Spectrum deftly takes visible light’s pulse

Is there really treasure at the end of the rainbow? Yes, say scientists who have fired ultrashort laser pulses into a novel fiber that transforms one color into many. The remarkable spectrum exiting the fiber, when applied as a sort of ruler, takes the ordeal out of measuring visible-light frequencies. Before long, every university and industrial laboratory may routinely use the technique to achieve precision now attained only at national standards laboratories, the inventors predict.

To many scientists, that improvement’s as good as gold. “Completely revolutionary,” comments Alan Madej of the National Research Council of Canada in Ottawa. “It’s the biggest development in precision electromagnetic measurement since people started to measure laser frequencies” in the early 1970s, he adds.

Ernst O. Göbel of Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, says the new method “puts absolute frequency measurements in the optical regime on totally new grounds.”

Since 1967, researchers have defined a second of time by counting out radiofrequency waves of a 9.2-gigahertz oscillation of the cesium-133 atom (SN: 8/7/99, p. 92). But scientists needing to precisely measure frequencies of early lasers found themselves in a bind: No electronic device counts fast enough to keep up with visible light’s ripples, which wiggle at least 46,000 times as fast as the cesium clock’s oscillation.

Instead, physicists laboriously created so-called frequency chains starting with the cesium signal. Each link, built from atomic clocks, other lasers, and electronic circuits, raised the frequency by a known ratio, ultimately stepping it up to the target’s optical frequency.

Now, all those intermediate links melt away, report researchers at JILA in Boulder, Colo.; Lucent Technologies’ Bell Labs in Murray Hill, N.J.; and the Max Planck Institute for Quantum Optics in Garching, Germany.

In an unusual fiber invented by two Bell Labs scientists, light pulses pick up new colors as they travel. Confinement by air tubes encircling the narrow glass core distorts pulses and adds colors, coinventor Robert S. Windeler explains.

The May 29 Physical Review Letters and April 28 Science describe a series of laser pulses taking a centimeters-long joyride through such a fiber. What emerges is pulses containing a spectrum-wide set of optical beams evenly spaced in frequency. Cued by a cesium standard, the rate of entering pulses sets the gap between the ticks of this ruler.

Led by Theodor W. Hänsch, who devised the method’s underlying concept, the Garching team has used the spectral ruler to measure certain hydrogen oscillations with unprecedented accuracy.

As the technique becomes commonplace, perhaps in a desktop instrument, the entire gamut of physical and biological sciences could benefit because so many scientific probes involve color detection, says coauthor Scott A. Diddams, now at the National Institute of Standards and Technology in Boulder. The method may also lead to more rigorous tests of whether fundamental physical constants—and hence the laws of physics themselves—vary slightly over time, adds JILA team leader John L. Hall.