# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 3 Junction Temperature Optical Sensing Techniques
### FBG Sensing Technique  
The thermography method gives average temperature distribution over an area and thus is incapable of detecting maximum temperature for a targeted point [111]. FBG is a very recent optical technique explored in the literature for measuring the junction temperature of power switching semiconductor devices. The working principle of FBG is based on the wavelength shift that occurs because of the variation in thermal profiles over the grating portion of the fiber sensor, and the wavelength properties can be characterized to represent the variation in junction temperature in SiC MOSFET and Si IGBTs. In a typical FBG, the manufacturer engraves the Bragg gratings into a single-mode fiber. When the light signal from a broadband source is fed to the fiber, the light signal known as the Bragg wavelength is reflected, depending on the refractive index and the grating period [112] and is given as:  
\[\lambda_{B}=2\;\;n_{eff}\;\Lambda \tag{9}\]  
where \(\Lambda\) is the grating pitch and \(n_{eff}\) is the effective refractive index of the single mode fiber. Whenever the external temperature around the grating portion of the fiber is varied, the thermo-optic effect alters the core refractive index [111], thus affecting the \(n_{eff}\), which shifts the central wavelength of the reflected signal accordingly. The relationship between the temperature variation and the wavelength shift of the FBG sensor is given by:  
\[\frac{\Delta\lambda}{\lambda}=\left[(+\;a_{f}\;+\;\xi)\Delta T\right]+(1-\;P_ {e}\;)\Delta\varepsilon \tag{10}\]  
where \(\Delta\lambda\) is the wavelength shift, \(\lambda\) is the initial center wavelength of the FBG, \(a_{f}\) is the thermal expansion coefficient of the fiber, \(\xi\) is the thermo-optic coefficient, and \(P_{e}\) is the strain-optic coefficient. Hence, Equation (10) considers wavelength shift due to the temperature and strain; however, in the case of junction temperature measurement, only the effect of temperature is required. The external strain on the FBG can be eliminated by using a tube or rigid housing, which additionally protects the sensor from mechanical damage [47]. In this case, Equation (10), can then be rewritten as:  
\[\frac{\Delta\lambda_{b}}{\lambda_{b}}\;=\;\;(a_{f}\;+\;\xi)\Delta T \tag{11}\]  
A typical measurement flow chart of the FBG sensing technique is shown in Figure 9. The setup involves bonding the FBG sensor to the semiconductor chips, usually inside modules, with thermal oil or glue to improve the thermal contact between the chip's surface and the fiber sensor [29; 113]. The fiber is then illuminated with a broadband light source (BBS) via an optical circulator, and the sampling rate is pre-selected depending on the interrogator type. The reflecting light signal wavelength shift can then be routed to the interrogator and examined for various pulse widths applied to the Si IGBT gate. Based on the predetermined sensitivity, the junction temperature due to conduction losses for long-duration pulses can be computed. However, the transient losses during switching and the peak power generated during ON-OFF switching losses may or may not be detected depending on the interrogator acquisition rate. Table 6 depicts the recent advancements in FBG sensors in the junction temperature sensing of power switching semiconductor devices. FBG sensors can be enclosed in a tube to improve the shear strength of the sensor; it should be calibrated to minimize measurement errors that may be introduced due to environmental factors. A temperature-wavelength relationship can then be obtained for the FBG sensor with the least square method (better fit with a linear equation of about 99.9% precision), and an accuracy of up to \(\pm\) 1 \({}^{\circ}\)C was reported at a temperature below 40 \({}^{\circ}\)C in [111]. The effect of airgap and the interrogator sampling rate was studied in experimental work by [PERSON] et al. [112].