Fig. 1. Piecewise link model of the few-mode fiber

Fig. 2. Splicing errors of optical fiber end facets

Fig. 3. Curves of normalized axial alignment errors versus mode power losses

Fig. 4. Coupling strength of quantum signals loaded on different modes. (a) Quantum signals are loaded on LP₀₁ mode; (b) quantum signals are loaded on LP_11a mode; (c) quantum signals are loaded on LP_11b mode

Fig. 5. System experimental setup diagram

Fig. 6. QBER as a function of mismatch distance and torsion angle of optical fiber core

Fig. 7. Relationship between fiber length and QBER