Laser-induced Fluorescence Doppler Spectroscopy using Asymmetric Optical Vortex Beams: Principles of the Advanced LIF Method

The traditional laser-induced fluorescence (LIF) Doppler spectroscopy method, which utilizes a Gaussian beam, has been widely employed in plasma experiments to measure the flow velocities of ions and neutral particles. However, we have always faced the inevitable problem that the measurable velocity components are limited to the direction of beam propagation. Recently, we proposed a new LIF method using an asymmetric optical vortex beam, which has non-uniform intensity and phase distributions in the azimuthal direction of the beam cross-section (K. Terasaka et al., Sci. Rep. 2024), and is hereafter referred to as aOVLIF. The twisted wavefront of the optical vortex beam provides a Doppler shift associated with the velocity perpendicular to the beam propagation direction, because the Doppler shift of the resonant absorption frequency of the particle is given by the scalar product of the phase gradient of the beam and the velocity vector of the particle. As a result, the LIF spectrum obtained by the aOVLIF method exhibits a frequency shift caused by the flow of particles across the beam. The aOVLIF method can even determine the three-dimensional flow vector without requiring movement of the optical system. Additionally, the frequency shift of the spectrum is independent of the measurement position along the beam propagation direction. We will present the principle of the aOVLIF method and the numerical results obtained with typical parameters of laboratory plasmas.