Alan Pak Tao Lau

The University of Hong Kong, Hong Kong, Hong Kong

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Publications (117)198.17 Total impact

  • IEEE Photonics Technology Letters 08/2015; 27(15):1-1. DOI:10.1109/LPT.2015.2432793 · 2.18 Impact Factor
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    ABSTRACT: Optical signal-to-noise ratio (OSNR) monitoring is indispensable for ensuring robust and flexible optical networks that provide failure diagnosis, dynamic lightpath provisioning and modulation format adaptation. We propose and experimentally demonstrate a low-cost, modulation-format-independent OSNR monitoring scheme utilizing reduced-complexity coherent receptions, electrical filtering and radio frequency (RF) power measurements. By measuring the RF power of the coherently received baseband signals at three different frequency components, the proposed OSNR monitor is also insensitive to spectral narrowing induced by cascaded wavelength selective switches (WSSs). We experimentally demonstrate accurate data-format-transparent and filtering-effect-insensitive OSNR monitoring for 25-Gbaud dual-polarization (DP-) transmissions with QPSK, 16-QAM and 64-QAM signals over various distances with different amount of filtering effects by cascaded WSSs. We further characterize the influence of different system parameters, such as the bandwidth of the electrical low-pass filter, the laser frequency offset and laser linewidth on the accuracy of the proposed OSNR monitor. The robustness of the proposed OSNR monitoring scheme to fiber nonlinearities, calibration parameter mismatches and variations of WSS parameters are also investigated.
    Optics Express 06/2015; 23(12):15971. DOI:10.1364/OE.23.015971 · 3.49 Impact Factor
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    ABSTRACT: High spectral efficiency modulation format based unrepeatered transmission systems using distributed Raman amplifier (DRA) have attracted much attention recently. To enhance the reach and optimize system performance, careful design of DRA is required based on the analysis of various types of impairments and their balance. In this paper, we study various pump RIN induced distortions on high spectral efficiency modulation formats. The vector theory of both 1st and higher-order stimulated Raman scattering (SRS) effect using Jones-matrix formalism is presented. The pump RIN will induce three types of distortion on high spectral efficiency signals: intensity noise stemming from SRS, phase noise stemming from cross phase modulation (XPM), and polarization crosstalk stemming from cross polarization modulation (XPolM). An analytical model for the statistical property of relative phase noise (RPN) in higher order DRA without dealing with complex vector theory is derived. The impact of pump RIN induced impairments are analyzed in polarization-multiplexed (PM)-QPSK and PM-16QAM-based unrepeatered systems simulations using 1st, 2nd and 3rd-order forward pumped Raman amplifier. It is shown that at realistic RIN levels, negligible impairments will be induced to PM-QPSK signals in 1st and 2nd order DRA, while non-negligible impairments will occur in 3rd order case. PM-16QAM signals suffer more penalties compared to PM-QPSK with the same on-off gain where both 2nd and 3rd order DRA will cause non-negligible performance degradations. We also investigate the performance of digital signal processing (DSP) algorithms to mitigate such impairments.
    Optics Express 05/2015; 23(9). DOI:10.1364/OE.23.011838 · 3.49 Impact Factor
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    ABSTRACT: We present a method to mitigate the chromatic dispersion (CD)-induced power fading effect (PFE) in high-speed and short-reach carrier-less amplitude and phase (CAP) systems using the degenerate four-wave mixing (DFWM) effect and a decision feedback equalizer (DFE). Theoretical and numerical investigations reveal that DFWM components produced by the interaction between the main carrier and the signal sideband help to mitigate PFE in direct detection systems. By optimizing the launch power, a maximum reach of 60 km in single mode fiber (SMF-e + ) at 1530nm is experimentally demonstrated for a 40 Gbit/s CAP32 system. In addition, we study the performance of a decision feedback equalizer (DFE) and a traditional linear equalizer (LE) in a channel with non-flat in-band frequency response. The superior PFE tolerance of DFE is experimentally validated, and thereby, the maximum reach is extended to 80 km. To the best of our knowledge, this is the twice the longest transmission distance reported so far for a single-carrier 40 Gbit/s CAP system around 1550 nm.
