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Proceedings Paper

Signal competition in heterodyne interferometry
Author(s): Ombeline de La Rochefoucauld; Shyam M. Khanna; Elizabeth S. Olson
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Paper Abstract

The Organ of Corti is a complex structure with many reflecting surfaces characterized by a wide range of reflectivities. Heterodyne interferometry has been the primary technique for measuring motion of the cochlear sensory tissue for some time. We would like to know under what conditions reflections from out-of-focus surfaces affect the measured velocity of the in-focus surface. Heterodyne interferometry uses interference between two laser beams (object and reference). The velocity of the test object shifts the frequency of the object beam due to the Doppler effect. The heterodyne signal (a frequency modulated (FM) wave) is decoded using a frequency demodulator. By reviewing the theory of FM demodulation and showing tests with our Revox FM demodulator, we demonstrate that the influence of a secondary signal on a measurement depends on the modulation index (ratio of the frequency deviation (Δf=2Vo/λ) to the modulation frequency, fm where Vo is the velocity amplitude and λ is the laser wavelength). For high-modulation-index signals, the fundamental component of the FM demodulator output is not affected by a secondary signal unless the secondary signal's power is nearly as large as that of the primary signal. However, the output waveform can be distorted. For a low-modulation-index signal, a secondary competing signal can have a relatively large effect on the fundamental component of the output signal, but the output signal waveform is not distorted. The results underscore the benefit of steep optical sectioning to reduce contamination by out-of-focus signals.

Paper Details

Date Published: 21 June 2006
PDF: 10 pages
Proc. SPIE 6345, Seventh International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, 63450B (21 June 2006); doi: 10.1117/12.693112
Show Author Affiliations
Ombeline de La Rochefoucauld, Columbia Univ. (United States)
Shyam M. Khanna, Columbia Univ. (United States)
Elizabeth S. Olson, Columbia Univ. (United States)

Published in SPIE Proceedings Vol. 6345:
Seventh International Conference on Vibration Measurements by Laser Techniques: Advances and Applications
Enrico Primo Tomasini, Editor(s)

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