Variation-Aware Photonic System Modeling and Design
Optical interconnects provide high bandwidth, low power and low latency compared to traditional electrical interconnects, and is a promising substitute for the latter in short reach applications, such as data centers, inter-chip, and intra-chip communications. However, optical interconnects are very sensitive to temperature fluctuations and process variations. In photonic integrated circuits (PIC), process variation issues become more severe, and need to be addressed with great priority. This project explores variation-aware modeling, design and optimization techniques for photonic integrated systems, including:
Microring resonators are widely used in optical network-on-chips (ONoC). However, the microring resonance wavelength is highly sensitive to thermal and process variations. We propose four power-efficient tuning methods to compensate for the wavelength mismatch:
Variation-aware adaptive tuning
The performance of optical device varies significantly due to fabrication process variations and thermal variations. The power budget of the optical link can be set based on the worst case to guarantee the yield, yet wasting much energy. We propose an energy-efficient adaptive tuning technique that allocates just-enough power for each fabricated link using on-chip bit-error-rate test (BERT).
Compact models for microring modulators
Accurate compact models are developed for carrier-injection microring modulators, including static and dynamic, optical and electrical properties. The model can predict the modulators' quality factor, extinction ratio, and eye diagrams for different device geometries and driving conditions. The models are implemented in Verilog-A and are compatible with SPICE simulators.
Spatial pattern analysis of wafer-scale test data of microring modulator
Significant spatial patterns are identified from the optical and electrical test data of silicon microring modulators with heaters. The spatial patterns are related to fabrication process steps and would be helpful in process diagnose and uniformity improvement.
Dynamic laser power scaling (DLPS)
Laser power consumption is a significant portion of the total power consumption of an optical link. The laser is kept at the same output level even when there is little or no traffic. Based on the observation that the required laser power decreases when the link data rate decreases, we propose a dynamic scaling technique that decreases the laser power or turns off the laser when there is little or no network traffic.
Circuit-level simulation of silicon nanophotonic interconnects
We developed compact models for key components of a fabricated optical network-on-chip (ONoC). These models are implemented in both SPICE-compatible electronic design automation tools and dedicated photonic-circuit simulators. Model validation is conducted at both device and link levels, allowing the circuit designer to study the impact of individual device design on the overall link performance.
Transceiver optimal pairing
Based on the observation that different transmitter-receiver (Tx-Rx) pairs require distinct wavelength tuning distances and power consumptions, there is an opportunity to reduce the microring tuning power by optimally mix-and-matching a number of transceivers to form communication pairs. For two sets of fabricated devices, the transceiver pairs assigned by the optimal pairing technique reduce the average microring tuning power by 6% to 60%.
• Energy-efficient channel alignment
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. Simulations show that the proposed scheme significantly reduces the microring tuning power in the presence of denser comb lines, and can benefit the overall system energy efficiency despite some power wasted in unused laser comb lines.
Y. Wang, M. Seyedi, A., Wu, R., Hulme, J., Fiorentino, M., Beausoleil, R. G., and Cheng, K. - T., "Energy-Efficient Channel Alignment of DWDM Silicon Photonic Transceivers", in Design, Automation and Test in Europe (DATE), Dresden, Germany, 2018.
R. Wu, M. Seyedi, A., Wang, Y., Hulme, J., Fiorentino, M., Beausoleil, R. G., and Cheng, K. - T., "Pairing of Microring-based Silicon Photonic Transceivers for Tuning Power Optimization", in 23rd Asia and South Pacific Design Automation Conference (ASP-DAC), 2018.
R. Wu, Wang, Y., Zhang, Z., Zhang, C., Schow, C., Bowers, J. E., and Cheng, K. - T., "Compact Modeling and Circuit-Level Simulation of Silicon Nanophotonic Interconnects", in Design, Automation and Test in Europe (DATE), Lausanne, 2017.
Z. Zhang, Wu, R., Wang, Y., Zhang, C., Stanton, E. J., Schow, C. L., Cheng, K. - T., and Bowers, J. E., "Compact Modeling for Silicon Photonic Heterogeneously Integrated Circuits", Journal of Lightwave Technology (JLT), 2017.
F. Lan, Wu, R., Zhang, C., Pan, Y., and Cheng, K. - T. Tim, " DLPS: Dynamic Laser Power Scaling for Optical Network-on-Chip ", in Asia and South Pacific Design Automation Conference (ASP-DAC), 2017.
R. Wu, Chen, C. - H., M. Seyedi, A., Huang, T. - C., Fiorentino, M., Beausoleil, R. G., and Cheng, K. - T. Tim, " Large-Signal Model for Small-Size High-Speed Carrier-Injection Silicon Microring Modulator ", in OSA Integrated Photonics Research, Silicon, and Nano-Photonics Conference (IPR), 2016.
R. Wu, Chen, C. - H., Huang, T. - C., Beausoleil, R. G., and Cheng, K. - T. Tim, " Spatial Pattern Analysis of Process Variations in Silicon Microring Modulators ", in IEEE Optical Interconnect Conference (OI), 2016.
R. Wu, Chen, C. - H., Li, C., Huang, T. - C., Lan, F., Zhang, C., Pan, Y., Bowers, J. E., Beausoleil, R. G., and Cheng, K. - T., " Variation-Aware Adaptive Tuning for Nanophotonic Interconnects ", in International Conference on Computer-Aided Design (ICCAD), 2015.
R. Wu, Chen, C. - H., Fedeli, J. - M., Fournier, M., Cheng, K. - T. Tim, and Beausoleil, R. G., "Compact models for carrier-injection silicon microring modulators", Optics Express, vol. 23, no. 12, pp. 15545-15554, 2015.
J. Bovington, Wu, R., Cheng, K. - T. Tim, and Bowers, J. E., "Thermal stress implications in athermal TiO2 waveguides on a silicon substrate", Optics Express, vol. 22, pp. 661–666, 2014.
Y. Zheng, Lisherness, P., Gao, M., Bovington, J., Cheng, K. - T., Wang, H., and Yang, S., "Power-efficient calibration and reconfiguration for optical network-on-chip", Journal of Optical Communications and Networking (JOCN), vol. 4, pp. 955–966, 2012.
Y. Zheng, Lisherness, P., Gao, M., Bovington, J., Yang, S., and Cheng, K. - T. Tim, "Power-efficient calibration and reconfiguration for on-chip optical communication", in Design, Automation & Test in Europe Conference & Exhibition (DATE), 2012.