    Optics Express 05/2015; 23(9). DOI:10.1364/OE.23.011412 · 3.49 Impact Factor
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    ABSTRACT: A novel low-complexity two-stage digital feed-forward carrier phase estimation algorithm based on the rotation of constellation points to remove phase modulation for a 64-ary quadrature amplitude modulation (QAM) system is proposed and analyzed both experimentally and through numerical simulations. The first stage is composed of a Viterbi and Viterbi (V&V) block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge (TE) symbols. The second stage applies the V&V algorithm after the removal of phase modulation through rotation of constellation points. Comparison of the proposed scheme with constellation transformation, blind phase search (BPS) and BPS+MLE (maximum likelihood estimation) algorithm is also shown. For an OSNR penalty of 1 dB at bit error rate of $10^{-2}$ , the proposed scheme can tolerate a linewidth times symbol duration product ( $\Delta \hbox{$\nu $}\cdot T_{s}$ ) equal to $3.7\times 10^{-5}$ , making it possible to operate 32-GBd optical 64-QAM systems with current commercial tunable lasers.
    Journal of Lightwave Technology 05/2015; 33(9):1-1. DOI:10.1109/JLT.2015.2402441 · 2.86 Impact Factor
  • Zhaohui Li · Qi Sui · Tao Gui · Alan Pak Tao Lau
    SPIENewsroom 04/2015; DOI:10.1117/2.1201504.005808
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    ABSTRACT: The Silences of the Archives, the Reknown of the Story. The Martin Guerre affair has been told many times since Jean de Coras and Guillaume Lesueur published their stories in 1561. It is in many ways a perfect intrigue with uncanny resemblance, persuasive deception and a surprizing end when the two Martin stood face to face, memory to memory, before captivated judges and a guilty feeling Bertrande de Rols. The historian wanted to go beyond the known story in order to discover the world of the heroes. This research led to disappointments and surprizes as documents were discovered concerning the environment of Artigat’s inhabitants and bearing directly on the main characters thanks to notarial contracts. Along the way, study of the works of Coras and Lesueur took a new direction. Coming back to the affair a quarter century later did not result in finding new documents (some are perhaps still buried in Spanish archives), but by going back over her tracks, the historian could only be struck by the silences of the archives that refuse to reveal their secrets and, at the same time, by the possible openings they suggest, by the intuition that almost invisible threads link here and there characters and events.
    Optics Express 04/2015; 23(7). DOI:10.1364/OE.23.008409 · 3.49 Impact Factor
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    ABSTRACT: We study long-haul Quasi-Single-mode (QSM) systems in which signals are transmitted in the fundamental modes of a few-mode fiber (FMF) while keeping other system components such as amplifiers and receivers are kept single-moded. The large-effective-area nature of the FMF fundamental modes improves system nonlinear tolerance in the expense of mode coupling along FMF transmissions which induces multi-path interference (MPI) and needs to be compensated. We analytically investigate 6-spatial-polarization mode QSM transmission systems in presence of MPI and show that in the weak coupling regime, the QSM channel is a Gaussian random process in frequency. MPI compensation filters are derived and performance penalties due to MPI and signal loss from higher-order modes are characterized. We also experimentally demonstrate 256 Gb/s polarization multiplexed (PM)-16-QAM QSM transmissions over a record distance of 2600 km with 100-km span using decision directed least mean square (DD-LMS) algorithm for MPI compensation.
