Power Linear DACs (PLDACs) for Configuration and Control of Silicon Photonic Integrated Circuits

Silicon photonics has emerged as a key enabler for progressing integrated circuits in the post-Moore scaling era, whereby the advantages of photonics complement the mature and robust CMOS circuit. The hybrid integration of CMOS electronics and photonics realizes entirely novel system-level functionality. Photonic integrated circuits (PICs) extensively employ thermo-optic tuning for calibrating for process and temperature variations, and also for reconfiguration of these circuits. These thermo-optic phase-shifters, or microheaters, are driven by electronic digital-to-analog converters (DACs) which induce an optical phase-shift proportional to the power delivered. Thus, linear power sweeping capability is desired from the DAC. In this work, we introduce power linear DACs, or PLDACs, which are expected to become a pervasive block in hybrid CMOS-photonic circuits. The mostly-digital PLDAC designed in the TSMC 65nm LP CMOS technology comprises of a 4-bit $\Delta \Sigma$ modulator followed by a 4-bit current-steering DAC, a square root circuit, and the driver. The 12-bit PLDAC operates at an oversampled clock rate of 10MHz and delivers up to 24mW of power to doped-silicon microheaters in a SiP foundry process with an estimated silicon footprint of $305\mu \mathrm{m}\times 66\mu \mathrm{m}$.