Multiplexing Orbital Angular Momentum Modes Generator by Long-Period Fiber Grating Arrays

Orbital angular momentum (OAM) modes, characterized by helical phase fronts, enable powerful spatial manipulation of light for applications in optical communications, quantum information, and imaging. However, compact and flexible devices for generation of multiplexed OAM modes remain challenging. Here we propose and demonstrate a compact, reconfigurable long-period fiber grating (LPFG) array for multi-channel, multi-order OAM modes in a few-mode fiber. Femtosecond inscription enables micrometer-scale, highly localized refractive-index modulations (RIMs) that can be positioned azimuthally within the fiber core. Coupled-mode analysis reveals that the coupling coefficients for the mode conversion from $LP_{01}$ to $LP_{21},LP_{31}$ modes are strongly controlled by the RIM transverse position and modulation depth of the RIMs. By employing distinct grating periods, phase mismatch effectively suppresses inter-grating coupling and minimizes crosstalk, enabling independent channels. Experimentally, rotating-fiber inscription produces LPFG arrays comprising up to five parallel gratings with resonance channels spanning 1300–1700nm, achieving coupling efficiencies exceeding 95% and a minimum insertion loss of 0.60dB. With phase control, $OAM_{\pm 2}$ and $OAM_{\pm 3}$ modes are synthesized, including a five-channel third-order OAM configuration. The LPFG-array platform provides a scalable, low-loss solution for multiplexed OAM sources in mode-division multiplexing and integrated photonics.