Ultra-sensitive quasi-distributed temperature sensor based on an apodized fiber Bragg grating

This work targets a remarkable quasi-distributed temperature sensor based on an apodized fiber Bragg grating. To achieve this, the mathematical formula for a proposed apodization function is carried out and tested. Then, an optimization parametric process required to achieve the remarkable accuracy that is based on coupled mode theory (CMT) is done. A detailed investigation for the side lobe analysis, which is a primary judgment factor, especially in quasi-distributed configuration, is investigated. A comparison between elite selection of apodization profiles (extracted from related literatures) and the proposed modified-Nuttal profile is carried out covering reflectivity peak, full width half maximum (FWHM), and side lobe analysis. The optimization process concludes that the proposed modified-Nuttal profile with a length (L) of 15 mm and refractive index modulation amplitude (Δn) of 1.4×10^-4 is the optimum choice for single-stage and quasi-distributed temperature sensor networks. At previous values, the proposed profile achieves an acceptable reflectivity peak of 10^-0.426  dB, acceptable FWHM of 0.0808 nm, lowest side lobe maximum (SL max) of 7.037×10^-12  dB, lowest side lobe average (SL avg) of 3.883×10^-12  dB, and lowest side lobe suppression ratio (SLSR) of 1.875×10^-11  dB. These optimized characteristics lead to an accurate single-stage sensor with a temperature sensitivity of 0.0136 nm/°C. For the quasi-distributed scenario, a noteworthy total isolation of 91 dB is achieved without temperature, and an isolation of 4.83 dB is achieved while applying temperature of 110°C for a five-stage temperature-sensing network. Further investigation is made proving that consistency in choosing the apodization profile in the quasi-distributed network is mandatory. If the consistency condition is violated, the proposed profile still survives with a casualty of side lobe level rise of -73.2070  dB when adding uniform apodization and -46.4823  dB when adding Gaussian apodization to the five-stage modified-Nuttall temperature-sensing network.