This paper gives experimental and theoretical studies of the measurement of small vibrational-displacement-amplitudes such as ultrasonic vibrators. The measurement is based on the Doppler shift of a laser beam from the vibrating surface. A measurement method which employs a laser beam with multiple axial-modes and a Michelson-interferometer-type optical system, is presented. It determines the displacement amplitude through the ratio of the harmonics of the frequency spectrum of detected signal, for example, the fundamental to the third-harmonics. An experiment was performed to verify this method. The resultant sensitivity was about 200i at the vibrational frequency 39 kHz.
One of the most important problems, encountered in the analysis and design of underwater information transmission systems using ultrasonic waves, is how to quantitatively evaluate the effect of multi-path propagation due to surface reflection The author was provided with a probabaistic model to deal with the multi-path propagation problem. For the purpose of demonstrating the applicability of the model, the experiments were carried out in an unechoic water tank with a free surface. The results and the statistical analysis obtained by using a digital computer are described It has become evident that the multi-path propagation plays an important role in the trade-off among various design parameters. Therefore the concepts developed are to be applied to the design of an underwater telemetry system.