In this work, we present the computation of different integrated optical micro-electro-mechanical systems by applying the finite element method to the electromagnetic vectorial Helmholtz equation. The finite element solver combines state-of-the-art features with sophisticated symmetry handling and the perfectly matched layer approach to simulate open boundaries. The focus of investigation is on a photonic crystal slab for the purpose of a polarization selective filter in an integrated optical sensor system. The transmission characteristics of the filter is modeled with a finite element solver and the results are compared to analytic computations, published results, and the transfer matrix method for selected designs. A final design is found for maximum polarization selectivity at a given operation wavelength.