# 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 measured the signal power at 25 \({}^{\circ}\)C. The interrogator outputs of the received signal power reflected from the FBG-inscribed optical fibers, fabricated under different exposure times, are illustrated in Figure 7a-f. The FBGs fabricated using the laser power of 350 mW under a laser exposure of 1 s exhibited a received signal power of 2.9 dB at 1542 nm, as shown in Figure 7a. When the exposure time was increased to 3 s, the received signal power was increased to 7.4 dB, as illustrated in Figure 7b. With the increase in the exposure time of the femtosecond laser beam, the power of the received signal increased until the exposure time reached 15 s (Figure 7c-e). The highest signal power of 20.6 dB was detected for an exposure time of 15 s. However, when the exposure time was increased to 18 s, the signal power reduced down to 17.4 dB, as represented in Figure 7f. Thus, we can consider 15 s to be the optimal value of exposure time to fabricate a good-quality FBG sensor. Under a short exposure time, very few laser pulses overlapped in each irradiated spot. Thus, the change in the refractive index along with the width and depth of the grooves inside the gratings were  
Figure 7: Interrogator-assisted measurement of the received signal power (at \(\lambda_{B}\) = 1542 nm) reflected back from the 15-mm-long FBGs fabricated under different exposure times (\(t\)) of the laser beam using a laser power of 350 mW. (**a**) \(t\) = 1 s; (**b**) \(t\) = 3 s; (**c**) \(t\) = 6 s; (**d**) \(t\) = 12 s; (**e**) \(t\) = 15 s; (**f**) \(t\) = 20 s; (**g**) dependence of the received signal power on the exposure time of the laser beam. small. Consequently, the reflected signal power was low for short exposure times. Perfect FBGs were fabricated when the exposure time was 15 s. With the increase in the exposure time, the number of laser pulses irradiated in each spot increased, which in turn caused an increase in the refractive index, width, and depth of the gratings. Furthermore, the heat-affected zone around the gratings is expected to increase. As a consequence, the power of the reflected signal was decreased from the highest value for a laser exposure time of 18 s. We fabricated FBGs of different lengths inside the fiber core by means of femtosecond laser beam irradiation with a laser power of 350 mW and an exposure time of 15 s. The interrogator's output was measured at 25 \({}^{\circ}\)C. Figure 8a illustrates that a signal peak of 15.1 dB was detected at 1542 nm when the length of the FBG was only 3 mm. For a FBG with a length of 6 mm, we measured a signal peak of 17.4 dB, as shown in Figure 8b. The received signal power was 18 dB for a FBG with a length of 9 mm, as depicted in Figure 8c. A further increase in the received signal power was achieved by increasing the length of the FBG to 15 mm (Figure 8d,e), where the highest signal power of 20.6 dB was detected from the 15-mm-long FBG. However, when the FBG's length was 18 mm, the signal power decreased down to 16.2 dB, as shown in Figure 8f. The decrease in the signal power with a FBG length greater than 15 mm might have been due to the high losses introduced by the fiber core and FBGs. As a result, we can consider 15 mm to be the optimal length of the FBG for a good-quality FBG sensor. Figure 8g demonstrates the dependence of the received signal power on the length of the FBG. After accumulating all  
Figure 8: Interrogator-assisted measurement of the received signal power (at \(\lambda_{B}\) = 1542 nm) reflected back from FBGs of different lengths (L) fabricated by irradiating a femtosecond laser beam with a power of 350 mW for 15 s. (**a**) L = 3 mm; (**b**) L = 6 mm; (**c**) L = 9 mm; (**d**) L = 12 mm; (**e**) L = 15 mm; (**f**) L = 18 mm; (**g**) dependence of the received signal power on the length of the FBGs. the experimental results, we obtained the following optimal fabrication parameters: a laser power of 350 mW, an exposure time of 15 s, and a FBG length of 15 mm.