Confined optical phonons are investigated in short period (100) Si/Ge strained layer superlattices by Raman spectroscopy. The observed energies are closely related to the thickness of the corresponding Si and Ge slabs. The influence of the confinement effect on the mode energies is analysed with a model based on a simple picture of standing waves which fit into a single layer. No dependence of the modes on the thickness of the adjacent layers is found, confirming good confinement. The additional energy shift due to the built-in biaxial strain is determined from samples with different strain distributions but equal layer thicknesses.