Illuminating High Power Lasers

The industrial laser is no longer a "gee-whiz" technological wonder. It has become firmly established in metalcutting as the tool of choice for many applications. So established, in fact, that many fabrication shops are bringing back work they once sent out to laser specialty houses for processing.

Many familiar industry themes are driving this trend in shops. They include shorter lead times, quality control and demanding customer delivery schedules. But not insignificant in this trend is an increasing understanding, call it demystification, of the laser as a manufacturing tool. Consequently, shops are making the capital and personnel investments necessary to take control of their laser cutting applications.

For new laser users and those more experienced, continuous development of the laser itself has brought to market machines with increasing capabilities. At the same time, reductions in operating costs, made possible by technological developments, make a laser of 3 kilowatts (kW) comparable in operating costs to a laser of 2 kW. How is that possible?

In laser technology, it's not an apples-to-apples comparison. Moving above a laser power output of 2kW is technologically a bigger leap than many shops may realize. This article focuses on the technological issues that come into play with higher powered lasers. To find out about these lasers, we talked to Brian Jarvis, national product manager for LVD Corporation (Plainville CT), which markets high powered laser machine tools worldwide.

In The Beginning . . .

The first laser (Light Amplification by Stimulated Emission of Radiation) was developed in the late 1950s. It consisted of the same components and operated on the same basic principles as lasers made now.

Fundamentally, a laser consists of a cavity (glass tube) with mirrors at either end. One of the mirrors is 100 percent reflective, and the other is less than 100 percent reflective (usually 70 percent). Gas to be lased is pumped into the cavity and circulated by a turbine.

For our discussion, this gas is a mixture of helium, carbon dioxide and nitrogen gases--commonly called a CO2 laser. There are many other lasing materials available but, for higher powered, industrial lasers, the above CO2 mixture is the most common.

Without getting too deep into the physics of what's happening in a laser, lasers work because the atoms in the gas mixture are excited by an electrical power source or generator. When that happens, a photon of light is given off by stimulated atoms in the lasing gas mixture. These photons excite other atoms that then give up a photon, and very shortly, a chain reaction is formed.

The photons produced in the tube oscillate back and forth between the two mirrors until a portion of the power is allowed to pass through the partially reflective mirror. The beam that escapes from the partially reflective mirror becomes the cutting medium.

The nature of the photons (light) given off in a laser makes it a practical tool for manufacturers. Laser produced light is monochromatic--meaning it has the same frequency--and coherent. This means the frequency is in phase. Focusing the beam gives the laser its power.

Beyond 2 kW

In industrial metalcutting applications using CO2 lasers, two basic types come into play. These are CO2 lasers excited by direct current (DC) and CO2 lasers excited by radio frequency (RF). For industrial lasers up to approximately 2.2 kW, a DC excited laser is most commonly used. It's generally less expensive to manufacture and generally consumes less power than an RF excited laser.

However, once a laser moves into power outputs above about 2.2 kW, an RF excited device is recommended and has become more commonplace in the industry, says Mr. Jarvis. The primary difference between the two types is the location of the electrodes that excite the CO2 gas mixture. In a DC laser, the anode and cathode are located inside the glass plasma tube. An RF laser excites the gas with electrodes mounted externally to the plasma tube.

At the upper end of its power curve (approaching 2.2 kW), a DC excited laser gas mixture becomes more unstable. Pumping that much current directly through the cavity causes gas to begin to dissociate (unmix), which degrades the beam being output by the laser. This results in poor cutting performance by the laser machine tool.

It's this small difference in excitation method that allows the RF CO2 laser to produce usable and stable beams up to 10 kW.

Weighing The Costs

Traditionally, a DC laser had several things going for it in the marketplace compared to RF excited designs. Typically, DC lasers use less electrical power to excite the lasing gas. On the down side, it consumes more gas. With the electrode inside the tube, heat and dissociation require replenishing the gas more often. A fair approximation of run cost for a 2 kW DC excited laser is about $8 to $12 an hour, says Mr. Jarvis.

Until recently, an RF laser generated power with vacuum tubes. It cost more to run than a DC design--even in the same output range--because it used significantly more electricity. What's changed is the use of solid circuitry to produce the RF excitation in place of vacuum tube technology.

An RF laser still uses more power to excite the lasing gas but the percentage difference between it and the DC design is less with solid state electronics. This cost difference is offset further, because with the power source outside the laser cavity, less gas is usedhalf as much in some lasers. The RF laser actually runs cooler, which keeps the gas mixture viable longer.

