Application of Azimuthal Doppler Shifts for Transverse Flow Velocity Measurements

We have developed a novel method, Optical Vortex Laser Absorption Spectroscopy (OVLAS), specifically for measuring transverse flow velocities in plasma. Conventional Tunable Diode Laser Absorption Spectroscopy (TDLAS) has a limitation in the measurement direction, with sensitivity restricted to the laser's propagation direction. This limitation is particularly problematic when attempting to observe particles moving perpendicular to the material surface during plasma-material interactions. We overcome this limitation by replacing the probe beam with an optical vortex beam, thus introducing the concept of the azimuthal Doppler shift for transverse flow velocity measurement. In our setup, an optical vortex beam was introduced into a flowing inductively coupled plasma (ICP) and derived the azimuthal Doppler shift distribution from the absorption spectra. These spectra are constructed based on the intensity variation in the beam cross-section. The results confirm that the azimuthal Doppler shift varies sinusoidally in the azimuthal direction, allowing the calculation of transverse flow velocities. For flow velocities exceeding 70 m/s, we observed measurement errors of less than 15%, with a mean absolute percentage error (MAPE) below 8%.