J. Eur. Opt. Society-Rapid Publ. 2025, 21, 12 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025007 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
Freeform surfaces manufactured with a combination of ultra-fine grinding and plasma jet polishing
Heike Müller 1,*
, Sebastian Henkel 2, Christian Schulze 2, Samson Frank 2, Jens Bliedtner 2, and Thomas Arnold 1, 3
1 |
Leibniz Institute of Surface Engineering( IOM), Permoserstraße 15, 04318 Leipzig, Germany |
2 |
EAH Jena University of Applied Sciences, Department SciTec, Carl-Zeiss-Promenade 2, 07745 Jena, Germany |
3 |
Institute of Manufacturing, TU Dresden, 01062 Dresden, Germany |
Received 12 December 2024 / Accepted 3 February 2025
Abstract. 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).
Keywords: Ultra-fine grinding, Atmospheric plasma jet, Plasma jet polishing, Process chain, Freeform optics, Fused silica.
1 Introduction
Optical freeform surfaces are becoming increasingly important as they serve a pivotal role in the miniaturization of optical assemblies and the enhancement of optical imaging quality. The implementation of rotationally symmetrical and cylindrical optical functional surfaces would necessitate the positioning of a considerable number of lenses in a series, which would inevitably result in an increased prevalence of aberrations and a notable loss of intensity.
Freeform optical elements optimized for a specific purpose are applied to generate the requisite intensity distribution at the desired receiver position. Consequently, a system with the same performance can be achieved with a significantly reduced number of optical elements, or even higher performance can be attained. Thus, freeform surfaces are capable of addressing the diverse requirements of optical assemblies and developments in photonic technologies [ 1 – 3 ].
* Corresponding author: heike. mueller @ iom-leipzig. de
However, manufacturing processes must also be adapted to meet these demands. The conventional production chain for optical elements, which relies on mechanically abrasive processes using classic tools such as cup or shell tools, must be supplemented or replaced by newer technologies. The use of various processes with sub-aperture tools and improvements in system technology for ultra-precision machining is beneficial in this regard.
The shape generation of refractive, optical freeform elements is usually determined by the application of computerized numerical controlled( CNC) grinding processes using multi-axes machines and tools with punctual engagement. The grinding process, which employs bonded grain with a geometrically undefined cutting edge, is a crucial technique in optical production, as it can be used to realize the geometric shaping of the component as well as to produce polishable surfaces with a specific surface structure, thanks to its high precision [ 4 ]. Due to the brittle-hard properties of inorganic non-metallic materials, a large number of lateral, radial and axial microcracks form from
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