Ultrasonovision: an interferometric device for ultrasonic visualization and measurement

Gabor suggested measuring acoustic wavefronts by optically measuring the displacement of a reflective membrance coupled to the medium of propagation. We have implemented this suggestion and measured acoustic wavefronts in water at frequencies from 0.5 to 5 MHz and at intensities as low as 250 nW/cm2. The response is linear to several W/cm2. The membrane is so tightly coupled to the medium that its displacement equals the displacement amplitude of the wave to within 1% up to angles of incidence of 40°. Pictures of the acoustic fields were obtained in near real time (several frames/sec) with wavelength-limited lateral resolution. The arrangement used is that of a Michelson interferometer, with one mirror rigid and external to the acoustic field and the other a thin (∼8 μ) metalized plastic film (or pellicle) suspended normal to and in the path of the acoustic wave. A narrow laser beam is raster scanned across the interferometer, and so the pellicle. The photocurrent from a photodiode has an acoustic frequency component proportional to the pellicle displacement. This component may be measured directly or used to brightness-modulate a CRT and thus produce an image of the sound field.