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.

A high-sensitivity curvature sensing configuration is implemented by using a fiber Mach-Zehnder interferometer (MZI) with D-shaped fiber Bragg grating (FBG). A segment of D-shaped fiber is fusion spliced into a single mode fiber at both sides, and then a short FBG is inscribed in the D-shaped fiber. The fiber device yields a significant spectrum sensitivity as high as 87.7 nm/m$^-1$ to the ultra-low curvature range from 0 to 0.3 m$^-1$, and can distinguish the orientation of curvature experienced by the fiber as the attenuation dip producing either a blue or red wavelength shift, by virtue of the asymmetry of D-shaped fiber cladding. In addition, by tracking both resonant wavelengths of the MZI and embedded FBG, the temperature and curvature can be measured simultaneously.

We have demonstrated an ultra-broadband fiber mode converter based on ${{\rm CO}_2}$-laser inscribed length apodized phase-shifted long-period gratings (LPGs) with a three-section linear length apodization profile, where a $\pi $-phase shift was introduced between two adjacent grating sections. The grating parameters were optimized theoretically to achieve the broadband mode conversion between the ${{\rm LP}_{01}}$ mode and ${{\rm LP}_{11}}$ mode. The demonstrated device provides mode conversion efficiency higher than 90% over an ultra- broad bandwidth of $ {\sim} {182}\;{\rm nm}$. The insertion loss of the LPGs is negligible. Orbital angular momentum modes with left- and right-handed circular polarization can be generated from the demonstrated ultra-broadband mode converter successfully.

We report a simple method for generating optical vortex beams (OVB) from an all-fiber laser. An intracavity mode converter based on few-mode fiber long-period gratings was used to enable the OVB operation. The intensity profiles and the wavefront helicity of the output laser beams were characterized by analyzing the spatial intensity profiles of the laser emission and its self-interference. Selection of the wavefront handedness can be achieved simply by adjusting the intracavity polarization controller. The laser achieved ~50 mW of output power for both first-order and second-order vortex modes generation with the corresponding slope efficiencies of 8.12% and 7.91%, respectively. The corresponding mode conversion efficiency of 98% and 99.5% are for the LP11 mode and LP21 mode, individually. The linearly polarized orbital angular momentum beams with $l=\pm 1$ were characterized experimentally by adjusting a polarizer at the laser output.

We demonstrate broadband and low-loss three-mode and six-mode scramblers employing ${\mathrm{CO}}_2$-laser inscribed long-period gratings (LPGs) for space-division multiplexing. Step-index (SI) few-mode fibers are used to avoid mode coupling to the cladding modes. We characterize the mode scramblers using a swept-wavelength interferometer. Mode-dependent loss (MDL) and modal transfer matrices over the $C+L$ band are presented. Demonstrated LPGs with negligible MDL and low insertion loss contributed to high-performance ${\mathrm{CO}}_2$-laser inscription. The total MDLs induced by the SI fiber with LPGs in three-mode and six-mode scramblers are measured to be 2 and 4 dB, respectively.

We demonstrate high-order mode conversion in a few-mode fiber (FMF) via C O 2 laser inscribed long-period fiber gratings (LPFGs) at both the 1.55 µm and 2 µm wavebands. At the 1.55 µm waveband, five high-order core modes (the L P 11 , L P 21 , L P 02 , L P 31 , and L P 12 modes) can be coupled from the L P 01 mode with high efficiency by the FMF-LPFGs. The orbital angular momentum beams with different topological charges ( ± 1 , ± 2 , ± 3 ) are experimentally generated by adjusting the polarization controllers. At the 2 µm waveband, three high-order modes (the L P 11 , L P 21 , and L P 02 mode) can be coupled by the FMF-LPFGs with different grating periods. The proposed LPFG-based mode converters could have a potential prospects in mode-division multiplexing and multiwindow broadband optical communication applications.