Acousto-optic frequency switching for single-camera planar Doppler velocimetry

A single-camera Planar Doppler velocimetry system has been demonstrated, in which illumination beams at two closely-spaced optical frequencies are derived from a single Argon-ion laser. The frequency of one beam lies on an absorption line of iodine vapour, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and the Doppler-shifted scattered light passes through an iodine cell onto a single solid-state camera. Light scattered from the measurement plane from the beam with its wavelength set to be off the absorption line is not affected by its passage through the cell, and provides a reference image, while that from the beam on the absorption line encodes the velocity information as a variation in transmission dependent upon the Doppler shift. The complex imaging system required for conventional PDV is eliminated, since superposition of the reference and signal images is automatic. The two beams are generated by an optical arrangement incorporating an acousto-optic modulator, and are transmitted to the region of interest by a single mode optical fibre, which ensures both a smooth Gaussian illumination profile and co-linearity of the beams. The system is demonstrated using a spinning disc and a velocity resolution of +/-1 ms-1 is achieved.