Laser-induced Fluorescence Doppler Spectroscopy using Asymmetric Optical Vortex Beams: Advances in Practical Doppler Shift Detection in Laboratory Plasma

The asymmetric optical vortex laser-induced fluorescence (aOVLIF) method has been proposed for measuring flow velocity components perpendicular to the laser beam direction in plasma diagnostics. It uses an optical vortex beam with an azimuthally asymmetric intensity profile. The twisted wavefront generates a transverse phase gradient across the beam cross-section. Particles moving across the beam experience an additional Doppler shift in the resonant absorption condition, proportional to the scalar product of the local phase gradient and their velocity vector. Experimental realization requires precise control of the beam's spatial phase and intensity, achieved using a computer-generated hologram (CGH) on a spatial light modulator (SLM). A symmetric vortex beam with a topological charge of 100 has a circular intensity pattern. However, a slight displacement of the CGH introduces asymmetry, resulting in an asymmetric vortex beam. Though the intensity becomes distorted, the transverse phase gradient remains clear, inducing directional Doppler shifts. For example, a 600 μm-diameter beam with a topological charge of 100 can yield a 50 MHz shift at a transverse velocity of 1 km/s. We present preliminary experimental results validating the aOVLIF method and its potential to extend conventional LIF diagnostics.