Irshaad Fatadin

Blind Equalization and Carrier Phase Recovery in a 16-QAM Optical Coherent System

Blind equalization and carrier phase recovery in a simulated 14 Gbaud 16-QAM optical coherent system are investigated. Equalization techniques to compensate for linear transmission impairments are presented using the constant modulus algorithm (CMA), the recursive least-squares (RLS)-CMA, and the radius directed equalization (RDE). With 7 T/2-spaced taps, the RDE and the RLS-CMA can compensate up to 1000 ps/nm of CD in the 16-QAM coherent system with performances comparable to the decision-directed (DD) equalizer. We show that the RDE is a promising technique for blind equalization in a 16-QAM coherent system with lower complexity than the RLS-CMA. Blind carrier phase recovery is investigated in a decision-directed-mode. We show that the blind carrier phase recovery algorithm can recover the Square-16-QAM constellation for laser beat linewidths of DeltanuTs ~ 10-4 in a polarization-multiplexed (POLMUX) 16-QAM coherent system with the RDE algorithm giving better overall performance than the CMA when compensating for CD and differential group delay (DGD). Finally, the dynamical characteristics of the equalizers to track endless polarization rotations are discussed. With the adaptation parameters optimized, the equalizers can track angular rate of rotation ~ 105 rad/s.

Laser Linewidth Tolerance for 16-QAM Coherent Optical Systems Using QPSK Partitioning

The laser linewidth tolerance for 16-ary quadrature amplitude modulation (16-QAM) coherent optical systems is investigated using a quaternary phase-shift-keying (QPSK) partition scheme. The different stages needed to partition the square-16-QAM into QPSK constellations for carrier phase estimation are discussed. It is shown that at 1 dB above sensitivity at a bit-error rate of 10-3, a combined linewidths symbol duration product of 1 × 10-4 is tolerable. The performance of the algorithm with different bits resolution in the analog-to-digital converter is also presented.

Compensation of Quadrature Imbalance in an Optical QPSK Coherent Receiver

This letter explores the Gram-Schmidt orthogonalization procedure (GSOP) for compensation of quadrature imbalance in an optical 90deg hybrid. We present computer simulations for an optical QPSK communication system using a digital coherent receiver and investigate the impact of quadrature imbalance on the required optical signal-to-noise ratio for the receiver and the frequency estimation algorithm. We then demonstrate the improvement which can be achieved using the GSOP, including the impact of quantization in the digital coherent receiver. Finally, we show that the GSOP can equally be applied to polarization-division multiplexed systems, applying the GSOP in conjunction with the constant modulus algorithm to demultiplex a PDM-QPSK signal.

Compensation of Frequency Offset for 16-QAM Optical Coherent Systems Using QPSK Partitioning

The performance of a feed-forward carrier frequency recovery algorithm for 16-quadrature amplitude modulation (QAM) optical coherent systems is investigated using the quaternary phase-shift keying (QPSK) partition scheme. The frequency estimation technique, which employs only a subset of the 16-QAM symbols with the QPSK partition, is demonstrated to be effective when applied prior to the carrier phase recovery. Frequency offset of 10-1 times the symbol rate is shown to be tolerable at a bit-error rate (BER) of 10-3 with combined linewidths symbol duration products of 10-4 .

Impact of phase to amplitude noise conversion in coherent optical systems with digital dispersion compensation

The impact of phase to amplitude noise conversion for QPSK, 16-QAM, and 64-QAM coherent optical systems are investigated with electronically-compensated chromatic dispersion (CD). The electronic equalizer is shown to convert the phase noise from the local oscillator (LO) to amplitude noise, limiting the amount of CD that can ideally be compensated digitally. The simulation results demonstrate that the performance of coherent systems can significantly be degraded with digitally compensated CD and LO phase noise. The maximum tolerable LO linewidth is also investigated for the different modulation formats and found to become increasingly stringent for longer transmission distance and higher symbol rate.

Compensation of Frequency Offset for Differentially Encoded 16- and 64-QAM in the Presence of Laser Phase Noise

The compensation of frequency offset for differentially encoded 16- and 64-ary quadrature amplitude modulation (QAM) in the presence of laser phase noise is investigated. Differential encoding is employed to solve the four-fold phase ambiguity problem in a nondata-aided transmission system with square QAM constellations. Simulation results show that frequency offset and phase noise can successfully be compensated using a second-order digital filter loop for the square QAM constellations.

Numerical simulation of intensity and phase noise from extracted parameters for CW DFB lasers

A self-consistent numerical approach is demonstrated to analyze intensity and phase noise from experimentally extracted parameters for a continuous-wave distributed feedback (DFB) laser. The approach takes into account the intrinsic fluctuations of the photon number, carrier number, and phase. Values for the parameters appearing in the rate equations are extracted from the measured relative intensity noise spectra and linewidth of the laser. The simulation of the frequency spectra of intensity and phase noise of the DFB laser are performed by fast Fourier transform and exhibit good agreement with experimental results. The model presented here can readily be extended for the purpose of system simulations

Influence of Pulse Shape in 112-Gb/s WDM PDM-QPSK Transmission

In this work, we investigated the influence of pulse shape on the transmission performance of polarization-division-multiplexed quadrature-phase-shift keying modulation format at 112 Gb/s in a ten-channel wavelength-division-multiplexed (WDM) transmission experiment with 50-GHz channel spacing. Nonreturn-to-zero (NRZ) and return-to-zero with 50% duty cycle (RZ50) were compared. RZ50 was found to have better performance in both single-channel and WDM experiments. Compared with NRZ, the use of RZ50 yielded an increase in reach from 6560 to 7760 km in the single-channel experiment (corresponding to an increase in reach by 18%); in the case of WDM. the reach was extended from 5920 to 7360 km (corresponding to a 24% increase in reach).

Carrier Phase Recovery for 16-QAM Using QPSK Partitioning and Sliding Window Averaging

This letter presents the carrier phase recovery for 16-ary quadrature amplitude modulation (16-QAM) optical coherent systems using the quaternary phase-shift keying (QPSK) partitioning with sliding window averaging and differential decoding. We assess the increase in linewidth tolerance achievable with sliding window averaging as opposed to block averaging. Simulation results demonstrate that combined linewidth symbol duration product, Δv · T_s, 10^-4 is tolerable at the target bit error ratio (BER) of 10^-2 and 10^-3 for a penalty of 0.6 and 0.8 dB, respectively, compared with the theoretical limit with differential decoding. The impact of analog-to-digital converter (ADC) resolution on the performance of the QPSK partitioning algorithm is also investigated. Finally, the performance of the algorithm using the measured phase noise for a distributed feedback (DFB) laser is presented for different values of Δv · T_s. We show that for Δv · T_s > 10^-4, the penalty of block averaging is > 0.5 dB with respect to sliding window averaging at the target BER of 10^-3 with the measured phase noise. The degradation associated with block averaging at the target BER of 10^-2 is shown to be less significant compared to sliding window averaging.

Differential carrier phase recovery for QPSK optical coherent systems with integrated tunable lasers

The performance of a differential carrier phase recovery algorithm is investigated for the quadrature phase shift keying (QPSK) modulation format with an integrated tunable laser. The phase noise of the widely-tunable laser measured using a digital coherent receiver is shown to exhibit significant drift compared to a standard distributed feedback (DFB) laser due to enhanced low frequency noise component. The simulated performance of the differential algorithm is compared to the Viterbi-Viterbi phase estimation at different baud rates using the measured phase noise for the integrated tunable laser.