Low-Complexity Coherent Optical Transceiver IQ Imbalances Calibration Enabled by Cross-Power Spectrum Method

In this paper, an in-field and low-complexity calibration scheme based on cross-power-spectrum (CPS) analysis for in-phase and quadrature (IQ) imperfections of coherent optical transceivers is proposed. This CPS method allows for high-precision estimation of time delays by calculating the cross-spectrum between two signals and extracting phase information. Its computation process only requires FFT operations and simple linear fitting, contributing to lower complexity and simpler implementation for estimating IQ skew compared to other schemes. The robustness of the proposed scheme is investigated through simulation against the mutual influence of IQ skews, amplitude imbalances, and phase imbalances of transceivers, as well as various channel impairments such as local oscillator frequency offset, phase noise, and fiber chromatic dispersion. Moreover, simultaneous calibration of transceiver IQ skews, amplitude imbalances, and phase imbalances in dual-polarization after fiber transmission has been experimentally conducted. The measurement errors for Tx/Rx IQ skews, amplitude imbalances, and phase imbalances are less than 0.2 ps, 0.5 dB, and 1°, respectively. The proposed CPS scheme exhibits a substantial dynamic measurement range of up to ±200 ps for the skew of the coherent optical transceivers. The aforementioned methods effectively compensate for the IQ skews (1.6 ps/4ps), amplitude imbalances (0.58 dB/0.23 dB), and phase imbalances (−2.1°/0.8°) of the Tx/Rx in experimental scenarios involving 20/40Gbaud 16/64 QAM signal transmissions over an 80 km standard single-mode fiber. Thanks to the proposed scheme, the compensated signal exhibits an optical signal-to-noise ratio reduction of about 3 dB at the hard-decision forward error correction threshold and 4 dB at the soft-decision forward error correction threshold for the 20 GBaud 16 QAM and 40 GBaud 16 QAM signals, respectively.