  Fourier-transform infrared spectroscopy (FTIR) is the golden standard of
mid-infrared (MIR) molecular spectroscopic analysis through optically-encoded
vibrational signatures. Michelson-type FTIR and MIR dual-comb spectrometers
allow us to simultaneously investigate multiple molecular species via the
broadband and high-resolution spectroscopic capabilities. However, these are
not applicable to high-speed measurements due to the low temporal resolution
which is fundamentally limited by the signal-to-noise ratio (SNR). In this
study, we develop a high-speed FTIR spectroscopy technique called
phase-controlled Fourier-transform infrared spectroscopy (PC-FTIR) that has the
capability to measure MIR absorption spectra at a rate of above 10 kHz. PC-FTIR
demonstrates the high scan rate with a high SNR for various spectral bandwidths
by arbitrarily adjusting the instrumental spectral resolution. As a proof of
principle demonstration, we measure high-speed mixing dynamics of two liquids
at a rate of 24 kHz. We also measure MIR spectra of gas-phase molecules with
higher spectral resolution at a rate of 12 kHz. This high-speed MIR
spectrometer could be especially useful for measuring non-repetitive fast
phenomena and acquiring a large amount spectral data within a short time.
