Paul Sechrist and Sri Venkat
Ever since the first use of a CO2 laser in an industrial materials-processing application, engineers have worked to improve the performance, reduce size and operating costs, and increase reliability and processing efficiency of these lasers. Developments have included radio frequency (RF) and pulsed dc excitation, new sealed plasma tubes, improvements in pumping flowing-gas lasers, and robust all-metal construction.
For example, the current generation of low-to-medium-power (<500 W average power), sealed, pulsed slab-discharge CO2 lasers are much smaller in size as compared with flowing-gas lasers and yet can provide considerable materials-processing power.
Shrinking laser footprints
The two principal components in a CO2 laser are the laser head, or resonator, and the power supply. Head size is a function of its design, output power, and efficiency. Higher output means a bigger laser, as does a less efficient design. Also, with the trend toward miniaturization of the power supply's electronic component with reduced thermal constraints, power supplies have recently become smaller. Their size also depends on the pumping mechanism required by the design of the laser system (dc-excited vs. RF-excited).
Also contributing to the size reduction of today's sealed slab-discharge laser (SSDL) designs is the use of a parallel-plate discharge, instead of the typical cylindrical symmetry, which requires a long discharge tube. The slab system's discharge, which is roughly rectangular, produces high beam power from a small volume, with the internal optics converting this efficiently into output power.
Compactness means the slab devices can mount directly onto a numerical control (NC) machine. A SSDL rated at >150 W-the equivalent of >400 W in a flowing-gas laser-can be as small as 40 x 8 x 6 in. and weigh less than 65 lb. It also is easier to mount the device to a gantry because a simple umbilical connects the laser head to the power supply. Gas lines are eliminated, and the laser and its power supply can be separated as much as 50 ft, which is useful with large travel gantries. An equivalent flowing-gas medium-power CO2 laser for cutting thin metal or ceramic is often too bulky to mount directly on a mill spindle or gantry (see Fig. 1). The laser beam must therefore be delivered through an articulated arm that contains several beam-folding mirrors.
The key to high performance from a SSDL is the high peak-effective power, excellent pulse shape, and beam quality. Depending on the design, some SSDLs can cut, mark, or weld with two to three times the efficiency of conventional flowing-gas CO2 lasers of equivalent power. It has been demonstrated that a SSDL with 500-W nominal power can process the same thickness of mild steel at virtually the same speed as high-performance flowing-gas lasers rated at 1 kW (see table above).
A 500-W SSDL can deliver pulses with 1500-W peak power. The beam is highly focused (0.075-mm diameter), with instantaneous intensity more than three times that of a comparable flowing-gas laser. This high intensity speeds up the vaporization of the work material, especially metals, to allow greater processing speeds and deeper cuts. Furthermore, some of the advanced models of SSDLs available today can pulse at repitition rates up to 100 kHz (see Fig. 2). Pulse customization can mean faster changeover between multiple materials-processing applications (see Fig. 3).
Newer sealed slab lasers can often process materials once considered difficult or impossible for systems with lower average powers. Some 100-W units, for instance, will begin processing 1.5-mm-thick mild steel by first piercing through the surface in a high-peak-power, low-repetition-rate mode. While maintaining high peak power, they can then switch to a higher-repetition-rate mode to complete the process with high-speed cutting. Without this dual capability, a conventional CO2 laser would require at least 300 W to perform the same task.
Processing speeds on all materials (metal and nonmetal) can be greatly enhanced with use of a smaller focused spot. This will produce sharper, cleaner cuts or welds and a minimum heat-affected zone (see Fig. 4). The "perfect" laser beam, with M2 = 1, is circular or Gaussian, with spatial propagation to infinity without diverging.
FIGURE 4. Sharp, clean cut made with a sealed slab-discharge laser can often eliminate the need for secondary processing to clean up the part edge. Surface roughness of this 304SS part, 2 mm thick, was 50 µin. after a single pass of the 500-W SSDL.
The SSDL inherently produces a beam with M2 in the range 1.1 to 1.5, compared to the 1.2-3.0 typical with other existing industrial CO2 lasers. The area of the focused spot on the work surface can thus be more than four times smaller with the pulsed slab device, which would increase the power density by a factor of four and shorten the ablation time.
Different materials applications demand different repetition rates. Metal cutting is best achieved at 1 to 2 kHz, while repetition rates as high as 100 kHz are best for nonmetal applications and metal-welding applications. One way to increase the materials-processing speed is to pulse the laser beam at a high repetition rate (up to 100 kHz). This technique has previously been used with great success with flowing-gas CO2 lasers, although at slower rates.
Pulsing the output beam also allows the laser to process thicker or tougher material than could otherwise be cut, although at a slower rate. For this pulsing process to be effective, the rise-and-fall time of the laser pulses must be short compared to the pulse duration. The laser power must reach a critical level for processing to begin. Below this level, the beam merely heats or melts the material, wasting valuable laser power and increasing the size of the heat-affected zone.
The lower limit on rise-and-fall time of a CO2 laser is ultimately a function of the nature of the electrical discharge within the laser. A SSDL has a rise-and-fall time of less than 50 µs. Consequently, by pulsing such a laser at a 50% duty cycle and 5-kHz repetition rate, the peak power will be up to three times that of the average power.
Optimizing operating costs
Operating costs for a CO2 laser are affected by several direct and indirect cost factors. Direct costs are determined by laser longevity and reliability, electricity and water consumption, and CO2 gas consumption.
Current state-of-the-art completely sealed (hard-seal) SSDLs feature robust, all-metal construction and sealed optics. Lifetimes more than 20,000 h between refurbish requirements, with no maintenance expected during this period, have been demonstrated. Lower electrical costs are also characteristic of the sealed slab-discharge laser. A 500-W pulsed SSDL can deliver more processing power than a flowing-gas 1500-W laser, while drawing less electrical power than a 300-W flowing-gas device. In addition, no external CO2 gas supply is required, and the cooling system is a closed-circuit design, which eliminates both the costs of consumable CO2 gas and cooling water.
Indirect factors reducing laser operating costs are the absence of a beam delivery system, the small size, and the fact that no vacuum pumps are required. For instance, the capability to mount the laser directly over the work surface means there are fewer optics. Moreover, the device's mirror does not need to be replaced or adjusted, unlike with many flowing-gas lasers, which have output and internal mirrors that require periodic replacement and realignment.
Throughout the materials-processing operations, a more-efficient laser design can translate into time savings. For example, warm-up time is reduced compared to other laser types, and, in many cases, secondary processing is not necessary because the reduced heat-affected zone makes the laser cut edge acceptable as final quality.
CO2 laser technology remains a dynamic field. Technical advances continue in response to market demands for better lasers. SSDLs have become the dominant choice for many applications that until recently could only be done by much-higher-power systems.
PAUL SECHRIST is vice president, CO2 Business Unit, and SRI VENKAT is a CO2 product manager with Coherent Inc., Laser Group, 5100 Patrick Henry Drive, Santa Clara, CA 95054; e-mail: [email protected]; [email protected].