The usefulness of guided wave optics as a tool for characterizing general purpose thin films is demonstrated in two cases of practical interest, i.e., silicon on sapphire and amorphous silicon on glass. Important results are as follows: (i) optical wave propagation at &lgr;=1.15 &mgr;m has been observed along a few millimeters in silicon layers; (ii) silicon on sapphire layers show a very high photoelastically induced quasiuniaxial birefringence (&Dgr;n≃0.02); (iii) amorphous silicon layers exhibit a nonrectangular index profile, a high birefringence (&Dgr;n≃0.02), and finally an anomalously low value of their mean ordinary index (n≃3.04±0.01).