A new design for on-chip laser optical connections which could give a huge speed boost to computers has been unveiled by electrical engineers from the University of Wisconsin-Madison and the University of Texas at Arlington. The findings were published in the Nature Photonics journal’s July 22, 2012 issue.
The surface-emitting optically pumped laser measures only 2 micrometers in height, smaller than the width of a human hair. Such a vastly lower profile might one day make it cheaper and easier for manufacturers to combine high-speed optical data connections into microprocessors powering the next generation of computing.
Laser on a Chip Breakthrough
Conventionally, edge-emitter lasers have been considered the only candidate for on-chip optical links. But, because mirrors are hard to form in such lasers, and since the lasers occupy a large chip area, researchers have been hard-pressed to find a practical way to monolithically integrate the mirrors on silicon chips.
Surface-emitting lasers needed for high-speed optical links between computer cores could be 20 to 30 micrometers tall, somewhat bigger than one hole in the mesh of a car’s oil filter. However the research engineers state that on a 1.5-micrometer wavelength optically connected chip, lasers of that size dwarf their silicon surroundings.
“It sits tall on the chip, like a tower,” according to Zhenqiang Ma, UW-Madison professor of electrical and computer engineering. “That is definitely not acceptable.”
Reflective Photonic Crystal Mirrors
As a solution, the researchers suggest replacing layers and layers of reflectors necessary in the usual distributed Bragg reflector laser design with two highly reflective photonic crystal mirrors. Made of compound semiconductor quantum well materials, each mirror is held in place with silicon nanomembranes, exceptionally thin layers of silicon.
“We apply a nanomembrane transfer printing process to accomplish this goal,” says Weidong Zhou, UT Arlington professor of electrical engineering.
A single layer of photonic crystal is equivalent to about 15 to 30 layers of dielectric reflectors found in conventional lasers. Consequently, manufacturers could manufacture 2-micrometer-high lasers for data links with performance that could equal current designs.
Module to Module Optical Links
In addition to their bigger size, reflectors for conventional lasers are composed of materials grown exclusively at very high temperatures. As such, they can damage the chip they are placed upon during production. Since fabrication via transfer printing can occur at much lower temperatures, Zhou and Ma hope their laser design can be used to place optical links on silicon chips with much fewer wasted materials, time and effort.
Optical data links are already a reality at the biggest scales of data networks. The backbone of the Internet is composed mainly of fiber-optic links between countries, cities and nieghborhoods. Yet at present, data moves over to slower metal connections and wiring when it travels from a regional hub to your house, computer and ultimately between the CPU cores in the processor powering your machine.
“In the future, you’ll see a move to optical at each step,” Ma says. “The last step is within the chip, module to module optical links on the chip itself.”