CO2 Lasers Make the Cut

Dr. Holger Schlüter

With system revenues well above $2 billion last year, laser cutting remains the major industrial market for high-power laser sources. In fact, advances in machine dynamics and beam delivery systems have led to cutting speeds, and ultimately production costs, that make lasers unrivaled for flexible cutting of sheet metal up to 1 in. thick. Plasma and water jet cutting are orders of magnitude slower and are limited to excessive thickness or nonmetals. Punch presses, which laser cutting systems can now rival in productivity, do not offer the flexibility of tool-free sheet metal fabrication.

Fueled by developments in radio-frequency (RF) technology, by turbo radial blowers with actively controlled magnetic bearings and by adaptive resonator optics, compact high-power CO₂ systems are the tool of choice for many cutting applications. They should retain this cutting edge for some time to come — in part because the beam-parameter product (the possible intensity over a given depth of field) at any given CW power remains unprecedented among commercial high-power laser systems.

Although CO₂ laser systems can offer power in excess of 20 kW, available intensity at the workpiece for a given depth of field — the Rayleigh range, defined as the distance over which the focus intensity decreases by a factor of 2 — approaches its maximum near 5 kW.¹ Over the past 20 years, this optimum has shifted from 1 kW of maximum processing power in 1980 to 5 kW in 2000, with the addition of about a kilowatt of laser power approximately every five years (Figure 1).

Figure 1. Multikilowatt solid-state lasers are chosen whenever flexible beam delivery is a priority, while high-power CO₂ systems for the moment retain an economic advantage in conventional 2-D sheet metal cutting.

Advances continue with the recent industrial introduction of the metal vapor plasma cutting process for sheet metal up to 10 gauge (3 mm). The significantly higher energy transfer efficiency of this process promotes cutting speeds that are three times higher at these gauges. Because the melt dynamics requires a certain minimum feed rate and the light-matter interaction needs a specific intensity range, the introduction of this process to two-dimensional sheet metal cutting was possible only through the combination of advances in source development and machine dynamics.

Higher-power processing

Today the market volume for multikilowatt CO₂ and solid-state lasers is approaching equality. The distance between the available power for these two systems has always been about one order of magnitude, which is comparable to the difference in their electrical-to-optical (wall-plug) efficiency as late as 1998 (2 to 3 percent for lamp-pumped solid-state devices and 10 to 15 percent for CO₂ lasers). The commercial introduction of diode-pumped high-power solid-state lasers with output power of 4 to 6 kW began to change this equation. Although this dramatic change has made no visible impact on the shop floor as yet, the advance in laser power for solid-state devices may increase in the years to come.

Modern CO₂ lasers typically require an investment about two to three times lower than solid-state lasers (regardless of whether lamp- or diode-pumped) with comparable power, but multikilowatt solid-state lasers with fiber optic beam delivery have the advantage whenever flexible, cost-effective beam delivery is a priority. Still, CO₂ lasers in the single-kilowatt range should continue to dominate laser cutting for at least a decade because of their unique power and intensity at unrivaled costs per watt.

In the 1980s, transverse-flow CO₂ lasers rapidly approached power levels in excess of 10 kW. However, customer requirements, such as high beam quality, minimum floor space, low maintenance and process repeatability and control, have prevented the prevalence of these laser systems in highly productive environments. The fairly recent introduction of diffusion-cooled high-power CO₂ lasers adds another flavor to the market, although these systems have yet to find widespread acceptance in laser cutting.

To understand some of these issues, it helps to examine the beam-parameter product of modern fast-axial-flow CO₂ laser systems for different power levels (Figure 2). The beam-parameter product is a more generally applicable form of the well-known diffraction limit number M², because it allows one to compare the focus radius w₀ at a given depth of field z_R for lasers with different wavelengths. At 10.6 μm, a CO₂ laser with an M² of 1 has a beam-parameter product of approximately 3, while an Nd:YAG laser at 1.064 μm yields roughly 0.3 at the diffraction limit.

Figure 2. The beam-parameter product w₀θ_f of modern CO₂ lasers is in a range of 3 to 5 up to power levels of 5 kW. Red and orange areas indicate the parameter range for solid-state lasers. The effect of the beam-parameter product of CO₂ lasers on the intensity at the workpiece (blue line) is quite dramatic. Because the intensity is inversely proportional to the square of the beam-parameter product at a given depth of field z_R, the higher output power cannot make up for the increase in the product above 5 kW.

