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International Crystal Labs Describes Laser Optics Maintenance



Sensitive laser optics are invariably found in vulnerable locations in industrial laser systems. These optics are pounded by spatter during normal use in cutting, marking or welding ("CMW") applications. In a CO2 laser cutting system a lens focuses the laser beam on a substrate. These lenses are typically made from ZnSe and form the barrier between spatter generated in the cutting or marking process and the chain of sensitive and expensive optics that lead back to the resonator. Welding and some marking CO2 lasers use mirrors to focus the beam. The performance of a focusing lens is enhanced by an anti-reflective coating that increases the power transmitted through the lens and also prevents reflection back into the resonator. Mirrors are first surface coated to prevent absorption losses, so unlike a household mirror with a reflective second surface that is protected by glass, the mirror coating is directly exposed to ambient conditions bound to include spatter in the welding or marking environment. Proper cleaning will extend the life of CO2 laser lens and mirrors and also of YAG optics.

Spatter build up on a ZnSe focusing lens or a mirror causes the quality of the cutting, marking or welding that the laser is performing to deteriorate over time. Performance degradation in laser optics is caused to some extent by light scattering, but most of it results from heat buildup as the optic absorbs more energy because of contamination. As heat builds up in the optic it distorts in shape, thereby altering the focus and power is lost from absorption. There is a progressive degradation of weld, cut and marking quality. As the quality of CMW degrades the point is reached where spatter damage to the laser optic is so severe that it must be replaced due to the power loss and focus distortion. If the optics are not timely replaced there can be severe consequences. ZnSe lenses, for example, can melt or explode from absorbing too much heat, consequently annihilating the protective barrier between spatter and the rest of the optics in the system leading back to the resonator and resulting in a catastrophic chain reaction failure mode that causes many thousands of dollars of damage and takes days if not weeks of downtime to repair.

Laser optics should be cleaned using a solvent applied with either lens tissue or a special cloth provided in a polishing kit by an optics supplier. The optics must be cleaned delicately, not scrubbed, as aggressive cleaning will ruin the AR coating (which is invisible to the naked eye) on a lens or window and the first surface coating on a mirror. The same technique applies YAG windows, Zones lenses and mirrors. At International Crystal Laboratories we prefer heptane for cleaning because it is safer than acetone and the vapors are easier to tolerate, but acetone can also be used on some optics. Both acetone and heptanes are readily available from laboratory supply houses such as Aldrich, Alfa Aesar and Fisher. Optical cleaning kits are available for many optics dealers, but the composition of these kits varies considerably from one dealer to the next. Although most OEMs recommend the use of lens tissue to clean optics, the author's personal experience is that lens tissue is useless on the shop floor and that laser techs have much better success cleaning optics using an appropriate micro fiber cloth stretched tightly over a flat glass substrate and a hand held air blower.

A significant proportion of YAG laser systems incorporate means to use protective disposable windows to protect the other optics in the system. YAG lasers can use AR coated BK-7 or fused silica windows as spatter shields. CO2 laser optics can also be protected with disposable alkali halide windows such as Potassium Chloride (KCl) or Sodium Chloride (NaCl) Lens Savers windows. CO2 laser cutting applications normally employ high pressure assist gases such as nitrogen (which keeps the cut in a stainless steel substrate from turning blue) and oxygen (which enhances the cut of mild steel) and which blow off excess metal slag from the kerf of the cut. In these applications, the protective window must operate in a neutral pressure environment to prevent it from breaking. The mounting system for protective windows must also allow air circulation between the window and the lens to eliminate heat buildup that may distort laser focus (US Patents 5,898,522 and 6,270,222). Protective windows generally cost less than 10% of the price of the optic that they are designed to protect and they are intended to be disposable, but they can be cleaned in the same manner as other laser optics to extend their useful lives.

Employing good optics maintenance techniques richly rewards laser shops. Optics replacement costs can drop 10 to 20% from proper and timely cleaning and by as much as 70% when use of protective windows is coupled with an optics cleaning maintenance program.

Authored by Robert Herpst, International Crystal Laboratories

For more information contact:

Robert Herpst

International Crystal Laboratories

11 Erie Street

Garfield, NJ 07026

973-478-8944

info@lenssavers.com

www.lenssavers.com

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