DDM Laser

Light Amplification by Stimulated Emission of Radiation

Appendix

PROBLEMS AT THE FOCUS IN C02 LASER PROCESSING SYSTEMS

THE LENS MOUNT
Is the lens mounted with the correct orientation? For meniscus lenses the concave surface should be towards the workpiece.

  1. Is the lens securely fixed (does not move or ‘rattle’)?
  2. Check that the lens mount shows no signs of heating-up by a mis-aligned beam, or by an oversized beam. Signs of problems are removal of anodising (burnt-off by the laser beam), with debris evaporated onto the upper surface of the lens.

  3. Check that the lens clamp ring (or equivalent) is not over-tightened. Over-tightening imparts massive stresses and leads to a significant reduction in laser damage threshold.

  4. Does the lens mount have adequate cooling? If applicable, check that the water flow is not blocked.

  5. Was the lens (diameter) a tight fit in the lens mount? Mechanical parts should have approx 0.1mm / 0.004″ dia. clearance. If the clearance is inadequate, then laser-heating of the lens plus water-cooling of the mount can cause massive mechanical stresses, causing immediate degradation in function of the lens, and early lens failure due to cracking and/or burn-out.

  6. If lens breakage is a problem, check that the mount is correctly designed. If a mount with spherically curved spacers or clamp rings is used, then the mount design is probably the culprit. Absolutely avoid over-tightening until a correctly designed mount can be obtained.

  7. Check that the clamp ring is not cross-threaded and so seating unevenly on the lens.

  8. Dirty mounts/clamp rings can cause lens breakage, especially in conjunction with the use of curved spacers/clamp rings, over-tightening, and tight-fitting lenses. Mounting a lens onto a ‘large’ single particle, typically 50-100um in size, has been observed to cause linear, central cracking of thick ZnSe lenses.

  9. Check the cleaning procedure before installation of the lens. If a modern substitute for CFC-based solvents has been used then invisible residues, lasting for hours, can remain on the lens surfaces and cause significant performance degradation. Ideally, clean lenses using water-free acetone.

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