What laser instrument measures vibration velocity from the Doppler shift of reflected light?

A laser instrument that measures vibration velocity from the Doppler shift of reflected light is a laser Doppler vibrometer.

An LDV directs a laser beam at a moving or vibrating surface. Motion along the beam changes the frequency of the scattered light through the Doppler effect. Optical heterodyne detection compares the returned light with a reference beam, allowing the instrument to recover the target’s velocity as a function of time without touching the surface.

This non-contact method is valuable when attaching a sensor would alter the motion, or when the target is very small, hot, delicate, or difficult to reach. Applications include speaker and instrument testing, aircraft and bridge vibration studies, hard-disk diagnostics, MEMS research, and medical measurements of structures such as the eardrum.

A common LDV uses a beamsplitter and often a frequency-shifted reference produced by a Bragg cell or acousto-optic modulator. Unlike a scanning tunneling microscope, it does not image atomic surfaces; unlike a Fabry–Perot etalon, it is primarily a motion-measuring instrument.