Freeform surfaces manufactured with a combination of ultra-fine grinding and plasma jet polishing

In order to meet the increasing requirements of freeform production, a new process chain is presented that combines grinding with thermally induced plasma jet polishing. This is demonstrated by an example of a self-designed Alvarez lens geometry made of fused silica glass. A 3-stage grinding process is applied, comprising coarse pre-shaping, fine freeform shape generation, and ultra-fine first surface smoothing. Grinding is performed using a 5-axis computerized numerical controlled machining process with spherical diamond grinding tools. After grinding, the surface quality is sufficient for subsequent plasma jet polishing, in terms of shape accuracy, mid-spatial frequency errors, and roughness. The surface shape is retained during polishing while simultaneously achieving a high surface quality and meeting the roughness requirements for optical applications. The requisite improvements to the grinding processes, as well as a parameter optimization for plasma jet polishing are presented. Moreover, the application of a convolution function, which describes plasma jet polishing theoretically, helps to reduce the number of samples necessary to optimize the process chain with respect to favorable transfer points, thereby perfectly linking the processes. The novel process chain offers a versatile and efficient approach for the fabrication of optical freeform surfaces with a final roughness of Sq < 0.5 nm (white light interferometer, 50×).