Reductions in power requirements for solid state RF lasers has made a 3 kW RF cost about the same--$8 to $12 per hour--to operate as a 2 kW DC excited laser. "It used to be a general rule, comparing a vacuum tube RF and a DC laser, that the RF uses approximately twice the power and half the gas. With increased power consumption efficiency of transistorized RF, it's now a cost wash," says Mr. Jarvis. "This is a big reason why shops are moving toward the higher power lasers. It translates into more productivity at the same cost."

What's More Power Give You?

Until the advent of solid state electronics in RF excited lasers, 2 kW was pretty much the power threshold for efficiently used metalcutting lasers. However, because operating costs for the RF excited CO2 laser have been brought into line, shops are asking for more power.

"What's not well understood is that power increases in lasers are not linear," says Mr. Jarvis. "Doubling the power does not mean doubling the cutting rate." Higher power does, however, allow a shop to cut thicker materials, up to about 3/4-inch mild steel plate and 3/8-inch stainlessoxide free. It also lets shops cut thinner materials faster.

How fast? Well, if a shop is cutting light gage sheet metal with a 1.5 kW laser and upgrades to a 3 kW, the productivity increase is about 10 to 15 percent. This is probably not justifiable due to the higher price of the 3 kW laser. While operating costs for the lasers are comparable, it costs more to buy the higher power laser.

"Where it does make sense to consider higher power is in shops that are cutting a variety of material thicknesses and types," says Mr. Jarvis. For example, in plate thicknesses of 12 gage, 1/4, 3/8, and 1/2 inches, a 3 kW laser will give about a 15 to 20 percent increase in productivity over a 1.5 kW unit. In heavier gages, the same 3 kW RF laser can deliver a 20 to 30 percent productivity increase over a 2 kW DC laser.

"Now if a job shop works in both sheet metal and larger plate, the 10 to 15 percent increase in sheet metal production, coupled with a 30 percent increase in heavier plate, makes the high power laser look quite justifiable," says Mr. Jarvis. "That's what we're seeing in the marketplace. The bigger, RF excited lasers are not going eliminate DC CO2 units in sheet metal, but they do make sense for shops that work in both sheet and plate and are looking to increase productivity."

The added flexibility to run different gages is particularly attractive to job shops, because of increased potential market for these shops.

Mode Quality

Mode quality in lasers is the energy distribution of the beam through its entire power curve. From 1 to 100 percent, mode quality describes how well the energy distribution remains reliable under different power levels. It's critical in high power lasers that a stable discharge from the cavity be maintained throughout the entire power range.

For lasers up to 2 kW, a stable output design is used most often to create the working beam. "Basically in a stable output design, the beam diverges or spreads out from the cavity," says Mr. Jarvis. "The shape of the beam, in profile, is like a bell curveoften referred to as a Gaussian mode. Technically, its shape is TEM 00, which stands for transverse electromagnetic 00."

Much above 2 kW, it is not uncommon for resonators to use an unstable output design. "Unstable is an unfortunate term," says Mr. Jarvis. "The high power beam simply passes through a different area, taking power from the edge of the output mirror. This creates a different beam profile than a stable output laser. But, it's in no way inferior."

For high power lasers, the power often is taken from around the edges of the output mirror rather than from the entire output mirror surface. The optic delivery system conveys the output laser beam to the workpiece. The farther the beam travels, the more diverging (spreading) occurs.

To get the most cutting from the beam, a device often is used to concentrate the diverging beam back into a tight bundle. It's called a collimator. A collimator is an optic, in addition to the internal machine optics, and is generally found on RF high powered lasers or on higher powered DC lasers.

Lasers using an unstable design have a smaller mirror in the center of the output mirror. The beam profile created by this output mirror is called a TEM 01. Because in this mode most of the laser power is around the periphery of the beam, with less concentrated in the center, TEM 01 is also referred to as a doughnut mode. This beam shape distributes the high power of 3 kW and higher lasers over a larger area, giving consistent beam quality through the entire power rangefrom 1 to 100 percent.

Focusing On The Work

A laser beam is focused onto the workpiece to be cut. To get maximum productivity and the best cut quality, it's a good idea to match the focal length of the working laser beam to the depth of cut. Generally a focal length of 5 inches is sufficient for sheet metal and thin gage plate. However, as shops move into heavier plate work, focal length of the laser beam begins to affect the cut quality and edge parallelism.

