Influence of the frequency response of the photodetector on the heterodyne interferometer-based sound pressure standards in water

The heterodyne interferometer has been used to realize the sound pressure unit directly and absolutely for underwater acoustics and ultrasound, which is considered as the primary standard of the hydrophone calibration instead of the reciprocity method. The widely used demodulation methods are the zero-crossing method and the arctangent method. Recent studies show that the frequency response of the utilized photodetector (PD) in the heterodyne interferometer also significantly influences the results of the sound pressure realization using the two demodulation methods, especially for the high-intensity focused ultrasound application, which is investigated in this paper. Simulations are performed to obtain general conclusions using different types of low-pass filters to simulate the nonideal frequency responses of the PD. Also, experimental results of the frequency response of the utilized PD are then used to analyze the induced relative error of the demodulated acoustic particle velocity so as to evaluate the related measurement uncertainty of the sound pressure realization. The simulation method is useful to choose the required PDs and evaluate the related measurement uncertainty induced by their frequency responses for the optical sound pressure standards and other optical measurement applications.