On-chip photonic digital-to-analog converter by phase-change-based bit control

In this paper, we propose an N-bit optical digital-to-analog converter (DAC) by integrating N pairs of 2 × 2 phase-change-based on-chip photonic switches and 2 × 2 multimode interference (MMI) splitters. The on-chip photonic switch is constructed by integrating the wavelength-selectable racetrack micro-ring resonator (MRR) and the phase change chalcogenide Ge2Sb2Se4Te (GSST). The GSST-integrated switch utilizes a racetrack resonator configuration for the accurate modulation of the resonant wavelength to prevent intercoupling between adjacent units. For electrothermal heating of the GSST film to trigger its phase transition for switchable control, an indium tin oxide (ITO) heater with a bowtie-shaped structure is integrated into the racetrack resonator. By numerical calculations, we demonstrate that an 8 V voltage pulse of 300 ns duration, with an energy consumption of 18.45 nJ, can transition the optical state from OFF state to ON state. Another 6 V voltage pulse of 250 ns duration, followed by a 4 V pulse of varying duration, with a total energy consumption of 34.78 nJ, can switch the optical state from ON state to OFF state. The asymmetric structure of the 2 × 2 MMI shows ultra-high transmittance approaching 50% in the through port (connected to the next order of MMI), enabling the creation of multistage cascaded MMI splitters with an output light power ratio close to 50%. Our results show that this configuration potentially offers a feasible solution for applications of optical digital-to-analog converters.