Precision metrology with coherent dual frequency combs

Summary form only given. Frequency combs have enabled a wide range of applications because of their unique combination of coherence and broad bandwidth. The comb provides 100,000´s of comb teeth at precise, well-known optical frequencies that can support broadband accurate spectroscopy or multi-wavelength interfometric ranging. However, the full advantages of the comb can be realized as a source for these and other applications only with a matched detection approach. In other words, the detection must fully resolve each individual transmitted comb tooth. It is possible to achieve this tooth-by-tooth detection through multi-heterodyne interference against a second, or dual, coherent frequency comb. Dual comb systems for spectroscopy have been pursued for several years now in different implementations. When implemented with fully coherent frequency combs over sufficiently long integration times, dual comb spectroscopy enables measurements of the absorbance and phase spectra from a gas with a resolution limited only by the comb teeth linewidth (and any system nonlinearities), yielding a frequency accuracy and precision far in excess of conventional FTIR systems and a bandwidth far in excess of tunable laser spectrometers. In our recent work, we have moved beyond the 1550-nm region to demonstrate coherent dual-comb spectroscopy over much broader near-infrared spectra and even into the mid-infrared. This system has enabled precise and accurate line-center measurements of methane, acetylene and water. In the future, coherent dual-comb spectroscopy is promising for very precise, accurate and broadband lineshape measurements, which can support accurate measurements of trace gas concentrations in air.