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Laser Technology

Optimal laser optics

By Russ Olexa

With new high-powered lasers moving into job shops and more laser companies introducing them, are users experiencing problems with the mirrors and lenses that don’t last as long as lower-power lasers? The answer is yes and no.

"We’ve certainly seen these high-powered lasers have an effect on optics," says Guy Goulet, technical services manager for Centricut, West Lebanon, N.H., a company that sells a broad line of lenses and mirrors for lasers. "The first thing is the heat absorption rate. As any laser beam, no matter what the wattage is, passes through a focusing lens, a certain amount of the energy is absorbed into the lens. It’s this absorption that causes a thermal breakdown that will cause the lens not necessarily to fail but to be consumed. So it stands to reason that the higher a laser’s wattage, the more absorption the lens has, and it will break down more quickly. When you’re up into the 7,000-watt range, you’ll go through more lenses than you would for a laser that’s 4 kW or less.

"Second, if you’re cutting thick aluminum or stainless steel--and these 7-kW machines can now cut up to 11/4-in. stainless steel--you’re cutting with nitrogen," he says. "The pressure that’s required of the nitrogen in order to cut, say, at 11/4-in. thickness puts a tremendous amount of physical stress on the lens. You’re talking 600 psi or more at large flow rates of 3,600 to 3,800 cubic ft. per hour. Just the physical stress of all that pressure against the lens in the cutting head certainly will have an effect on it. We’ve seen more cracked lenses than we have in the past. It’s not substantial, but it’s definitely becoming more prevalent."

Proper parameters
Mark Taggart, managing director of Laser Mechanisms Inc., Farmington Hills, Mich., a company that works with OEMs in the industrial, military and medical laser fields to provide beam delivery solutions, says it’s much more than a mirror or lens problem when there are difficulties with these components. "I believe it’s more of the process parameters of actually running the laser [instead of problems with the lens or mirrors]," he says. Taggart defines the process parameters as the ability to set up the laser for the best results for a given cut or material.

"For high-powered lasers, there are multiple process variances, such as the pressure of the gas, the type of gas, the type of material being cut, if it’s a spatter application where there’s a lot of debris coming up from the material, as well as other parameters," he explains. "There are many different variables that cause lens failure, beyond using a high-powered resonator. For instance, you could get spatter when cutting thick steel. But, it really depends on how the process of cutting it is manipulated. If you use a 2.5-in. focal length lens that’s close to the work, you’re much more likely to get spatter from the process than you are with a 7.5-in. or 10-in. focal length lens, based on the separation.

"You have your energy that can let you cut thicker plate or thinner plate faster," says Taggart. "You manipulate the laser beam to accommodate this process. You can focus at the top, middle or bottom of the material, depending on all of these different requirements. If all of these process conditions aren’t correctly done, especially cleaning the lens properly and at the appropriate intervals, you can have significantly reduced focusing lens life."

But James Rogowski, managing director at Trumpf Canada, Mississauga, Ontario, says that Trumpf has been building higher-wattage lasers for some time. The company will introduce a new 7-kW CO2 laser at the Fabtech International & AWS Welding Show in November. "I subscribe to both views," says Rogowski. "First of all, it’s true that when you have more power, there’s more stress put on the optics and mirrors. But I also subscribe to the fact that if you do your due diligence to the laser, there’s nothing to worry about. However, you have to take into consideration that the manufacturers of these optics have done a wonderful job of keeping up with the higher powers. So there’s new coatings, new manufacturing technology and new products that they’ve come out with within the last few years that have certainly allowed them to successfully evolve to the higher-power outputs.

"Every lens and mirror that we’ve put in our lasers has changed over the last four years," he continues. "Different types of coatings on the zinc selenide optics have helped quite a bit for the clarity and wear on them. They repel the heat and energy much better today, and you get a longer life out of the optic. The optic’s size and finish have changed, too. The finish is critical on a copper resonator mirror. If you were going to take one and just rough turn it on a lathe and look at the finish under a microscope, you’ll have little peaks and valleys. This is where the energy that comes from the light gets trapped. And if you can make the mirror’s surface as smooth as possible, or more smooth than what’s been done, then less energy will be absorbed into it. The quality of the mirrors has improved tremendously with optics companies using advanced ways to finish them."

