False-color images of the "fingerprints" of molecular iodine, each taken under different experimental conditions using a NIST frequency brush created with an ultrafast visible laser. The squares within each frame reveal the frequency and intensity of light from individual "bristles" of the brush. The variation in the colors reveal where the iodine absorbs specific optical frequencies. Credit: S. Diddams/NIST
The new technique will enable scientists to measure and manipulate optical frequencies in a massively parallel manner.
NIST physicists and collaborators were the first to compare the operation of multiple femtosecond frequency combs, thereby demonstrating reproducibility, and to verify that both the starting position of a comb and the spacing between the teeth can be controlled precisely. NIST scientists also have demonstrated the most precise synthesis ever of optical frequencies, generating specific colors with a reproducibility of 19 digits. The experiments are a significant step toward next-generation “atomic clocks” based on optical rather than microwave frequencies.
NIST staff and collaborators also have extended the reach of frequency combs. One project extended the wavelength coverage 1,000 nanometers (a measure for wavelengths of light) farther into the infrared than ever before, while another effort at JILA created the world’s first frequency comb in the extreme ultraviolet. In addition, NIST has shown that extremely stable microwave signals can be generated from optical frequency combs.
Frequency combs have dramatically simplified and improved the accuracy of frequency metrology. They also are making it possible to build optical atomic clocks, expected to be as much as 100 times more accurate than today’s best time-keeping systems. Better clocks will lead to studies of, for example, the stability of the constants of nature over time, and enable improved technology for advanced communications and precision navigation systems, such as next-generation global positioning systems.
Today’s best atomic clocks, and the international definition of the second, are based on the natural oscillations of the cesium atom, a frequency in the microwave region of the electromagnetic spectrum. Optical combs provide the equivalent of regularly spaced "gears" that can be used to link the slower "ticks" of microwave-based atomic clocks to the much faster, more precise "ticks" of optical clocks (see graphic below).
Highly accurate measurements of frequencies are also essential for many other advanced fields of science that require the identification or manipulation of atoms or molecules, such as detection of toxic biochemical agents, studies of ultrafast dynamics and quantum computing. As scientists continue to improve frequency comb technology and make it easier to use, it may be applied in many other research fields and technologies, from medical tests in doctor’s offices, to synchronization of advanced telecommunications systems, to remote detection and range measurements for manufacturing or defense applications.