The production of complex shaped optical elements like non-standard aspheres, acylinders, or freeform elements are highly demanded. Thus, optical manufacturing technologies need to be developed for optical systems to design freeform surfaces. Reactive Plasma Jet (RPJ) is one of the most promising tools for freeform generation of fused silica, SiC, ULE® and silicon. However, there are severe limitations when this technique is used for the surface machining of optical glasses like N-BK7®. The chemical interaction between plasma generated active species and metal components of N-BK7 induces the formation of a residual layer in the plasma-surface contact zone and surrounding which can degrade the capability of acquiring the required surface profile. It is shown that elevated surface temperature can modify the residual layer leading to higher predictability of freeform machining results.
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.
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