Fig. 1. Schematic diagram of the RCF cross-section.

Fig. 2. Colormap of minimal Δneff between adjacent modes as a function of V and ρ. The curves show the corresponding cut-offs for each mode in the calculation region.

Fig. 3. Intensity distributions and electrical field orientations for the first higher-order eigenmodes, along with intensity and phase distributions for the composite modes in the RCF with different bending radii R. (a) Straight fiber, (b) R=10  mm, and (c) R=6  mm.

Fig. 4. (a) Effective refractive indices neff and (b) effective refractive index difference Δneff as a function of bending radius R.

Fig. 5. Conversion length LC as a function of bending radius R for different ρ and V.

Fig. 6. (a) Optical microscope image of fiber cross section for fiber 1. (b) Intensity profiles and spiral interference patterns for OAM±1,1 modes after propagating 2 m in fiber 1.

Fig. 7. (a) Experimental setup of the bending test. LD, semiconductor laser; CL, fiber collimator; BE, beam expander; Pol, polarizer; M, mirror; SLM, liquid crystal spatial light modulator; QWP, quarter-wave plate; LE, lens; HWP, half-wave plate; BS, non-polarizing beam-splitter. (b) Bend-post of the RCF. L is the length of the bending section, θ is bending angle, and R is the bending radius. (c) Output intensity profiles and corresponding interference patterns of fiber 1 for straight fiber, R=9  mm and L=20  mm, and R=9  mm and L=LC=43  mm, respectively.

Fig. 8. Experimentally measured and simulated values of bending lengths versus bending radii for three RCFs.