Application of multi-wavelength m-lines spectroscopy for optical analysis of sol-gel prepared waveguide thin films

M-lines spectroscopy (MLS) is an accurate, to within 10^-3- 10^-4, nondestructive technique for measuring optogeometric parameters (refractive index and thickness) of thin planar step-index waveguide films. Two exciting polarizations can be used and information about the film anisotropy derived. Usually, MLS uses only one wavelength, which may be disadvantageous in some cases. We have developed an MLS setup that includes a set of lasers emitting in the range of 405 to 1550 nm to conduct multi-wavelength MLS (MWMLS). MWMLS offers an opportunity to obtain more detailed optical information, e.g., index profiles and dispersion curves, especially important for sol-gel prepared waveguide thin films that are relatively porous and whose structure depends on the annealing treatment. The paper presents a detailed description of the MWMLS setup. By using sol-gel prepared waveguide thin films of Y₂O ₃, HfO₂:Eu³⁺, and TiO₂, optical measurements are exemplified. Proceeding from the measurements, the advantages and limitations of the method are discussed.