Fully integrated low-cost photonic DAC based on MMI splitters and MRRs

We present the design and simulation of a 4-bit single-wavelength coherent photonic digital-to-analog converter (pDAC) targeting silicon photonic integration for photonic neural network accelerator applications. The architecture integrates a 1×4 asymmetric multimode interference (MMI) power splitter, four single-bus microring modulators (MRMs), and a 4×1 MMI combiner on a 270 nm rib waveguide platform. Operating on a single wavelength, the design exploits field superposition at the output combiner to synthesize 16 linearly spaced power levels, enabling π-phase encoding for signed weight representation, a capability unavailable to wavelength-division multiplexed (WDM) pDAC architectures where fields at different wavelengths cannot destructively interfere. To eliminate the dedicated active phase tuners required by prior coherent pDAC implementations, we introduce an amplitude-phase-aware voltage lookup algorithm that exploits the intrinsic voltage-dependent phase response of the MRMs, jointly solving for per-ring voltages that linearize the coherent output without auxiliary phase correction hardware. MRM doping profiles are co-optimized via NSGA-II to achieve VπL = 0.39 V·cm. System-level simulation confirms 16 monotonically increasing output levels at 1550.641 nm, a 30 GHz modulation bandwidth, ±0.25 LSB INL, −2.2 dB end-to-end insertion loss, and a photonic footprint of ∼4,600 µm2.