Research on space-division multiplexing (SDM) came to prominence in early 2010 being primarily proposed as a means of multiplying the information-carrying capacity of optical fibers at the same time as increasing efficiency through resource sharing. Proposed SDM transmission systems range from parallel single-mode fibers with shared amplifier pump lasers to the full spatial integration of transceiver hardware, signal processing, and amplification around a fiber with over 100 spatial channels comprising multiple cores each carrying multiple modes. In this paper, we review progress in SDM research. We first outline the main classifications and features of novel SDM fibers such as multicore fibers (MCFs), multimode fibers, few-mode MCFs, and coupled-core MCFs. We review research achievements of each fiber type before discussing digital-signal processing, amplifier technology, and milestones of transmission and networking demonstrations. Finally, we draw comparisons between fiber types before discussing the current trends and speculate on future developments and applications beyond optical data transmission.

We investigate optical transmission of an ultra-wideband signal in a standard single mode fiber. Using a near continuous optical bandwidth exceeding 157 nm across the S-, C- and L-bands, we combine doped-fiber amplifiers covering S, C and L-bands with distributed Raman amplification to enable high-quality transmission of polarization division multiplexed (PDM)-256-quadrature-amplitude modulation (QAM) signals over a 54 km standard single-mode fiber. We receive 793 × 24.5 GBd signals from 1466.34 nm to 1623.57 nm and measure a data rate estimated from the generalized mutual information (GMI) of 256.4 Tb/s and an LDPC decoded throughput of 244.3 Tb/s. The measured data rates exceed the highest previously measured in a single mode fiber, showing the potential for S-band transmission to enhance achievable data rates in optical fibers.

Space-Division Multiplexing (SDM) has been intensively proposed in recent years to overcome the capacity limitations of current single-mode fiber infrastructure and to increase the efficiency of optical transmission systems. Few-mode fibers offer the potential of transmitting several signals at the same wavelength over different modes of a fiber with one single core. In this work, we demonstrate the potential of using fewmode fiber based SDM systems for long-haul transmission. We analyze system limitations such as mode-dependent loss and Kerreffect based nonlinear signal distortions for different modulation formats and distances up to 3500 km. We find that modedependent loss poses the major limitation in the analyzed transmission system, masking most intermodal nonlinear interactions.

We experimentally investigate high-capacity fewmode fiber transmission for short and medium-haul optical links. In separate experiments, we demonstrate C + L band transmission of 283 Tbit/s over a single 30 km span and recirculating loop transmission of 159 Tbit/s over 1045 km graded-index three mode fiber. The first experiment reached a data-rate per fiber mode within 90% of the record data-rates reported in the same transmission bands for single-mode fibers. The second experiment demonstrated the feasibility of reaching high datarates over long distance few-mode fiber transmission, despite strong impairments due to mode-dependent loss and differential mode delay.

We demonstrate the first continuous variable quantum key distribution (CV-QKD) system with fully digital phase and polarization tracking, enabling the first wavelength division multiplexing experiment for CV-QKD. We transmit 194 × 0.5 GHz channels with 25 GHz channel spacing, demonstrating an aggregate estimated asymptotic secret key rate of 172.6 Mbit/s over 25 km.

This paper extends a recent demonstration of 1.2 Pb/s transmission using a few-mode multi-core fiber with a cladding diameter of 160 mm. The fiber was a 3.37 km, 4-core, 3-mode few-mode multi-core fiber with spatial multiplexing and demultiplexing performed with specially designed mode-selective couplers. We transmitted 368 wavelength division multiplexed spatial super channels across the C and L bands. We selected polarization-division multiplexed (PDM)-256-quadrature amplitude modulation (QAM) format to maximize the generalized mutual information of the transmitted signals, which were decoded using punctured soft-decision forward error correction with a code rate granularity of 0.01. The maximum crosstalk of the system was -31 dB between LP11 modes of adjacent cores, leading to a Q-factor penalty of approximately 0.5 dB. We estimated the system mode-dependent loss values ranging from 3 dB to 5 dB across C and L bands with negligible wavelength dependence.