Fiber Damage Threshold Raised

Kevin Robinson

The flexibility of fiber optics promises to improve laser machining processes, but high-energy lasers damage the fiber and affect the beam quality. Now researchers from Heriot-Watt University, Liverpool University of Liverpool and GSI Lumonics Ltd. of Rugby have shown that the light from a CO₂ laser can improve the fiber's laser damage threshold to the point that it can deliver the high peak powers necessary for percussion drilling.

"Using a fiber allows easy manipulation of the beam, which is particularly useful when processing complex 3-D workpieces," explained Duncan P. Hand of Heriot-Watt's department of physics. "It also allows a single laser to be easily multiplexed between two or more machines."

The fiber delivery of high-energy laser pulses would be a boon to laser machining. Researchers have demonstrated that treating fiber with a CO₂ laser improves its damage threshold. Image courtesy of GSI Lumonics Ltd.

Percussion drilling, in which multiple pulses of laser energy punch holes in the part, is more efficient than trepan drilling, in which the beam cuts out a hole like a saw. Percussion drilling requires higher peak power pulses, however. Because optical fiber always has imperfections on the polished ends, these higher powers damage the fiber very quickly, making it unusable.

Basing their work on findings that CO₂ laser treatment improves the damage threshold for nanosecond pulses, the researchers examined the suitability of the treatment for the micro- to millisecond pulses needed for percussion drilling. They cleaved and polished the ends of a fiber with a 400-µm core diameter using commercial polishing equipment, and determined the damage threshold for the fiber with an Nd:YAG laser. They then irradiated the polished ends of new fibers with CO₂ laser light and tested them with an Nd:YAG.

Fiber At Work

Treatment with as little as 1 W of laser power increased the damage threshold by more than an order of magnitude. The study, which the researchers reported in the Nov. 20, 2000, issue of Applied Optics, suggests that cleaved fiber absorbs CO₂ laser light in a thin layer close to the surface. This heats the fiber tip and smooths the scratches and pits that remain from the cleaving and polishing process.

Hand said that the treatment improves the fiber's damage threshold to that of commercially polished fused silica. The fiber tips resisted damage for up to 3 minutes at Nd:YAG pulse energies of up to 55 J, significantly greater than those used for percussion drilling.

The group's next step is to prove that the treatment will work in a commercial environment. "We believe that 2 to 3 minutes is enough to investigate any long time-constant thermal effects," Hand said, "and that it should be possible to operate continuously for very long periods of time."