Observation of Raman–Nath optical diffraction in the phase grating plane

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Abstract
Previous studies of Raman-Nath optical diffraction have emphasized the characteristics of the light distribution in either the nearfield or the farfield of the optical phase grating produced by an acoustic beam. The plane in which this grating is located can be imaged outside of the acoustic medium and experimentally studied with a photodetector. This paper describes measurements of the light intensity variations which occur in the image plane containing the optical phase grating after spatial filtering is performed. Theoretical results are presented for values of the Raman-Nath phase parameter v .ltoreq. 6 when the negative optical diffraction orders are removed by a spatial filter. Experimental results are also presented from a 2.5-MHz transducer radiating into distilled water which confirm the theoretical predictions. It is possible to use this technique to assess the frequency response of an optical system over a large frequency range since frequencies as high as 4 times the fundamental are generated in the optical intensity variation. For small values of the phase parameter, the approach should be useful for observing the temporal waveforms of broadband ultrasonic pulses of the type frequently encountered in medical diagnostic systems.

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