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Lasers ♦ news digest


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)


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.


“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 complete functioning clock at its Beverly, Mass., location.


“The work between the three organisations was never ‘thrown over the wall,’” said Sandia manager Charles Sullivan, using an expression that has come to mean complete separation of effort. “There was tight integration from beginning to end of the project.”


Nevertheless, the reduced power consumption that was key to creating the smaller unit required, in addition to a completely new architecture, a VCSEL rather than the previous tool of choice, a rubidium- based atomic vapour lamp.


“It took a few watts to excite the rubidium lamp into a plasma-like state,” Serkland said. “Use of the VCSEL reduced that power consumption by more than a thousand times to just two milliwatts.” Serkland’s success in attaining this huge power reduction caused some in the clock business to refer to him as “the VCSEL wizard.”


The way the clock keeps time may best be imagined by considering two tuning forks. If the forks vary only slightly in size, a series of regular beats are produced when both forks vibrate. The same principle works in the new clock.


The VCSEL, in addition to being efficient, inexpensive, stable and low-power, is able to produce a very fine, single-frequency beam. The laser frequency, at 335 THz (894.6 nms), is midway between two hyperfine emission levels of the caesium atom, separated in terms of energy like the two differently sized tuning forks.


One level is 4.6 GHz above and the other 4.6 GHz below the laser frequency. (Hyperfine lines are the energy signatures of atoms.) A tiny microwave generator sends an oscillating frequency that alternates adding and subtracting energy from the incoming laser carrier frequency. Thus, the laser’s single beam produces two waves at both hyperfine emission energies. When they interact, the emitted waves produce (like two tuning forks of different sizes) a series of ‘beats’ through a process known


October 2011 www.compoundsemiconductor.net 145


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