Faster repetition gives comb teeth you can see

Hank Hogan, hank@hankhogan.com

Frequency combs have been the basis for a Nobel Prize, but their “teeth” have never been visible – until now. Thanks to recent advances in laser technology, researchers at the University of Konstanz and the US National Institute of Standards & Technology (NIST) in Boulder, Colo., have created a frequency comb with teeth – individual comb frequencies – that can be seen by eye with a grating and a microscope.

NIST physicist Scott Diddams said the output from the comb is essentially the same as that of 50,000 frequency-stabilized lasers spanning the spectrum from 470 to 1130 nm. The researchers hope to use this array of stable frequencies in spectroscopy experiments aimed at identifying and controlling the quantum state of specific gases, he added.

A frequency comb with teeth that can be seen is now possible, thanks to new laser technology. Showing the entire comb would comprise 1500 such photos. Courtesy of Scott Diddams, NIST, and Albrecht Bartels, Gigaoptics GmbH.

“This could have applications for everything from basic research to chemical analysis to the detection of trace gases important for environmental or security monitoring,” Diddams said. “Another interesting application is using the comb to calibrate high-resolution astronomical spectrographs. Such a tool could be helpful to astronomers searching for planets around other stars.”

A third use involves the generation of optical and microwave waveforms with very low timing jitter. This last application exploits the frequency control of the comb as well as the ability to address and control many of its individual modes.

As for why such a comb can now be built, Albrecht Bartels, research group leader at the university and CEO of Gigaoptics GmbH, pointed to two advances. The first is the creation of a femtosecond laser with a 10-GHz repetition rate, a critical threshold for visibility.

“The 10-GHz repetition rate is important because it spreads the emitted frequency comb teeth wide enough in frequency space to be resolved with a grating spectrometer,” he said.

Besides being visible, he said, the wider spacing makes it possible to manipulate the comb on a mode-by-mode basis. It is even possible to isolate single modes and make them individually available to a spectrometer, enabling previously impossible applications.

The second enabling development was microstructured fiber technology, which allows the creation of a white light quasicontinuous spectrum when laser pulses are sent through the fiber. A key advance here was that the group achieved very efficient coupling of the laser to the continuum generation fiber, an improvement that was needed because the peak power intensity fell by the same ratio as the pulse rate went up.

The technology for such a laser has been around for years but had been applied to 1-GHz systems. It was a challenge to shrink it in size – and thereby increase the repetition rate – by a factor of 10 while still maintaining femtosecond pulse operation, Bartels said.

As for the future, the researchers do not envision the new technology making it into the field anytime soon. “We are at the early stages of these applications. There is still plenty of development at a basic level to be carried out in the lab over the coming years,” Diddams said.

The group described its work in the Oct. 30, 2009, issue of Science, with Bartels the lead author.