Christian Wördehoff

Frequency and Phase Estimation for Coherent QPSK Transmission With Unlocked DFB Lasers

This letter presents a hardware-efficient frequency estimator and an advanced phase estimation algorithm capable of tracking the phase noise of a 10-GBaud optical quadrature phase-shift-keying transmission system with standard distributed-feedback lasers in the presence of a frequency mismatch up to 1.2 GHz. This algorithm allows us to implement a digital coherent receiver without an analog frequency control circuit.

Ultra-Fast Adaptive Digital Polarization Control in a Realtime Coherent Polarization-Multiplexed QPSK Receiver

A digital polarization control system integrated in a 2.8 Gbit/s realtime polarization-multiplexed coherent QPSK system compensates for endless polarization changes having a maximum gradient of 3.5 krad/s (12 krad/s) with 1 dB (3.9 dB) loss in receiver sensitivity.

Fast Adaptive Polarization and PDL Tracking in a Real-Time FPGA-Based Coherent PolDM-QPSK Receiver

Fast polarization changes of 40 krad/s and 6-dB polarization-dependent loss (PDL) are tracked in a 2.8-Gb/s real-time coherent quadrature phase-shift keying receiver. The tolerance against fast polarization changes and PDL is measured for different polarization control time constants. The sensitivity penalty of the receiver at a polarization change speed of 40 krad/s is 0.7 dB at bit-error rate (BER) of 1times 10-3, with a BER floor of 6.1times10-7. With an additional PDL of 6 dB, these figures become 1.7 dB and 9.6 times 10-6, respectively.

Hardware-Efficient Phase Estimation for Digital Coherent Transmission With Star Constellation QAM

In order to build optical transmission systems for 100 Gb/s and above on a single wavelength division multiplexing channel, transition from quadrature phase-shift keying (QPSK) to higher quadrature amplitude modulation (QAM) formats seems to be inevitable. Instead of the usual square constellation QAM, we focus on regular star constellation QAM formats and present a hardware-efficient phase-estimation algorithm that is suitable for all regular star constellations, together with preliminary simulation results.

Realtime 40 krad/s polarization tracking with 6 dB PDL in digital synchronous polarization-multiplexed QPSK receiver

Fast polarization changes of 40 krad/s (and 6 dB PDL) are tracked in a 2.8 Gb/s realtime coherent QPSK receiver with 0.7 dB (2.4 dB) sensitivity degradation.

Frequency Estimation and Compensation for Coherent QPSK Transmission with DFB Lasers

We present a hardware-efficient combined frequency and phase estimator. It is capable of tracking phase noise of 10 GBaud optical QPSK transmission systems with DFB lasers and frequency mismatch up to 0.8 GHz.

Realtime phase tracking with multiplier-free barycenter approximation in digital synchronous QPSK receiver for coherent detection

A multiplier-free phase estimation algorithm for coherent QPSK detection is presented. The barycenter approach is modified by a selectivity mechanism that improves performance. In addition to recently published measurement results, theoretical derivations and simulation results are provided.

32-krad/s polarization and 3-dB PDL tracking in a realtime digital coherent polarization-multiplexed QPSK receiver

The tolerance against fast polarization changes and PDL of a digital coherent QPSK receiver is determined in a 2.8 Gb/s realtime polarization-multiplexed transmission experiment. The sensitivity penalty for polarization changes with a speed of 32 krad/s on the Poincare sphere is 0.5 dB.

Nonlinear effect of IQ modulator in a realtime synchronous 16-QAM transmission system

Different operation points of a 16-QAM modulator are tested in realtime. An optimal condition is found which minimizes BER. The FEC threshold was reached for a receiver input power below -30 dBm.

Realtime implementation of square 16-QAM transmission system

Combination of quadrature amplitude modulation with coherent detection is attractive for optical transmission systems, since it permits an increase of data rate without increasing the symbol rate or the required bandwidth. 16-point Quadrature Amplitude Modulation (16-QAM) is most interesting in this context. In-phase (I) and quadrature (Q) signals transmit 2 bit each. Together with polarization division multiplex this amounts to 8 bit/symbol. 2.5 Gbit/s synchronous coherent 16-QAM data is transmitted and received in a realtime intradyne setup with BER below FEC (7% overhead) threshold. A phase noise tolerant feedforward carrier recovery concept with hardware-efficient implementation was tested. Transmission was error-free in a back-to-back electrical test for various PRBS lengths. The carrier recovery does not contain any feedback loop and is therefore highly tolerant against laser phase noise.