Wing-Chau Ng

Wideband wavelength conversion of 16 Gbaud 16-QAM and 5 Gbaud 64-QAM signals in a semiconductor optical amplifier

We demonstrate wavelength conversion based on four-wave mixing in a semiconductor optical amplifier of signals with quadrature amplitude modulation (QAM). We first demonstrate wavelength conversion of 16 Gbaud 16-QAM signals over the entire C-band using two co-polarized pumps with low power penalty at the forward error correction threshold (FEC) for a wide range of input optical-signal-to-noise-ratio (OSNR). We also demonstrate for the first time wavelength conversion of 5 Gbaud 64-QAM signals in a semiconductor optical amplifier with bit-error rate below the FEC threshold over the entire C-band and investigate the dependence of the power penalty on input OSNR with a single pump configuration.

BER Performance of Coherent Optical Communications Systems Employing Monolithic Tunable Lasers With Excess Phase Noise

We analyze the bit error rate (BER) performance of optical coherent communication systems employing multi-section monolithic tunable lasers with large excess phase noise. The BER of the coherent system utilizing a second-order decision-directed digital phase-locked loop (DD-PLL) for phase tracking was computed numerically. Experimental results have also been demonstrated with a 16-quadratic-amplitude modulation coherent system deploying a sampled-grating distributed Bragg reflector laser at 16 Gbaud that confirms the performance of the second-order DD-PLL in tracking the excess phase noise.

Reduction of MIMO-FIR taps via SOP-estimation in stokes space for 100 Gbps short reach applications

We propose a novel parallelizable DSP architecture based on coarse Stokes space SOP estimation, which reduces the number of MIMO cross taps, and avoids timing recovery failure due to differential group delay at certain SOPs.

An optimized 16-QAM constellation for mitigating impairments of phase noise and limited transmitter ENOB in optical coherent detection systems

A new 16-QAM constellation is proposed combatting laser phase noise and limited resolution of high speed digital-to-analog converters. Experimental results show small but consistent performance improvement at low and high baud-rates compared with square and other reported constellations.

Overcoming Phase Sensitivity in Real-Time Parallel DSP for Optical Coherent Communications: Optically Filtered Lasers

Implementation of real-time giga-Baud optical coherent systems for single-carrier higher-level modulation format such as 64-quadrature amplitude modulation (QAM) depends heavily on phase tracking. For offline digital signal processing, decision-directed phase recovery is performed at symbol rate with the best performance and the least computational effort compared among other best-known algorithms. However, in real-time systems, hardware parallelization and pipelining delay on feedback path pose stringer requirement on the linewidth, or the frequency noise spectral level of laser sources. This leads to the paucity of experiments demonstrating real-time phase tracking for 64- or higher QAM. In this paper, we experimentally investigate the impact of optically-filtered lasers on parallel and pipelined phase tracking in a single-carrier 5 Gbaud 64-QAM back-to-back coherent system. For parallelization levels higher than 24, the optically-filtered laser shows more than 2 dB improvement in optical signal-to-noise ratio penalty compared to that of the same laser without optical filtering.

Tracking excess noise from a monolithic tunable laser in coherent communication systems

We demonstrate the tracking of white-FM noise and 1/f^α excess noise from a SGDBR laser in a coherent system. The performance of 2^nd-order phase-locked loop was experimentally examined in 16-QAM system at 16 Gbaud.

Optically-filtered laser source enabling improved phase tracking in coherent transmission systems

We report phase tracking performance using an external cavity laser with and without optical filtering of high-frequency phase noise with an ultra-narrowband fiber Bragg grating. Phase tracking improvement is characterized when using a commercial, integrated coherent receiver. Noise suppression reduces tracking variance in a Wiener filter by 5 dB.

Parallel and Pipelined Decision-Directed Phase Recovery for 64-QAM in the Presence of Sinusoidal Tones

We experimentally investigate the impact of sinusoidal laser phase on parallel and pipelined decision-directed phase recovery in a 5 Gbaud 64-QAM system, including the effects of frequency offset compensation and equalization.

Impact of Sinusoidal Tones on Parallel Decision-Directed Phase Recovery for 64-QAM

Coherent optical communications at Gbaud rates require parallelization in digital signal processing, however the feedback delay introduced by pipelining challenges phase recovery. Parallelized phase recovery creates a greater sensitivity to residual frequency offset induced by the presence of sinusoidal tones in the source. We experimentally investigate the impact of sinusoidal laser phase noise on parallel decision-directed phase recovery in a 5 Gb 64-quadrature amplitude modulation system, including the effects of frequency offset compensation and equalization.