WO2007063343A1 - Laser processing tool - Google Patents

Laser processing tool Download PDF

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Publication number
WO2007063343A1
WO2007063343A1 PCT/GB2006/050417 GB2006050417W WO2007063343A1 WO 2007063343 A1 WO2007063343 A1 WO 2007063343A1 GB 2006050417 W GB2006050417 W GB 2006050417W WO 2007063343 A1 WO2007063343 A1 WO 2007063343A1
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WO
WIPO (PCT)
Prior art keywords
laser
beam splitter
optical
illumination
compensating
Prior art date
Application number
PCT/GB2006/050417
Other languages
French (fr)
Inventor
Antony Phillips
Original Assignee
Gsi Group Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Gsi Group Limited filed Critical Gsi Group Limited
Publication of WO2007063343A1 publication Critical patent/WO2007063343A1/en

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/03Observing, e.g. monitoring, the workpiece
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/03Observing, e.g. monitoring, the workpiece
    • B23K26/032Observing, e.g. monitoring, the workpiece using optical means
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/0665Shaping the laser beam, e.g. by masks or multi-focusing by beam condensation on the workpiece, e.g. for focusing

Definitions

  • This invention relates to laser processing tools.
  • it relates to astigmatism compensation in a laser processing tool or apparatus.
  • a laser processing apparatus typically comprises a laser directed through an optical system which focuses the laser onto a workpiece where the laser is used to process the workpiece.
  • the processing operations may be welding, drilling, and so on. It is often required to be able to visually view the workpiece and the site where the laser impinges upon the workpiece and this is often done by utilising some of the same optics that the laser beam is focused by to transmit illumination in the visual spectrum from the work site to a camera. This may then be used to monitor the processing operation, to initially focus the laser beam and for other monitoring and control operations.
  • the camera or other optical system (and in this specification the term “camera” will include any optical viewing system or monitoring system) shares some of the optics of the laser system so that it can view the same target point.
  • a typical system of this type is shown in Figure 1.
  • a laser 1 is applied to input optics 2 which collimates the laser beam to an optical element 3 from where it is sent to a focussing optical system 4 for focussing onto a workpiece 5.
  • An illumination source (light source) 8 directed directly at the workpiece 5 and the reflected light from this passes back through optical system 4 and to beam splitter 3 where it is passed to a further optical system 6 which focuses the light onto a camera or other optical receiver or recorder 7.
  • the beam from the laser 1 can be collimated (eg by collimator 2) so that it is a collimated, parallel, beam by the time it reaches beam splitter 3 and onwards to the workpiece.
  • collimator 2 e.g. collimator 2
  • a fairly large distance is required between the beam splitter and the laser 1.
  • this required dimension is too large to fit into a required laser beam head to be used in relatively compact spaces.
  • the distance d between the beam splitter and laser which is necessary in order for the beam to be collimated at the splitter is too long. This distance may be reduced by allowing the laser beam to still be divergent when it reaches the beam splitter and such an arrangement is shown in Figure 2.
  • the distance dl between the laser and the beam splitter is much less than that in Figure 1.
  • the beam is divergent at the splitter then collimation is necessary between the splitter and the workpiece and this is achieved by the addition of a an optical element 9 between the beam splitter 3 and focusing lens arrangement 4.
  • the present invention arose in an attempt to provide a cost efficient method to improve the image quality in the visual optical path of a laser system.
  • the present invention further arose in an attempt to provide an improved compact laser head.
  • an optical system comprising a laser path, target illumination and a camera path, wherein a beam splitter introducing astigmatism is included in a common part of the laser and camera paths and further comprising a compensating optical element mounted between the beam splitter and a camera or optical output, wherein the further element is tilted relative to the beam splitter to compensate for said astigmatism.
  • an illumination means is applied to the additional optical element such that the illumination is passed from this element through the beam splitter to the workpiece and then reflected back through the beam splitter, the orthogonal element to the camera.
  • an optical element eg LED, tungsten light source or other light source
  • the astigmatism compensating element is a partial reflecting mirror, such as a partially silvered mirror.
