Distributed strain measurements using fiber Bragg gratings in small-diameter optical fiber and low-coherence reflectometry.

Optical low coherence reflectometry and fiber Bragg gratings written in small diameter (50 micrometer) optical fibers were used for measurements of non-homogenous internal strain fields inside an epoxy specimen with sub-grating length resolution. The results were compared with measurements using Fiber Bragg gratings in standard size (125 micrometer) single mode fibers and show that smaller fibers are less intrusive at stress heterogeneities.

Distributed measurements of fiber birefringence and diametric load using optical low-coherence reflectometry and fiber gratings.

Polarization sensitive optical low-coherence reflectometry (OLCR) is used for measuring the complex fiber Bragg gratings (FBG) reflection coefficient. We determine the beat length directly from oscillations in the OLCR amplitude with a resolution of 10(-6) and a spatial resolution only limited by the minimum beat length or the coherence length of the light source. Using the OLCR amplitude and phase in combination with an inverse scattering algorithm the birefringence is retrieved with a resolution of 2x10(-5) while the spatial resolution is 25 mum. The two developed techniques are applied for measuring position, magnitude and footprint of induced birefringence of an FBG under uniform and non-uniform diametric loading.