Abstract
A new method of in-plane vibration measurement is possible by using an optical heterodyne technique which detects the frequency shift of light reflected from a point in motion. If a point in a vibrating surface is illuminated with two laser beams whose light frequencies are slightly shifted from each other in the direction symmetrical with respect to the normal of the surface and the intensity at the point is measured with a photomultiplier through an optical system, the output signal of a limiter-discriminator connected to the terminal of the photomultiplier has an amplitude proportional to the frequency-amplitude product of in-plane vibration. By using these principles, a prototype instrument for the measurement of in-plane vibration is constructed and a vibration amplitude of 10 Å at 20 kHz is measured in real time together with its direction.