Low-power diode lasers expand atomic-clock applications
Kathy Kincade
Researchers from Westinghouse Electric Corp.`s Science & Technology Center (Pittsburgh, PA) are developing the first diode-pumped atomic clock intended for commercial use. Westinghouse claims its miniature laser-pumped cesium-cell frequency standard is smaller, less costly, and more efficient than any currently available atomic clock. When formally introduced in 1997, the device is expected to bring high-accuracy time-keeping capabilities to a number of applications for which this technology has previously been too expensive and cumbersome.
"We`re talking about a clock that could revolutionize communications and location applications," says Irving Liberman, manager of Westinghouse`s time-standards program. "It will be one-tenth the size and 100 times lighter than any atomic time standard now available, and it will draw one-tenth the power (about one-third of a watt). We also project that it will cost one-tenth as much as atomic clocks now on the market," which range from $3000-$10,000 for a rubidium-cell clock to $20,000-$60,000 for a cesium-beam clock.
Because diode-laser prices are so volume-dependent, the actual price of the Westinghouse clock will be determined largely by the cost of the laser. "We need only a microwatt diode, but they are hard to find," says Liberman. Westinghouse has tried several types of diode lasers and currently favors vertical-cavity devices because of their lower power requirements.
Historically, accuracy has been the most important feature of atomic clocks. "NIST [the National Institute of Standards and Technology] is the nation`s timekeeper, and they want the best," says Liberman. "After performance, size, power, and cost are secondary issues for them." For example, the glass cylinder in NIST-7 (a cesium-beam clock that is NIST`s first optically pumped clock) is 10 ft long and 1 1/2 ft in diameter. In addition, though NIST-7 uses a diode laser for pumping, "the setu¥takes u¥a small bench," says Liberman.
Other cesium-beam and rubidium-cell clocks employ even more complex pumping devices. Until NIST-7, all NIST atomic clocks isolated the atoms by passing the cesium vapor through a magnetic field. And though rubidium-cell clocks are optically pumped, they use a lam¥fitted with an isotopic filter to eliminate undesired wavelengths.
Performance versus practicality
In contrast, the most advanced version of the Westinghouse clock will be about the size and weight of a walnut (see photo). The device combines an integrated electronic circuit with an aspirin-sized, cylindrical glass cell filled with cesium vapor. An 852-nm diode laser pumps the cesium atoms to a higher energy level, producing a population imbalance. A microwave electromagnetic field excites electrons in the cesium vapor, partially restoring the atoms to their base energy level when the circuit`s frequency precisely matches the resonant frequency (about 9.2 gigahertz) of the cesium atoms.
What the Westinghouse clock gains in size and cost, however, it loses in accuracy--relatively speaking, that is. For example, NIST-7 will neither gain nor lose one millionth of second in a year, which makes it invaluable for setting and maintaining time and frequency standards. The Westinghouse clock, on the other hand, boasts a millionth of a second accuracy in a day.
"NIST is going for performance, while we are going for practicality," says Liberman. "We are trying to bring atomic clocks to the world."
However, the Westinghouse clock is still 1000 times more accurate than comparably priced crystal oscillators, which are used in many applications considered appropriate for a compact atomic clock. This accuracy is primarily the result of keeping the diode laser centered at 852.1 nm (the cesium absorption line). In fact, controlling the wavelength is the key operational element of this clock, says Liberman. Westinghouse researchers first began developing their proprietary stabilization technique in the 1980s as part of a blue-green submarine-communications project for the Navy. "We developed a thallium-cesium atomic-line filter that we planned to pum¥with a laser," he says. "We then learned how to stabilize lasers on the cesium line."
The proprietary nature of this technique keeps Liberman from disclosing exactly how it works. "I can say only that the wavelength is a function of its temperature and the current. By carefully controlling the temperature and the current in feedback loops, we are able to stabilize the wavelength to that current."
Potential markets
While the Westinghouse clock is not likely to make crystal oscillators obsolete, it has the potential to become a leading contender in several applications. "We have identified two major commercial markets so far: long-distance telephone systems and cellular telephone systems," says Liberman. "Both are on the order of tens of thousands of clocks per year."
The military is also interested in this technology; in fact, the project is supported by a three-year, $3 million contract that is managed by the US Air Force Wright Laboratory Avionics Directorate and funded by the Advanced Research Projects Agency. "Atomic clocks have the potential to increase the accuracy, reliability, and computational speed of global positioning systems," adds Liberman, "but currently available clocks are too expensive and power hungry. That is one of the main reasons the military is interested in our device."
Westinghouse is also working with NIST through a Cooperative Research and Development Agreement.