The enhanced interaction of an optical probing beam with a traveling surface acoustic wave (SAW) using either a separate stationary rectangular phase or a holographic (HOE) phase reference gratings is reported. SUMMARY Grating enhanced probing of SAW using both amplitude and phase structures located on the same reflecting plane as the SAW was investigated analytically and experimentally by Koniotskii and Black) This paper extends this work to include a separation distance L between the reference grating and the plane of the SAW. A spatial frequency analysis in the Ranian-Nath diffraction regime was performed for both transmission and reflection configurations. The small signal case where the reference structure is strongly modulated in comparison to the phase modulation of the traveling space-time grating produced by the SAW is treated. The double grating interaction is found to provide a heterodyne-type sensitivity and spatial selectivity.