Focused vortex beam generator suitable for optical fiber spanners in a complex liquid environment.

We experimentally demonstrated an all-fiber focused vortex beam (FVB) generator which was prepared by milling a spiral zone plate (SZP) on the Au-coated end face of a hybrid fiber by focused ion beam (FIB). In this generator, the fundamental modes propagating in the hybrid fiber are focused while being modulated into high-order orbital angular momentum (OAM) mode by the SZP at the end face. The focus length and topological charge were designed and then were both theoretically and experimentally verified. The results show that, the obtained characteristics of the FVB agree with the designed ones. The measured diameters of the focal spots are 2.2 µm, 4.4 µm, and 5.2 µm for the FVB with the topological charge of 0, 1, and 2, respectively. The simulated results show that the proposed FVB generators can maintain good focusing characteristics in different liquids, so it is a good candidate for optical fiber spanner use in a complex liquid environment. Moreover, the processing efficiency of the proposed FVB generators is nearly ten times higher than that of the previously reported ones due to the Au-coated film.