technology VCSELs
those built with integrated optics, due to the far shorter wavelength of electrons than photons. Billions and billions of dollars have been poured into the development of silicon microelectronics, but all that funding cannot overcome a fundamental limitation: Optics is much more energy-efficient for interconnects once the bandwidth-length product exceeds a certain value, which is now considered to be relatively low. This limitation of microelectronics is propelling the world towards optical interconnects while one key issue is still to be resolved: How to realize the electrical to optical transition? The existing technologies employed in long- haul links are too expensive and energy hungry to be suitable for use in short and ultra-short optical links.
Turning to VCSELs
The most suitable, simplest high-speed optical source is the directly modulated laser. If this is to combine excellent beam quality with low cost and frugal use of energy, the VCSEL has to be the technology of choice. Used in short optical interconnects, VCSELs must excel on three fronts: They must deliver high serial bandwidths, allow dense packaging, and operate without cooling.
System design rules, such as Amdahl’s law, can reveal the required serial bandwidth. Applying this law shows that it is possible to avoid bottlenecks in a supercomputer by matching computational power to interconnect bandwidth and memory capacity. However, simply adding more links to increase bandwidth can lead to penalties, in terms of complexity and cost. What’s more, with one particular technology, there is a limit to the number of links that can be connected. In 2011, these considerations led system designers from Google to state that 40 Gbit/s will be the desired bandwidth for their next-generation data-centers.
resulting from the postponed transition from copper to optics. Concerns are so severe that they have led to the publication of articles with headlines such as The End of Supercomputing. Such tories have appeared because it is possible to show that all of the world’s electrical energy generation could go into just bits and bytes, by extrapolating current growth in bandwidth demand, using today´s values for energy consumption per transmitted bit for supercomputers and data-centers.
The ill informed may try to brush aside this looming catastrophe, arguing that humanity doesn’t need supercomputers. What they don’t know is that the Internet is run by large data-centers – when you press a single button for an Internet search, 300 computers scurry away to deliver rapid suggestions. How can these computers communicate, before transmitting the information to the end user? That’s right! By interconnects. Thousands of them.
All theses computers are built around microelectronics, which yield circuits that are far more compact than
To enable system scalability, the optical chips employed in these links must be housed in very dense packaging. One of the strengths of the VCSEL is its small footprint – it is an order of magnitude smaller than it edge-emitting cousins. It is also compatible with a compact hybrid package design, such as that adopted in the IBM TERABUS project. Here, a tiny package resulted from
Keys to successful VCSEL research
include a high yield full-wafer fabrication process, systematic design variation and automated evaluation
June 2012
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