  • the invention provides an optical head comprising a laser or laser input, a beam splitter positioned to receive diverging radiation from the laser, a camera or optical output and one or more optical elements for compensating for astigmatism introduced by the beam splitter.
  • the apparatus may comprise a light source adapted to illuminate directly (ie impinge directly upon) the astigmatism compensating element.
  • the placing of the light source to illuminate element 10 directly is of significant benefit firstly in obtaining a compact head and also in positioning of the illumination source.
  • problems of alignment can arise. This can be a particular problem when, as is common, a workpiece is moved towards or away from the beam forming optics and this has, in previously proposed systems, required the illumination source to be moved or the light level from the illumination source would change as the workpiece is moved, which is again undesirable.
  • movement of the workpiece does not effect the amount of illumination on the workpiece itself since the illumination is transmitted along the same path as the laser radiation.
  • Figure 1 shows a previously proposed optical arrangement
  • Figure 2 shows a further previously proposed arrangement
  • Figure 3 shows an optical arrangement with astigmatism compensation
  • Figure 4 shows an optical arrangement as in Figure 3 with an illumination source
  • Figure 5 shows a simulated ray diagram of a previously proposed optical system
  • Figure 6 shows simulated images from the simulated system of Figure 5;
  • Figure 7 shows a simulation of an optical system including astigmatism compensation;
  • Figure 8 shows simulated images obtained from the system of Figure 7;
  • Figure 9 shows an illumination source attached directly to a focusing mechanism
  • Figure 10 shows a detail of the view of Figure 9; and Figure 11 shows a laser head.
  • FIGS 3 and 4 show laser/optical head arrangements according to the present invention.
  • a laser and viewing optics arrangement which includes astigmatism compensation.
  • a laser 1 may be any type of laser but may typically be a solid state laser such as a Nd: YAG laser.
  • the laser generates an output in the form of diverging beam 20 which is redirected by a beam splitter 3 to focusing optics 4, 9 as described with reference to Figure 2.
  • the beam is focused to a target position on a workpiece 5.
  • a camera 7 is also used to visually monitor the workpiece and target site and this receives illumination, as before, via optical systems 4, 9 and beam splitter 3.
  • a further astigmatism compensating element 10 is applied between the beam splitter and camera 7 or input camera optics (not shown).
  • the astigmatism compensation element is an element which is tilted to the beam splitter.
  • it is a plane parallel plate which is tilted so as to be orthogonal to the plane of tilt of the beam splitter 3 and this corrects for all or substantially all of the astigmatism (and/or coma) introduced by the beam splitter.
  • the plate may be inclined at 45° and rotated by 90° compared to the beam splitter.
  • the beam splitter itself will generally be a plane parallel plate.
  • the compensating element may be partially reflecting mirror, eg partially silvered.
  • the laser may be introduced from a fibre optic cable receiving at its remote end a beam from a remote laser.
  • the target may be illuminated by a LED or other light source shining directly upon the target site.
  • the illumination source is most preferably applied directly to the compensating element 10.
  • Figure 4 shows an embodiment in which a light source 11 is used to illuminate directly the compensating element 10.
  • the light source may be any suitable light source such as one or more LEDs, a tungsten light source or others. Light from the LED is reflected off element 10 in a direction of beam splitting plate 3 and to the target 5. It is then reflected back off the target back through the optical system and then through plate 10 to the camera or other optical system 7.
  • the light source may be included within a laser head arrangement (shown schematically by the dashed lines H in Figure 4). Since the head is also made compact by being able to utilise a diverging laser and therefore arranging for the laser output to be closer to the beam splitter 3, this enables a very compact laser head or laser delivery module to be obtained.
  • the laser 1 may form part of the head or the laser may be a separate item and may be applied to a suitable input to the head or may be applied via an optical fibre (not explicitly shown) from a remote laser source as is known in the art. This is indicated by first dot-dash line Hl.
  • the camera or other visual recording apparatus may form part of the head arrangement or may be separate (as indicated by dot-dash line H2). Again, the light passed through element 10 may be applied to an input of an optical fibre (not shown) and transmitted through this fibre to a remote camera, computer or other monitoring equipment.
