Development Of A Miniature Objective Based On Calcium Fluoride And Zinc Sulfide Lenses For Ultrafast Laser Microsurgery Applications

Calcium fluoride and zinc sulfide crystals exhibit broad multispectral transparency and are used for a multitude of optical applications. However, they both belong to a soft brittle crystal category and are mechanically challenging to process. Thus, a fabrication protocol using a single-point diamond turning method was developed, which can yield surfaces with optical quality for these materials. By using a diamond turning tool with low waviness (<= 100-nm waviness) and by using feed rates of 1 and 0.2 mu m/rev, CaF2 and ZnS surfaces with surface roughness of 6 and 5 nm were manufactured, respectively. Design and fabrication of a miniature objective with 0.5 NA are also presented, which was developed for the ultrafast laser microsurgery application. The objective consists of CaF2 and ZnS lenses to withstand the high power from ultra-short laser pulses and to provide a tightly focused beam with 1.23 mu m 1/e(2) spot radius for laser ablation. Based on the spot size measurement of the objective, its performance was considered to be near diffraction limited. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)