High diffraction order cladding modes of helical long-period gratings inscribed by CO₂ laser

The helical long-period gratings (HLPGs) with resonance at high diffraction order are fabricated in single-mode fiber using a CO₂ laser. A series of HLPGs with different pitches are fabricated, and the phase-matching curves of the HLPGs with first and second diffraction orders are presented based on the experimental results. The temperature, surrounding refractive index (SRI), and torsion-sensing characteristics of the HLPGs with different diffraction orders have been investigated experimentally. The maximum torsion sensitivity of resonance at the second diffraction order is about 0.228 nm/(rad/m), which is twice as high as that of the first diffraction order cladding mode. The HLPG offers great potential to perform simultaneous multiparameter measurement due to the resonance dips at different diffraction orders having quite different sensitivities to temperature, SRI, and torsion.