# A Temperature Fiber Sensor Based on Tapered Fiber Bragg Grating Fabricated by Femtosecond Laser
## 3 Device Fabrication  
As mentioned above, the optical fiber taper is a crucial component for temperature sensitivity improvement. The taper, with 25 \(\upmu\)m diameter waist and 1000 \(\upmu\)m length, was fabricated using a commercial fusion splicer (80S, Fujikura Ltd., Tokyo, Japan). In our experiment, the taper with relatively short and asymmetrical transition regions is preferred. Here we propose a reliable and effective method of fabricating an asymmetrical taper using a basic fusion splicer from two identical optical fibers. In order to ensure that the taper will have the required spectral properties, which can be further optimized for specific sensing applications, the transmission spectrum of the fiber taper was monitored during the whole tapering procedure. Firstly, the fiber coatings of two standard single-mode fibers (SMF-28e+, Corning Ltd., New York, NY, USA) were wiped out at about 10 mm and were cleaved flat. Then the fiber taper was fabricated by elongating it during an arc-discharge provided by the fusion splicer. A constant tension was applied to the fiber throughout the whole tapering procedure, which results in the asymmetric taper profile. The waist diameter and the length of the fiber taper are controlled by adjusting the discharge current, the duration time and pre-stress. As in this procedure, using a lower discharge intensity is likely to obtain a longer taper with a thinner waist, however, it is more fragile. After comparison, the discharge intensity was set to \(-50\) bit (bit is a default unit of discharge intensity of the fusion splicer) and the arc time was set to 900 ms in the tapering procedure. Finally, the fiber taper with 25 \(\upmu\)m diameter and 1000 \(\upmu\)m length was fabricated, where the FBG will later be inscribed on. The microscopic image of the fiber taper before inscription is shown in Figure 2, which was obtained from a commercial microscope (WDK2010-Z, Cewei Guangdian Ltd., Shanghai, China). Figure 1: Light propagation and interference mode of the proposed tapered FBG. Figure 2: Microscopic image of the fiber taper (10x, NA = 0.3). The schematic of the experimental setup for FBG inscription on the tapered fiber region by femtosecond laser is shown in Figure 3, which includes a femtosecond laser system (Astrella, Coherent Inc., Santa Clara, CA, USA), a broadband amplified spontaneous emission (ASE) source (MPD-3303, Mywave Instrument Co. Ltd., Shenzhen, China), and an optical spectrum analyzer (AQ6370D, Yokogawa Test & Measurement Corporation, Tokyo, Japan). The inscription process can be observed through the microscope objective lens. The motorized two-dimensional translation stage is used to keep the fiber axis coinciding with the laser focus. A circulator is used for light propagation. The broadband light source could offer light in the experiment. The OSA is used to observe the evolution of the FBG reflection spectrum during the grating inscription. Experimentally, a Ti: Sapphire amplifier laser system (Astrella, Coherent Inc., Santa Clara, CA, USA) with an operation wavelength of 800 nm, pulse duration of 35 fs, and a laser repetition rate of 1 kHz was adopted. To precisely control the refractive index modulation, the laser beam was firstly filtered through a pin-hole to form a smooth Gaussian beam with the diameter adjusted to 4 mm. Then, the laser beam was focused into the taper through a high numerical aperture oil-immersion objective (Plan-Apochromat, Zeiss, Oberkochen, Germany). The femtosecond laser fabrication platform is shown in Figure 4, where the two-dimensional translation stage XMS100, (Newport Industries Ltd., Miami, FL, USA) is controlled by PC. In order to avoid the focus distortion caused by the fiber cylindrical lens effect, the fiber is immersed in the refractive index liquid drops.