One of the key devices in monolithically integrated coherent receivers for polarization diversity systems is the semiconductor waveguide TE/TM mode converter. A multimodal analysis method of the conversion mechanisms in these devices is presented. This method can deal with asymmetric structures that are the actual case in fabricated devices. A two-dimensional vectorial finite element method is employed for the evaluation of guided modes. The first design of a buried waveguide TE/TM converter in InP/InGaAsP is proposed and simulated. The influence of the multimodal effects is discussed.