Behavior Model and Nonlinear Distortion Characterization of PAM Optical DAC Transmitter

Optical digital-to-analog converter (DAC) transmitters exhibit great potential for saving power consumption and reducing footprint. However, the novel architecture of these optical transmitters introduces risks to signal performance due to inconsistencies in the responses among modulation segments. In this paper, a bit-dependent impulse model is proposed to characterize both the linear behavior and nonlinear distortions of N-bit pulse amplitude modulation (PAM) optical DAC transmitters. By applying minimum mean square error (MSE) criterion to the proposed model, the linear behavior, nonlinear noise spectrum, and nonlinear signal-to-noise ratio (SNR) are derived. Experiments on a Si-based PAM4 optical DAC transmitter, incorporating various impairments, demonstrate that the proposed model and distortion theory accurately characterize the linear response and nonlinear noise. Across all 48 investigated scenarios, the normalized mean square errors (NMSEs) between theoretical best linear approximations and measured values are below –17.5 dB. Nonlinear SNRs are also predicted with small errors of 0.65 dB across a 15 dB dynamic range. Furthermore, the proposed distortion theory provides a quantifiable criterion for bit-segment mapping, which can mitigate the nonlinearity of optical DAC transmitters.