Measurement of multi-point displacements by optical fiber specklegram sensor

Abstract

The evaluation of an optical fiber specklegram sensor on the assessment of multi-point displacements is reported. The light emitted by a 663 nm laser source is launched into the multimode fiber, resulting in an output speckle field that is detect by a CCD. The fiber is attached to 3 microbending transducers for specklegram modulation, being the intensity distribution referenced to the possible fiber statuses in order to calculate the relative speckle field changes. Finally, the obtained data is processed by artificial neural networks for retrieving the magnitude of each input displacement. The methodology was tested on the interrogation of 2 or 3 transducers, yielding maximum errors of 0.29 μm and 0.5 μm, respectively, regarding a 30 μm range. Although the specklegram sensitivity is reduced for a large number of attached devices, the methodology can be applied on the monitoring of small sensor arrays, providing a reliable alternative for time and wavelength-based multiplexing schemes.