During laser operation with several kilowatts, the focusing lens is heated because it absorbs a small portion of the laser power. Absorption takes place mainly in the AR coatings and at the dirt on the lens. On a new clean lens with standard AR coating, absorption is typically 0.2% of the incoming laser power.
However, a lens with Ophir’s Black Magic coating has absorption of 0.15%. When used in a laser cutting machine, absorption increases gradually due to increasing amounts of dirt on the lower surface. When the lens needs to be replaced, absorption usually is in the range 0.3% to 0.4%.
Heating of the lens causes additional surface curvature due to thermal expansion and increases the refractive index of the lens material. Consequently, the lens’ focal length becomes shorter and the focus position cannot be predicted exactly because it depends on many parameters such as laser power, intervals laser on/off and cleanliness of lens among others.
The lens material may not get heated uniformly due to dirt particles on the lens, causing the areas close to the dirt to get heated. This impacts the focusing properties, increasing the focus diameter and lowering the cutting quality. When a ‘critical’ amount of dirt accumulates on the lens, it needs to be replaced.
Use of lenses with reduced absorption such as Ophir Clear Magic lens can make the focal length more stable, improving the reliability of the cutting process.
Key features of Ophir Clear Magic lens:
- Guaranteed absorption <0.13%
- Maximum focus stability
- Clear coating allows HeNe beams to be seen on the work piece
- Can be tested with polarising filters to detect thermally-induced stress
- Recommended and approved by leading OEMs
- Used for all high powered CO2 lasers including those over 5KW
- Industry-leading tough coating for excellent durability
- Ability to withstand back spatter
- Easier to clean and maintain
- Resistant to humidity
- Radioactive-free coating
- Excels in cutting aluminium and stainless steel
- Superior cost-benefit ratio