An improved heterodyne laser interferometer for use in studies of cochlear mechanics

A displacement-sensitive heterodyne laser interferometer is described which is suitable to measure the sound-evoked motion of various structures in the intact, living cochlea. Data are included to demonstrate the interferometer's low noise-floor ( < 1 pm Hz (-0.5)), wide bandwidth (from d.c. to 63 kHz) and linear dynamic range ( > 300 microm). The interferometer's optical sensitivity is sufficient to detect the motion of the basilar membrane and various components of the organ of Corti without the use of artificial reflectors: the reflectivity needed to achieve a 10 pm Hz(-0.5) noise-floor is approximately 0.0001%, and the optical sectioning depth is approximately 23 microm. These features make the interferometer ideally suited to measure acoustic vibrations in the living inner ear, but there may also be applications in other fields of neuroscience.