Beyond 40 MHz frontier: the future technologies for calibration and sensing of acoustic fields

Several techniques that are suitable for calibration of ultrasound hydrophones in the frequency range beyond 20 MHz are briefly reviewed. Attention is largely focused on substitution techniques because in contrast to the primary methods they are not as laborious and can be carried out relatively quickly. The techniques discussed include two acoustic procedures using swept frequency systems, an approach based on nonlinear propagation of acoustic waves in water medium, an acousto-optic technique that employs fiber optic sensors and a novel technique that is based on Time Reversed Acoustic (TRA) approach. It is shown that these techniques are capable of extending the frequency range in which the hydrophones can be calibrated up to 100 MHz. The importance of spatial averaging correction and its impact on the acoustic output measurements is also pointed out.