Spatiotemporal coupling in spectral broadening and pulse compression of Laguerre-Gaussian vortex pulses in a multi-pass cell

Spectral broadening in multi-pass cells (MPC) enables post-compression of high-energy femtosecond pulses, while structured optical pulses carrying orbital angular momentum are additionally limited by spatiotemporal coupling during nonlinear propagation. Here we investigate the nonlinear evolution and compression of Laguerre-Gaussian pulses in an MPC under different initial amplitude and phase perturbations using a unified propagation model. We show that spectral uniformity and compression quality are weakly affected by transverse amplitude modulation but are strongly degraded by initial nonlinear phase distortions. The compression limit is governed not by the nonlinear phase magnitude itself, but predominantly by frequency-dependent wavefront distortions quantified by the spatiotemporal Strehl ratio. These results establish clear physical criteria for high-fidelity post-compression of structured ultrafast pulses.