High-Spatial-Resolution High-Temperature Sensor Based on Ultra-Short Fiber Bragg Gratings With Dual-Wavelength Differential Detection
Kuikui Guo, Jun He, He Li, Baijie Xu, Shengzhen Lu, Shen Liu, Xizhen Xu, Gaixia Xu, Yiping Wang
Abstract
Type II fiber Bragg gratings (FBGs) inscribed with femtosecond ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> s) laser possess significant potential for high temperature sensing. In this work, we propose and demonstrate a method for fabricating a parallel-integrated ultra-short type II FBG (PI-US-FBG) by using an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> s laser point-by-point technology. The PI-US-FBG, featuring by an ultra-short grating length of 80 µm, consists of six identical FBGs parallel-inscribed into a fiber core at different radial positions in the same cross section. The fabricated PI-US-FBG exhibits a broadband Gaussian-shape spectrum with a low reflectivity of ∼10%, an ultra-low out-of-band insertion loss of 0.01 dB, and a large full width at half maximum bandwidth of 9.4 nm. Moreover, this PI-US-FBG could be used as a high temperature sensor with a wide measurement range from 25 to 1000 °C, and an excellent linearity was achieved with a dual-wavelength differential detection. The temperature sensitivity could be increased from 0.00316 to 0.00945 (dB/ °C) by enlarging the wavelength spacing of the tunable laser. In addition, two cascaded PI-US-FBGs were used to precisely measure the temperature field distribution in a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser spot with a very high spatial resolution of 100 µm. Hence, the proposed PI-US-FBGs could be used for a large-scale fiber sensor network, especially in future distributed temperature measurement with a high spatial resolution.