Spatial period is an important characteristic parameter in the design and fabrication of continuous phase plate (CPP). The smaller the minimum spatial period, the more freedom of CPP design and the more difficult of CPP manufacture. The minimum spatial period of continuous phase plate determines removal function size of tool in high-precision machining of continuous phase plate. Based on chemical reaction, atmospheric pressure plasma processing (APPP) is a non-contact and high efficiency material removal method. The removal function of APPP is nonlinear with dwell time because of chemical reaction rate affected by temperature. The dwell-time algorithm of variable removal function was proposed in order to solve the nonlinear removal function. APPP for fabrication of continuous phase plate with small spatial period is introduced in this paper. Finally, APPP with variable removal function dwell time algorithm was used to fabrication a continuous phase plate with spatial period of 8mm, surface peak-valley (PV) more than 790nm, wavefront gradient root-mean-square (RMS) of 1.07um/cm. The results show that residual surface error between designed surface and measured surface root-mean-square (RMS) is down to 50 nm. The variable removal function in APPP for fabrication of continuous phase plate with small spatial period is validated.
As a new type of laser gain medium, sesquioxide crystal has significant advantages in thermal conductivity, phonon energy and other properties. It has an important application in improving the performance of picosecond/femtosecond laser. However, due to its hard and brittle characteristics, the machining process is prone to pitting, cracks, microcracks, sub surface defects and other damages, which brings great difficulties and challenges to its polishing process. In order to solve the above problems, a new chemical mechanical polishing slurry is proposed, which includes alumina, cerium oxide, aluminum sulfate, urea and pure water. After polishing with this slurry, the surface roughness of the crystal is better than 0.4nm, the damage depth of the sub surface is better than 1.5 μm, the surface shape is close to 1/10λ, and the material removal rate is up to 48nm / min. Based on the analysis of the results of small angle grazing X-ray diffraction and X-ray photoelectron spectroscopy (XPS), this paper describes the production of ReOHCO3 by the reaction of urea and Re2O3 in the new polishing slurry. The hydration layer composed of ScOOH and Sc2(SO4)3 is formed on the surface of alumina/ aluminum sulfate and crystal, and the soft product is removed by ceria to form a smooth and low damage crystal surface.
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.
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