Towards Coherent Detection in SDM-based Optical Access Networks

• To tackle the continued increase in capacity demand in optical access, self-homodyne coherent detection (SHCD) is being considered as a candidate solution, utilizing a spatial division multiplexing (SDM) in a low-crosstalk multicore fiber (MCF) [1, 2] and high-order modulation formats such as quadrature phase-shift keying (QPSK), 16-levels quadrature amplitude modulation (16-QAM) or even 64-levels QAM (64-QAM) . The latest imposes strict requirements on both the performance of digital signal processing (DSP) and the spectral purity of the free running lasers [4–6]. Although a SHCD scheme relaxes the laser linewidth requirements, eliminates the need for a carrier frequency tracking, the necessity for a carrier phase estimation remains if high-order constellations are considered. The reasons for that are (1) residual phase noise in a system and (2) a closer location of the constellation points in the complex plane, which leads to a significantly lower phase noise tolerance as compared to QPSK or even 16-QAM signals . Therefore, it is of crucial importance to propose a low-complexity (and low-latency) phase estimation algorithms for these scenarios as well, especially because traditional algorithms cannot be directly employed for 64QAM. During this presentation,first, we overview our recent activities on designing and experimental validation of a high-performance and a low-complexity DSP block, focusing on carrier phase recovery (CPR) suitable for circular and square m-QAM constellations. For more information about the efficient CPR schemes for m-QAM, we recommend theOFC2017 and ACP2017 proceedings.Second, after the efficient CPR scheme is explained, we provide an insight into a solution of SDM-based optical access network where self-homodyne coherent detection (SHCD) scheme with our novel low-complexity CPR is implemented for a 28-Gbaud 16-QAM and 64- QAM transmission in downstream links with a goal to improve their spectral efficiency and to increase a phase-noise tolerance, which paves the way towards higher cost-efficiency. The proposed architecture together with the experimental demonstration is given inCLEO2018 conference proceedings

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