  • a remote camera computer or other monitoring equipment.
  • Such camera or other monitoring equipment and this is known generically in the specification as a camera although it may include any type of equipment including a computer or other processing or light monitoring equipment.
  • the beam splitting plate 3 and compensating plate 10 are both preferably at 45° to the beam path between the target and camera. Both are orthogonal to each other. That is, their tilt planes are at 90° to each other. If plate 3 is at a different angle than 45° than the plate 10 may also need to be at a different angle. The angles will easily be able to be determined by the man skilled in the art.
  • Figure 5 shows a computer simulation of a prior art optical system, similar to that of Figure 2, in which an astigmatism inducing plate 3a but no astigmatism compensation is used.
  • Figure 6 shows the simulated spot images that were obtained from this.
  • the images which are generally schematic, astigmatism is introduced (the images are not circular spots but are clearly elongate). This is because, due to the astigmatism the beam is focused differently in the X and Y directions. This is of course undesirable and leads to problems in focusing and clarity of image which might effect the efficiency of laser processing.
  • Figure 7 shows a simulation of a system according to the present invention in which a further element 10a is introduced which is tilted orthogonally to element 3 a.
  • the system shows simulated ray diagrams and Figure 8 shows the images that are likely to be obtained. It is immediately seen that these are much more circular than the generally elongate images of Figure 6 and therefore that astigmatism is much reduced or, at best, completely eliminated.
  • the spot diagrams of Figures 6 and 8 give the geometrical image blur formed by the lens when imaging a point object at three different image heights, on-axis, 2mm and 3mm to simulate the image quality at the edge of field of the CDD camera (1/3" format).
  • the shape of the spot diagrams gives an indication of the aberration present in the optical system. Astigmatism is particularly easy to spot in Figure 6 due to the elongation of the spot in one axis.
  • a set of rays are traced from a point object and traced through the entrance pupil of the imaging system.
  • the entrance pupil is divided up into a number of segments and each ray that is traced is directed to each segment.
  • the ray then continues through the optical system refracting at all surfaces on the way until they eventually cross the image plane.
  • the spot diagram is a plot of the rays in the X-Y plane at the image. In a perfectly corrected system the spot diagram will produce a single spot where all the rays come together at the image plane.
  • the geometrical spot diagram is a measure of the geometrical aberrations and does not take account of diffraction.
  • the compensation element which also serves as a reflector for the illumination source is preferably attached directly to the camera mount which also acts as a focusing mechanism.
  • the compensating element 10 is attached to a camera mount 20 and this can moved longitudinal relative to the process head in order to bring the compensating element towards or away from other optical elements (not shown in Figure 9) for focusing.
  • a range of focus lenses of various focal lengths are provided to give the required laser spot size and depth of focus to suite the particular laser processing application.
  • a camera focusing mechanism is then useful to compensate for the variation in camera focus following a change in focus lens.
  • FIG. 10 is an enlarged view of this part of the mechanism and clearly shows the illumination means in the form of an LED 21 mounted to project through a hole 22 to transmit towards the compensating element 10. Light from the element is then reflected down towards the remainder of the optical assembly as shown in many drawings towards the workpiece. Instead of a single LED, a plurality of LEDs may be used or other illumination means.
  • the illumination sources are attached directly to the focusing mechanism to avoid relative movement between the illumination source and the compensation element. Such relative movement is to be avoided to prevent misalignment of the illumination source to the optical axis of the laser and camera paths.
  • the LED is most preferably a surface mount LED to retain a compact design.
  • the level of LED illumination is most preferably adjustable to account for variations in camera sensitivity and the difference in light intensity reflected back from different surfaces of the workpiece. For example, a matt black surface will reflect back a different light intensity to a highly polished reflective surface. Many means for controlling illumination intensity will be apparent but this may be via a standard LED dimming circuit.
  • the best position of the LED relative to the recollimating lens depends on the type of LED used. To obtain a uniform illumination distribution at the workpiece it is often necessary to position the LED away from the focal plane of the recollimating lens.
