Silicon photonics move to rule networks

Developments in silicon photonics have moved the technology into the mainstream, according to presenters at the 2008 Optical Fiber Communication Conference and Exposition.

The move from copper twisted pairs—which consume from seven or more watts to transmit and receive 10Gbit/s of network traffic—to optical fibers can cut power consumption tenfold, proponents say. Moving further to silicon-based optical components could cut power by half again while driving down costs with CMOS integration.

CMOS gains
"Using silicon in optical networks is no longer just research," said Karen Liu, a VP at Ovum RHK, the telecom research arm of Datamonitor PLC. "Companies are now fabricating optical components in CMOS, which should lower costs and power consumption."

Both IBM's T.J. Watson Research Center and Intel Research have shown silicon lasers and waveguides designed to facilitate optical communications on future CMOS chips. And last fall, Luxtera Inc. announced it was sampling the first CMOS chip designed to plug-and-play with existing optical networks.

Now Lightwire Inc. has also announced it is sampling a CMOS photonics transceiver aimed at replacing existing 220m line-replaceable modules (LRM) with small form factor pluggable modules based on silicon.

Different approaches
"Luxtera and Lightwire are answering different questions about what silicon can be used for in optical networks," said Liu. "Luxtera is answering the question of whether both electrical and optical components can be integrated onto the same chip, and Lightwire is answering the question of whether they should be."

Luxtera's single chip device integrates both an LRM's electrical transceivers and the optical waveguides onto a CMOS chip; Lightwire has divided the labor between a 65nm chip for electrical processing and a 130nm chip dedicated exclusively to the optical functions.

"By using two chips instead of one, we think we can move more quickly to multichannel 40Gbit and 100Gbit versions," said Lightwire founder and chief technology officer Kal Shastri. "Our existing 130nm optical chip design will also work at 40Gbit, so we just have to refine the electrical chip."

So far, Lightwire's strategy appears to be working since the company claims its single channel chip consumes only 400mW, compared to 500mW per channel for Luxtera's four-channel device. When Lightwire moves to a four-channel device, it claims power consumption will be half that of Luxtera's—1W compared to 2W.

Beam splitting
Both Luxtera and Lightwire, as well as IBM and Intel, use a Mach-Zehnder interferometer (MZI) to modulate the laser. Unlike IBM and Intel, which are also trying to craft a laser in silicon, both Luxtera and Lightwire use standard discrete lasers to feed a silicon waveguide, which funnels the beam into the MZI.

An MZI works by splitting a beam into two parallel paths, one of which is modulated with an electrical gate that changes the index of refraction in that path by concentrating the charge. When the two paths are recombined at the end of the interferometer, their phase difference translates into the bits that encode the electrical signal within the light exiting the chip.

IBM, Intel and Luxtera use a traditional MZI architecture with a p-n junction concentrating the charge. This requires the interferometer's two paths to be several millimeters long. Lightwire has developed a patented 3D architecture that stacks the p- and n-type materials, separated by 20A to 24A of silicon dioxide, to form a MOS capacitor, which Lightwire calls a semiconductor-insulator-semiconductor capacitor.

Lightwire claims its capacitor has more surface area to concentrate a charge than a p-n junction, enabling it to reduce the length of the interferometer's two paths to 0.5mm.

"Our charge is concentrated smack in the middle of the device," said Shastri. "As a result we get the maximum phase shift for unit length. Plus, we can use low CMOS voltages of 1.2V instead of 3.3V or 5V, like our competitors."

Up and coming
Lightwire also claims to be able to scale its device more easily, merely by going to thinner gate oxides. That's different from p-n junction-based MZI designs that don't scale well to smaller sizes.

"By going to thinner gate oxides, we can increase the reflective index of the device, and thus can scale down its length very easily," said Shastri.

Lightwire will next design multichannel devices, including both four- and ten-channel devices. They will use a single laser to drive the optical chip, which will then split and modulate each channel separately.

"We will have samples of our current part available in March, and production units in second half of 2008," said Vijay Albuquerque, Lightwire's CEO. "Plus, we have a roadmap that leads directly to 40- and 100Gbit/s parts over the next 12 to 24 months."

Lightwire's CMOS photonics process was jointly developed with the Singapore's Institute of Microelectronics and is fabricated by Chartered Semiconductor Manufacturing Ltd.

- R. Colin Johnson
EE Times




Related information


* What is CMOS?
Complementary Metal Oxide Semiconductor, or commonly referred to as CMOS, is a type of IC which includes microprocessor, microcontroller, static RAM and other digital logic circuits.