The Mark 4 all-fiber Doppler velocity interferometer system launched by Martin, Froeschner & Associates measures and records instantaneous velocity vs. time histories and can track accelerations and vibrations of almost any surface. It delivers turnkey reliability and subnanosecond time resolution. With delay leg lengths of up to 30 m, it can resolve velocity changes as small as 0.1 m/s. Features include full quadrature, a push-pull configuration and a coaxially fiber-coupled visible laser source to facilitate target spotting. Light from the system’s high-purity laser is focused onto the target by a 5- to 50-mm effective focal length variable-focus lens, and the reflected Doppler-shifted light is collected by the same lens and returned by the same fiber to the interferometer for analysis.