Fig. 1. Schematic diagram of fiber structure and refractive index profile for (a) single-mode fiber (SMF) and (b) ring-core fiber (RCF); (c) schematic diagram of fusion splicing and tapering scheme for exciting the LP01 mode in the RCF with high efficiency.

Fig. 2. Simulation of light propagation and coupling power evolution along the tapered fibers with different stretched lengths: (a) 5000 μm and (b) 16,000 μm.

Fig. 3. Theoretical optimization of coupling efficiency in the relationship of waist radius.

Fig. 4. Experimental setup of characterizing coupling efficiency and mode changes in the SMF–RCF taper.

Fig. 5. Transmission of the SMF–RCF taper: (a) RCF connected to optical spectrum analyzer (OSA) directly; (b) RCF connected to the OSA with an SMF.

Fig. 6. The influence of wavelength on coupling efficiency of modes in the RCF from 1530 nm to 1570 nm.

Fig. 7. (a) Experimental setup of orbital angular moment (OAM) generation and test system. (b) Mode field and interference fringe of the LP01 mode in the RCF. (c) Mode field and interference fringe of OAM+1. (d) Mode field and interference fringe of OAM−1.