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 also are 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 20 to 24 angstroms 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 half a millimeter.
"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 1.2 volts instead of 3.3 or 5 volts, like our competitors."
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 100-gigabit per second 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.