CO2 gas lasers still dominate sheet metal cutting, but they aren’t the only game in town anymore. Solid-state lasers like the fiber and disk varieties--which use diode lasers to pump, or excite, the lasing medium--continue to have considerable market success. And now fabricators may have a new type of solid-state system to consider for sheet metal cutting: the direct-diode laser.
Not too long ago, cutting most sheet metal gauges efficiently with a direct-diode laser seemed like a far-fetched concept, and for good reason. Historically direct-diode lasers haven’t had adequate beam quality to do the job. That’s because there has always been a trade-off between high power and high beam quality. In a direct-diode setup, you couldn’t have both.
“If you look at an individual diode laser emitter, it actually has the highest beam quality you may ever need, but you need to gang a large number of diodes together to get the powers you need to cut sheet metal. But if you have lasers with the same type of wavelength, [in a direct-diode arrangement] you previously couldn’t combine them to have the same amount of overlap in space and direction. So you ended up with a poor-quality beam that could not be used for efficient sheet metal cutting.”
So said Parviz Tayebati, CEO of TeraDiode, Littleton, Mass. The company is working on a decade-old concept that could bring a high-power direct-diode laser into the sheet metal cutting arena.
In the early 2000s three researchers at MIT’s Lincoln Laboratory--Dr. Tso Yee Fan, Dr. Antonio Sanchez-Rubio, and Dr. Bien Chann--were conducting research for the Defense Department, which wanted a way to make very efficient, high-powered lasers. The three, aided by Dr. Robin Huang, developed a direct-diode laser concept that uses what’s called wavelength beam combining, or WBC. In 2009 Chann and Huang left Lincoln Laboratory and launched TeraDiode to commercialize the technology.
In a traditional direct-diode setup, the beam quality degrades when you combine beams. This happens because when identical wavelengths combine, they don’t travel in the same direction or merge neatly to produce a small spot size. In low-powered direct-diode systems, this degradation is relatively minor, but it becomes more significant as more diodes are ganged together for more power.
The Lincoln Laboratory scientists took a different approach. Diode lasers with identical wavelengths don’t combine well, but what if each diode emitter had a slightly different wavelength? And what if these diode emitters (specifically, an array on a diode bar) were placed at just the right distance from a diffraction grating, a device that splits and diffracts light? If placed properly, the diffraction grating, along with a lens and a partially reflective mirror, could take this energy from these diode laser emitters and produce one very powerful and consistent laser beam. Essentially, the system combines the similar but slightly different wavelength beams from an array of diodes into one powerful beam with waves going the same direction and to the same spot.
This is the basic idea behind WBC direct-diode technology. The company has produced 500-W modules that can be ganged together to produce a 2-kW or 4-kW, 970-nanometer-wavelength (that is, almost 1 micron), high-brightness laser, with a beam quality rating (BPP, or beam parameter product) of 2.5 millimeter-milliradian. The lower the BPP number, the higher the beam quality.
Sheet metal cutting requires high beam quality, but there’s a range. A beam quality that is too high actually can be detrimental. Incredibly high energy density in a beam’s center can produce an extremely narrow heat-affected zone that isn’t always effective when you need to evacuate molten metal from a kerf, a problem that’s especially apparent when processing a thick workpiece. But as sources explained, the company has produced lasers for a range of beam-quality specifications, between BPP 2.5 and 5 mm-mrad.
So is there a limit as to how many 500-W modules can be ganged together, and still maintain that beam quality window suitable for processing metal? “The beam quality for the 500-W module is very high, so [when ganging together modules] up to about 6 kW, we don’t need to do anything more,” Tayebati said. “We just combine the power.”
According to the company, the laser has similar cutting attributes as other solid-state, 1-micron-wavelength lasers. It cuts extremely quickly on gauge sheet but slows when cutting thicker stock. Because it’s a direct-diode system, it exhibits 40 percent wall plug efficiency; of all the electrical power that feeds into the laser system, 40 percent goes into the beam. And like its solid-state cousins, multikilowatt direct-diode systems are quite compact; a 2-kW direct-diode laser power supply is a little bigger than a welding power source.
Tayebati added that by using different diode laser emitters, the technology can be adapted to produce high-powered beams of different wavelengths, not just 1-micron beams. At this writing, the company is working on a direct-diode, 450-nm blue laser that could become a less expensive alternative to excimer lasers used in semiconductor processing. Also in the works, at the other end of the spectrum, is a 4-micron infrared laser for applications in defense and plastic processing.
As for the near-1-micron WBC technology for sheet metal applications, the direct-diode laser has moved out of the alpha testing phases and into the beta phase. “We’re now in the process of commercializing 500-W, 2-kW, and 4-kW versions,” Tayebati said. “We expect beta samples to ship during the fourth quarter of 2012 through the second quarter of 2013.”
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