Note: using an optical phase-locked loop in heterodyne velocimetry.

An optical phase-locked loop was introduced in heterodyne velocimetry to lock the differential frequency between a fiber laser and an external cavity diode laser. An uncertainty less than 1 MHz of the locked beat frequency was achieved during several microseconds, corresponding to a velocity uncertainty at 0.1 m/s level for 1550 nm light. In this way, a measurement with higher precision and better time resolution simultaneously can be obtained during a transient process. Three proof-of-principles shots were performed to measure elastic wave-induced vibrations on surfaces of steel films with submillimeter thicknesses. The surface velocity fluctuations were probed with amplitudes of about 2 m/s and periods of tens of nanoseconds, and propagating times and sound velocities of waves were also well analyzed at a time scale 0.5 μs. A velocity resolution of 0.1 m/s level and a temporal resolution of a few nanoseconds were achieved in these measurements.