The multi-beam laser Doppler vibrometer (MB-LDV) has been successfully used for acoustic landmine detection in field experiments at an Army test site. Using the MB-LDV in a continuously scanning mode significantly reduces the time of the measurement. However, continuous motion of a laser beam across the ground surface generates noise at the vibrometer output due to dynamic speckles. This speckle noise defines the noise floor and the probability of detection of the system. This paper studies the origins of speckle noise for a continuously scanning LDV. The structure of the speckle field exhibits points of phase singularity that normally coincide with signal dropouts. The signal dropouts and phase singularities can cause spikes in the demodulated velocity signal, which increase the noise in the velocity signal. The response of FM demodulators to input signals causing spikes in the LDV output are investigated in this paper. Methods of spike reduction in the LDV signals have been developed and experimentally investigated.
A Laser Transit Anemometer (LTA) system for measurement of velocity, flow angle and turbulence in the rotating blade rows of turbomachinery and for other applications has been developed by the authors (SDL). Advanced optical, electronic, and computer components make the system sensitive to submicron scatterers, provide automated data collection and permit probing close to blade and wall surfaces, in some cases without seeding the flow. This paper describes new and practical developments of the optical system and presents test results from axial compressor measurements. Based upon these test results, improvements were incorporated into the system and these improvements are discussed briefly.
A method is described of applying a variable frequency-shift to a laser beam, by means of Bragg cells, in which the direction of the output diffracted beam is unaffected by the magnitude of the applied shift. Applications of the technique to reference-beam and Doppler-difference forms of laser anemometry are discussed together with methods of obtaining increased dynamic range.
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