Rigorous modal solutions of tapered spot-size converters, using buried core and deep-etched structures, through semiconductor fabrication techniques, are presented. Results for devices where both the primary and the mode forming secondary waveguides are operating close to their modal cutoff are presented, by using a full-vectorial finite elementbased modal solution and a full-vectorial beam propagation approach, also based on the finite element method. Mode
beating in the tapered section, the expansion of the spot-size and the consequent enhancement of the coupling to an optical fiber are also reported. Numerically simulated results indicate that, overall, a narrow far-field profile with a higher coupling efficiency, along with a better relaxation of the fabrication tolerances for the SSC designs presented here, can be achieved.