We report about the determination of the axial velocity component by a laser Doppler velocity profile sensor that is based on two superposed fanlike interference fringe systems. Evaluation of the ratio of the Doppler frequencies obtained from each fringe system yields the lateral velocity component and the axial position inside the fringe system. Inclined particle trajectories result in chirped burst signals, where the change of the Doppler frequency in one burst signal is directly related to the axial velocity component. For one single tracer particle it is possible to determine (i) the lateral velocity component, (ii) the axial velocity component including the direction, and (iii) the axial position of the tracer trajectory. In this paper we present the measurement principle and report about results from simulation and experiments. An uncertainty of the axial velocity component of about 3% and a spatial resolution in the micrometer range were achieved. Possible applications of the sensor lie in three-component velocity measurements of flow fields where only one optical access is available.
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