We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO₂ laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP₀₁ mode and LP₁₁ mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.

We demonstrate the fabrication of the helical long-period grating (HLPG) in a two-mode fiber (TMF) with CO ₂ laser. The torsion characteristics of the helical gratings are experimentally investigated. It was found that the resonance wavelength of the TMF-HLPG linearly shifts with a twist sensitivity of 0.47 nm/(rad/m), which is an order magnitude higher than that of the conventional long-period fiber gratings (LPFGs). The temperature sensitivity of the TMF-HPFG was measured to be 23.9 pm/°C, which is one third of that of the conventional LPFGs. The proposed TMF-HLPG-based twist sensor would have a promising application for the twist monitor.

We investigated an all-fiber mode converter based on long-period fiber gratings (LPFGs) written in the few-mode fiber. Mode conversion between the fundamental core mode and different higher-order core modes (${\mathrm{LP}}_{11}$, ${\mathrm{LP}}_{21}$, and ${\mathrm{LP}}_{02}$ modes) can be realized via a single LPFG with an efficiency of 99% at the resonance wavelength. Moreover, optimized mode conversion between the ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{21}$ modes can be realized by cascading two LPFGs with different grating pitches. The maximum conversion efficiency is estimated to be $\sim 99.5\%$ at 1553 nm. The orbital angular momentum states with different topological charges ($\pm 1,\pm 2$) are demonstrated experimentally. The all-fiber LPFG mode converters could have promising applications in the mode-division multiplexing optical communications.

We demonstrate a simple and highly sensitive optical fiber relative humidity (RH) sensor based on a long-period fiber grating (LPFG) coated with polyethylene glycol (PEG)/polyvinyl alcohol (PVA) composite films. The resonance wavelength of the LPFG is sensitive to environmental humidity due to the change in effective refractive index caused by the strong surface absorption and desorption of the porous PEG/PVA coatings. The sensor is sensitive in a wide range from 50% to 95% RH, with a highest sensitivity of 2.485 nm/%RH in the range 50–75% RH. The proposed RH sensor has the advantages of compact size, good reversibility, and stability, which makes it attractive for high-humidity environments.

We demonstrate the fabrication of helical long-period gratings (HLPGs) in a polarization-maintaining fiber using CO2-laser. The maximum polarization dependent loss (PDL) of the grating was measured to be more than 30 dB at the resonance wavelength. The HLPG was found to have a high refractive index sensitivity of -7248.6 nm/RIU when the surrounding refractive index changes in the range of 1.4470-1.4600, temperature sensitivity of 158.8 pm/°C, and strain sensitivity of 3.17 pm/μE. The twist sensitivities based on intensity variation were measured to be 5.98 dB/ (rad/m) (co-directional twist) and -5.97 dB/ (rad/m) (contra-directional twist), respectively. The demonstrated grating devices have promising applications as high-sensitivity fiber sensors, PDL compensator etc.

We demonstrated the fabrication of bandwidth tunable ultra-broadband mode converters based on CO₂-laser inscribed long-period fiber gratings (LPFGs) and helical long-period gratings (HLPGs) in a two-mode fiber (TMF). The simulation and experimental results show that there is a dual-resonance coupling from LP₀₁ to LP₁₁ core mode at the dispersion turning point. The mode converters based on the TMF-LPFG and TMF-HLPG provide a 10-dB bandwidth of ∼300 nm and ∼297 nm, respectively, which covers O + E+S + C band. The 1^st order orbital angular momentum (OAM) mode based on TMF-LPFG was generated by adjusting the polarization controllers (PCs), while the 1^st order OAM mode can be generated directly by the TMF-HLPG. When the twist rate is varied from -36 rad/m ∼ 36 rad/m, the tunable range of the 10-dB bandwidth is ∼52 nm and ∼91 nm for the LPFG and HLPG mode converters, respectively. The ultra-broadband mode converter can be adopted as a bandwidth tunable mode converter, which can be applied in ultra-broadband mode-division-multiplexing transmission systems and optical fiber sensing systems based on few-mode fibers.