Planar Doppler velocimetry using a Mach-Zehnder interferometric filter

A planar Doppler velocimetry system to measure flow velocity fields is described. The technique uses a Mach-Zehnder interferometric filter to convert Doppler frequency shifts into intensity variations. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. An experimental arrangement is described that incorporates a phase-locking system designed to stabilise the interferometric filter. Two methods to process the interference fringe images are presented; the first uses the shift of the fringe pattern to determine the Doppler shift along profiles. The second provides a full-field measurement of the Doppler shift by determining the phase at each pixel in the images. Results are presented here for measurements of velocity fields on a rotating disc with maximum velocities, at the edge, of +/- 70m/s. Measurements on a seeded air jet with a nozzle diameter of 20mm and an exit velocity of similar to 85m/s are also presented.