Fig. 1. Schematic diagram of cascaded MSCs.

Fig. 2. SMPM algorithm flow. (a) Schematic illustration of the composite waveguide structure. (b) Mode patterns of CV modes and SMs. (c) Amplitude and phase spectra of the input mode expanded into SMs. (d) Variation of phase differences between SMs during transmission (with SM1 as a reference). (e) Changes in mode field during transmission, side view, and slice view. (f) Amplitude and phase spectra of the output field expanded into CV modes. (g) Energy evolution of CV modes during transmission.

Fig. 3. (a1) and (a2) Refractive index profiles of SMF and FMF for MSC fabrication. (b1)–(b3) Schematic structures of the fiber taper and MSC. (c) TDCs for SMF and FMF, phase-matching points for MSC, and manufacturing parameters.

Fig. 4. Process for determining DoF. (a) Coupling lengths and taper diameters of MSC0, MSC1, and MSC2. (b) Variation of coupling length with DoF. (c) Simulation of the mode conversion process in MSCs.

Fig. 5. (a) Schematic diagram of the energy-level structure of the coupling region in MSC. (b) The energy-level structure of SMF and FMF in MSC0–MSC2.

Fig. 6. (a) Schematic representation of the rear-stage MSC acting as a mode combiner. (b) Insertion loss and output mode purity after injecting different modes into MSCs. (c1–c3), (d1–d3), (e1–e3) Evolution of mode fields, along with changes in loss and purity, when injecting different modes into the FMF input port of MSC0–MSC2.

Fig. 7. (a) Experimental setup for MDMUX testing. (b) Measured insertion loss of the MDMUX. The fluctuations near 1635 nm are attributed to the instability of the light source.

Fig. 8. Output OAM mode fields and interferometric vortices of the MDMUX with topological charges of 0, ±1, and ±2, analysis results of mode components at 1550 nm, as well as a comparison between the recovered mode field and the original mode field.