Optical flow sensor through near-field grating diffraction

A novel optical flow-meter has been developed, especially designed for very-high flow speed. The system is a variant of laser Doppler velocimeter (LDV), realized by means of an optical phase-grating. Standard LDV generates a fringes pattern by means of two laser beams, intersecting at an angle. For fast flow rates, the fringe period has to be very long, in order to avoid too high output frequencies, therefore a very low angle is required. For the realization of such pattern we propose a different approach, employing an optical grating, working in the near-field range. This technique has been demonstrated by measuring the flow-speed distribution in a water-jet machine.