Baruch A. Fuchs

Diamond turning of L-arginine phosphate, a new organic nonlinear crystal

We have demonstrated that single point diamond turning can be used to generate high optical quality finished surfaces on a new organic nonlinear crystal, L-arginine phosphate (LAP). The proper choice of cutting conditions can produce surfaces with <5-A rms local roughness. Local softening or melting near the cutting tool tip may play a key role in the machining process by ensuring that material is removed by ductile cutting rather than brittle fracture. At the same time, the low melting temperature of LAP makes lubrication and cooling especially important to prevent extensive melting and tool fouling. In spite of the presence of a weak cleavage plane in LAP, the surface quality is relatively insensitive to crystallographic orientation. Tool wear is apparently negligible, so that surface flatness is governed by the stability of the diamond turning machine. These results suggest that it may be possible to fabricate large aperture LAP harmonic converters for use in inertial confinement fusion lasers.

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.

Removal rates of fused silica with cerium oxide/pitch polishing

The polishing removal rates of fused silica under 20 test conditions have been characterized by the resulting Preston coefficients and an inspection of surface quality using a Nomarski microscope and an optical heterodyne profilometer. Preston coefficient values of > 16 X 10-14 cm3/dyne-cm have been established, approximately a factor of 10 higher than cited previously. However, operational variables can reduce removal rates significantly. Process parameters are shown to have an effect on surface quality that has been related to surface deposits. The effects of chemical additives on removal rates were of special interest. Of importance here is not necessarily the introduction of such additives in practice. The results may improve our understanding of the chemistry of polishing.

Processes for the elimination of fogging on KDP crystals prior to and during use in laser systems

The rate of surface fogging on KDP crystals has been associated with several parameters in the diamond turning and subsequent cleaning process. Fogging can also occur during use because of environmental degradation. We have determined that reaction of the KDP surfaces with additives in the diamond turning oil has been a major factor in fogging and have found a replacement oil that eliminates this effect. We have also developed an antireflective silicone- sol coating that also reduces surface environmental deterioration during use by a factor of four.

Diamond turning of lithium niobate for optical applications

We have investigated the surfae finishing of lithium niobate by using the single-point diamond turning technique. Surface finishes of better than 5 nm rms on z-oriented samples have been achieved. However, tool wear and spalling are much more significant with lithium niobate than with materials such as the crystals KDP and LAP. We present preliminary results comparing the optical damage thresholds of polished and diamond-turned samples.

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.