As shown in Fig. 1, an optical heterodyne technique can be successfully applied for in-plane vibration measurement. The purpose of this paper is to make clear the design method and technical details to construct a measuring instrument based on this technique. For the purpose, a prototype apparatus is constructed and its performance is studied experimentally. It is ascertained that an in-plane vibration with an amplitude of 10Å at 20 kHz can be measured together with the velocity vector distribution. Figs. 2 and 3 show schematic diagrams of the optical system and an experimental setup respectively. Figs. 4 through 8 are photographs and schematic diagrams of main components. In order to determine the optimum conditions of the optical imaging system shown in Fig. 2, studies on mutual relations among beat signal amplitude, diameter of pinhole, and mean diameter of speckle are made. Fig. 10 shows the results of these studies and it is concluded that the diameter of pinhole should be a few times as large as that of speckle to obtain adequate beat signals. Figs. 13 and 15 are the results of performance test.
