Sensing Characteristics of Helical Long-Period Gratings Written in the Double-Clad Fiber by CO₂ Laser

Abstract

We demonstrate the fabrication of helical long-period gratings (HLPGs) with the periodic helical index modulation by twisting the double-clad fiber (DCF) while the fiber was exposed by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. Torsion, bending, surrounding refractive index, temperature and axial strain characteristics of the fabricated DCF-HLPG were investigated experimentally. The DCF-HLPG was found to have a high torsion sensitivity of -0.384 nm/(rad/m) and bending sensitivity of 18.112 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> due to the asymmetric helical index modulation across the fiber section induced by the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser heating.