Economical generation of high-quality optical vortices with gradual-width Fermat spiral slit mask

A general model for creating an optical vortex using a variable-width Fermat spiral slit (FSS) mask was developed. The generated optical vortex demonstrates much better quality than those produced by slits with a uniform width. The depth of focus was nearly doubled, which can be explained by discretizing the gradual-width FSS. To demonstrate the generation robustness, the slits were printed on a transparent slide, and an output vortex beam with greater than 98% intensity purity was obtained. With the high quality and simplicity of this vortex generation method, its analysis and generation scheme are expected to be extended to generation of plasmonic vortices and short-wavelength light sources such as X-rays and electron beams. Generation of high-quality optical vortex beams at an almost negligible cost will make investigation much more accessible.