We present two implementations of fast, discrete incoherent optical frequency-domain reflectometers (I-OFDR) for the interrogation of equally-spaced fiber Bragg grating (FBG) arrays, based on the determination of the array's radiofrequency (RF) response at a sparse number of frequencies. FBG reflectivities are determined by use of the inverse discrete Fourier transform (IDFT) of the sparse RF response, in a dynamic range limited by crosstalk induced by FBG positioning errors. The first implementation employs the complete, vector RF response at a number of frequencies equal to the number N of FBGs in the array. In the second, the introduction of a reference reflector allows for an interrogation using the power (phaseless) RF response in 4N − 1 frequencies. Demodulation based on IDFT leads to total interrogation times determined by the network analyzer scan time, which can be as low as 10 µs per FBG. Depending on the interrogation technique, electrical bandwidth requirements are 1-2 GHz in our array with 10cm separation. We implemented both techniques in a N = 10 array, inducing decays in reflectivity by 10 dB in one or several FBGs. Unambiguous detection of FBG decays was obtained in both interrogation methods. Additional tests performed on the measured reflectivities also show that measurement linearity is preserved in the 10-dB decay range. As discrete I-OFDR systems, the proposed techniques show the possibility to reach compromises between interrogation time and dynamic range or accuracy in reflectivity measurements, using the number of interrogation frequencies and the sensor topology.
The rise of optical fiber communications in the last three decades has been accompanied with the development of a large number of sensors based on this technology, overcoming in many aspects the characteris-tics of the current electrical sensing systems. Optical fiber sensors pro-vide a wealth of applications in a variety of fields, ranging from struc-tural health monitoring to detection of chemical species. In this article, after enumerating the principal benefits of optical fiber sensors, it is presented a review about the two key aspects of opti-cal fiber sensing system, namely the sensing devices and their interroga-tion methods, respectively. In both parts, the main devices and methods have been included. In the devices section, examples of single-point, multi-point and dis-tributed sensing elements are given. In the same way, a number of inter-rogation methods are briefly described, including standard spectral analysis, interferometry, and time-domain and frequency-domain meth-ods for both point and distributed optical fiber sensing systems.
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