But what does someone who’s on both sides of the fence have to say about lenses or higher-power lasers? Todd Jacobson, technical consultant with the Laser Maintenance Group, St. Charles, Ill., sees every manufacturer’s laser in the field and repairs them. He’s seen just about every problem that can occur with laser optics.

"With more power, you’ll see more thermal lensing taking place on the output coupler or the cutting lens," says Jacobson. An output coupler reflects the laser light within the resonator and allows a portion of it, usually about 50 percent, to pass through from the resonator to the cutting head. "These two optics, the coupler and focusing lens, especially have an issue," he explains. "With lower power, the lens is more forgiving. If you have a little spot of dirt or haze on them, there’s not enough power to really thermal lens and break it down. With a high-power laser, keeping them clean becomes very important."

Jacobson notes that when a focusing lens has thermal lensing, the power from the laser light is absorbed into it, and then it starts expanding from the heat. Instead of just passing all the light energy through, the thermal lensing will cause the optics to curve and improperly focus the beam rather than letting all the light energy pass straight through. The more it heats up, the more it will change focus.

"If you keep the lens clean, it’s not an issue," he says. "When it comes to lenses cracking, we’ve seen that it’s not cracking from the high pressure introduced to expel the melted material in the cut. There’s cracking because the focusing lens will have a film on its surface. It might have a hydrocarbon film buildup on it from the cutting process. This will cause it to absorb the energy, which in turn will cause it to crack and fail. Again, it’s a cleaning problem."

So how often should a lens be cleaned? "At minimum, each operator should start off with a clean lens on his shift," says Jacobson. "If they’re just cutting mild steel for the day, they can probably cut all day without having to clean it, even though mild steel is a dirty process because at times the steel will have oil on it. But, they’re still cutting with less power. Another issue arises when companies cut with nitrogen or shop air because they’re using full power all the time, and the lens can get a hydrocarbon buildup that’s invisible to the eye. The only way to see it is with a polarizer. You can definitely smell it, though. A normal lens should have no odor. When they start to go bad, people describe the odor as a rotten egg or garlic."

Jacobson says that training is a big issue for any laser machine because without proper training, an operator can make any laser that’s in perfect condition cut poorly. "When you get into the higher-powered lasers, people buy these for thicker materials like aluminum and stainless," he says. "These materials require a greater skill level to cut."

Laser life span
Jeff Hahn, national product manager for Mitsubishi Lasers, part of MC Machinery Systems Inc., Wood Dale, Ill., says his company is now offering a 6-kW CO2 laser. He remarks that the life of a lens in a higher-powered laser is going to be shorter, but it depends on the application and what a company is doing with it.

For instance, Hahn says that on a 4-kW laser, if aluminum is being cut more so than mild steel, the laser will run at a higher power and slower feed rate because of the reflectivity of the material. The output power of the laser will be greater in this application. Therefore, lens life will also be dictated by the cutting application.

At Prima North America Inc., Chicopee, Mass., Pieter Schwarzenbach, vice president of laser technology, says that the company has just introduced a new 5-kW CO2 laser and hasn’t seen any problems with mirrors or lens life. "We’re looking for the best optics available on the market," he says. "We stress optic cleanliness because we want to prevent any contamination in the CO2 resonator gas stream. Any contaminants in the gas stream may influence the resonator optics, but not so much on the lens. That’s why we perform the resonator assembly in a new, clean room. You also have to be careful not to contaminate the lens from oily cooling air that is delivered through the beam. In the material piercing process, you don’t want to have particles contaminating the lens. The process must be controlled by using the proper parameters for piercing pressure and piercing conditions."

John Annessi is the production manager at Prestige Metal Products Inc., Antioch, Ill., a job shop that laser cuts 0.005-in. shim stock and up to 1-in. steel. It has a 5.2-kW CO2 Bystronic laser and a 2-kW CO2 Cincinnati laser. Annessi says the company used the 2-kW laser to cut 1/4-in. stainless steel but was pushing it to its limits. Prestige Metal Products bought the higher-power Bystronic laser to give it more cutting latitude.

