Asymmetric optical vortex laser-induced fluorescence (aOVLIF) method

We have been developing a novel method for flow-velocity measurement using optical vortex beams. When an optical vortex beam with azimuthally symmetric intensity distribution is applied to the laser-induced fluorescence (LIF) method, it is possible to measure the velocity of flow traversing the beam, which is, in principle, impossible with conventional plane-wave-like beams. However, information on the direction is lost because the additional azimuthal Doppler shift is observed as the broadening of the LIF spectrum [1]. Recently, we have found that asymmetry in the azimuthal intensity distribution of optical vortex beams enables the flow measurement that includes the direction. In this case, the azimuthal Doppler shift gives a frequency shift to the LIF spectrum. On the other hand, asymmetric optical vortex beams are known to rotate their intensity distribution while propagating. The effect of the rotation on the aOVLIF measurement was evaluated numerically and found to be practically negligible. Details, including preliminary experimental results, will be reported at the meeting.

[1] S. Yoshimura et al., Jpn. J. Appl. Phys. 59, SHHB04 (2020).