Near-Field Grating for high-speed Laser Doppler Velocimetry

In order to measure very high fluid speed we developed a novel Laser Doppler Velocimeter (LDV), realized by means of an optical phase-grating. Conventional LDV creates a pattern of light fringes by means of the interference between two laser beams, colliding at an angle α. When the speed is very high, the fringe period has to be very long in order to avoid too high output frequencies. For the realization of such pattern we propose an optical grating, working in the near-field range. By this technique we successfully measured the speed distribution of a water-jet machine.