High-resolution, high-accuracy dual comb spectroscopy with over 40 THz bandwidth
Alexander M. Zolot, Fabrizio R. Giorgetta, Esther Baumann, William C. Swann, Jeff Nicholson, Ian R. Coddington, Nathan R. Newbury
Most spectroscopic instruments directly measure optical wavelength, which is converted to frequency and calibrated against spectral features that have traditionally been measured using complicated frequency chain methods. In the past decade optical frequency combs, generated via stabilization of mode locked lasers pulse trains, have greatly simplified the measurement of optical frequencies. Comb sources have also received attention for direct spectroscopic measurements due to their many unique properties, including: broad bandwidth, a large number of discrete spectral components (i.e. comb teeth), high degree of spectral coherence, facile mixing to other spectral regions, and potential for highly accurate frequency measurements. We have realized many of these potential benefits by interfering two combs with different repetition rates, derived from erbium fiber ring lasers, on a detector to form a type of Fourier transform spectrometer (FTS). Instead of a scanning Michelson interferometer, as used in traditional FTS, the difference in comb repetition rates produces a multiheterodyne signal between the two interfering pulse trains. This setup functions purely in the time/frequency domain and is therefore impervious to the alignment and dispersion effects that limit the calibration and linearity of traditional FTS systems. Because the combs are rigorously referenced to absolute frequency standards, each tooth frequency is known with kHz-level accuracy and the uncertainty in molecular line centers should be limited only by the absorbance signal-to-noise ratio and any nonlinearities in the detection. We confirm this accuracy to sub-MHz levels by comparison to lines previously measured through saturated absorption spectroscopy.
59th Annual Western Spectroscopy Association Conference