The performance of acousto-optic cells for laser-Doppler anemometry

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Abstract
The performance of water filled acousto-optic cells is quantified with particular reference to their use for laser-Doppler anemometry. With the cell operating at a frequency of 9.2 MHz, the intensities of the +1 and -1 order beams are shown to account for 76% of the transmitted light intensity for an electrical power input of 0.56W. The influence of the incident angle and the performance of a cell at frequencies close to but different from the fundamental frequency are quantified. Operation of the cells at a frequency of 30 MHz, i.e. close to a third harmonic, is shown to require significantly greater electrical power input but can without introducing thermal connection effects, allow the transmission of a first and zero order beams with equal intensities and a total loss of 8% of the incident intensity. To show that an acousto-optic cell can be used as a beam splitter and frequency shifter, measurements of three components of velocity are reported in the region of recirculation immediately downstream of the centre disc of an annular jet.