Continuous phase plates (CPPs) are essential diffractive optical elements in the light path of laser-driven inertial confinement fusion (ICF) systems, such as the National Ignition Facility [1
], Laser Megajoule [2
] and the SG-III laser facility [3
]. The continuously varying structured topography of CPPs can modulate the incident laser to realize beam shaping and smoothing, and thus achieve the uniform illumination of the target surface [4
]. The complexity of surface topography (with small spatial periods and large surface gradients) makes it difficult to fabricate/structure CPPs with high efficiency and accuracy. Magnetorheological finishing (MRF) has been used to fabricate large-aperture CPPs [5
], in which the spatial periods of microstructures are usually larger than 4 mm, and the peak to valley (PV) of the structure height is as large as several microns. Smaller structures on CPPs are difficult for MRF due to the limitation of tool sizes. Ion beam figuring (IBF) has the potential to figure structures down to 1 mm and different sizes of removal spots can be achieved with a shielding diaphragm. However, the low removal rate limits its application to large and steep CPPs [6
Atmospheric pressure plasma processing (APPP) is a promising technique for the modification, decontamination, and etching of polymers and glasses [7
]. Recently, APPP has received a great deal of interest in optical fabrication because of its deterministic high material removal rate, controlled millimeter tool spot, and no subsurface damage. It is based on pure chemical reactions between the surface atoms of silicon-based materials and reactive fluorine radicals generated by the plasma at atmospheric pressure, which avoids the introduction of damage to the processed surface and significantly lowers the processing cost. Jourdain et al. [14
] adopted the reactive atom plasma process for the figuring of large telescope optics. An inductively coupled type plasma torch with a De-Laval nozzle was used to generate a Gaussian removal footprint. For the management of heat transfer, an adapted tool-path strategy was combined with an iterative figuring procedure. Due to the high thermal nonlinear effect of inductively coupled plasma, Dai et al. [15
] developed an algorithm based on the nested pulsed iterative method to compensate for this time-varying non-linearity by varying the dwell time. With the compensated dwell time, the surface error converged from 4.556 λ PV (peak-to-valley) to 0.839 λ PV in one iterative figuring. More commonly, capacitively coupled plasma is adopted for high precision processing applications. Meister and Arnold [16
] investigated the atmospheric plasma jet machining of fused silica. A three-dimensional finite element heat transfer model was built to consider spatio-temporal variations of the surface temperature and temperature-dependent material removal. The figuring convergence was improved by an iterative correction of the targeted removal according to the modelling results. Sun et al. [17
] investigated the etching characteristics of plasma chemical vaporization for reaction-sintered SiC by optimizing the gas composition. Experiments showed that a large surface roughness resulted from the different etching rates of the different components in SiC. By applying the optimum gas composition, a smooth surface was obtained after plasma etching if the etching rate of the Si component was equal to that of the SiC component. Deng et al. [18
] combined plasma chemical vaporization machining and plasma-assisted polishing. Plasma chemical vaporization machining was performed to remove the subsurface damage layer, while plasma-assisted polishing (including plasma modification and soft abrasive polishing), was performed for damage-free surface finishing. The results indicated that a flat and scratch-free surface with a root mean square (RMS) roughness of 0.6 nm was obtained.
The stable and controllable Gaussian-shape removal function makes APPP possible to fabricate structured surfaces with high accuracy and efficiency. Also, multi-aperture processing (with multiple-scale removal functions) is enabled to target different scales of surface features to increase the overall machining efficiency. However, little research has been reported on structuring CPPs with the multi-aperture APPP technique. In this paper, a multi-aperture APPP method was proposed for CPP structuring. The APPP system was first introduced, and its removal characteristics were investigated in two aspects: the repeatability and robustness. Then, a mathematical model for multi-aperture APPP dwell time solution was established and simulation analysis was performed to study the structuring efficiency and accuracy. Finally, the experimental processing was carried out to validate the effectiveness of the proposed multi-aperture APPP to structure CPPs.