# A Temperature Fiber Sensor Based on Tapered Fiber Bragg Grating Fabricated by Femtosecond Laser
## 4 Results and Discussion  
In the following analysis, peak A (1540.25 nm) and peak B (1583.62 nm) are chosen for comparison. The temperature experiment is performed in the class-1000 cleanroom and the temperature-sensing system is shown in Figure 7. Inside the furnace chamber, there is a platform. When the sensor is under the temperature test, it is placed on the upper surface of the platform without fixation. There are two drilling holes on the front and back furnace doors, which could support the optical fiber yet without strain applied on the proposed sensor. The reflection spectra are monitored during the temperature experiment. Both the heating and cooling processes are tested. The temperature range provided by the heating chamber is from 30 \({}^{\circ}\)C to 350 \({}^{\circ}\)C, and the reflection spectra is recorded by a step of 10 \({}^{\circ}\)C. Each reflection spectrum is measured after 20 min staying at a specific temperature to guarantee thermal equilibrium. Figure 8 shows the wavelength shifts of the tapered FBG and the reference FBG (peak A and B) with the temperature variation. For clarity, the spectra from 1520 nm to 1610 nm at 30 \({}^{\circ}\)C and 350 \({}^{\circ}\)C are demonstrated in Figure 8a. The detailed reflection spectra of peak A and B are demonstrated in Figure 8b-e. For clarity, the spectra are exhibited every 20 \({}^{\circ}\)C. The temperature sensitivity analyses of both FBGs are shown in Figure 8f. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Peak Index** & **Grating Pitch (nm)** & **Grating Length (\(\mu\)m)** & **Wavelength (nm)** & **Output Power (dBm)** & **\(-3\) dB Fractional Bandwidth (nm)** \\ \hline A & 535 & 1000 & 1540.25 & \(-32.54\) & 2.75 \\ B & 550 & 1000 & 1583.62 & \(-38.25\) & 1.12 \\ C & 550 & 1000 & 1583.62 & \(-38.28\) & 1.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: FBG information of the reflection spectra evolution. Figure 7: Temperature-sensing system. It can be seen in Figure 8 that the characterized wavelengths of the tapered FBG and the reference FBG shift when the temperature increases, yet with different wavelength variations. When temperature increases from 30 \({}^{\circ}\)C to 350 \({}^{\circ}\)C, red shifts happen for both FBGs. When the temperature decreases  
Figure 8: Reflection spectra and temperature-sensitivity analysis of the proposed sensor. (**a**) Reflection spectra at 30 \({}^{\circ}\)C and 350 \({}^{\circ}\)C for comparison; (**b**) Detailed reflection spectra of peak A during the heating process from 30 \({}^{\circ}\)C to 350 \({}^{\circ}\)C; (**c**) Detailed reflection spectra of peak A during the cooling process from 350\({}^{\circ}\)C to 30 \({}^{\circ}\)C; (**d**) Detailed reflection spectra of peak B during the heating process from 30 \({}^{\circ}\)C to 350 \({}^{\circ}\)C; (**e**) Detailed reflection spectra of peak B during the cooling process from 350 \({}^{\circ}\)C to 30 \({}^{\circ}\)C; (**f**) Temperature characterization of peak A and B.  
from 350 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, blue shifts happen for both FBGs. From Figure 8a, the wavelength variation of the tapered FBG in the 30-350 \({}^{\circ}\)C range is 3.91 nm, while that of the reference FBG is 3.54 nm. This is because the tapered FBG exposes more to the ambient, resulting in a more sensitive wavelength variation. According to Figure 8b-e, both FBGs shift linearly along with the temperature change. For further discussion, the temperature-wavelength relationships of both FBGs are analyzed in Figure 8f. For each FBG, the characterized wavelengths at specific temperatures in the heating and cooling processes are shown in one subfigure. The experimental results are marked in blue or red circles, and the linear fitting evaluation is used to obtain the sensitivities, where the slope represents the temperature sensitivity and the R-square represents the fitting evaluation. As shown in Figure 8f, both FBGs demonstrate good linearity along with the temperature variation, and R-squares are quite satisfactory.