Probability-Maintained Notch Sequences for 64QAM

A practical and accurate method for coherent optical transmitter assessment is crucial. Compared with conventional receiver-based methods which rely on expensive high-speed oscilloscopes to capture signal waveforms, spectrum analyzer-based approaches offer a low-cost alternative in which the stimulus plays an important role. In this study, the probability-maintained (PM) notch sequence as a novel stimulus is proposed, whose probability density function is identical to that of a standard 64-array quadrature amplitude modulation signal and whose power spectral density has a deep notch of 25 dB. By applying the proposed PM notch sequence, the noise spectrum of the coherent optical transmitter can be measured using an optical spectrum analyzer, in which the regrowth frequency components in the notch region of a double-side PM notch indicate the nonlinear distortion while the spectrum difference between double-side and single-side PM notches reveals the in-phase\/quadrature imbalance. The performance of the coherent optical transmitter is assessed by establishing an equivalent additive noise model based on the measured noise spectrum. The proposed method demonstrates an accuracy within 0.3 dB compared to the Q factor obtained by a coherent optical receiver.