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The results of a detailed theoretical study on collinear guided wave to leaky wave acoustooptic (AO) interactions in proton-exchanged LiNbO/sub 3/ (PE:LiNbO/sub 3/) planar waveguides are presented. The guided-to-leaky mode conversion for an input optical beam at the wavelength of 632.8 nm by the induced diffraction grating from a collinear surface acoustic wave (SAW) is analyzed using a generalized multimode formulation of the coupled mode theory. Mode conversion efficiency and AO bandwidth have been calculated as functions of acoustic frequency, interaction length, guiding layer thickness, and acoustic drive power density for three cuts of the LiNbO/sub 3/ substrate. High performance configurations that are desirable for application to demultiplexing and switching in optical communication systems are identified, and the corresponding channel capacity and frequency resolution are determined. For example, it was shown that the X-cut configuration features the highest mode conversion efficiency. However, a relatively small AO bandwidth is associated with this configuration. Both high mode conversion efficiency and large AO bandwidth can be accomplished at the guiding layer thickness of 1.0 /spl mu/m. A TM/sub o//spl rarr/TE/sub /spl nu// mode conversion efficiency as high as 42% together with an AO bandwidth of approximately 70 MHz can be achieved in the Z-cut waveguide at the guiding layer thickness of 1.0 /spl mu/m, acoustic drive power density of 50 mW/mm, interaction length of 40 mm, and acoustic frequency of 460 MHz. The corresponding channel capacity and frequency resolution are 745 and 0.09 MHz, respectively. Measured mode conversion efficiencies as high as 90 and 78% obtained at the acoustic frequencies of 107 and 367 MHz using the X-cut substrate and the Y-propagation SAW have verified the theoretical prediction on the mode conversion efficiencies.