- Laser Micromachining Keeps the Olympic Torch Burning
Anne L. Fischer
Prior to every Olympic Games, a handheld torch is lighted by the sun shining through a parabolic mirror at the original site of the event in Olympia, Greece. In a relay that has come to symbolize peace throughout the world, thousands of runners carry the torch to its destination: the city hosting the games. While the lighting and relay have gone relatively unaltered through the years, the technology behind the flame has changed.
A laser beam micromachines holes in the flow controllers of the relay torches used for the 2004 Olympic Games
Design requirements mandate that each torch lights quickly, runs for at least 15 minutes, operates in all kinds of weather, is lightweight and mass-producible at low cost, produces a flame visible to the naked eye as well as to video cameras, and produces no smoke or toxins.
For each of the modern Olympic Games,the host country designs a new torch. The burner assembly for the 2000 Olympics in Sydney, Australia, for example, was designed by a team at the University of Adelaide in Australia and was manufactured by G.A. & L. Harrington Pty. Ltd. in Sydney.
A researcher analyzes one of the flow controllers used in the Olympic relay torches. The monitor shows a magnified view of a flow controller compared with a human hair (the dark line).
One challenge they faced was that the flow control system for the torch’s fuel had to be as small and lightweight as possible to keep the overall weight low. The tiny hole in each controller, situated in a 250-µm-thick brass sheet, had to be perfectly round and could vary only about 1 µm. If the hole were too large, the flame would burn too quickly and not last the 10 minutes required to complete each leg of the relay. It also would not maintain the required flame height of 25 cm, which offers maximum visibility. If the hole were too small, however, wind or rain could snuff out the flame.
The challenge was in mass-producing the flow controllers with holes of consistent size. Nonlaser drilling methods were incompatible with the project because they could not always make holes within the desired tolerances.
The manufacturers enlisted the help of the Centre for Lasers and Applications at Macquarie University in Sydney, which used direct-write laser micromachining to develop a method suitable for fabricating the flow controllers. In particular, it developed a process based on simple trepanning that produced high-quality holes while limiting heat deposition inside the controllers.
The technique involves focusing a stationary laser beam as the target is moved in a minute circular fashion. It works like a woodpecker’s beak, with the laser emitting 7000 pulses per second, forming a perfectly round hole in ~10 s. In practice, the laser’s wavelength, average and peak power, fluence and translation speeds must be optimized to efficiently ablate the metal substrate without producing surface contamination, which can block the small hole, or thermal loading, which can cause swelling and irregularities in the hole’s shape.
The designers used a laboratory-built copper vapor laser that produced ~60-ns green pulses, offering average powers of ~15 W of high beam quality, which they focused onto a 5-µm spot. Under these conditions, explosive melt ejection mechanisms resulted in the physical removal of molten metal droplets from the substrate surface.
According to the center’s Michael Withford, they judged the performance characteristics of this laser to be superior to that of other high-power lasers. However, as a result of recent improvements in commercially available devices, they are producing high average power/high beam quality at visible wavelengths with a solid-state, 532-nm Nd:YAG laser from Lightwave Electronics Corp. of San Jose, Calif. (acquired by JDS Uniphase).
The design for the flow controller for the 2000 Olympics was so successful that the manufacturer won the contract for the 2004 games.
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