is transmitted and the remainder of the measurement beam  32  is reflected and absorbed. In this way a damped measurement beam  32  can be obtained, having an intensity that is far less than 1% of the intensity of the laser beam  12 ′. 
         [0038]    The damped measurement beam  32  is focused by a lens assembly  40  onto the light sensor (not shown). At first glance, the focusing may appear at first glance to contradict the objective given above, namely to limit the intensity of the measurement beam  32  on the light sensor (not shown). In fact, however, experiments by the inventor have shown however that such a focusing is advantageous, as it can ensure that the total energy of the measurement beam  32  is also actually detected by the light sensor (not shown). If the measurement beam  32  were not focused, it may happen in practice that, due to an offset of the measurement beam  32 , a part of the cross-section of the measurement beam  32  lies outside of the light sensor and is not taken into account during regulation. By using the Brewster-element  36  together with the half-mirror  28 , the measurement beam  32  can be damped to such an extent that its intensity on the light sensor, in spite of the focusing, is not damaging to it. 
         [0039]    The intensity of the measurement beam  32  is input via the signal lead  46   a  into the regulation unit  44  as an actual value of the laser beam intensity. Via the signal lead  46   d  a desired or set value of the laser power is input into the regulation unit  44 . The desired value could be for example a temporally constant desired output power of the working beam  30 , which is thereby stabilized in time by means of the regulation unit  10 . The desired value input could also be a time-dependent power profile, as will be described in further detail with reference to  FIGS. 3 and 4 . 
         [0040]    The regulation unit  44  compares the actual power value from the power measurement device  34  with the desired power value, and a PID-regulator determines from this comparison a control signal or position signal which is fed to the first and second galvanometric motor  29 ,  36  via signal leads  46   b  or  46   c . The position signals are of a type such that the two transparent plates  16 ,  22  are constantly rotated synchronously in opposite directions, so that the relation α=−β (see  FIG. 2 ) is always maintained. 
         [0041]      FIG. 3  shows a laser scanning system  48  comprising a laser source  50 , which in the exemplary embodiment shown is formed by a CO 2 -laser and emits a laser beam  12 , and the regulation system  10  of  FIG. 1 , which receives the laser beam  12  and guides a working beam  30 , the power of which being regulated to a desired value, into a deflection device  52 . The deflection device  52  deflects the working beam  30  into a deflected beam  30 ′, and scans a surface of the work-piece  54  therewith. The system  10  and the deflection device  52  are connected to a computer  56 . 
         [0042]    In  FIG. 4  essential elements of the deflection unit  52  are shown. The deflection unit  52  comprises a Y-deflection mirror  58 , which is driven by a galvanometric motor  60 , and a X-deflection mirror  62 , which is driven by a galvanometric motor  64 . The galvanometric motors  60 ,  64  of the deflection system  52  are controlled by the computer  56 , in order to scan the surface of the work-piece  54  with the deflected laser beam  30 ′. When scanning the work-piece  54  the intensity of the working beam  30  is regulated. For example the intensity of the working beam  30 ′ can be increased to counteract a defocusing of the deflected working beam  30 ′, which occurs if the point of incidence on the target surface of the work-piece  54  is a long distance away from the centre of the surface. This kind of defocusing is known by the term “field-flattening”. By increasing the intensity of the working beam  30 ′ at points where the focusing is less sharp, this defocusing can be counteracted. Also, the power of the working laser beam  30  can be adapted to the scan velocity. At a high scan velocity, the power is increased, while at a low scan velocity, for example during changes of direction when marking or cutting corners, it is reduced. In the exemplary embodiment shown, this is achieved by having the computer  56  feed a suitable desired-value profile into the system via the signal lead  46   d  during the scanning. 
         [0043]    As can be seen from  FIG. 4 , the driving means of the X- and Y-mirrors  62 ,  58  is similar to the driving means of the first and second transparent plates  16 ,  22 . In the ideal case, even identical galvanometric motors can be used. From this structural similarity, not only can costs be saved, but the response times of the deflection system  52  and the regulation system  10  and very similar, so that these components are optimally matched to each other and a speed of regulating the laser light intensity is achieved as seems to be barely achievable with conventional Brewster-elements. 
         [0044]    The features shown in the present description, claims and drawings can be relevant both separately and in arbitrary combination for the implementation of the invention in the various embodiments. 
       LIST OF REFERENCE MARKS 
       [0000]    
       
           10  System for regulating the power of a laser beam 
           12 ,  12 ′ Laser beam 
           14  Base plate 
           16  first transparent plate 
           18  first axis 
           20  first galvanometric motor 
           22  second transparent plate 
           24  second axis 
           26  second galvanometric motor 
           28  half-mirror 
           30  working laser beam 
           32  measurement laser beam 
           34  power measuring device 
           36  Brewster-element 
           38  cooling element 
           40  lens assembly 
           42  energy absorber 
           44  regulation unit 
           46   s - 46   d  signal leads 
           48  laser scanning system 
           50  CO 2 -laser 
           52  deflection device 
           54  work-piece 
           56  computer 
           58  Y-mirror 
           60  galvanometric motor 
           62  X-mirror 
           64  galvanometric motor