Photonic integrated circuits suffer from large process variations. Effective and accurate characterization of the variation patterns is a critical task for enabling the development of novel techniques to alleviate the variation challenges. In this study, we propose a hierarchical approach that effectively decomposes the spatial variations of silicon microring-based optical transceivers into wafer-level, intra-die, and inter-die components. We then demonstrate that the characterized variation models can be used to generate trustworthy synthetic data for architecture- and system-level solutions for variation alleviation. We further demonstrate the utility of our variation characterization method for accurate yield prediction based on partial measurement data.

Microring-based silicon photonic transceivers are promising to resolve the communication bottleneck of future high-performance computing systems. To rectify process variations in microring resonance wavelengths, thermal tuning is usually preferred over electrical tuning due to its preservation of extinction ratios and quality factors. However, the low energy efficiency of resistive thermal tuners results in nontrivial tuning cost and overall energy consumption of the transceiver. In this study, we propose a hybrid tuning strategy which involves both thermal and electrical tuning. Our strategy determines the tuning direction of each resonance wavelength with the goal of optimizing the transceiver energy efficiency without compromising signal integrity. Formulated as an integer programming problem and solved by a genetic algorithm, our tuning strategy yields 32%~53% savings of overall energy per bit for measured data of 5-channel transceivers at 5~10 Gb/s per channel, and up to 24% saving for synthetic data of 30-channel transceivers, generated based on the process variation models built upon measured data. We further investigated a polynomial-time approximation method which achieves over 100x speedup in tuning scheme computation, while still maintaining considerable energy-per-bit savings.

Nanophotonic interconnects have started replacing traditional electrical interconnects in data centers for rack-level communications and demonstrated great potential at board and chip levels. However, microring-based silicon photonic transceivers, an important element for nanophotonic interconnects, are very sensitive to fabrication process variations, and require power hungry wavelength tuning. In this paper, we apply efficient optimization algorithms to mix-and-match a pool of fabricated transceiver devices with the objective of minimizing the overall tuning power. This optimal pairing technique, applied during the production stage, reduce power consumption for wavelength tuning. For two sets of fabricated devices, the pairs of transceivers assigned by the optimal pairing technique reduce the tuning power by 6% to 60%. We further evaluate the method on synthetic data sets that are generated from a well-established process variation model. Our experimental results show that even greater power saving can be achieved when more fabricated devices are available for pairing and the runtime of the optimization algorithm is quite scalable.

The comb laser-driven microring-based dense wavelength division multiplexing silicon photonics is a promising candidate for next-generation optical interconnects. However, existing solutions for exploring the power-performance trade-off of such systems have been restricted to a limited design space, resulting from the unnecessary constraints of using an identical spacing for laser comb lines and microring channels, and of utilizing consecutive laser comb lines for data transmission. We propose an energy-efficient channel alignment scheme that aligns the microring channels to a subset of laser comb lines that are non-uniformly distributed in the free spectrum range of the microrings. Based on a well-established process variation model, our simulations show that the proposed scheme significantly reduces the microring tuning power in the presence of denser comb lines. The power saved from microring tuning can improve the overall system energy efficiency despite some power wasted in unused laser comb lines. We further conducted a case study for design space exploration using the proposed channel alignment scheme, seeking the most energy-efficient configuration in order to achieve a target aggregated data rate.