  • the front surface of the compensating element 10 is coated to provide 50% reflection at 45° incidents over a bandwidth 450 nm to 650 nm to provide the reflective path from the LED illumination source 21 to the workpiece, and 50% transmission at 45° incidents from 450 nm to 650 nm to provide the visible path back from the illuminated workpiece to the camera.
  • the back face of the compensating element 10 is most preferably anti-reflection coated over the visible bandwidth 450 nm to 650 nm to avoid ghosting and glare problems.
  • a laser beam collimated by a lens of focal length 100 mm before the beam splitter would result in a typical distance D between the beam splitter and the end of the fibre delivered laser of about 130 mm.
  • the diameter of the laser beam at the beam splitter would be 24 mm and this would result in a large beam splitter for accommodating the full diameter angled at 45°.
  • the arrangement of Figure 2 allows the laser beam to be divergent when it reaches the beam splitter.
  • This arrangement allows the distance D from the fibre to the beam splitter to be reduced to 60 mm.
  • the diameter of the laser beam at 60 mm from the end of the fibre is 14.5 mm which therefore allows a substantial reduction to the size of the beam splitter.
  • the resulting design is therefore a compact and lightweight processing head which is highly advantageous for most processing applications.
  • Figure 11 shows a machine head. This illustrates an input for a laser L and for a camera C, the beam splitter 3 and the compensating element 10. It also shows how the illumination path can be divergent through the beam splitter 3, via the element 10, or conversely convergent in the camera path reflected back from the workpiece through element 10 via the beam splitter 3.

Abstract

An optical system is disclosed, comprising a laser path, target illumination (11,21) and a camera path, wherein a beam splitter (3) introducing astigmatism is included in the laser path and camera path and further comprising a compensating optical element (10) mounted between the beam splitter (3) and a camera (7) or optical output, wherein the further element is tilted orthogonally relative to the beam splitter to compensate for said astigmatism. A compact laser delivery head is attainable.

Description

Laser Processing Tool
This invention relates to laser processing tools. In particular, it relates to astigmatism compensation in a laser processing tool or apparatus.
A laser processing apparatus typically comprises a laser directed through an optical system which focuses the laser onto a workpiece where the laser is used to process the workpiece. The processing operations may be welding, drilling, and so on. It is often required to be able to visually view the workpiece and the site where the laser impinges upon the workpiece and this is often done by utilising some of the same optics that the laser beam is focused by to transmit illumination in the visual spectrum from the work site to a camera. This may then be used to monitor the processing operation, to initially focus the laser beam and for other monitoring and control operations.
Most conveniently, the camera or other optical system (and in this specification the term "camera" will include any optical viewing system or monitoring system) shares some of the optics of the laser system so that it can view the same target point.
A typical system of this type is shown in Figure 1. A laser 1 is applied to input optics 2 which collimates the laser beam to an optical element 3 from where it is sent to a focussing optical system 4 for focussing onto a workpiece 5. An illumination source (light source) 8 directed directly at the workpiece 5 and the reflected light from this passes back through optical system 4 and to beam splitter 3 where it is passed to a further optical system 6 which focuses the light onto a camera or other optical receiver or recorder 7.
These types of systems can work fine provided that the beam from the laser 1 can be collimated (eg by collimator 2) so that it is a collimated, parallel, beam by the time it reaches beam splitter 3 and onwards to the workpiece. In order to do this, a fairly large distance is required between the beam splitter and the laser 1. Often, this required dimension is too large to fit into a required laser beam head to be used in relatively compact spaces. As shown in Figure 1, the distance d between the beam splitter and laser which is necessary in order for the beam to be collimated at the splitter is too long. This distance may be reduced by allowing the laser beam to still be divergent when it reaches the beam splitter and such an arrangement is shown in Figure 2. In this case, the distance dl between the laser and the beam splitter is much less than that in Figure 1. However, if the beam is divergent at the splitter then collimation is necessary between the splitter and the workpiece and this is achieved by the addition of a an optical element 9 between the beam splitter 3 and focusing lens arrangement 4.
