Novel fiber microprobe comprising a nanostructured gradient index optical vortex phase mask and single-mode fiber was investigated numerically and experimentally. The mask which, consists of spatially distributed sub-wavelength nanorods, was fabricated via the modified stack-and-draw fiber drawing technique. The mask has flat input and output surfaces and transverse dimensions of a standard single-mode fiber. The microprobe allows efficient generation of high-quality optical vortex beams in different environments such as air and water.