Precision asphere and freeform optics manufacturing using plasma jet machining technology

Abstract

Atmospheric Plasma Jet Machining is a non-conventional deterministic sub-aperture surface processing technique that employs a local chemical etching process for material removal. It was shown to have great technological potential for the manufacturing and correction of precision optical surfaces made preferably from fused silica, ULE(R), silicon, or silicon carbide. A plasma based processing chain suitable for freeform generation or surface correction comprises high-rate plasma etching for freeform generation (removal rate 1-30 mm3/min), bonnet polishing for smoothing, plasma fine correction (removal rate 0.01-1 mm3/min) to achieve minimal surface figure error and a post-polishing step utilizing a soft tool. Since plasma jets are not only a source of reactive species but also of heat, the chemical reaction during processing depends on the resulting local surface temperature distribution. A coupled model involving finite elements for temperature field analysis and sophisticated dwell time calculation algorithms has been developed to simulate the complex interplay of surface heating and material removal. With it, the convergence of the etching process is significantly increased. Sub-aperture polishing on freeforms suffers from inhomogeneities in material removal depending on local surface curvatures. An analytical approach has been chosen to estimate the material removal during polishing and appropriate measures have been undertaken to preserve the plasma-generated surface shape. By combination of the processing steps and applying the theoretical modeling fast and efficient precision freeform manufacturing aiming at residual errors of less than 30 nm PV becomes possible.