# Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors
## 4 Results and Discussion
### Reproducibility of the Fiber Bragg Gratings  
To examine the reproducibility of the FBGs inside the fiber core, we fabricated FBGs inside ten (10) F-doped silica fibers under same irradiation conditions: a femtosecond laser power of 350 mW, an exposure time of 15 s, and a FBG length of 5 mm. Afterwards, we measured the received signal power using an interrogator at 25 \({}^{\circ}\)C. The received signal power from the nine FBG-incorporated F-doped optical fiber samples is illustrated in Figure 9.  
Figure 9: Received signal power from the FBG-inscribed F-doped optical fiber samples fabricated by IR femtosecond laser irradiated phase mask-assisted fabrication technology with a laser power of 350 mW, an exposure time of 15 s, and a FBG length of 5 mm. (**a**–**i**) Received signal power from nine optical fiber samples at the Bragg wavelength of 1542 nm; (**j**) plot of the received signal power for different optical fiber samples. A negligible difference in signal strength (\(\sim\)0.1 dB) at 1542 nm was detected from the FBG-incorporated optical fibers. These results indicate excellent reproducibility of FBG sensors using the proposed femtosecond laser irradiated phase mask-assisted fabrication technology. We examined the radiation resistance of the IR femtosecond (IR FS) laser-fabricated F-doped FBG sensors by placing them inside a Gamma chamber with Co\({}^{60}\)\(\gamma\)-rays radiated at 6000 gray/hour, where the accumulated radiation dose was \(10^{5}\) Gy. In addition, we investigated the temperature sensitivity of the F-doped FBG sensors and the power of the reflected signal at the Bragg wavelength under different temperatures. The performance of the IR FS laser-inscribed F-doped FBG sensors were compared with the IR FS and UV femtosecond laser (UV FS)-fabricated Ge-doped FBG sensors. Figure 10 demonstrates the radiation resistance, temperature sensitivity, and impact of temperature on the reflected signal power of the different kinds of F/Ge-doped FBG sensors. It is worth mentioning that Figure 10 confirms a maximum wavelength shift of -5.5 pm under extreme Gamma radiation (\(0^{5}\) Gy) in the IR FS laser-fabricated F-doped FBG sensors, which was the lowest among the F/Ge-doped FBG sensors fabricated by IR or UV femtosecond lasers. Under the same amount of radiation, the commercial IR FS  
Figure 10: Radiation resistence, temperature sensitivity, and impact of temperature on the reflected signal power of different kinds of F/Ge-doped FBG sensors fabricated by IR and UV femtosecond lasers. (**a**) Bragg wavelength shift under Gamma radiation; (**b**) impact of temperature on the power of the reflected signal at the Bragg wavelength; (**c**) temperature sensitivity. laser-inscribed Ge-doped FBG sensor showed a wavelength shift of 11.25 pm. The worst scenario was observed for the commercial UV FS laser-fabricated Ge-doped FBG sensors, where a wavelength shift of \(\sim\)50 pm was observed under 10\({}^{5}\) Gy of Gamma radiation. Due to the extreme Gamma radiation inside the Gamma chamber, the temperature of the surrounding environment of the FBG sensors increased significantly, which is a common phenomenon during atomic energy generation. Thus, it is desirable to utilize a stable FBG sensor that exhibits a high reflected signal power at the Bragg wavelength. The UV FS laser-fabricated Ge-doped FBG sensors showed a significant reduction in the reflected signal power (\(-\)3 dBm) at the Bragg wavelength of 1542 nm when the surrounding temperature was increased from 25 \({}^{\circ}\)C to 335 \({}^{\circ}\)C, as evident from Figure 10b. In contrast, the IR FS laser-fabricated FBG sensors showed excellent consistency in the reflected signal power even at high temperatures such as 335 \({}^{\circ}\)C where the F-doped FBG sensors showed the best results with a peak reflected signal power of 0.7 dBm.