Ultrasonic vibration-assisted sculpturing of large-scale 3D optical microstructure arrays with small theoretical error and reliable dynamic response

Freeform micro-structures can exhibit advanced and useful functions of optical components by guiding beam path. Growing interest has been shown in fabricating large-scale 3D freeform micro-structures on hard-to-cut materials as mould for mass production. Currently, ultrasonic vibration-assisted diamond turning is a feasible and efficient technique to fabricate these structures made of steel. However, the tool path linear error increases significantly with the increase in workpiece dimension in fast tool/slow slide servo diamond turning of microstructures with sharp edges. In this research, using Mirrax 40 steel as the mould material and applying a high-frequency ultrasonic tooling system (UTS2) to ease the machining of steel workpiece. To reduce linear error in machining large-scale 3D microstructures, diamond sculpturing instead of diamond turning was applied. This machining technology is called ultrasonic vibration-assisted sculpturing (UVAS). The principle of UVAS, tool path determination, surface generation, tool parameter selection as well as linear error of the tool path and form error induced by ultrasonic vibration kinematics, were comprehensively investigated, in order to fabricate an aspheric microlens array and a freeform microlens array with small theoretical error and reliable slide dynamic response. Furthermore, a large-scale freeform microlens array was fabricated and a measurement method was used to evaluate the machining accuracy. UVAS achieved a form error of less than 0.5 μm and an arithmetic surface roughness Sa of 5 nm for the large-scale freeform microlens array.