Non-Intrusive Characterization and Efficient Compensation of Transceiver Frequency Responses for Coherent Optical Communication System

The escalation of symbol rates in next-generation coherent optical systems (e.g., 1.6 Tb/s and beyond) significantly exacerbates influence of transceiver frequency response impairments (FRIs). The system performance is degraded by FRIs of both transmitter (Tx) and receiver (Rx), being difficult to discriminate the individual contribution of each subsystem. Nevertheless, such discrimination is essential for the optimization of state-of-the-art transceiver designs, enabling effective mitigation of impairment propagation. To address this technical challenge, we propose a non-intrusive scheme for the simultaneous characterization and separation of Tx and Rx frequency responses (FRs). By leveraging tone-shift characteristics induced by frequency offset combined with multi-tone probe signals, the proposed method digitally separates the FRs of the Tx and Rx without requiring auxiliary components. Experimental results demonstrate the proposed scheme achieves high estimation accuracy, with deviations below 1 dB in amplitude and 0.25 rd in phase across a 20-dB bandwidth of 35 GHz. Furthermore, using the monitored responses for targeted pre- and post-compensation in a 69-GBaud DP-16QAM transmission system yields up to a 2-dB improvement in OSNR sensitivity at the 7% HD-FEC threshold. In addition, when combined with a reduced-tap MIMO equalizer, the proposed scheme still preserves a 1-dB OSNR sensitivity gain while reducing the equalization complexity by approximately 44%.. Therefore, the proposed scheme provides a practical solution for in-field transceiver characterization, paving the way for simplified DSP and enhanced performance in future ultra-high-speed coherent optical interfaces.