This paper explains key innovations that allow monitoring of detailed spectral features of an FBG in response to impact loading. The new system demonstrates capture of FBG spectral data at rates of 100 kHz. Rapid capture of the entire reflection spectrum at such high reading rates shows important features that are missed when using systems that merely track changes in the peak location of the spectrum. The update rate of 100 kHz allows resolution of features that occur on transient time scales as short as 10 µs. This paper gives a detailed description of the unique features of the apparatus and processes used to capture the data at such a rapid rate. Furthermore, we demonstrate this interrogation scheme on a composite laminate system during impact.

We demonstrate the measurement of wavelength hopping in dynamic fiber Bragg grating (FBG) sensor measurements and its effect on the interpretation of the dynamic behavior of a composite laminate. Strain measurements are performed with FBG sensors embedded in laminates, subjected to low-velocity impacts, with data acquired using a commercial peak wavelength following controller and a high-speed full-spectral interrogator recently developed by the authors. The peak follower response is theoretically predicted from the full-spectral interrogator measurements. We demonstrate that dynamic wavelength hopping does occur, that it changes the apparent dynamic behavior of the composite and that it can be directly predicted from the dynamic spectral distortion. We also demonstrate that full-spectral data acquisition at speeds lower than those required to fully resolve the dynamic event creates apparent measurement errors due to wavelength hopping as well.

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This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows important features that are missed when using methods that merely track changes in the peak location of the spectrum. The essential feature of the new system is that it incorporates a MEMs tunable filter driven by a variable frequency openloop sinusoidal source. The paper demonstrates the new system on a laminated composite system under impact loading.

In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor subjected to a non-uniform static strain state and simultaneously exposed to vibration loading. The full spectral response of the sensor is interrogated in reflection at 100 kHz during two loading cases: with and without an added vibration load spectrum. The static tensile loading is increased between each test, in order to increase the magnitude of the non-uniform strain field applied to the FBG sensor. The spectral distortion due to non-uniform strain is observed to change once the sensor is exposed to a nontransient 150 Hz vibration spectrum. With high-speed full spectral interrogation, it is potentially possible to separate this vibration-induced spectral change from spectral distortions due to non-uniform strain. Such spectral distortion contains valuable information on the static damage state of the surrounding host material.