Planar optical waveguides formed by Si ion implantation into PECVD SiO2 have been characterized by the dark mode spectroscopy method at a wavelength of 0.6328 mu m. The measured effective index values of the guided modes have been used to investigate the optical properties of the core layers of the waveguides after different pre-implantation treatments. It was found that annealing the specimens before implantation, affected both the refractive index and thickness of the core layers. In the annealed specimens a thicker core layer and a larger relative refractive index difference between the core and the buffer layer resulted.