Laser interferometer microscope for the measurement of nanometer vibrational displacements of a light‐scattering microscopic object

A laser interferometer microscope (LIM) has been developed to measure the amplitude and phase of submicroscopic vibrational displacements. The instrument is based on a combination of differential optical interferometry, optical heterodyning, and high‐resolution incident light microscopy. Diffusely backscattered laser light of a vibrating microscopic object (φ≊1 μm) suffices to operate the LIM and to measure vibrational displacements down to fractions of a nanometer for frequencies ranging from 10 Hz to a few kHz. Calibration of the LIM is relatively simple and does not depend on the intensity of the scattered light. Since the instrument is not sensitive to small background motions or drift of the equilibrium position of the object, it is very suitable for measurements on biological objects in vivo. Application of the LIM to the fish lateral line organ enabled the measurement of vibrational displacements of the cupula from below 0.3 nm to about 300 nm.