More than 110-nm broadband mode converter based on dual-resonance coupling mechanism in long period fiber gratings

Abstract We propose and demonstrate the broadband generation of orbital angular momentum (OAM) beams with high efficiency based on a two-mode fiber long period grating (TMF-LPG) on account of a dual-resonance coupling mechanism. Theoretical and experimental investigations reveal that a dual-resonance coupling phenomenon exists because of the special phase matching condition between LP 01 and LP 11 core modes in the two-mode fiber. When a specific grating period is chosen, the dual resonance wavelengths gather together and the TMF-LPG could act as a broadband mode converter. An LPG with 15.0 dB bandwidth (corresponding to conversion efficiency greater than 96%) of 118.2 nm and 10.0 dB bandwidth (corresponding to conversion efficiency greater than 90%) of 156.4 nm is achieved in experiment. Furthermore, simulation results indicate that the operation wavelength can be extended to other wavelengths if a TMF with appropriate core radius is chosen to fabricate the LPG.