Discrimination of Temperature and Strain by Characterizing Two Femtosecond Laser-Written Coincident Sapphire Fiber Bragg Gratings for Harsh Environment Applications

In this study, two co-incident sapphire fiber Bragg gratings (SFBGs) were successfully inscribed utilizing a femtosecond (fs) laser to achieve a high fringe contrast interferogram. These two SFBGs employ a unique configuration, one parallel to the center axis, called p-SFBG, and the other forming an angle from the center-axis, called a-SFBG, allowing for simultaneous strain and temperature measurements with low crosstalk. As a proof of concept, p-SFBG and a-SFBG using line-by-line method are characterized, which are shorter in length (i.e., 1.5 mm), producing reflectivity of ~3 dB. This effort demonstrates the use of two coincident SFBGs forming an angle of 2.29° between them within an extremely highly multimode sapphire fiber. The p-SFBG exhibits superior strain sensitivity (1.8 pm/ $\mu \varepsilon $ ) in contrast to the a-SFBG (0.4 pm/ $\mu \varepsilon $ ). Their efficacy was evaluated under applied strains reaching up to $1000~\mu \varepsilon $ . Both SFBGs underwent thorough characterization for long-term stability at elevated temperatures, enduring exposure at $1600~^{\circ }$ C for 24 h. The novelty of the sensor lies in the fact that a-SFBG exhibits extremely low sensitivity compared to p-SFBG, which plays an important role in eliminating the crosstalk between co-incident SFBGs. This configuration addresses challenges related to temperature and strain crosstalk, facilitating simultaneous measurements via sensitivity matrix in harsh environmental conditions. This achievement represents a significant advancement in the field of optical sensing, paving new avenues for simultaneous temperature and strain measurement, particularly for applications in harsh environments.