Ultrahigh-Resolution Direct-Frequency-Comb Spectrometer

At low pressures, molecular spectra typically exhibit a large number of very narrow absorption features spread across a relatively wide spectral bandwidth. In order to obtain an accurate estimate of the line shapes, widths, and depths of these absorption features it is a requirement that one performs spectroscopy that is both densely spaced and high resolution across the entire spectral range. Here we demonstrate an approach that delivers on this need: it makes use of an optical frequency comb as the light source along with an optical filter cavity and a dispersive spectrometer to acquire high-resolution spectra with dense spectral sampling. The technique has the potential to acquire spectra with a resolution limit imposed solely by the frequency stability of the comb. As a proof of concept we measure the absorption spectrum of acetylene with a 7.86-MHz spectral spacing over 3.4-THz (25 nm) spectral range. This represents more than 430 000 individual spectral measurements, which are each individually free from the instrumentation broadening that limits conventional spectrometers. Using this spectrum, we extract the thermodynamic properties (temperature, pressure, and molecular density) of the gas sample with high accuracy and precision while simultaneously performing gas compositional analysis.