Measurements of the instantaneous velocity difference and the local velocity with a fiber-optic coupler

New optical arrangements with two single-mode input fibers and a fiber-opti c coupler are devised to measure the instantaneous velocity difference and the local velocity. The fibers and the coupler are polarization-preserving to guarantee a high signal-to-noise ratio. When the two input fibers are us ed to collect the scattered light with the same momentum-transfer vector bu t from two spatially separated regions in a flow, the obtained signals inte rfere when combined via the fiber-optic coupler. The resultant light receiv ed by a photomultiplier tube contains a cross-beat frequency proportional t o the velocity difference between the two measuring points. If the two inpu t fibers are used to collect the scattered light from a common scattering r egion but with two different momentum-transfer vectors, then the resultant light contains a self-heat frequency proportional to the local velocity at the measuring point. The experiment shows that both the cross-beat and the self-beat signals are large and that the standard laser Doppler signal proc essor can be used to measure the velocity difference and the local velocity in real time. The new technique will have various applications in the gene ral area of fluid dynamics. (C) 2001 Optical Society of America.