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Optical Engineering

Quantitative phase demodulation from a free-running Michelson interferometer by using a novel quadrature moiré grating with misalignment error compensation
Author(s): HoPui Ho; K.-C. Lo; Y.Y. Hung
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Paper Abstract

This paper describes a novel technique to perform fast phase demodulation in Michelson interferometers without any need for phase locking or heterodyning, thus enabling quantitative detection of vibrations in a noncontacting mode. The new scheme greatly simplifies the optical design of laser interferometers used for nondestructive evaluation of materials. It uses a matched moiré grating to transform the interference fringe pattern formed between the reference and signal beams into intensity modulation. It one divides the filter into four quadrants so that the gratings in each quadrant have been shifted to represent an equivalent phase difference of π/2, the intensity signals collected behind the four quadrants effectively represent four simultaneous data points due to phase stepping, covering the full cycle. This means that a unique phase measurement can be obtained at any arbitrary phase position, and thus the interferometer may be left to operate in free-running mode. Since all the four signals originate from the same laser source, appropriate differential signal processing can be incorporated to remove intensity noise due to fluctuations in the laser source. In addition, we have studied the effect of angular misalignment between the grating filter and the interference fringe pattern. Our results show that such misalignments may be compensated through a back-substitution algorithm. This further enhances the practical benefits of our scheme in real situations.

Paper Details

Date Published: 1 July 2006
PDF: 7 pages
Opt. Eng. 45(7) 075601 doi: 10.1117/1.2227010
Published in: Optical Engineering Volume 45, Issue 7
Show Author Affiliations
HoPui Ho, The Chinese Univ. of Hong Kong (Hong Kong China)
K.-C. Lo, City Univ. of Hong Kong (Hong Kong China)
Y.Y. Hung, City Univ. of Hong Kong (Hong Kong China)


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