# Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors
## 3 Materials and Methods
### Experimental Details  
FBGs were inscribed inside the core of the F-doped optical fibers by means of a Ti:Sapphire femtosecond laser (APRI, Femto-K1) operating at the central wavelength (\(\lambda\)  
Figure 1: Basic principles of (**a**) FBG sensors for the measurement of temperature sensitivity; (**b**) the laser-assisted phase mask technique for fabricating FBG sensors.  
of 800 nm. This laser produced ultra-short laser pulses with a pulse width (\(\tau\)) of 22 fs at a pulse repetition rate of 1 kHz. The power of the laser pulses coming out from a linearly polarized femtosecond laser system was attenuated by a 360\({}^{\circ}\) rotatable quartz phase \(\lambda/2\) wave plate connected to a polarizing beam splitter transmitting a \(p\)-polarized laser beam. Figure 2a represents the image of the femtosecond laser system.  
The femtosecond laser beam was passed through a pinhole and expanded by a beam expander (Edmund; magnification: \(\times\)1.88 and \(\times\)2.25), as illustrated in Figure 2b,c. Figure 3 depicts the beam profile of the laser beam, captured by a beam profiler ([PERSON]; BC106N-VIS/M, Newton, NJ, USA), before the beam expander and after different magnifications of the beam expander. Afterwards, the laser beam was passed through one of the three cylindrical lenses having focal lengths of 12.7 mm ([PERSON]; LJ1942L2-B), 15 mm ([PERSON]; LJ1636L2-B), and 19 mm ([PERSON]; LJ1095L1-B). Because of the excellent performance in fabricating Bragg gratings in fused silica glass, we selected the beam expander with a magnification of \(\times\)2.25 and the cylindrical lens with a 19 mm focal length to fabricate FBGs in optical fibers (Figure 2c).  
Figure 2: Experimental setup. (**a**) Image of the femtosecond laser system; (**b**) schematic diagram of the fabrication process of Bragg gratings inside fused silica glass; (**c**) schematic diagram of the fabrication process of FBGs inside the core of optical fibers; (**d**) measurement technique of the transmitted and reflected signals; (**e**) measurement technique of the radiation resistance and temperature sensitivity.  
For fabricating FBGs inside the fiber core, the incident laser beam was passed through a \(+1/-1\)-order phase mask ([PERSON] photonics, Farum, Denmark) having a period of 2132 nm. This beam was centered onto the fiber core and scanned horizontally over the cross-section of the fiber core. For optimization, the distance of the phase mask from the sample surface of the fused silica glass was varied from 0 \(\upmu\)m to 1500 \(\upmu\)m. Since excellent gratings were detected for a distance of 1000 \(\upmu\)m in the fused silica glass, we selected 1000 \(\upmu\)m as the distance between the optical fiber surface and the phase mask during the fabrication of the radiation-resistant FBGs sensors. To investigate the influence of the irradiation conditions of the laser beam on the FBG signal strength, we controlled the laser power (measured after the cylindrical lens) from 150 mW to 650 mW. Furthermore, the exposure time of the optical fiber samples under the laser beam was varied between 1 s and 20 s. In contrast, we varied the grating lengths from 3 mm to 18 mm under the optimized laser parameters. We also examined the reproducibility of the proposed technique for fabricating FBG sensors. Moreover, we compared the performance of the femtosecond laser-fabricated FBG sensors with the most widely used commercial ultraviolet (UV) laser-fabricated FBG sensors.