24 July 2012
Traditionally, edge-emitter lasers are considered as the candidate for on-chip optical links. However, because mirrors are hard to form in these lasers and because the lasers occupy a large chip area, researchers have been challenged to find a practical way to monolithically integrate the mirrors on silicon chips.
Now, electrical engineers at The University of Texas at Arlington and at the University of Wisconsin-Madison have created a laser at just 2 µm to combat this challenge. The device’s lower profile could make it cheaper and easier for manufacturers to integrate high-speed optical data connections into the microprocessors powering the next generation of computers.
Surface-emitting lasers necessary for high-speed optical links between computer cores could be 20 to 30 µm tall, slightly bigger than one hole in the mesh of a car’s oil filter. Yet the research team’s engineers say that, on a 1.5-µm-wavelength optically connected chip, lasers of that size dwarf their silicon surroundings.
“It sits tall on the chip, like a tower,” said Zhenqiang Ma, a UW-Madison professor of electrical and computer engineering. “That is definitely not acceptable.”
Integrating light into silicon chips, which are not efficient light emitters, was a challenge, said Weidong Zhou, a UT-Arlington professor of electrical engineering.
Zhou and Ma collaborated to shrink on-chip lasers. They proposed replacing layers of reflectors necessary in the traditional distributed Bragg reflector design with two highly reflective photonic crystal mirrors. Each mirror, composed of compound semiconductor quantum well materials, is held in place with silicon nanomembranes.
One layer of photonic crystal is equal to about 15 to 30 layers of dielectric reflectors found in conventional lasers. As a result, manufacturers could fabricate 2-µm-high lasers for data links with a performance that could equal current designs.
Besides their larger size, reflectors for conventional lasers are made of materials grown only at very high temperatures, which means they can damage the chip they are placed on during production. Fabrication via transfer printing, on the other hand, occurs at much lower temperatures, so the scientists’ laser design can be used to place optical links on silicon chips with much less wasted material, time and effort.
Although optical data links already exist at the largest scale of data networks, the data moves over slower metal connections and wiring as it travels from a regional hub to your home, computer and between the CPU cores within the processor powering your machine.
“In the future, you’ll see a move to optical at each step,” Ma said. “The last step is within the chip, module to module optical links on the chip itself.”
The two engineers founded Semerane Inc. to implement their production process in functional on-chip photonic crystal membrane lasers that could eventually be part of the next-generation high-speed computer processors. Their hope is to make data links more practically available.
“Eventually, a CPU core in America could be connected to another CPU core in Asia, with optical connections all along the chain,” Ma said.
The findings, which appeared July 22 in Nature Photonics, was funded by the US Air Force Office of Scientific Research, Army Research Office and DARPA.