Intel Detects More Silicon Photonics

It's been almost two years since Intel Corp. (Nasdaq: INTC) announced any "breakthroughs" in silicon photonics. Well, we can't let that stand, can we?
Fear not. Intel is publishing its latest results today in the journal Nature Photonics, describing an Avalanche Photodiode (APD) -- a type of detector for the receiving end of an optical link -- that's made of silicon (but not completely -- there's also a germanium layer).

Intel's silicon photonics efforts so far have focused on modulators and lasers. The APD is new ground, not just because it's a different part, but because its performance outdoes "any equivalent device in a III-V-based or exotic material," says Mario Paniccia, an Intel fellow and director of the company's Photonics Technology Lab. (III-V, or "three-five," refers to a class of compounds such as indium phosphide (InP) or gallium arsenide.)

That's a first. Silicon photonics have always been a tradeoff. The devices would be easier to integrate and cheaper to manufacture, since they can be built using complementary metal-oxide semiconductor (CMOS) techniques that are commonplace in the chip world -- but the performance suffers. Intel has been aiming for CMOS devices with 90 percent of the performance of InP ones.

The APDs aren't anywhere close to being a marketable product, by the way. "This is a research result. It's actually a very new result, Paniccia says.
Like any big company, Intel has started and ended its share of

Why? It so happens, Intel's silicon photonics work was being done at what is now Numonyx's fab. Intel found it easiest to just keep the operation in place, Paniccia says. So, the engineers technically work at Numonyx and build their devices on the same Numonyx production lines that are churning out high-volume memory chips.
Meanwhile, silicon photonics are reaching the commercial stage, mainly in the form of active optical cables for data centers.

Going the distance Intel's silicon photonics efforts are aimed mostly at short-reach connections, but the APD could easily be applied to a telecom network. The devices usually get mentioned in the context of long-haul spans, partly because they're too expensive to use elsewhere -- $200 to $300 apiece, Paniccia says.

The advantage of an APD is that a weaker light source can generate a sufficient current. That means you can take some liberties on the transmission side -- moving the source a farther distance away, for instance. Among the possible applications Paniccia cited was the fiber-to-the-home network, where APDs could conceivably be used to extend the reach of fiber links.
Performance for APDs can be measured in the gain-bandwidth product -- that is, the device's gain multiplied by the speed of the connection, which comes out to a fixed number measured in Hertz. (Note that this means the gain goes down as the bandwidth gets faster.)
For an indium phosphide APD, that gain-bandwidth product is around 120 GHz, Intel says. Intel's silicon APD is showing 340 GHz, implying that it would have better gain than InP devices.
Intel didn't specify the speed it's aiming for with APD, but the company is shooting high with its marketing, saying a silicon APD could be an aid in 40-Gbit/s networks. That would be quite a leap, as APDs are only available in speeds up to 2.5 Gbit/s today.
"A 40-Gbit/s APD might be really pushing it, but as something they're talking about for the future, it might be reasonable," says Ali Abouzari, vice president of sales for CyOptics Inc.
To describe which part of the APD is made of silicon, it's helpful to look at how an APD works. A normal photodiode receives a photon of light and produces an electron/hole pair (you can think of a "hole" as the opposite of an electron), creating electrical current. An APD adds a multiplication region where that reaction gets amplified, creating many more electron/hole pairs and a stronger current.

Intel used silicon for the multiplication region. But to absorb the photon and get the process started, Intel needed germanium, because silicon is transparent to the infrared wavelengths used in communications. Silicon can't "catch" the light.

Plenty of challenges exist with this approach. One is that the silicon and germanium atoms form lattices that don't quite match up, and that can cause some current to leak out even when there's no light present. Intel is still working on getting that "dark current" down, Paniccia says.