Abstract:A microwave photonic technique is presented to interrogate a 5 m-long fiber, containing 9 mm-long weak FBGs having 10 mm grating separation, used as a distributed sensor. Hot-spots as small as 20 cm have been measured with an accuracy of 0.5 cm
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.
Optical sensors are photonic devices sensitive to different magnitudes that are used precisely to measure, in an absolute or a relative way, these magnitudes. These optical sensors are nowadays used to measure temperature, pressure, strain, humidity or the presence of a particular gas. In the past few decades a multitude of photonic sensors and different interrogation techniques have been developed, which had a great impact in dozens of different fields. One of the best examples is civil architecture, in which photonic sensors play a fundamental role in order to monitor the condition of the structures.
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