Laser Technique Polishes Glass Lenses

R. Winn Hardin

Glass's high infrared absorption could prove a boon to a new laser technique that can polish both symmetric and asymmetric glass lenses faster than mechanical techniques.
Ferran Laguarta and his associates at the Universitat Politècnica de Catalunya may have solved the light source, monitoring and thermal stress issues that have prevented large-area laser polishing of fused silica lenses. Essentially, laser polishing heats the surface of a glass lens after it has been ground to the proper shape. The heated surface material then flows because of gravity and surface tension to fill the minute ridges formed by grinding machines.
Because it is a noncontact polishing method, the researchers maintain that it could prove even more important for nonspherical and nonrevolution optical polished surfaces.

Large, even surfaces
In 1994, the group polished 80-mm² laser-grade fused silica, using a high-power CO₂ laser. A conventional oven first heated the glass to 580 °C to reduce the chance of cracks created by the fast heating from the laser. (The viscosity of glass drops severely at 650 °C.) A multifaceted vibrating mirror distributed the 10.6-µm light evenly across the surface, exposing the glass for 0.4 to 9 s.
To smooth the glass without destroying its curvature, the system has to heat the surface to a depth greater than the peak-to-trough distance (500 nm) without raising the temperature of the remaining material above 650 °C. Infrared radiation can achieve this because it is readily absorbed by glass at 10.6 µm.
After the surface absorbed the laser beam and turned liquid, the researchers realized a reduction in the glass's rms surface deviation from 500 to 1 nm.
"It could be of interest because it could substantially reduce the polishing time," said Gary Perrone, production manager for the optical house Janos Technology Inc. of Townshend, Vt.
It takes a lens about 8 h to go through the polishing process, he said.
Recently, the Spanish group improved the system to cover 5000- mm² glass surfaces. Using a similar laser with a maximum power of 2 kW, a dual-faceted lens, half of which is attached to a piezoelectric actuator, folds the nearly Gaussian beam into a vertical line. Quickly vibrating the mirror averages the interference fringes, and cylindrical ZnSe optics allow the system to switch beam widths from 2 to 18 mm.
Interferometric measurements showed that the process resulted in a significant drop in surface roughness but had little effect on the wavy formations with gradual slopes from the grinding process.