- Electrical Engineering
- Communications & Networking
- Electrical Interconnect and Packaging
- VLSI Design
2012 IEDM postdeadline paper
2012 CLEO Plenary talk
2012 IEEE Comm. Mag., Silicon Nanophotonics Beyond 100G
2011 IBM R&D Journal: Technologies for Exascale systems
2010 SEMICON Talk: CMOS Nanophotonics for Exascale
2008 ECOC Tutorial: On-Chip Si Nanophotonics
In the premiere January 2007 issue of the scientific journal Nature Photonics, the IBM team demonstrated optical buffering of 10 bits of information on a tiny device on a silicon chip that could allow one day supercomputers to utilize optical interconnects to achieve better performance.
This demonstration is a major milestone in development of all-optical buffers for optical interconnects, which is one of the major thrusts of the Silicon Integrated Nanophotonics project. Implementation of dense optical interconnects in systems, servers, and inside the box is severely limited by lack of a reliable scheme to obtain optical buffering at the switching nodes. To avoid data congestion in the switching fabric multiple electrical-to-optical (EO) and OE conversions are necessary to convert optical signals to electrical and store the packets in memory until the congestion is resolved. If optical buffering will become a reality this might be a major breakthrough in the development of interconnect network. Avoiding multiple EO/OE conversion might not only drive the cost/performance significantly below corresponding metrics for electrical cables, but also might result in a better signal integrity and lower overall power consumption.
Long delays can be achieved simply by using an optical waveguide whose length is long enough to delay optical signals. However the footprint of such a delay line is large and could not be used for on-chip integration. Resonantly enhancing the delay hence making the device footprint small is of great importance for on-chip applications. For example, when the waveguide is curved to form a ring, light is forced to circle multiple times in the ring at resonance frequencies, lengthening the delay.
The demonstrated device is capable to store optical information for data streams as fast at 20Gbps (Giga-bit per second). The device is comprised of up to 100 identical micro-ring resonators that are all working in–tune to produce very large optical delays in excess of 500ps. Most of these demonstrated numbers represent at least a factor of 10 improvements with respect to previous demonstrations.
Utilization of nanophotonic silicon waveguides and ultra-compact micro-rings with the radius as small as just a few microns allows to shrink the device area to miniscule 0.03mm2 that is becoming comparable to the CMOS microelectronics circuits. Interestingly enough the actual demonstrated buffering capacity of 10bits on a footprint that is as small as 0.03mm2 is only about 10 percent of the surface storage density of the floppy disk. This buffering density for optical signals is exceeding any analogous reports by at least a factor of 100-1000. This advancement could potentially lead to integrating hundreds of these devices on one computer chip. Demonstrated 10bits of information is already enough to encode and buffer one single ASCII character. However to be useful for on-chip optical interconnects 100 to 1000 bits delay is necessary. The level of miniaturization of a demonstrated delay line in principle does allow to achieve this in a very small footprint on a chip.
This work is also an important technological advancement in the broad area of nanophotonics. To demonstrate the experimentally functional delay lines composed of multiple cascaded ring resonators significant efforts were applied to design lossless coupling between the resonators, to minimize sidewall and bending losses, and to control geometric parameters of all resonators in a cascade to within just a few atomic layers of silicon. Excellent control of critical dimensions is demonstrated in a device with up to 100 coupled rings with frequency misalignment that is only 0.05% of the central wavelength. Our development of an optical delay line is based on several achievements including record low-loss SOI nanophotonic waveguides (loss 1.6dB/cm), low-loss sharp bends (loss 0.004dB/turn for 6um radius) and innovative design of the ring resonators for reduced coupling losses.
This work was partially supported by the Defense Advanced Research Projects Agency (DARPA) through the Defense Sciences Office program Slowing, Storing and Processing Light.