Temporal Reference Acoustical Holography

Abstract

This paper describes a new form of holography which obviates the need for a reference wave (real or electronically simulated). An experiment is described where reconstructions were obtained from a temporal reference acoustical hologram which was recorded at 4.8 MHz using a Sokolov ultrasound camera system. An optical method for recording a large aperture temporal reference acoustical hologram which is currently under development at the Douglas Advanced Research Laboratories, uses a double pulsed ruby laser system which introduces a quarter wavelength shift in the optical reference (or object) beam between the two pulses which are timed half an acoustic cycle apart. The quarter wavelength shift allows a subfringe interferometric hologram of an acoustically vibrating surface to be recorded, where, under appropriate conditions the intensity of the reconstructed surface is linearly proportional to the displacement of the surface between the two pulses. The pulsed laser interferometric hologram is thus an optical hologram of an acoustical temporal reference hologram. A second method of optically recording temporal reference acoustical holograms which employs a scanning cw laser is described. The important advantages expected of both these optical techniques are: (1) the total recording time is short (half an acoustic cycle) and thus the techniques are more tolerant of object motion; (2) the area of the temporal reference acoustical hologram will be significantly larger than obtainable with conventionl methods; (3) the necessity and complexity of electrons detection (large arrays or scanning) is eliminated and replaced by the high bandwidth recording capability of optical holography; (4) direct remote sensing of the acoustical object wave is achieved.