Submillimeter-length single-helix chiral grating in high-NA fiber for robust third-order OAM generation

We demonstrate a submillimeter-length single-helix chiral grating embedded in a high-numerical-aperture single-mode fiber (HNA-SMF) for efficient generation of third-order orbital angular momentum (OAM), specifically the OAM_{±3, 12} mode. This design facilitates enhanced coupling of higher azimuthal modes due to significant perturbations arising from both the geometric effect of the thin-core offset under high-NA conditions and the elasto-optic effect induced by intense helical stress with a small twist pitch. As a result, we achieve an unprecedented device length of 0.88 mm with a resonance dip of -28 dB, corresponding to a high conversion efficiency of 99.8%-the shortest among all fiber grating-based OAM generators reported to date, to the best of our knowledge. It also exhibits excellent robustness against external disturbances, demonstrating low responsivities to torsion, strain, and temperature at 12 pm/(rad/m), 0.5 pm/με, and 24 pm/°C, respectively. These features make it promising for applications in OAM-based particle sensing and manipulation, particularly for chiral compounds.