Patent Publication Number: US-2003227614-A1

Title: Laser machining apparatus with automatic focusing

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention relates generally to a laser machining or engraving apparatus. The invention relates in particular to a laser machining apparatus including an autofocus arrangement for maintaining a laser beam focused on the base of a feature being machined as the depth of the feature changes during the machining.  
       DISCUSSION OF BACKGROUND ART  
       [0002] Lasers are being increasingly used for precise operations in laser marking and laser machining. In such operations, laser radiation is usually focused into a focal spot on the surface of a material being marked or machined and delivered as in a sequence of pulses. The amount of material removed is dependent, among other factors, on the power intensity of the laser radiation in the focal spot and the number of pulses delivered.  
       [0003] Several problems may be encountered in performing such laser machining operations. By way of example, one problem frequently encountered, particularly in machining relatively deep features in a material, is that as soon as material being machined is removed by the action of optimally focused radiation, the base of the feature being machined will no longer be in the plane of optimal focus. Accordingly, the power intensity of the machining beam at the instantaneous plane of machining will decrease with increasing depth of machining. This can lead to problems in controlling the depth and size of machined features.  
       [0004] A problem could also be experienced in attempting to machine a plurality of identical-sized features on a non-plane surface. Such a non-plane surface may be a surface that is intentionally contoured, or a surface that is nominally plane but has spatial variations from perfect planarity comparable to or greater than the depth of focus or the Rayleigh range of the focused laser radiation.  
       [0005] Another problem in laser machining a feature in a material is not knowing how deep the feature is at any instant during the machining. In machining such features, it can be important to stop machining at a precise depth. Prior art machining methods rely on controlling the reproducibility of laser power from pulse to pulse in a sequence of pulses and from one sequence of pulses to the next, and relying on delivering a predetermined number of pulses to machine a feature of a desired depth. Significant progress has been made in controlling such pulse sequences, however, this approach still presents certain problems. One such problem is that the rate of removal of material may vary with depth of a feature being machined. This variation can be expected to be different from material to material. This can lead to a need for extensive calibration efforts being required for each different operation in each different material to be machined.  
       [0006] There is a need for laser machining apparatus that provides a solution to one or more of the above-discussed problems. Preferably, the apparatus should at least be capable of monitoring the depth of a feature being machined.  
       SUMMARY OF THE INVENTION  
       [0007] In one aspect, the present invention is directed to method of laser machining a plurality of features in an object. The method is carried out using apparatus including a laser for providing laser radiation and an optical system for delivering the laser radiation to the object. Power of the laser radiation is adjustable into first and second ranges. A power in the first power range is insufficient to remove material from the object. A power in the second power range is sufficient to remove material from the object. The optical system has a selectively variable focal length. The optical system is arranged to receive a portion of the laser beam delivered to the object that is reflected from the object. The optical system includes a detector arrangement for determining from the reflected portion of the laser radiation whether or not the laser radiation is focused on the object.  
       [0008] In one preferred embodiment of the method, the power of the laser radiation is adjusted to the first range. The first-power-range laser radiation is delivered by the optical system to a first location on the object. The detector arrangement determines whether or not the first-power-range laser radiation is focused on the object. If the detector arrangement determines that the first-power-range laser radiation is not focused on the object, the focal length of the optical system is varied until the detector arrangement determines that the first-power-range radiation is focused on the object. After the detector arrangement determines that the first-power-range laser radiation is focused on the object, the power of the laser radiation is adjusted to the second power range and material is removed from the object using the second-power-range laser radiation until a feature is machined in the object at the first location. After the feature is machined at the first location the power of the laser beam is readjusted to the first power range, and the first-power-range laser radiation is delivered to a second location on the object. If the detector arrangement determines that the first-power-range radiation is not focused on the object, the focal length of the optical system is varied until the detector arrangement determines that the first-power-range radiation is focused on the object. After the detector arrangement determines that the first power-level-laser radiation is focused on the object, the power of the laser radiation is adjusted to the second power range, and material is removed from the object using the second-power-range laser radiation until a feature is machined in the object at the second location.  
