Single-crystal germanium is an excellent optical material in the infrared wavelength range. The development of germanium Fresnel lenses not only improves the optical imaging quality but also enables the miniaturization of optical systems. In the present work, we developed a ductile-mode micro grooving process for fabricating Fresnel lenses on germanium. We used a sharply pointed diamond tool to generate the micro Fresnel structures under three-axis ultraprecision numerical control. By adopting a small angle between the cutting edge and the tangent of the objective surface, this method enables the uniform thinning of the undeformed chip thickness to the nanometric range, and thus provides complete ductile regime machining of brittle materials. Under the present conditions, a Fresnel lens which has a form error of 0.5 νm and surface roughness of 20-50 nm Ry (peak-to-valley) was fabricated successfully during a single tool pass.
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