On the optimum spatial code of a laser anemometer

The question of the optimum spatial code of a light scattering laser anemometer is reexamined using the Fisher number as a figure of merit. We show that field distributions that are related by a Fourier transform yield the same Fisher number. The well known differential Doppler and time-of-flight configurations are therefore equally good. Comparison with a diffraction limited upper bound on the Fisher number shows that these configurations are both suboptimal. We point out that the optimum code is likely to be Fourier self-reciprocal, and show that a self-reciprocal hybrid combination of the Doppler and time-of-flight configuration yields a spatial code that is superior to those used previously.