Fiber-optic laser transit velocimeters

The design and performance of a dual-beam transit (two-spot) velocimeter using fiber-guided light input and output are briefly discussed. With a 10-micron spot diameter at the fiber input and a numerical aperture (NA of 1/15, an output spot diameter of approximately 25 microns or less and an NA of 1/4 (approximately the limiting NA of the fiber) are obtained. This spot is significantly less than the fiber diameter, well obtainable with graded-index fibers. The instrument has been successfully tested in flight and used to measure the flow in wind tunnel wall boundary layers and the efflux from a model engine thrust reverser.