The coherence, frequency accuracy, and broad optical bandwidth of frequency combs have enabled applications ranging from supporting optical clocks, to time/frequency metrology, to microwave generation, to precision molecular spectroscopy and beyond (1{4). I will focus on two very difierent applications that both rely on frequency combs to provide higher precision and accuracy than conventional methods. Both applications require the use of two or more frequency combs and ultimately require operation outside the laboratory. Such applications are only possible with the continued evoluation of flber-based frequency combs (5). First, I will discuss coherent dual-comb spectroscopy. In dual-comb spectroscopy, the complex linear response of a sample is probed at each tooth of the frequency comb via massively parallel multi-heterodyne detection. In particular, we are focus on the use of dual-comb spectroscopy for atmospheric sensing over open air paths. Dual-comb spectroscopy (DCS) ofiers several favor- able characteristics for intermediate-path sensing, including a coherent, collimated light source capable of traversing long path lengths (0.1{10km), and simultaneous broad spectral coverage with extremely high resolution to enable coverage of the absorbing wavelength regions of several species with negligible instrument-derived distortion to the absorption spectrum. I will discuss a recent demonstration of the use of DCS for simultaneous, quantitative measurements of several greenhouse gas absorbers over a 2-km path above the NIST Boulder campus using near-infrared (1.5{1.7"m) frequency combs. The second application supports state-of-the-art optical clocks or oscillators, which have reached remarkable levels of stability and accuracy (6{9). We have developed a technique to allow for comparison of two such optical clocks over a free-space link. This method essentially follows the conventional two-way time-frequency (TWTFT) technique but implemented in the optical domain rather than the rf domain. Through the two-way exchange of coherent optical pulse trains over turbulent, but nevertheless reciprocal, free-space links, one can completely cancel any timing noise associated with turbulence. Recent experiments conducted over a turbulent, 4-km free space link at the NIST Boulder campus have verifled the low residual instability and high accuracy possible with this method (10). With further development, this approach should enable the synchronization of two distant optical locks. This free-space approach can supplement current flber-optic based approaches in situations when the remote clock is not located at the end of a deployed flber link. In the future, such techniques might also support the use of optical clocks in satellite platforms. 1 REFERENCES
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