Effect of element beam quality degradation on coherently combined vortex beams and rotational Doppler velocimetry.

Vortex beams are important for rotational Doppler velocimetry, but the influence of element beam quality degradation on coherently combined vortex beams remains insufficiently clarified. Based on a compact honeycomb phased-array geometry with the central channel closed, this work uses the surrounding 18 effective sub-apertures to form coherently combined vortex beams. A radial high-order-mode degradation model described by M2 is introduced. The target vortex-mode purity ηl is defined by the normalized full complex-field overlap with the M2=1.0 reference vortex field. Unlike prior studies mainly focused on turbulence-induced OAM degradation, adaptive beam formation, or rotational Doppler velocimetry algorithms, this work isolates the transmitting-element beam-quality tolerance and quantifies its transfer to Doppler peak identifiability. The results show that ηl decreases from 1.000 at M2=1.0 to 0.0305 at M2=3.0, and this degradation further affects rotational Doppler velocimetry performance under turbulence propagation.