# Femtosecond-Laser-Assisted Fabrication of Radiation-Resistant Fiber Bragg Grating Sensors
## 1 Introduction  
A fiber Bragg grating is a kind of distributed Bragg reflector, typically a few millimeters in length, which is fabricated inside the fiber core to reflect light at the Bragg wavelength and transmit all other wavelengths. Since their discovery by [PERSON] et al., fiber Bragg gratings (FBGs) have been considered a promising tool in diverse fields of science and technology [1]. Because of their high-quality optical signal, stability, low cost, and ease of fabrication, FBGs have become an indispensable component in optical sensors, optical networks, wavelength division multiplexing (WDM) systems, photonic signal processing, and industrial applications [2; 3; 4; 5; 6]. Depending on the application area, six different structures of FBGs have been reported so far: uniform, Gaussian apodized, raised-cosine apodized, chirped, discrete phase shift, and superstructure [7]. Among the various application areas, the most remarkable is sensing. During the last four decades, a broad range of FBG sensors have been proposed by researchers: strain sensors [8; 9; 10]; chemical sensors [11; 12]; bending sensors [13; 14]; medical sensors [15; 16]; and temperature sensors [17; 18; 19]. The application of FBG sensors in the medical sector shows much promise. [PERSON] et al. proposed an ultra-fast polymer optical fiber Bragg grating-based sensor for monitoring the human heartbeat and respiratory functions [15]. [PERSON] et al. proposed a polymer optical fiber-based FBG sensor for monitoring human physiological and body functions such as respiration rate, heart rate, foot pressure, and joint movement [16]. In addition to their other sensing applications, FBG sensors have remarkable applications in space science and the power industry, especially the atomic energy industry. However, the quality of most of these FBG sensors degrades with time due to exposure tohigh amounts of radiation [20; 21]. [PERSON] reported the temperature and gamma-ray (\(\gamma\)-ray) dependency of optical attenuation in FBG-inscribed silica glass fibers [20]. The attenuation caused by \(\gamma\)-ray irradiation and the temperature variation were measured as 0.185 dB/m and 27.5 \(\times\) 10\({}^{-4}\) dB/\({}^{\circ}\)C, respectively. [PERSON] and his research group detected a 0.25 dB reduction in optical power from FBG temperature sensors due to the long-term radiation effect [21]. As a result, there is huge demand for radiation-resistant sensors. To overcome the impact of radiation on FBG sensors present in the atomic energy industry and space science, researchers have proposed different techniques for fabricating radiation-resistant FBG sensors [22; 23]. [PERSON] et al. proposed a radiation-resistant FBG sensor system where the FBG sensors written into a photosensitive fiber were connected to a specially designed radiation-resistant optical fiber and shielded by stainless steel tubing. Still, the FBG sensors showed a radiation-induced absorption (RIA) of \(\sim\)2.2 dB/km [22]. [PERSON] et al. proposed radiation-resistant FBG sensors in random air-line fibers for temperature sensing in nuclear reactor cores. Although the authors claimed that they had developed radiation-resistant FBG sensors, a reduction in the peak signal strength of \(\sim\)5 dB was detected in the FBG sensors due to long-term exposure to heavy radiation. In addition, a wavelength shift of \(\sim\)5.5 nm was observed in FBG sensors under long-term radiation [23]. As a result, the development of a stable radiation-resistant FBG sensor remains a challenge to researchers. Femtosecond lasers have been considered a versatile tool for fabricating micro/nano-structures on the surface of and inside transparent materials [24; 25; 26; 27; 28]. Because of their focusing capability inside the fiber core, researchers consider femtosecond lasers to be the most promising technology for fabricating FBG sensors. Initially, femtosecond laser sources operating in the ultraviolet (UV) wavelength range were considered for fabricating FBG sensors [29; 30; 31].