Spatiotemporal couplings in ultrashort vortex pulses: role of dispersion and modal conversion

Spatiotemporal couplings (STCs) in ultrashort optical pulses have been extensively studied for beams with simple spatial structure; however, their characterization in vortex pulses remains fundamentally more complex due to phase singularities and nontrivial modal structure. In this work, we develop a systematic methodology for the quantitative analysis of STCs in ultrashort vortex pulses, enabling consistent characterization of spatial chirp, angular dispersion, and pulse-front tilt in beams with nontrivial phase topology. The approach is formulated to provide results directly comparable to experiments and is applicable to structured beams beyond the Gaussian paradigm. Applied to vortex pulses generated by spiral phase plates and spiral axicons, the results reveal that STCs originate from the interplay between chromatic dispersion and wavelength-dependent modal conversion, which redistributes spectral components across spatial modes. In particular, the spectral dependence of conversion efficiency governs the magnitude of these distortions. As a representative case, broadband achromatic spiral phase plates exhibit a nearly threefold larger efficiency bandwidth ( ≈ 250 nm at 550 nm) compared to conventional designs ( ≈ 90 nm), resulting in significantly reduced STCs. These findings provide physical insight and design guidelines for ultrafast structured light.