In this study, we measured acousto-optic (AO) characteristics of integrated thin film structure $(PZT/TiO_x/SiO_2/SiN_x/SiO_2/Si)$ and analyzed the propagation ratio of surface acoustic power and AO figure of merit of $SiN_x$ film for Bragg diffraction type AO device applications. AO Bragg diffraction was induced by AO interaction between surface acoustic wave (SAW) and guided optical beam in the integrated structure. The unique point of this device structure was that we intended to fabricate a AO device using conventional Si (100) substrate and PZT film known as a best piezoelectric material. In first experiment (set #1), we varied AO quality factor (Q=$\pi$, 2$\pi$ and 4$\pi$) at the same conditions of fabrication methods and structure factors such as film thickness and device size. We measured the intensity variation of optical beam increasing acoustic power and estimated AO diffraction efficiency($\zeta$), the ratio of optical intensities of diffracted and incident optical beams. AO diffraction efficiency in the three samples (Q= $\pi$, 2$\pi$ and 4$\pi$) were 4.4, 4.7 and 8.1 % at the acoustic power of 60, 44 and 91 mW, respectively. To analyze the propagation ratio of surface acoustic power and AO figure of merit of $SiN_x$ film, we fabricated new AO devices (set #2, 4 samples) structures with the main variables of thickness in top $SiO_2$ and PZT films. AO diffraction efficiency of these devices were 6∼9% at the same acoustic power (84 mW). As the thickness of total films in AO device was increased, AO diffraction efficiency was decreased. These observations may be due to the decrease of surface acoustic power when it propagating to the thickness direction. Comparing AO diffraction efficiency of theses devices, the propagation ratio of surface acoustic power to the thickness direction was analyzed in top $SiO_2$ and PZT film, respectively. Using AO diffraction efficiency and propagation ratio of surface acoustic power in AO devices (set #2), we calculated total propagation ratio (40∼60%) of surface acoustic power from PZT to SiN_x film and AO figure of merit $(M_2=0.75 \times 10^{-18} s^{3}/g)$ of $SiN_x$ film.
