Cen Xia

Space-division multiplexing: the next frontier in optical communication

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Supermodes for optical transmission

In this paper, the concept of supermode is introduced for long-distance optical transmission systems. The supermodes exploit coupling between the cores of a multi-core fiber, in which the core-to-core distance is much shorter than that in conventional multi-core fiber. The use of supermodes leads to a larger mode effective area and higher mode density than the conventional multi-core fiber. Through simulations, we show that the proposed coupled multi-core fiber allows lower modal dependent loss, mode coupling and differential modal group delay than few-mode fibers. These properties suggest that the coupled multi-core fiber could be a good candidate for both spatial division multiplexing and single-mode operation.

Hole-Assisted Few-Mode Multicore Fiber for High-Density Space-Division Multiplexing

A seven-core few-mode multicore fiber in which each core supports both the LP01 mode and the two degenerate LP11 modes has been designed and fabricated for the first time, to the best of our knowledge. The hole-assisted structure enables low inter-core crosstalk and high mode density at the same time. LP01 inter-core crosstalk has been measured to be lower than -60 dB/km. LP11 inter-core crosstalk has been measured to be around -40 dB/km using a different setup. The LP11 free-space excitation-induced crosstalk is simulated and analyzed. This fiber allows multiplexed transmission of 21 spatial modes per polarization per wavelength. Data transmission in LP01/LP11 mode over 1 km of this fiber has been demonstrated with negligible penalty.

Time-division-multiplexed few-mode passive optical network.

We demonstrate the first few-mode-fiber based passive optical network, effectively utilizing mode multiplexing to eliminate combining loss for upstream traffic. Error-free performance has been achieved for 20-km low-crosstalk 3-mode transmission in a commercial GPON system carrying live Ethernet traffic. The alternative approach of low modal group delay is also analyzed with simulation results over 10 modes.

Adaptive frequency-domain equalization for the transmission of the fundamental mode in a few-mode fiber

We propose and experimentally demonstrate single-carrier adaptive frequency-domain equalization (SC-FDE) to mitigate multipath interference (MPI) for the transmission of the fundamental mode in a few-mode fiber. The FDE approach reduces computational complexity significantly compared to the time-domain equalization (TDE) approach while maintaining the same performance. Both FDE and TDE methods are evaluated by simulating long-haul fundamental-mode transmission using a few-mode fiber. For the fundamental mode operation, the required tap length of the equalizer depends on the differential mode group delay (DMGD) of a single span rather than DMGD of the entire link.

Demonstration of world's first few-mode GPON

We demonstrate the first few-mode-fiber based passive optical network, effectively utilizing mode multiplexing to eliminate combining loss for upstream traffic. Error-free performance has been achieved for 20-km 3-mode transmission in a commercial GPON system carrying live Ethernet traffic.

Structured directional coupler pair for multiplexing of degenerate modes

A technique for multiplexing degenerate modes in circular multimode fibers using the structure directional coupler pair is presented. The same device can be used for demultiplexing of degenerated modes in combination with MIMO processing.

First Demonstration of Six-Mode PON Achieving a Record Gain of 4 dB in Upstream Transmission Loss Budget

The power budget and costs are the two primary concerns for access networks. A major challenge is to minimize upstream power combining loss to increase the power budget. Spatial modes multiplexing offers the possibility to minimize upstream combining loss without significant added costs. We demonstrate the first integrated six-mode passive optical network, utilizing spatial modes to eliminate upstream combining loss. A record 4-dB net gain in power budget was achieved by fusion splicing a photonic lantern with a FMF, in contrast to the traditional power combining scheme with a single-mode power combiner/splitter. BERs <;10-9 were obtained for all the six modes enabling Ethernet transmission using commercial GPON equipment. The packet loss of all six modal channels was tested for 12 h. Finally, we discuss some special issues of few-mode PON regarding its practical application.

Coupled-core multi-core fibers: High-spatial-density optical transmission fibers with low differential modal properties

The characteristics of coupled-core multi-core fibers (CC-MCF) are reviewed. Random mode coupling and low differential group delay properties of the CC-MCF are discussed, and relationships between differential modal group delay, bends, and twists are also investigated.

Multi-channel nonlinearity compensation of PDM-QPSK signals in dispersion-managed transmission using dispersion-folded digital backward propagation.

We demonstrate nonlinearity compensation of 37.5-GHz-spaced 128-Gb/s PDM-QPSK signals using dispersion-folded digital-backward-propagation and a spectrally-sliced receiver that simultaneously receives three WDM signals, showing mitigation of intra-channel and interchannel nonlinear effects in a 2560-km dispersion-managed TWRS-fiber link.