The zero-order Raman-Nath method for visualizing the longitudinal character of acoustic beams

Acoustic fields have been visualized in a plane parallel to the path of propagation using schlieren optics. A similar method is described which also uses light diffracted by the periodic density variation induced in the medium by the acoustic field. This diffraction phenomenon was described in a paper by Raman and Nath. Whereas, in the schlieren system, the zero-order beam is intercepted by a stop and the higher orderr ays make up the image, in the method described here, all orders contribute to the image, including the zero order. This is made possible by a phase sensitive detector which is used to measure the diffracted light. The sound field is scanned in one leg of an optical Michelson interferometer. The zero-order beam is decreased by the energy diffracted intoh igher orders, and further, that portiono f the light remaining in the zero order thatis interacting with the acoustic beam is shiftedi n phase. Thus the zero-order method offersth e possibility of greater sensitivity than thsec hlieren system.