Zur Kalibrierung eines piezooptischen Faserhydrophons für diagnostische Ultraschallfelder
Ultrasonic imaging uses higher ultrasound frequencies and power levels to achieve better spacial resolution and image quality. Potential hazard for patients have to be evaluated on the basis of relevant acoustic parameters. Measurements of the acoustic pressure with piezoelectric hydrophones is only reliable up to 40 MHz because of minimum sensor size. This work improves a pizooptical measurement method for acoustic pressure. Acoustic pressure changes the refractive index of water. This is determined by measuring laser power, reflected at a fiber/water interface. To improve calibration uncertainty for that kind of measurement to the level of piezoelectric hydrophones (11% - 14%) was the primary goal. Therefore a piezooptic fiber hydrophone with single mode fiber and single mode light source (laser diode) is developed. The active area of the used single mode fiber type has a diameter of 5 µm. Since the fiber influences sound field propagation due to reflection and diffraction, sensitivity of the device is frequency dependent. These effects are compensated by mathmatic modelling. By calculaing the exact curve progression of the factor of reflection depending on the refractive index gradient at the fiber/water interface, calibration uncertainty is improved from 18% to below 11%. Results are validated by intercomparison with several calibarted piezoelectric hydrophones. Measurements are carried out on different ultrasound imagaing systems.
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