A pair of breakthroughs in the field of silicon photonics by researchers at the University of Colorado Boulder, the Massachusetts Institute of Technology and Micron Technology Inc. could allow for the trajectory of exponential improvement in microprocessors that began nearly half a century ago鈥攌nown as Moore鈥檚 Law鈥攖o continue well into the future, allowing for increasingly faster electronics, from supercomputers to laptops to smartphones.
The research team, led by CU-Boulder researcher Milos Popovic, an assistant professor of electrical, computer and energy engineering, developed a new technique that allows microprocessors to use light, instead of electrical wires, to communicate with transistors on a single chip, a system that could lead to extremely energy-efficient computing and a continued skyrocketing of computing speed into the future.
Popovic and his colleagues created two different optical modulators鈥攕tructures that detect electrical signals and translate them into optical waves鈥攖hat can be fabricated within the same processes already used in industry to create today鈥檚 state-of-the-art electronic microprocessors. The modulators are described in a recent issue of the journal Optics Letters.
First laid out in 1965, Moore鈥檚 Law predicted that the size of the transistors used in microprocessors could be shrunk by half about every two years for the same production cost, allowing twice as many transistors to be placed on the same-sized silicon chip. The net effect would be a doubling of computing speed every couple of years.
The projection has held true until relatively recently. While transistors continue to get smaller, halving their size today no longer leads to a doubling of computing speed. That鈥檚 because the limiting factor in microelectronics is now the power that鈥檚 needed to keep the microprocessors running. The vast amount of electricity required to flip on and off tiny, densely packed transistors causes excessive heat buildup.
鈥淭he transistors will keep shrinking and they鈥檒l be able to continue giving you more and more computing performance,鈥� Popovic said. 鈥淏ut in order to be able to actually take advantage of that you need to enable energy-efficient communication links.鈥�
Microelectronics also are limited by the fact that placing electrical wires that carry data too closely together can result in 鈥渃ross talk鈥� between the wires.
In the last half-dozen years, microprocessor manufacturers, such as Intel, have been able to continue increasing computing speed by packing more than one microprocessor into a single chip to create multiple 鈥渃ores.鈥� But that technique is limited by the amount of communication that then becomes necessary between the microprocessors, which also requires hefty electricity consumption.
Using light waves instead of electrical wires for microprocessor communication functions could eliminate the limitations now faced by conventional microprocessors and extend Moore鈥檚 Law into the future, Popovic said.
Optical communication circuits, known as photonics, have two main advantages over communication that relies on conventional wires: Using light has the potential to be brutally energy efficient, and a single fiber-optic strand can carry a thousand different wavelengths of light at the same time, allowing for multiple communications to be carried simultaneously in a small space and eliminating cross talk.
Optical communication is already the foundation of the Internet and the majority of phone lines. But to make optical communication an economically viable option for microprocessors, the photonics technology has to be fabricated in the same foundries that are being used to create the microprocessors. Photonics have to be integrated side-by-side with the electronics in order to get buy-in from the microprocessor industry, Popovic said.
鈥淚n order to convince the semiconductor industry to incorporate photonics into microelectronics you need to make it so that the billions of dollars of existing infrastructure does not need to be wiped out and redone,鈥� Popovic said.
Last year, Popovic collaborated with scientists at MIT to show, for the first time, that such integration is possible. 鈥淲e are building photonics inside the exact same process that they build microelectronics in,鈥� Popovic said. 鈥淲e use this fabrication process and instead of making just electrical circuits, we make photonics next to the electrical circuits so they can talk to each other.鈥�
In two papers published last month in Optics Letters with CU-Boulder postdoctoral researcher Jeffrey Shainline as lead author, the research team refined their original photonic-electronic chip further, detailing how the crucial optical modulator, which encodes data on streams of light, could be improved to become more energy efficient. That optical modulator is compatible with a manufacturing process鈥攌nown as Silicon-on-Insulator Complementary Metal-Oxide-Semiconductor, or SOI CMOS鈥攗sed to create state-of-the-art multicore microprocessors such as the IBM Power7 and Cell, which is used in the Sony PlayStation 3.
The researchers also detailed a second type of optical modulator that could be used in a different chip-manufacturing process, called bulk CMOS, which is used to make memory chips and the majority of the world鈥檚 high-end microprocessors.
Vladimir Stojanovic, who leads one of the MIT teams collaborating on the project and who is the lead principal investigator for the overall research program, said the group鈥檚 work on optical modulators is a significant step forward.
鈥淥n top of the energy-efficiency and bandwidth-density advantages of silicon-photonics over electrical wires, photonics integrated into CMOS processes with no process changes provides enormous cost-benefits and advantage over traditional photonic systems,鈥� Stojanovic said.
The CU-led effort is a part of a larger project on building a complete photonic processor-memory system, which includes research teams from MIT led by Stojanovic, Rajeev Ram and Michael Watts, a team from Micron Technology led by Roy Meade and a team from the University of California, Berkeley, led by Krste Asanovic.听 The research was funded by the Defense Advanced Research Projects Agency and the National Science Foundation.
Contact:
Milos Popovic, 303-492-5304
Milos.Popovic@colorado.edu
Laura Snider, CU media relations, 303-735-0528
Laura.Snider@colorado.edu
From left, Jeff Shainline, a postdoctoral researcher; Milos Popovic, an assistant professor of electrical, computer and energy engineering; and Mark Wade, a graduate student, discuss the silicon wafer containing the photonic-electronic microchips they designed. Photo by Casey Cass, CU-Boulder.
