Showing papers by "Kazuro Kikuchi published in 2013"

Compensation for In-Phase/Quadrature Imbalance in Coherent-Receiver Front End for Optical Quadrature Amplitude Modulation

Abstract: We propose a novel method of compensation for imbalance between in-phase (I) and quadrature (Q) channels in the front-end circuit of digital coherent optical receivers. Adaptive finite-impulse-response (FIR) filters in the butterfly configuration, which are commonly used for signal equalization and polarization demultiplexing, are modified so as to allow for adjustment of any imbalance between the IQ channels. IQ imbalances under consideration include the gain mismatch, the phase mismatch, and the timing-delay skew. Computer simulations for the dual-polarization quadrature-amplitude-modulation (QAM) format up to an order of 256 show that such IQ imbalances can severely degrade the system performance, especially for higher order QAM; however, using the proposed scheme, we can compensate for them without any significant penalty over a wide range of imbalances.

Frequency stabilization of multiple semiconductor lasers for Nyquist-WDM transmission systems

Abstract: We propose a technique for stabilizing frequencies of multiple semiconductor lasers, which can reduce the fluctuation of the frequency spacing to below 3MHz. Using our scheme, we demonstrate 10-GBaud Nyquist-WDM PDM 16QAM transmission over 1,000km.

Blind equalization and carrier-phase recovery in QPSK coherent optical receivers based on modified constant-modulus algorithm

Abstract: We propose and experimentally verify a fully-blind scheme for equalization and carrier-phase recovery in QPSK coherent optical receivers, which is based on the modified CMA. Its computational complexity is much less than the conventional approach.

Novel Polarization-diversity Scheme Based on Mutual Phase Conjugation for Fiber-nonlinearity Mitigation in Ultra-long Coherent Optical Transmission Systems

Abstract: We propose a novel method of fiber-nonlinearity mitigation for ultra-long optical transmission systems, which employs two polarization modes mutually phase-conjugated. Simulations and experiments show that in the nonlinear region, this diversity method significantly outperforms both of the singleand dual-polarization schemes at the same bit rate.

Influence of channel misalignment of time-interleaved DAC on sensitivity degradation in coherent optical receivers

Abstract: The time-interleaved architecture is indispensable for high-speed digital-to-analog converters (DACs) in coherent optical transmitters. After analyzing the impact of misalignment of time-interleaved channels, we measure receiver-sensitivity degradation by such misalignment using QPSK and 16-QAM signals.

Dual-stage decision-directed phase estimator enabling perfect frequency-offset elimination in digital coherent optical receivers

Abstract: We propose a dual-stage phase estimator controlled by the decision-directed LMS algorithm in digital coherent optical receivers. Simulations and experiments show that our proposed scheme can perfectly eliminate arbitrary frequency offsets in higher-order QAM systems.

Wavelength Demultiplexing of Nyquist WDM Signals under Large Frequency Offsets in Digital Coherent Receivers

Abstract: We propose a novel wavelength-demultiplexing algorithm applicable to Nyquist WDM systems. The proposed scheme features the ability of perfect wavelength demultiplexing even under the frequency offset half as large as the symbol rate. Through simulations and experiments, effectiveness of our scheme is demonstrated in 16-QAM Nyquist WDM systems.

Proposal of optical-sampling-based constellation monitor for DP-QPSK signals

Abstract: We propose a novel linear optical-sampling system for monitoring the constellation diagram of DP-QPSK signals. A polarization-constellation in the Stokes space is obtained with phase-noise-free optical-sampling measurements and converted to an IQ-constellation diagram.