Short and Sharp--Spectroscopy with Frequency Combs

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Science  21 Jan 2005:
Vol. 307, Issue 5708, pp. 364-365
DOI: 10.1126/science.1108159

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Many researchers have long assumed that continuous (or single-mode) lasers are sharper than pulsed lasers and are thus better able to resolve narrow spectral features of atoms and molecules. However, single-mode lasers are difficult to construct at short wavelengths, for example in the extreme-ultraviolet. Sharp lasers at these wavelengths have thus been difficult or impossible to come by. In his Perspective, Udem highlights an approach that may circumvent this problem. He discusses the report by Witte et al., who have used a train of coherent laser pulses rather than just a single laser pulse. The resulting spectral lines are narrower than the spectral width of a single pulse. This use of the laser's "harmonics" may enable high-resolution spectroscopy at wavelengths where no single-mode lasers are available.