Effect of Laguerre-Gaussian beam propagation on TDLAS spectra observed using optical vortex

The optical vortex (OV) is the center of the Laguerre-Gaussian (LG) mode, and its wavefront has a spiral shape. The particles in the OV feels the additional azimuthal doppler effect induced by the spiral wavefront. We are developing an optical vortex laser absorption spectroscopy (OVLAS) to detect the lateral particle flow using the azimuthal Doppler shift. Since the Doppler shift in the LG beam depends on the position in the beam cross-section, the fundamental LG beam is deformed inevitably by the non-uniform absorption. The deformed beam structure can be decomposed by the series of the LG modes. Since the Gouy phase shift depends on the order of the LG mode, the non-uniform beam structure represented by the superposition of different orders of the LG beams is varied with beam propagation. This unwanted change in beam structure deforms the Doppler spectrum obtained with OVLAS, so its effect must be clarified. We will discuss the detail of the propagation effect in the presentation.