Fibre-optic laser Doppler anemometer with Bragg frequency shift utilising polarisation-preserving single-mode fibre

Abstract

A fibre-optic laser Doppler anemometer (LDA) utilising a single-mode polarisation-preserving fibre as a (transmitting) link is discussed. Since these fibres are bimodal in the sense that they can sustain two modes with orthogonal polarisations, they can be used to transmit the two beams of a normal LDA in which one beam is frequency shifted. By rotating the polarisation of one beam 90 degrees before coupling it to the fibre, the two beams propagate independently through the fibre and can be separated again at the fibre exit. The performance of this type of fibre in a laser Doppler set-up is investigated and experimental results are given. A miniature optical measuring head using this principle and operating in backscatter is described. The backscattered light is collected by a conventional multi-mode fibre.