Silicon photonics (SiP) is emerging as a leading candidate for addressing the power and bandwidth challenges of next generation on- and off-chip interconnects due to its compatibility with complementary-metal-oxide-semiconductor (CMOS) process, ultra-small device footprint, low power consumption and high bandwidth. Small footprint allows fitting hundreds to thousands of these devices on a single chip, thus allowing true scalability of the i/o bandwidth. As the power consumption is proportional to the device volume (through the amount of electrical carriers that need be injected), the power consumption of such devices, at 10 fJ/bit, is also orders of magnitudes smaller than other structures. Hypothetically, this in turn enables < 1 pJ/bit link power efficiencies.
A significant part of our research is devoted to investigating the characteristics of various nano-photonic devices for application in telecom and datacom systems. All components necessary for a transmission link have been demonstrated and characterized including transceivers, modulators, filters and receivers. Ongoing research efforts are directed towards realizing dense WDM, various modulation formats for increased laser power utilization and further improving and optimizing device characteristics in collaboration with device design groups.
Microring devices have the advantage of small-footprint, energy efficient operation, and intrinsic WDM compatibility. They are especially promising for use in the CMOS-compatible silicon photonics platform to replace electrical interconnects in next generation datacenters and supercomputers. One challenge facing commercial usage of microrings is their thermal sensitivity. Because of the high thermal optic coefficient of silicon the resonance location of microrings shift around 0.8 nm/K. Our group has pioneered active thermal feedback systems based on dithering, where a small periodic signal applied to the heater is used to derive the location of the microring resonance. The dither technique overcomes the symmetry of the microring resonance and allows simple feedback circuitry to be used. This technique has been applied to thermally stabilize multiple microrings at the same time for WDM operation.
Fast thermal initialization of microring devices is also possible by leveraging the dithering method. Due to fabrication variation, the initial resonance location of a microring will be slightly different from the intended operating laser wavelength in the system. Thus an initialization sequence is necessary to lock the microrings to the lasers. We demonstrated that the initialization process can be accomplished within an order of magnitude of the thermal time constant of the microring system.
High-port-count optical space switches, long of interest in long-haul systems, have found renewed interest in data centers to provide high bandwidth connections on demand. We have demonstrated the applicability of SiP Mach-Zehnder and microring based device for optical switching and we continue to study their scalability.
In our endeavor to investigate practical microring resonator-based dynamic switching functionalities within this silicon platform, we have characterized a multi-wavelength 1×2 switch, capable of switching optical signals with data rates approaching terabits-per-second. Subsequently, we examined a 2×2 switch, which also enables dynamic message routing of wavelength-parallel broadband photonic signals. Combining the demonstrated 1×2 and 2×2 switching functionalities, we then investigated a thermally active 4×4 switch, which enables non-blocking switching functionality within complex network architectures.
In addition to the design and implementation of these components, we have characterized their predominant signal-degrading impairments in an effort to further improve the performance of interconnection networks derived from future versions of these devices. Narrowband filtering of high-speed data signals is an important consideration when passing data through high-quality-factor microring resonators, as is done in the aforementioned multi-wavelength switches. We have experimentally and numerically characterized the power penalty induced by this effect, and applied the numerical simulator to a wide variety of other higher order transfer functions realizable with these devices.
Advances in multicore architectures have produced increased parallelism in computation, and as a result data movement rather than raw computation has come to determine system performance in data centers and high performance computational systems. The fundamental limitation in increased data movement is energy consumption: for electronic links, power dissipation scales with data throughput. Optical data transfer over silicon photonics offers a promising alternative as wave division multiplexing allows for high bandwidths and the bit-rate transparency of photonic systems means power consumption does not scale with data throughput. While point-to-point optical data transfer is a fairly mature area, data-dependent traffic exhibits several aspects of dynamic path dependence in computational systems. Frequently path changes may occur, which requires a switching fabric to respond to these demands in real-time with reasonable network utilization. For a photonic switching fabric to be of practical use, it must be energy efficient, have short reconfiguration times, maintain optical signal quality, and be scalable to larger switching sizes.
We are working towards developing and testing programmable switching fabrics composed of Mach Zehnder Interferometers (MZIs) and microring resonators. Single devices can act as 2x2 switching elements, whose switching behavior can be tuned with an applied voltage. Since volume devices are subject to manufacturing tolerances, device characterizations are necessary to accurately determine the applied voltages necessary for the appropriate behavior. By intelligently connecting multiple MZIs or microrings, larger switching fabrics can be constructed. We are using FPGAs as a central logic controller-dubbed the arbiter-connected to analog-to-digital and digital-to-analog electronics used to quickly actuate physical configurations of many photonic devices. Our arbitration system includes a higher control layer that is compatible with software and provides a control message passing interface.