Directional discrimination in Doppler anemometry (LDA) without frequency shifting using twinned optical fibres in the receiving optics

We present a simple method for directional discrimination in laser Doppler anemometry (LDA) without using frequency shifting components. The measuring volume is imaged onto a twinned pair of receiving fibres, each of semi-circular cross section, which permit the separate detection of the leading and trailing part of the burst signal, thus revealing the sign of the velocity component without the need for frequency shifting elements. Besides being very straightforward and inexpensive, this method is particularly apt for fibre coupled LDA measuring devices.