2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/165598Soliton propagation is usually described in the ``slowly varying envelope approximation'' (SVEA) regime, which is not applicable for ultrashort pulses. We present theoretical results and numerical simulations for both NLS and parametric ($χ^{(2)}$) ultrashort solitons in the ``generalised few-cycle envelope approximation'' (GFEA) regime, demonstrating their altered propagation.4 pages, 4 figuresOpticstext