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Proceedings Paper

Evaluation Of An OTDR Microbend Distributed Sensor
Author(s): J. S. Schoenwald; R. H. Rattner; P. M. Beckham
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Paper Abstract

We review the performance of a microbend fiber optical time domain reflectometry (OTDR) based distributed strain sensor. A testbed has been developed to evaluate and compare this system to conventional discrete strain gauge sensors. The testbed consists of an aluminum cantilever beam to which have been attached several pairs of strain gauges (for reference) and several microbend transducers. A single standard multimode optical fiber is threaded through the microbend transducers. The fiber is pulse illuminated by a commercial OTDR instrument. The microbend losses are detected as step changes in the returned OTDR signal wave form. The transducers consist of small pairs of opposing sets of teeth, one set mounted on the flexible beam testbed, the other machined into a small, rigid cantilever bar mounted to produce scissor action as the beam flexes. The transducers are biased to produce a step loss with the beam in its neutral position. The direction of beam deformation is then determined by the increase or decrease in the step size. Several transducers along the beam, incorporating the same fiber, permit the acquisition of strain measurements from multiple locations on the structure by processing a single OTDR wave form. The waveform is obtained as the logarithm of the Rayleigh backscattered intensity. The change in the step response of the logarithmic signal caused by each transducer is linear in the in-plane strain produced by displacing the tip of the beam. Repeatability, hysteresis and sensitivity are dependent on the nature and thickness of the fiber coating, signal processing, transducer design, and other less significant factors. We have demonstrated a resolution of 2-3 microstrains over a range of 120 microstrains using transducers with 2 mm pitch in tooth spacing and a 2.5 cm interaction length, mounted on cantilever bars 5 cm in length, and averaging multiple wave forms to improve signal-to-noise ratio.

Paper Details

Date Published: 16 January 1989
PDF: 8 pages
Proc. SPIE 0986, Fiber Optic Smart Structures and Skins, (16 January 1989); doi: 10.1117/12.948898
Show Author Affiliations
J. S. Schoenwald, Rockwell International Science Center (United States)
R. H. Rattner, Rockwell International Science Center (United States)
P. M. Beckham, Rockwell International Science Center (United States)

Published in SPIE Proceedings Vol. 0986:
Fiber Optic Smart Structures and Skins
Eric Udd, Editor(s)

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