We present the design of a parabolic multimode waveguide used as a spot size width converter. The designed structure allows reducing the input spot size width an order of magnitude using a very short structure with respect to linear or conventional tapers. An illustrative example of spot width reduction from 10 microns to a Si wire of 1 micron with a small length in the order of 17 microns and a coupling efficiency of ∼90% is presented. Using a linear taper structure with a taper length >100 μm, the coupled power is still less than that obtained by the parabolic multimode interference (PMMI) guide. This means that the PMMI offers a length reduction of more than 5× with respect to the linear taper to achieve the same spot reduction.