This said, it is astonishing that CO₂ lasers offer the best beam-parameter products. Commercial multikilowatt solid-state devices (lamp- or diode-pumped) have beam-parameter products five to 10 times higher than CO₂ lasers of comparable power. The reason for this is that they are based on rare-earth-doped single crystal rods (typically Nd:YAG). In these extended solid laser media, the temperature distribution caused by the waste heat of the pump process accounts for several effects on the optical resonator, such as thermal lensing and thermally induced birefringence, which are not easy to compensate for.

Now examine the average intensity possible with current CO₂ lasers at a given power level, assuming a standard 5-in. focus head with a beam diameter of 14 mm on the lens. This choice of boundary conditions keeps depth of field z_R constant. For laser cutting, the depth of field is as important as the intensity. The required intensity must be held throughout the full depth of the cutting kerf. With sheet thickness up to 1 in., this requires substantial depth of field. Therefore, for industrial sheet metal cutting, beam-parameter products must be 6 mm·rad or below, with the highest available power yielding the maximum cutting speeds.

Currently, the beam-parameter product of a fast-axial-flow CO₂ laser increases to a value above 10 at about 6 kW, decreasing the available intensity at the workpiece below that required for productive laser cutting. The effect of the reduced value is much stronger than the increased laser power, because the intensity scales with the inverse square of the focus diameter and, therefore, with the inverse square of the beam-parameter product.

As the power and intensity of modern CO₂ laser sources increase, the power handling capability of the focusing optics also must advance. Recent developments in low absorption coatings for ZnSe lenses have been as important for laser cutting technology as the advances of the laser sources themselves.

This year will see the commercial introduction of diode-pumped solid-state lasers based on disc-shaped laser media rather than on rods, which will dramatically lower the beam-parameter product of multikilowatt systems based on this technology. Still, the price per watt for CO₂ lasers remains unrivaled even by such systems. This is why Trumpf engineers believe that advances in CO₂ laser technology will ensure its place in laser cutting for some time.

Laser source technology

Several recent technological advances in support systems are key to CO₂ laser performance in sheet metal cutting (Figure 3). Because gas lasers are characterized by a laser medium with very low thermal conductivity, efficient heat removal is a necessity. This requirement led to the development of transverse-flow lasers, where gas exchange involves huge fans and gas flow perpendicular to the optical axis. So far, though, this concept has not produced the necessary beam-parameter products and beam stability for successful laser cutting.

Figure 3. DC-coupled discharges, common in early gas laser systems, require the electrodes to be placed inside the gas volume. This creates a possible source of contamination due to cathode sputtering. RF excitation allows placing the electrodes outside of the gas discharge volume (left). Modern gas lasers utilize turbo radial blowers with actively controlled magnetic bearings (right) and thus have a lubricant-free gas system.

For this reason, fast-axial-flow gas lasers dominate the laser cutting industry. Here, gas flows parallel to the optical axis of the resonator. This allows limiting the laser medium to the mode volume of the desired mode; for instance, diffraction-limited TEM₀₀. Restricting mode volume are the fused silica tubes with tightly controlled tube diameters (on the order of 10 to 20 mm) and folded laser resonators 5 to 6 m long.

In such a system, the cross section of the fused silica tube and, ultimately, the cross section of the desired mode restrict gas flow. Speed of flow must be subsonic, and the maximum flow rate in a fast-axial-flow CO₂ laser will depend on the maximum allowable mode diameter. Currently, diffraction-limited beam quality is not accessible at powers above roughly 5 kW, but further advances in gas dynamics and adaptive optical resonators should extend this limit to higher power levels.

With flow speeds approaching fractions of the speed of sound, any particles in the laser medium would ultimately destroy the resonator optics. Therefore, manufacturers assemble modern CO₂ lasers in cleanrooms, with great attention paid to particle contamination.

Because the excitation of the laser gas itself can be a source of particles, early DC-excited gas discharges were replaced with capacitively coupled RF discharges where the electrodes could effectively be placed outside the laser medium. In this case, the energy couples to the discharge through the dielectric fused silica tubes. The DC excitation requires the anode and cathode to be placed inside the laser medium, producing particles by cathode sputtering.

