CO2 Technology Continues to Evolve
Dan Nash, Coherent Inc.
The first commercial application for CO2 lasers was wire stripping — using a large, flowing gas system that required regular repair and maintenance. Today, the latest developments for this type of laser include a system scheduled for launch into space; it is small, lightweight and capable of five years of continuous operation. Things have changed.
When selecting laser technology for almost any application, systems integrators and end users both look for low cost and high reliability. Along with the various CO2 configurations, most integrators also consider other options, particularly diode-pumped solid-state laser systems. But because the size, efficiency, reliability and lifetime of both laser types are similar, selection usually depends on which wavelength is best suited for the application. For instance, when processing brittle materials such as ceramics and glasses, or oxides, organic materials and plastics, CO2 lasers have an edge because of the high material absorption in their wavelength range.
After selecting the wavelength region, systems designers can choose from several approaches that have evolved to adapt CO2 technology to meet the various needs of different applications.
In the first CO2 lasers, a DC voltage applied to opposing electrodes excited the gas. This simple design was inexpensive and effective, but it had its limitations. The discharge had to be convectively cooled by flowing gas to obtain a reasonable output power of 100 W per meter of discharge length. Electrode sputtering was also a problem in that it limited the operating lifetimes of sealed-off lasers to approximately 1000 hours or less. Although this technology is still used today, it is effective only at high power levels.
The introduction of radio-frequency excitation in the early 1980s was a major technology shift in that it allowed diffusion cooling of the discharge. Diffusion cooling occurs when the “hot” molecules of the discharge “diffuse” to the water-cooled electrodes, where they are conductively cooled. Without the need to convectively cool the discharge, the rotating machinery that had been used to flow the gas was eliminated. This, along with reducing electrode sputtering problems, dramatically boosted system lifetime and enabled the output power of radio-frequency-excited sealed lasers to increase from a few watts to more than several kilowatts. Today, they can have a lifetime exceeding 20,000 hours, without maintenance.
Over the years, several CO2 laser configurations have evolved. The slab configuration consists of water-cooled electrodes and curved optics. The beam is confined along one axis, and the electrodes and curved optics are along the other. Efficient cooling of the plasma, combined with the fact that the output power will scale with the area of the electrodes, allows this configuration to produce several kilowatts cost-effectively.
The sealed-off version of the slab laser is housed in a small package that provides significant average and peak pulse powers and requires no maintenance (see figure). Flowing-gas CO2 slab lasers achieve the highest powers, up to 40 kW, but these configurations typically require maintenance of mechanical blowers and have costly consumables.
Slab designs allow a reduced footprint for CO2 systems as compared with conventional fast-axial-flow configurations.
The waveguide configuration uses ceramic waveguides that have a small cross-sectional area, confining the beam in two directions. This produces a high mode quality such that no additional optics are required to produce M2 values close to 1.0. The waveguide also allows the use of flat optics, which facilitates folding of the cavity, easily enabling compact systems in the 200-W range. The relatively high gas pressures used in the small cross section also enable generation of output pulses with fast rise and fall times for modulation exceeding 100 kHz. This allows quicker, more uniform and more predictable ignition of the laser plasma — an important feature for lasers used for rapid prototyping, marking or engraving applications.
Pumping up the applications
A Q-switched CO2 laser is possible with a variation of the waveguide configuration. This involves adding an electro-optic modulator inside the laser cavity to perform the Q-switching function. The laser can then obtain pulse widths approaching 0.1 μs with peak powers greater than 100 kW at pulse rates of up to 100 kHz. The short, controlled pulses obtained from the Q-switched laser minimize the heat-affected zone on the work surface, which is a significant advantage because it enables the user to work with plastics, glass, ceramics and printed-circuit-board materials.
The unique characteristics of CO2 lasers also are being exploited in sensing applications. CO2 laser transmitters for heterodyne remote sensing systems achieve optical frequency stability below 1 kHz. This laser type also tends to oscillate on one line, which makes it attractive in coherent ladar systems. It also is finding use as a pump source in emerging terahertz lasers. Flexibility comes in part because the lasers can achieve pulse widths of 15 to 20 ns by employing simultaneous Q-switching and cavity dumping techniques.
In essence, the evolution of CO2 lasers has been driven by the needs of the applications for which the 9- to 11-μm wavelengths are the best choices. Cost and reliability have been key elements, but technical characteristics such as power, peak power, pulse repetition frequency and pulse shape have led to important improvements. As the number of materials processing, scientific and remote sensing applications continues to grow and new types of applications are discovered, we can expect to see CO2 laser technology meet the challenges and adapt to these demands.
Meet the author
Dan Nash is manager of CO2 product marketing at Coherent Inc. in Bloomfield, Conn.; e-mail: firstname.lastname@example.org.
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