# A Temperature Fiber Sensor Based on Tapered Fiber Bragg Grating Fabricated by Femtosecond Laser
## 1 Introduction  
Over the decades, optical fiber sensors have attracted growing interest for various applications, since they offer advantages including their small size, light weight, electromagnetic immunity, high stability, and the possibility of remote interrogation in sensor arrays [1]. These advantages are suitable for safe and efficient structural health monitoring and control. Examples where temperatures need to be accurately measured include biomedical cell culture and transformation, chemical and biological microfluidic systems, down-hole drillings for oil and gas exploration, and aircraft engines [2; 3; 4]. Therefore, a large variety of fiber optic temperature sensors have been envisioned and demonstrated for accurate temperature measurement. The sensitive structures include the fiber Bragg grating (FBG) [5; 6; 7], long period fiber grating (LPFG) [8], Fabry-Perot cavity [9; 10; 11], Mach-Zehnder interferometer [12; 13; 14], microfiber-based structures [3; 15], etc. Among these fiber sensing structures, FBG has narrow band filtering characteristics at certain wavelengths, which guarantees the desired precision for fiber sensing, fiber laser, and fiber communication. FBG is a periodic refractive index modulation along the optical fiber, which can be fabricated by exposing the fiber to an intense periodic optical laser. Generally, FBGs are created in photo-sensitive glass using ultraviolet laser, in which case, hydrogen loading is preferred for enhanced photosensitivity, since pure silica is non-photosensitive [16; 17]. This inconvenience is challenging to realizing a complex grating structure as well as to expand the practical functions. However, femtosecond laser can remain efficient in fabricating non-photosensitive optical materials, which exhibits obvious advantages in FBG fabrication [18; 19; 20; 21; 22]. The femtosecond laser interacts with dielectricmaterial through a high nonlinear photoionization mechanism, leading to the modification which shares no relevance to the photosensitivity of the material [23; 24; 25; 26; 27; 28]. As for sensitivity-intensified fiber structures, a tapered fiber region exposes more contacting areas to the ambient and enhances the evanescent field, thus improving the sensitivity to external parameters more than conventional non-tapering optical fiber, which is very important for achieving heat monitoring and control more precisely and efficiently [15; 29; 30; 31; 32]. Relevant literatures have been reported in the past few years. Particularly, in 2013, [PERSON] et al. [33] reported a temperature-compensated refractive-index sensor using a single FBG in an abrupt fiber taper. The taper diameter is 12 \(\upmu\)m and the taper length is 10 mm. The FBG were inscribed in the tapered region using a 193 nm ArF excimer laser and a phase mask with period of 1089.2 nm. With such a diameter-length ratio in the tapered region, the sensor had three different peaks for a single FBG, and the temperature characterization was used for measurement compensation. In 2017, [PERSON] et al. [34] reported a label-free detection of cancer biomarkers using an in-line taper fiber-optic interferometer and a FBG. The FBG was inscribed in a standard fiber using a 193 nm ArF excimer laser and a phase mask with a period of 1070.49 nm, and the taper was later drawn near the FBG. The temperature sensitivities for taper and FBG were 10.7 pm/\({}^{\circ}\)C and 9.5 pm/\({}^{\circ}\)C, respectively. In 2018, [PERSON] et al. [35] reported a temperature-independent refractive index measurement using a FBG on an 18 \(\upmu\)m diameter abrupt tapered tip. The FBG was inscribed in the tapered region using an 267 nm femtosecond laser and a phase mask with a period of 1075 nm. The taper was then broken at one of the transition regions in order to create the tip.