Broadband frequency-comb spectroscopy

An optical frequency comb is a collection of exceedingly sharp and precisely spaced radiation “teeth” in the frequency domain, generated by a train of light pulses from a mode-locked femtosecond laser. More than 10⁵ such teeth, each one a narrowband oscillator, can be packed into a few-hundred-nanometer spectral range. The problem for spectroscopists, however, lies in separately detecting each individual comb tooth. Three physicists at NIST in Boulder, Colorado, have now done that. The trio sent the signal comb through a gas sample to be analyzed. A second, minutely different comb was used for reference, and the resulting “heterodyne beat” between the two generated a third comb in the RF range where each tooth could be easily resolved and mapped back to the original signal. In that way, the group measured the spectrum of hydrogen cyanide gas over a wavelength range of 125 nm. In just a few seconds,...