Progress and prospect of ion flow velocity measurement using optical vortex laser-induced fluorescence method*

The ion flow measurement using the laser-induced fluorescence (LIF) method is based on the Doppler effect that modifies the absorption condition. Since the Doppler shift is given by the dot product of the phase gradient and the flow velocity, when a plane-wave-like beam with a phase gradient only in the direction of propagation is used, the measurable velocity is limited to the component projected on the beam propagation direction. This one-dimensionality can be overcome by using optical vortex beams with the azimuthal phase gradient; the Doppler shift now has two main terms, i.e., the translational Doppler shift and the azimuthal one. Optical vortex laser absorption spectroscopy has successfully used this azimuthal Doppler shift to measure the flow of neutral particles crossing the beam perpendicularly [1]. In the optical vortex LIF method, the shift of the LIF spectrum due to the azimuthal Doppler shift can be observed by asymmetrizing the intensity distribution of the optical vortex beam [2]. Progress to date and future developments of the optical vortex LIF method will be discussed in the poster.

[1] H. Minagawa et al., Sci. Rep. 13, 15400 (2023).

[2] K. Terasaka et al., Sci. Rep. 14, 2005 (2024).