2×2 MIMO Equalizer Enabled Transmitter Side IQ Imbalance Compensation for Optical Single Sideband Direct Detection System

Recently, optical single sideband (SSB) based direct detection (DD) has been considered as a promising candidate for inter data center interconnects (inter-DCIs). Kramers-Kronig (KK) receiver is always applied to eliminate the signal-to-signal beating interference (SSBI) caused by the square law-detection of a single-ended photodiode (PD) in SSB DD system. The electro-to optical (E/O) conversion of the complex signal can be realized by IQ modulator (IQM). However, IQ imbalance exists at the transmitter since the in-phase and quadrature-phase ports of IQM are driven by the real and imaginary parts of the signal, respectively. In this paper, we show the theoretical analysis of the transmitter side (Tx-side) IQ imbalance includes time delay, amplitude mismatch and phase mismatch. Tx-side IQ imbalance will seriously deteriorate the system performance. To deal with the impairment caused by Tx-side IQ imbalance, a 2×2 real-value multiple-input multiple-output (MIMO) equalizer is proposed and implemented, and the system with the impact of these three factors are investigated in simulation, respectively. Besides,a transmission of 180 Gb/s/λ discrete Fourier transform spread (DFT-S) orthogonal frequency division multiplexing (OFDM) signal over 80-km single mode fiber (SMF) is experimentally demonstrated with bit error rate (BER) below the soft-decision forward error correction (SD-FEC) threshold of 2 × 10 ⁻² enabled by the proposed 2 × 2 MIMO equalizer.