Laser damage of fused silica is a critical problem that limits the fluence increase of high-power laser systems. The reaction of plasma processing of fused silica was analyzed. The redeposited layer was removed and scratches of the fused silica were passivated by plasma to increase the laser-induced damage threshold (LIDT). It was found that the deposition generated during the etching process adheres to the surface of fused silica to limit the further increase in the LIDT. The LIDT of fused silica tends to increase and then decrease as fused silica is processed by atmospheric pressure plasma. Accordingly, a competitive evolution for etching and deposition affecting the LIDT of fused silica was established. The etching and deposition relationship analyzed by the model can effectively increase the LIDT of fused silica without deteriorating the original surface of fused silica by choosing an appropriate dwell time.
KEYWORDS: Atmospheric plasma, Plasma, Diffractive optical elements, High power lasers, Polishing, Chemical analysis, Chemical reactions, Surface finishing, Chemical elements
Continuous phase plate (CPP) is an important diffractive optical element, which is widely used in high power laser devices. The continuous phase plate with a small aperture period of 4 mm is processed by the atmospheric pressure plasma polishing (APPP). Through the study of the reaction mechanism, it is found that the removal volume has a non-linear relationship with the dwell time, which will lead to machining errors. Based on this, a dwell time compensation method is proposed, and the machining program is generated according to this relationship. A 70mm × 70mm × 20mm continuous phase plate was fabricated by using the processing program generated by this method. The processing time was 4.5h, and the surface residual converged to 57.188nm RMS. The experimental results show that the method can effectively calculate the removal function under different dwell time, and significantly improve the machining accuracy.
Reaction-sintered silicon carbide (RS-SiC) has been widely applied in space telescope mirrors, reflector, microelectronic mechanical systems due to its excellent properties. These applications require high surface accuracy and quality, but traditional mechanical contact polishing methods are hard to process this material due to the hardness and poor machinability. In this paper, atmospheric pressure plasma processing (APPP) which is a non-contact optical manufacturing technique with highly efficient and subsurface damage-free has been proposed to process this material, and optimal process parameter mainly about influence on removal function for machining processing RS-SiC have been found. The sample was observed and analyzed by scanning white-light interferometer (SWLI), which showed the calculated Peak- Maximum removal rate (MRR) and Volume-MRR were 12.526μm/min and 0.1298mm3 /min. Meanwhile, the surface chemical composition of RS-SiC was also investigated by XPS to reveal the plasma etching processes. The results illustrated that small number of radicals CxFy were introduced onto the RS-SiC surface during the plasma process which could be generated during the process of the reaction gas CF4 being excited. The process parameters and analysis of surface chemical composition in this paper will guide the further processing of RS-SiC.
The surface figure control of the conventional annular polishing system is realized ordinarily by the interaction between the conditioner and the lap. The surface profile of the pitch lap corrected by the marble conditioner has been measured and analyzed as a function of kinematics, loading conditions, and polishing time. The surface profile measuring equipment of the large lap based on laser alignment was developed with the accuracy of about 1μm. The conditioning mechanism of the conditioner is simply determined by the kinematics and fully fitting principle, but the unexpected surface profile deviation of the lap emerged frequently due to numerous influencing factors including the geometrical relationship, the pressure distribution at the conditioner/lap interface. Both factors are quantitatively evaluated and described, and have been combined to develop a spatial and temporal model to simulate the surface profile evolution of pitch lap. The simulations are consistent with the experiments. This study is an important step toward deterministic full-aperture annular polishing, providing a beneficial guidance for the surface profile correction of the pitch lap.
The response characteristics of the pitch polishing pad under the pressure of conditioner is the key factor which affect the control efficacy of surface figure in annular polishing. The effect of the environmental temperature and humidity, rotation speed of the pad, uploading ratio and eccentricity of the conditioner on the response characteristics of pitch polishing pad were experimental researched. To this aim, a control system of the environmental temperature and humidity on the 2.8m-aperture annular polishing machine is established. The control precision of the local environmental temperature upon the pad is up to ±0.1℃, and the control precision of humidity is up to ±1% RH. The experimental results indicate that the matching degree of the pad and the conditioner decides the changing rule of the surface figure of workpiece with the eccentricity of the conditioner. The new adjustable parameters including the environmental temperature and humidity and the uploading ratio of the conditioner were introduced to change the matching degree and improve the controllability of the of the surface figure. Under the optimized process, the long-term stability of the surface figure of the meter-scale optical element is realized using the 2.8m annular polishing machine, which is an important step for the final realization of the deterministic processing in the large-aperture annular polishing.
An ideal pitch button blocking process determines the level of workpiece deformation, especially the high-aspect-ratio optics, during the blocking process and process of polishing later. We have studied the pitch button blocking process by Finite Element Analysis (FEA) according to the thermoelastic equation. Meanwhile, the optimized pitch button blocking has been gotten by FEA which includes the thickness and material of blocking plate, as well as the radius, arrangement, elastic modulus and coefficient of thermal expansion of pitch buttons. The numerical simulation of Nd:glass (Ø100 mm×2 mm thickness) shows that the surface figure change (ΔPV) which is induced by the thermal stress during pitch button blocking process is influenced seriously by the thickness of blocking plate.
Dynamic Mechanical Analysis (DMA) and Bending Beam Rheometer (BBR) are applied to detect the property of the polishing pitch and get the creep compliance combining the craftwork of polishing. The simulation of workpieces’ surface in polishing is completed based on the Preston formula, Boltzmann superposition principle and Boltzmann superposition principle. A V-curve is gotten on studying the PV’s time history of the polished workpiece. Meanwhile, a confirmatory experiment whose parameter is agree with the simulation is completed, which the optimum result of PV, 0.3λ (λ=623.8nm) is similar with the optimum result of PV by simulation, 0.27 λ. A resemble simulated surface is gotten on the same PV with the experiment.
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