Laser Technology

Don't lose focus

By Sue Roberts

Cutting power of CO2 lasers has jumped dramatically since the first gas-assisted laser cuts were experimentally made in 1967. Thicker metal is now being cut, and cuts are made faster, putting heavy-duty requirements on every aspect of the machine tools.

By all indications, the upward power trend has not topped out at the 6kW-level systems now on the market.

This higher power equals extra challenges for laser optics, including the lens, the final optic in the laser's path. Manufacturers of the cutting machines, the optics themselves and the protective mirrors focus on quality and durability as the keys to keeping up with the laser-cutting requirements, continually making the process easier and more cost effective regardless of the wattage used.

One of the challenges for CO2 focusing lenses, especially in high-powered lasers, is to minimize heat absorption from the beam. Less absorption means less distortion of the lens due to thermal expansion. When the lens heats up in the center, it expands and creates a shorter radius of curvature for that surface, which in effect shortens the focal length of the lens and moves the focused spot position. Expansion and contraction from heating will also reduce the life of the lens itself.

Shape matters
No matter whom you talk with, the first and absolute requirement for a CO2 laser lens is a pure, high-quality Zinc Selenide (ZnSe) crystal. Second, grinding and cutting processes producing either a meniscus or a plano-convex lens from the raw material need to be precise.

Meniscus lenses have one concave surface and one convex surface, creating a smaller beam diameter and smaller spot size that produces the same power output in a smaller area than the plano-convex lenses. Mounted down, with the convex surface toward the material being processed, the meniscus lenses offer a high-accuracy cut and cutting speeds about 5 percent faster than the plano-convex lenses.

Plano-convex lenses have one flat surface and one outward-curving, convex surface. Typically used for high-turnover applications and cutting thicker metals, the lenses have a broader cut width that allows the laser's assist gas to ease the cutting process and provide a straight edge. Greater depth of field provided by these lenses maintains a straight, taperless edge in thicker materials.

The type of lens is usually specified by the machine manufacturer. European laser manufacturers typically choose meniscus lenses with accuracy as the primary goal. American manufacturers tend to use the plano-convex lenses with material-thickness cutting abilities and economy as significant factors.

Losing reflectivity, adding durability
Dennis Cope, president of optics manufacturer Ophir Optics Inc., says, "One of the challenges with optics of this nature, especially for high-powered lasers, is that you want very little absorption. And you want the lens surface to be as clean and defect-free as possible. You don't want the lens to develop any areas that absorb the energy, because those areas can get hot very quickly and that leads to failure of the lens.

"Once the lens begins to heat non-uniformly the laser beam can be affected, because the refractive index of the Zinc Selenide changes with temperature," Cope explains. "If severe, this may lead to fracturing of the lens due to the mechanical stresses that are induced within the lens from the non-uniform heating."

When the infrared laser beam passes through the lens there is some reflection from each surface. Anti-reflective (AR) coatings, applied to both surfaces of the lens, minimize reflectivity and allow more energy to be transmitted through the optic.

Cope points out that without AR coatings, surface reflection from lenses can be up to 30 percent in Zinc Selenide and up to 50 percent in other infrared materials. This reflected energy does not pass through the lens and therefore can't be used to do the cutting work. In addition, without AR coatings, there would be more laser light internally reflected between the two lens surfaces, further inhibiting the laser's power to the workpiece.

Several layers of different coatings precisely applied to the lenses work together to reduce surface reflection to nearly zero percent. Ophir's most recent coating for lenses, Black Magic, is a low-absorption coating that is durable and easy to clean, in many cases doubling the life of the lens.

Durability of the AR lens coating protects against damage in what can be harsh industrial environments, preventing scratches during cleaning and improving the ability to safely remove contaminants from the lens surface.

"An anti-reflective coating is a series of thin layers that vary in refractive index and are designed so that they don't reflect the infrared energy, but allow it to continue to pass through the lens with almost no reflection loss," Cope explains.

Keep it clean
At Prima North America Inc., Pieter Schwarzenbach, vice president of laser technology, echoes the importance of good substrate material and high-quality AR coatings for the meniscus lenses used in the Prima laser cutting machines. He also emphasizes the importance of keeping all of the optics free of sparks, smoke and spatter, as well as contaminants from the assist gas.

"Users should make sure that the lens stays clean by using clean assist gasses," he says. "Sometimes people talk about using compressed air as assist gas. If that compressed air is contaminated with oil or water, that contamination will affect the lifetime of the lens substantially."

Schwarzenbach recommends that rather than cleaning laser resonator optics, including the lens, on a regular basis, after every 1,000 hours or 2,000 hours of cutting, clean them only when there is a reduction in performance after 5,000 hours to 8,000 hours. "In some cases we observed that the cleaning process does more harm than good. If the optics are good, don't touch them."

Protective windows
Protecting the delicacy of the lenses within the industrial environment has led to the development of disposable windows designed as protective shields that are mounted between the lens and the cutting surface.

Robert Herpst, managing director at Lens Savers, a division of International Crystals Laboratories, says, "What makes these windows work is a system that lets the assist gas found in laser cutters circulate and equalize the pressure around the window which keeps the windows from breaking. The windows are mounted in a way that lets the pressure equalize around them so you get a high pressure on both sides of the window."

He continues, "Windows are used by people who have spatter problems, which are fairly common in a lot of laser-cutting applications such as galvanized steel and aluminum or when piercing thick material."

Spatter on a Zinc Selenide lens heats up faster than the lens material and transfers that heat to the portion of the lens where it rests. The spot of uneven heat can initially lead to power loss and focus distortion, and ultimately to the melting of the lens and a catastrophic failure. Because the focusing lens is also the seal at the end of a laser's optic system, a lens failure can lead to the contamination of the mirrors and lenses throughout the system, causing costly production delays and optics replacements.

Lens Savers is typically called upon by end-users to retrofit the window systems to their existing laser-cutting equipment. Protective windows, which began as an option for lower-pressure environments like laser welding, are now available to be applied to laser-cutting systems using high-pressure assist gases of virtually any wattage, focal length and brand of laser available.

Windows have been installed in "pretty much everything that's out there," according to Herpst. "A lot of the business involves working with the customers to find a way to get one of these into their system and get the assist gas and pressure to equalize."

Success in manufacturing CO2 laser lenses is a distinct science. According to Cope, "It's not magic, but it is very good technology with excellent control over all the processes." Properly choosing, maintaining and protecting those lenses just might be the magic that leads to efficient laser-cutting operations. FFJ


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