Recent works in our laboratories investigated the microstructure of titania films prepared from a colloidal solution and used as planar waveguides. The transmission electron microscopy including high resolution observations together with waveguide Raman spectroscopy, especially in the very low frequency range, showed a strong influence of the heat-treatment procedure on the films' morphology. In addition, atomic force microscopy provided valuable information on their surface roughness. At lower temperatures (450 degree(s)C), the films' structure consists of a mixture of amorphous TiO2 and anatase nanocrystals (4.5 - 5 nm) and its surface is smooth. On the contrary, when increasing the annealing temperature, the nanocrystals grow (50 nm at 1000 degree(s)C), pores (5 - 20 nm) are clearly distinguished and the films' waveguiding properties disappear. Here, we discuss the optical behavior of these films and its relationship to microstructure. It is clearly shown that a high degree of crystallization induces an important porosity, a rough surface and thus dramatically affects the films' waveguiding properties. Finally, we report a relatively simple strategy to stabilize the amorphous phase and control the crystallization in order to improve the optical quality of the waveguides.