Chip-Scale Atomic Clock
The ultimate in precision--the cesium clock--has been miniaturized
The standard by which clocks in the United States are set is a cesium-based atomic clock in Boulder, Colo., that loses less than a nanosecond per day. It’s the size of a small car and draws roughly a kilowatt of power. Less-accurate commercial atomic clocks, which keep time by watching the vibrations of atoms, are typically the size of a suitcase, though not nearly as portable as one. But now, for the first time, atomic clock accuracy is available in a form small enough and power-efficient enough for backpack-size battery-powered devices. Someday an atomic clock might even fit into a smartphone.
Earlier this year, Symmetricom, of San Jose, Calif., introduced the first commercial chip-scale atomic clock, the SA.45s. It measures 4 by 3.5 by 1.1 centimeters, weighs 35 grams, and draws a paltry 115 milliwatts. The tiny clock is accurate to within about less than half a microsecond per day.
All the inner workings of a full-scale atomic clock have been shoehorned into the chip-scale version, according to Steve Fossi, Symmetricom’s director of new business development. A resonance cell containing cesium 133 and a buffer gas is heated until a moderately dense vapor of cesium is distributed throughout the cell. The vapor is illuminated with light from a semiconductor laser, which is modulated at a frequency near the 9.192-gigahertz natural oscillation frequency of the cesium atoms. Once the beam drives the atoms into an oscillating state, they absorb less of the light, and the photons transmitted through the cell can be used to determine whether the laser beam’s modulation frequency coincides with the resonant frequency of the atoms. A servomotor can then lock the modulation frequency of the laser to the atomic resonance, holding the clock's output stable.
Collapsing all that into a chip was no mean feat, says Fossi. The resonance cell, where the cesium atoms are heated to a vapor, had to be extremely small, for example. "Our design team had to develop from scratch an engineered MEMS [microelectromechanical systems] product to achieve the cell’s current 2-cubic-millimeter volume," says Fossi.
MEMS micromachining for the clock’s core components made a dramatic difference in the its efficiency, says John Kitching, who runs the atomic devices and instrumentation group at the U.S. National Institute of Standards and Technology, or NIST. "Until seven or eight years ago, manufacturers were still using glass-blowing techniques to form the airtight enclosures for the cesium," recalls Kitching, whose NIST group did pioneering work from 2001 to 2006 on the miniaturization of atomic clocks and was the first to demonstrate an atomic clock based on microfabricated components. "This meant that making cells significantly smaller than about 1 cubic centimeter was extremely difficult. Shrinking them to a cubic millimeter or two dramatically reduced the amount of heat needed for the clock to reach its operating temperature, which aided tremendously in the device reaching its current power specs," says Kitching, who was not involved in the development of the Symmetricom device.
Kitching also notes that switching to a new kind of vertical-cavity surface-emitting laser to heat up the resonance cell was an equally important step. Before this, he says, atomic clocks typically used discharge lamps, which need a lot of power. It took several years of development before the laser’s wavelength was stable enough to keep the clock’s frequency within a range of 10 x 10-10 hertz across the device’s –10 °C to 70 °C operating temperature.
There are several applications to which the chip-scale atomic clock is well suited, but it’s most likely to show up first in GPS receivers. Today a GPS device must be able to see four satellites to get a fix on its location. Adding an atomic clock would let a receiver get by with links to just three satellites, says Fossi. "And if you don’t care about altitude, you could get by with two. So it would help in a lot of urban canyon situations and improve a GPS unit’s performance in terms of the time it takes to get a fix on the satellites," he says.
The U.S. military is already interested in the clock, according to Fossi, and it’s being readied for use in GPS for vehicles and airplanes. Symmetricom is working with military contractors, which Fossi would not name, on a backpack version as well. The eventual goal is a handheld GPS device with atomic clock accuracy.
Another potentially important application is in undersea oil and gas exploration. When prospecting beneath the ocean, gas companies lay out a grid of sound and motion sensors on the ocean floor. A boat on the surface blasts pulses of sound through the water into the earth below. The pulse reflects off the different layers of sediment and rock, and the sensors time-stamp the echoes using a hyperaccurate built-in clock. The processed data allows engineers to construct a picture of the composition of the layers beneath the ocean floor. The quality of that picture depends on how accurate the time-stamping is. Symmetricom’s chip-scale atomic clock would improve that accuracy by 10 to 30 times and consume only 20 percent of the power drawn by the oven-controlled crystal oscillators typically used in this application, says Fossi.
Fossi says it will take a while before the chip shows up in consumer electronics. At US $1500, it costs much more than most gadgets do, and it’s still far too large and draws too much power to be shoehorned into a smartphone. "If you look far out into the future, you could envision that happening, but there’s a lot of engineering we would have to do," he says.