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  • Better Optical Modulators Boost Silicon Photonics
Oct 2013
BOULDER, Colo., & CAMBRIDGE, Mass., Oct. 3, 2013 — Improvements to two optical modulators are seen as a major step toward a major goal in silicon photonics: enabling microprocessors to use light instead of electrical signals to communicate with transistors on a chip.

The pair of optical modulators was made at the University of Colorado at Boulder, MIT and Micron Technology Inc., and the team said that the work could allow for the trajectory of exponential improvement in microprocessors that began nearly half a century ago — known as Moore's Law — to continue well into the future, allowing for increasingly faster electronics, from supercomputers to laptops to smartphones.

A silicon wafer containing the photonic-electronic microchips designed by the research team, which includes scientists from CU-Boulder, MIT, Micron and UC Berkeley. Introducing photonics into electronic microprocessors could extend Moore's Law well into the future. Courtesy of Milos Popovic. 

The research team, led by CU-Boulder researcher Milos Popovic, an assistant professor of electrical, computer and energy engineering, developed a new light-based technique for microprocessors. He and his colleagues created two different optical modulators — structures that detect electrical signals and translate them into optical waves — that can be fabricated using standard industrial processes for microprocessors.

In 1965, Intel co-founder Gordon Moore predicted that the size of transistors used in microprocessors could be shrunk by half about every two years for the same production cost, allowing twice as many to be placed on the same-sized silicon chip. The net effect would be a doubling of computing speed every couple of years, and his projection held true for more than 40 years.

While transistors continue to get smaller, halving their size today no longer leads to a doubling of computing speed. The limiting factor is the power 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.

A microchip that contains both photonics and electronics is tested at CU-Boulder researcher Milos Popovic's lab. Photo by Casey Cass, CU-Boulder.

"The transistors will keep shrinking, and they'll be able to continue giving you more and more computing performance," Popovic said. "But in order to be able to actually take advantage of that, you need to enable energy-efficient communication links."

Another limitation with microelectronics is that positioning electrical wires carrying data too closely can result in crosstalk between the wires.

In the past half-dozen years, microprocessor manufacturers such as Intel have continued increasing computing speed by packing more than one microprocessor into a single chip, creating multiple "cores." But that technique is limited by the amount of communication that becomes necessary between the microprocessors, requiring hefty electricity consumption.

Using lightwaves instead of electrical wires could eliminate these limitations and extend Moore's Law into the future, Popovic said.

A digital rendering of an optical modulator that can be fabricated using standard industry processing for making today's state-of-the-art electronic microprocessors. Courtesy of Milos Popovic.

To make optical communication an economically viable option for microprocessors, the photonic technology must be fabricated in the same foundries that are used to create the microprocessors. Photonics must be integrated side-by-side with the electronics to get buy-in from the microprocessor industry, Popovic said.

"In 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," he said.

Last year, Popovic collaborated with scientists at MIT to show, for the first time, that such integration is possible. "We are building photonics inside the exact same process that they build microelectronics in," Popovic said. "We 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."

In two papers published in August in Optics Letters ( and with CU-Boulder postdoctoral researcher Jeffrey Shainline as lead author, the research team refined its original photonic-electronic chip further, detailing how the crucial optical modulator could be improved to become more energy-efficient. That optical modulator is compatible with a manufacturing process — known as silicon-on-insulator CMOS, or SOI CMOS — used 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's high-end microprocessors.

Members of the research team — from left, Jeff Shainline, Milos Popovic and Mark Wade — discuss the photonic-electronic microchip they developed. Courtesy of Casey Cass, CU-Boulder.

Vladimir Stojanovic, leader of one of the MIT teams collaborating on the project and lead principal investigator for the overall research program, said the group's work on optical modulators is a significant step forward.

"On 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," 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. The research was funded by DARPA and the National Science Foundation.

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optical modulator
A multilayered thin-film device used to modulate transmitted light in integrated photonic circuits.
An electronic device consisting of a semiconductor material, generally germanium or silicon, and used for rectification, amplification and switching. Its mode of operation utilizes transmission across the junction of the donor electrons and holes.
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