Lei Li

Multiplier-Free Intrachannel Nonlinearity Compensating Algorithm Operating at Symbol Rate

Intrachannel nonlinearity is considered a major distortion in high-capacity transmission systems particular for the case without inline optical chromatic dispersion compensation. In this paper, a low-complexity intrachannel nonlinear compensator operating at the symbol rate is proposed based on the nonlinear perturbation predistortion for the dual-polarization quadrature phase-shift keying (DP-QPSK) systems. Compared with the widely studied backpropagation algorithm, the proposed algorithm achieves comparable performance with significantly reduced complexity and halved sampling speed in digital signal processing and digital-to-analog converters. The proposed algorithm is demonstrated in a 43 Gb/s DP-QPSK transmission experiment over 1500 km. In addition to the experimental demonstration, numerical simulation verifies that the proposed algorithm is quite robust by tolerating significant uncertainties of link parameters and span-by-span inhomogeneity in the links.

Experimental demonstration of 448-Gbps+ DMT transmission over 30-km SMF

We have experimentally demonstrated 469-Gbps transmission over 30-km SMF using DMT on a LAN-WDM system for 400-Gbps Ethernet. Only four directly modulated lasers and direct detectors as optical devices were necessary to achieve the results.

100 Gb/s optical IM-DD transmission with 10G-class devices enabled by 65 GSamples/s CMOS DAC core

100 Gb/s optical IM-DD transmission over 10 km SMF is experimentally demonstrated by using discrete multi-tone and CMOS DAC core. The limiting factors towards further reach extension are investigated.

Implementation efficient nonlinear equalizer based on correlated digital backpropagation

One stage per span is commonly considered as the lower boundary for implementation complexity of nonlinear equalizer based on digital backpropagation. The proposed method overcomes this boundary and improves the efficiency by about four times.

Wide-range, Accurate and Simple Digital Frequency Offset Compensator for Optical Coherent Receivers

We present a digital frequency offset compensator for optical coherent QPSK receivers. The compensation accuracy reaches ± 30 MHz while the range covers ± 5 GHz. This would realize coherent receivers with common off-the-shelf tunable lasers.

Nonlinear polarization crosstalk canceller for dual-polarization digital coherent receivers

A novel method is proposed to compensate polarization crosstalk induced by interchannel nonlinearity. Experiments have demonstrated that the proposed method provides over 1 dB performance improvement for 112 Gb/s dual-polarization QPSK coherent receivers.

Simple Fiber Model for Determination of XPM Effects

Previous investigations have revealed that the impairments of the cross-phase modulation (XPM) in dense wavelength-division multiplexing (DWDM) systems include two aspects: the XPM-induced phase noise and the XPM-induced polarization scattering. Such XPM phenomena are strongly dependent on the transmission system configurations and are nonintuitive. In this paper, a simple fiber model is proposed to facilitate the determination of XPM effects. By employing this model, the phase noise and polarization scattering are calculated based on the system configurations and the time-consuming computation by the split step Fourier method is avoided. The proposed model is verified by the dual polarization quadrature phase-shift keying coherent DWDM experiments and simulations in terms of the variance and autocorrelations of phase noise and polarization crosstalk as well as their dependence on relative polarization states and the overall Q-impairment. The proposed model helps deeper analysis of XPM phenomena, and eventually, leads to development of various XPM mitigation methods through transmission system design and coherent receiver DSP.

Laser-Linewidth-Tolerant Feed-Forward Carrier Phase Estimator With Reduced Complexity for QAM

An improved feed-forward carrier phase estimation scheme with further complexity reduction is presented and numerically verified for quadrature amplitude modulation (QAM) formats. Based on the blind phase search (BPS) algorithm, a general two-stage configuration is first derived by removing several restrictions assumed in the prior art. Two guidelines for determining the design parameters are then proposed to minimize the computational complexity while mitigating the pattern effect and maintaining the tolerance to laser linewidth. Taking square 64QAM as an example, the computational efforts can be reduced by a factor of 4 while keeping the same tolerance to laser linewidth as compared to a single-stage BPS estimator.

Improvements to Digital Carrier Phase Recovery Algorithm for High-Performance Optical Coherent Receivers

The Viterbi-and-Viterbi (V-V) algorithm is widely used to recover the carrier phase in optical digital coherent receivers. For simplicity, the basic V-V algorithm assumes constant carrier phase within the average duration. However, this basic assumption is probably violated by factors such as laser frequency offset and nonlinear XPM. In order to improve the basic V-V carrier phase recovery, five methods are introduced, verified, and analyzed. All these methods are compatible with parallel implementation that is mandatory for a realistic DSP circuit. The Q-improvement brought by each algorithm is analyzed together with the complexity of each. Among the five methods, the laser frequency offset compensation expands the tolerable frequency offset to 0.37 symbol rate, and the optimum weighted averaging in conjunction with normalization processing improves the Q-value by 2 dB under severe XPM condition.

Multiplier-free Phase Recovery for Optical Coherent Receivers

We demonstrate a multiplier-free pre-decision phase recovery for optical coherent receivers. The complexity is much lower than previous mth power phase recovery, while simulation and experiment show that the performance is virtually same.