Advanced modulations and DSP enabling high-speed coherent communication using large linewidth lasers

Abstract

Technology roadmap reveals the trend for coherent communication systems that support high capacity and advanced modulations to shift from long-haul towards metro networks, where the traffic loads require the number of transceivers in a much larger scale [1]. Low-cost, small-footprint monolithically integrated semiconductor lasers with relatively large phase noise (PN) are preferred over the costly narrow linewidth external cavity lasers in such context, provided no degradation in system performance. However, there are issues that need to be carefully considered and tackled before any practical implementation. We summarize several reasons for the commonly used combined transmitter and local oscillator (Tx/LO) laser linewidth failing to be a reasonable figure of merit in evaluating the performance in such systems. Firstly, the non-white nature of the frequency noise of semiconductor lasers results in different impairments compared to lasers with white frequency noise, even though both having equal linewidth [2, 3]. Secondly, there exist different types of impairments to the transmitted signal between the PN of the Tx and LO lasers in a post digital chromatic dispersion (CD) compensated system: the PN of the Tx laser directly converts to the phase distortions in the signal constellations that could be corrected with carrier phase recovery (CPR) algorithms in digital signal processing (DSP), while the LO phase noise inter-mix with the sidebands of the dispersed signal and results in signal distortions in both the phase and amplitude dimensions that can't be effectively compensated with CPR. In this presentation, we will summarize our recent reported works on advanced phase noise-tolerant modulations, DSP algorithms and system design [4–7], to address the above discussed issues respectively.