A composite optical waveguide (OWG) was fabricated by use of the masked sputtering method to deposit a tapered TiO<sub>2</sub> film onto a single-mode potassium ion-exchanged (PIE) planar waveguide. Using the slab OWG spectroscopy apparatus, including a xenon lamp and a charge-coupled device (CCD) spectrometer, transmission spectra of the composite OWG with different superstrates were measured in the wavelength range 400 to 900 nm. The findings show that the composite OWG transmission spectrum is sensitive to the refractive index of the liquid overlaid on the TiO<sub>2</sub> film. The sensing mechanism is considered to be the liquid-index-induced change in surface-scattering loss for the guided mode coupled adiabatically from the PIE layer into the TiO<sub>2</sub> film. The composite OWG transmission spectrum and its response to liquid index exhibit strong polarization dependence due to large differences in properties between the TE<sub>0</sub> and TM<sub>0</sub> modes in the TiO<sub>2</sub> film. The composite OWG transmission spectroscopy as a new refractometry was analyzed experimentally and theoretically.

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