Two single-mode fibers collect light with the same scattered wave vector from two spatially separated regions in a sample. These regions are illuminated by a single coherent laser beam, so that the collected signals interfere when combined by means of a fiber-optic coupler, before they are directed to a photomultiplier tube. The fibers and the coupler are polarization preserving to guarantee a high signal-to-noise ratio. The measured intensity fluctuations are used to determine the velocity difference δv(L) for spatial separations L in the sample. Specifically, an intensity autocorrelation function is calculated theoretically for rigid body rotation and is tested experimentally. Experimental results span two orders of magnitude in L and agree with theoretical predictions with an error of less than 5%. This new technique will be very useful in the study of turbulent flow and particle settling dynamics.
© 1998 Optical Society of America
(030.7060) Coherence and statistical optics : Turbulence
(060.1810) Fiber optics and optical communications : Buffers, couplers, routers, switches, and multiplexers
(060.2420) Fiber optics and optical communications : Fibers, polarization-maintaining
Yixue Du, Bruce J. Ackerson, and Penger Tong, "Velocity difference measurement with a fiber-optic coupler," J. Opt. Soc. Am. A 15, 2433-2439 (1998)