Meniscus lens element Clear Magic™

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Absorption, Thermal Lensing, and Clear Magic Dianne Devereaux-Michael, CO2 Sales Manager, Ophir Optics During laser operation with several kilowatts, the focusing lens is heated because it absorbs a small portion of the laser power. The anti-reflection (AR) coating was developed for CO2 lenses many years ago when lasers were lower powered than they are now. They were the best coatings available for many years. Now, of course, the average laser machine is no longer 1K to 2.5K, but can be up to 5 or 6K or more. These equipment improvements required that new optical coatings be developed to handle the thermal demands of higher power. Absorption takes place mainly in the AR coatings or because of dirt on the lens. A brand new, clean lens with standard AR coating typically has absorption of 0.2% of the incoming laser power. A CO2 laser lens, such as Ophir's Black Magic™, is a low absorption lens (lower than 0.15%). During use in a CO2 laser cutting machine, absorption rises 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%. The best coating for a CO2 lens in the industry is Ophir’s Clear Magic™. This coating is guaranteed to be lower than 0.13% absorption When a laser is cutting it creates heat, which causes additional surface curvature due to thermal expansion and increases the refractive index of the lens material (ZnSe). As a consequence of these effects, the lens focal length becomes shorter and the focus position cannot be predicted exactly because it depends on many parameters, like laser power, intervals laser on/off, cleanliness of lens, and others.