Spatial multiplexing using multiple-input multiple-output signal processing

Wavelength-division multiplexing (WDM) has been the workhorse of data networks since the early 1990s, enabling ubiquitous and a ffordable data services with unabated exponential tra ffic growth. Today, commercial WDM systems can carry close to 10 Tbit/s over a single fi ber, and research experiments have reached the 100-Tbit/s mark. Over the past few years, however, WDM capacities have approached the nonlinear Shannon limit to within a factor of 2. In order to further scale network capacities and to avoid a looming "capacity crunch", space has been identifi ed as the only known physical dimension yet unexploited for optical modulation and multiplexing. Space-division multiplexing (SDM) may use parallel strands of single-mode fiber, uncoupled or coupled cores of multi-core fiber, or individual modes of multi-mode waveguides. In this context, integration at various levels (including optical ampli fiers, transponders, networking elements, and transmission fiber CAPEX and OPEX) is essential to continue the reduction in cost and energy consumption per transported information bit that has allowed the Internet to thrive. Integration, however, inherently comes at the expense of crosstalk. If crosstalk rises to levels where it cannot be treated as a transmission impairment any more, multiple-input multiple-output (MIMO) digital signal processing (DSP) techniques have to be used to manage crosstalk in highly integrated SDM systems. At the beginning of an exciting new era in optical communications, we review fundamentals as well as practical experimental aspects of MIMO-SDM: We discuss the importance of selectively addressing all modes of a coupled-mode SDM channel at transmitter and receiver in order to achieve reliable capacity gains and show that reasonable levels of mode-dependent loss (MDL) are acceptable without much loss of channel capacity. We then introduce MIMO-DSP techniques as an extension of familiar algorithms used in polarization-division multiplexed (PDM) digital coherent receivers and discuss their functionality and scalability. Finally, we review the design of mode multiplexers (MMUXs) that allow for the mapping of the individual transmission signals onto an orthogonal basis of waveguide modes, and discuss their performance in experimental demonstrations. Although MIMO-SDM has been experimentally proven to be a feasible technique for reliable high-capacity long-haul fiber transmission beyond the nonlinear Shannon limit of single-mode fi ber, signi cant research, development, and standardization eff orts have to be invested to enable a smooth upgrade path from today's single-mode systems to MIMO-SDM based optical communication networks.