Cycle-slip-resilient high-precision transmitter IQ skew estimation for ultra-high baud-rate digital subcarrier...

In ultra-high baud-rate coherent digital subcarrier multiplexing (DSM) systems, transmitter IQ skew is a critical impairment that needs to be accurately estimated and subsequently compensated. Here, we propose a high-precision transmitter IQ skew estimation scheme with strong cycle-slip resilience. The scheme employs a specially designed training sequence (TS) and utilizes a Hampel filter (HF) at the receiver to process the extracted phase information. The proposed scheme can effectively suppress phase outliers to avoid cycle slips, significantly improving the estimation accuracy of IQ skew. Experimental verification of 120 GBaud DP-16QAM DSM signals with 4 subcarriers transmission over 100 km standard single-mode fiber (SSMF) verifies that the proposed scheme eliminates cycle slips caused by noise-induced phase outliers within an optical signal-to-noise ratio (OSNR) range of 17~27 dB, in comparison with the traditional approach without HF. Consequently, the estimation accuracy is improved from 0.3 ps to less than 0.1 ps, and the corresponding compensation based on the estimated value achieves ~0.4 dB OSNR gain, closely approaching the performance with an ideal IQ skew compensation limit.