Note that in the present specification where single lenses are shown, or 'elements' are described, in practice these may be single elements or may be a combination of several lenses, mirrors or other optical elements.
The addition of further elements 9 enables the laser to be properly focused onto the workpiece but this then means that the laser beam splitter 3 introduces astigmatism to the visual image at the camera since the visual image is converging (as shown by the ray lines) when it reaches the beam splitter 3. In effect, this is because one side of the beam (bl) will reach the beam splitter 3 at a different angle to the other side of the beam (b2). These are then differently refracted by the beam splitter and so astigmatism is introduced. To correct for this, up to now, fairly expensive correction lenses have been used to improve image quality at the camera, or possibly software techniques may be used. These are expensive and do not produce good results.
The present invention arose in an attempt to provide a cost efficient method to improve the image quality in the visual optical path of a laser system.
The present invention further arose in an attempt to provide an improved compact laser head.
According to the present invention there is provided an optical system comprising a laser path, target illumination and a camera path, wherein a beam splitter introducing astigmatism is included in a common part of the laser and camera paths and further comprising a compensating optical element mounted between the beam splitter and a camera or optical output, wherein the further element is tilted relative to the beam splitter to compensate for said astigmatism.
The addition of a further optical element or system angled orthogonally to the beam splitter so as to correct the astigmatism introduced by the beam splitter. Thus, a compact laser/optical arrangement can be achieved in which the laser output need not be parallel when it reaches the beam splitter and so the laser can be close to the beam splitter.
Preferably, an illumination means is applied to the additional optical element such that the illumination is passed from this element through the beam splitter to the workpiece and then reflected back through the beam splitter, the orthogonal element to the camera.
The placing of an optical element (eg LED, tungsten light source or other light source) direct to the further optical element in a very compact optical head assembly, including the optical element to be formed.
Preferably, the astigmatism compensating element is a partial reflecting mirror, such as a partially silvered mirror.
In a further aspect, the invention provides an optical head comprising a laser or laser input, a beam splitter positioned to receive diverging radiation from the laser, a camera or optical output and one or more optical elements for compensating for astigmatism introduced by the beam splitter.
The apparatus may comprise a light source adapted to illuminate directly (ie impinge directly upon) the astigmatism compensating element.
The placing of the light source to illuminate element 10 directly is of significant benefit firstly in obtaining a compact head and also in positioning of the illumination source. In previously proposed systems, where illumination is applied directly to the workpiece 5 (as shown in Figures 1 and 2) then problems of alignment can arise. This can be a particular problem when, as is common, a workpiece is moved towards or away from the beam forming optics and this has, in previously proposed systems, required the illumination source to be moved or the light level from the illumination source would change as the workpiece is moved, which is again undesirable. By placing the illumination source to be reflected and/or refracted directly by part of the optical system, then movement of the workpiece does not effect the amount of illumination on the workpiece itself since the illumination is transmitted along the same path as the laser radiation.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 shows a previously proposed optical arrangement; Figure 2 shows a further previously proposed arrangement;
Figure 3 shows an optical arrangement with astigmatism compensation;
Figure 4 shows an optical arrangement as in Figure 3 with an illumination source;
Figure 5 shows a simulated ray diagram of a previously proposed optical system;
Figure 6 shows simulated images from the simulated system of Figure 5; Figure 7 shows a simulation of an optical system including astigmatism compensation;
Figure 8 shows simulated images obtained from the system of Figure 7;
Figure 9 shows an illumination source attached directly to a focusing mechanism;
Figure 10 shows a detail of the view of Figure 9; and Figure 11 shows a laser head.
Figures 3 and 4 show laser/optical head arrangements according to the present invention.