       [0009] In another aspect of the invention, each of the features has a predetermined depth, and the focal length of the optical system is varied by moving one or more of the optical elements of the optical system. The material-removing operation for machining a feature includes removing material from the object with laser radiation power adjusted into the second power range, then, with the laser radiation adjusted into the first power range, moving the one or more optical elements to vary the focal length of the optical system until the detector arrangement determines that the first-power-range radiation is focused on the object. The instant depth of the feature being machined is determined from the optical-element movement and compared with the predetermined depth. If the instant depth is less than the predetermined depth, the material removal and depth determining steps are repeated until the instant depth is about equal to the predetermined depth.  
       [0010] In a preferred embodiment of the apparatus, the detector arrangement includes an optical arrangement for dividing the reflected portion of the laser radiation into first and second parts. All of the first part of the reflected radiation is directed onto a first detector to provide a first electronic signal. The second part of the reflected radiation is directed through a focusing lens onto a pinhole aperture and a second detector is located behind the pinhole aperture to receive a portion of the second part of the reflected radiation transmitted through the pinhole aperture, thereby providing a second electronic signal. The pinhole aperture is located in a position with respect to the focusing lens and the optical system is arranged such that when laser radiation is focused on the object, the ratio of the second electronic signal to the first electronic signal has a maximum value. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.  
     [0012]FIG. 1 schematically illustrates one preferred embodiment of laser engraving apparatus in accordance with the present invention including a laser providing laser radiation and an optical system for directing the laser radiation to an object to be engraved, the optical system having a scanning arrangement for directing laser radiation to from one location to another on the object to be engraved.  
     [0013]FIG. 2 is a block diagram schematically illustrating electronic components and their interconnection in an electronic controller for controlling the apparatus of FIG. 1.  
     [0014]FIG. 3 is a timing diagram schematically illustrating the interrelationship of electronic signals in the controller of FIG. 2.  
     [0015]FIG. 4 schematically illustrates another preferred embodiment of laser engraving apparatus in accordance with the present invention similar to the apparatus of FIG. 1 but wherein the optical system does not include the scanning arrangement and laser radiation is delivered from one location to another on the object to be engraved by moving the object from one position to another with respect to the optical system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0016] Referring now to the drawings, wherein like features are designated by like reference numerals, FIG. 1 schematically illustrates a preferred embodiment of laser engraving apparatus  14  in accordance with the present invention. Apparatus  14  includes a laser  16 , a laser power supply  18 , and an optical system  20 . A beam of laser radiation  22  from laser  18  is expanded and collimated by passing the beam through a negative lens  24  and then through a positive lens  26 . The expanded collimated beam is passed through a polarizing beamsplitter  28 . On passing through polarizing beamsplitter  28  a relatively small portion, for example, about one percent is reflected as a beam  22 M from reflecting face  28 A of the polarizing beamsplitter and focused by a lens  30  onto a detector  32 . The detected beam portion is represented by a signal I m , which is used by electronic circuitry in a controller  70 , described in detail further hereinbelow, for providing a measure of power in beam  22 .  
     [0017] A plane polarized beam  22 P exits polarizing beamsplitter  28  and is passed through a lens group  34  including a fixed, positive lens element  36  and a negative lens element  38  that is movable with respect to lens element  36  as indicated by double arrow A. Varying the axial position of the lens element  38  varies the focus of the optical system  20 .  
     [0018] It is preferable that laser radiation from laser  16  is plane polarized. This will typically be the case for most solid-state lasers including frequency-converted lasers. Typically, commercially available polarizing beam splitters have sufficient stress birefringence that sufficient radiation will be reflected from the polarizing beamsplitter to provide beam  22 M.  
     [0019] If laser radiation from laser  16  is not plane polarized, it will be polarized by polarizing beamsplitter  28 . In this case, about 50 percent of the laser radiation will be reflected in beam  22 M and some attenuation of the beam may be required to avoid overloading detector  32 .  
