Fiber Bragg grating (FBG) sensors have been shown to be a good means of nondestructive monitoring of the stress and/or strain of the materials in which they are embedded. Many FBG transverse stress/strain measurement systems can resolve only a single stress and/or strain value for the entire length of the FBG and often require the use of polarization-maintaining fiber. We demonstrate a new method for measuring the two components of transverse stress with high spatial resolution in a distributed FBG sensor. A directional compressive load is applied by placing weights on top of the FBG, creating a transverse stress in the core of the FBG. Small metallic strips are placed under the FBG to create a localized stress in the FBG. The relative index of refraction as a function of position in the FBG is determined with a low-coherence Michelson interferometer and a layer-peeling algorithm. With this method we are able to measure changes in the refractive index with resolution better than 5x10-6, limited by the signal-to-noise ratio of the measurement system, with a spatial resolution of 16 μm. To determine transverse stress, we repeat the measurement for four different polarization states. A four-state analysis is then used to determine the birefringence as a function of position in the grating. This measurement assumes that the applied transverse load is much larger than any other birefringence in the grating, so that the principal axes do not change with position in the grating. This measurement offers the advantage that it can be implemented with a simple layer-peeling algorithm, and it does not require the use of expensive polarization maintaining fiber. Measurements of the externally induced birefringence agree well with values predicted by the stress-optic properties and the geometry of the fiber.
We demonstrate a novel technique to determine the size of Mie scatterers with high sensitivity. Our technique is based on spectral domain optical coherence tomography measurements of the dispersion that is induced by the scattering process. We use both Mie scattering theory and dispersion measurements of phantoms to show that the scattering dispersion is very sensitive to small changes in the size and/or refractive index of the scatterer.
We discuss the fundamental limits of fiber Bragg grating (FBG) wavelength metrology. High-accuracy wavelength measurements are critical for FBG strain sensors because a wavelength measurement uncertainty as small as 1 pm leads to an uncertainty of nearly 1 microstrain. We compare the measurement uncertainties for several common wavelength measurement systems, including tunable laser, optical spectrum analyzer (OSA), and interferometric. We show that when using an OSA it is difficult to achieve a measurement uncertainty better than 10 pm, and if the OSA is not accurately calibrated to a known wavelength reference, then the wavelength measurement uncertainty can be as large as 1 nm. We describe the uncertainties involved in determining peak and/or centroid wavelength from a measured data set. We also discuss calibration references for FBG sensor interrogation units. Wavelength references that are based on molecular absorption lines are often an excellent choice for FBG sensor calibration. However, some interrogation units require a wavelength reference unit based on narrow reflection lines rather than absorption lines. We investigated the application of athermally packaged FBGs as wavelength references, but we found that their wavelengths will drift with time and can undergo large jumps. We concluded that it is difficult to achieve stability better than 4 pm/year in athermally packaged FBGs.
We discuss the current status of fiber Bragg grating (FBG) sensor metrology. High-accuracy wavelength measurements are critical for FBG strain sensors, because wavelength measurement uncertainties even as small as 1 pm lead to an uncertainty of nearly 1 microstrain. We administered an FBG wavelength measurements round robin in which twelve participants measured the spectral reflectance of four FBGs; we found that the measured peak wavelengths of a single FBG varied by as much as 35 pm. We have determined that this variation results from two factors: the uncertainty of the measurement systems and the wavelength instability of the athermally packaged FBGs. We found that the wavelengths of athermally packaged gratings will drift with time and can undergo large jumps and that it is difficult to achieve stability better than 4 pm/year. We discuss the measurement uncertainties for several common wavelength measurement systems, including tunable laser, optical spectrum analyzer (OSA), and interferometric measurements. We show that when using an OSA, as many of the round-robin participants did, it is difficult to achieve a measurement uncertainty better than 10 pm, and if the OSA is not accurately calibrated to a known wavelength reference; then wavelength measurement uncertainty can be as large as 1 nm. Wavelength references that are based on molecular absorption lines are ideal for calibrating OSAs, but they are not optimal for calibrating FBG interrogation units where a reference based on reflection lines rather than absorption lines is often preferred. We discuss other possible candidates for wavelength references for FBG sensor interrogation units.
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