Vijitha Herath

Coherent optical communication: Towards realtime systems at 40 Gbit/s and beyond

Coherent optical communication systems promise superior performance, but their realization in real time also poses big technical challenges. After an introduction the potential of coherent optical transmission systems is shown as manifested in offline experiments. Then we present key components that are necessary to realize these systems in real time. We review recent achievements in realtime coherent communication and finally present the results of a realtime QPSK transmission system with a 3x3 coupler in the receiver. The achieved BER at a data rate of 1.4 Gbit/s is well below the FEC threshold.

A performance comparison of nonlinear phase noise tolerant constellation diagrams

Nonlinear phase noise in a fiber optical communication system with 16 Quadrature Amplitude Modulation (16-QAM) has been analyzed in this work. Three new constellation schemes are compared with the post compensation using mean shift rotation method. The new constellations focus on optimizing the ratio between the amplitudes in the 16-QAM and placing the points of 16-QAM in different patterns to minimize the effect of the nonlinear phase noise. In previous works, the best performance was achieved with circular constellation without any additional compensation techniques. The performance is improved by improved circular constellation with optimized radii, five shells circular constellation and improved five-shell circular constellation in this work. The improved five-shell circular constellation diagram improves the bit error rate performance by one order of magnitude from the existing circular constellation at the transmission distance of 3000 km. This result is due to the fact that moderate amplitude signals are more tolerance to nonlinear phase noise.

Real-time Digital Carrier & Data Recovery for a Synchronous Optical Quadrature Phase Shift Keying Transmission System

This paper focuses on the recent progress in coherent optical communication using advanced digital signal processing (DSP) technology. In particular the design of DSP components is presented which are required to realize a 10 Gbaud synchronous optical quadrature phase shift keying (QPSK) transmission system. Additionally the measurement results of a preliminary synchronous QPSK transmission setup with distributed feedback (DFB) lasers and real-time digital in-phase and quadrature (I&Q) receiver at a data rate of 1.4 Gbit/s are shown. The minimum bit error ratio (BER) was 1.7ldr10-5, lower than ever reported before for a real-time system with DFB lasers.

9.5 Gbit/s 20 channel 1∶8 DEMUX for a coherent optical receiver DSPU ASIC input interface

This paper presents the design of an input interface to a CMOS DSPU of an optical coherent QPSK with polarization multiplex receiver. The interface consists of a 20 channel 1:8 DEMUX. Source Coupled FET logic (SCFL) and CMOS logic were used in the design. The interface converts 10 Gbit/s input data rate to 1.25 Gbit/s at the output. The interface gives an open eye diagram at the output up to 9.5 Gbit/s input data rate. The system consumes 7.9 mW/channel.Gb/s. 130 nm bulk CMOS technology was used in the design.

A simple mean clock skew estimation algorithm for clock distribution networks in presence of random process variations and nonuniform substrate temperature

This paper presents a simple algorithm to estimate mean clock skew of a well balanced H-tree clock distribution network. The clock skew is a result of random process variations and nonuniform die temperature. The available literature separately discusses how to estimate clock skew in the presence of random process variations and nonuniform die temperature. This work combines the effects of both phenomena into a single equation using some simplifying assumptions. The derived equation can recursively apply to a well-balanced H-tree clock distribution network to estimate the mean clock skew. The proposed method estimates the mean clock skew with a high degree of accuracy. The delay distributions are assumed normal in the calculation. The computational effort needed for the proposed algorithm is O(N).

Realtime digital golarization and carrier recovery in a polarization-multiplexed optical QPSK transmission

This paper presents a phase estimation algorithm for a synchronous optical QPSK transmission system. The algorithm has been used in a digital signal processing unit for realtime carrier and data recovery. It has further been combined with polarization multiplex and electronic polarization control.

Realtime Optical Synchronous QPSK Transmission with DFB lasers

This paper focuses on the design of the components required to realize a 10 Gbaud synchronous optical quadrature phase shift keying (QPSK) transmission system. These are a 5 bit 10 Gsamples/s analog-to-digital converter and a digital signal processing unit for carrier and data recovery.

Performance comparison of optical receivers using different filtering algorithms and modulation schemes

This paper compares the performance of optical receivers that use phase shift keying (PSK) and low pass filtering techniques. The filtering techniques that are discussed in this paper are Average filtering, Viterbi & Viterbi algorithm and Weighted Viterbi & Viterbi algorithm. Step-by-step implementation of the optical transmission system with AWGN and linear phase noise is discussed. Low pass filtering is done by obtaining the sum of most recent (2N+1) samples with suitable coefficients, where N is the Filter Width of the low pass filter. The suitability of the QPSK modulation scheme is observed by comparing QPSK modulation scheme with BPSK modulation scheme and 8-PSK modulation scheme.

Multiplier-free realtime phase tracking in digital synchronous QPSK receiver for coherent optical detection

A multiplier-free phase estimation algorithm based on the barycentre approach for coherent QPSK detection is presented. A selectivity mechanism was developed that significantly improved its performance. In addition to recently published measurement results, theoretical background and new simulation results are presented.