     [0020] After exiting lens  34 , beam  22 P, still plane-polarized, passes through a quarter-wave plate  40  which causes the beam to become circularly polarized. The circularly polarized beam  22 C is reflected by a galvanometer scan mirror  42  through a flat-field positive lens  44  which focuses the circularly polarized beam  22 C onto a workpiece  46  to be engraved. Galvanometer scan mirror  42  is rotatable as indicated by arrows B and is one of two such mirrors used for scanning focused beam  22 C over workpiece  46  in two different axes. As such galvanometer scanning mirror arrangements are well known in the art to which the present invention pertains, only one such mirror is shown in FIG. 1 for simplicity of illustration.  
     [0021] A portion of beam  22 C focused on workpiece  46  is reflected as a beam  22 C′, still circularly polarized, back through lens  44  to scan mirror  42 . Scan mirror  42  directs circularly polarized beam  22 C′ through quarter-wave plate  40 . This causes the circularly polarized beam to become a plane polarized beam  22 S, polarized in a plane perpendicular to the polarization plane of beam  22 P. Beam  22 S passes through lens group  34  into polarizing beamsplitter  28  and is reflected from face  28 A of the polarizing beamsplitter onto beamsplitter  50 .  
     [0022] A portion  22 S′ of beam  22 S is reflected by beamsplitter  50  through a positive lens  52 , which focuses beam  22 S′ onto a detector  54 . The power in this beam portion is represented by an electronic signal I t  from detector  54 . Another portion  22 S″ of beam  22 S is transmitted through beamsplitter  50  and is focused by a lens  56  through a pinhole aperture  58  in a plate  60  onto a detector  62 . The power in the portion of beam  22 S″ that passes through pinhole  58  is represented by an electronic signal I f  from detector  60 . The ratio of I f :I t  provides a measure of the amount of beam  22 S″ that passes through pinhole  58  relative to the total reflected power. Processing of signals I f  and I t  is performed by the above-discussed controller  70 . One preferred arrangement of controller  70  is described in detail further hereinbelow with reference to FIG. 2.  
     [0023] It is particularly important that the amount of radiation passing through pinhole  58  is measured as the ratio I f :I t . This provides that the measurement is not significantly affected by changes in laser power or the reflectivity of a surface being engraved. It is also important that reflected beam  22 C′ is collected by the same optical elements used to deliver beam  22 C to the workpiece. This provides that small mounts of misalignment of these optical elements do not have any significant effect on the position of beam  22 S″ on pinhole  58 . Lenses  56  and pinhole  58  in plate  60  are adjusted in position such that beam  22 S″ is focused onto pinhole  58  when beam  22 C is focused on a surface that will reflect the beam back along its original path. At this position, the ratio I f :I t  is maximized. Adjustment of the pinhole aperture can be observed by an observer&#39;s eye  53  via beamsplitter  50 .  
     [0024] In one example of an engraving operation using apparatus  20 , the position of lens  38  is adjusted such that beam  22 C passes through lens  44  and is focused initially at a point  62  on upper surface  46 A of workpiece  46  in a plane  64  coincident with the upper surface of the workpiece. As engraving proceeds at point  62 , beam  22 C penetrates into the workpiece and the part of the workpiece on which the beam is incident (the base of the feature being engraved) moves toward a plane  66  below plane  64 , i.e., below the plane of initial focus. Correspondingly, the position of the focus of beam  22 S″ moves and is no longer focused on pinhole  58 . Because of this, the amount of light in beam  22 S″ penetrating pinhole  58 , and accordingly the ratio I f :I t  is reduced. Lens  38  is moved until the ratio I f :I t  is again maximized. When the ratio is maximized, beam  22 C is again sharply focused on that portion of workpiece  64  instantly being engraved, i.e., on the base of the feature being engraved. Apparatus  20  can be calibrated such that the movement of lens  38  can be used as a measure of the movement of the position of the beam focus and correspondingly the depth of an engraved feature.  
     [0025] It should be noted here that lens group  34  represents one of the simplest of lens groups for changing the focus of optical system  20  and has only one moving lens. Those skilled in the art may devise more complex lens groups having more than two lenses in total, or more than one movable lens, without departing from the spirit and scope of the present invention.  
