Out-of-band exclusive frequency-domain MIMO equalization with the reduced-overlap-and-save method for scalable mode multiplexed signal transmission.

We present computationally-efficient implementation techniques of multiple-input multiple-output (MIMO) frequency-domain equalization (FDE) that deals with a modal dispersion effect underlying in long-haul mode-division-multiplexed (MDM) signal transmission. Computational complexity reduction comes from twofold approaches: the first is out-of-band-exclusive (OBE) FDE for removing redundant computations in MIMO-FDE processing by taking a practical signal bandwidth occupation into account. A second appoarch, referred to as overlapping-reduced (OR) FDE, further reduces the complexity by modifying an overlapping ratio in the overlap-and-save FDE method to below 0.5. We show that OBE-FDE reduces complexity for processing with a larger number of spatial channels compared with the conventional FDE, and that the overlapping ratio is minimized up to 0.25 by the OR-FDE approach. Also shown is an experimental demonstration using the results of three-mode MDM transmission with different inter-mode coupling scenarios, verifying that the complexity is decreased by up to 37 % with negligible deterioration in signal performance by using the proposed method.

MIMO carrier phase recovery for carrier-asynchronous SDM-MIMO reception based on the extended Kalman filter.

We propose a novel phase recovery scheme designed for coherent space division multiplexing (SDM) systems with independently-operated asynchronous light sources. The proposed scheme is based on the approach of the extended Kalman filter and is referred to as multiple-input multiple-output carrier phase recovery (MIMO-CPR). In the minimum mean squared error (MMSE) sense, it simultaneously and optimally obtains estimates of the multiple phase errors arising from phase-unlocked asynchronous light sources. To ensure the scheme's application for SDM fibers with a time-varying property, we also describe a modification to incorporate a MIMO equalization scheme and analyze the computational complexity. The performance of the proposed MIMO-CPR scheme is investigated through numerical simulation, which shows that it has a tolerance for the sum linewidth symbol duration product of up to 3.4 × 10-4, 1.0 × 10-4 and 2.2 × 10-5 for QPSK, 16QAM, and 64QAM signals, respectively, if 1-dB optical signal-to-noise ratio (OSNR) penalty is allowable to achieve BER of 10-3. Transmission experimental results using three spatial modes in a 51-km-long few-mode fiber (FMF) also verify the applicability of the MIMO-CPR scheme to carrier-asynchronous coherent SDM-MIMO systems.

Hybrid cladding-pumped multicore EDFA/Raman amplification for space division multiplexing transmission systems.

We propose and demonstrate a hybrid cladding-pumped multicore erbium-doped fiber amplifier (EDFA) and distributed Raman amplification for space division multiplexing transmission systems. The cladding-pumped multicore EDFA is used to efficiently amplify signals in multiple cores simultaneously, while Raman pumping is used to control loss in each core individually. We construct an in-line amplified 7-core transmission line, and show that distributed Raman amplification can compensate loss variation between cores. Furthermore, we transmit 46 WDM PDM-16QAM signals over a long distance of greater than 1000 km and demonstrate good transmission performance.

Simultaneous nonlinearity mitigation in 92 × 180-Gbit/s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation.

We experimentally demonstrated the simultaneous nonlinearity mitigation of PDM-16QAM WDM signals using complementary-spectrally-inverted optical phase conjugation (CSI-OPC). We achieved reserved-band-less, guard-band-less, and polarization independent OPC based on periodically poled LiNbO3 waveguides. By employing the CSI-OPC, 2.325-THz-band (93 × 25 GHz) complementary spectral inversion was achieved while retaining the original WDM bandwidth. A Q2-factor improvement of over 0.4 dB and a 5120 km transmission with a Q2-factor above the FEC limit were confirmed using a 10-channel WDM transmission at the signal band center and signal band edge. We then demonstrated the mitigation of the nonlinear impairments in a 3840 km long-haul WDM signal transmission for all 92-channel 180-Gbit/s PDM-16QAM quasi-Nyquist-WDM signals.

Filtering-tolerant transmission by the Walsh-Hadamard transform for super-channel beyond 100 Gb/s.

Super-channel transmission is a promising solution to increase the capacity of a channel beyond 100 Gb/s in next-generation optical networks. The performance of a super-channel comprising multiple subcarriers, however, degrades if optical filtering distortions occur in particular subcarriers. In this paper, we propose a method that improves super-channel performance by dispersing the distortions over all subcarriers. We also numerically demonstrate that the method effectively mitigates the filtering-induced penalty suffered by super-channels.

Experimental demonstration of PDL penalty reduction by wavelength-interleaving transmission.

Experiments and numerical simulation demonstrate the validity of wavelength-interleaving (WI) transmission for reducing the penalty induced by polarization dependent loss (PDL) through the method of extreme value statistics. It is confirmed that applying the WI technique across n (n>1) channels can effectively reduce PDL-induced Q-penalty or outage probability.

1000-km 7-core fiber transmission of 10 x 96-Gb/s PDM-16QAM using Raman amplification with 6.5 W per fiber.

We demonstrate 7-core fiber transmission of 10 x 96-Gb/s PDM-16QAM signals over 1000-km using distributed Raman amplification (DRA). DRA gain of 9-12 dB and equivalent noise figure of less than 1 dB are achieved in all cores. We also prove the feasibility of high power multi-core fiber transmission with per fiber power of 6.5 W.