Manufacturing a Long-Period Grating with Periodic Thermal Diffusion Technology on High-NA Fiber and Its Application as a High-Temperature Sensor

Abstract

We demonstrated a kind of long-period fiber grating (LPFG), which is manufactured with a thermal diffusion treatment. The LPFG was inscribed on an ultrahigh-numerical-aperture (UHNA) fiber, highly doped with Ge and P, which was able to easily diffuse at high temperatures within a few seconds. We analyzed how the elements diffused at a high temperature over 1300 °C in the UHNA fiber. Then we developed a periodically heated technology with a CO2 laser, which was able to cause the diffusion of the elements to constitute the modulations of an LPFG. With this technology, there is little damage to the outer structure of the fiber, which is different from the traditional LPFG, as it is periodically tapered. Since the LPFG itself was manufactured under high temperature, it can withstand higher temperatures than traditional LPFGs. Furthermore, the LPFG presents a higher sensitivity to high temperature due to the large amount of Ge doping, which is approximately 100 pm/°C. In addition, the LPFG shows insensitivity to the changing of the environment’s refractive index and strain.

Switchable Ytterbium Fiber Laser Based on a Symmetrical Long-Period Fiber Grating

“…Furthermore, due to the thermal expansion of the core, the mode field diameter (MFD) is also locally expanded at each taper location [62]. The germanium in the core is diffused toward the cladding [63] increasing the MFD as is shown in [64], in which were observed two bright points of irradiation on an axial cut-off in the middle of the thermally-expanded core, similar to LP11 mode. Then, due to the micro-tapering of the cladding, the evanescent portion of the electromagnetic field is spreading out to interact with the environment close to the surface of the fiber, enhancing the sensitivity.…”

High-Resolution Temperature Sensor Based on Single-Frequency Ring Fiber Laser via Optical Heterodyne Spectroscopy Technology

“…Optical fiber sensors have been widely investigated due to their attractive properties of anti-electromagnetic interference, high sensitivity and the facilitation of long-distance sensing. For the past few decades, various fiber structures, such as fiber Bragg grating (FBG), long period grating (LPG), Fabry-Perot interferometer (FPI), photonic crystal fiber (PCF) and Sagnac interferometer (SI), have been developed for sensing parameters, including temperature, strain, refractive index (RI) and so on [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ].…”