Enhancing the Azimuthal Mode Couplings in a Helical Fiber Grating by Using Phase Sampling

In a single-helix helical long-period fiber grating (SHLPG), due to the inherent one-fold azimuthal symmetric structure of the fiber core, it is very hard to obtain an efficient high-order azimuthal mode coupling. In this letter, we have proposed and demonstrated theoretically a phase-sampling approach to overcome this obstacle, which is realized by periodically inserting phase shifts in the azimuthal direction of the SHLPG. As results of the proposed method, high-efficient couplings between the fundamental mode and the second- and third-order azimuthal modes in a SHLPG have been successfully demonstrated. The proposed method can be applied to generate and convert high-order orbital angular momentum modes even with the SHLPG.