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We demonstrated a two-dimensional vector-bending sensor by use of fiber Bragg gratings (FBGs) inscribed in a homogeneous seven-core fiber. Seven FBGs were simultaneously inscribed in each of all seven cores using a modified Talbot interferometer and a lens scanning method. The vector bending response of six outer-core FBGs was investigated at all 360° directions with a step size of 15°. The bending sensitivities of the six outer-core FBGs display six perfect '8'-shaped patterns in a polar-coordinate system. That is, they exhibit strong bending-direction dependence with a maximum sensitivity of 59.47 pm/m. The orientation and amplitude of the vector bending can be reconstructed using measured Bragg wavelength shifts of any two off-diagonal outer-core FBGs. So, the six outer-core FBGs have 12 combinations for bend reconstruction, which can be averaged across multiple reconstructions to develop an accurate two-dimensional vector bending sensor. The average relative error was lower than 4.5% for reconstructed amplitude and less than 2.8% for reconstructed orientation angle θ. Moreover, the seven-core FBGs offer several advantages such as a compact structure, fabrication flexibility, and the temperature compensating ability of central-core FBG.

Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments.

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Ultra-weak fiber Bragg grating (UWFBG) arrays are key elements for constructing large-scale quasi-distributed sensing networks for structural health monitoring. Conventional methods for creating UWFBG arrays are based on in-line UV exposure during fiber drawing. However, the UV-induced UWFBG arrays cannot withstand a high temperature above 450 °C. Here, we report for the first time, to the best of our knowledge, a new method for fabricating high-temperature-resistant UWFBG arrays by using a femtosecond laser point-by-point (PbP) technology. UWFBGs with a low peak reflectivity of ∼ - 45 dB (corresponding to ∼ 0.0032%) were successfully fabricated in a conventional single-mode fiber (SMF) by femtosecond laser PbP inscription through fiber coating. Moreover, the influences of grating length, laser pulse energy, and grating order on the UWFBGs were studied, and a grating length of 1 mm, a pulse energy of 29.2 nJ, and a grating order of 120 were used for fabricating the UWFBGs. And then, a long-term high-temperature annealing was carried out, and the results show that the UWFBGs can withstand a high temperature of 1000 °C and have an excellent thermal repeatability with a sensitivity of 18.2 pm/°C at 1000 °C. A UWFBG array consisting of 200 identical UWFBGs was successfully fabricated along a 2 m-long conventional SMF with an interval of 10 mm, and interrogated with an optical frequency domain reflectometer (OFDR). Distributed high-temperature sensing up to 1000 °C was demonstrated by using the fabricated UWFBG array and OFDR demodulation. As such, the proposed femtosecond laser-inscribed UWFBG array is promising for distributed high-temperature sensing in hash environments, such as aerospace vehicles, nuclear plants, and smelting furnaces.