The study of second-order coupling of cladding modes of helical long-period gratings inscribed by commercial welding machine

A presentation based on the helical long-period grating (H-LPFG) affected by the change of temperature and twist has been demonstrated. There are two resonant peaks near 1451 nm (resonant peak 1) and 1519 nm (resonant peak 2) when the pitch length is about 757 μm. The resonant peak 1 is caused by second-order diffraction coupling between the fundamental mode and the LP 110 cladding mode, and the resonant peak 2 is caused by first-order diffraction coupling between the fundamental mode and the LP 15 cladding mode. Their temperature sensitivities are 72 pm/℃ and 45 pm/℃, respectively. The resonant peak 1 twist sensitivities are − 138 nm·mm/rad with co-direction twist and 133 nm·mm/rad with contrary-direction twist, but the resonant peak 2 is insensitive to twist. This indicates that at the resonant peak 2, the H-LPFG can measure temperature without cross-impact by twist. Therefore, this can be applied to measure temperature and twist at the same time.