Optical flow sensor through near-field grating diffraction

• Design of a non-standard optical method for flow measurement, employing an optical grating in the near-field region, in order to develop optical fringes with large spatial period.
• Measurement of very-high flow speed (∼900 m/s) with a compact and low-cost equipment.
• The realized measurement sensor is able to work also in a very difficult environment (such for the water-jet application).
• The proposed sensor can measure the speed distribution of the waterjet flow, as demonstrated by a comparison with a custom made laser Doppler velocimeter.

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 α. 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 α 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.