Optimized fabrication of curved surfaces by a FIB for direct focusing with glass fibres

A focused ion beam (FIB) was used to fabricate Fresnel phase lenses on optical fibre tips. The influence of dwell time and the scanning strategy to produce parabolic structures in silicon was investigated, because these parameters have a strong influence on the shape of the fabricated structures. The lens shape was characterized by atomic force microscopy and it was shown that the FIB does not roughen the surfaces. The optical performance of the lenses was characterized by scanning near field optical microscopy (SNOM) and the results were compared to simulations taking into account fabrication imperfections of the Fresnel lenses. At a wavelength of 840 nm a spot size of 740 nm FWHM was achieved.