A third-order digital pre-distortion for PAM4 short-reach optical transport systems

We propose and experimentally demonstrate a third-order digital pre-distortion (DPD) scheme to pre-compensate for the third-order nonlinear impairment introduced by the Mach–Zehnder modulator (MZM) in intensity modulation and direct detection (IMDD) optical transmission systems. In our DPD scheme, the modulator driving signal is pre-distorted by the third power of the signal. The coefficient is computed based on the Taylor expression of the MZM power modulation transfer function. We conduct an experimental demonstration to assess the DPD scheme gain in C-band 50 Gb/s four-level pulse amplitude modulation (PAM4) IMDD 20 km standard single-mode fiber (SSMF) systems with the utilization of a second-order Volterra series feed-forward equalizer-based decision feedback equalizer (VFE-DFE) at the receiver side to compensate for the distortion introduced by the chromatic dispersion of optical fiber and the square-law detection of a Photodetector. Compared to VFE-DFE, our DPD scheme combined with VFE-DFE achieves a 1.0 dB improvement of receiver sensitivity and improves the optimum operating voltage from 400 mV to 500 mV at a bit error ratio (BER) of 1×10⁻³.