High-speed laser velocimeter realized by a near-field grating

A novel Laser Doppler Velocimeter (LDV) has been developed, by means of an optical phase-grating. Standard LDV generates a fringes pattern by means of two laser beams, colliding at a set angle. When the speed is very high, the fringe period has to be very long, in order to avoid too high output frequencies, therefore a very low angle should be 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 speed distribution of a water-jet machine.