Simultaneous Measurement of Direction-Recognizable Curvature and Temperature Based on Symmetry Exposing Tilt FBG

A symmetry tilt fiber Bragg grating (STFBG) sensor is demonstrated for simultaneous measurement of directional recognizable curvature and temperature utilizing an upgraded tilt fiber Bragg grating (TFBG) writing system. Two TFBG structures are written on the opposite side of a single-mode fiber (SMF) core. More light is restricted in the fiber core due to the secondary reflection of the grating region. When the grating region is curved, the intensity of cladding modes can be enhanced drastically. Thus, compared with the traditional TFBG, the intensity variation of STFBG coupled modes is strengthened, which results in high sensitivity. A high curvature sensitivity of up to 6.172 dB/m−1 is achieved with intensity demodulation. Benefiting from the symmetry characteristics, the STFBG can implement a direction-recognizable curvature sensing in the measurement region of 0°–90°. Meanwhile, based on wavelength demodulation, the sensor can also realize temperature sensing with a sensitivity of 0.010 nm/°C, which does not influence curvature measurement.