Highly birefringent fiber Bragg grating have been widely used for multi-parameter measurements such as torsion and strain. Here, we propose and demonstrate a novel highly birefringent cladding fiber Bragg grating (Hi-Bi CFBG) fabricated for simultaneous measurement of torsion and strain at high temperature. After optimization of fabrication parameters, the Hi-Bi CFBG with a high birefringence of 2.2 × 10-4 and a low reflection less than 1% was successfully fabricated in a conventional single-mode fiber by using a femtosecond laser direct writing technology. This Hi-Bi CFBG consists of sawtooth periodic refractive index modulation fabricated in the fiber cladding. The significant polarization splitting of the reflection peak of the Hi-Bi CFBG is 233 pm induced by strong birefringence. And then, a simultaneous measurement of torsion and strain at high temperature of 700 °C was carry out, and the results show that the fiber torsion angle and direction can be deduced by monitoring the variation of the reflection difference between the two polarizationpeaks and the fiber strain can be detected by monitoring the wavelength shift of one of the polarization-peaks. The Hi-Bi CFBG exhibited a high torsion sensitivity of up to 80.02 dB/(deg/mm) and a strain sensitivity of 1.06 pm/με at high temperature of 700 °C. As such, the proposed femtosecond-laser-inscribed Hi-Bi CFBG can be used as a mechanical sensor in many areas, especially in intelligent health monitoring at extreme environments
A method, i.e., vector projections, using two outer cores of multicore fiber without calibration and OFDR was proposed to realize three-dimensional (3D) shape sensing. Compared with traditional shape sensing method, the vector projections method based on two outer cores of multicore fiber, decreased the number of fiber cores, which eliminating the requirement for fiber cores arrangement. Experimental results show that the mean standard deviation (SD) of bending orientation is 2.84° and the mean SD of bending radius is 1.70 mm. This method offers the potentiality of dynamic shape sensing and provide reference for the researchers who focus on optical fiber secondary manufacture such as FBG arrays inscription to achieving shape sensing.
We propose and demonstrate a fiber Bragg grating (FBG) array inscribed with femtosecond laser point-by-point (PbP) method for quasi-distributed high-temperature sensing. Via optimization of grating length, femtosecond laser energy and grating pitch, a wavelength-division-multiplexed (WDM) FBG array including nine FBGs with various Bragg wavelengths ranging from 1510 nm to 1590 nm and an identical ultra-weak FBG array including 60 FBGs with a peak reflectivity of ~0.1‰ were fabricated. After annealing at 700°C for nearly 200 hours, the FBG exhibited a an extremely low Bragg wavelength shift (i.e., -2 pm/h). In addition, the modulation of WDM FBG array was studied. A generalized calibration curve was applied to the FBG array and the maximum fitting error of 27 pm and the temperature measurement accuracy of ± 1.8°C were achieved. And then, the fabricated WDM FBG array was used to realize the quasi-distributed high-temperature sensing up to 700°C in the static and dynamic environment. Such WDM FBG array could be developed for quasi-distributed high-temperature sensing in metallurgical, chemical, and aviation industries.
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