Optical freeform generation by laser machining and plasma-assisted polishing

Thomas Arnold,Anne Maiwald,Georg Böhm,Klaus Zimmer,Martin Erhrhard,O Fähnle,A Vasdekis

doi:10.1051/epjconf/201921503003

Thomas Arnold, Anne Maiwald + Show 5 more

Open Access

https://doi.org/10.1051/epjconf/201921503003

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Abstract

Tailored optical freeform lenses are required for different applications. Sub-aperture deterministic machining techniques such as plasma jet machining have shown great potential to generate freeform surfaces. However, depending on the required local slopes of the surface shape geometrical limitations occur due to the lateral tool function width. In the paper an alternative approach to fabricate freeform shapes exhibiting steep local slopes is presented. A first step involves a dwell time based fs-laser ablation process to generate the surface contour on a fused silica sample. Since the resulting roughness after laser machining lies in the range of 400 nm RMS which does not match optical requirements a subsequent plasma jet based polishing step is performed where micro-roughness is drastically reduced to values below 0.3 nm RMS.

Highlights

Today, an increasing demand for complex shaped optical elements like non-standard aspheres, acylinders, or freeform elements is observed
Since the resulting roughness after laser machining lies in the range of 400 nm RMS which does not match optical requirements a subsequent plasma jet based polishing step is performed where micro-roughness is drastically reduced to values below 0.3 nm RMS
Sub-aperture polishing often leads to significant shape alteration, in the case of significant variations of local surface curvature, which originates from the corresponding local variation of polishing removal rates

Summary

Introduction

An increasing demand for complex shaped optical elements like non-standard aspheres, acylinders, or freeform elements is observed. In cases, where highly individualized and tailored single piece optics or small batches are required, replication techniques like moulding are hardly appropriate for production. Classical manufacturing of such optics generally involves intense sub-aperture polishing after a freeform grinding process to remove sub-surface damage (SSD). The process chain comprises ultra-short pulse laser machining for shape generation by ablation on fused silica windows, followed by a plasma jet polishing step. This process chain is targeting the fabrication of individual small and medium-sized optics with diameters of 1 mm – 25 mm that exhibit steep surface slopes

Experimental

Surface machining procedure