Mitigation and Monitoring of the Impact of Extinction Ratio of IQ-Modulator on Nyquist M-QAM Signals

We present the analysis of the distortion of optical constellations, induced by IQ-modulator with finite extinction ratio, for NRZ and Nyquist M-QAM signals. We demonstrate that constellation points of NRZ 16-QAM signal shift nonlinearly, whereas constellation points of Nyquist 16-QAM experience linear shift, which is accompanied with appearance of residual carrier in the optical spectrum. Mitigation of this error in Nyquist M-QAM signal is achieved by simple bias adjustment based on monitoring of low-pass filtered RF power at the transmitter output.