Abstract: In this paper a new laser Doppler velocimeter for extremely low velocity is introduced. The spacial light modulation and time dividing technique are used to clean-up low frequency noise which is close to the measurand. The principle of the method, optical diagram, and signal processing system are described. The measuring range is (300μm/s-0.02μm/s), and the accuracy is higher than 2%. An experimental result of a heating soldering iron Is given.

Key words: low velocity measurement, Laser Doppler anemometer