A new method of spectroscopy has been demonstrated at NIST-Boulder using dual optical frequency combs. The full spectrum of a gas (both absorption and dispersion) is measured over a broad spectral region and with frequency accuracy that can reach 1 Hz. The spectroscopic measurement is equivalent to laser spectroscopy with 155,000 individual single frequency lasers, but is accomplished instead with two frequency combs. Unlike conventional Fourier Transform Spectroscopy, the system has no moving mechanical parts and minimal optical alignment. Finally, the spectrum is measured rapidly so the technique should allow for high-resolution spectroscopy on dynamical systems.
The system is based on fiber-laser frequency combs, whose output forms a comb of lines spanning a wide optical spectrum. When the frequency comb is sent through a gas cell, a given comb line will be absorbed (or phase shifted) by the gas if it lies on a resonance of one of the gas molecules. The challenge in extracting the gas spectrum lies in “reading out” the amplitude and phase change separately on each individual comb line. To solve this challenge, a second phase-locked comb was mixed with the transmitted comb, thereby translating the optical spectrum directly into the RF. The present experiment interrogates the effect of the absorption from the gas on 155,000 comb lines, spanning a wavelength range of 125 nm, with a frequency resolution 6 orders of magnitude better than other spectroscopic techniques. This work represents by far the largest number of frequency comb teeth that have been individually observed. It is described in more detail in the article by I. Coddington, W. C. Swann, N. R. Newbury, Phys. Rev. Lett., 100, 103902 (2008).
Contact: Nathan Newbury, phone 303-497-4227
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