    Optics Express 02/2015; 23(3). DOI:10.1364/OE.23.003156 · 3.49 Impact Factor
  • Kangping Zhong · Xian Zhou · Tao Gui · Li Tao · Yuliang Gao · Wei Chen · Jiangwei Man · Li Zeng · Alan Pak Tao Lau · Chao Lu
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    ABSTRACT: Advanced modulation formats combined with digital signal processing and direct detection is a promising way to realize high capacity, low cost and power efficient short reach optical transmission system. In this paper, we present a detailed investigation on the performance of three advanced modulation formats for 100 Gb/s short reach transmission system. They are PAM-4, CAP-16 and DMT. The detailed digital signal processing required for each modulation format is presented. Comprehensive simulations are carried out to evaluate the performance of each modulation format in terms of received optical power, transmitter bandwidth, relative intensity noise and thermal noise. The performance of each modulation format is also experimentally studied. To the best of our knowledge, we report the first demonstration of a 112 Gb/s transmission over 10km of SSMF employing single band CAP-16 with EML. Finally, a comparison of computational complexity of DSP for the three formats is presented.
    Optics Express 01/2015; 23(2). DOI:10.1364/OE.23.001176 · 3.49 Impact Factor
  • Yuliang Gao · Elwin Ha · Alan Pak Tao Lau · Chao Lu · Xiaogeng Xu · Liangchuan Li
    Optics Express 12/2014; 22(25):31167-79. DOI:10.1364/OE.22.031167 · 3.49 Impact Factor
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    Xian Zhou · Kangping Zhong · Yuliang Gao · Chao Lu · Alan Pak Tao Lau · Keping Long
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    ABSTRACT: Modulation format independence is one of the key challenges in digital signal processing (DSP) techniques for future elastic optical transmissions. We proposed a modulation-format-independent blind phase search (MFI-BPS) algorithm for square M-ary quadrature amplitude modulation (M-QAM) systems, in which modulation format recognition (MFR) and carrier phase estimation (CPE), are included and implemented both in a feed-forward manner. Comprehensive simulation and the experimental studies on 224 Gbit/s polarization multiplexing 16-QAM (PM-16QAM) systems demonstrate the feasibility and the effectiveness of the proposed MFI-BPS algorithm.
    Optics Express 10/2014; 22(20). DOI:10.1364/OE.22.024044 · 3.49 Impact Factor
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    Xuebing Zhang · Zhaohui Li · Jianping Li · Changyuan Yu · Alan Pak Tao Lau · Chao Lu
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    ABSTRACT: A low-cost coherent receiver using two 2×3 optical hybrids and single-ended detection is proposed for long-reach optical access network. This structure can detect the two polarization components of polarization division multiplexing (PDM) signals. Polarization de-multiplexing and signal-to-signal beat interference (SSBI) cancellation are realized by using only three photodiodes. Simulation results for 40 Gb/s PDM-OFDM transmissions indicate that the low-cost coherent receiver has 3.2 dB optical signal-to-noise ratio difference compared with the theoretical value.
    Optics Letters 09/2014; 39(18). DOI:10.1364/OL.39.005248 · 3.18 Impact Factor
  • Xian Zhou · Chao Lu · A.P.T. Lau · Keping Long
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    ABSTRACT: Minimum distance blind phase search (BPS) algorithm has good tolerance to laser linewidth but with high computational complexity. In this letter, a novel substitution algorithm, i.e., symmetry BPS (S-BPS) is proposed for arbitrary square M-ary quadrature amplitude modulation (M-QAM) coherent optical systems. Based on the symmetry of square M-QAM, it can reduce the complexity directly by simplifying the phase error detection in the real plane and without multiplication. Taking square 4-QAM, 16-QAM, and 64-QAM as examples, the effectiveness of the proposed S-BPS algorithm is demonstrated by numerical simulations.
    IEEE Photonics Technology Letters 09/2014; 26(18):1863-1866. DOI:10.1109/LPT.2014.2332358 · 2.18 Impact Factor
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    ABSTRACT: We propose a novel technique for simultaneous multi-impairment monitoring and autonomous bit-rate and modulation format identification (BR-MFI) in next-generation heterogeneous fiber-optic communication networks by using principal component analysis-based pattern recognition on asynchronous delay-tap plots. The results of numerical simulations performed for three commonly used modulation formats at two different bit-rates each demonstrate simultaneous and independent monitoring of optical signal-to-noise ratio, chromatic dispersion, and differential group delay with mean errors of 1 dB, 4 ps/nm, and 1.6 ps, respectively, without knowing the signal's bit-rate and modulation format. Similarly, the results for joint BR-MFI validate accurate identification of all the bit-rates and modulation formats despite the presence of various network impairments. The effects of fiber nonli-nearity and transmitter variations on the performance of the proposed technique are also investigated.
