Estimation for IQ skew of A Transmitter in Digital Coherent Communication Systems

To satisfy the ever-increasing traffic demand in a cost-effective manner, high-baud transceivers need to be developed. 64 Gbaud transceivers are being introduced to commercial systems and research on beyond 100 Gbaud transceivers is in progress. High-baud signals are susceptible to IQ skew at the coherent transceiver since the symbol duration is extremely short. IQ skew can be eliminated by the filter used for polarization-impairment compensation provided that the filter has real-number tap coefficients [1] . However, the elimination of transmitter IQ skew becomes impossible if there is frequency mismatch between the transmitter laser and local oscillator. To compensate for transmitter IQ skew, additional filters need to be implemented in the receiver [2] ; this inevitably increases the receiver cost. Therefore, we need a practical fabrication process that can eliminate transmitter IQ skew. We propose here a novel method for estimating transmitter IQ skew, in which the transmitter-IQ-skew value is rigorously determined in a theoretical manner from the tap coefficients of the adaptive filters used.