01 Mar 2007
Ansheng Liu, principal engineer at Intel's Photonics Technology Lab, talks to optics.org about the company's latest high-speed silicon modulator.
Scientists at Intel's labs at California and Jerusalem have built a highly scalable silicon modulator capable of reaching speeds of up to 30 Gbit/s (Optics Express 15 660). The prototype modulator is 8 mm long, 0.5 mm thick, and has an insertion loss of 7 dB, which is comparable with commercially available lithium niobate and indium phosphate modulators.
The design is based on a Mach Zehnder interferometer, with a reverse-biased p–n diode embedded in each of the two arms. It works by rapidly injecting and removing free carriers to alter the refractive index of silicon, and uses optical interference to control the amount of light leaving the device.
The Intel team first demonstrated a 10 Gbit/s silicon modulator back in April 2005, but Liu says that pushing the technology to higher speeds has proved extremely difficult. That's because the relatively large p–n junction capacitance and parasitic effects of the metal contact effectively place an upper limit on the speed that can be achieved.
To solve this problem, the team has used a so-called travelling wave design, in which the optical and electrical signals are always moving at the same speed, and in the same direction. Here, the optical signal is carried on the silicon waveguide, while the electrical signal travels on a coplanar aluminium waveguide.
"One of the biggest challenges was in keeping the attenuation of the RF waveguide low, and at the same time matching the propagation speeds of the electrical and optical signals," said Liu.
Existing modulators operating at 40 Gbit/s exploit materials such as indium phosphide (InP), which are expensive and difficult to fabricate. Intel is instead aiming for a single-die silicon nanophotonic solution, and has been developing the technology to fabricate all of the key optical building blocks from a standard CMOS process. Back in 2004 the chip giant announced a 1 GHz modulator, and then in September last year the company demonstrated an electrically pumped InP-silicon laser (see related stories).
But it's not just Intel who is looking to developing all-silicon solutions. Smaller players such as Luxtera and Infinera have also been aggressively pursuing research into integrated CMOS-based photonics solutions. The Defense Advanced Research Projects Agency (DARPA) has also initiated its Electronic & Photonic Integrated Circuits (EPIC) programme to help universities and start-ups to develop CMOS fabrication processes for nanophotonic devices such as lasers, wavelength converters, interconnects and optical amplifiers.
Liu concedes that any product featuring this modulator is still many years away. He says that this demonstration clears the way for data transfer between silicon photonic chips operating at speeds of 1 THz and beyond. "When the technology is available, we envisage transceivers (based on this modulator) being used for interconnects around Intel PCs as well as for traditional telephone services." Liu added.