Lasers ♦ news digest
VCSEL slashes power consumption in atomic clock
The GaAs based VCSEL operating at a wavelength of 894 nm operates at only 2 mW, and consumes over a thousand times less power than the conventional light source used in atomic clocks, a rubidium-based atomic vapour lamp.
A matchbook-sized atomic clock 100 times smaller than its commercial predecessors has been created by a team of researchers at Symmetricom Inc. Draper Laboratory and Sandia National Laboratories.
The portable Chip Scale Atomic Clock (CSAC), only about 1.5 inches on a side and less than a half-inch in depth, also requires 100 times less power than its predecessors; instead of 10 W, it uses only 100 mW.
“It’s the difference between lugging around a device powered by a car battery and one powered by two AA batteries,” said Sandia lead investigator Darwin Serkland.
Despite common implications of the word “atomic,” the clock does not use radioactivity as an energy source. Instead, where an old-fashioned alarm clock uses a spring-powered series of gears to tick off seconds, a CSAC counts the frequency of electromagnetic waves emitted by caesium atoms struck by a tiny laser beam to determine the passage of time.
The clock is suited for use by miners far underground or divers engaged in deep-sea explorations, who would normally not receive GPS signals which are blocked by natural barriers. It enables them to plan precise operations with remote colleagues who also have atomic clocks, because their timing would deviate from each other by less than one millionth of a second in a day.
A CSAC timekeeper would also be invaluable to experts using electromagnetic interference to prevent telephone signals from detonating improvised explosive devices, or IEDs. Again, where GPS signals were blocked, a CSAC timekeeper would still function.
On a nationwide scale, relay stations for cross- country phone and data lines, which routinely break up messages into packets of information and send them by a variety of routes before reconstituting them correctly at the end of their voyages, would continue functioning during GPS outages.
The clock’s many uses, both military and commercial, are why the Defence Advanced Research Projects Agency (DARPA) funded the work from 2001 until the CSA Clock hit the commercial market in January.
Darwin Serkland measures the wavelength of the VCSEL. The image on the monitor (left) shows a bright circle of light emitted from a 894 nm VCSEL needed to drive the atomic clock. The objects that look like black baseball bats are tiny wire needles carrying milliampere currents. The round white plastic containers on Serkland’s workbench each contain about 5,000 VCSELs fabricated from one-quarter of a 3-inch diameter GaAs wafer. Each wafer is designed differently to yield a unique type of laser. (Photo by Randy Motoya)
“Because few DARPA technologies make it to full industrial commercialisation for dual-use applications, this is a very big deal,” said Gil Herrera, director of Sandia’s Microsystems and Engineering Sciences Application (MESA) centre. “CSAC now is a product with a data sheet and a price.”
Caesium atoms are housed in a container the size of a grain of rice developed by Cambridge, Mass.-based Draper Lab. The caesium atoms are interrogated by a light beam from a VCSEL, contributed by Sandia. Symmetricom, a leading atomic clock manufacturer, designed the electronic circuits and assembled the components into a
June 2011
www.compoundsemiconductor.net 149
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