An old dog doing new tricks
January 9, 2012
Although the solid-state laser is considered new in metal cutting processes, the first laser demonstration in a laboratory in 1960 was a solid-state laser. CO2 lasers turned out to be more practical for cutting metals, but solid-state lasers are making headway. Understanding laser equipment and the physics involved in generating a laser beam helps to explain the capability differences between CO2 and solid-state lasers.
Lately many in the metalworking industry have been discussing solid-state lasers. New offerings from equipment suppliers in this field have generated many questions among fabricators. How are solid-state lasers different from conventional lasers? What are the advantages and drawbacks of each? Which type is best-suited to my operations?
You might already be aware that solid-state lasers can provide enhanced performance in cutting thin sheet metal and thin-wall tubing compared to traditional laser sources. They also have the potential to reduce operating costs for fabricators. Having said that, there is more to learn; specifically, what is driving this new wave of laser technology, and how can it benefit your manufacturing process?
It’s a good idea to start with the basics: What is a solid-state laser, and what makes it different from the CO2 laser that fabricators are familiar with?
At the risk of oversimplifying laser machines, every laser cutting tool consists of three components. The first is a pump source, which provides the energy to generate the laser beam. The second is what laser engineers refer to as a gain medium, which is the material that provides the photons of light that create the laser beam. The third component is the optical resonator, which is a set of aligned mirrors that reflect and amplify the photons, creating an intense, directional beam of light.
How is the light from a laser different from light emitted by an ordinary source, such as a light bulb? First, ordinary light contains many different wavelengths, or colors of light, and rays travel in all directions. A laser beam consists of light rays with a single wavelength and the rays travel roughly parallel to each other in the same direction. In short, laser light is monochromatic and coherent.
This is a simplified way of looking at a laser, but sufficient to understand the differences among various laser types.
The pump source in a CO2 laser is usually high-voltage DC or, in the case of RF-excited lasers, radio waves. The gain medium is the CO2 gas, which is pumped with this DC or RF energy to excite it. When CO2molecules reach a high enough quantum state, due to the addition of this pump energy, they begin to emit photons of light. A mirror at each end of the resonator reflects the light, giving it intensity and coherence.
The same principles apply to a solid-state laser, but the particulars change a bit. In solid-state lasers, the pump energy is actually light: Low-power diode lasers, which operate by converting electrical current into light, are stacked together in groups to provide the power supply for the lasing process. Instead of CO2 gas, the gain medium is a solid material (hence the term solid state), usually a crystal of yttrium aluminum garnet (YAG) doped, or coated, with an active material, most often the elements neodymium (Nd:YAG) or ytterbium (Yb:YAG).
It is worth noting at this point that these solid-state lasers are not new. The first laser demonstrated in a laboratory in 1960 used a solid-state gain medium, a ruby crystal in the shape of a rod.
The pump sources available at the time weren’t very advanced. Early lasers were limited to producing brief flashes of light. They had no practical application until they could generate continuous waves of light, which was to follow quickly. In industrial applications, solid-state laser sources have long been the mainstay of robotic laser welding and 3-D laser cutting.
The key difference between gas lasers and solid-state lasers is the frequency of light they create.
A CO2 laser produces light with a wavelength of approximately 10.6 microns (µm), which places it in the mid-infrared spectrum of electromagnetic radiation. A solid-state laser generates light at a much higher frequency. The wavelength is approximately 1µm, placing it in the near-infrared spectrum. Differences in the way these wavelengths of light interact with matter lead to major differences in how the beam is directed to the cutting head and how the beam cuts material.
Beam Delivery. The pathway that delivers infrared light of a CO2 laser to the cutting area usually is a bellows pressurized with compressed air or nitrogen gas to keep out any dust or particulate, as well as a set of mirrors that direct light to the cutting head. The near-infrared light generated by a solid-state laser can be channeled and directed by a flexible fiber-optic cable. This cable can bend with the motion of a robot in three dimensions, which is why equipment that articulates in 3-D uses this laser source—it’s not practical to create this sort of cutting path with reflecting mirrors in fixed locations (see Figure 1).
Cutting Capabilities. This leads to a final question: If a solid-state laser has a clear advantage in terms of the beam delivery, why has it not been used for laser cutting all along? The two reasons concern the limitations of flash lamp technology and the characteristics of near-infrared light.
First, the flash lamps traditionally used for pumping these lasers aren’t very efficient. They consume a lot of electricity and have a short lifespan, thus requiring frequent replacement. Recent advances in pump source technology have resulted in replacing flash lamps with diode lasers, which have low power consumption and a long working lifespan.
The other reason concerns how laser light interacts with the metal it is intended to cut. The absorption rate of near-infrared energy by a piece of steel is higher than it is for mid-infrared energy. This means that the steel melts faster under this kind of laser light. Because the laser cutting process relies on using light energy to melt material and an assist gas to blow the molten material out of the cut kerf, the faster melting capability translates into faster cut speeds.
However, the near-infrared beam’s capability drops off quickly as the material thickness increases. To determine the laser source with the greatest productivity, it is necessary to evaluate each application individually (see Figure 2).
To summarize, the CO2 laser’s advantage is that it is versatile; it’s well-suited to cutting many thicknesses. For processing thin sheet metal or thin-walled tubular, the solid-state laser is faster.
What does all this mean to the average fabricator? If your focus is processing thin sheet metal material, whether flat sheets, tube, pipe, or profiles, the solid-state laser has the potential to improve your processing times and reduce the complexity of your machine tools, both resulting in lower costs per part. It also demonstrates how after 50 years of development and innovation, the laser as a manufacturing tool continues to evolve and open up new avenues to profitability.