Fig. 4.

Broadband spectroscopic measurement by PC-FTS based on a galvanometric scanner. a Continuous interferograms of H¹²C¹⁴N molecules measured with the mode-locked laser. The single-sided interferograms are continuously obtained at a scan rate of 300 Hz (the corresponding temporal interval is 3.3 ms). The inset shows modulations due to the free-induction decay. b A broadband spectrum obtained by Fourier-transforming a single interferogram. The non-averaged broadband spectrum covering 7 THz shows absorption lines of HCN molecules at a spectral resolution of 10.1 GHz with a high signal-to-noise ratio. The structured spectral profile comes from that of the mode-locked fiber laser