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.

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.

Band division multiplexing technology is a promising medium-term transitional solution to deal with the traffic flood, by leveraging the existing optical fiber transmission infrastructure. Additionally, the deployment of multi-band optical networks requires updating and upgrading key components, such as multi-band amplifiers and multi-band optical cross-connects. From cost view, it would be a promising way for operators to expand transmission capacity on some of the links with high priority, which are used heavily. Therefore, the application scenario of this paper is positioned in the C-C+L hybrid optical networks. In this scenario, the introduction of the L-band brings additional inter-band stimulated Raman scattering (ISRS) which is a non-negligible nonlinear effect, leading new constraint for service provisioning. This paper proposes an ISRS-aware routing, band, modulation, and spectrum assignment (RBMSA) scheme, considering the impact of ISRS generated by new requests on existing requests. By implementing the SNR re-verification, not only the transmission quality of the current request can be realized, but also the service quality of the existing requests will not be seriously degraded to be unacceptable. A two-dimensional model of request bandwidth and duration is established for band selection. Priority is provided for arranging the requests in the L-band when it is available. For each single request, when the least number of slots required by adopting different modulation formats is the same, lower-order modulation is preferred to improve its SNR tolerance of this request under the ISRS impact. Simulation results indicate that the proposed scheme can effectively decrease the blocking probability, and improve the channel SNR for the successfully established requests.

Recently optical amplification technologies for mode-division multiplexing (MDM) transmission systems and networks have attracted great interest. In this paper, we firstly propose a compact structure of few-mode Erbium-doped fiber amplifier (FM-EDFA) for weakly-coupled MDM amplification, which is characterized by weakly-coupled few-mode Erbium-doped fiber (EDF), cascaded mode-selective couplers (MSCs) for spatial mode control of pump lights, and few-mode wavelength multiplexer (FM-WMUX) for pump/signal coupling. Compared to previous studies, the proposed FM-EDFA has a similar structure with commercial single-mode EDFA (SM-EDFA) and can support weakly-coupled MDM transmission without inducing large modal crosstalk. Then related components are designed and fabricated for a 2-mode proposed FM-EDFA. The mode-field mismatching between transmission FMF and EDF under central alignment is investigated by simulation. Thanks to the effective suppression of modal crosstalk for the optical components, the transmission FMF, the EDF and their coupling, we experimentally demonstrate the first weakly-coupled MDM-WDM amplification and transmission over 15-km 2-mode FMF with 10-Gb/s optical on-off keying (OOK) modulation and direct detection. The proposed FM-EDFA can be extended to MDM transmission systems and networks supporting more modes.

Weakly-coupled mode division multiplexing (MDM) has been intensively investigated in recent years. In order to be compatible with conventional intensity modulation and direct detection systems, effective solutions to handle degenerate non-circularly-symmetric linearly-polarized (LP) modes ( $\text{LP}_{lm{\rm{a}}}$ and $\text{LP}_{lm{\rm{b}}}$ modes, l ≥ 1) of weakly-coupled few-mode fibers (FMFs) are highly desired. In this paper, a novel degenerate-mode-selective coupler (DMSC) consisting of an input FMF and an output two-mode fiber (TMF) is proposed based on coupling mechanisms of a FMF directional coupler. Optical powers in both $\text{LP}_{lm{\rm{a}}}$ and $\text{LP}_{lm{\rm{b}}}$ modes of the FMF of $\text{LP}_{lm}$ DMSC can be selectively and simultaneously transferred to the LP _11a and LP _11b modes of the TMF. Therefore, the DMSC can demultiplex degenerate non-circularly-symmetric LP modes without any digital signal processing (DSP). Necessary conditions for the DMSC are theoretically investigated. LP ₁₁ and LP ₂₁ DMSCs are numerically simulated to verify the principles and fabricated in the form of fused-type fiber couplers. Compared with regular mode-selective couplers (MSCs), the fabricated DMSCs significantly improve the power stability for demultiplexing randomly-rotated non-circularly-symmetric LP modes. Based on a mode demultiplexer composed of cascaded LP ₀₂ MSC, LP ₂₁ DMSC, LP ₁₁ DMSC and LP ₀₁ MSC, 4-LP-mode MDM transmission over 10-km of weakly-coupled FMF is demonstrated with 10-Gb/s on-off-keying and DSP-free direct detection.