Simultaneously precise frequency response and IQ skew calibration in a self-homodyne coherent optical transmission system

Abstract

The self-homodyne coherent detection (SHCD) system is becoming more popular in intra-data center applications nowadays. However, for a high-speed SHCD system, the device imperfection such as transmitter (Tx) and receiver (Rx) side in-phase (I)/quadrature-phase (Q) time skew and bandwidth limitation will greatly restrict the transmission performance. The current mainstream calibration methods for traditional optical transceivers rely on the effect of frequency offset and phase noise to separate the Tx and Rx imperfection, which is not compatible with the SHCD system. In this paper, we have proposed and demonstrated a highly precise calibration method that can be applied in dual-polarization (DP) SHCD system. Based on the specially designed multi-tone signals, the amplitude/phase frequency response (AFR/PFR) of the transceiver and the Tx/Rx IQ skew can be obtained by just one measurement even after long-distance fiber transmission. By using a 4 MHz linewidth distributed feedback (DFB) laser, a DP SHCD transmission system combined with a 20 GHz optical transceiver and two 10 km standard single-mode fibers is experimentally constructed. The test results indicate that the measurement error of the AFR/PFR and Tx/Rx skew are within ±1dB/±0.15rad and ±0.3ps respectively, and the dynamic range for IQ skew calibration can reach dozens of picoseconds. The measured bit error rate value of 46GBaud DP-16QAM signals/35GBaud DP-64QAM signals are improved from 2.30e-2 to 2.18e-3/9.59e-2 to 2.20e-2 with the help of the proposed calibration method.