From the Labs: Information Technology

Faster Silicon Laser
A new design could yield a more practical light source for ­telecommunications networks

SOURCE: “Mode-Locked Silicon ­Evanescent Lasers”
Brian R. Koch et al.
Optics Express 15: 11225-11233

RESULTS: Researchers have designed a stable, electrically pumped silicon-based laser that emits ultrashort pulses of light at a frequency of 40 gigahertz.

WHY IT MATTERS: In modern telecommunications networks, bits of information are carried by laser light. Currently, the lasers that generate the light are made in dedicated indium phosphorous clean rooms. Silicon-based lasers that could be made on existing high-volume semiconductor manufacturing lines would be much cheaper. Until now, silicon lasers have been incapable of emitting pulses of light that are short enough and have high enough frequencies for use in telecommunications networks. The researchers hope that the new ­silicon-­based device could replace the costlier lasers now used in optical networks.

METHODS: The construction of the device begins with a wafer in which a layer of silicon dioxide is sandwiched between two layers of silicon. In the top layer of silicon, the researchers etch a channel called a waveguide. To the top of the wafer they bond strips of indium phosphide; when current is applied to electrical contacts, the strips emit light that bounces back and forth inside the waveguide. A small amount of the light sneaks back into the indium phosphide, where it is amplified and emerges as laser light. In order to control the pulses of light emitted by the laser, the researchers had to make sure that the waveguides were of a precise length, and that light-amplifying and light-­absorbing regions of the device were electrically isolated from each other.

NEXT STEPS: Currently, the laser’s performance drops at the high temperatures that can be characteristic of network hardware. The researchers need to modify the device so it can withstand these temperatures, and it will have to pass other tests of robustness. In addition, the researchers are exploring the best way to combine the laser with other components, such as modulators, to make silicon-based photonic chips.

Why Wi-Fi Fails
A diagnostic system determines where and why buildingwide
systems falter

SOURCE: “Automating Cross-Layer Diagnosis of Enterprise Wireless ­Networks”
Yu-Chung Cheng et al.
Proceedings of the ACM Sigcomm ­Conference, Kyoto, Japan, August 2007

RESULTS: Researchers at the University of California, San Diego, have developed a system that tracks wireless traffic in a building and determines precisely what causes signals to dip, traffic to slow, and laptops to get kicked off the network.

WHY IT MATTERS: Wi-Fi tends to be unreliable. A number of factors can interfere with a signal, from hardware malfunctions and software bugs to interference from microwave ovens and cordless phones. What’s more, the degree of influence these factors have can change quickly, making Wi‑Fi failures difficult to anticipate and diagnose. An efficient way to pinpoint problems would make them much easier to correct.

METHODS: The researchers installed 192 radios to monitor traffic throughout the university’s computer science building. To infer wireless activity that wasn’t measured directly, they developed novel algorithms that extracted clues from the measured data. Using both the measured and the inferred data, they were able to determine how much each interfering factor contributed to Wi-Fi problems. The researchers think the technology could be implemented quickly. They say manufacturers could easily equip routers with traffic-monitoring hardware, along with software that analyzes network activity.

NEXT STEPS: The researchers will explore the technical challenges of deploying the system and maintaining constant network analysis.