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Researchers at Intel have pushed the boundaries of silicon photonics once again by developing the first avalanche photodetector (APD). The silicon-based device, which Intel claims could reduce costs and improve the performance of existing commercially available optical devices, promises to revolutionize how multiple processor cores communicate within computing systems. Intel has published the results of the research upon which the APD was developed in a recent issue of Nature Photonics.
APD’s underlying technology uses standard silicon to transmit and receive optical data among computers and 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. According to Intel engineers, silicon photonics–based technology is the optimal cost-efficient method to dramatically increase communication speeds between devices powered by multiple processor cores.
The APD is a light sensor that amplifies weak signals as light is directed onto silicon. “Intel's APD converts the light beams into electrical signals,” says Yimin Kang, a senior researcher at Intel, who also noted that until now manufacturers paid more than $100 for a single device of this kind. Kang also said that the product was specially designed to be robust, hence the usage of silicon — a relatively inexpensive and tested commodity that can produce devices equivalent to mature, commercially available indium phosphide (InP) APDs.
Last year, Intel announced the development of a photodetector made from germanium and silicon. The device, which had a bandwidth of 31 gigahertz, made use of germanium’s capability to efficiently detect light in the near infrared, which is the standard for communications. However, design defects compromised the product’s electrical performance and prompted Intel to explore a slightly different approach. 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, principal engineer at Intel, explains the technology behind the novel device: “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 in the electronics industry that less-expensive silicon photonics produce inferior results. While this is true in many cases, Intel claims it is not so with APDs. In fact, 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.
APD utilizes silicon and CMOS processing to attain a "gain-bandwidth product" of 340GHz — a breakthrough achievement, according to Intel engineers. 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 door to lower the cost of 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 traditional, more expensive optical materials such as indium phosphide, say the engineers. 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 how silicon can be used to create very high performing optical devices. He also noted that, apart from optical communication, the silicon-based APDs could be employed in other areas such as sensing, imaging, quantum cryptography and biological applications.
The research was conducted by Intel in collaboration with the Defense Advanced Research Projects Agency (DARPA) and Numonyx, a manufacturer of memory solutions based on Switzerland. During APD development, Intel consulted with a number of experts, among them Professor John Bowers of the University of California, 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 the company is now looking at two potential extensions to the new technology. The first would be to develop a wave guide–based APD, which could improve the absorption at wavelengths up to about 1600 nanometers and allow for easy integration with other optical devices, such as demultiplexers and attenuators. Intel also hopes to lower the operational voltage of the industry standard to “something more common in consumer electronics, to open up a much broader user base,” Morse says.
Researchers at Intel believe that commercial optics are just a couple of years away, which is leading the company to aggressively drive optics in its upcoming platforms.
Anuradha Menon and Sarah Gingichashvili write for the electronic magazine The Future of Things at www.thefutureofthings.com.