Hardware-Efficient and Reliable Coherent DSCM Systems Enabled by Single-Pilot-Tone-Based Polarization Demultiplexing

Abstract: Recently, coherent digital subcarrier multiplexing (DSCM) technology has become an attractive solution for next-generation ultra-high-speed datacenter interconnects (DCIs). To meet the requirements of low-cost and low-power consumption in DCI applications, a comprehensive simplification of the coherent DSCM system has been investigated. The pilot-tone-based polarization demultiplexing (PT-PDM) technique, known for its low-power consumption and ultra-fast polarization tracking capabilities, has emerged as a compelling alternative to the power-hungry N-tap adaptive multi-input multiple-output (MIMO) equalizer. However, the effectiveness of this PT-PDM technique is extremely vulnerable to the receiver-side XY-skew (Rx-XY-skew), which is revealed in this paper for the first time. Then, a pilot-tone-enabled modified Godard phase detector (PT-MGPD) scheme is proposed to realize Rx-XY-skew estimation, serving as the prerequisite for the successful implementation of the PT-PDM and simplification of the adaptive equalizer. Both the simulation and experiment are conducted to evaluate the accuracy of the proposed PT-MGPD scheme. The results prove it can achieve accurate estimation with an error of less than 0.3ps. Besides, a low-complexity, high-spectral-efficiency, and ultra-fast polarization demultiplexing method based on a single pilot tone (SPT) is proposed for the DSCM system in this work. Based on the proposed PT-MGPD and SPT schemes, the conventional N-tap MIMO equalizer served for each subcarrier can be successfully pruned into two polarization-independent single-input single-output equalizers, and there is no performance penalty even if the polarization rotation speed reaches 10Mrad/s. According to the results, the proposed schemes provide a hardware-efficient and reliable coherent DSCM solution for next-generation ultra-high-speed DCIs.