Finally, an optical frequency comb that visibly lives up to its name.
Scientists at the University of Konstanz in Germany and the National Institute of Standards andTechnology (NIST) in the United States have built the first optical frequency comb—a tool for preciselymeasuring different frequencies of visible light—that actually looks like a comb.
As described in the Oct. 30 issue of Science,* the "teeth" of the new frequency comb are separatedenough that when viewed with a simple optical system—a grating and microscope—the human eye cansee each of the approximately 50,000 teeth spanning the visible color spectrum from red to blue. Afrequency comb with such well-separated, visibly distinct teeth will be an important tool for a wide rangeof applications in astronomy, communications and many other areas.
A basis for the 2005 Nobel Prize in physics, frequency combsare now commonplace in research laboratories and next-generation atomic clocks. But until now, comb teeth have been so closely spaced that they were distinguishable only with specialized equipment and great effort, and the light never looked like the evenly striped pattern of the namesake comb to the human eye.
Each tooth of the comb is a different frequency, or color (although the human eye can't distinguish thevery small color differences between nearby teeth). A frequency comb can be used like a ruler tomeasure the light emitted by lasers, atoms, stars or other objects with extraordinarily high precision. Otherfrequency combs with finer spacing are highly useful tools, but the new comb with more visibly separatedteeth will be more effective in many applications such as calibrating astronomical instruments.
These are photographs of four different regions of the new optical frequency comb. The light is filteredthrough a grating spectrometer and photographed with a digital camera through a microscope.Each visible line or "tooth" is an individual frequency in the comb, which spans the visiblespectrum from red to blue. More than 1,500 such photos would need to be lined up to show theentire comb.
(Photo Credit: S. Diddams/NIST)
The new comb is produced by a dime-sized laser that generates super-fast, super-short pulses ofhigh-power light containing tens of thousands of different frequencies. As in any frequency comb, theproperties of the light over time are converted to tick marks or teeth, with each tooth representing aprogressively higher number of oscillations of light waves per unit of time. The shorter the pulses of laserlight, the broader the range of frequencies produced. In the new comb described in Science, the laserpulses are even shorter and repeated 10 to 100 times faster than in typical frequency combs. The laseremits 10 billion pulses per second, with each pulse lasting about 40 femtoseconds, or quadrillionths of asecond, producing extra-wide spacing between individual comb teeth.
Another unusual feature of the new comb is efficient coupling of the laser pulses into a "nonlinear"optical fiber, which dramatically expands the spectrum of frequencies in the comb. Since details of theunusually powerful dime-sized laser were first published in 2008, scientists have doubled the averagepulse power directed into the fiber, enabling the comb to reach blue colors for the first time, producing aspectrum across a range of wavelengths from 470 to 1130 nanometers, from blue to infrared. The 50,000individual colors become visible when the light emitted from the fiber is filtered through a gratingspectrometer, a common laboratory instrument that acts like a souped-up prism.
The broad spectrum spanned by the comb—unusual for such a fast pulse rate—enables all thefrequencies to be stabilized, using a NIST-developed technique that directly links optical and radiofrequencies. Stabilization is crucial for applications.
The ability to directly observe and use individual comb teeth will open up important applications inastronomy, studies of interactions between light and matter, and precision control of high-speed opticaland microwave signals for communications, according to the paper. NIST scientists previously haveshown, for example, that this type of frequency comb could boost the sensitivity of astronomical toolssearching for other Earthlike planets as much as a hundredfold. In addition, the new comb could be usefulin a NIST project to develop optical signal-processing techniques, which could dramatically expand thecapabilities of communications, surveillance, optical pattern recognition, remote sensing and high-speedcomputing technologies.
The laser was built by Albrecht Bartels at the Center for Applied Photonics of the University ofKonstanz. The frequency comb was built and demonstrated in the lab of NIST physicist Scott Diddams inBoulder, Colo.
Source: National Institute of Standards and Technology (NIST)