Simultaneous monitoring of transmitter IQ impairments in digital subcarrier systems

High-speed digital subcarrier multiplexing (DSCM) coherent optical system is more sensitive to practical impairments in electrical and optical components, particularly the transmitter (Tx) in-phase/quadrature (IQ) impairments, including IQ amplitude imbalance, phase imbalance, and timing skew. In this paper, we propose a joint estimation based on paired subcarriers (JE-PS) with a designed training sequence for Tx IQ impairments. The training sequence on one pair of symmetric subcarriers is designed to be negative conjugate, and another pair of symmetric subcarriers is conjugate. Then we can use two subcarriers located in the negative or positive frequency region to extract the Tx IQ amplitude imbalance and IQ skew in the frequency domain. Since the IQ skew and subcarrier frequency induce extra phase interference, we use negative and positive paired subcarriers to eliminate the interference and calculate the IQ phase imbalance. Theoretical analysis and simulation results indicate that JE-PS methods are insensitive to the modulation format and laser phase noise. Besides, it is also robust to the transceiver IQ impairments and ASE noise. Finally, the JE-PS methods are experimentally verified in a 50-Gbaud four-subcarrier PM-16QAM DSCM signal transmission system. The estimation accuracy of Tx IQ amplitude, phase imbalance, or skew is within 0.25 dB, 1°, or 0.2ps, respectively.