Referring to Figure 3, a laser and viewing optics arrangement is shown which includes astigmatism compensation. A laser 1 may be any type of laser but may typically be a solid state laser such as a Nd: YAG laser. The laser generates an output in the form of diverging beam 20 which is redirected by a beam splitter 3 to focusing optics 4, 9 as described with reference to Figure 2. The beam is focused to a target position on a workpiece 5. Here it may be used to process the workpiece such as through drilling, welding or other operations. A camera 7 is also used to visually monitor the workpiece and target site and this receives illumination, as before, via optical systems 4, 9 and beam splitter 3. In order to correct for the astigmatism caused by the beam splitter and the diverging nature of the light when it hits the splitter 3, a further astigmatism compensating element 10 is applied between the beam splitter and camera 7 or input camera optics (not shown).
The astigmatism compensation element is an element which is tilted to the beam splitter. In preferred embodiments of the invention, it is a plane parallel plate which is tilted so as to be orthogonal to the plane of tilt of the beam splitter 3 and this corrects for all or substantially all of the astigmatism (and/or coma) introduced by the beam splitter. The plate may be inclined at 45° and rotated by 90° compared to the beam splitter.
The beam splitter itself will generally be a plane parallel plate.
The compensating element may be partially reflecting mirror, eg partially silvered.
Note that instead of a laser itself, the laser may be introduced from a fibre optic cable receiving at its remote end a beam from a remote laser.
In some embodiments, the target may be illuminated by a LED or other light source shining directly upon the target site. However, in order to obtain a compact tool, the illumination source is most preferably applied directly to the compensating element 10. Figure 4 shows an embodiment in which a light source 11 is used to illuminate directly the compensating element 10. The light source may be any suitable light source such as one or more LEDs, a tungsten light source or others. Light from the LED is reflected off element 10 in a direction of beam splitting plate 3 and to the target 5. It is then reflected back off the target back through the optical system and then through plate 10 to the camera or other optical system 7.
This has a significant advantage in that the light source may be included within a laser head arrangement (shown schematically by the dashed lines H in Figure 4). Since the head is also made compact by being able to utilise a diverging laser and therefore arranging for the laser output to be closer to the beam splitter 3, this enables a very compact laser head or laser delivery module to be obtained. The laser 1 may form part of the head or the laser may be a separate item and may be applied to a suitable input to the head or may be applied via an optical fibre (not explicitly shown) from a remote laser source as is known in the art. This is indicated by first dot-dash line Hl.
The camera or other visual recording apparatus may form part of the head arrangement or may be separate (as indicated by dot-dash line H2). Again, the light passed through element 10 may be applied to an input of an optical fibre (not shown) and transmitted through this fibre to a remote camera, computer or other monitoring equipment. Such camera or other monitoring equipment and this is known generically in the specification as a camera although it may include any type of equipment including a computer or other processing or light monitoring equipment.
In an embodiment of the invention, the beam splitting plate 3 and compensating plate 10 are both preferably at 45° to the beam path between the target and camera. Both are orthogonal to each other. That is, their tilt planes are at 90° to each other. If plate 3 is at a different angle than 45° than the plate 10 may also need to be at a different angle. The angles will easily be able to be determined by the man skilled in the art.
Figure 5 shows a computer simulation of a prior art optical system, similar to that of Figure 2, in which an astigmatism inducing plate 3a but no astigmatism compensation is used. Figure 6 shows the simulated spot images that were obtained from this. As is shown by the images, which are generally schematic, astigmatism is introduced (the images are not circular spots but are clearly elongate). This is because, due to the astigmatism the beam is focused differently in the X and Y directions. This is of course undesirable and leads to problems in focusing and clarity of image which might effect the efficiency of laser processing.
Figure 7 shows a simulation of a system according to the present invention in which a further element 10a is introduced which is tilted orthogonally to element 3 a. The system shows simulated ray diagrams and Figure 8 shows the images that are likely to be obtained. It is immediately seen that these are much more circular than the generally elongate images of Figure 6 and therefore that astigmatism is much reduced or, at best, completely eliminated.
The spot diagrams of Figures 6 and 8 give the geometrical image blur formed by the lens when imaging a point object at three different image heights, on-axis, 2mm and 3mm to simulate the image quality at the edge of field of the CDD camera (1/3" format).
The shape of the spot diagrams gives an indication of the aberration present in the optical system. Astigmatism is particularly easy to spot in Figure 6 due to the elongation of the spot in one axis.