     [0026] In a complex lens group including more than one movable lens element, typically, all of the movable lens elements are moved synchronously by rotating a single sleeve including cam slots that move the movable lenses. Rotary movement of this sleeve can be effected by a shaft encoder or the like and interpreted as axial lens motion for maximizing the ratio I f :I t . Such a rotating sleeve and cam slot can be used, of course, to move a single lens such as lens  38 . In such an arrangement the amount of rotation of the sleeve necessary to refocus beam  22 C is used as a measure of the movement of one or a group of lens elements. One or more elements may also be moved by sliding a single sleeve linearly along the optical axis of the lens elements. In this description and in the claims appended hereto the terminology “moving one or more lens elements” is meant to include axially moving a single lens element or synchronously axially moving a group of elements with the motion of a single rotary or linear translation mechanism.  
     [0027] If, after engraving in one position on workpiece  46 , beam  22  is scanned to another position on surface  46 A of workpiece  46 , it is most likely that beam  22 C would not be at its sharpest focus at the new position. This being the case, lens  38  is moved again to maximize the ratio I f :I t  before engraving commences. In this way, the beam can be brought to its sharpest focus even if workpiece  46  has an irregular surface, i.e., if points on surface  46 A of workpiece  46  are not coplanar.  
     [0028] It should be noted here that while apparatus  16  is described as including a scanning mirror  42  for directing beam  22 C to selected locations on workpiece  48  (such as location  63  indicated by dotted lines  22 C), this should not be construed as limiting the present invention. Those skilled in the art to which the present invention pertains will recognize that moving beam  22 C to different locations on the workpiece could be accomplished by providing a fixed turning mirror in place of mirror  42 , thereby providing a fixed orientation of beam  22 C, and by moving workpiece  46  relative to beam  22 C, by means of a translation stage or the like.  
     [0029] Referring now to FIGS. 2 and 3, with continuing reference to FIG. 1, electronic controller  70  includes a microprocessor  72  for processing signals I f , I t , and I m  and providing therefrom an analog output for moving lens  38  of zoom lens group  34  (see FIG. 1). One preferred microprocessor is a Model 68HC11 micro controller available from Motorola, Inc., of Phoenix, Ariz. Microprocessor  72  includes a random access memory (RAM)  74 , which is used to store in-process variables and an electronically erasable programmable read only memory (EEPROM)  76  which is used to store operating software and related constants for operating the microprocessor and for processing signals. A digital to analog (D/A) converter  78  provides an analog signal for operating a servo driver  80  that is used to move lens  38  of variable-focus lens group  34 . A personal computer  82  is in communication with microprocessor  72  via a port  84 . Personal computer  82  is used for controlling laser  16  as well as for other functions discussed further hereinbelow.  
     [0030] In a preferred embodiment of apparatus  16 , laser  18  is a pulsed laser and radiation in beam  22  is in the form of a sequence of pulses of laser radiation. Accordingly, controller  70  is arranged to process signals I f , I t , and I m  in the form of such pulses. Pulse signals I f , I t , and I m  from detectors  62 ,  54 , and  32 , respectively, are first amplified by amplifiers  86 ,  88 , and  90 , respectively. The output of amplifiers  86 ,  88 , and  90  is connected to sample and hold (S/H) circuits  92 ,  94 , and  96 , respectively. The output of sample and hold (S/H) circuits  92 ,  94 , and  96  is connected to analog to digital (A/D) converters  100 ,  102 , and  104  respectively.  
     [0031] The amplified signals I f , I t , and I m  are sampled at their maximum value, digitized by the A/D circuits, and passed to microprocessor  72  for processing . A preferred method of effecting this sampling is set forth below. The method is applicable for any of the signals I f , I t  and I m .  
     [0032] The sampling method is synchronized by a synchronization signal (Sync). The synchronization signal is supplied from power supply  20  of laser  18  (see FIG. 1) and indicates that the laser has delivered a laser pulse at time t 1  (see FIG. 3). The synchronization signal triggers a delay circuit  106  (see FIG. 2). In response to the triggering, circuit  106  generates a signal S D  (see FIG. 3,) the falling edge of which, at time t 2 , coincides with the time at which the laser pulse has a maximum value. This falling edge of signal S D  triggers a monostable multivibrator (MMV) circuit  108  (see FIG. 2) that stretches the pulse in time, resulting in a hold signal S H  (see FIG. 3) that controls the sample and hold circuits  92 ,  94 , and  96 . During the time that signal S H  is applied to the sample and hold circuits, the sampled signal is held at the maximum value read by the sample and hold circuits at the leading edge of the S H  signal.  
