The figure depicts the basic optical layouts for (1) conventional laser Doppler velocimetry (LDV) and (2) a newer method of LDV based partly on the use of two optical fibers. Whereas conventional LDV involves the use of at least two lasers aimed in specified direction and detection of light scattered to one detector in almost any direction, the newer method involves only one laser and fiber-optic receptors that collected light scattered in two specified directions.
In conventional LDV, two coherent laser beams are made to intersect in a small measurement volume, where they interfere. As a seed particle entrained in a flow passes through the measurement volume, the laser light reflected by the particle is modulated at a frequency proportional to the spatial frequency of the interference fringes and the component of velocity perpendicular to the interference fringes. More specifically, the modulation frequency is given by Δf = (1/2Π)v·Δk, where v is the velocity and Δk = k1 – k2 is the difference between the wave vectors of the two laser beams. Because Δf is independent of the direction in which the light is scattered, a photodetector can be placed in any convenient position to receive the scattered light. The output of the photodetector is processed to extract Δf and thus the component of v in the k1 – k2direction.
In the newer method, the measurement volume lies in a small region somewhere along a single illuminating laser beam, but in this case, the measurement volume lies at (and is defined by) the intersection of the laser beam and the lines of sight of two fiber-optic receptors. To obtain a high signal-to-noise ratio, these receptors are constructed in the form of polarization-preserving, single-mode optical fibers. Scattered light collected by these receptors is combined in a fiber-optic coupler and delivered to a photodetector, where interference between the beams scattered in the two directions gives rise to a Doppler beat frequency. This beat frequency is given by the same equation as that for the modulation frequency in conventional LDV, except that in this case, Δk = k1 – k2 is the difference between the wave vectors (at the laser wavelength) defined by the lines of sight from the measurement volume to the input ends of the two fiber-optic receptors. In this case, Δf is independent of the direction of the laser beam; hence, it is possible to illuminate the measurement volume from any convenient direction. The beat frequency can be measured by use of a standard LDV signal processor, a commercial digital photon correlator, or a fast digital correlator.
Unlike in conventional LDV, it is not necessary to add more lasers operating at different wavelengths and aimed in different directions in order to be able to measure additional velocity components. Instead, it suffices to add pairs of fiber-optic receptors aimed to define the corresponding orthogonal wave-vector differences, plus the signal-processing equipment needed to extract the beat-frequency outputs of the additional receptors.
This work was done by Penger Tong, Bruce J. Ackerson, and Walter I. Goldburg of Oklahoma State University for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17136.