Modification of laser-induced fluorescence spectrum by additional azimuthal Doppler effect in optical vortex beams

Laser-induced fluorescence (LIF) method is a powerful tool to measure flow velocities of atomic and ionic species in a plasma. Optical vortex beams, which carry orbital angular momentum of light, have a potential to expand the capability of flow velocity measurements with LIF method, because an atom moving in an optical vortex beam experiences additional Doppler effect in the azimuthal direction. The LIF spectra obtained by substituting a commonly-used plane-wave-like beam with an optical vortex beam have been numerically evaluated. It is demonstrated that modification of the spectrum can be used to detect a fast flow perpendicular to the laser path, which is impossible to be observed by conventional method in principle. The use of standard deviation of the spectrum as an index of perpendicular flow velocity is also discussed.