Zn-doped TiO2 (0,3,7% wt.) thin films were obtained by Sol-Gel method, deposited by spin-coating process on glass substrates, then heat treated at 500 °C for 90 min. Deposited films (Zn: TiO2) have been characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV–vis transmittance spectroscopy and m-lines spectroscopy (MLS). XRD analysis of the undoped TiO2 films showed the formation of both phases (anatase and brookite) with A(101) plan as preferential orientation. Increase of zinc concentration inhibited the crystal growth in the TiO2 films. SEM pictures of undoped TiO2 and Zn doped films indicate that zinc contents can lead to significant microstructure changes of TiO2 films. The average transmittance and the optical band gap of the films are 70–90% in the wavelength range 350–900 nm and 3.6–3.51 eV, respectively. M-lines spectroscopy put into evidence that Zn: TiO2 planar waveguides support single confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. MLS measurements proved that doping zinc in TiO2 increased the index refraction of TiO2 thin films which enhances the waveguiding properties. Simultaneously, the birefringence decreases with increasing concentrations up to 7 wt%.Graphical Abstract