# Multipoint Thermal Sensing System for Power Semiconductor Devices Utilizing Fiber Bragg Gratings
## 4 Experimental Setup  
_FBG Array Installation and Multiplexing_  
The inverter circuit was powered by a three-phase AC supply to drive the electronic load, which increased the IGBT's temperature by exposing the FBG array sensors to different types of static thermal stresses. The FBG sensors are placed between the IGBT baseplate and the heatsink inside the inverter. Post-installation calibration of the sensor is necessary to ensure that the Bragg wavelength recorded during the air-interface calibration as shown in Figure 5 is maintained for the IGBT-FBG interface. Initially, we utilized the installation setting (FIBER1) depicted in Figure 6c.  
After the sensors were calibrated, the fiber was threaded through the transparent inverter's housing opening to the IGBTs. Figure 7a shows the table  
Figure 6: (**a**) FBG array in a single optical fiber. (**b**) Installation settings of the sensors in the inverter circuit (**c**) FIBER1; vertical sensor configuration along the rectifier/IGBT baseplate. (**d**) FIBER2; horizontal sensor configuration along the rectifier/IGBTs baseplate.  
where the FBG port is attached to the inverter's plastic casing for ease of connection and disconnection. To determine the optimal placement for thermal sensing, the performance of the two setups (FIBER1 and FIBER2) was compared. The Bragg wavelength at varying temperatures was recorded and validated using measurements from an IR thermal camera (FLIR E53) positioned 10 cm away from the baseplate of the invertor.  
As shown in Figure 7b, the red circle at the center indicates the specific location where the IR camera measurement was taken, corresponding to the position of the FBG sensor. This visual representation shows the spatial alignment between the two measurement techniques. The IR camera imaged the surface temperature of 26.2 \({}^{\circ}\)C along the baseplate, which corresponds to an average temperature value of 25.51 \({}^{\circ}\)C for FBG1, estimated based on the observed reflected wavelength on the optical spectrum analyzer for three different measurements. The same process was repeated for the FIBER2 configuration. In both configurations, it is apparent that the three FBGs have good linearity, with an R\({}^{2}\) value of greater than 0.999. Table 1 presents the estimated parameters for the second-order polynomial fit curves and the mean squared error (MSE) of the sensors at the ambient temperature of 21.9 \({}^{\circ}\)C.  
The three sensors, namely FBG1, FBG2, and FBG3, exhibit sensitivities of 9.1, 9.3, and 9.93 pm/\({}^{\circ}\)C, respectively. The maximum sensitivity shift reported is roughly 2 pm. A slight deviation in sensor sensitivity was observed compared to pre-installation calibration, likely due to strain induced on the FBG sensor when sandwiched between the copper baseplate and heatsink.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Settings**} & \multicolumn{3}{c}{**FIBER1**} & \multicolumn{3}{c}{**FIBER2**} \\ \cline{2-9}  & **Intercept** & **Ambient Temp. (\({}^{\circ}\)C)** & **MSE** & **R\({}^{2}\)** & **Intercept** & **MSE** & **R\({}^{2}\)** & **Ambient Temp. (\({}^{\circ}\)C)** \\ \hline FBG1 & 1529.7 & 21.9 & 0.00628 & 0.9997 & 1529.7 & 0.00679 & 0.9996 & 21.9 \\ FBG2 & 1539.85 & 21.9 & 0.00329 & 0.9999 & 1539.86 & 0.00321 & 0.9999 & 21.9 \\ FBG3 & 1555.04 & 21.9 & 0.0044 & 0.9999 & 1555.01 & 0.00497 & 0.9998 & 21.9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Fitting curve parameters for fiber configurations.  
Figure 7: (**a**) Tabletop experimental setup. (**b**) Thermal imaging of the IGBT.