1120-channel OAM-MDM-WDM transmission over a 100-km single-span ring-core fiber using low-complexity 4×4 MIMO equalization.

A successful transmission of 14 multiplexed orbital angular momentum (OAM) channels each carrying 80 wavelengths over a 100-km single-span ring-core fiber (RCF) is experimentally demonstrated. Each transmission channel is modulated by a 20-GBaud quadrature phase-shift keying (QPSK) signal, achieving a record spectral-efficiency-distance product of 1870 (bit/s/Hz)·km for the single-core RCF based mode division multiplexing (MDM) transmissions. In addition, only low-complexity 2×2 or 4×4 multiple-input multiple-output (MIMO) equalization with time-domain equalization tap number no more than 25 is required to deal with the crosstalk among the highly degenerate intra-MG modes at the receiving end of the demonstrated OAM-MDM-WDM system, showing great potential in large-capacity and relatively long-distance MDM transmission with low digital signal processing (DSP) complexity.

1-Pbps orbital angular momentum fibre-optic transmission.

Space-division multiplexing (SDM), as a main candidate for future ultra-high capacity fibre-optic communications, needs to address limitations to its scalability imposed by computation-intensive multi-input multi-output (MIMO) digital signal processing (DSP) required to eliminate the crosstalk caused by optical coupling between multiplexed spatial channels. By exploiting the unique propagation characteristics of orbital angular momentum (OAM) modes in ring core fibres (RCFs), a system that combines SDM and C + L band dense wavelength-division multiplexing (DWDM) in a 34 km 7-core RCF is demonstrated to transport a total of 24960 channels with a raw (net) capacity of 1.223 (1.02) Peta-bit s-1 (Pbps) and a spectral efficiency of 156.8 (130.7) bit s-1 Hz-1. Remarkably for such a high channel count, the system only uses fixed-size 4 × 4 MIMO DSP modules with no more than 25 time-domain taps. Such ultra-low MIMO complexity is enabled by the simultaneous weak coupling among fibre cores and amongst non-degenerate OAM mode groups within each core that have a fixed number of 4 modes. These results take the capacity of OAM-based fibre-optic communications links over the 1 Pbps milestone for the first time. They also simultaneously represent the lowest MIMO complexity and the 2nd smallest fibre cladding diameter amongst reported few-mode multicore-fibre (FM-MCF) SDM systems of >1 Pbps capacity. We believe these results represent a major step forward in SDM transmission, as they manifest the significant potentials for further up-scaling the capacity per optical fibre whilst keeping MIMO processing to an ultra-low complexity level and in a modularly expandable fashion.

Spin-orbit coupling suppressed high-capacity dual-step-index ring-core OAM fiber.

We design and fabricate a dual-step-index ring-core fiber (RCF) for orbital angular momentum (OAM) mode transmission. It has been proven that the proposed novel, to the best of our knowledge, dual-step-index ring-core structure can suppress the spin-orbit coupling and cut off the radial higher-order modes when the mode number becomes very large. In experiments, we demonstrate that the fabricated fiber can support OAM8,1 with the interferometric method, where four higher-order mode groups are weakly-coupled. We also measure the loss of each mode according to the cut-back method and the loss can achieve <0.3 dB/km for the OAM modes with an order from |l| = 1 to |l| = 5. The exploration of this novel optical fiber structure may provide ideas and knowledge for the improvement of the optical fiber communication capacity.

Design, fabrication, and characterization of a low-index center and trench-assisted 7-ring-core 5-mode-group fiber for dense space-division multiplexing.

An optimized design of 7-ring-core 5-mode-group fiber for mode-group-based dense space-division multiplexing (DSDM) is proposed. It is found that decreasing the refractive index of the center and trench of each ring core can increase the available ring-core thickness and meanwhile suppress the radial higher-order modes. Based on the simulation, a fiber, with a thicker ring core and a relatively low refractive index contrast at the ring-core boundaries, is found of higher mode purity and lower macro-bending-caused mode coupling. In the experiment, the seven cores of the fabricated fiber have low transmission losses around -0.25 dB/km with only a few fluctuations. The three higher-order mode groups (MG2,1, MG3,1, and MG4,1) are verified to be in a weak-coupling state (crosstalk of which are less than -12 dB) over a transmission length of 23 km.

Long-haul intermodal-MIMO-free MDM transmission based on a weakly coupled multiple-ring-core few-mode fiber.

Mode-division multiplexing (MDM) technique based on few-mode fibers (FMFs) can achieve multiplicative growth in single-fiber capacity by using different linearly polarized (LP) modes or mode groups as spatial channels. However, its deployment is seriously impeded because multiple-input multiple-output digital signal processing (MIMO-DSP) with huge computational load must be adopted to combat intermodal crosstalk for long-haul FMF transmission. In this paper, we present an intermodal-MIMO-free MDM transmission scheme based on weakly coupled multiple-ring-core FMF, which achieves ultralow distributed modal crosstalk (DMC) so that the signal in each LP mode can be independently received by single-LP-mode MIMO-DSP even after hundreds-of-kilometer transmission. Evaluation method for the required DMC levels is proposed and different transmission reaches are investigated by simulation. By adopting an improved method for quantitative DMC measurement, we show that the required DMC level for long-haul transmission is feasible. Finally, we experimentally demonstrate 1800-km LP01/LP02 multiplexed transmission and 525-km LP01/LP21/LP02 multiplexed transmission only adopting 2×2 or 4×4 MIMO-DSP. The proposed scheme may pave the way to practical applications of long-haul MDM techniques for the first time.

Long-distance transmission of quantum key distribution coexisting with classical optical communication over a weakly-coupled few-mode fiber.

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.