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New optical arrangements with two single-mode input fibers and a fiber-opticcoupler are devised to measure the instantaneous velocity difference and localvelocity. The fibers and the coupler are polarization-preserving to guarantee ahigh signal-to-noise ratio. When the two input fibers are used to collect thescattered light with the same momentum transfer vector but from two spatiallyseparated regions in a flow, the obtained signals interfere when combined viathe fiber-optic coupler. The resultant light received by a photomultiplier tubecontains a cross-beat frequency proportional to the velocity difference betweenthe two measuring points. If the two input fibers are used to collect thescattered light from a common scattering region but with two different momentumtransfer vectors, the resultant light then contains a self-beat frequencyproportional to the local velocity at the measuring point. The experiment showsthat both the cross-beat and self-beat signals are large and the standard laserDoppler signal processor can be used to measure the velocity difference andlocal velocity in real time. The new technique will have various applicationsin the general area of fluid dynamics.

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