Scientists at the University of Konstanz in Germany and the National
Institute of Standards and Technology (NIST) in the United States have built
the first optical frequency comb-a tool for precisely measuring different
frequencies of visible light-that actually looks like a comb.
These are 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 1,500 such photos would need to be lined up to show the entire comb. Credit: S. Diddams/NIST
As described in the Oct. 30 issue of Science,* the "teeth" of the
new frequency comb 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 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 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-sized laser that generates super-fast, super-short
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 light waves per unit of time. The shorter the pulses
of laser light, the broader the range of frequencies produced. In the new comb
described in 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 nanometers, from 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 like 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.