Broadband OAM Mode Generator Based on Helical Long-Period Grating Inscribed by Automated CO₂-Laser

We demonstrate the fabrication of a broadband helical long-period grating (HLPG) in a double-cladding fiber. The proposed method uses a double-side CO2-laser automated inscription technique that allows for the rotation and moving of the fiber during laser irradiation. Gratings with different pitches have been fabricated, covering a broadband wavelength range. A 10-dB bandwidth of 160 nm was achieved for the grating with a resonance wavelength of 1554.2 nm, which featured a compact length of 5.98 mm. Additionally, by splicing the HLPG with two-mode fiber (TMF), the HLPG-TMF structure could convert the fundamental mode to the first-order orbital angular momentum (OAM) core mode with a bandwidth of more than 130 nm. The proposed HLPG could be adopted as a broadband tunable mode converter, OAM generator, and broadband fiber filter, which can be used in fiber communication and sensing systems.