     [0033] The S H  signal is also connected to a digital input port (not explicitly shown) of microprocessor  72 . The rising edge of this signal, at time t 2 , provides a signal Tr in  (see FIG. 2) that prepares a program (software) stored in the microprocessor to accept from AID converters  100 ,  102 , and  104 , digital signals representative of signals I f , I t , and I m , and to process those signals. After a small delay, within the hold time of signal S H  (at time t 3  in FIG. 3), the program generates a signal Tr out  and that signal is delivered by microprocessor  72  to the A/D converters. On receipt of the signal by the converters, A/D conversion (digitization) is initiated for the amplified signals held in the S/H circuits. The digitized signals are delivered to the microprocessor for processing. At time t 4 , and the end of the hold period of S/H circuits  92 ,  94 , and  96 , the S/H circuits are returned to their sample state. Subsequent pulses are similarly sampled beginning in FIG. 3 at times T 5  and T 6 .  
     [0034] From the digitized values of I f , I t , and I m , the microprocessor computes the value of the ratio I f :I t , which, as noted above, is the principle value used to control system  16  for maintaining the focus of the system at the base of a feature being machined. Motion of lens  38  to adjust the focus of the system is effectuated by a lens driver  80 , which requires an analog signal. This signal is generated by microprocessor  72  via a digital to analog (D/A) converter  76  (see FIG. 2).  
     [0035] In one preferred sequence of operations for making an engraving of a predetermined depth at location on workpiece  46 , the predetermined depth is stored in microprocessor  72 . A plurality of pulses, each thereof having insufficient power to remove material from the workpiece is delivered to a selected engraving location on the workpiece. These pulses may be referred to as scanning pulses and apparatus  16  may be referred to as being in the depth-scan mode. During the delivery of these pulses, lens  38  is moved incrementally until the ratio I f :I t  is at a maximum, indicating that beam  22 C is focused on the surface of the workpiece.  
     [0036] When beam  22 C has been focused, one or more laser pulses having a predetermined power sufficient to remove material from the workpiece is delivered to the engraving location. These pulses may be termed engraving pulses. The number of engraving pulses is selected, according to preprogrammed data on the removal depth of material as function of pulse power, to remove less than the predetermined depth of material from the workpiece. Following the delivery of the engraving pulses the ratio I f :I t  is no longer at a maximum. Apparatus  16  is again set to the depth-scan mode and scanning pulses are delivered while incrementally moving lens  38  until the ratio I f :I t  is again at a maximum. The amount of movement of the lens is interpreted as a current engraving depth and compared with the desired, predetermined engraving depth by microprocessor  72 . Microprocessor  72  then decides if one or more additional engraving pulses must be delivered. If more engraving pulses are delivered, the above-discussed sequence of operations is repeated until the desired engraving depth has been reached. Once the desired engraving depth has been reached, beam  22 C may be moved to a new location on the workpiece and the above-described sequence of operations repeated, beginning by moving lens  38  to maximize the ratio and focusing beam  22 C at the new engraving location.  
     [0037] It is preferable that laser  18  be arranged such that the power output of the laser can be switched rapidly without a significant change in the beam quality of the laser. This provides that when beam  22 C is focused in the depth scan mode, the beam will remain focused when the power is switched to the engraving or machining mode. One possible arrangement is to arrange laser  18  as a Q-switched continuously-optically-pumped, solid-state, pulsed laser with selectively variable pulse repetition rate, optical pumping power is held constant, and peak pulse power is varied by varying the pulse repetition rate in a manner such that the average power extracted from the solid-state gain-medium of the laser is essentially constant. This provides that thermal conditions in the solid-state gain medium and, accordingly, beam quality, remain essentially constant. Such a laser is described in detail in U.S. patent application No. 09/416,354 the complete disclosure of which is hereby incorporated by reference. In another arrangement of a Q-switched continuously-optically-pumped, solid-state, pulsed laser with selectively variable pulse-repetition rate optical pumping power is held constant and the laser resonator is arranged to deliver continuous wave (CW) radiation when pulses are not being delivered and between pulses when pulses are being delivered. Such a laser is described in detail in U.S. patent application No. 10/001,681 the complete disclosure of which is hereby incorporated by reference. Both of these pulsed laser arrangements are available in an Avia™ model laser, from Coherent® Inc. of Santa Clara, Calif.  
