# Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors
## 4 Results and Discussion
### Impact of Fabrication Parameters on the Reflected FBG Signal  
We studied the impact of various fabrication parameters on the performance of the FBG sensors. We fabricated 15-mm-long FBGs inside the core of the optical fibers by irradiating the infrared (IR) femtosecond laser beam for 15 s where the laser power, measured immediately after the cylindrical lens, was varied from 150 mW to 650 mW. We measured the strength of the received signal power at the Bragg wavelength of 1542 nm that returned from the FBGs using the interrogator, where the measurement was carried out at room temperature, i.e., 25 \({}^{\circ}\)C. Figure 6a-f show images of the received signal power from different FBG sensors fabricated under different laser powers. We detected a substantial difference in the received signal for variation in the irradiation power of the IR femtosecond laser beam. Figure 5: Characteristics of the FBGs fabricated inside the fiber core. (**a**) Image of the FBGs; (**b**) reflected signal from the FBGs at the Bragg wavelength of 1542 nm; (**c**) transmission spectrum of the FBG-incorporated optical fiber. The received signal power was negligible when the FBGs were fabricated at a laser power of 150 mW, as depicted in Figure 6a. The power of the received signal was 10 dB when the laser power was increased to 250 mW for fabricating FBGs inside the fiber core (Figure 6b). The highest received signal power of 20.6 dB was detected in the FBG sensors fabricated by the femtosecond laser beam at a laser power of 350 mW, as represented in Figure 6c. The received signal power started decreasing again for the FBGs fabricated by a laser power higher than 350 mW, where the received signal power was 16.8 dB for 450 mW, 15.7 dB for 550 mW, and 15.4 dB for 650 mW (Figure 6d-f). The quality and precision of the FBGs are directly dependent on the laser power of the IR femtosecond laser beam. Femtosecond laser irradiation causes a refractive index change in the irradiated area, which is expected to be around 1.9 \(\times\) 10\({}^{-3}\) for our sample fiber with the laser parameters used during our experiments [34]. The amount of refractive index change is highly dependent on the laser intensity, where a large refractive index change is observed for a high laser intensity. A change in the refractive index can slightly alter the Bragg wavelength. In addition, the reflectivity of the signal depends on the change in the refractive index and the length of the gratings [36]. At low laser power, the change in the refractive index was low and FBGs might not have formed everywhere inside the fiber core, which caused the received signal power to be low. From the experimental results, we can infer that the best-quality FBGs were formed under the optimal laser power. When the laser power is higher than the optimal value, we expect an increase in the refractive index and heat-affected zone around the gratings, which in  
Figure 6: Interrogator-assisted measurement of the received signal power (at \(\lambda_{B}\) = 1542 nm) reflected back from the 15-mm-long FBGs fabricated under different laser powers (P) for an exposure time of 15 s. (**a**) P = 150 mW; (**b**) P = 250 mW; (**c**) P = 350 mW; (**d**) P = 450 mW; (**e**) P = 550 mW; (**f**) P = 650 mW; (**g**) dependence of the received signal power on the irradiation power of the laser beam. turn degrades the quality of the gratings. Hence, the power of the received signal varied with the irradiation power of the femtosecond laser beam and showed the largest peak of 20.6 dB at the optimal laser power of 350 mW. Figure 6g plots the dependence of the received signal power on the irradiation power of the femtosecond laser beam at the Bragg wavelength of 1542 nm. After we found the optimal laser power (350 mW) of the femtosecond laser beam, we varied the exposure time of the laser beam from 1 s to 20 s for the fabrication of 15-mm-long FBGs inside the fiber core.