Protecting your optic investment, and your laser’s productivity
Todd Jacobson, Technical Consultant, Laser Maintenance Group

Reflecting back to 1989 when I entered into this career eager to apply my degree in Laser Technology, mainstream fabrication lasers were typical low powered 800 and 1200 watts. With today’s lasers now at 5000 and 6000 watts, I find there are still common areas of maintenance and problems between them. However, some of these issues are exponentially magnified with the higher wattage.

We will cover routine maintenance items that the majority of operators or maintenance personal can comfortably handle. However, until you have been properly trained, or even when in doubt you should always have a trained technician complete or assist you with the tasks.

We will start this issue with a topic that is commonly over looked, and explain the pros and cons of your decisions in handling this. The Focusing lens, we all have them, but do we take care of them as we should?
Let’s start with the pros: One school of thought states, “Only clean your optic when you can see that it’s dirty.” doing so will minimize the chance of scratching the thin coating of thorium fluoride applied to the surface. Scratches will increase light absorption and scatter. Additionally, cleaning optics takes valuable time away from the production schedule.

On the opposing side, failing to periodically inspect the lens could have a multitude of unwanted and potentially costly results. Two conditions can exist; a visibly dirty lens, or a lens with a hydrocarbon film not seen by the naked eye. Although you might not be able to see it, you can smell the difference. A clean lens will have no odor while a contaminated lens may take on an odor described as rotten eggs with the intensity depending upon the level of contamination.

Contamination (regardless of the type) will cause thermal lensing. This is a condition where the lens starts absorbing more of the precious power we pay extra for when buying our laser systems. The result is a focal point that drifts due to the temperature dependent index of refraction, ultimately causing dross on our parts. In response to a poor cut, the operator will adjust cutting parameters, in a futile effort to correct the problem. Unfortunately, he will continue to adjust, trying to compensate for a forever-changing focal point all the while getting further and further away from good parameters. Your operator will eventually be frustrated, the manufacture gets sub standard parts and, or extra labor costs to correct the parts. While struggling to get acceptable parts, production levels are reduced and good cutting parameters lost. Once the operator cleans or replaces the lens the suffering is only half over. Now the operator will have to adjust parameters settings in order to get them back to their original position, continuing the painful time lost to reduced production.

If the lens is ignored long enough the optic will stress and end in a catastrophic failure. You can use a Polarized Stress tester to check for damage. Typical cutting lens are made of Zinc Selenide (ZnSe). 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 is setting up diffraction patterns for your eye to see.

To detect stress regions in the lens, place the lens between the polarized stress tester and hold 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 optic manufactures such as Ophir Optic’s MSDS for a complete report. Besides the health issue, your system will require extended down time and costly repair to thoroughly clean the debris from your beam delivery guide and undoubtedly replace burned components in the cutting head.

One of the biggest 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 your hourly costs. Lens inspection should be one of the first things your operator checks when there are problems with the cut quality. It’s time well spent considering the consequences if you do not.