Laser-induced fluorescence measurements using optical vortex beams

Flow velocities of ions and neutrals are important parameters for characterizing plasmas. We have been developing novel methods for flow-velocity measurement based on optical vortex laser spectroscopy. In addition to usual longitudinal Doppler effect, an atom moving in an optical vortex beam experiences the azimuthal Doppler effect, which arises from azimuthal phase gradient around its singularity point. Therefore, it is expected that the Doppler spectrum contain information on the flow perpendicular to the beam path. We have so far studied optical-vortex laser absorption spectroscopy to measure neutral flow. Because metastable excited neutrals are abundant in weakly ionized laboratory plasmas, the absorption spectrum can be obtained by single path absorption. However, it is not applicable to flow measurement of ions due to very slight absorption. Here we consider laser-induced fluorescence (LIF) measurements using optical vortex beams to expand the capability of optical vortex plasma diagnostics. Comparison of the shapes of LIF spectra taken with optical vortex beams and with conventional plain-wave like beams will be given and discussed.