Helical Long-Period Fiber Gratings as Wavelength-Tunable Orbital Angular Momentum Mode Generators

High-quality helical long-period fiber gratings (HLPFGs) with a coupling strength greater than 30 dB and an insertion loss less than 0.5 dB are formed by applying CO2 laser to expose an optical fiber that is simultaneously translated and rotated to relieve the residual stress of the fiber along a helical pathway, rather than twisting the fiber under a high-temperature condition. The mode coupling characteristics of this type of HLPFG are investigated for the first time. Experimental results demonstrate that the helical refractive-index modulation of the HLPFGs can generate light with orbital angular momentum (OAM) modes, where the chirality of the OAM modes is insensitive to the polarization state of the input light. The HLPFGs provide excellent torsion characteristics, and the resonant wavelengths of OAM modes can be linearly tuned when applying torsions.