Diode-assisted microheaters for programmable photonic integrated circuits

Thermo-optically tunable Mach-Zehnder interferometer (MZI) switches have been popular for building robust designs of programmable photonic integrated circuits (PPICs) for various applications. In general, microheaters integrated on both arms of the MZIs are preferred to minimize power consumption by accounting for phase errors between MZI arms arising from fabrication-induced process variabilities. However, only one of the two microheaters integrated in MZI arms is chosen to drive, for achieving desired phase imbalance with lower power consumption. Again, integrating microheaters on both arms doubles the electronic control channels and as many bond pads, posing challenges for scalability and packaging. To address this, we propose a diode-assisted microheater (DAM) architecture that enables selective driving of one of the two microheaters in an MZI without increasing the pad count or control complexity. The DAM is implemented on a standard silicon photonics technology platform and is fully compatible with foundry processes. Comprehensive electrothermal simulations and subsequent experimental results confirm the CMOS foundry fabrication feasibility of such diode-assisted microheater designs. The integrated diode design exhibits low series resistance and high breakdown voltage, ensuring efficient forward operation and negligible reverse leakage. Both the diode and microheater resistances were modelled as functions of the forward current, showing excellent agreement with experimental data. DC and AC characterizations reveal that the DAM achieves efficient thermal tuning with a total power dissipation of ∼50 mW at ±3 V and a rise/fall time of ∼20 μs. The DAM's performance was validated in a 1 × 4 power splitter comprising cascaded 1 × 2 MZIs, fabricated through a commercial silicon photonics foundry. Using DAM-based tuning, we achieved precise phase correction, yielding uniform power splitting with deviations of only ±0.015 dB at 1550 nm and ±0.025 dB across a 40 nm operating wavelength bandwidth. Furthermore, by employing bipolar driving and selectively activating one of the MZI arms, we experimentally demonstrated a reduction in power consumption exceeding 50% when compared with the experimental results for the same PPIC by assuming only one microheater was integrated in MZIs. These results demonstrate that the proposed DAM architecture offers a scalable, power-efficient, and foundry-compatible solution for next-generation PPICs.