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Applied Optics

Applied Optics


  • Vol. 12, Iss. 6 — Jun. 1, 1973
  • pp: 1145–1156

Two-Component Dual-Scatter Laser Doppler Velocimeter with Frequency Burst Signal Readout

D. B. Brayton, H. T. Kalb, and F. L. Crosswy  »View Author Affiliations

Applied Optics, Vol. 12, Issue 6, pp. 1145-1156 (1973)

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A dual-scatter laser Doppler velocimeter (LDV) system designed for measuring wind tunnel flow velocity is described. The system simultaneously measures two orthogonal velocity components of a flowing fluid at a common point in the flow. Essential single-velocity component dual-scatter concepts are presented to simplify the description of the more sophisticated two-component system. To implement the two-component system three laser beams with a 0°, 45°, and 90° polarization plane relationship are focused to a common point in the flow by the system-transmitting optics. The beams interfere to form two perpendicular sets of interference fringe planes that are orthogonally polarized. The system-receiving optics collect and separate the orthogonally polarized components of laser radiation scattered from micron-size particles moving with the flowing fluid through the fringes. The system requires no artificial seeding, since intrinsic test section aerosols are utilized for radiation scattering. The passage of each scatter particle through the interference fringes simultaneously produces two frequency-burst-type photodetected signals, the frequencies of which are directly proportional to two perpendicular components of particle velocity. The system photodetection, signal-conditioning, and data acquisition instrumentation is specifically designed to process the frequency burst information in the time domain as opposed to spectrum analysis or frequency domain processing. The system was initially evaluated in an AEDC wind tunnel operating over a Mach number range from 0.6 to 1.5. The LDV and calculated wind tunnel mean velocity data agreed to within 1.25%; flow direction deviations of a few milliradians were resolved.

© 1973 Optical Society of America

Original Manuscript: August 20, 1971
Published: June 1, 1973

D. B. Brayton, H. T. Kalb, and F. L. Crosswy, "Two-Component Dual-Scatter Laser Doppler Velocimeter with Frequency Burst Signal Readout," Appl. Opt. 12, 1145-1156 (1973)

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  1. J. W. Foreman, Appl. Opt. 6, 821 (1967). [CrossRef] [PubMed]
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  9. The concept of threshold laser power level as discussed in the latter part of Sec. II.A for the dual-scatter LDV is also applicable to the reference-beam LDV. For all reference-beam configurations except for that of back-scattered radiation collection, the scattering direction must be severely restricted in range due to frequency broadening considerations.1–6 Thus, except for back scattering, the reference-beam LDV requires a higher threshold laser power level.
  10. D. B. Brayton, W. H. Goethert, Trans. Instrum. Soc. Am. 10, 40 (1971).
  11. C. M. Penny, I.E.E.E. J. Quantum Mech. QE5, 318 (1968).
  12. H. Kogelnik, Bell Syst. Tech. J. XX, 467 (March1965).
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  14. M. Kerker, The Scattering of Light (Academic Press, New York, 1969).
  15. Let it be assumed that wedge refraction caused by the poor surface flatness of an ordinary window will cause the direction of each illuminating beam to be deviated by an average angular amount aw in a random direction. One can then show that the beam separation distance at the window DW must satisfy DW ≪ NF λ/αW if substantial beam overlap at the beam crossover point is to be maintained. The quantity αW has been experimentally determined to be about 1 × 104 radian by examining the crossover point after inserting many ordinary plate glass and plexiglass windows.
  16. D. B. Brayton, W. M. Farmer, “Small Particle Signal Characteristics of a Dual Scatter Laser Doppler Velocimeter,” Appl. Opt., to be published.
  17. F. H. Smith, A. E. Lennert, J. O. Hornkohl, AEDC-TR-71-165 (1971).

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