Abstract
A new and more convenient technique using lasers has been developed to measure small vibrations of an object under observation. A mathematical analysis of the system is presented, which shows that it is possible to recover the information not only on the velocity or the position of the object, but also on its sense of direction. The instrument consists of a sophisticated Michelson interferometer, where the reference mirror is vibrated sinusoidally. The photodetector output signal contains components of the multiples of the mirror drive frequency modulated by the target vibration. A theoretical analysis shows that complete target information is available from this signal after suitable processing. Within certain limits, fluctuation of the intensity of the returned light does not affect this measurement and, therefore, the surface quality of the target is of secondary importance. The distance of the target from the laser, the permissible bandwidth of operation, and the accuracy of the instrument are discussed. Some consideration is given to various industrial applications of the technique, and it is compared to other methods where possible.