Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/04—Automatically aligning, aiming or focusing the laser beam, e.g. using the back-scattered light

Definitions

• the present invention relates to an apparatus for determining the position of the focal point in a laser machining system in which a laser beam is transmitted via an optical fibre to a workpiece to be machined by the laser beam.
• laser machining systems are frequently used for cutting, welding and marking operations in the manufacturing industry- Thanks to the recent development of optical fibre technique such laser machining stations are designed with flexible transmission of the laser beam to the work- piece by means of optical fibres.
• a machining head is mounted in the arm of an articulated robot for manipulating the laser beam relative to the workpiece.
• the z coordinate position of the focal point is often dis- placed in high power machining systems if the optical system is not the best. Such displacement has usually not been considered in prior adjustment systems, due to the fact that the adjustments in such cases must be carried out at full laser power conditions.
• figure 1 illustrates an optical fibre machining head
• figure 2 illustrates the shape of an focused laser beam
• figure 3 illustrates a detector for determining the waist of the laser beam in x, y and z coordinates
• figure 4 illustrates three different cases (A, B and C) for the image of the end surface of the fibre on a mirror
• figure 5A is a first example of the detection of back-reflected radiation according to the invention.
• figure 5B is another example of the detection of back-reflected radiation according to the invention.
• figure 6A illustrates an embodiment with a so-called beam dump for determining the position of the focal point at full laser power condition
• figure 6B is another example with a direct liquid cooling for determining the position of the focal point at full laser power condition.
• Figure 1 illustrates schematically an optical fibre machining head. A laser beam is transmitted from a laser source, not illustrated here, via an optical fibre to the workpiece to be machined. The end surface 1 of the optical fibre, i e the end surface of the fibre faced to the work- piece, is imaged on the workpiece 3 by means of optics 2.
• the shape of the focused laser beam is illustrated in figure 2.
• the beam converges to a minimal sectional area and then diverges with the same angle.
• the object of the invention is to determine this waist 4 in the z coordinate as well as in the x and y cordinates.
• Figure 3 illustrates a detector for determining the x, y and z coordinates.
• the main part is a position sensitive detector plate 5 covered by a mirror 6 and possibly a damping filter 7 therebetween.
• the mirror has a specific transmission for the wave-length in question.
• the x and y coordinates are then determined by means of the position sensitive detector plate 5, which for instance could be of the type lateral photo diode or a CCD-array.
• a damping filter 7 between the mirror 6 and the detector plate 5.
• the z coordinate (i e perpendicular to the plane of the detector plate) can be determined by adjusting either the detector or the focusing optics along the z axis and then take into account that part of the radiation which is reflected back through the fibre to the laser.
• Figure 4A illustrates a case in which the image of the end surface of the fibre is located below the surface of the mirror. The back-reflected image is then located behind (above in the figure) the end position of the fibre, and there is only a small part of the reflected laser radiation which is transmitted back through the fibre.
• Figure 4C illustrates a corresponding situation in which the image of the end surface of the optical fibre is located above the surface of the mirror. The back-reflected image is then located iri front of (below in the figure) the end surface of the fibre and, again, there is only a small part of the reflected radiation that is transmitted back through the fibre.
• Figure 4B illustrates a case in which the image of the end surface of the optical fibre is located on the surface of the mirror.
• the back-reflected image will be located on the end surface of the optical fibre and substantially all of the back-re- fleeted radiation (beam power) will then also be transmitted back through the fibre.
• the maximum of the back-reflected radiation i e the radiation that emerges from the end surface of the optical fibre on the laser side, the z coordinate position of the focus point could be determined.
• the detection itself of the back-reflected radiation can be carried out in different ways, for instance:
• FIG. 5A The principle is illustrated in figure 5A.
• An angular, anti-reflex coated plate 10 is arranged in the beam path between the laser 8 and the outgoing fibre 9. Back-reflected radiation is detected through the residual reflection on the plate by means of a detector 11.
• damping unit 12 in the form of one or more partial transmissive mirror plates are arranged in the beam path between the laser 8 and the outgoing fibre 9.
• the damping unit 12 is angulated as in figure 5A and the back-reflected radiation will be reflected by the damping unit 12 and detected by detector 11. Detection of back-reflected radiation, alternative 3:
• the principle is illustrated in figure 5C.
• the laser beam is deviated by means of a mirror 13 towards the optical fibre 9.
• the mirror has a small amount of optical leakage which means that back-reflected radiation can be detected behind the mirror by means of a detector 11.
• the detector 11 which detects the residual radiation can be of the photo diode type or a thermal sensor. Such detectors are known per se and will not be described in any detail here.
• a detecting mirror 16 When determining the z coordinate position of the focal point for a laser operating at full power it is preferred to use a detecting mirror 16 having a low reflection only.
• the reflecting surface of the mirror must withstand high power densities, which means that the type of material is critical. In case of Nd-YAG laser radiation, uncoated quartz is recommended as it has a reflection of 3,5 % only. If also the back side of the mirror is cooled by means of a suitable liquid, the reflection on the rear surface will not disturb the measuring operation.
• a so-called beam dump 14 is arranged below the measuring mirror
• an alternative embodiment with a direct liquid coolant 15 is illustrated.

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• Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Mechanical Engineering (AREA)
• Laser Beam Processing (AREA)

Priority Applications (4)

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Publications (1)

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Citations (7)

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• 1998
  • 1998-05-07 SE SE9801590A patent/SE9801590L/ not_active Application Discontinuation
• 1999
  • 1999-05-05 WO PCT/SE1999/000756 patent/WO1999059764A1/en not_active Application Discontinuation
  • 1999-05-05 AU AU43053/99A patent/AU4305399A/en not_active Abandoned
  • 1999-05-05 US US09/674,805 patent/US6528761B1/en not_active Expired - Fee Related
  • 1999-05-05 JP JP2000549417A patent/JP2002515343A/ja active Pending
  • 1999-05-05 EP EP99952082A patent/EP1091824A1/en not_active Withdrawn

Patent Citations (7)

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