Diffraction effect on spectral line shape of optical vortex Doppler absorption spectroscopy

Since the optical vortex has twisted wavefront, the motion in the wave field causes the Doppler shift in all the three-dimensional directions. We are developing the optical vortex Doppler absorption spectroscopy that is sensitive to the movement in the direction across the optical path. Since the Doppler shift in the optical vortex field depends on the position with respect to the beam center, which is the phase singularity, the observation of local absorption is required for the optical vortex Doppler absorption spectroscopy. However, as the laser propagates from the plasma to the imaging device, the small spatial structure is deformed due to the diffraction. In order to avoid the diffraction effect, a 4f imaging system was constructed and the absorbed beam was transferred to the imaging device using the lens system. In this presentation, we discuss the influence of the diffraction on the spectral line shape and the imaging system to avoid it.