Experimental Demonstration of Simultaneously Precise Tx and Rx Skew Calibration for Coherent Optical Transceiver

High precise IQ skew calibration of the coherent optical transceiver is crucial for high baud rate and high-order modulation formats optical transmission system. In this paper, a novel high precise optical coherent transceiver calibration method based on the specially designed interleaved in-phase (I) and quadrature-phase (Q) multi-tone signals is proposed. This method can simultaneously characterize the transmitter (Tx) and receiver (Rx) side IQ skew by just one measurement without using any additional device or training process. Because of the non-overlapping characteristic of these interleaved I and Q signals in the frequency domain, the IQ crosstalk in the coherent optical transceiver can be effectively avoided, contributing to high-precision calibration. Since the calibration process only focuses on the phase frequency response characteristic, the dynamic range of IQ skew calibration can easily reach dozens of picoseconds. Theoretical analysis and simulation results indicate that the measurement accuracy of the Tx skew and Rx skew is within 0.2 ps in the proposed scheme, and the dynamic range of IQ skew calibration can reach 40 ps. With the help of this IQ skew calibration method, the transmission performance improvement is experimentally verified in both single-polarization 30 Gbaud 32QAM signal and dual-polarization 30 Gbaud 16QAM signal transmission systems.