The laser beam produced after passing through the machine focusing lens is "hourglass" shaped. The waist of the hourglass is the area of maximum power concentration. That's where the laser cuts best. Focal length is measured from the lens to the center of the focus waist. Common focal lengths are 3.75 and 5 inches for thin and sheet metal, and 7.5 and 10 inches for thicker plate.

A 5-inch focal length beam has a shorter "waist' than does a 7.5-inch focal length. In addition, the angles that form the top and bottom of the hourglass are more severe in a shorter focal length. While this is not a problem in thin gage material, it can result in poor edge quality for thicker materials. That's why matching focal length to material thickness is important.

For sheet-metal-only lasers, a single focal length (usually 5 inches) is sufficient. For high power laser applications, especially in shops that cut many different gages, it's recommended that relatively simple lens changeovers be available for the machine tool, says Mr. Jarvis. Using the proper focal length beam gives better quality cuts and optimizes production speeds.

Bigger Laser Machines

Larger table sizes are also part of the manufacturing trend to higher power lasers, according to Mr. Jarvis. In addition to more production flexibility for a wider range of gage sizes and higher processing rates, shops are asking for more work surface area to maximize the number of parts per sheet.

Traditional 96-inch by 48-inch sheet capacities are being stretched to 180 inches by 72 inches. "LVD has made machines with a work surface of 20 by 10 feet," says Mr. Jarvis.

Demand for these big machines is coming from shops that cut out large parts and prefer not to weld two smaller pieces. These larger machines also reflect better use of nesting software by manufacturers and product kitting practices. Shops are able to get complete kits of parts for a product or group of products from a single gage stock than changing to another gage and making complete kits from it.

"It boils down to the amount of time a shop has to produce its parts," says TEM00

Gaussian

Mr. Jarvis. "If one can cut a single part from a sheet instead of having to make two pieces and weld them together the shop's ahead. Likewise if a shop gets more parts from a single nesting, it too is ahead of the game."

Getting The Beam To The Work

Bigger laser machines can create unique beam delivery problems. Because it's generally impractical to carry the laser itself on the machine axes, a system to deliver the beam to the working zone of the machine is necessary.

For small travels, beam delivery generally consists of a tube with angular mirrors that direct the beam from the laser cavity directly to the work. Extremes between travels are not significant enough to affect beam integrity. But when a machine is capable of moving 200 plus inches from the laser source, beam distortion and divergence become problematic.

"Laser users are looking for a consistent beam quality and power over the full travel of a machine," says Mr. Jarvis. "To deliver this, beam length compensation devices are commonly used on larger machines." Beam length compensation is accomplished by using adaptive optics or fixed beam length devices.

Adaptive optics automatically move components within the beam delivery system based on programmed parameters or cutting head location. Constant beam length devices ensure the beam length is the same regardless of where the cutter head is located in the work zone. Beam compensation devices work to maintain power and focal positioning over long travel distances.

Other Considerations

Heat is a major output of high powered lasers. A chiller is required on any CO2 laser to keep the recirculating gas and optics from overheating. High power lasers use higher capacity chiller systems to keep the beam delivery system from getting too hot and to maintain laser stability.

For most laser cutting applications, an assist gas is induced into the cutting zone through a nozzle. It may be oxygen, nitrogen or air, depending on the material being processed. It's purpose is to help the laser beam penetrate the workpiece, facilitate absorption of the beam into the material and to blow molten material from the cut zone. For cutting heavy plate in mild steel, getting the assist gas into the work zone requires relatively low pressure. "On the other hand, cutting thick stainless steels, oxide-free, can require pressures of 300-500 psi with nitrogen generally used as the assist gas," says Mr. Jarvis. "A delivery system capable of sufficient pressure and volume for the nitrogen gas is an important consideration for shops that work with thicker stainless steels."

Maintenance considerations for higher powered lasers are also a consideration. While the solid state RF excited beam generation has less maintenance associated with it than DC or other laser systems, generally the skill level necessary for maintaining the unit is higher.

Is It For You?

Higher powered lasers are viable for manufacturing. It's important to understand the differences between processing with high power lasers and using 2 kW or less. Applying the higher power gives a shop more flexibility and higher processing speed potential. It's technology that's ready to go to work.