Wavelength Tunable Orbital Angular Momentum Mode Convertor Based on Helical Long-Period Grating Inscribed in Six-Mode Fiber Using CO2 Laser

We demonstrated the fabrication of orbital angular momentum (OAM) mode convertors (MCs) operating at 1.3-2.1-mu m waveband based on the helical long-period gratings (HLPGs) inscribed in the few-mode fiber (FMF) using a CO2 laser. The +/- 1st, +/- 2nd, and +/- 3rd OAM mode can be directly generated by the HLPGs with efficiencies of more than 90%. The torsion sensitivities of the first-, second-, and third-order OAM modes in 2-mu m waveband are 0.52, 0.55, and 0.36 nm/(rad/m), which has increased by 5.8 times, 2.8 times, and 1.6 times compared to that in 1.3-mu m waveband, respectively. The strain sensitivities of the first-, second-, and third-order OAM modes in 2-mu m waveband are -4.2, -4.0, and -4.9 pm/mu epsilon, which has increased by 2.3 times, 2 times, and 2.2 times compared to that in 1.3-mu m waveband, respectively. The proposed OAM mode MCs based on the HLPGs can function as not only promotion of optical communication system but also high-torsion sensitivity sensors in the sensing field.