    Journal of Optical Communications and Networking 05/2014; 6(5):441-448. DOI:10.1364/JOCN.6.000441 · 1.55 Impact Factor
  • Lingchen Huang · Dawei Wang · Alan Pak Tao Lau · Chao Lu · Sailing He
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    ABSTRACT: The performances of various blind timing phase estimators (TPE) for digital coherent receiver are analyzed. The equivalence among four TPE algorithms is analytically presented, showing that two TPE algorithms applying squaring pre-filters are in fact identical. Three TPE algorithms applicable to Nyquist signals are proposed based on the equivalence analysis. In addition, the impact of receiver bandwidths, spectrum weighting bandwidths and signal timing phases on TPE performance are investigated. The definition of sampling diversity and the analysis of sampling diversity gain for four pulse shapes are presented. The effect of sampling diversity is observed and verified via both simulations and experiments.
    Optics Express 03/2014; 22(6):6749-63. DOI:10.1364/OE.22.006749 · 3.49 Impact Factor
  • Li Tao · Yiguang Wang · Yuliang Gao · Alan Pak Tao Lau · Chao Lu
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    ABSTRACT: We experimentally demonstrate a 10.5 Gb/s CAP128 system over 40 km SSMF using DML and direct-detection with a hybrid MMA/DD-LMS equalization scheme. The influences of RIN and laser linewidths of the DML are also investigated.
    Optical Fiber Communication Conference; 03/2014
  • Jie Liu · Zhenhua Dong · Kang Ping Zhong · Alan Pak Tao Lau · Chao Lu · Yanzhao Lu
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    ABSTRACT: We propose a modulation format identification technique based on extracting and identifying specific features of received signal power distributions for digital coherent receivers. Simulation and experimental results demonstrate successful identification among six common modulation formats.
    Optical Fiber Communication Conference; 03/2014
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    ABSTRACT: We experimentally demonstrate 256 Gb/s PM-16-QAM transmission up to 2600 km by using 100-km spans of few-mode fibers (FMF) operating in quasi-single-mode (QSM) transmission and using a DD-LMS algorithm to compensate multi-path interference (MPI).
    Optical Fiber Communication Conference; 03/2014
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    ABSTRACT: A low complexity and linewidth tolerant two-stage carrier phase estimation (CPE) by using QPSK partitioning and quasi-multiplier free algorithm is proposed for DP-16QAM signal. The performance, linewidth tolerance the algorithm are numerically and experimentally demonstrated.
    Optical Fiber Communication Conference; 03/2014
  • Yuliang Gao · Qunbi Zhuge · David V. Plant · Chao Lu · Alan Pak tao Lau
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    ABSTRACT: We propose a blind and universal DSP platform containing a new joint timing phase and frequency offset estimation technique. The quick convergence speed enables fast receiver initializations for flexible transmissions with arbitrary modulation formats.
    Optical Fiber Communication Conference; 03/2014

Publication Stats

837 Citations
198.17 Total Impact Points


  • 2009–2014
    • The University of Hong Kong
      • Department of Electrical and Electronic Engineering
      Hong Kong, Hong Kong
    • The Hong Kong Polytechnic University
      • • Photonics Research Centre
      • • Department of Electrical Engineering
      • • Department of Electronic and Information Engineering
      Hong Kong, Hong Kong
  • 2010–2011
    • Jinan University (Guangzhou, China)
      • Department of Biochemistry
      Shengcheng, Guangdong, China
    • Tianjin University of Technology
      T’ien-ching-shih, Tianjin Shi, China
  • 2006–2009
    • Stanford University
      • Department of Electrical Engineering
      Palo Alto, California, United States
  • 2004–2007
    • University of Toronto
      • Department of Electrical and Computer Engineering
      Toronto, Ontario, Canada