A set of rays are traced from a point object and traced through the entrance pupil of the imaging system. The entrance pupil is divided up into a number of segments and each ray that is traced is directed to each segment. The ray then continues through the optical system refracting at all surfaces on the way until they eventually cross the image plane. The spot diagram is a plot of the rays in the X-Y plane at the image. In a perfectly corrected system the spot diagram will produce a single spot where all the rays come together at the image plane. The geometrical spot diagram is a measure of the geometrical aberrations and does not take account of diffraction.
To keep the design of the process head compact, the compensation element, which also serves as a reflector for the illumination source is preferably attached directly to the camera mount which also acts as a focusing mechanism. This is shown schematically in Figure 9 where the compensating element 10 is attached to a camera mount 20 and this can moved longitudinal relative to the process head in order to bring the compensating element towards or away from other optical elements (not shown in Figure 9) for focusing.
A range of focus lenses of various focal lengths are provided to give the required laser spot size and depth of focus to suite the particular laser processing application. A camera focusing mechanism is then useful to compensate for the variation in camera focus following a change in focus lens.
An illumination source 21 is mounted directly to the focusing mechanism so as to move therewith. Figure 10 is an enlarged view of this part of the mechanism and clearly shows the illumination means in the form of an LED 21 mounted to project through a hole 22 to transmit towards the compensating element 10. Light from the element is then reflected down towards the remainder of the optical assembly as shown in many drawings towards the workpiece. Instead of a single LED, a plurality of LEDs may be used or other illumination means.
The illumination sources are attached directly to the focusing mechanism to avoid relative movement between the illumination source and the compensation element. Such relative movement is to be avoided to prevent misalignment of the illumination source to the optical axis of the laser and camera paths.
The LED is most preferably a surface mount LED to retain a compact design.
The level of LED illumination is most preferably adjustable to account for variations in camera sensitivity and the difference in light intensity reflected back from different surfaces of the workpiece. For example, a matt black surface will reflect back a different light intensity to a highly polished reflective surface. Many means for controlling illumination intensity will be apparent but this may be via a standard LED dimming circuit. The best position of the LED relative to the recollimating lens depends on the type of LED used. To obtain a uniform illumination distribution at the workpiece it is often necessary to position the LED away from the focal plane of the recollimating lens.
In a preferred embodiment, the front surface of the compensating element 10 is coated to provide 50% reflection at 45° incidents over a bandwidth 450 nm to 650 nm to provide the reflective path from the LED illumination source 21 to the workpiece, and 50% transmission at 45° incidents from 450 nm to 650 nm to provide the visible path back from the illuminated workpiece to the camera.
The back face of the compensating element 10 is most preferably anti-reflection coated over the visible bandwidth 450 nm to 650 nm to avoid ghosting and glare problems.
A laser beam collimated by a lens of focal length 100 mm before the beam splitter would result in a typical distance D between the beam splitter and the end of the fibre delivered laser of about 130 mm. The diameter of the laser beam at the beam splitter would be 24 mm and this would result in a large beam splitter for accommodating the full diameter angled at 45°.
The arrangement of Figure 2 allows the laser beam to be divergent when it reaches the beam splitter. This arrangement allows the distance D from the fibre to the beam splitter to be reduced to 60 mm. The diameter of the laser beam at 60 mm from the end of the fibre is 14.5 mm which therefore allows a substantial reduction to the size of the beam splitter.
The resulting design is therefore a compact and lightweight processing head which is highly advantageous for most processing applications.
Figure 11 shows a machine head. This illustrates an input for a laser L and for a camera C, the beam splitter 3 and the compensating element 10. It also shows how the illumination path can be divergent through the beam splitter 3, via the element 10, or conversely convergent in the camera path reflected back from the workpiece through element 10 via the beam splitter 3.

Claims

Claims
1. An optical system comprising a laser path, target illumination and a camera path, wherein a beam splitter introducing astigmatism is included in a common part of the laser and camera paths and further comprising a compensating optical element mounted between the beam splitter and a camera or optical output, wherein the further element is tilted relative to the beam splitter to compensate for said astigmatism.
