The Design and Simulation of High-order OAM Mode Couplers

We propose a fiber coupler that supports 7-order orbital angular momentum (OAM) modes with adjustable coupling efficiency. The coupler is composed of an OAM air-core fiber (ACF) and a single mode fiber (SMF) that are side-polished. The high mode field distribution and effective index of the ACF are calculated based on the beam propagation method (BPM). At the wavelength of 1500nm, phase matching between the fundamental mode and specific modes in the ACF is achieved by appropriate tapering of the single mode. The launched fundamental mode
$\mathbf{LP}_{\mathbf{01}}$
couples to the higher order modes
$(\mathbf{OAM}_{\mathbf{7},\mathbf{1}})$
with coupling efficiency over 90%. Additionally, in order to achieve effective excitation of the 7-order OAM mode and flexible adjustment of coupling efficiency, we discuss the effects of coupling length and core-core pitch on coupling efficiency and coupling mode. The proposed mode coupler is expected to find potential applications in all-fiber optical signal processing, multiplexing and sensing systems.