Determination of the nonlinear ultrasonic parameter beta using a Michelson interferometer

We describe an accurate optical technique for determining the nonlinear ultrasonic parameter beta. Our technique uses a path-stabilized Michelson laser interferometer, in contrast to more conventional methods which use piezoelectric or capacitive detectors. Features include high spatial resolution (typically 20 mu m), flat bandpass response over a wide range (>30 MHz) and simple self-calibration. We demonstrate the technique using a fused silica disc 19.15 mm thick and a fundamental frequency of 10 MHz. For comparison, beta was also obtained for the same specimen using capacitive and piezoelectric detectors, Errors and correction factors for all three methods are discussed; in particular, we note the importance of including ultrasonic diffraction effects. When such corrections are applied, we obtain the following results: \beta(interf)\ = 11.4 +/- 0.4, \beta(capac)\ = 12.0 +/- 0.4 and \beta(piezo)\ = 10.2 +/- 0.5. These values are in reasonable agreement with each other as well as with published values of \beta\ = 11-14.