Absolute Calibration of Hydrophones Using Heterodyne Interferometry and Zero-crossing Signal Demodulation

The current primary method for hydrophone calibration is the three-transducer spherical-wave reciprocity at NPL, covering the frequency range between 1 kHz up to 500 kHz with expanded uncertainties as low as ± 0.5 dB. Optical methods can provide an alternative method for primary calibration as they directly measure the acoustic particle velocity at a specific point in the field. The hydrophone is then placed at the exact same point and can be calibrated directly referenced to the acoustic Pascal, with traceability to the wavelength of the light source used. This paper discusses the optical development of a heterodyne interferometer for such purpose. A transducer is placed in a water tank while an acoustically transparent pellicle is mounted at a certain distance. As sound propagates, the pellicle follows the movement of sound. A heterodyne interferometer with an 80 MHz carrier provides a reference beam while the measurement beam probes the pellicle. The reflected measurement beam is then mixed back with the reference and the output of the interferometer is a frequency modulated voltage signal with modulation depth corresponding to the amplitude of the acoustic field. This paper explains the optical arrangement required for the measurements and considers the resulting signal demodulation based on the zero-crossing point method that enables the calculation of the Doppler shift and hence the velocity due to the sound field.