Acousto-optical sound pulse analysis under Raman-Nath conditions and beyond

The acousto-optical interaction of a plane monochromatic light wave with a plane sound pulse, consisting of a continuous frequency spectrum, is consid ered. A new integro-differential equation, describing the phenomenon, is de rived. This equation is firstly solved in the Raman-Nath regime. A general expression for the intensity in the optical near field (Fresnel zone) is gi ven. For the case of a weak sound field and close to the sound column, it i s shown that the light intensity, as a function of time, is proportional to the second derivative of the sound pulse signal. Secondly this equation is solved beyond the Raman-Nath regime. The obtained expressions can he very useful for optical near field measurements of sound pulses with a discrete or continuous spectrum, that consists out of higher frequencies than in the restricted Raman-Nath case.