High-Order Mode Suppression in Sapphire Fiber Bragg Gratings Based on Mode Field Matching Fusion Splicing for Single-Mode Demodulation Systems

Solving the multimode issue of sapphire fiber is of great significance for improving the performance and extending the application fields of sapphire fiber-based high-temperature sensors. Here, a mode field matching fusion splicing method between sapphire fiber and single-mode fiber was reported to obtain a quasi-single mode reflection spectrum from sapphire fiber Bragg grating (SFBG). By optimizing the splicing parameters, an overlapped double conical (ODC) splicing structure between sapphire fiber and single-mode fiber could be formed. The mode field matching between sapphire fiber and single-mode fiber could be well achieved by this structure, realizing the suppression of high-order modes in the reflection spectrum. A quasi-single mode reflection spectrum from SFBG was obtained based on this method with a full-width at half-maximum of 0.3 nm and a signal-to-noise ratio (SNR) of 8 dB. Further improvement has been made on this fusion splicing method by using pretapered single-mode fiber, and the SNR reached 14 dB. In addition, we combined our SFBG sensor with a commercial single-mode interrogator and achieved real-time temperature monitoring up to 1160 °C.