Dean P. Spacht and Anthony J. DeMaria
Recent developments have reduced the heat-affected zone in certain materials processing applications and simplified integration of lasers into robotic systems.
FIGURE 1. The SPL series of CO2 lasers from DEOS rely on an electro-optic modulator to produce 100-ns Q-switched pulses without a long decay tail.
The carbon-dioxide (CO2) laser is considered by many to be the most mature industrial laser because of its long, successful history in materials processing applications from laser cutting and welding to surface marking. Despite this maturity, important innovations continue to be made in commercial CO2 laser technology, particularly in the low and medium power ranges, where sealed laser heads combine high reliability and low operating costs. Two recent advances include Q-switched lasers with pulse durations shorter than 100 ns and air-cooled laser heads with powers up to 100 W. These developments, which improve the performance and economics of existing applications, also enable CO2 systems to expand into applications that were formerly the realm of higher-cost laser types.
Shorter pulses, colder processing
The RF-excited, sealed CO2 laser produces a well-behaved TEM00 beam, operates in continuous wave (CW) or pulsed modes, delivers years of maintenance-free operation, and offers wall-plug efficiencies up to 10%. The laser head also can be very compact and relatively inexpensive. Output is in the mid-infrared (IR) with a wavelength between 9 and 11 µm depending on internal optics. This long wavelength, which is strongly absorbed by a variety of work materials, is a major reason for the widespread use of CO2 lasers in materials processing applications.
The long wavelength, however, can be a potential disadvantage with CO2 lasers. Specifically, material removal occurs through intense local heating. This can damage the surrounding area, producing a heat-affected zone (HAZ). In plastics and paper, the damage can take the form of charring. In ceramics, thermal damage manifests as microcracking and local buildup of a glassy phase. For micromachining, marking, and other high-performance applications, it is imperative to minimize the size of the HAZ, by reducing the amount of the input laser energy that ends up as peripheral heat flow.
The HAZ can be reduced by using a small focused spot size and operating the CO2 laser in a fast pulsed mode, rather than continuous wave. Essentially, the tight focus confines the energy to the desired target area, and the short pulse duration reduces the time during which the heat can flow into surrounding material.
FIGURE 2. This view of a folded-waveguide laser illustrates how direct access to the ground electrode allows the option of air-cooling by providing the capability to force air through the channels formed by the cooling fins and the outer case of the laser.
As with most gas lasers, it is reasonably straightforward to design a sealed CO2 laser to produce a beam with an M2 value of 1.2 or less, thereby allowing small focused spots. Fast pulsing has proved to be another matter. The simplest way to pulse the laser is to switch the RF power source used to excite the CO2 plasma. This can produce pulses as short as 10 µs. With the correct duty cycle, resulting peak powers can be slightly higher than the laser's cw power level. The materials processing performance of such lasers is definitely an improvement over a cw laser, but still results in a significant HAZ in plastics and delicate materials.
A shorter pulse is also possible with the use of an internal electro-optic modulator as a Q-switch (or cavity dumper). In the past, Q-switching was limited to military and high-value applications, because the cadmium-telluride (CdTe) modulator crystals had to be carefully selected for zero flaws. Even then, the modulators only had a short lifetime. DEOS has eliminated these drawbacks in its SPL laser series through the use of a modulator with a patented window design that produces a very high optical damage threshold with regular grade CdTe crystals.
In the SPL lasers, the modulator is switched a second time at the end of each pulse to remove the long tail characteristic of Q-switched CO2 pulses. As a result, the laser produces pulse durations of only 100 ns with very short rise and fall times (see Fig. 1). One benefit is the dramatic HAZ reduction in plastics, such as kaptan and polyimide, which allows the creation of finer cuts and holes. Moreover, paper can be perforated and cut with virtually no charring.
Just as important, the short pulse duration produces very high peak power. With a pulse energy up to 0.5 mJ, the peak power can be greater than 3 kW from a compact laser head rated at 20-W average power. This high peak power allows the laser to process difficult materials such as fiberglass composites, and even ceramics, which would normally require a laser rated at several hundred watts or more. The Q-switch also supports pulse repetition rates of up to 100 kHz, which helps boost the throughput of industrial processes.
