PORTLAND, Ore. Silicon photonics could someday replace $10,000 optical-to-electrical-to-optical converters with $1 CMOS chips and enable on-chip data communications with photons, not the electrons used today.
But the road to silicon photonics is fraught with peril. Doomsayers originally insisted that silicon's indirect bandgap forever favored direct bandgap materials like indium-gallium-arsenide (InGaAs), but history proved those compounds were too difficult to integrate.
Now, researchers at IBM's Thomas J. Watson Research Center (Yorktown Heights, N.Y.) say they've clinched the case for silicon photonics. IBM will announce new world's records Friday (Dec. 22) for silicon photonics speed, density and bandwidth. IBM also said its demonstration chip, a CMOS optical delay line, proves that silicon photonics will ultimately achieve the "holy grail" optics on chips at integration levels rivaling those of electronic CMOS integration.
"We have demonstrated for the first time, that we can build silicon-on-chip delay lines in a footprint of 0.03 millimeters2chip shows that now we can really begin to think about integrating optics and CMOS together."
The previous world's record in silicon photonics, held by Alcatel-Lucent's Bell Laboratories cast an optical delay line into an on-chip ring resonator measuring a few millimeters. By contrast, IBM's ring resonators are 6 microns in diameter.
"The previous delay lines based on ring resonators occupied a few square centimeters on a chip, making our delay line with micron-sized rings orders of magnitude (more than 100 times) smaller," said Yurii Vlasov, who is also a member of the research staff. "But it is not the small size of our device, even, that is most impressive," he said. "Rather it is the level of control we achieved in our waveguide dimensions the gap between the waveguide and the ring, for instance, has to be precisely controlled to within just a few nanometers, making our device the world's first to qualify as silicon optical nanotechnology."
IBM crafted its on-chip optical delay line from a 200-mm silicon-on-insulator (SOI) wafer with 226 nm of silicon atop a 2-micron-thick buried oxide layer, creating photonic-wire waveguides with submicron cross sections (510 x 226 nm) capable of single-mode propagation at the all-important 1.55-micron wavelength favored by networks today, with about 1.7 db of loss per centimeter.
The resulting CMOS chip achieved nanosecond-scale delays, which could work in network connections today to reduce congestion at busy switches. Each ring resonator slows the light passing through it by forcing it to circle around many times before passing along to the next ring. By ganging 100 ring resonators in series, IBM also demonstrated the feasibility of on-chip photonic silicon delays lines.
"In principle, optical delay is a very useful function for networks to reduce traffic congestion at the intersections of the switching nodes where today it is usually resolved by translating optical information into electrical and storing it electrically in a DRAM," said Xia.
"The light goes around each of the 100-ring resonators between 60 and 80 times to achieve the long delay time," said Vlasov.