We investigate interplay between the residual and applied chirp of optical duobinary modulated signals in order to improve transmission performance. To find the best performance for 10-Gb/s optical duobinary transmission systems,we consider the residual chirp accompanying from the finite extinction ratio,the applied chirp adjusted by the applied voltage ratio (the chirp parameter) between two electrodes of LiNbO3 modulators,and the bandwidth of electrical low-pass filters used in duobinary transmitters. The simulation results suggest that nearly zero chirp during the mark (`1') period and large peak chirp at the middle of the space (`0') provide the best transmission performance. This zero chirp around marks and high peak chirp at the middle of each space can be controlled by the applied voltage ratio between two electrodes of modulator and the filter bandwidth, respectively.
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