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Frequency Comb Shows Teeth

Photonics.com
Nov 2009
GAITHERSBURG, Md., & KONSTANZ, Germany, Nov. 2, 2009 – The first optical frequency comb that actually looks like a comb has been built and demonstrated by scientists in the US and Germany.

The “teeth” of the new frequency comb – a tool for precisely measuring different frequencies of visible light – are separated enough that, when viewed with a simple optical system (a grating and microscope), the human eye can see each of the approximately 50,000 teeth spanning the visible color spectrum from red to blue. A frequency comb with such well-separated, visibly distinct teeth will be an important tool for a wide range of applications in astronomy, communications and many other areas.

A basis for the 2005 Nobel Prize in physics, frequency combs are 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 to the human eye, the light never looked like the evenly striped pattern of the namesake comb.

OpticalFrequencyComb.jpg


Photographs of four different regions of the new optical frequency comb. The light is filtered through 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 visible spectrum from red to blue. More than 1500 such photos would need to be lined up to show the entire comb. (Image: S. Diddams/NIST)
Each tooth of the comb is a different frequency, or color (although the human eye can’t distinguish the very small color differences between nearby teeth). A frequency comb can be used like a ruler to measure the light emitted by lasers, atoms, stars or other objects with extraordinarily high precision. Other frequency combs with finer spacing are highly useful tools, but the new comb with more visibly separated teeth will be more effective in many applications such as calibrating astronomical instruments.

The new comb is produced by a dime-size laser that generates superfast, supershort pulses of high-power light containing tens of thousands of different frequencies. As in any frequency comb, the properties of the light over time are converted to tick marks or teeth, with each tooth representing a progressively higher number of oscillations of lightwaves per unit of time. The shorter the pulses of laser light, the broader the range of frequencies produced.

In the new comb described in the Oct. 30 issue of Science, the laser pulses are even shorter and repeated 10 to 100 times faster than in typical frequency combs. The laser emits 10 billion pulses per second, with each pulse lasting about 40 femtoseconds, or quadrillionths of a second, 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 the unusually powerful dime-sized laser were first published in 2008, scientists have doubled the average pulse power directed into the fiber, enabling the comb to reach blue colors for the first time, producing a spectrum across a range of wavelengths from 470 to 1130 nm (blue to infrared). The 50,000 individual colors become visible when the light emitted from the fiber is filtered through a grating spectrometer, a common laboratory instrument that acts as a souped-up prism.

The broad spectrum spanned by the comb – unusual for such a fast pulse rate – enables all the frequencies to be stabilized using a NIST-developed technique that directly links optical and radio frequencies. Stabilization is crucial for applications.

The ability to directly observe and use individual comb teeth will open up important applications in astronomy, studies of interactions between light and matter, and precision control of high-speed optical and microwave signals for communications, according to the paper.

NIST scientists previously have shown, for example, that this type of frequency comb could boost the sensitivity of astronomical tools searching for other Earthlike planets as much as a hundredfold. In addition, the new comb could be useful in a NIST project to develop optical signal-processing techniques, which could dramatically expand the capabilities of communications, surveillance, optical pattern recognition, remote sensing and high-speed computing technologies.

The laser was built by Albrecht Bartels at the Center for Applied Photonics of the University of Konstanz. The frequency comb was built and demonstrated in the lab of NIST physicist Scott Diddams in Boulder, Colo.

For more information, visit: www.nist.gov







GLOSSARY
grating
A framework or latticework having an even arrangement of rods, or any other long narrow objects with interstices between them, used to disperse light or other radiation by interference between wave trains from the interstices. The ability of a grating to separate wavelengths (chromatic resolving power) is expressed as being equal to the number of lines in the grating.
microscope
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
optical
Pertaining to optics and the phenomena of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
remote sensing
Technique that utilizes electromagnetic energy to detect and quantify information about an object that is not in contact with the sensing apparatus.
spectrum
See optical spectrum; visible spectrum.
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