Real-Time Transverse Flow Velocity Measurement using Optical Vortex

We are developing optical vortex laser absorption spectroscopy (OVLAS), which replaces the probe beam of tunable diode laser absorption spectroscopy (TDLAS) with an optical vortex beam. OVLAS can measure the velocity of metastable atoms across the beam, which is challenging with conventional TDLAS. So far, we have measured the transverse flow velocity of metastable atoms by analyzing the azimuthal Doppler shift distribution on the beam cross-section. Since the measurement requires hundreds of 2D absorption images taken during laser frequency sweeping, the time resolution is limited by data transfer from the camera to a PC. Observing the temporal variation of the velocity distribution is crucial for understanding plasma dynamics, so improving OVLAS's time resolution is important. To reduce data acquisition time, we replace the camera with a quadrant photodiode (QPD). Additionally, the laser frequency is slightly modulated during the sweeping of the probe laser's wavelength, and resulting differentiated signals from the QPD are obtained using lock-in detection synchronized to the frequency modulation. The transverse flow velocity is instantly obtained by analyzing the difference in the zero-crossing frequency of the differentiated signals for each channel of the QPD. The details of the real-time transverse flow velocity measurement will be reported in this presentation.