International Technical Groups
OE Reports 180 - December 1998
COIL systems offer optimum in power, beam quality
by Yvonne CartsPowell
The most powerful lasers in the world are buildingsized devices used for fusion research. The majority of highpower lasers, however, are industrial: carbondioxide and Nd:YAG lasers have long been workhorses for cutting and drilling metals, while excimer lasers (and solid state lasers harmonically converted in the UV) are important for creating and processing microelectronics. One of the most intriguing recent highpower laser developments has been a chemical laser.
The chemical oxygeniodine laser (COIL) was developed for military applications by the Air Force in 1977, but its unique capabilities make it attractive for industrial processing. The laser emits kilowatt powers, with a good quality, focusable beam at an IR wavelength (1.315 µm) that can easily be transmitted by optical fiber. In addition, the experimental laser has the potential for scaling to higher powers (tens of kilowatts). William P. Latham, of the Air Force Research Laboratory (Kirtland AFB, NM), says, "we've been working on materials-processing applications for four years." The 1.315-µm wavelength is a transition for atomic iodine.
The laser works by creating an electronically excited state of oxygen, called oxygen singlet delta. Energy from the singlet oxygen (which has a remarkably long spontaneous lifetime of about 45 minutes) transfers to atomic iodine. The excited iodine then emits light. The starting chemicals for the reaction include an aqueous mixture of hydrogen peroxide and potassium hydroxide, gaseous chlorine, and molecular iodine. The traces of chlorine and iodine in the exhaust are removed from the exhaust gases using standard halogen scrubbers. The by-products are potassium salt, water, and oxygen.
Although the chemical reactions are relatively straightforward, the device is not simple. There are a variety of design issues for the laser cavity, not least of which is the mixing of the chemicals. Whereas many flowing gas or liquid lasers operate at relatively slow flow speeds, in the COIL, the reaction times of the chemicals dictate flow velocities near the speed of sound. Several papers at the upcoming Photonics West '99 Symposia (23-29 January) describe supersonic chemical flow speeds.
Despite the complicated plumbing, the COIL is attractive for highpower industrial applications because it has advantages over both carbondioxide and Nd:YAG lasers. High power carbondioxide lasers also use flowing gas designs, and also produce good quality (near diffractionlimited) beams. The main advantage of the COIL over a CO2 laser is the wavelength: very little of the COIL wavelength is absorbed by fused silica fibers, but much of it is absorbed by metals. In other words, the beam can be efficiently transmitted to, and then used to cut, a metal workpiece. Beams from carbon dioxide lasers, with a wavelength of 10.6 µm, are beyond the transmission range of fused silica, and are much more difficult to maneuver to the workpiece. The wavelengths from a Nd:YAG laser can be fibertransmitted but, at high powers, these solid state lasers suffer from thermal distortion (the crystal heats up and changes the optical properties of the cavity, generally degrading the beam's quality and power).
Because COIL is a lowpressure flowinggas laser, heat is removed from the lasing medium very quickly. Also, the design can be scaled up to achieve higher powers. The Air Force Research Laboratory has demonstrated scaling to 40 kW. In a paper on COIL, Latham and coauthor Brian Quillen assert, "COIL stands as the one and only laser candidate that does not have issues that prohibit scaling to very high powers."
A 1kW COIL has been used in industrial cutting demonstrations in Japan. The Air Force Research Lab's current testbed is the amusingly acronymed Research Assessment, Device Improvement Chemical Laser (RADICL), a 20kW laser. One experiment at the Air Force facility showed transmission of 7.36 kW of laser power through a 900µm diameter fiber, a world record. (For comparison, YAG lasers have transmitted up to 2.57 kW of power through fiber optics). Damaging fibers and laser optics is a real possibility for highpower lasers. The fibers used for the COIL experiments, however, were offtheshelf items. Latham and Quillen comment that, "currently, the COIL laser is the only laser that can be scaled to powers over 10 kW and be delivered through a fiber."
The U.S. and Japan are not the only countries where COIL research is taking place. Researchers from countries including Russia, the Czech Republic, India, and Korea will also present papers on COIL at Photonics West '99. Many of the papers will focus on understanding the energy transfers and optimizing the design of this sort of laser.
Yvonne Carts-Powell, based in Boston, writes about optoelectronics and the Internet.
For further resources, see the following:
"Cutting Performance of a Chemical Oxygen-Iodine Laser," William P. Latham, James A. Rothenflue, et al. Proc. of SPIE, Vol. 3268, 1998, p.130.
Gas and Chemical Lasers and Intense Beam Applications, Proc of SPIE, Vol. 3268, 1998.
Gas Flow and Chemical Laser and High-Power Laser Conference, Proc of SPIE, Vol. 3092, 1996.
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