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The determination of the peak reflection of Bragg wavelengths and their respective shifts is the basis of FBG sensing because the detected reflected wavelengths can be converted to parameters such as temperature, strain, and other indirect parameters. The main challenge when it comes to FBG sensing is how to demodulate its wavelength changes. FBG sensors have been used in techniques to interrogate these sensors and to detect Bragg wavelength shifts as a function of the parameter being measured. The interrogation techniques used for FBGs can be divided into two basic methods: detecting the reflection spectra and then recognizing each peak representing each sensor, or converting wavelength variations into optical power intensities and then further converting optical power into voltage with a photodetector. One can detect the Bragg wavelength displacement either in the frequency domain (by inspecting the spectra) or in the time domain (by conversion into power amplitude), a process known as intensity-based demodulation. The easiest way to apply the former method uses an OSA, which directly measures the reflection spectrum of the FBG. The latter method applies tunable filters or tunable lasers to “read” the input frequency spectra and convert them into a variable voltage output. This chapter presents the most used techniques, especially those that can be easily implemented in the laboratory or that are simple enough for use in the field. Section 6.2 addresses the conventional demodulation methods, those used by most FBG laboratories and commercial ones. The following sections discuss general techniques, such as intensity based and frequency domain methods.
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