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Ultra-precise surface machining of N-BK7 using microwave-driven reactive plasma jet machining

Chemically reactive Plasma Jet Machining (PJM) is a contactless and efficient surface machining technique increasingly applied to the surface shape generation and error correction of various optical elements. However, the application of fluorine-based PJM to surface machining of N-BK7® is challenging since the chemical interaction between reactive plasma species and metal components of N-BK7 induces a residual layer in the contact zone and surrounding of the plasma-treated area. It was noticed that a residual layer degrades the ability of obtaining the prerequisite surface profile and causes a nonlinear and hardly predictable removal behavior with respect to the etching time. In this paper, extensive studies are conducted for relaxing constraints in applying the fluorine-based PJM to the surface machining of N-BK7, particularly regarding to the manufacture of freeform optical elements. In this regard, the chemical composition of residual layer is evaluated by using SEM/EDX analysis aiming at clarifying the chemical kinetics between plasma generated active particles and the N-BK7 surface atoms. Furthermore, the etching behavior of N-BK7 is compared with Fused Silica to verify the optimality of obtained results. Finally, the area machining is tested at different plasma dwell times to evaluate the predictability and regularity of results.