Project Overview 
Data centers and High-Performance Computing (HPC) systems are an essential part of our cyberinfrastructure. While the energy consumption of data centers worldwide is currently estimated at 26 GW, the amount of data we process is growing at ~2 dB per year. Unless we drastically improve the energy efficiency, the energy consumption in data centers worldwide will increase to ~2600 GW in the next decade. Unfortunately, traditional electronic data center designs today remain inefficient in energy utilization due to poor efficiency in electronics and poor communication architectures. Due to limitations in port-count and bandwidth of the electronic switches, the inefficiency of the cascaded switch stages compounds, especially in terms of latency, throughput, scalability, and power consumptions. Utilizing the classical electronic infrastructure current and future data centers are not going to achieve the 20-30MW expected results in terms of energy in 2020 [1, 2]. Current HPC Data Centers use a single architecture (e.g. 3D Torus [3], HyperX [4], and Fat-Tree [5, 6]) to serve various applications. To improve the performance and efficiency of these systems, it is important to observe that multiple heterogeneous applications running inside HPC Data Centers present different communication patterns among the computing nodes. To optimize each application performance, it would be desirable to match the inter-node communication network to the specific application [7]. Hence, the best design should be capable of reconfiguring the network based on various communication patterns of heterogeneous applications. Such reconfiguration is difficult in electrically interconnected systems with fixed cabling but it can be possible when using integrated optical switching technologies as passive and active wavelength-routing in Arrayed Waveguide Grating Routers (AWGRs) [8, 9].

Figure 1. Multi-hop Legacy Interconnect Architecture (from [6]). Hierarchical all-to-all architecture using wavelength routing in AWGRs. AWGR All-to-All working principle.

The energy cost of moving data, particularly for long distances and high data-rates, is becoming one of the dominant factor for the energy consumption, greater than the computational energy cost. Furthermore, a significant part of power inefficiency in communication comes from poorly adapting to the communication and traffic patterns. Hence, most of the time, current communication systems are consuming energy even when no meaningful bits need to be transported. This means that most of the energy consumption in current data center architectures, is quite independent of the load, and thus load balancing/scheduling techniques cannot guarantee significant energy savings. Finally, close integration of photonics with electronic ICs, or ‘embedded silicon photonics’ is viewed as a transformational technology which enables energy-efficient, high throughput, and cost-effective data communications. By introducing embedded photonics it is possible to intimately integrate photonics and electronics via optical interposers and silicon photonics with 2.5D and 3D integration. Embedded photonics will significantly impact the energy efficiency and the cost of chip-to-chip, board-to-board, and rack-to-rack data communications. 

Current Research Activities 
This project pursues scalable, high-throughput and energy-efficient computing systems realized by incorporating high-radix silicon photonic switches, multi-socket blade integration, application-aware reconfigurability of interconnection topology and resource management. In particular, UC Davis research team has been pursuing the following research efforts: 
1. Design, fabrication and testing of high-radix silicon photonic switches exploiting contentionless wavelength routing in arrayed waveguide grating routers (AWGRs) as a building block for all-to-all interconnection inside a multi-socket blade (MSB). Through the reconfigurability of the AWGR switch we will achieve reconfigurable optical interconnection between the MSBs depending on the application needs. 

Figure 2. Photo fabricated chip consists of two 8 x 8 AWGR optical switches integrated with silicon photonic transmitter and receiver arrays. The section ‘SiN AWGR’ is an all-to-all 8×8 optical switch utilizing a silicon nitride based 8×8 AWGR, and the section ‘Si AWGR’ is an 8×8 optical switch utilizing a silicon based 8×8 AWGR. (b) Measured transmission spectra of the SiN AWGR from input port 4. All the powers are normalized to test straight waveguide transmission.


Figure 3. (a) SiN AWGR, (b) top: SiN AWGR waveguides array; bottom: SiN- Si waveguides; (c) AWGR star coupler and free propagation region; (d) Integrated modulator.


 Figure 4. Optically interconnected Multi-Socket Board. The Multi-Socket Board (MSB) architecture based on passive AWGR all-to-all interconnection. Each socket has p+µ transmitters and receivers for intra- and inter-board communications.

2. Development of a scalable system-wide benchmarking simulator for optimizing (and reconfiguring) the interconnection topology and resource allocations for given (dynamically changing) applications, including energy-efficient communication by making use of dynamic frequency and voltage scaling (DVFS). 

Figure 5. Execution time (a) and energy (b) results for an optical AWGR-based MSB normalized to the electronic baseline. The first bars in the figures represent the baseline; the second ones the optical configuration with the CDR; the third ones represent the optical configuration with source synchronous and 2-level voltage and frequency scaling based on link utilization and, finally, the forth ones the optical configuration with source synchronous and 3-level voltage and frequency scaling based on buffer thresholds.


3. Optical-electrical printed circuit board (OE-PCBs) and interposers that exploit 3D photonics coupled to silicon photonic switches and transceivers, thus achieving photonic and electronic interconnects between electronic ICs and silicon photonics. 

Figure 6. (a) An OE-PCB containing multiple silicon photonic optical interposer and electronic ICs, and (b) is its top view (from optical interposer side) and (c) shows its back view (from the OE-PCB side). Figure 4 (d) shows a sequence of integration of the optical interposer and the OE-PCB which contain matching grooves and optical waveguides.
Figure 7. (Left) 3D integration of E-ICs on an active optical interposer with an active optical layer to be mounted on an OE-PCB. Perspective view of the electronic IC and active silicon photonic interposers with C4 bumps ready for integration on an OE-PCB. [Right] Coupling and misalignment tolerance between the optical interposer and a silicon photonic die consisting of negative tapers indicating +/- 1 µm lateral misalignment tolerance.

4. Design, simulate, and develop DRAM with embedded 3D electronic-photonic-integrated-circuits (3D EPICs) for 4x reduced latency and 4x reduced power consumption in processor-memory average access time (AMAT) interconnected by AWGRs. Figure 12 shows details of the configurations of SiPh-FGDRAMs. The Optical TSV is based on the 3D silicon photonics recently experimentally demonstrated the PI’s group [28]. As Figure 12 (c) illustrates, the Optical TSV consists of 90 degree vertical couplers and vertical U-turn silicon photonic devices which consist of a silicon photonic vertical via and a 45o reflector attached to a waveguide end. This vertical coupler can also be used for interlayer coupling in a multi-layer silicon photonic 3D integrated circuit by placing a matching vertical coupler face-to-face.

Figure 8. (a) shows 3D stacking of silicon-photonic-electronic DRAM dies with the silicon photonic logic layer at bottom vertically interconnected by through-silicon-optical-vias (TSOVs). (b) proposed 3D EPIC DRAM utilizing silicon-photonic CMOS DRAM mini-bank dies in multi-stacks including the silicon photonic logic layer at bottom. (c) A schematic of a vertical U-turn silicon photonic device formed by two silicon photonic 45o reflectors and a vertical via (d) FDTD simulation of optical propagation through the vertical U-turn silicon photonic device.

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