  Broadband dual-comb spectroscopy (DCS) based on portable mode-locked fiber
frequency combs is a powerful tool for in situ, calibration free, multi-species
spectroscopy. While the acquisition of a single spectrum with mode-locked DCS
typically takes microseconds to milliseconds, the applications of these
spectrometers have generally been limited to systems and processes with time
changes on the order of seconds or minutes due to the need to average many
spectra to reach a high signal-to-noise ratio (SNR). Here, we demonstrate
high-speed, continuous, fiber mode-locked laser DCS with down to 11 $\mu$s time
resolution. We achieve this by filtering the comb spectra using portable
Fabry-Perot cavities to generate filtered combs with 1 GHz tooth spacing. The 1
GHz spacing increases the DCS acquisition speed and SNR for a given optical
bandwidth while retaining a sufficient spacing to resolve absorption features
over a wide range of conditions. We measure spectra of methane inside a rapid
compression machine throughout the 16 ms compression cycle with 133 cm$^{-1}$
bandwidth (4000 comb teeth) and 1.4 ms time resolution by spectrally filtering
one of the combs. By filtering both combs, we measured a single-shot, 25
cm$^{-1}$ (750 comb teeth) spectrum of CO around 6330 cm$^{-1}$ in 11 $\mu$s.
The technique enables simultaneously high-speed and high-resolution DCS
measurements, and can be applied anywhere within the octave-spanning spectrum
of robust and portable fiber mode-locked frequency combs.
