| 摘要 |
In the disclosed method and structure a pulse of a sound filed to be mapped is illuminated at at least two extremes values at different moments with a laser beam of short duration compared to the period of the illuminated sound pulse and interference produced between the resulting reflected beam and a reference beam. A holographic image is formed. The relative phase of the laser pulse and the reference is changed between the individual illuminations by a preselectable amount which deviates from pi and which suppresses the optical and/or acoustical noise. Specifically, a time delay initiates the first illumination process when an extreme value is reached at the beginning of the sound pulse to be mapped. A counter is started by this time delay and counts the half and quarter waves of the sound pulse. The counter initiates an additional illumination process after a preselectable odd number has been reached. A phase shifter changes the relative phase between the object beam and the reference beam.
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