A novel in-service signal quality monitoring technique for nonreturn-to-zero differential phase-shift keying signals using asynchronous amplitude histogram evaluation has been demonstrated. As a result of phase-to-amplitude modulation, there are three Gaussian distributions in the amplitude histogram. The widths of the Gaussian distributions are related to the amount of amplified spontaneous emission, while the separations between the Gaussian distributions increase monotonically with fiber dispersion. The amount of dispersion and optical signal-to-noise ratio can, thus, be directly extracted and discriminated from the amplitude histogram.
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Li et al. (2005) studied this question.