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A commercial FBG with a center wavelength of 1553.865 nm, 0.24 nm bandwidth, and >97% reflectivity was used for the purpose. It was used to investigate the thermal response by determining its sensitivity and accuracy. The indoor temperature (laboratory) system was increased by applying the heat of liquids. For the FBG sensor system in outdoor environment, sunlight was used as the natural temperature source. Figure 4.1 shows the general schematic diagram of the FBG temperature sensor system. The simplest system consisted of a light source (e.g., tunable laser source), a spectrum analyzer (e.g., optical spectrum analyzer), and an FBG. For indoor measurement, different types of liquid were used, which were distilled water and 0.1 M NaCl solution. Different focusing elements were used for the FBG sensor measurements in outdoor area, which were convex and hand lens with focal lengths of 20 and 15 cm, respectively.