Characterization of optical vortex beam in free space and optical fiber

Optical vortex beam has been used in many applications such as nanoscale imaging, sensing, and so on due to its unique azimuthal phase distribution. Due to the helical wavefront nature, the vortex beam carrying orbital angular momentum (OAM) has also been used to distinguish the enantiomers of the chiral molecule. However, coupling efficiency remains a problem due to the size mismatch of the beam and the molecule. In our work, vortex fibers with plasmonic nanostructures are used to nano-focus the vortex beam to enhance the coupling between light and matter. To achieve this goal, cylindrical vector and OAM beams have been characterized in free space and through vortex fiber, a polarization-maintaining ring core fiber. The helicity characterization and polarization analysis of the vortex beams were performed. The spiral and fork-shaped interference patterns were observed with the helicity measurement showing the direction of the phase fronts. The free-space vortex beam was coupled and transmitted through the vortex fiber with a pure and stable output beam. Nano-focusing of the vortex beam was tested by the fabrication of a nanostructure on the core of the vortex fiber facet. The structure could enhance the coupling efficiency of the beam with chiral molecules.