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Article: Protecting your laser’s optics
By Todd Jacobson, Technical Consultant, Laser Maintenance Group 

One routine laser maintenance task that most operators and maintenance personal can easily handle is cleaning the focusing lens. However, until an operator is properly trained, a qualified technician should always assist with this job.
A focusing lens keeps a laser running efficiently and productively. A dirty lens can cost a
company a lot of money. But should it be cleaned as often as an operator thinks it should?
One school of thought is to only clean your optic when you can see that it’s dirty. This will minimize the chance of scratching the thin coating of thorium fluoride on the surface.
Scratches will increase the absorption of light and scatter it. Also, cleaning optics takes time away from the production schedule.
On the other hand, failing to inspect the lens periodically could produce unwanted and potentially costly results. Two conditions might exist: a visibly dirty lens or a lens with an invisible hydrocarbon film.
Although you might not be able to see the difference, you can smell it. A clean lens will have no odor while a contaminated lens might smell like rotten eggs.
Regardless of the contamination type, it will cause thermal lensing. The result is a focal point that drifts due to the temperature-dependent index of refraction, ultimately causing dross on parts. In response to a poor cut, the operator will adjust cutting parameters in a futile effort to correct the problem. The operator’s frustration level increases and the manufacturer gets sub-standard parts and extra labor costs to correct them. While the struggle continues, production levels fall and proven cutting parameters are lost. Once the operator cleans or replaces the lens, the struggle is only half over. Now he’ll have to adjust parameter settings to get them back to their original position, increasing the time lost in production.
If the lens, typically made from zinc selenide (ZnSe), is ignored long enough, the optic will stress and break catastrophically. A polarized stress tester can check for damage.
The unstressed crystal structure in a lens contains dipole moments whose sum at their vertex is equal to zero. In the stressed crystal, there are domains where the sum of the dipole moments are not zero and therefore are polarized. When viewed with a polarizer stress tester, these domains will look like shadow lines or darkened regions. The polarizer sets up visible diffraction patterns.
To detect stress regions in the lens, place the lens between the polarized stress tester and hold it in front of a good light source. Then slowly rotate the assembly 360 degrees, looking for dark regions or distinct patterns.
Catastrophic failure will turn the ZnSe lens into a dangerous yellow powder. This powder is a serious health hazard requiring immediate evacuation of the area. Please refer to an optic manufacturer’s MSDS information, such as Ophir Optic’s, for a complete report. Besides the health issue, your system will require extended downtime and costly repair to thoroughly clean the debris from your beam delivery guide, and undoubtedly you will have to replace components in the cutting head.
One of the factors in determining your lens inspection interval is the type of materials you process. This interval should be frequent enough to provide a consistent process, therefore reducing hourly costs. Lens inspection should be one of the first things your operator checks when cut-quality problems arise. It’s time well spent considering the alternative.