Acoustical Holograms Using Phase Information Only

Abstract

Optical signals require square-law detectors and hence direct phase measurement is not feasible without the use of a coherent reference beam. On the other hand, acoustic beams can be directly converted to useful electrical signals with linear transducers. Consequently, an external reference beam can be provided electronically rather than acoustically to form an acoustical hologram. A scheme is described where a second external electronic reference beam, phase-locked with the signal beam, is used in combination with the first electronic reference to record only the phase of the signal in an acoustical hologram. All fringes in such a hologram have the same visibility, and the resulting hologram reconstructs the original two-dimensional object. Improved reconstructions were obtained when appropriate spatial filtering eliminated the interference from the conjugate image in a narrow field, coaxial (Gabor) type hologram. The optical analogue of acoustical phase-only holography, is to record an optical hologram on a photographic film having linear characteristics, and variable gamma depending on the signal intensity.