Sensitive Acoustic Vibration Measurements by Optical Heterodyne Detection of Scattered Laser Light

Abstract

A sensitive method for measurement of small vibrations at acoustical frequencies has been devised using optical heterodyne spectroscopy to detect the Doppler shifts of laser light diffusely scattered from the vibration object. Beginning with the geometry of a Michelson interferometer, we replace one mirror by the scatterer and mix the reference and scattered beam on a small-area photomultiplier cathode. With only simple optical alignment, acoustic vibration spectra can be measured without perturbations even on very small-size objects, since no attachment such as an interferometer mirror is required. Vibration amplitudes as low as 0.01 nm and up to ∼100 nm can be detected with a precision of a few percent in a few seconds averaging time by phase sensitive detection of the optical beat note at the excitation frequency. Application to mechanically unstable subjects such as the auditory organs of living biological specimens is facilitated by incorporation of low-frequency feedback control of the optical path length and provision for continuous amplitude calibration. The complete system has been used in measurements of tympanal membrane movements of acoustic receptor organs. [Supported by NSF and NIH].