Xingyu Zhou

On the capacity improvement achieved by bandwidth-variable transceivers in meshed optical networks with cascaded ROADMs

We investigate the capacity improvement achieved by bandwidth-variable transceivers (BVTs) in fiber links with cascaded reconfigurable optical add and drop multiplexers (ROADMs). It is shown that BVTs with flexibilities in both symbol rate and spectral efficiency enable one to optimize capacity by balancing optical filtering tolerance and signal required signal-to-noise ratio (SNR). Compared with a fixed symbol rate transceiver with standard quadrature amplitude modulations (QAMs), BVTs can increase average capacity by up to 17% in both simulations and experiments.

Modulation format identification aided hitless flexible coherent transceiver

We propose a hitless flexible coherent transceiver enabled by a novel modulation format identification (MFI) scheme for dynamic agile optical networks. The modulation format transparent digital signal processing (DSP) is realized by a block-wise decision-directed least-mean-square (DD-LMS) equalizer for channel tracking, and a pilot symbol aided superscalar phase locked loop (PLL) for carrier phase estimation (CPE). For the MFI, the modulation format information is encoded onto the pilot symbols initially used for CPE. Therefore, the proposed MFI method does not require extra overhead. Moreover, it can identify arbitrary modulation formats including multi-dimensional formats, and it enables tracking of the format change for short data blocks. The performance of the proposed hitless flexible coherent transceiver is successfully evaluated with five modulation formats including QPSK, 16QAM, 64QAM, Hybrid QPSK/8QAM and set-partitioning (SP)-512-QAM. We show that the proposed MFI method induces a negligible performance penalty. Moreover, we experimentally demonstrate that such a hitless transceiver can adapt to fast block-by-block modulation format switching. Finally, the performance improvement of the proposed MFI method is experimentally verified with respect to other commonly used MFI methods.

Equalization-Enhanced Phase Noise in Stokes-Vector Direct Detection Systems

In this work, we provide both numerical and experimental investigations of equalization-enhanced phase noise (EEPN) in Stokes-vector direct detection (SV-DD) systems. We show that the influence of EEPN cannot be neglected in high symbol rate SV-DD systems after transmission over several hundred kilometers of fibers when electronic chromatic dispersion compensation and lasers with linewidths of the order of Megahertz are employed. Simulation results are presented to evaluate the dependence of EEPN effects on laser linewidth, transmission distance, and symbol rate. Experiments are then conducted in a 56-Gbaud 16-quadrature amplitude modulation (QAM) SV-DD system to demonstrate the EEPN-induced performance degradation after 320-km transmission over standard single mode fiber (SSMF).

Multi-subcarrier flexible bit-loading enabled capacity improvement in meshed optical networks with cascaded ROADMs

We propose to use adaptive bit loading based on time-domain hybrid QAM (TDHQ) to maximize the capacity of subcarrier-multiplexing (SCM) systems in meshed optical networks with cascaded reconfigurable optical add and drop multiplexers (ROADMs). Note that the capacity is defined as the achievable net bit rate at the soft-decision FEC threshold of BER = 2 × 10-2 in this work. The capacity improvement is first numerically and experimentally demonstrated in a 4-subcarrier SCM system with an aggregate symbol rate of 34.94 Gbaud. Compared with the conventional SCM system using uniform standard QAM, the proposed system can achieve an average capacity increase of 31.75% and 26.1% over various link conditions in simulations and experiments, respectively. Furthermore, we demonstrate that the proposed SCM system can better approach the channel capacity in the presence of narrow inline optical filtering. An average capacity improvement of 7.59% is also reported over all 17 ROADMs cases from 1 to 17 by simulations at OSNR = 21 dB, compared with its single carrier counterpart using TDHQ.

Low-Complexity One-Step Digital Back-Propagation for Single Span High-Capacity Coherent Transmissions

A low-complexity one-step digital back-propagation (DBP) scheme is proposed to mitigate intrachannel and interchannel fiber nonlinearities in high-capacity single span transmissions. Compared with conventional coherent receivers, the only additional calculation is a low-complexity nonlinear phase noise compensation stage operated between bulk chromatic dispersion (CD) compensation and adaptive equalizer. The required number of real additions and multiplications per sample are only (6N + 2) and (2N + 6), respectively, where N is the number of compensated wavelength-division-multiplexing (WDM) channels. With the proposed one-step DBP, we demonstrate that the nonlinear noise can be suppressed by >60% in single channel experiments and >40% in seven-channel WDM simulations.