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Weak acousto-optical interaction and normal light incidence provided, light diffraction tomography has proved to be a useful tool for mapping ultrasonic fields. Beyond this range, for normal incidence, data acquisition and evaluation has turned out to be far more difficult, which is mainly due to the fact that in this case the light intensity is no longer a function of only one variable but of two. Furthermore, the measurement of the acoustic phase is affected by additional optical phase shifts. A method has been developed which facilitates, by means of Bragg incidence, the quantitative mapping of ultrasonic fields beyond the range of weak acousto-optical interaction. The awkwardness of data acquisition at normal incidence could be avoided. As a result, the favourable properties of light diffraction tomography, e.g. the mapping of ultrasonic fields without retroaction and with high spatial resolution, can be utilized in a frequency range which has not been accessible so far. This in turn might open new perspectives for the calibration of hydrophones in an extended frequency range up to about 50 MHz. |
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