PROBLEMS AT THE FOCUS IN C02 LASER PROCESSING SYSTEMS
The focusing lens is often the first item to be blamed when a laser system fails to provide an adequate cut or weld. This is rather strange, since over many years of observation by optical suppliers and skilled laser engineers, it has been found that the properties of C02 laser focusing lenses are almost invariably not the cause of problems in laser processing. There are very many reasons why process failure or component failure can occur, some more subtle than others.
Just about the only intrinsic reason for process failure ‘due to the lens’ would be supplier or customer misidentification of the required item, perhaps resulting in the use of a lens of incorrect focal length or of inadequate thickness for the assist gas pressure in use.
THE INCOMING BEAM
When system performance is not as expected, these are some of the features of the laser beam, incident upon the lens, that should be checked:-
- Is the beam correctly aligned to the centre of the lens?
- Is there beamclipping at the lens? Are the low energy ‘tails’ of the beam intensity distribution missing the lens mount?
Is the beam diameter within normal operating limits at the lens? (Ie: check for upstream thermal lensing which often reduces the beam diameter at the lens, or ‘ballooning’, perhaps due to atmospheric contamination in the beam path by solvents or other gaseous contaminants).
- Is the beam mode structure of normal operating quality? Is the beam cross-section reasonably circular?
Are there pointing instabilities in the beam? Beam pointing instability can be due to degraded laser cavity optics or unwanted movements in upstream beam delivery optics, especially mirrors.