Race for the Future

Researchers at Intel have pushed the boundaries of silicon photonics once again by developing the first avalanche photodetector.

The APD is a silicon-based device that Intel claims could reduce costs and improve the performance of existing commercially available optical devices, revolutionizing how multiple-processor cores communicate. According to Intel engineers, who published the results of their research in a recent issue of Nature Photonics, silicon photonics technology is the optimal cost-efficient method for increasing communication speeds between devices powered by multiple-processor cores.

The underlying technology uses standard silicon to transmit and receive optical data between computers and between computers other electronic devices, aiming to provide a reliable platform for future bandwidth needs of data-intensive computing applications such as remote medicine and 3D virtual worlds.

The APD directs light onto silicon to amplify weak signals. “Intel’s APD converts the light beams into electrical signals,” says Yimin Kang, a senior researcher at company. Until now, manufacturers typically have paid more than $100 apiece for a single device of this kind. Kang says the silicon, a relatively inexpensive and tested commodity, provides for a robust device equivalent to mature, commercially available APDs that use indium phosphide (InP).

Signal Amplification

Last year, Intel announced the development of a photodetector made from germanium and silicon. The 31-gigahertz device leveraged germanium’s ability to efficiently detect light in the near infrared, which is the standard for communications. But design defects compromised the product’s electrical performance and prompted Intel to explore other approaches. The new photodetector has built-in amplification, which according to the company makes the product much more useful in detecting signals when minimal light falls on the detector.

Mike Morse, a principal engineer at Intel, explains how the APD works: “First, a negative and a positive charge [electrons and holes, in semiconductor terminology] are created when the light strikes the detector. The electron is accelerated by an electric field until it attains a high enough energy to slam into a silicon atom and create another pair of positive and negative charges. Each time this happens, the number of total electrons doubles, until this ‘avalanche’ of charges is collected by the detection electronics.”

It is largely accepted within the electronics industry that less-expensive silicon photonics produce inferior results. But Intel claims that’s not the case with its APDs. Company engineers say that silicon’s properties allow for higher gain with less excess noise than that recorded in InP devices. Moreover, the new approach, they say, also results in higher sensitivity, a metric defined as the smallest amount of optical power falling on the detector needed to maintain a low bit error rate.

Intel uses silicon and CMOS processing to attain the APD’s “gain-bandwidth product” of 340GHz. The gain-bandwidth product is a standard measure for APD performance that multiplies the device’s amplification capability (gain) by the fastest speed signal that can be detected (bandwidth). This opens the reducing the price point for optical links running at data rates of 40 gigabits per second or faster and proves, for the first time, that a silicon photonics device can exceed the performance of a device made with more expensive optical materials, the Intel engineers contend. They add that higher speeds, along with lower power and noise levels, are essential in applications related to supercomputing, data center communications, consumer electronics, automotive sensors and medical diagnostics.

According to Dr. Mario Paniccia, director of Intel’s Photonics Technology Lab, this research demonstrates once again that silicon can be used to create high-performing optical devices. He also says that silicon APDs offer potential uses in other areas, such as sensing, imaging, quantum cryptography and biological applications.

Federal Assist

Intel conducted its research in collaboration with the Defense Advanced Research Projects Agency and Numonyx, a Swiss manufacturer of memory solutions. During APD development, Intel consulted with a number of experts, among them professor John Bowers of the University of California at Santa Barbara, who has assisted engineers with testing the new device. “This APD utilizes the inherently superior characteristics of silicon for high-speed amplification to create world-class optical technology,” says Bowers.

Morse reports that Intel is now looking at two potential extensions of the technology. The first would be a wave-guide APD, which could improve the absorption at wavelengths up to about 1,600 nanometers and allow for integration with other optical devices, such as demultiplexers and attenuators. Intel also wants to lower the operational voltage of the industry standard to “something more common in consumer electronics, to open up a much broader user base,” he says.

Researchers at Intel consider commercial optics as technology on the near horizon — just a couple of years out — which is driving their efforts to develop optics for upcoming platforms.