Plane-wave model and experimental measurements of the directivity of a Fabry-Perot, polymer film, ultrasound sensor

Optical detection of ultrasound is popular due to the small element sizes that can be achieved. One method exploits the thickness change of a Fabry-Perot (FP) interferometer caused by the passage of an acoustic wave to modulate a laser beam. This detection method can have greater sensitivity than piezoelectric detectors for sub-millimeter element sizes. The directivity of FP sensors and the smallest achievable effective element size are examined here. A plane-wave model of the frequency-dependent directional response of the sensor, based on Brekhovskikh’s work on elastic waves in layered media, is described and validated against experimental directivity measurements made over a frequency range of 15 MHz and from normal incidence to 80 deg. In terms of applications, the model may be used to provide a noise-free response function that can be deconvolved from sound field measurements in order to improve accuracy in high-frequency metrology and imaging applications, or, for example, as a predictive tool to improve sensor design. Here, the smallest achievable effective element radius was investigated by comparing the directivity with that of a rigid circular pressure transducer, and found to be ∼0.9d, where d is the thickness of the FP interferometer. [Funding was provided by the EPSRC, UK]