Fig. 5.

High-scan-rate broadband spectroscopic measurement by PC-FTS based on a polygonal scanner. a Continuous interferograms of ¹²C₂H₂ molecules measured with the mode-locked laser. The burst of the interferogram appears every 83 µs, corresponding to a scan rate of 12 kHz. Clear modulations of the molecular free-induction decay are observed. b Non-averaged transmission spectra corresponding to the interferograms shown in Fig. 5a. Each spectrum covers over 1.5 THz with a resolution of 11.5 GHz. The absorption lines of C₂H₂ are clearly observed. The noise in the spectra can mainly be attributed to the side-modulations of the sinc function caused by the rectangular Fourier-transform windowing. Apodization (not applied here) can reduce the side-modulations at the expense of the spectral resolution. c Comparison between a 20-averaged transmission spectrum and a computationally calculated spectrum based on HITRAN database. The standard deviation of the residual is 1.7%