17 June 2009 Spectral peak tracking for enhanced fiber optic sensing
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Abstract
Within this work, we describe our newly developed interrogation scheme for fiber optic sensing applications. This measurement system will be utilized in Ariane launchers for monitoring temperature and mechanical stress distribution during flight. The acquired sensing data can be used to control propulsion unit an thrusters and thereby adapt the flight path in a way that damage on the launcher is prevented. In order to detect the peak wavelength of e.g. fiber Bragg grating (FBG) sensors, a tunable laser source based on a modulated-grating laser diode is able to scan through a more than 40nm wide spectrum in the infrared region. Several sensors with different spectral answers can be placed inside one sensor fiber and then interrogated sequentially. The magnitudes of the reflected intensities depend on the actual sensor position that is determined by the measurand (e.g. temperature). One single sensor is scanned by a variable number of spectral sampling points and the spectral answer of the sensor is then calculated by centroid algorithms. Depending an the spectral width of one sensor, the number of sensors that shall be interrogated and the required sampling points per sensor, a maximum sampling frequency of 240kHz is achievable with our hardware. Contrary to comparable systems, our interrogator is capable of switching to any available wavelength of its spectrum within a couple of nanoseconds. Therefore standard continuous sweeping through the entire spectrum is not necessary. This results in a new measurement scheme, wherein spectral gaps between consecutive sensors do not need to be scanned and can be skipped. Since most of the spectrum consists of the gaps between the sensors, overall measurement time is thereby reduced significantly. One problem arises from this measurement scheme: Due to the fact that the sensor's spectral answers vary in time, a special algorithm for tracking the spectral movement has to be implemented. The scope of this work is the description, implementation and assessment of this new peak tracking procedure. After describing the measurement setup, we will therefore explain the algorithm behind the peak tracking measurement. Afterwards the simulation process is explained and results are shown. Performance obtained by peak tracking compared to standard continuous wavelength scanning is evaluated in detail and further development steps which are necessary to obtain a fully sophisticated interrogation systems are discussed.
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Markus P. Plattner, C. N. Brand, T. Mair, S. Schupfer, T. C. Buck, A. W. Koch, "Spectral peak tracking for enhanced fiber optic sensing", Proc. SPIE 7389, Optical Measurement Systems for Industrial Inspection VI, 738923 (17 June 2009); doi: 10.1117/12.827668; https://doi.org/10.1117/12.827668
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