Sensitivity Amplification of High Temperature Fiber Sensor Based on Bragg Hollow-Core Fiber and Harmonic Vernier Effect

An ultrasensitive optical fiber sensor based on the Fabry-Perot interferometer (FPI) and the optical Vernier effect formed by an anti-resonant Bragg hollow core fiber (BHCF) and a single mode fiber (SMF) cascade structure is proposed for measuring ultrahigh temperatures. This compact all-fiber structure can be easily fabricated by splicing one end of the BHCF with the SMF used as a light guide, and the other end with the SMF used as a sensor and cut to a specific length. The Vernier effect is formed by the reflection of three beams of light from the SMF-BHCF fusion interface and the two air/glass interfaces at both ends of the sensing SMF. When the length of the two optical fibers meets certain conditions, high-order harmonics Vernier effect can be formed, and the temperature sensitivity can be improved. For the harmonic order ${i}= {1}$ , the temperature sensitivity of the proposed fiber optic sensor is 0.2977 nm/°C in the temperature range of 795–830 °C, and the magnification ${M}$ factor is about 27.59. The fabrication of the optic fiber sensor is straightforward, reproducible, stable, small size, and low-cost, which exhibits potential applications in various industrial processes.