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18 Apr 2011
Grating-based beam combination yields unparalleled scale-up of power for metal cutting and directed energy applications.
Diode-pumped lasers, whether in disk, fiber, or YAG form, have enjoyed great success, but for materials processing applications in the real world, direct-diode systems are unequaled. That's the view of David Sossen, CEO of TeraDiode, a Massachusetts start-up that has developed a means of combining the output of multiple diode emitters into a single beam of extremely high brightness and quality.
"Direct-diode lasers are the holy grail," said Sossen. "Their efficiency, compactness and relatively low cost have always been attractive, but a combination of lower power and poorer beam quality compared to alternative systems has limited their penetration into industrial applications such as metal cutting and welding."
In its TeraDrive system, TeraDiode tackles these deficiencies through a technique called wavelength beam combining (WBC), which allows the spatial superposition of several independent diode-laser external cavities.
"WBC is based on a diffraction process," explained Sossen. "The outputs from separate laser diodes having different wavelength characteristics are brought together through a [Fourier] transform lens, and then passed through a diffraction grating that can be transmissive or reflective."
The diffraction grating's ability to convert different wavelengths to different angles is used to provide feedback to each emitter, via the transformation lens, and a laser resonator is formed between the highly-reflective coated back facet of the emitter, and the output coupler.
"In effect we are using the diffraction grating in the opposite way to its usual function," noted Sossen. "Instead of dispersing light over many angles, we are passing the beams into the diffraction grating at a very careful alignment of different wavelengths, and having them emerge in the same place, superimposed in space."
Easy scale-up
The result is a self-compensating external cavity laser, which can be easily scaled-up through the addition of more diode elements to the array without sacrificing the output beam quality. WBC keeps the center wavelength stabilized with respect to changes in power and temperature, a critical factor for pumping of solid state and fiber lasers. In theory, the maximum power available from a TeraDrive resonator is limited only by thermal considerations.
"The WBC process is very accommodating of imperfections in the quality of the beam from the emitter," said Sossen. "In practice, we can relatively easily build a laser around any emitter of suitable quality and reliability. Most of our work is currently at wavelengths of 900-1000 nm, but WBC can be applied to wavelengths from 400 to 4000 nm, depending on the application."
TeraDiode has demonstrated a fiber-coupled prototype producing an output power of a little over 1000 W from a 200 micron fiber with a numerical aperture of 0.18. That equates to a brightness of more than 31 MW/cm2/sr, some five times brighter than a commercially available 1 kW direct-diode laser can provide, according to the company.
At that brightness level, materials processing applications become realistic propositions. "That beam is bright enough to perform cutting and welding operations on the common gauge metals used in the bulk of industrial markets, operations which are mainly carried out with CO2 lasers at present," said Sossen.
Damage protection
In metal cutting, another contributing advantage for the TeraDiode laser is its relative insensitivity to back-reflection from the workpiece, which can present difficulties in other systems. The external cavity design not only protects the diode lasers from being damaged directly, but also prevents any disruption of the WBC process and its own reliance on reflection back to the diodes.
The end result can be impressive metal-cutting ability. In demonstrations, a 900 W TeraDrive laser successfully cut a 6.65 mm-thick mild steel sheet, which the company believes to be the thickest metal section cut to date using a diode laser (watch TeraDiode's cutting and welding video).
Higher brightnesses are in the pipeline: TeraDiode expects to launch kilowatt-class direct-diode lasers capable of delivering 1000 MW/cm2/sr in the second half of 2011. These brightness levels are claimed to equal or even exceed that of comparable fiber lasers.
"The broad materials processing market is our largest target," commented Sossen. "This includes metals, but also materials such as plastics, paper, wood, cardboard and others. Even within metals there is a wide range of different candidates, from ferrous metals to copper, stainless steel and nickel alloys, requiring different output powers and beam qualities."
Matching CO2 cut quality
One constant across metal cutting applications is the need to satisfy end user's quality requirements, which in this case means matching the quality of cut achieved by the CO2 lasers still employed by two-thirds of the market.
As Sossen noted, "The quality of the cut, called the edge quality, is critical. Automotive applications, to name one obvious example, insist on a very high-quality, burr-free cut surface for subsequent processing. The challenge for a new entrant is to meet that standard, and do so with a machine that can run for perhaps as many as 8000 hours per year."
Beyond materials processing, several other markets for extremely bright direct-diode lasers could open up, including medical imaging and digital projection. TeraDiode has its origins in a US Department of Defense project at MIT Lincoln Laboratory, and now that the technology is ready for wide adoption, military and security applications are also likely.
"We have already won four contracts to carry out early-stage work on military projects including range finding, illumination of targets and counter-measures," said Sossen. "Directed energy weapons are also a possibility in the future, as we believe that TeraDiode has a pathway to achieve the very high performance and laser parameters needed there too."
Ultimately, that could result in a lucrative set of applications for TeraDiode, which attracted Series A venture funding from Stata Venture Partners in September 2009. The global market for direct-diode lasers of the extreme brightness and high quality allowed by the WBC approach could add up to between one and two billion dollars, believes Sossen.
About the Author
Tim Hayes is a freelance journalist based in Bristol, UK.
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