Phase noise performance of the multilevel interferometric direct detection receivers

We analyze the phase noise contribution to the photocurrent of the multilevel direct detection receiver based on delay line interferometer (DLI) and balanced photodetector. We show that the phase noise influence in multilevel systems is much stronger compared to that in binary systems as a result of the DLI configuration. The characteristic function of the photocurrent is derived for arbitrary-delay line interferometer configuration, balanced photodetector, and post-detection low-pass filter. The characteristic function is used to estimate the symbol error rate of a D8PSK system. The results are validated by Monte-Carlo simulations.