Receiver in-phase and quadrature skew tolerant baud-rate timing recovery scheme for short-reach coherent optical interconnection.

A baud-rate sampling timing recovery (TR) scheme with receiver IQ skew tolerance is proposed and experimentally demonstrated. The proposed scheme performs independent TR for the in-phase and quadrature (IQ) tributary signals, thereby tracking the sampling phase error while naturally compensating for receiver IQ skew. The robustness of the IQ-independent TR to frequency offset (FO) and phase noise is theoretically analyzed. To address IQ misalignment caused by the IQ-independent TR, the use of pseudo-noise (PN) sequences for IQ frame synchronization is proposed. The proposed scheme achieves accurate timing recovery with hardware-efficient baud-rate sampling in the presence of receiver IQ skew, laying the foundation for stable performance of subsequent baud-rate equalization. The performance of the scheme is validated in a 56 GBaud polarization division multiplexed (PDM) 16QAM coherent experimental system. Experimental results demonstrate that the proposed scheme achieves similar BER performance to the modified Gardner + real-valued multiple-input multiple-output (RVMIMO) (@2 SPS) scheme. Moreover, the proposed scheme exhibits robustness to arbitrary IQ skew compared to the ABSPD + RVMIMO (@1 SPS) scheme.

Cost-effective and robust DSP scheme for a short-reach coherent system in the presence of transmitter IQ skew and chromatic dispersion.

A cost-effective and robust digital signal processing (DSP) scheme is proposed and demonstrated experimentally in a coherent 61 GBaud PDM 16QAM system. In our scheme, multi-stage DSP blocks are used to deal with channel effects, transceiver in-phase and quadrature (IQ) skew, and phase noise. A 4×4 real-valued multiple-input multiple-output (RV-MIMO) with N1 taps is for polarization recovery and receiver IQ skew calibration. After frequency offset compensation, two 2×2 RV-MIMO with N2 taps are used to compensate for chromatic dispersion (CD), inter-symbol interference, transmitter IQ skew, and phase noise. Finally, the residual phase noise is eliminated by the maximum likelihood (ML) estimator. The experimental results indicate that the proposed scheme provides better received optical power sensitivity and CD tolerance than the existing simplified DSP schemes. In addition, the proposed scheme can tolerate transmitter IQ skew up to 7 ps in a 10 km case, which outperforms both simplified and conventional DSP schemes. Meanwhile, the proposed scheme can keep the same transceiver IQ skew and CD tolerance and has reduced complexity by 25% after 10 km links, compared to 4×4 RV-MIMO followed by a transmitter skew compensator. To the best of our knowledge, the proposed scheme is the most cost-effective solution for a high baud rate datacenter interconnects where transmitter IQ skew and CD have to be dealt with.

Cost-effective digital coherent short-reach transmission system with D8QAM and low-complexity DSP.

We propose a cost-effective digital coherent scheme with low-complexity digital signal processing (DSP) for short-reach optical interconnection. Differential 8-ary quadrature amplitude modulation (D8QAM) with 1-decision-aided adaptive differential decoding bypasses carrier recovery and enables cycle-slip-free operation. We experimentally demonstrate that the receiver sensitivity of 400-Gb/s D8QAM is insensitive to the laser type, and is the same as 400-Gb/s 16QAM in the case of 2-km transmission with a distributed feedback (DFB) laser. The proposed adaptive equalizer (AEQ) using real-valued finite impulse response (FIR) filters and shorter tap lengths for the real-imaginary filters allows hardware-efficient implementation with high robustness to the receiver-side timing skew. In the case of 400-Gb/s D8QAM 10-km transmission, our AEQ achieves comparable performance as conventional 4×4 real-valued multi-input multi-output (MIMO) and the existing simplified AEQs with complexity reduction of 50% and 14% respectively.

Chromatic dispersion mitigation using a SEFDM-based diversity technique for IM/DD long reach optical links.

Spectral efficient frequency division multiplexing (SEFDM) can offer a higher spectral efficiency (SE) than orthogonal frequency division multiplexing (OFDM). In this work, we propose a diversity technique based on SEFDM for beyond 100-Gb/s optical intensity modulation and direct detection (IM/DD) long reach (LR) applications. We mathematically demonstrate that the self-created inter-carrier interference of SEFDM signals can be reused to achieve a diversity gain on each sub-carrier and, in turn, improve the tolerance to power fading induced by chromatic dispersion (CD) in IM/DD LR links. Based on the proposed diversity technique, we further demonstrated a 112-Gb/s SEFDM transmission over 80-km standard single-mode fiber, using only 28-GHz bandwidth and modulation format of up to 16-QAM. Experimental results show that SEFDM with the proposed diversity technique performs robust against CD effects and outperforms the conventional OFDM with adaptive bit and power loading of the same bandwidth and data rate, which validates the superiority of the proposed SEFDM in optical IM/DD LR transmissions.

Advanced modulation formats for 400-Gbps short-reach optical inter-connection.

Besides the long-haul optical networks covering over thousands of kilometers for backbone transmission, short reach optical networks (SR-ONs) are widely deployed in metro-area for aggregation and accessing. The SR-ONs include the metro optical transport networks (Metro-OTN), optical access networks or other optical inter-connection systems with even shorter distance. As predicted, the growing bandwidth demanding from SR-ONs will be much more than that from the long-haul optical networks in the near future. Besides, there are tremendous amounts of optical terminals and end-users in SR-ONs compared with the long-haul transmission systems and thus will induce large cost and huge energy consumption. So, the power and cost efficiency should be the key consideration for SR-ONs besides the transmission performance. To improve the power and cost efficiency in SR-ONs, advanced modulations and detection techniques based on low power, low cost and integrated optical modulators should be utilized. In this paper, different advanced modulation formats have been discussed. 56Gbps PAM4, 112Gbps poly-binary and 100Gbps DMT that can be used to realize 400-Gbps SR-ONs for different applications have also been demonstrated respectively. In addition, low-cost and low-power opto-electronic components suitable for SR-ONs, the impairments induced by all kinds of defects and bandwidth limitation of opto-electronic components and the corresponding compensation techniques based on DSP algorithms have also been discussed in the experiments.