     [0038] Those skilled in the art will recognize that apparatus  16  is useful in machining a plurality of nominally identical features even if the depth of feature is not monitored during machining of the feature, for example, if factors such as careful control of laser pulse delivery and knowledge of machining characteristics of a material are relied on to predict how many pulses are required to provide a desired depth of each feature. If these factors are relied on for depth control, focusing beam  22 C at an initial feature location and refocusing the beam at each other feature location can provide that the machining (focal) spot condition is essentially the same prior to machining each feature.  
     [0039] As noted above, the depth of removal of material per delivered engraving pulse from workpiece  46  is a function of the power in that engraving pulse. It is also a function, inter alia, of the material of the workpiece, the reflectivity of the surface of the workpiece being engraved, and the depth in any feature at which engraving is taking place. Because of this, it can be useful to monitor pulse power and reflectivity during an engraving operation as well as monitoring the engraving depth.  
     [0040] During an engraving operation, signal I m  provides a measure of laser pulse power and signal I t  provides a measure of the power of the fraction of the pulse power reflected from workpiece  46 . The ratio I t :I m  provides a measure of the reflectivity of the workpiece. The amount by which lens  38  must be moved to maximize the ratio I f /I m  provides a measure of the engraving depth, as discussed above. Accordingly, microprocessor  72  can be programmed to monitor pulse power during an engraving operation and to compute actual engraving depth as a function of pulse power and reflectivity for the material of workpiece  46 . This data can be used at an intermediate stage of an engraving operation to update any stored data on these functions. The updated data can then be used to more accurately compute how many pulses are required to complete a subsequent or final stage of the engraving operation.  
     [0041] While apparatus  16  is described above as a laser engraving apparatus, the apparatus is useful as simply a measuring apparatus, for example, for determining the surface contour of an object such as workpiece  46 . In a preferred surface contour determining operation, apparatus  14  is operated entirely in the depth scan mode, i.e., all laser radiation delivered by the apparatus has insufficient power to remove material from the workpiece. FIG. 4 schematically illustrates a preferred embodiment  15  of the present invention arranged for determining a surface contour. Apparatus  15  is similar to apparatus  14  of FIG. 1, but includes a fixed mirror  43  in place of scanning mirror  42  of apparatus  14 . Beam  22 C is moved to a starting location  82  on surface  86 A of workpiece  86 , and a plurality of scanning pulses is delivered to the starting location. During the delivery of these pulses, lens  38  is moved incrementally until the ratio I f :I t  is at a maximum, indicating that beam  22 C is focused on the surface of the workpiece. Workpiece  46  is then translated with respect to beam  22 C, as indicated by the dotted outline of the workpiece. As a result of this, beam  22 C is accordingly moved to a new location  83  on the surface of the workpiece. Lens  38  is moved if necessary, to maximize the ratio I f :I t  and refocus beam  22 C on the surface of the workpiece. The amount by which lens  38  must be moved to refocus beam  22 C is interpreted as the difference in surface height between starting location  82  and the new location  83 . The relocating and refocusing operations are repeated at a plurality of locations on the surface of the workpiece to determine a surface contour map of the surface.  
     [0042] While computing a surface contour map for a workpiece, it is possible to monitor the surface reflectivity of the workpiece at each surface-height measuring location, thereby providing a map of the variation of reflectivity over the surface.  
     [0043] The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, by the embodiments described herein. Rather the invention is limited only by the claims appended hereto. For example, the detector arrangement need not necessarily be limited to the illustrated pinhole arrangement. Those skilled in the art would be aware of a variety of techniques for determining beam focus by monitoring the reflected beam. Some examples of focus measurement systems can be found in the following U.S. patents, each of which is incorporated herein by reference: U.S. Pat. Nos. 5,978,074; 5,910,842 and 6,052,478.