Examining air cooling
Like all gas lasers, the sealed CO2 laser also generates considerable heat in the sealed laser head, which must be removed to maintain laser efficiency. In lasers with output power exceeding 50 W, this heat is usually removed by flowing cooling water through the laser head. In the case of slab lasers, the water flows through one of the slab electrodes to ensure efficient cooling. The water-cooling device is normally a closed-circuit system with a chiller hidden in the laser power supply, which can make it more difficult to integrate the lasers into certain machining operations. This cooling method also may be undesirable in some applications.
FIGURE 3. This plane view of a folded-waveguide laser is of a typical five-mirror, NV cavity configuration.
One way around the cooling issue involves the use of an oscillator configuration that delivers all the size and efficiency benefits of a slab laser design, but also allows direct access to the ground electrode and facilitates air-cooling when required (see Fig. 2). From a cooling standpoint, the key feature is that the ground electrode mates directly to the aluminum body of the laser via a series of fins. Either water or forced air (from internal fans) can flow through the cooling channels formed by these fins. With this approach, DEOS already has supplied 100-W air-cooled lasers for several applications. This configuration is also scalable to higher output powers.
In practice, the choice of cooling with water or air is application specific. In cleaner applications, OEM integrators and their end users usually prefer the air-cooled configuration because of its simplicity and capability for compact integration. The latest laser-based engraving systems, for example, are now somewhat near the size of a small photocopying machine.
In dirtier operating environments, water cooling is typically preferred, since dirt and dust could collect in the air flow channels over time, reducing heat flow and diminishing laser efficiency. A typical example of such an application is laser marking of consumer products.
Shrinking laser heads
Industrial applications also are benefiting from the continued reduction in the ratio of the size of the laser head to the output power. While slab discharge was an important breakthrough in this regard, this approach requires the use of an internal spatial filter to optimize output beam quality and water cooling. The SPL laser design offers the small size benefits typical of slab discharge, but produces a better output beam. There is no need for a spatial filter in the laser head because the plasma discharge is produced in ceramic grooves that form a folded waveguide. The most typical configuration of this waveguide is in the form of the letters NV with five cavity mirrors and one output coupler, although cavities with even more folds are not uncommon (see Fig. 3).
This type of monolithic folded resonator results in a compact, lightweight package attractive for robotic applications. Until recently, this market was completely dominated by fiber-delivered Nd:YAG lasers, which can have a higher cost of ownership, as well as problems related to the tendency of the delivery fibers to degrade due to the high laser fluence. Although CO2 laser output cannot be readily transmitted through optical fibers, the latest air-cooled sealed lasers are now small enough to mount directly on a robot arm. For example, a major auto manufacturer is now using such a setup to cut headlamp mounting holes in plastic. This 20-lb laser produces an average power of 100 W, but measures only 21 x 4.5 x 3 in.
Wavelength selectivity and laser longevity
The use of multiple laser mirrors in a folded cavity has another important benefit. By using wavelength selective coatings on these mirrors, it is possible to force the laser to oscillate at only one wavelength. The output thus can be factory selected at 10.6, 10.2, 9.6, or 9.3 µm, which can be beneficial when the laser is to be used for remote sensing applications or trace-gas detection. This feature is also useful in plastics processing, in which 9.3-µm radiation produces much better cuts and holes than the longer-wavelength lines.
There have been other important advances as well in recent years. For instance, manufacturers of sealed CO2 lasers have continued to improve laser reliability and longevity, maximizing the value of these technical innovations for the end user. In the early days of sealed CO2 lasers, it was not uncommon to return the laser every six months for gas refilling. Now, with the latest metal ceramic cavities, novel sealing techniques, and internal gas reservoirs, these lasers offer typical lifetimes of 20,000 hr or more, with power degradation of less than 10% per year.
All in all, these recent innovations in sealed CO2 lasers are delivering important benefits to OEM systems integrators and their end users on several fronts. The lasers can reduce HAZ in problematic work material, as well as provide added features such as a lower cost of ownership and a smaller size that supports easier integration into manufacturing lines. These benefits should ensure that this established laser technology remains a dominant force in materials processing applications.
DEAN P. SPACHT is sales and marketing manager, and ANTHONY J. DEMARIA is chairman and CEO at DEOS Inc., Bloomfield, CT; email: [email protected].