Highly birefringent cladding fiber Bragg grating for simultaneous torsion and strain sensing at high temperature

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