Polishing System Takes Pressure off Making Aspheres

Aaron J. Hand

As the use of aspherical optical components broadens, the industry is trying to overcome the challenges that the processing of those surfaces present. Researchers at the Delft University of Technology have developed a finishing process that uses a low-pressure slurry jet to locally shape and polish optical surfaces of complex shapes in brittle materials.

Fizeau interferograms of a flat polished glass sample show the influence of the workpiece rotation and nozzle angle in fluid jet polishing experiments. In the left image, the ring was generated with the workpiece rotating at 1 Hz and the nozzle held at a 0° angle with the surface normal, the comet-shaped spot at 0 Hz and 80° and the C-shaped hollow at 0 Hz and 45°. In the close-up on the right, the hollow was generated at 0 Hz and 0°.

Fluid jet polishing, which began as a doctoral research project in the university's Optics Research Group, applies a slurry jet to optical surfaces. Unlike abrasive slurry jet systems -- which use pressures of anywhere from 70 to 5400 bar -- fluid jet polishing applies less than 6 bar, resembling the pressures usually obtained during superpolishing or elastic emission polishing. However, unlike elastic emission polishing, the new processing tool does not come in contact with the optical surface.
The combination of low pressures and noncontact processing is an ideal one, according to Oliver W. Fähnle, who received his PhD with honors in September after completing his thesis on the process. "Kinematic noncontact means no tool wear, debris removal and cooling," he said. "And low pressure allows for polishing operation and does not place high demands on the applied slurry jet system."
Fähnle -- guided by Hedser van Brug, a staff member of the Optics Research Group -- used the fluid jet system to polish a BK 7 sample from a surface roughness of 350 to 25 nm rms, and to change the shape of a polished BK 7 sample while maintaining its surface roughness of 1.6 nm rms. His experiments used a premixed slurry of water and 10 percent #800 SiC grinding compound that streamed through nozzles measuring 25.4 mm long and 0.84 mm in diameter. These parameters, as well as others such as nozzle position and angle, can be varied for the desired effect.
The parameters' variability could give fluid jet polishing some advantages over magnetorheological finishing, which polishes with a magnetic-sensitive slurry that is stiffened locally by a magnetic field. While magnetorheological finishing has a fixed polishing spot shape, the spot shape and size with fluid jet polishing depends on the nozzle geometry and therefore can be optimized, Fähnle noted.
In addition, manufacturers can choose the slurry to achieve varying degrees of fast material removal or polishing, he said.
Originally funded by the TNO Institute of Applied Physics, the fluid jet polishing investigations -- including research into various surface materials, slurry compositions and other setup parameters -- have been transferred to TNO. The method is in its early stages of development, Fähnle noted, and the patent is still pending. But the research group is discussing the system's potential with industrial partners, considering possible cooperation for commercial development.