Freeform optics especially Continues Phase Plate, are greatly demanded in high power laser systems. Fabrication of them with high efficiency and low sub-surface damage becomes a major challenge. Atmospheric pressure plasma processing using radicals produced by plasma torch, which has strong potential for machining freeform optics with millimeter and sub-millimeter spatial resolution, is proposed. This paper presents a novel plasma torch designed for fabricating freeform optics. This integral compact torch is comprised of needle electrode and micro-hole electrode. Millimeter-scale plasma jet of different size can be obtained by changing the micro-hole electrode. To analyze the process distribution of process gas of the jet flow, flow simulation is performed using COMSOL Multiphysics software. A set of single factor experiments are conducted to reveal the relationship between removal characteristics and flow rate of He, CF4 and O2. The removal rate with adding O2 increases 7 times of that without O2, and the spectrum change of the plasma jet is observed. It indicates that O2 improves removal rate through consuming existing CFx to produce more F radicals. The removal rate and full width at half maximum (FWHM) of typical removal function is about 19 μm/min-2.2 mm and 18 μm/min-1.1 mm respectively.
In recent years, major projects, such as National Ignition Facility and Laser Mégajoule, have generated great demands for large aperture optics with high surface accuracy and low Subsurface Damage (SSD) at the mean time. In order to remove SSD and improve surface quality, optics is fabricated by sub-aperture polishing. However, the efficiency of the sub-aperture polishing has been a bottleneck step for the optics manufacturing. Atmospheric Pressure Plasma Processing (APPP) as an alternate method offers high potential for speeding up the polishing process. This technique is based on chemical etching, hence there is no physical contact and no damage is induced. In this paper, a fast polishing machine tool is presented which is designed for fast polishing of the large aperture optics using APPP. This machine tool employs 3PRS-XY hybrid structure as its framework. There is a platform in the 3PRS parallel module to support the plasma generating system. And the large work piece is placed on the XY stage. In order to realize the complex motion trajectory for polishing the freeform optics, five axis of the tool operate simultaneously. To overcome the complexity of inverse kinematics calculation, a dedicated motion control system is also designed for speeding up the motion response. For high removal rate, the individual influence of several key processing parameters is investigated. And under specific production condition, this machine tool offers a high material over 30mm3/min for fused silica substrates. This results shows that APPP machine tool has a strong potential for fast polishing large optics without introducing SSD.
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