2. A system as claimed in Claim 1, wherein the laser beam is divergent when it first reaches the beam splitter.
3. A system as claimed in Claim 1 or 2, wherein the compensating element is orthogonal to the beam splitter.
4. A system as claimed in Claim 3, where the compensation element is inclined at 45° and rotated by 90°.
5. A system as claimed in any preceding claim, wherein the compensating element moves with, and as part of, a focusing mechanism.
6. A system as claimed in any preceding claim, including an illumination source providing illumination to the workpiece via the compensating element and directed towards said element.
7. A system as claimed in Claim 6, wherein the compensating element acts as a reflector for the illumination source.
8. A system as claimed in Claim 6 or 7, wherein the illumination source is mounted in a fixed relationship to the compensating element, so as to avoid relative movement between the source and element.
9. A system as claimed in Claim 8 when dependent upon Claim 5, wherein the illumination source moves with the focusing mechanism.
10. A system as claimed in any of Claims 6 to 9, including a recollimation lens, and wherein the illumination source is positioned away from the focal plane of the recollimating lens.
11. A system as claimed in any of Claims 6 to 10, wherein the illumination source is one or more LEDs.
12. A system as claimed in any of Claims 6 to 11, wherein the illuminating means is adjustable.
13. A system as claimed in Claim 12, including a dimming circuit.
14. A system as claimed in any preceding claim, wherein the beam splitter is at 45° to direction of the laser path at the workpiece.
15. A system as claimed in any preceding claim, wherein the compensating element is a plane parallel element.
16. A system as claimed in Claim 15, wherein the compensating element is a partial reflecting mirror.
17. A compact laser delivery head , including an optical system as claimed in any preceding claim.
18. A laser delivery head as claimed in Claim 17, including an optical output for transmitting the optical beam to a remote camera, mounting or processing apparatus.
19. A laser delivery head as claimed in Claim 17 or 18, including a laser.
20. A laser delivery head as claimed in Claim 17, 18 or 19, including an input for receiving a laser beam.
21. A laser delivery head as claimed in any of Claims 17 to 19, including a light source mounted within the head.
22. A laser delivery head as claimed in Claim 21, wherein the light source is directed towards the compensating elements.
23. A method of compensating for astigmatism caused by using a tilted element in a laser delivery systems, comprising providing an additional element tilted orthogonally to the first tilted element.
24. A method as claimed in Claim 23, including providing an illumination means impinging upon the additional element.
25. A method as claimed in Claim 23 or 24, including a focussing means, wherein the additional element is moved together with the focussing mechanism, whereby the system is focussed without any relative movement occurring between the additional element and the focussing system.
26. A method as claimed in claim 25 wherein the illumination means is also moved together with the additional element and focussing system.
27. A method as claimed in Claim 23, used in a laser delivery apparatus which is optically monitored by an optical illumination system using over the same optical system.
28. An optical head including a mechanism as claimed in any of Claims 1 to 22.
PCT/GB2006/050417 2005-11-30 2006-11-28 Laser processing tool WO2007063343A1 (en)

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DE102020112472A1 (en) 2020-05-07 2021-11-11 Jenoptik Optical Systems Gmbh Objective for a material processing device, material processing device and method for operating a material processing device
CN113814559A (en) * 2021-08-18 2021-12-21 深圳市大族数控科技股份有限公司 Cylindrical mirror-based laser beam astigmatism compensation method and device and storage medium
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Publication number Priority date Publication date Assignee Title
DE102020112472A1 (en) 2020-05-07 2021-11-11 Jenoptik Optical Systems Gmbh Objective for a material processing device, material processing device and method for operating a material processing device
DE102021204313A1 (en) 2021-04-29 2022-11-03 3D-Micromac Ag Process and system for manufacturing microstructured components
CN113814559A (en) * 2021-08-18 2021-12-21 深圳市大族数控科技股份有限公司 Cylindrical mirror-based laser beam astigmatism compensation method and device and storage medium

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