Vibration measurements using interferometry
Interferometry can measure movements on the sub-nm scale. We use low-coherence light, achieved by a broad band of wavelengths, and lenses to restrict the interferometric measurement to a local region of interest.
An optical coherence tomography (OCT) system is used to image the cochlear membrane of the inner ear, the ribbon of tissue down which sound waves propagate. The noise floor is 30 pm vibration, and in vivo vibrations are typical 0.1-10 nm. Figure: The vibrations of the cochlear membrane, a thin 100-µm-thick ribbon stretched between two bone supports. OC = organ of Corti. RM = Riessner's membrane.
A similar OCT system is being developed as a hand-held otoscope to measure vibrations of the middle ear in the hearing clinic. Conductive hearing loss due to the ossicles (small ear bones) not properly moving in response to sound can be evaluated, without surgical violation of the typanic membrane. OCT can see through the typmanic membrane. Figure: TM = typmanic membrane. IN = incus. Ma = malleus.