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Analysis of Azimuthal Doppler Shift of Anisotropically Absorbed Laguerre-Gaussian Beam Propagating in Transverse Flow

Hiroki MINAGAWA, Shinji YOSHIMURA^1,2), Kenichiro TERASAKA³⁾ and Mitsutoshi ARAMAKI

College of Industrial Technology, Nihon University, Narashino, Chiba 275-8575, Japan

1)

National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, Gifu 509-5292, Japan

2)

Center for Low-temperature Plasma Sciences, Nagoya University, Nagoya 464-8601, Japan

3)

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

(Received 27 July 2022 / Accepted 13 August 2022 / Published 14 September 2022)

Abstract

The particle flux onto a material is an important parameter in the study of the plasma-material interaction. With conventional Doppler spectroscopy, it is difficult to measure the flow velocity perpendicular to the material because only the velocity component projected on the wave number vector can be measured. To overcome the limitation, we are developing a transverse flow measurement method using the azimuthal Doppler shift of a Laguerre-Gaussian (LG) beam absorption spectroscopy. In this paper, the feasibility of this spectroscopy method has been examined by numerical analysis. The LG beam is anisotropically absorbed in the transverse flow due to the azimuthal Doppler shift. Since the anisotropic LG beam rotates with propagation, the spatial structure of resonance absorption in plasma and the intensity structure of the LG beam that has propagated through the plasma are inevitably different in the LG beam absorption spectroscopy. It was shown that the deviation from the original azimuthal Doppler shift is reduced to several percent at the position where the intensity distribution of the LG beam reaches its maximum value. Therefore, the transverse flow can be measured with sufficient accuracy by properly selecting the position on the beam cross-section used to evaluate the azimuthal Doppler shift.

Copyright (c) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

Laguerre-Gaussian beam, optical vortex, azimuthal Doppler shift, angular spectrum method, tunable diode laser absorption spectroscopy

DOI: 10.1585/pfr.17.1401099

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