Optical Fiber Sensor With Double Tubes for Accurate Strain and Temperature Measurement Under High Temperature up to 1000 °C

In this work, we present a discriminative optical fiber sensor for temperature and strain measurement. The sensor comprises of two cascaded thermal regenerated Fiber Bragg gratings (RFBGs) incorporated with two fused silica capillary tubes. The RFBG ₂ is loosely enclosed in a fine fused silica tube and made solely sensitivity to temperature whereas the RFBG ₁ sensor still retains its sensitivity to both strain and temperature. These properties have made the discriminative measurement accurate and directive. The experimental results indicate that temperature response is linear in the range of 100 – 1000°C with the sensitivity of ~15.7 pm/°C. Besides, it presents good repeatability in strain detection at high temperatures (300 °C – 900 °C). The incorporation of the two fine glass tubes has enhanced the modified RFBG’s strain sensitivity to as high as ~5.46 pm/ $\mu \varepsilon $ in the measurement range of $0~\mu \varepsilon $ to 120 $\mu \varepsilon $ at 600 °C, which is about five times higher than that of common RFBG strain sensors. The sensitivity can be further enhanced by manipulating the parameters of the sensor’s structure.