Abstract
As for light diffraction by fixed gratings, one can distinguish between the Fraunhofer region (farfield) and the Fresnel region (nearfield) of the diffracted light. Whereas the individual farfield diffraction orders have constant light intensity, at least if the ultrasonic wave is a progressive one, the intensity in the nearfield of the diffracted light is modulated in both time and space. This intensity modulation is due to the mutual interferences of the various diffracted light waves and may be very complex, even for moderate ultrasonic amplitudes. Moreover, the intensity distribution in the nearfield of the diffracted light appears to be much more sensitive to variations of the ultrasonic power than the individual Fraunhofer intensities, which provides a useful technique for observing weak ultrasonic waves. In addition, the intensity measurement in the nearfield requires a simple experimental arrangement.
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