Dynamic orbital angular momentum mode coupler based on D-shaped photonic crystal fibers

We propose and demonstrate an orbital angular momentum (OAM) mode coupler based on a D-shaped photonic crystal fiber (PCF). The device consists of a single-mode D-shaped PCF coupled with a multimode D-shaped PCF, designed to generate and transmit OAM modes in fiber communication systems. Numerical simulations based on mode coupling theory and supermode interference theory demonstrate that under the excitation of linearly and circularly polarized fundamental modes in the single-mode PCF, various OAM modes can be efficiently excited in the multimode hollow PCF. Specifically, under fundamental mode excitation with an operating wavelength of 1550 nm and a coupling length of 5 mm, the coupling efficiency of the ±1 order OAM modes reaches 99.9%, and the maximum mode purity exceeds 99.22%. We also demonstrate that the coupling efficiency achieves 92.6%, and the maximum mode purity is up to 96.48% for the ±2 order OAM modes. Additionally, by adjusting the refractive index of the central air hole, 100 nm spectral tunability with high coupling efficiency of 90% can be achieved in the 1550-1820 nm range. The proposed structure provides a novel, to our knowledge, and simplified method for generating and dynamically regulating OAM based on D-shaped PCF.