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New metric for IQ imbalance compensation in optical QPSK coherent systems
Authors:Trung-Hien Nguyen  Pascal Scalart  Mathilde Gay  Laurent Bramerie  Christophe Peucheret  Fausto Gomez-Agis  Olivier Sentieys  Jean-Claude Simon  Michel Joindot
Affiliation:1.FOTON Laboratory, CNRS, ENSSAT,University of Rennes 1,Lannion,France;2.INRIA/IRISA,University of Rennes 1,Rennes,France;3.OPERA Department,Université Libre de Bruxelles,Brussels,Belgium;4.Department of Electrical Engineering,Technische Universiteit Eindhoven,Eindhoven,The Netherlands
Abstract:We report on a simple alternative method for the compensation of quadrature imbalance in optical quadrature phase-shift-keying (QPSK) coherent systems. By introducing a new metric, the phase imbalance can be determined and compensated. The proposed method is theoretically and numerically analyzed. In particular, it is shown that the method exhibits a small bias of estimated phase imbalance value. Thanks to its deterministic property, this bias can be simply compensated by incorporating at the receiver a phase rotator (or phase shift) whose value can be determined based on an analytical analysis. Moreover, the algorithm is also experimentally validated through bit-error-rate and error vector magnitude (EVM) measurements. A good agreement on the performance of the proposed method with that of the Gram–Schmidt orthogonalization procedure is shown in a 20-Gbit/s optical QPSK experiment. The robustness of both methods was verified with up to (30^circ ) phase imbalance by comparing the signal with and without phase imbalance compensation. A 10% reduction in EVM is achieved with our method for a high phase imbalance of (30^circ ), while the implementation complexity can be reduced owing to the suppression of the use of square-root operators.
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