Proceedings Article | 25 January 2012
Proc. SPIE. 8284, Next-Generation Optical Communication: Components, Sub-Systems, and Systems
KEYWORDS: Beam splitters, Digital signal processing, Optical amplifiers, Multiplexing, Single mode fibers, Raman spectroscopy, Fiber couplers, Space division multiplexing, Waveguide modes, Multiplexers
The capacity of optical transmission systems has increased dramatically since their first deployments in the mid
1970s . 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. With the high levels of integration needed
for economic deployment, space-division multiplexing may exhibit large crosstalk between the supported fiber
modes. We propose to use coherent multiple-input multiple-output (MIMO) digital signal processing (DSP), a
technique widely used in wireless communication, to compensate crosstalk present in spatial multiplexing over
fibers. 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 fibers 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). Details for both experiments will be presented, including the description of the
supported polarization- and spatial modes of the fiber, the mode multiplexers used to launch and detect the
modes, and the MIMO DSP algorithm used to recover the channels.
