Helical Long-Period Grating Through a Commercial CO₂ Laser Fiber Processing Machine for Torsion Sensor Applications

In this work, we report the production of helical long-period gratings (HLPGs) sensors in standard single-mode fibers (SMFs), using a commercial CO2 laser fiber processing machine. This allows to postprocess the optical fiber easily, allowing the production of HLPGs in a repeatable and controllable way. The helical refractive index profile was achieved through the rotational and linear motion of the SMF related to the CO2 laser beam. The HLPG was fabricated with periods ranging between 600 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$850~\mu $ </tex-math></inline-formula>m. Results showed the appearance of two sharp dip resonances at the C-band, having a maximum dip to noise level of ≈17 dB. The polarization-dependent loss was also tested, showing values near 2.5 dB. The HLPGs were characterized for torsion in the clockwise (CW) and counterclockwise (CCW) directions, showing the capability to discriminate each direction and presenting an average sensitivity of ≈47.8 nm/(rad/mm). Temperature tests were also performed to check the cross-sensitivity, showing a sensitivity of 38.2 pm/°C, resulting in a negligible error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8\times 10^{-{4}}$ </tex-math></inline-formula> (rad/mm)/°C.