Plasma jet polishing of ground freeform optics is presented. Accurate measurement of local maximum surface temperature and a closed-loop for temperature-based power control is necessary to achieve form-preserving uniform surface polishing. Microroughness can be significantly reduced in one step. The roughness after plasma jet polishing in higher spatial frequencies strongly depends on the extend of sub-surface damage and grinding marks.
Plasma Jet assisted smoothing of rough ground optical surfaces is presented. An accurate temperature regime during the process is inevitable to achieve a uniformly smoothed surface. The possibilities for in-process temperature control are demonstrated on the example of Fused Silica and N-BK7® polishing. For both materials, the surface roughness RMS value can be significantly reduced by a factor of 20 to 1000 depending on the material and the initial ground state. An annealing step after smoothing is necessary to minimize birefringence caused by internal stress. The achievement of the existing requirements for precision optics is demonstrated.
Atmospheric Plasma Jet Machining is performed on Borosilicate Crown Glass. A fluorine containing plasma jet is suitable for the etching of the material. A substrate surface temperature of about 325°C during processing is necessary for a controlled removal. The figure error can be corrected by a dwell time based deterministic process. The resulting surface roughness depends on the surface temperature of the processed sample.
Plasma Jet Machining is an established process in ultra-precision surface manufacturing. Removal of several nanometers up to millimeters can be achieved using the atmospheric pressure reactive plasma jet as a non-mechanical tool. Surface form measuring techniques have to be improved equally, to further enhance the deterministic machining. Exact knowledge of the instrument transfer function is necessary to distinguish measurement artefacts and reliable measurement results. Precise sinusoidal surface structures prepared by plasma jet etching can be used as calibration elements to determine the instrument transfer function, e.g. slope-measuring devices like Nanometer Optical component measuring Machine (NOM). The steps for manufacturing such calibration elements including theoretical considerations, adjustment of the plasma jet parameters and implementations on different substrates are presented. Finally, a chirped sinusoidal structure on a singlecrystalline silicon slab is fabricated.
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