Fiber Space-Division Multiplexed Transmission over Few-Mode- and Coupled-Core Fiber Based on Coherent MIMO Digital Signal Processing

The capacity in long-haul transmission has been increased steadily by three orders of magnitude over the last twenty years. However, studies show that the theoretical capacity limit of single-mode fiber is about to be reached, and space-division multiplexing has been proposed to overcome this limit. At the high levels of integration needed for economic deployment, space-division multiplexing will ultimately exhibit strong crosstalk between the supported fiber modes. We propose to use coherent multiple-input multiple-output (MIMO) digital signal processing (DSP), as widely used in wireless communication, to compensate this crosstalk. According to MIMO theory, crosstalk in multi-mode transmission systems can be completely reversed if the crosstalk is described by a unitary transformation. For optical fiber this is fulfilled if all available fiber modes can be selectively excited and if all the modes are coherently detected at the end of the fiber, provided that mode dependent loss is negligible. We successfully applied the technique to demonstrate the transmission of six independent mode-multiplexed 20-Gbaud QPSK signals over a single, optically amplified span of 137-km few-mode fiber (FMF). Further, in a multi-span experiment, we reach 1200 km by transmitting over a 3-core coupled-core fiber (CCF). Characteristics of both fiber types will be presented in detail, including the description of the supported polarization- and spatial modes, the mode multiplexers used to launch an detect the modes, and the MIMO DSP algorithm used to recover the channels.