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.
We summarize theoretical formulae for determining the intrinsic loss of few-mode fibers and compare intrinsic losses among step-index and parabolic graded-index few-mode fibers (FMF) with different dopants, and with single-mode fibers (SMF). The low-loss characteristics of pure-silica-core FMFs, in comparison with SMFs, give rise to additional motivations for pursuing mode-division multiplexed transmission over parallel-SMF transmission, and quasi-single-mode (QSM) transmission over ultra-large-area SMF transmission.
We report the design of 12-LP-mode (21-spatial-mode) fiber with a low differential mode group delay (DMGD), a large effective area, and a low bending loss that is adapted to a mode-division-multiplexing system over the C+L band. Based on the designed fiber parameters, we characterize the few-mode fiber (FMF) with the DMGD, an effective area, and a bending loss. Over the C+L band, the maximum DMGD is 0.106 ps/m, and the effective area is in the range of 150∼485 μm 2. The bending loss of the designed FMF reduces to zero when the bending radius is greater than 9 mm, and the DMGD is below 0.0413 ps/m at the wavelength of 1550 nm.
We experimentally demonstrate adaptive variable-forgetting-factor (VFF) recursive-least-square frequency-domain equalization (RLS-FDE) for mode-division multiplexing. The VFF-RLS-FDE algorithm improves convergence speed as transmission distance increases. For MDM transmission over a 1,000-km few-mode fiber, the convergence speed was increased by 18.7 times in comparison with LMS-FDE. In the meantime, the convergence error performance of VFF-RLS is also much superior to LMS while fixed-forgetting-factor RLS has a 0.2 dB penalty compared with LMS. The proposed VFF-RLS algorithm achieves better performances than conventional RLS in terms of convergence speed and convergence error.
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