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The shield is up

Understanding how debris shields can extend the life of high-power laser optics

Throughout industry, high-power laser systems are used for cutting sheet metal among other industrial applications. These systems’ sensitive optical components are prone to damage from the errant debris created during operation. As the laser beam size increases, so too does the risk of damage to the laser optics. Even the smallest dust fragment can reduce the performance of the equipment.

Debris shields combat these issues. Also called protective windows or cover slides, debris shields are specifically designed to prevent damage to the optical components in laser beam delivery systems caused by dust, shrapnel, fumes, fragments and other pollutants. By keeping contaminants away, debris shields extend the life of the high-power optics.

“Think of it like a windshield for your laser,” says Tara L. King, sales manager for United Lens Co. “It’s protecting everything behind it. When people are using their lasers to cut sheet metal, for example, it is hazardous for them when the metal slag flies back and is kicking all over the place.”

ULC’s debris shields are designed specifically for fiber lasers. They are flat on both sides. The type of attachment, such as a drawer-type set up, depends on the machine configuration and is specific to the laser. The shields are produced in several diameters ranging from 5 mm to 135 mm with a typical thickness range from 1.5 mm to 3 mm and a thickness tolerance range from ±0.20 mm to ±0.020 mm. The 55-mm-dia. shield, for example, has a thickness of 1.5 mm and a tolerance of +0.15/-0.0.

“It just depends how the laser equipment manufacturer needs the debris shield to fit in,” King says. “The shields are customized to the specific needs of the laser manufacturers. We are a build-to-print job shop for optics so we can make any kind of modification.”

Shield features

ULC’s debris shields have ultra-low reflectivity that can be set to the desired wavelength, including 1,070 nm, which is a common industry wavelength. The shields also feature transmitted wave front distortion.

“The transmitted wave front distortion is how well the light passes through,” King says. “TWFD tells us how much the light deviates from one surface to another on a minute level. We want the TWFD to be as good as it can be for debris shields so the laser beam can go through without any interference.”

The shields also have a surface accuracy of 2λ, λ/4 or λ/20, depending on part geometry.

“Surface accuracy typically refers to how flat a piece of glass is, and, in the case of these debris shields, is measured in TWFD instead of reflected wave front distortion because most important is how well the beam travels through the debris shield,” King explains.

A surface quality (scratch and dig) of 10-5 is achievable with the debris shields. The scratch and dig standards of military spec Mil-PRF-13830B refer to cosmetic defects. The debris shields also have a high damage threshold, which means a long life for continuous wave (CW) and pulsed lasers. The shields are independently certified to withstand power densities up to 28J/cm2 @ Nd:Yag pulsed, 10 ns/20 Hz laser damage threshold testing.

In general, the more parallel the two surfaces of the optic are, the better the beam will travel through them. Some shields have a parallelism of less than 10 arc seconds.

Founded in 2016, United Lens is celebrating its 100-year anniversary. Watch the video to learn more about how the company has grown throughout the years.

Bottoms and tops

The two most common debris shield substrates are fused silica, which has good chemical resistance, UV transparency and nearly zero thermal expansion, and fused quartz, which has a wide operating temperature range, resistance to thermal shock and low coefficient of thermal expansion. Additional material types are also available.

In addition to various material types, specialized coatings, such as anti-reflective (AR) coatings, are available. AR coating wavelengths range from 248 nm to 2,500 nm, which is good for minimizing heat in pulsed and CW lasers.

“We can tweak the coating if necessary,” King adds. “If the customer needs the coating to work a specific way at a certain wavelength, we can provide that.”

The life of the debris shield depends on the user’s process and atmosphere and the type of material being processed. “Users can go through a shield every couple of hours or for some it may take a week,” King says.

ULC’s debris shield longevity can be prolonged with proper maintenance. Rotating the debris shield to a new location after each use means the shield can be used multiple times. ULC recommends the user try to avoid shooting only the center of the workpiece and instead shoot off-center and at the outside, much like the motions of the hands on a clock.

As this New Year begins, it is of note that 2016 marks ULC’s centennial anniversary. Founded in 1916 by Italian immigrant Fileno DiGregorio, ULC has been under the same continuous family ownership and is on its fourth generation.

The company began as a modest producer of molded spectacle lens blanks to become a producer of thin film-coated, polished and precision-machined optics. The ISO 9001:2008 certified company supplies the laser industry with machined flow tubes, spacer cavities, laser windows, debris shields, high damage threshold reflectors, finished mirrors and other optic components. Additionally, ULC’s inventory is always stocked with diverse materials for fast turnaround times.

United Lens Co.