The optical phonon modes in short-period GaAs/AlAs superlattices were investigated by means of Raman spectroscopy and far-infrared reflection spectroscopy using a grating coupler technique. Grating couplers provide a wave vector transfer parallel to the layers, enabling study of in-plane dispersion of the phonon modes in the superlattice structure. The mixing of confined and interface optical modes was observed as the in-plane phonon wave vector was varied. The experimental frequencies of the superlattice optical modes obtained from infrared and Raman spectra are in good agreement with calculations performed using a dielectric continuum model.