Abstract
Cylindrical vector (CV) mode as the eigenmodeof optical fibers is a strong candidate for high-capacity mode-division multiplexing (MDM) systems. Based on the Kramers–Kronig (K-K) reception, we experimentally demonstrate a 448-Gb/s MDM transmission system using four CV eigenmodes (
$\boldsymbol{|l|=2}$
) over 3-km few-mode fiber (FMF), each mode carrying 28-GBaud 16-quadrature amplitude modulation (16-QAM) signals. In conventional MDM systems with
${\boldsymbol{M}}$
channels, blind
${\boldsymbol{N \times {N}}}$
${\boldsymbol{(N\leq {M})}}$
multiple-input multiple-output (MIMO) technique is implemented to demultiplex and equalize
${\boldsymbol{N}}$
channels after transmission. However, without appropriately initializing finite impulse response (FIR) filter taps in MIMO, the singularity may show up and result in
${\boldsymbol{N}}$
MIMO outputs converged to
${\boldsymbol{P}}$
${\boldsymbol{(P< {N})}}$
independent outputs with information missing. Here we employ the
${\boldsymbol{4\times 4}}$
multi-user constant modulus algorithm (MU-CMA) technique to avoid such singular problems in our MDM demonstration. To verify the reliability of MU-CMA, we test 7,408 signal sets (598 static sets and 13-hour 6,810 dynamic sets), and none of the singularities is observed. Specifically, 390 static sets out of 598 are added with an extra 3-dB mode-dependent loss (MDL), and the results of these sets prove the MDL-resistance of MU-CMA. Furthermore, the bit error rate (BER), MDL, and mode coupling for each mode of the proposed system are also investigated statically and dynamically. In the static case without extra MDL, all four CV mode channels can achieve
${\boldsymbol{7\%}}$
hard-decision forward error correction (FEC) threshold at BER of
${\boldsymbol{3.8\times 10^{-3}}}$
, under certain optical-to-signal noise ratio (OSNR). In dynamic cases, the averaged BER of four CV modes can also approach the FEC BER threshold. These experimental results verify the robustness of K-K reception and MU-CMA in CV-mode MDM systems. They also highlight the reliability and stability of CV-mode MDM systems, which show the enormous potential of the CV-mode MDM scheme employed in high-speed short-reach networks.
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