At first, Annessi was afraid of what the higher-power laser would do to his focusing lens. He broke two lenses in the first three months, but after that experience, the company never broke another one. Annessi made sure the cutting process parameters were correct on the Bystronic laser. "With the 2-kW machine, I never had a lens break," he says. "In the first three months that we ran the 5.2-kW machine, I shattered two lenses. We haven’t done it since, and it’s been two years. I think in the beginning we weren’t paying as much attention to the machine and its cutting process. Normally with the 2-kW machine, if you get a little dirt on the lens or contamination in the system, you can keep cutting and clean it up later. With more wattage, if you don’t take care of these issues right away, you’ll have problems. If you get something on top of your lens and you don’t get it cleaned up right away, then it’s burned in and can cause problems. Paying a lot of attention to the maintenance of the machine really seems to make a big difference in its performance."

He adds that by tweaking the cutting process on the Bystronic laser, it experienced the same lens life as the Cincinnati laser. "We could’ve had operator error when we first began using it because, with the new 5.2-kW machine, we were cutting thicker materials. When you get into thicker materials and higher wattage, you have to be more attentive to the little problems that you can have with the cutting process."

Output coupler mirrors
But what about the internal mirrors of the laser? Would a 5-kW or 7-kW laser put more stress on a mirror than a 4-kW laser?

"A higher-power laser puts more stress on optics because you have more power, and with higher power, you’ll have more absorption of energy into your optical mirror," says Rogowski. "The manufacturers have increased the quality and are using different coatings. One of the things that’s susceptible to this power is the output coupling that not only reflects the laser light but also allows it to pass through from the resonator to the cutting head. All lasers have output couplers. It’s the transmissive optic that lets some of the light out of the resonator. On a 5/8-in. diameter beam, about 50 percent of the light is let out for cutting.

"With higher-power lasers, we see areas that need improving," he says. "These couplers are in the 4,000-hour to 6,000-hour usage phase before replacement. What we’ve done is introduce a machine that includes the option of a new coating, a diamond coating to replace zinc selenide. Diamond won’t break down thermally. Laser light at 7 kW of power, hitting a diamond coating, does absolutely nothing to it. So thermally, we have a stable optic. So we’ve gone from 4,000 to 6,000 hours for replacement for a zinc selenide coupler to somewhere in the 20,000- to 30,000-hour range. Then we’re not even talking about replacement, we’re talking about taking it out and refurbishing it. The diamond will need to be buffed, and then it can be reused."

"Fortunately, most lasers have been using more copper mirrors," adds Goulet. "The manufacturers have been proactive about cooling these. The key for the optics to function properly is proper cooling. Of course, as the wattage goes up, the need for bigger cooling systems on the mirrors and the optics is increased. So, we’ve seen a shift to copper mirrors instead of the half-and-half silicon copper ones."

Lens care
Taggart says that the two key elements for refractive optics are choosing a good substrate and proper coating that adheres to that substrate. If someone can do this consistently, he can validate his optics’ recipe. "There are also some exotic attempts in the marketplace with optics that use special coatings and substrates for higher-powered lasers," says Taggart. "{These are unique solutions from individual optic companies to accommodate these lasers.

"But most importantly, how does the operator take care of the lens?" he asks. "Does he clean the lens by blowing it off and making sure there’s no debris on it before using it, as opposed to dragging and wiping an alcohol pad or whatever is specified by the optics company across it? If there are any particles on top of the lens, they’ll create a little scratch as a wipe or pad is dragged over the lens. Every time an operator improperly cleans an optic, it reduces its efficiency and life."

Along with keeping a lens clean, both Jacobson and Goulet recommend specific optics for higher-power lasers such as Ophir Optic’s Black Magic and II-VI’s MP5. Both use a low-absorption coating to give long focusing lens life for high-wattage lasers. FFJ

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