A method is presented which allows us to accurately determine geometric effects in QW devices. Finite element methods have been used in combination with perturbation techniques to determine the propagation characteristics of a single QW GaAs/AlGaAs ridge waveguide. The attenuation coefficient and phase shift are evaluated for both TE and TM polarizations. The results show that a simple slab model overestimates the attenuation and phase change for wavelengths smaller than the excitonic resonances of the QW and underestimates them for larger wavelengths.