Better optical modulators boost silicon photonics

An improved pair of optical modulators could be a big 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 optical modulators were made at the University of Colorado at Boulder, MIT and Micron Technology Inc., and their creators say that they could allow for Moore’s law – the trajectory of exponential improvement in microprocessors that began nearly a half-century ago – to continue well into the future, allowing for increasingly faster electronics, from supercomputers and laptops to smartphones.

This silicon wafer contains new photonic-electronic microchips. Introducing photonics into electronic microprocessors could extend Moore’s Law well into the future. Photo courtesy of Milos Popovic.

The research team, led by CU-Boulder researcher Milos Popovic, an assistant professor of electrical, computer and energy engineering, created two different optical modulators that can be fabricated using standard industrial processes for microprocessors.

As 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.

“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.”

A microchip that contains both photonics and electronics is tested at the lab of CU-Boulder researcher Milos Popovic. Photo courtesy of Casey Cass, CU-Boulder.

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, Popovic said.

To make optical communications an economically viable option for microprocessors, the technology must be fabricated in the same foundries that are used to create the microprocessors. “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.

In two papers published in August in Optics Letters (doi: 10.1364/ol.38.002729 and doi: 10.1364/ol.38.002657) 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.