P. Paul Hed

Optical glass fabrication technology. 2: Relationship between surface roughness and subsurface damage

For glass grinding conditions where fracture is the principal mechanism, a constancy is found between the depths of surface roughness and subsurface damage. For a range of experimental conditions we find the ratio of subsurface damage to surface roughness to be 6.2-6.4 for bound diamond abrasive grinding. This is larger than the value reported for loose abrasive grinding (3.7-4.0). The constancy of this value has great practical importance. From a measurement of the surface roughness one can obtain an accurate estimate of the damage layer thickness that must be eliminated by polishing or subsequent grinding operations.

Fine diamond turning of KDP crystals

Frequency doubling optical components for the Nova laser system are made from single-crystal potassium dihydrogen phosphate (KDP). While developing a manufacturing process for these components, we found that single-point diamond turning could be used to directly produce finished parts with no need for additional surface polishing. A surface roughness of better than 8-A rms and 36-A P-V was measured on a test sample generated with certain machine and tool parameters. Further improvement in surface finish may be possible by employing refined diamond turning procedures and equipment.

Examination of the polished surface character of fused silica.

Investigation of the surface character of fused silica polished with various compounds dispersed in water identified pH 4 as the optimum condition for high quality. Analyses support the conclusion that at this pH redeposition of hydrated material onto the surface during polishing is limited. Comparative polishing results for Zerodur are included. Improvement of the laser-damage threshold of a coating on the pH 4 polished fused silica is suggested.

Optical glass fabrication technology. 1: Fine grinding mechanism using bound diamond abrasives.

The mechanism for fine grinding using diamond tools is shown to depend on the properties of the glass, the acidity of the grinding fluid, as well as the chemical and mechanical properties of the abrasive bonding material. Knowledge of the mechanism is essential for the implementation of this technology for the deterministic fabrication of precision optical surfaces. We find that the fracture mechanism is preferred to plastic scratching for most applications.

Laser rod holder during grinding and polishing

A technique for clamping a laser rod during optical surfacing and polishing is described which is superior to the commonly used method of cementing the rod into a hole in a metal block. (AIP)

Improvement in polishing of fused-silica parts

High-power laser optics can require high-quality polished surfaces for damage resistance. Fabrication efficiency for producing quality surfaces becomes an important cost consideration. Our systematic study of fused-silica polishing conditions has led to the discovery that enhanced removal efficiency and higher quality optical surfaces can be achieved by decreasing the cerium oxide polishing slurry pH to 4. Both hydrochloric acid and citric acid have been used to lower the slurry pH with equal success.