Fig. 1. SE versus MIMO complexity of the SDM demonstrations utilizing lab-deployed FM-MCF spools or field-deployed MCF cables (data points are evaluated based on information reported from the cited references).

Fig. 2. (a) Cross-section diagram of the fabricated 7-RCF cable; (b) cross-sectional photo of the fabricated 7-RCF; (c) schematic diagram of the field-deployed fiber cable route; (d) optical fibers in the fiber unit; (e) field-deployed fiber cable in a fiber pipeline.

Fig. 3. (a) The designed and the fabricated RIPs of the ring-core in the 7-RCF; (b) the mode effective refractive index (neff) at 1550 nm of the designed and the fabricated RIPs.

Fig. 4. The measured mode-dependent attenuation of the 7-RCF spool before cabling and installed cable at 1550 nm.

Fig. 5. The measured XT of 7-RCF spools and installed cable at 1550 nm: (a) inter-MG XT within the one single fiber core; (b) inter-core XT.

Fig. 6. The measured DGD at 1550 nm of 7-RCF spools and installed cable. One averaged DGD value is given for OAM MGs |l|=0 and 1 in the measured results, as their impulse response merged into one Gaussian-distribution peak due to strong coupling between these two MGs after transmission.

Fig. 7. (a) The received normalized power fluctuations (or mode partition noise) of the OAM mode with a topological charge of l=3 in the central core of the field-deployed 7-RCF and the 7-RCF spool; (b) the normalized power fluctuation spectrum of the OAM mode in the central core of the field-deployed 7-RCF; (c) the normalized power fluctuation spectrum of the OAM mode in the central core of the 7-RCF spool.

Fig. 8. Experimental setup. ECL, external cavity laser; AWG, arbitrary waveform generator; EA, electrical amplifier; EDFA, erbium-doped fiber amplifier; LP, linear polarizer; SLM, spatial light modulator; MR, mirror; HWP, half-wave plate; BS, beam splitter; QWP, quarter-wave plate; PBS, polarization beam splitter; PBC, polarization beam combiner; Col., collimator; VPP, vortex phase plate; ICR, integrated coherent receiver. The intensity profiles of OAM MGs with |l|  = (a) 4, (b) 3, and (c) 2 and the phase masks of OAM MGs with |l|  = (d) 4, (e) 3, and (f) 2.

Fig. 9. The measured BERs of all channels after 5-km field-deployed 7-RCF bidirectional transmission: (a) forward transmission and (b) backward transmission; the absolute values of tap weights in 16 FIR filters of 4×4 MIMO equalizers to equalize the four modes belonging to OAM MGs |l|=3 at 1540 nm in the (c) forward transmission and (d) backward transmission.

Fig. 10. The measured SNRs under different backward transmission power with 8-dBm forward transmission power after (a) 5-km installed cable transmission and (b) 5-km theoretical transmission. lF, topological charge of the forward transmission OAM light beam; lB, topological charge of the backward transmission OAM light beam; ⟨IS-lo⟩2/⟨IRB-lo⟩2 represents the power ratio between the detected signal and RB noise after coherent detection (details can be found in Appendix B).

Fig. 11. Schematic setup of coherent detection using balanced detectors. LO, local oscillating light; PD, photodetector; EA, electrical amplifier.

Fig. 12. The convergence error of the CMA algorithm versus the number of iterations in CMA-based time domain equalizer updates.