Blower technology also is critical for achieving the enormous gas-flow rates required for efficient cooling of a multikilowatt CO₂ laser. But because the flow is restricted by the cross section of the quartz tubes, conventional axial fans used in transverse-flow lasers cannot produce the pressure difference necessary to drive the gas flow in the small fused silica tubes.

Until 1990, only bulky and extremely heavy roots blowers could produce the flow rates of several thousand cubic meters per hour at the necessary compression ratios of 1.3 to 1.4. Requiring extensive floor space and frequent maintenance, they were also a source of contamination because of the large bearings and the wear within the pump. The introduction of turbo radial blowers running at speeds of 40,000 rpm or more have made possible supercompact high-power gas lasers. Today a 10-kW CO₂ laser head needs no more than 20 cubic feet of space and has a footprint of approximately 15 square feet.

Most important for the recent development of lasers with powers in excess of 3 kW was the introduction of a completely oil-free gas system employing actively controlled magnetic bearings for the turbo radial blower. This technology ensures a contamination-free gas.

Beyond mechanics

The introduction of linear motors, in particular, also allowed end users to take advantage of the metal vapor plasma process. The possibility of accelerations of 20 m/s² or more was key for the use of the process in 2-D laser cutting machines. It is important to note that the minimum contour size that a machine can support depends inversely on the acceleration. For example, with an acceleration of 2 m/s², the smallest part feature that can be cut at 20 m/min has a tangential curvature of more than 50 mm, while the same speed can be sustained down to tangential curvatures of about 5 mm for accelerations of 20 m/s². Small features still must be cut using the conventional cutting technology, but the new process can handle a majority of features.

Figure 4. The recent multiplication of cutting speeds at a power of 5000 W was made possible through the introduction of the metal vapor plasma (MVP) cutting process (lower right). Conventional cutting depends on the absorption of the laser radiation on the molten material at the cutting front as shown in the drawing (lower left). During MVP cutting, a keyhole forms that is surrounded by molten material, increasing the coupling efficiency between the incident laser beam and the material.

To understand the metal vapor plasma process better, it helps to examine the basic laser cutting process, which creates a front of molten material in advance of the laser beam. Because of the steep angle of incidence between the molten material and the CO₂ laser beam, which approaches the Brewster angle, the absorption is much higher than for normal incidence. Removal of the molten material requires the use of high-pressure cutting gas. Reactive laser cutting using oxygen assist allows taking advantage of the exothermal reaction of the oxygen with the metal in addition to the laser power to boost feed rates. If an oxide-free cutting edge were required, one would switch to nitrogen cutting gas.

The metal vapor plasma that forms would normally shield the material from the laser energy, but processing with the beam above a specific intensity produces an unstable process with pulsating plasma formation. Above a certain minimum feed rate, however, a stable regime forms. In this case, the metal vapor plasma absorbs the laser beams. The high feed rate causes the molten material to form a closed contour around the plasma. The resulting keyhole in the material — very similar to the keyhole caused by deep-penetration laser welding — enhances process efficiency.

This process can be sustained in thin sheet metal — up to 10 gauge — only at high feed rates. Below a thickness-dependent feed rate, the process becomes too unstable to sustain a continuous cut. Although this process has been known for years as linear laser slitting, it wasn’t until the linear motor and other recent advances in machine dynamics that it became viable for 2-D laser cutting.²
Today, modern sheet metal cutting systems with RF-excited 5-kW laser resonators, lubricant-free turbo radial blowers and linear motor technology can achieve cutting speeds of 40 m/min in thin sheet metal. This yields a dramatic improvement in productivity and cost for modern sheet metal fabrication.

References

1. U. Habich et al (1996). Proc. XI International Symposium on Gas Flow and Chemical Lasers and High Power Laser Conference 3092, pp. 174-177.

2. K-U. Preißig, D. Petring and G. Herziger (1994). SPIE Proc. Series 2207, pp. 96-110.

Meet the author

Holger Schlüter is director of laser production and development for Trumpf Inc. in Farmington, Conn., and director of technology for Trumpf Photonics in Cranbury, N.J.