A Vector Curvature Sensor Based on Periodically Etching the V-Grooves on Assembled Long-Period Fiber Grating

In this paper, vector curvature measurement is realized by periodically etching V-grooves along one side of the optical fiber on the multi-mode fibers (MMFs) part of the SMF-MMF-SMF assembled long-period fiber grating (SMS-VLPFG). Because different bending directions will cause different degrees of deformation of the V-grooves, and then restrain the beam differently, the curvature can be vector demodulated by detecting the light loss. Based on intensity demodulation, the curvature measurement sensitivities at the 0°, 90°, and 180° positions are 11.9 dB/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, 9.7 dB/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, and -6.4 dB/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, respectively. The curvature sensitivity based on wavelength demodulation at the 0°, 90°, and 180° positions are -18.7 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, -12.0 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, and 7.3 nm/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, respectively. The measurement range is in the 0-1.1667 m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>. The asymmetric configuration of the sensor enables vector-sensing capability. The sensor structure is relatively compact, and the total length of the structure is only 3 mm. In the wide spectral range, The resonance effect is great and is conducive to clear demodulation. The sensor has low temperature crosstalk and temperature sensitivity is only -0.02 dB/°C (32.6 nm/°C) in the range of 20-110°C. This compact sensor based on intensity and wavelength modulation offers a novel approach for vector curvature sensing.