# Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors
## 1 Introduction
However, most researchers have utilized infrared (IR) femtosecond lasers for fabricating FBGs [32; 33; 34; 35; 36]. A large variety of fabrication techniques have been considered for FBG fabrication, including the point-by-point method [29; 32], the holographic method [31], the plane-by-plane method [33], and the phase mask method [34; 35; 36; 30]. Because of the low processing time and high precision, the phase mask method is the most widely used technique for fabricating FBG sensors. [PERSON] et al. demonstrated a UV femtosecond laser-assisted point-by-point inscription technique for the fabrication of FBG sensors used to monitor temperatures in the range between 500 \({}^{\circ}\)C and 1050 \({}^{\circ}\)C [29]. Another research group proposed for the first time the fabrication of Bragg gratings inside a photonic crystal fiber by a UV femtosecond laser using the phase mask technique for channel dropping, tunable filtering, and channel monitoring [30]. Bragg gratings were fabricated inside a germanosilicate optical fiber core by means of two UV femtosecond laser-assisted holographic technologies [31]. [PERSON] et al. reported the fabrication of a temperature-compensated FBG-based 3D shape sensor by means of a femtosecond laser-assisted point-by-point method [32]. The proposed sensor was designed for bend sensing up to a curvature of 2.5 \(\times\) 10\({}^{-2}\) mm\({}^{-1}\), temperature sensing up to 250 \({}^{\circ}\)C, and real-time 3D fiber shape sensing. A novel technique, namely, an IR femtosecond laser-assisted plane-by-plane method, was proposed by [PERSON] and his research group where the researchers were successful in measuring the bending sensitivity of fibers ranging between 0.046 nm/m\({}^{-1}\) and 0.049 nm/m\({}^{-1}\) with an error rate down to \(\sim\)1% [33]. [PERSON] et al. reported the formation of Bragg gratings inside an all-SiO\({}_{2}\) and Ge-doped fiber core by irradiating an IR femtosecond laser on top of a phase mask [34]. The fabricated gratings exhibited low polarization dependence and stable operation at temperatures up to 950 \({}^{\circ}\)C. Using the IR femtosecond laser-aided phase mask technique, FBGs were fabricated inside non-photosensitive fibers to achieve a transmission of -25 dB at the Bragg wavelength [35]. Most of these research works were limited to the characterization of the FBG sensors in terms of reflected power, RIA, and sensitivity and the demonstration of the fabrication technique. None of the reported research works examined the impact of laser parameters on the performance of the FBG sensors. In addition, the benefit of using an IR laser over an UV laser in respect of the performance of FBG sensors remains undiscovered. Thus, it is desirable to investigate the impact of laser wavelength and other laser parameters on the performance of the FBG sensors, especially radiation-resistant sensors. In this paper, we report the fabrication of radiation-resistant FBG sensors using the IR femtosecond laser-assisted phase mask technique. Acrylate-coated fluorine (F)-doped silica fibers were considered for producing radiation-resistant FBG sensors suitable for operating under heavy \(\gamma\)-ray radiation. The Bragg resonance was detected at 1542 nm. We investigated for the first time the influence of various fabrication conditions on the performance of the FBG sensors. To optimize the laser parameters for achieving the highest reflected signal power, we fabricated the FBG sensors by varying various laser parameters including beam diameter, focal length, laser power, and laser exposure time. To optimize the performance of the FBG sensors, we varied the laser power, laser exposure time, and length of the FBG under the optimized fabrication conditions. Furthermore, we compared the performance of the IR femtosecond laser-inscribed FBG sensors with the commercially available UV femtosecond laser beam-fabricated FBG sensors.